Download Sequence stratigraphy and platform evolution of Lower

Sequence stratigraphy and platform evolution of Lower–Middle
Devonian carbonates, eastern Great Basin
Maya Elrick Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131
ABSTRACT
Lower–Middle Devonian carbonates (270–
400 m thick) of the eastern Great Basin
were deposited along a low-energy, westward-thickening carbonate platform. Six regional facies representing peritidal, shallow
subtidal, stromatoporoid biostrome, deep
subtidal, slope, and basin environments
are recognized. Four third-order ('1.5–2.5
m.y. durations), transgressive-regressive sequences are identified across the platformto-basin transition based on deepening and
shallowing patterns in regional facies, intensity and stratigraphic distribution of
subaerial exposure features, and stacking
patterns of fourth- to fifth-order, upwardshallowing peritidal and subtidal cycles.
Transgressive systems tracts along the
basin/slope are characterized by upwarddeepening successions of proximal through
distal turbidites overlain by fine-grained,
hemipelagic deposits. Shallow-platform
transgressive systems tracts are composed
of stacks of thicker-than-average peritidal
cycles overlain by subtidal cycles or noncyclic deep subtidal facies. Maximum flooding
zones along the shallow platform are composed of stacked peritidal cycles dominated
by subtidal facies, noncyclic deep subtidal
facies, or distinct deeper subtidal units
within successions of restricted shallow
subtidal or peritidal facies. Highstand systems tracts along the basin/slope are composed of hemipelagic deposits overlain by
distal through proximal turbidites. Highstand systems tracts along the shallow platform are characterized by upward-shallowing succession of cyclic peritidal through
shallow subtidal facies.
Sequence boundary zones (2–16 m thick)
along the shallow platform are composed of
exposure-capped peritidal and subtidal cycles that exhibit upsection increases in the
proportion of tidal-flat subfacies and increases in the intensity of cycle-capping subaerial exposure features. Sequence boundary zones along the basin/slope (6 –20 m
thick) are composed of upward-shallowing
successions of proximal turbidites or by
platform-margin peloid shoal deposits; the
absence of exposure features and meterscale cycles within basin/slope sequence
boundary zones indicates that the combined
rates of third- through fifth-order sea-level
fall rates were less than tectonic subsidence
rates.
Sequence stratigraphic correlations between contrasting facies belts of the basin/slope (section NA) and the edge of the
shallow platform (section TM) were independently verified with high-resolution conodont and brachiopod biostratigraphy.
Correlation of sequences 1– 4 with transgressive-regressive sequences of similar age
in the western, midwestern, and eastern
United States, western Canada, and Europe
indicates they are eustatic in origin.
Systems-tract scale correlations across
the study area indicate that the platform
evolved from a homoclinal ramp to a distally steepened ramp, then into a flattopped platform (sequences 1–2). An incipiently drowned, intraplatform basin
developed during sequence 3 as the result of
third-order sea-level rise and differential
sediment accumulation rates between the
platform margin and intraplatform basin.
During deposition of highstand systems
tract 3, progradation infilled the intraplatform basin, resulting in a flat-topped platform. A distally steepened ramp developed
during transgressive systems tract/maximum flooding zone 4 and evolved into a flattopped platform during highstand systems
tract 4 deposition. The four sequences stack
in an aggradational to slightly progradational pattern (‘‘keep-up’’ style sedimentation) and are bound by sequence boundary
zones rather than unconformities, suggesting that greenhouse climate modes and second-order accommodation gains related to
the lower portion of the second-order
Kaskaskia sequence controlled sequencescale stacking patterns.
GSA Bulletin; April 1996; v. 108; no. 4; p. 392– 416; 12 figures; 1 table.
392
INTRODUCTION
Sequence stratigraphic concepts and terminology were originally defined from geometric relationships of seismic reflectors in
siliciclastic systems (Vail et al., 1977). These
original concepts have been refined and integrated with data from well logs, cores, and
outcrops, and have incorporated the effects
of high-frequency sea-level oscillations
(104–105 yr) to understand the internal architecture of depositional sequences (e.g.,
Wilgus et al., 1988; Goldhammer et al.,
1990, 1993; Montañez and Osleger, 1993).
These stratigraphic techniques have been
successfully applied to outcrops across carbonate platform-to-basin transitions where
exposures are laterally continuous and permit tracing of stratal geometries and boundaries physically or by photo mosaics (Sarg,
1988; Franseen et al., 1993; Sonnenfeld and
Cross, 1993). In structurally complex regions
or where exposures are limited, such as the
Great Basin, laterally continuous, undeformed outcrops are rare, and stratal geometries cannot be traced over long distances.
This paper discusses the sequence stratigraphy and evolution of a Lower–Middle Devonian carbonate platform-to-basin system
that developed in a passive-margin setting in
the eastern Great Basin of the western
United States (Fig. 1). The Lower–Middle
Devonian deposits are present in blockfaulted mountain ranges characteristic of
the Basin and Range Province; consequently, tracing of stratal geometries and
critical horizons is not possible between isolated mountain ranges. In addition, biostratigraphically diagnostic fossils are present
only in the deeper-water deposits, thus correlations based on high-resolution biostratigraphy are limited. The main objectives of this paper are (1) to illustrate how
one-dimensional stratal stacking patterns at
individual stratigraphic sections are used to
correlate depositional sequences across a
full platform-to-basin transition; (2) to interpret platform evolution at the systems-
SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES
ably below Middle–Upper Devonian platform carbonates (Guilmette Formation and
coeval deposits) (Fig. 2). These Devonian
deposits form the top of a 4- to 7.5-kmthick succession of uppermost Precambrian
through Devonian passive-margin carbonates and siliciclastics (Stewart and Poole,
1974; Bond and Kominz, 1984). During the
latest Devonian–Early Mississippian, eastward-directed thrusting related to the Antler orogeny juxtaposed lower–middle Paleozoic deep-water deposits over coeval
shallow-water deposits along the Roberts
Mountain thrust (Fig. 1; Roberts et al., 1958;
Johnson and Pendergast, 1981; Speed and
Sleep, 1982).
Eight stratigraphic sections were measured along the platform-to-basin transition
in Nevada (Fig. 1); six of the sections are
autochthonous with respect to the Antler orogenic thrust belt, whereas the two more basinal sections were transported eastward by
Antler-related and younger compressional
events.
REGIONAL FACIES
AND DEPOSITIONAL
INTERPRETATIONS
Figure 1. Map of study area with location of measured sections and line of cross section
shown in Figure 3. Regional extent of intraplatform basin modified from Johnson et al.
(1989). PR 5 Pahranagat Range; SC 5 Schell Creek Range; SP 5 Sheep Pass, southern
Egan Range; S 5 Sunnyside, southern Egan Range; CC 5 Cherry Creek Range; NM 5
Newark Mountain, Diamond Range; TM 5 Table Mountain, Mahogany Hills; NA 5 northern Antelope Range.
tract scale, which alternated between a distally steepened ramp and a flat-topped
platform; and (3) to interpret controls on
sequence-scale stacking patterns.
In an earlier paper (Elrick, 1995), the
stratigraphy of the Lower–Middle Devonian
deposits was discussed and interpreted in
the context of understanding the spatial and
temporal distribution and origin of high-frequency, upward-shallowing cycles or parasequences. For brevity in this paper, basic
facies are summarized in table format (Table 1) and only a brief discussion of interpreted depositional environments is provided to facilitate sequence stratigraphic
interpretations. The reader is referred to Elrick (1995) for detailed discussions of facies,
subfacies, meter-scale cycles, and cycle-generating mechanisms.
GEOLOGIC SETTING
The uppermost Lower–Middle Devonian
Simonson Dolomite and stratigraphic equiv-
alents (Fig. 2) were deposited along a westward-thickening carbonate platform that extended along depositional strike from
southern Canada to southeastern California, and '400 km across strike (Fig. 1; Osmond, 1954; Johnson et al., 1989, 1991).
Oceanic deposits lay to the west of the platform, and the partially emergent Transcontinental Arch lay to the east (Sandberg et al.,
1982; Johnson et al., 1989, 1991). Throughout the Early and Middle Devonian, the regional trend of the platform-to-basin slope
break was roughly north-south through central Nevada (Fig. 1; Johnson and Murphy,
1984; Johnson et al., 1989). This relatively
stable slope-break position had existed since
the Late Silurian (Llandovery) when faultinduced(?) downdropping caused the slope
break to backstep eastward '65 km (Johnson and Potter, 1975).
Middle Devonian deposits lie disconformably above Lower Devonian platform
dolomites (Sevy Dolomite) and conform-
Six regional facies are recognized across
the platform-to-basin transition; in order of
increasing water depths these facies are
peritidal, shallow subtidal, stromatoporoid
buildup, deep subtidal (including intraplatform basin), slope, and basinal; all but the
basinal and slope facies have been dolomitized. Meter-scale, upward-shallowing cycles (or parasequences) are present in peritidal through deep subtidal facies, but
cannot be discerned in basinal, slope, and
intraplatform basin facies. As a result of extensive dolomitization and recrystallization,
meter-scale cycles are obscured within the
coarse crystalline dolomite subfacies. Detailed descriptions of the regional facies and
meter-scale cycles are given in Elrick (1995)
and Table 1; brief environmental interpretations are given below.
Four, large-scale (tens to hundreds of
meters thick), transgressive-regressive sequences (sequences 1– 4) are recognized
and correlated across the platform-to-basin
transition using deepening and shallowing
patterns in regional facies, the intensity and
stratigraphic distribution of subaerial exposure features, and changes in meter-scale,
cycle stacking patterns. Middle Devonian
time scales of Odin et al. (1982), Palmer
(1983), and Harland et al. (1982, 1989) indicate the average duration of the sequences
Geological Society of America Bulletin, April 1996
393
M. ELRICK
is '1.5–2.5 m.y.; as such, they are third-order in scale (Goldhammer et al., 1993).
In this paper, the term platform is used in
a general sense for a thick succession of
mostly shallow-water carbonates and does
not imply any specific morphology (e.g.,
Wilson, 1975; Read, 1985). The term ramp
refers to a gently dipping surface (,18
slope) where shallow-water carbonates pass
gradually into deeper-water carbonates
(Ahr, 1973; Read, 1985). As a result of longterm changes in accommodation space, the
Middle Devonian carbonate platform
changed from a ramp to flat-topped platform, and the position of the shoreline
migrated; consequently, to simplify discussions, the terms ‘‘inner’’ and ‘‘outer’’ ramp/
platform refer to present geographic positions. The inner ramp/platform includes
stratigraphic sections in eastern Nevada
(sections PR, SC, S, and SP), and the outer
ramp/platform refers to sections in central
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Nevada (CC, NM, and TM) (Figs. 1 and 3).
The term shallow ramp or platform refers
collectively to inner through outer ramp/
platform sections because, at these locations, the deposits are composed dominantly
of shallow subtidal and peritidal facies. Basinal and slope facies occur only at section
NA; consequently this section is referred to
as ‘‘basinal.’’
When interpreting the regional cross section shown in Figure 3, it is important to
realize that inner platform sections are oriented parallel to depositional strike, whereas basinal and outer platform sections are
dip-oriented sections. Because the magnitude and direction of Tertiary Basin and
Range extension and Mesozoic contraction
are similar between sections along the inner
platform (sections PR, SC, SP, S, and CC;
Levy and Christie-Blick, 1989), the relative
distance between these sections does not
change appreciably when palinspastically
restored. Consequently, the inner platform
portion of the cross section was constructed
using present (or actual) distances. Distances between dip-oriented sections CC,
NM, TM, and NA were restored using data
from Levy and Christie-Blick (1989) and
Dobbs et al. (1993).
Peritidal Facies
Peritidal facies include thin and thick
laminite beds, laminated-wavy beds, and
monomict and polymict breccias (Table 1).
These subfacies are arranged into upwardshallowing, meter-scale peritidal (capped by
tidal-flat subfacies) and, less commonly, subtidal cycles (capped by subtidal subfacies).
Thin to thick laminites subfacies are
present across the shallow platform and
form caps (0.1–2.8 m thick) to peritidal cycles and thin, transgressive bases (,0.1 m
thick) to some cycles. Thin laminites repre-
Geological Society of America Bulletin, April 1996
SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES
sent full aggradation of the seafloor to highintertidal and supratidal water depths (Hardie and Shinn, 1986). Thick laminites were
deposited in lower intertidal to restricted,
shallowest subtidal environments as indicated by the conformable stratigraphic position below thin laminites, rare skeletal and
trace fossils, and rare interbedding with
thrombolites or LLH stromatolites. The
abundance of planar laminae indicates deposition from suspension settling and traction
transport as subtidal sediment was washed
onto the tidal flats during storms and high
tides (i.e., mechanical sedimentation dominated algal binding). Both thin and thick
laminites display abundant evidence of subaerial exposure including sediment-filled
dissolution cavities, horizontal and vertical
desiccation cracks, rubble and solution-collapse breccias, microkarst erosion surfaces,
textural homogenization resulting from in-
cipient pedogenesis, and alveolar structures
(Elrick, 1995).
Laminated-wavy beds (0.2–3.3 m thick)
are present at the base or in the middle part
of peritidal cycles across the shallow platform. Ripple cross-laminae attest to traction
transport and graded planar laminae indicate deposition from waning-flow, tidal, or
storm currents. Restricted, shallow subtidal
conditions are indicated by the paucity of
skeletal and trace fossils, the stratigraphic
position below tidal-flat subfacies, and by
the lack of subaerial exposure features.
Shallow subtidal environments have been
interpreted for similar facies (‘‘ribbon
rocks’’) by Demicco (1983), Osleger and
Read (1991), and Cowan and James (1993).
Monomict and polymict breccias are
present at the tops of some peritidal cycles
along the inner and outer ramp. Cycle-capping polymict breccia beds (0.08 – 4.0 m
thick) represent solution-collapse breccias
(e.g., Kahle, 1988; Knight and James, 1991)
that developed during high-frequency (104–
105 yr) sea-level falls related to meter-scale
cycle development (Elrick, 1995). No evidence of former evaporite minerals is observed, thus it is likely that more soluble
phases such as aragonite and/or high-Mg
calcite were preferentially leached. The variety of clast compositions indicates that at
least several subfacies were involved in solution-collapse events. At section PR, sequence 4 is composed of an '60-m-thick
polymict breccia unit (Fig. 3). The origin of
this thick breccia unit is well understood;
however, it is not likely related to high-frequency sea-level fluctuations, rather to events
postdating Simonson Dolomite deposition.
Monomict rubble breccias represent the
initial breakdown of bedrock and protosoil
formation during periods of subaerial expo-
Geological Society of America Bulletin, April 1996
395
M. ELRICK
ramp of sequence 1 (sections NA and TM).
The abundance of open-marine fossil types
(crinoids and brachiopods) and stratigraphic relationships with adjacent facies indicate
deposition in normal marine, shallow subtidal environments.
Stromatoporoid Buildup Facies
Figure 2. Chronostratigraphic and biostratigraphic chart for Lower and Middle Devonian formations from shallow-water inner-platform (eastern Nevada) to deeper-water outer-platform and basin (central Nevada). Vertical lined pattern indicates unconformity.
Modified from Kendall et al. (1983), Johnson et al. (1989), and Hurtubise (1989).
sure (e.g., Meyers, 1988; Knight and James,
1991). Fitted fabrics and host-attached
clasts represent in situ fragmentation and
dissolution of lithified bedrock by downward-flowing, undersaturated fluids.
Shallow Subtidal Facies
Shallow subtidal facies include burrowmottled mudstone/wackestone, peloid-ooid
packstone/grainstone, Amphipora wackestone/grainstone, coarse crystalline dolomite, and crinoidal packstone/grainstone
subfacies (Table 1). They are arranged into
peritidal and subtidal cycles.
Burrow-mottled
dolomudstone/wackestone (0.3- to 5.5-m-thick intervals) is common along the shallow platform and forms
the base or the middle of peritidal cycles,
and the tops of subtidal cycles. Deposition in
low-energy, moderately restricted, shallow
subtidal environments is indicated by the
abundance of Thalassinoides burrows, the
paucity of skeletal fossils, the stratigraphic
position below tidal-flat subfacies, and the
lack of subaerial exposure features.
Peloid-ooid packstone/grainstone is rare
and forms the base (,4 m thick) of some
peritidal cycles along the shallow platform
or forms thick, noncyclic intervals (.20 m
thick) along the outer platform (Fig. 3). The
deposits represent current- or wave-agitated, shallow subtidal environments as indicated by coarse grain size, degree of sorting, sedimentary structures, stratigraphic
396
position below tidal-flat subfacies, and analogies with modern oolitic and peloid grainstone deposits. Peloidal-ooid beds directly
overlain by tidal-flat deposits represent
fringing shoals, whereas thick, noncyclic intervals represent amalgamated shoals that
developed along the outer platform.
Amphipora
dolowackestone/grainstone
subfacies (0.7–3.0 m thick) is most common
along the outer platform and forms the base
or the middle of peritidal cycles. The deposits are defined by the high abundance of
Amphipora stromatoporoids. The presence
of unabraded beds containing only Amphipora fossils and the conformable association with overlying tidal-flat subfacies indicate deposition in relatively low-energy,
restricted, shallow subtidal environments
(Krebs, 1971; Wong and Oldershaw, 1980).
Local patches of current-aligned fossils and
grainstone layers suggest episodic, high-energy conditions.
Coarse crystalline dolomite (1- to 50-mthick intervals) is present along the inner
through outer ramp of sequence 1. Recrystallization and dolomitization have obscured most primary features; however, the
presence of planar laminae, fenestral fabrics, cross-bedding, stromatolites, and typical shallow-water fossils, and stratigraphic
relationships with adjacent facies suggest
deposition in shallow-subtidal to tidal-flat
environments.
Crinoidal packstone/grainstone (1- to 11m-thick intervals) is present along the outer
Stromatoporoid floatstone/boundstone
(0.3– 4.0 m thick) caps subtidal cycles along
the outer and inner platform, and isolated
stromatoporoid heads occur locally in deep
subtidal (intraplatform basin facies) and peloid-ooid packstone/grainstone subfacies.
The diversity of skeletal material in the matrix between stromatoporoid heads (brachiopods, corals, gastropods, rare bryozoans, and crinoids) and the stratigraphic
association with deep subtidal facies indicate deposition in deep subtidal, openmarine environments. The relatively finegrained texture of the matrix and the lack of
current- or wave-generated features suggest
the buildups did not act as wave-resistant
barriers or reefs; rather the buildups formed
low-relief biostromes.
Deep Subtidal Facies (Including
Intraplatform Basin Facies)
Deep subtidal facies are composed of bioturbated skeletal mudstone/wackestone and
are present mainly as thick, noncyclic intervals (12– 47 m thick) along the inner and
outer platform of sequence 3, and at the
base of sequence 1 (section NA; Fig. 3). In
particular, these facies form the base of
subtidal cycles developed along the inner
platform. The fine-grained texture, lack of
wave- or current-generated sedimentary
structures, and the presence of typical,
open-marine fauna indicate deposition below storm-wave base in normal marine environments. These deep subtidal facies lie
both seaward and landward of coeval shallow subtidal/peritidal facies (Fig. 3), indicating that a deeper-water intraplatform basin developed during sequence 3 (discussed
below).
Slope Facies
Slope facies are present only in the basinal section NA and are not arranged into
meter-scale cycles.
Coarse crinoidal packstone/grainstone
beds (,0.5 cm thick) are present in the uppermost part of each sequence and are interpreted as upper slope or proximal turbid-
Geological Society of America Bulletin, April 1996
SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES
Figure 3. Palinspastically restored cross section of the platform-to-basin transition. Location of measured sections is shown in Figure 1. Note that distances shown in Figure 1 differ from the cross section because of palinspastic restoration using data of Levy and
Christie-Blick (1989) and Dobbs et al. (1993). Note also that inner platform sections lie parallel to depositional strike, and outer platform
and basinal sections are dip oriented. Stratigraphic datum of cross section is the maximum flooding zone of sequence 3. Because a single
datum within sequence 3 was used to ‘‘hang’’ the entire succession, overlying and underlying sequences are slightly distorted. Only 5 of
the 11 formation and/or member boundaries coincide with sequence boundaries.
ites derived from outer platform and/or
upper-slope crinoid thickets.
Laminated pellet-peloid packstone/grainstone lenses (,0.4 m thick, less than a few
meters wide) are present in the basal and
middle parts of sequence 2 and represent
more distal turbidites deposited along the
lower slope and toe of slope as indicated by
the fine-grained texture and intercalation
with basinal facies.
Lime-clast conglomerates (0.3- to 5.0-mthick beds) contain tabular clasts composed
of slope and basinal rock types, and individual beds commonly grade laterally into
folded, slumped, or undeformed basin/slope
facies indicating only minor downslope
transport. This subfacies is interpreted to
represent debris-flow beds deposited along
Geological Society of America Bulletin, April 1996
397
M. ELRICK
the slope and toe of slope (e.g., Cook and
Taylor, 1977).
Basinal Facies
Basinal facies are present only at section
NA.
Limestone-marl rhythmites (5- to 80-mthick intervals) were deposited below stormwave base in poorly oxygenated basinal environments. Individual, submillimeter-thick,
graded laminae within limestone and marl
layers represent suspension deposition from
either single storm events or distal turbidity
currents with the lime mud and pelleted material being derived from the adjacent shallow platform region. The rhythmic alteration of limestone and marl layers likely
reflects climatically controlled changes in
(1) fluvial and/or eolian influx of terrigenous
material into the marine environment; (2)
storm tract location, which controlled the
transport of carbonate material derived
from the shallow platform; or (3) turbidity
current generation, which controlled the
transport of siliciclastic and carbonate material to the basin (Elrick, 1993; Elrick and
Hinnov, in press).
Platy argillaceous wackestone (0.2- to 30m-thick intervals) represents deposition below storm-wave base; the presence of small
burrows and sparse skeletal material indicates an increase in bottom-water oxygen
levels relative to the limestone-marl rhythmites. The graded laminae are interpreted
as the result of distal storm or distal turbidite deposition.
METER-SCALE
UPWARD-SHALLOWING CYCLES
Peritidal through deep subtidal facies are
arranged into meter-scale upward-shallowing peritidal cycles (capped by tidal-flat
laminites) and subtidal cycles (capped by
shallow subtidal subfacies). Detailed descriptions and interpretations of the cycles
are given in Elrick (1995) and are briefly
outlined below to aid in illustrating how cycle stacking patterns and intracycle facies
composition are utilized to identify and correlate sequences, systems tracts, and sequence boundaries.
Peritidal cycles (90% of the observed cycles) are present within each of the four
shallow platform sequences. They are composed of shallow subtidal subfacies gradationally overlain by tidal-flat facies. Approximately 80% of the peritidal cycle caps show
evidence of subaerial exposure (desiccation
398
cracks, sediment- and spar-filled dissolution
cavities, rubble and solution-collapse breccias, and incipient pedogenic features).
Nearly half of the peritidal cycles have a
basal transgressive unit (upward-deepening
facies trends) overlain by a typical regressive
unit (upward-shallowing facies trends);
these are termed transgressive-prone cycles.
Subtidal cycles are limited to the basal
and middle portions of sequences and are
laterally equivalent to updip peritidal cycles.
Two types of subtidal cycles are observed.
Exposed subtidal cycles are composed of an
upward-shallowing succession of shallow
subtidal subfacies, and cycle caps display evidence of subaerial exposure (sediment- and
spar-filled dissolution cavities and brecciation). Submerged subtidal cycles are characterized by deep subtidal facies gradationally
overlain by shallow subtidal subfacies; these
subtidal cycle caps show no evidence of subaerial exposure. Both subtidal cycle types indicate incomplete infilling of accommodation space, before sea-level fall in the case of
exposed subtidal cycles, and before the succeeding transgression for submerged cycles.
The average calculated duration of peritidal
and subtidal cycles using the Odin et al.
(1982), Palmer (1983), and Harland et al.
(1989) time scales is between '50 and 130
k.y.; as such, they are fourth- or fifth-order
in scale (Goldhammer et al., 1993).
The mechanism that best explains the
abundance of cycle-capping exposure features, transgressive-prone peritidal cycles,
and subtidal cycles correlative with updip
peritidal cycles is high-frequency (104–105
yr) sea-level fluctuations (see Elrick, 1995,
for detailed discussion). Exposure-capped
peritidal and subtidal cycles are interpreted
as the result of high-frequency sea level falling below the platform surface for one, or
more likely, several sea-level oscillations
(‘‘missed beats’’; Goldhammer et al., 1990).
Transgressive-prone cycles indicate that
subtidal sediment was being produced and
deposited during transgressions; that is, sedimentation lag times were minimal during
initial flooding of the platform. Subtidal cycles correlative with updip peritidal cycles
are interpreted to reflect incomplete shallowing and infilling of accommodation space
before the succeeding high-frequency rise in
sea level. Noncyclic intervals in the intraplatform basin, slope, and basin are interpreted as the result of the seafloor lying
too deep to resolve high-frequency sea-level
oscillations (subtidal missed beats).
The autogenic tidal-flat progradation
model of Ginsburg (1971) is precluded be-
cause it requires a period of nondeposition
combined with gradual basin subsidence to
cause transgression and relative deepening,
and the presence of abundant transgressiveprone cycles indicates deposition during
transgressions. Episodic subsidence (faultinduced downdropping; Cisne, 1986) is also
precluded as a cycle-generating model because of the abundance of transgressiveprone cycles, which indicate gradual deepening from tidal flat to subtidal water depths
rather than abrupt deepening, which would
be expected from fault-induced subsidence.
The preservation of upward-deepening facies trends at cycle bases also indicates that
sediment redistribution by waves or currents
during transgressions was minor (low-energy transgressions).
DEPOSITIONAL SEQUENCES AND
SYSTEMS TRACTS
Methods used to define depositional sequences in the Middle Devonian carbonate
deposits differ from those used to define
seismic-scale sequences, because onlapping
and downlapping stratal geometries are difficult to recognize along low paleoslopes
and where stratigraphic sections are widely
separated between isolated Basin and
Range fault blocks. Instead, the Middle Devonian sequences are identified from vertical and lateral changes in regional facies,
from changes in meter-scale cycle stacking
patterns (systematic vertical changes in cycle
subfacies and cycle thickness), and from the
stratigraphic distribution and intensity of cycle-capping subaerial exposure features.
Sequences were initially identified from vertical facies changes and cycle stacking patterns in individual stratigraphic sections,
then similar patterns were correlated between adjacent sections to identify lateral or
two-dimensional facies relationships of retrogradation, aggradation, and progradation.
The bed-by-bed resolution available from
this outcrop study reveals that the changes
between retrogradational and aggradational/progradation facies patterns (maximum
flooding surface) and progradational to retrogradational patterns (sequence boundaries) are gradational over meters to tens of
meters and are composed of stacked meterscale cycles along the shallow platform and
noncyclic intervals along the basin/slope.
These gradational or transitional maximum
flooding zones and sequence boundary zones
reflect the effects of repeated fourth- to
fifth-order sea-level fluctuations superposed
upon third-order relative sea-level events
Geological Society of America Bulletin, April 1996
SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES
(see discussions in Goldhammer et al., 1993,
and Montañez and Osleger, 1993). These
transitional zones contrast with boundaries
identified seismically, because typical seismic data has the spatial resolution of a few
tens of meters; consequently, boundaries
between retrogradational and aggradational/progradational facies patterns will appear as surfaces rather than stratigraphic
zones.
In the following sections, the stratigraphic, sedimentologic, and biostratigraphic criteria used to identify and correlate systems
tracts and their boundaries are described for
shallow platform, platform margin, and basin/slope regions for each of the sequences.
Figure 4A (and Figures 6A, 8A, and 10A
below) illustrates lateral and vertical relationships of regional facies, meter-scale cycle thickness trends, and systems tracts for
sequences 1– 4. Detailed stratigraphic columns of portions of each sequence are provided to illustrate the meter-scale details of
specific boundaries and systems tracts across
the platform-to-basin transect.
Sequence Boundaries
The concept of sequences and sequence
boundaries were originally defined from
seismic-scale siliciclastic systems deposited
on shelves with recognizable shelf-slope
breaks (Vail et al., 1977). Defining type 1
versus type 2 sequence boundaries based on
the seaward extent and intensity of unconformity development was, as a consequence,
relatively straightforward. More recent sequence stratigraphic studies along carbonate ramps that lack distinct slope breaks and
in systems recording various orders of sealevel oscillation highlight the fact that type 1
versus type 2 sequence boundaries are difficult to define (Burchette and Wright, 1992;
Goldhammer et al., 1993; Montañez and
Osleger, 1993). Instead, the boundaries
along these types of platforms are typically
conformable stratigraphic intervals representing transitions between transgressive
(retrogradational) and regressive (progradational) depositional patterns (transitional
sequence boundaries of Montañez and Osleger, 1993). These transitional boundaries
are several meters to tens of meters thick
and are composed of stacked, high-frequency cycles rather than discrete, laterally
traceable surfaces.
Along the Middle Devonian platform, no
evidence of major penetrative karstification
or truncation related to prolonged (several
hundreds of thousands to millions of years)
subaerial exposure is recognized. Instead,
sequences are bounded by stratigraphic intervals (meters to tens of meters thick) of
exposure-capped cycles. Within these intervals, cycle caps typically exhibit progressive
upsection increases in the intensity of subaerial exposure features (positive chronosequences of Wright, 1994). For example,
shallow platform sequence boundary zones
are typically composed of three to six peritidal cycles; the caps to all of the cycles exhibit textural homogenization (laminite recrystallization of Elrick, 1995) and alveolar
structures related to incipient pedogenesis.
In addition to pedogenic features, the cycles
toward the top of the succession also display
dissolution cavities filled with yellow-weathering argillaceous dolomite (cavities may extend 1 m below cycle cap). The cycles at the
top of the succession may be capped by monomict rubble breccias that reflect extensive
dissolution and fragmentation of the exposed surface. Each exposure-capped cycle
is interpreted to represent a drop in fourthto fifth-order sea level below the platform
surface and exposure for time periods on the
order of 103–105 yr; successions of exposurecapped cycles indicate that the shallow
platform was affected by multiple episodes
of subaerial exposure during a long-term
accommodation minimum rather than a single, long-lived exposure event. These stratigraphic and diagenetic relationships indicate that the rates of third-order sea-level
fall were less than tectonic subsidence rates
along the shallow platform, resulting in the
development of third-order sequence boundary zones rather than third-order sequence
bounding unconformities. Physical tracing
of shallow platform sequence boundary
zones into downdip basin/slope regions is
not possible because of discontinuous outcrop exposures; however, basin/slope deposits show distinct shallowing-then-deepening
facies changes over stratigraphic intervals of
tens of meters, and these can be lithologically and biostratigraphically correlated to
similar facies patterns observed along the
shallow platform.
Depositional Sequence 1
Sequence 1 is 30 –75 m thick and includes
the Denay Limestone, upper Coils Creek
Limestone, middle and upper members of
the Sadler Ranch Formation, the coarse
crystalline members of the Oxyoke Canyon
Formation, and Simonson Dolomite (Fig. 3).
Along the shallow ramp, deposits are so
strongly dolomitized and recrystallized that
meter-scale cycles are difficult to discern;
however, large-scale (several tens of meters), transgressive-regressive facies trends
can be identified and correlated to patterns
observed in undolomitized deposits along
the ramp margin and basin/slope. The base
of sequence 1 is a widespread unconformity
that separates the Lower Devonian Sevy
Dolomite from the uppermost Lower–Middle Devonian Simonson Dolomite (Johnson
and Murphy, 1984); traced basinward
(west), the unconformity becomes a conformable succession in the Oxyoke Canyon
and Sadler Ranch Formations (Kendall et
al., 1983; Fig. 2).
At sections NA and TM, an upsection gradation from crinoidal packstone/grainstone
(shallow subtidal subfacies) to overlying darkgray, skeletal wackestone (deep subtidal facies) represents upward-deepening of the
transgressive systems tract (sections NA and
TM, Fig. 4B). Along the rest of the shallow
ramp, the basal Lower Devonian sequence
boundary is directly overlain by a succession
of sandy coarse crystalline to coarse crystalline dolomite (coarse crystalline members of
the Oxyoke Canyon Formation and Simonson Dolomite). This subfacies contains local planar laminae, cross-stratification, and
stromatolitic horizons indicating deposition
in shallow subtidal to peritidal environments;
as such, they represent a transgression over
the unconformity.
At section NA, shallowing related to the
overlying highstand systems tract is indicated by dark-gray, skeletal wackestone
(deep subtidal facies) grading into laminated pellet-peloid pack-/grainstone (distal
turbidites), and capped by coarse-grained,
graded crinoidal packstone/grainstone (proximal turbidites) (section NA, Fig. 4B). Shallowing at section TM is represented by
dark-gray, skeletal wackestone (deep subtidal facies) gradationally overlain by medium-bedded, crinoidal wackestone/packstone (shallow subtidal subfacies). Updip
along the shallow ramp, the transition between retrogradational (transgressive systems tract) and aggradational/progradational (highstand systems tract) deposition
cannot be discerned in the coarse crystalline
dolomite; however, late highstand systems
tract 1 is not as strongly recrystallized as the
underlying early highstand systems tract 1.
Consequently, a 6- to 28-m-thick interval of
stacked peritidal cycles is recognized (sections PR and CC, Fig. 4B).
Sequence boundary zone 2 along the shallow ramp is identified as a 6- to 13-m-thick
interval of exposure-capped peritidal cycles.
Geological Society of America Bulletin, April 1996
399
Figure 4. (A) Cross section of sequence 1 illustrating systems tracts, boundaries, and changes in
meter-scale cycle thickness. Symbol explanations shown in Figure 3. Dolomitization and recrystallization along the majority of the inner and outer ramp obscures meter-scale cycle recognition except
during the late highstand systems tract 1. (B) Partial stratigraphic columns illustrating details utilized to interpret systems tracts and boundaries. Scale in meters.
400
Geological Society of America Bulletin, April 1996
The two to six cycles within this interval
show upsection increases in the proportion
of tidal-flat subfacies, and each cycle cap displays evidence of prolonged subaerial exposure. For example, at the innermost ramp
section PR, the caps of the upper four peritidal cycles defining sequence boundary
zone 2 display sediment-filled dissolution
cavities that extend 0.5–1.2 m below cycle
tops; these features are absent in the underlying peritidal cycle caps (section PR,
Fig. 4B). At each of the shallow ramp locations, sequence boundary zone 2 is directly
overlain by a succession of peritidal cycles
dominated by subtidal subfacies (section
PR, Fig. 4B). At sections TM and NM, the
stratigraphic interval corresponding to sequence boundary zone 2 is covered by Quaternary colluvium.
At section NA (basin/slope), minimum
accommodation related to sequence boundary zone 2 development is marked by the
upsection transition from distal turbidites
(laminated pellet-peloid pack-/grainstone)
to proximal turbidites (coarse-grained,
graded crinoidal packstone/grainstone).
This '20-m-thick upward-shallowing succession is abruptly overlain by basinal facies
intercalated with debris flows composed of
slope-derived clasts (section NA, Fig. 4B).
There is no evidence for subaerial exposure
within this interval, implying that the combined rate of third-order and fourth- to
fifth-order sea-level fall was less than tectonic subsidence along the basin/slope.
Platform Evolution During Sequence 1
Figure 4. (Continued).
The early transgressive systems tract of
sequence 1 is characterized by peritidal to
shallow subtidal facies passing seaward into
shallow subtidal facies with no detectable
break in slope; that is, a homoclinal ramp
morphology (Fig. 5A). During maximum
flooding (maximum flooding zone 1), a
slight break in slope developed between sections NM and TM, evidenced by the deposition of shallow subtidal subfacies deposited at section NM, while sub-wave-base
facies were deposited at the adjacent section
TM (Fig. 5B). The lack of sediment gravity
flow deposits within deep subtidal facies at
sections TM and NA indicates that the gradient along this newly formed slope was not
steep enough to generate allochthonous deposits. During deposition of highstand systems tract 1, the slope break migrated seaward to between sections TM and NA, and
the slope gradient steepened enough to generate abundant distal to proximal turbidites;
that is, a distally steepened ramp (Fig. 5C).
Geological Society of America Bulletin, April 1996
401
M. ELRICK
Figure 5. Schematic depositional profiles of ramp evolution during sequence 1. Symbols
as in Figure 7; TM 5 approximate location of section TM. (A) During early transgressive
systems tract 1, a homoclinal ramp morphology developed. (B) During maximum flooding
zone 1, a slight break in slope developed between sections NM and TM (incipient distally
steepened ramp). (C) During deposition of highstand systems tract 1, the slope break
prograded seaward to between sections TM and NA, and a distally steepened ramp morphology developed.
To recapitulate, during sequence 1 the
platform evolved from a homoclinal ramp
during transgressive systems tract 1, to a
ramp with a slight break in slope between
sections NM and TM during the maximum
flooding zone (incipient distally steepened
ramp), to a distally steepened ramp with the
slope break between section TM and NA
during highstand systems tract 1. During the
late transgressive systems tract 1, when the
rate of third-order accommodation gain was
the greatest, relatively rapid shallow-water
sedimentation rates along the shallow ramp
kept pace with the accommodation gain,
while deeper subtidal accumulation rates
lagged behind the increase in accommodation. As a result, the gradient between shallow and deeper water environments steepened to generate a ramp with a slight break
in slope. Progradation during highstand sys-
402
tems tract 1 resulted in the westward migration of the slope break and a concurrent
steepening of the slope gradient.
Depositional Sequence 2
Sequence 2 is 95–180 m thick and includes
part of lower Denay Limestone, Sentinel
Mountain Dolomite, and the lower alternating member of the Simonson Dolomite
(Fig. 3). The sequence is characterized by
deepening and retrogradation of basinal,
slope, and subtidal through peritidal facies,
followed by shallowing and progradation of
shallow subtidal through peritidal facies
(Fig. 6A). Isolated stromatoporoid buildups developed along the platform margin
throughout sequence development.
An upward-deepening succession (transgressive systems tract 2) along the basin/
slope (section NA) is recorded by an abrupt
vertical change from coarse crinoidal turbidites (underlying sequence boundary zone 2)
to platy, argillaceous wackestone (basinal
facies) intercalated with lime-clast conglomerates (slope-derived debris flow beds) (section NA, Fig. 4A). Continued deep-water
conditions (late transgressive systems tract
2) is recorded by a thick, monotonous succession of limestone-marl rhythmites (basinal facies). Updip at section TM, a succession
of peritidal cycles overlain by exposed subtidal cycles reflects an increase in accommodation space related to transgressive systems
tract 2 development. Along the outer and
inner platform, evidence for upward-deepening is more subtle than that displayed in
downdip regions, but is indicated by successions of peritidal cycles that display upsection increases in the proportion of subtidal
subfacies and an associated upsection increase in the abundance and diversity of
skeletal fossils within the subtidal units (section CC, Fig. 4B); both characteristics suggest an increase in third-order accommodation space.
The maximum flooding zone separating
transgressive systems tract 2 from highstand
systems tract 2 is not recognized along the
basin/slope because of poor outcrop exposure. At section TM, facies representing
maximum water depths occur within a single
peritidal cycle whose base is composed of
dark-gray, stromatactis-bearing dolowackestone with in-growth-position colonial corals (1.5-m-thick interval); in comparison,
underlying (transgressive systems tract) and
overlying (highstand systems tract) cycles
contain subtidal units composed of bioturbated Amphipora wackestone (restricted
shallow subtidal subfacies). Along the inner
and outer platform, maximum deepening is
represented by a 7- to 20-m-thick succession
of subtidal and peritidal cycles that are dominated by shallow subtidal units (burrowmottled wackestone); in contrast, underlying and overlying cycles are composed
dominantly of tidal-flat subfacies (section
PR, Fig. 6B). Within maximum flooding
zone 2, a thin marker bed (,0.5 m) composed of Stringocephalus brachiopod– bearing wackestone can be correlated between
four shallow platform stratigraphic sections
or '140 km along depositional strike
(Fig. 6A); the presence of this marker bed
attests to the isochronous nature of the maximum flooding zone.
Shallowing along the basin/slope (highstand systems tract 2) is subtle but is represented by thin-bedded limestone-marl
Geological Society of America Bulletin, April 1996
SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES
rhythmites (basinal facies) grading upsection into medium-bedded, nodular rhythmites interbedded with laminated peloid
packstone/grainstone (distal turbidites) (section NA, Fig. 6B). Along the platform margin, shallowing is evidenced by a succession
of upward-thinning, exposed subtidal cycles
reflecting a decrease in third-order accommodation space. Shallowing related to the
highstand systems tract along the outer and
inner platform is evidenced in individual
sections by successions of peritidal cycles
dominated by tidal-flat subfacies (sections
NM and SP, Fig. 6B), and regionally by the
progradation of a thick peritidal wedge over
shallow subtidal facies (Fig. 6A).
Sequence boundary zone 3, along the
shallow platform, is recognized as a 7- to
16-m-thick interval of peritidal cycles whose
caps display sediment-filled dissolution cavities, rubble and solution-collapse breccias,
and microkarst erosion surfaces. These exposure-capped cycles are immediately overlain by noncyclic, intraplatform basin facies
or a succession of submerged subtidal cycles
(sections NM and SP, Fig. 6B). Minimum
accommodation along the platform margin
is difficult to recognize from regional facies
patterns alone; however, the highstand systems tract succession of exposed subtidal
cycles is immediately overlain by an upward-thickening succession of subtidal and
peritidal cycles that are dominated by deeper-water subtidal subfacies (section TM,
Fig. 6B).
Along the basin/slope, the third-order accommodation minimum is subtle, but is recorded as an 8-m-thick interval of mediumbedded, nodular limestone-marl rhythmites
(with sparse fossils) that are capped by a
thin crinoidal packstone/grainstone bed.
This succession is abruptly overlain by a
thick interval of thin-bedded limestone-marl
rhythmites (basinal facies) of the overlying
transgressive systems tract 3 (section NA,
Fig. 6B).
Platform Evolution During Sequence 2
Transgressive systems tract 2 inherited
the distally steepened ramp morphology of
the underlying sequence (Fig. 7A). A slight
seaward dip along the shallow ramp during
transgressive systems tract 2, maximum
flooding zone 2, and early highstand systems
tract 2 is indicated by a thicker accumulation
of shallow subtidal facies (composed of peritidal cycles dominated by shallow subtidal
units) along the outer ramp than the inner
ramp (Fig. 6A). A flat-topped platform mor-
phology evolved during late highstand systems tract 2, indicated by the presence of
cyclic peritidal facies across the entire shallow-platform region (Fig. 7B). A flat-topped
morphology is also indicated by the fact that
during the succeeding transgressive systems
tract 3, deepening is expressed by similar
facies deposited over the entire region
(Fig. 3).
The development of peritidal cycles
across the entire shallow platform
throughout sequence 2 deposition indicates
that water depths in this region never exceeded shallow subtidal water depths (,20
m), and the platform remained nearly aggraded to sea level throughout sequence development (‘‘keep-up’’ style deposition of
Schlager, 1981). The flat-topped or aggraded nature of the platform, particularly
during highstand systems tract 2, indicates
that sediment accumulation rates kept pace
with accommodation space gains. As a consequence of the shallow platform remaining
nearly aggraded to sea level throughout sequence development, large portions of the
shallow platform were subaerially exposed
during fourth- to fifth-order sea-level lowstands. Until additional accommodation
space was generated by continued subsidence or by succeeding higher amplitude
fourth- to fifth-order sea-level oscillations,
the position of the shallow platform remained above the effects of the smaller amplitude oscillations, and the sedimentary
record of one or more high-frequency sealevel oscillations was not recorded (‘‘missed
beats’’; Elrick, 1995).
Depositional Sequence 3
Sequence 3 is 90 –130 m thick and includes the Denay Limestone, Woodpecker
Limestone, basal Bay State Dolomite, and
the brown cliff and basal part of the lower
alternating members of the Simonson Dolomite (Fig. 3). The sequence is characterized by abrupt and extensive deepening atop
the underlying sequence boundary zone 3
and by the formation of a deep subtidal, intraplatform basin along the former flattopped platform. During deepening, stromatoporoid buildups developed along the
inner platform (as far landward as section
PR), while deep subtidal facies of the intraplatform basin (Woodpecker Limestone
and parts of the brown cliff member) were
deposited as far landward as section S
(Fig. 8A). This deepening was followed by
relatively abrupt shallowing and prograda-
tion by peritidal, shallow subtidal, and slope
facies (Fig. 8A).
Initial deepening marking transgressive
systems tract 3 along the basin/slope is indicated by a 50-m-thick interval of limestone-marl rhythmites (basinal facies) immediately overlying the underlying sequence
boundary zone 3 (section NA, Fig. 6B).
Deepening along the platform margin (section TM) is recorded by a succession of
thicker-than-average subtidal and peritidal
cycles, which are dominated by subtidal
units reflecting the increase in accommodation space (section TM, Fig. 6B). At sections
NM and CC, abrupt deepening is indicated
by noncyclic intraplatform basin facies
(Woodpecker Limestone) lying immediately
above peritidal cycles of the underlying sequence boundary zone 3 (section NM,
Fig. 6B). At sections SP, S, and SC, a succession of submerged subtidal cycles grading
up into noncyclic intraplatform basin facies
(brown cliff member) indicates an increase
in third-order accommodation or transgressive systems tract 3 deposition (section SP,
Fig. 6B). At the innermost platform section
(section PR), exposure-capped peritidal cycles of the underlying sequence boundary
zone are abruptly overlain by stromatoporoid buildup facies, indicating an increase in
accommodation space (section PR, Fig. 8B).
Distinct facies changes signaling maximum flooding along the basin/slope is not
observed because of poor exposure. Maximum water depths at section TM are recorded in a single peritidal cycle containing
a 5-m-thick unit of platy and bulbous stromatoporoid floatstone/boundstone (moderately deep subtidal facies) (section TM,
Fig. 8B). Along the intraplatform basin,
maximum water depths are represented by a
5- to 17-m-thick interval of thin-bedded,
noncyclic, deep subtidal facies that contain a
greater abundance of open-marine fossils
(crinoids, whole brachiopods, tentaculitids,
and bryozoans) than surrounding intraplatform basin facies (section SP, Fig. 8B). At
the most landward section PR, maximum
flooding zone 3 is similar to that developed
at section TM; maximum water depths occur
within a single subtidal cycle containing a
1.5-m-thick unit composed of platy stromatoporoid boundstone; underlying and overlying cycles deepen only to shallow subtidal
water depths represented by Amphipora
packstone and burrow-mottled wackestone
subfacies.
Shallowing related to highstand systems
tract 3 is indicated along the basin/slope by
an upward-shallowing succession of thin-
Geological Society of America Bulletin, April 1996
403
Figure 6. (A) Cross section of sequence 2; symbols as in Figure 3. (B)
Partial stratigraphic columns illustrating details utilized to interpret
systems tracts and boundaries. Symbols as in Figure 4B. Scale in meters.
404
Geological Society of America Bulletin, April 1996
SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES
Figure 6. (Continued).
bedded, limestone-marl rhythmites (basinal
facies) grading into medium-bedded, nodular rhythmites with intercalated lime-clast
conglomerates (upper slope-derived debrisflow beds), and overlain by well-sorted peloid grainstones (platform-margin shoals)
(section NA, Fig. 8B). Along the platform
margin (section TM), highstand systems
tract 3 is characterized by a succession of
upward-thinning peritidal cycles that exhibit
a concurrent increase in the proportion of
tidal-flat subfacies, which reflects a decrease
in third-order accommodation space. At
sections CC and NM, intraplatform basin
facies are overlain by a 4- to 25-m-thick interval of noncyclic, peloid-ooid packstone/
grainstone. On the landward (southeastern)
side of the intraplatform basin, shallowing
related to highstand systems tract 3 is indicated by peritidal, shallow subtidal, and
stromatoporoid buildup facies prograding
seaward over intraplatform basin facies
(sections S and SP, Fig. 8B). Progradation
from both sides on the basin margin resulted
in infilling and shallowing of much of the
accommodation space and allowed cyclic
peritidal facies to prograde across the
former intraplatform basin region.
The development of an intraplatform basin during transgressive systems tract 3 and
maximum flooding zone 3 is interpreted
from the presence of noncyclic, deep subtidal facies lying landward (or east) of cyclic
peritidal and shallow subtidal facies along
the platform margin (section TM, Fig. 8A).
Regionally, these intraplatform basin facies
Geological Society of America Bulletin, April 1996
405
M. ELRICK
tion cavities extending '0.3 m down from
cycle tops, rubble breccias, and microkarst
erosion surfaces (sections CC and S, Fig. 8B).
Along the platform margin, sequence boundary zone 4 is represented by a 7-m-thick interval composed of a polymict breccia bed
(1– 4 m thick) overlain by three exposurecapped peritidal cycles.
Along the basin/slope, sequence boundary zone 4 is represented by a 12-m-thick
interval of peloid grainstones (platformmargin shoals) that are abruptly overlain by
platy, argillaceous wackestone (basinal facies) of the overlying transgressive systems
tract 4 (section NA, Fig. 8B). Progradation
of platform-margin shoals over former basin/slope environments indicates that the
slope break now lay west of the study area;
prior to this, the slope break lay between
sections TM and NA.
Platform Evolution During Sequence 3
Figure 7. Schematic depositional profile of platform morphology during sequence 2.
TM 5 section TM. (A) A distally steepened ramp morphology developed during the late
transgressive systems tract 1/maximum flooding zone 1 with shallow subtidal and stromatoporoid buildup facies deposited along the outer ramp. (B) Seaward progradation of
cyclic peritidal facies during late highstand systems tract 2 generated a flat-topped platform morphology with a slope break lying between sections TM and NA. The gradient along
the slope was steep enough to generate minor sediment gravity flow deposits.
are present from southeastern California to
the Idaho-Nevada border and in an '20- to
100-km-wide facies belt (Fig. 1). Intraplatform basin facies were deposited in deeper
and more open-marine waters than coeval
platform deposits as suggested by (1) the
lack of coarse-grained textures or wave- or
current-reworked sedimentary structures,
(2) the lack of meter-scale cycles or subaerial exposure features, and (3) the presence
of typical open-marine fossils (crinoids, colonial corals, tentaculitids, and bryozoans).
While the platform margin was clearly top-
406
ographically higher than the intraplatform
basin, it did not act as an energy barrier because platform margin deposits lack typical
high-energy sedimentary structures/textures
(i.e., cross-beds, winnowed grainstones, and
wave-resistant reef structures).
Sequence Boundary Zone 4. Along the
shallow platform, minimum third-order accommodation is recorded in an 8- to 12-mthick zone of peritidal cycles whose caps
show abundant evidence for prolonged subaerial exposure such as alveolar structures,
desiccation cracks, sediment-filled dissolu-
A profound change in platform morphology occurs at the base of sequence 3. The
relatively flat-topped platform of the underlying sequence deepens abruptly landward
of section TM and substorm–wave-base water depths were attained across the majority
of the study area (Fig. 9A). During deposition of transgressive systems tract 3 and
maximum flooding zone 3, sediment accumulation rates did not keep pace with accommodation gains resulting in the development of an incipiently drowned
intraplatform basin. It is not clear why accumulation rates at section TM were able to
kept pace with accommodation gains while
rates landward of this region lagged behind
accommodation gains.
The decrease in accommodation space
during highstand systems tract 3 resulted in
the infilling of the intraplatform basin from
both margins (Fig. 9B). Infilling of the intraplatform basin was complete by late
highstand systems tract 3; subsequently, a
relatively flat-topped platform morphology
developed landward of section TM (Fig. 9C).
Depositional Sequence 4
Sequence 4 (55–70 m thick) includes the
Denay Limestone, the middle portion of the
Bay State Dolomite, and the upper portion
of the upper alternating member (Fig. 3).
Regional facies patterns are characterized
by minor deepening and retrogradation of
basin/slope and shallow subtidal facies, followed by basinward progradation of peritidal through shallow subtidal facies
Geological Society of America Bulletin, April 1996
SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES
(Fig. 10B). The most landward section (section PR) is composed wholly of polymict
karst breccia obscuring facies trends.
Upward-deepening facies relationships
related to transgressive systems tract 4 is
well developed along the basin/slope, while
platform-margin and shallow platform regions display only subtle changes in regional
facies trends. Upward-deepening and retrogradation defining transgressive systems
tract 4 along the basin/slope is indicated by
platform-margin grainstones abruptly overlain by a 5-m-thick interval of platy, argillaceous wackestone (basinal facies) (section
NA, Fig. 8B). Maximum flooding presumably occurs within this thin interval because it
is immediately overlain by platform marginderived debris flow beds, then peloid grainstone shoal deposits (highstand systems
tract 4). Upward-deepening facies trends at
section TM are subtle; the stratigraphic interval above the underlying sequence
boundary zone 4 is composed of a succession of peritidal cycles that have a higher
proportion of subtidal facies than the underlying succession of peritidal cycles. Maximum water depths (maximum flooding
zone 4) at section TM are recorded in a single, exposed subtidal cycle that contains a
0.5-m-thick brachiopod wackestone unit;
subtidal units in underlying (transgressive
systems tract) and overlying (highstand systems tract) cycles are composed of shallow
and/or restricted subtidal deposits (Amphipora wackestone or wavy-laminated
beds) (section TM, Fig. 10B). Along the
shallow platform, increases in third-order
accommodation (transgressive systems tract
4) are indicated by a succession of peritidal
cycles that are dominated by subtidal units
at sections SC, SP, and CC. Maximum flooding zone 4 is best developed along the outer
platform where subtidal units within peritidal cycles contain more open-marine fossils
(Stringocephalus brachiopods, colonial corals, and in-growth-position stromatoporoids) than underlying and overlying peritidal cycles (subtidal units composed of
Amphipora wackestone and wavy-laminated
subtidal subfacies) (section CC, Fig. 10B).
Distinct shallowing and highstand systems
tract development along the basin/slope are
indicated by slope facies overlain by a 60m-thick interval of peloid grainstones with
up to 10% quartz sand (section NA,
Fig. 10B). Along the shallow platform (including section TM), highstand systems
tract 4 is composed of stacked peritidal cycles that are dominated by tidal-flat subfacies (section TM, Fig. 10B). The upper
10 –13 m of the highstand systems tract at
sections S and SC are composed of ooid
grainstone– based peritidal cycles (section S,
Fig. 10B).
Sequence Boundary Zone 5. Minimum accommodation along the shallow platform is
defined by the vertical change from exposure-capped peritidal cycles to an '15-mthick interval of noncyclic shallow subtidal
facies (sections S and TM, Fig. 10B). Along
the inner platform, this shallow subtidaldominated interval represents the basal Fox
Mountain member of the Simonson Dolomite (Fig. 2).
At section NM, the upper 7 m of highstand systems tract 4 is composed of lightbeige, coarse crystalline peloidal dolomite;
this is abruptly overlain by platy, argillaceous wackestone (basinal facies) interbedded with platform margin– derived debris
flow beds (slope facies) (section NA,
Fig. 10B) and indicates a rapid increase in
third-order accommodation space related to
the overlying sequence. The upward-deepening succession above sequence boundary
zone 5 represents the Taghanic onlap of
Johnson (1970), a relatively long-lasting
transgression recognized in the western
United States, New York, Belgium, and
Germany (Johnson et al., 1985).
Platform Evolution During Sequence 4
During transgressive systems tract 4 and
maximum flooding zone 4, a distally steepened ramp developed with the break in
slope between sections TM and NA
(Fig. 11A). A slight westward-dipping gradient along the outer ramp is indicated by
thicker successions of shallow subtidal facies
at sections TM and NM than updip section
CC; in a landward direction, the transgressive systems tract 4/maximum flooding zone
4 is composed of cyclic peritidal facies, indicating that the inner part of the platform
was relatively flat topped. During deposition
of highstand systems tract 4, cyclic peritidal
facies prograded seaward across the rest of
the platform, and a flat-topped morphology
was reestablished across the majority of the
study area. Grainstone shoals developed between sections TM and NA and prograded
westward over underlying basin/slope facies
during highstand systems tract 4 (Fig. 11B);
consequently, the location of the platform
margin and the gradient along the slope cannot be discerned within the study area.
The vertical succession of facies at section
NA from highstand systems tract 3 to highstand systems tract 4 (platform-margin
grainstones, basinal facies, overlain by platform-margin grainstones), indicates that
third-order accommodation gains during
transgressive systems tract 4/maximum
flooding zone 4 were great enough to establish subwave-base water depths atop former
grainstone shoals, and that the slope between the platform margin and basin during
transgressive systems tract 4/maximum
flooding zone 4 was gentle enough to permit
the shoals to prograde seaward across the
slope and infill the remaining topography.
These relationships indicate that the volume
of sediment produced at section NA was
great enough to infill the underlying topography and permit progradation of the slope
break. Slopes greater than a few degrees
would likely have restricted seaward progradation of autochthonous grainstones because of the greater water depths and gradients developed along the slope.
BIOSTRATIGRAPHIC CORRELATION
OF SEQUENCES
Biostratigraphically diagnostic (at the
substage level) conodonts and brachiopods
are present only in basin/slope (section NA)
and some platform-margin deposits (section
TM); intraplatform basin through peritidal
facies, which compose the majority of the
sequences, contain fossils that can only constrain the deposits to a Middle–Late Devonian age (i.e., Stringocephalus brachiopods).
The high-resolution biostratigraphic control
available in the basin/slope section and the
adjacent platform-margin section permits
the correlation between contrasting facies
belts; that is, noncyclic subwave-base deposits (section NA) can be correlated biostratigraphically with cyclic, shallow subtidal and
peritidal deposits (section TM). Correlation
from the platform margin landward across
the rest of the shallow platform is based on
distinct vertical and lateral facies patterns,
cycle stacking patterns, and subaerial exposure features described above for individual
systems tracts.
The transgressive systems tracts and highstand systems tracts of sequences 1– 4 along
the basin/slope have been biostratigraphically dated using conodonts and/or brachiopods (Johnson et al., 1980, 1989, in press);
transgressive systems tracts have been dated
in sequences 1 and 4 at section TM (Fig. 3)
(Johnson et al., 1980). The maximum flooding zone 1 at section NA is in the serotinus
conodont Zone (late Emsian; Fig. 3; Johnson et al., 1980). At section TM, the maximum flooding zone 1 is represented by the
Geological Society of America Bulletin, April 1996
407
Figure 8. (A) Cross section of sequence 3; symbols as in Figure 3. Note absence of cycles
within the broad intraplatform basin. (B) Partial stratigraphic columns illustrating details
utilized to interpret systems tracts and boundaries. Symbols as in Figure 4A. Scale in
meters.
408
Geological Society of America Bulletin, April 1996
basal Sadler Ranch Formation, which is in
the serotinus Zone (Fig. 3; Kendall et al.,
1983). The Sadler Ranch Formation interfingers with the Oxyoke Canyon Formation
(Fig. 3; Kendall et al., 1983) which, in turn,
interfingers landward (east) with the coarse
crystalline member of the Simonson Dolomite (Johnson et al., 1985, 1989). The last
two units directly overlie the regional Lower
Devonian unconformity and show similar
transgressive and regressive facies patterns
as the biostratigraphically controlled portion of sequence 1 at section TM.
The transgressive systems tract 4/maximum flooding zone 4 along the basin/slope
occurs within the ensensis conodont Zone
(Johnson et al., 1980, in press). At section
TM, the maximum flooding zone 4 occurs
within western United States Faunal Interval 20 (containing Geranocephalus truncatus
brachiopods; Johnson, 1978); Faunal Interval 20 immediately overlies the ensensis
Zone, indicating that initial deepening (early transgressive systems tract 4) is within the
ensensis Zone.
The biostratigraphic and sequence stratigraphic correlations of sequences 1 and 4
bracket the ages of intervening sequences 2
and 3. Consequently, conodont dates from
the transgressive systems tract and/or maximum flooding zone of sequences 2 and 3
along the basin/slope indicate that sequence
2 is in the middle costatus Zone, and sequence 3 is in the middle kockelianus Zone
(Fig. 3; Johnson et al., in press).
Each of the biostratigraphically dated systems tracts at section NA has been recognized and dated in Idaho and Montana, the
Figure 8. (Continued).
Geological Society of America Bulletin, April 1996
409
M. ELRICK
midwestern and eastern United States, western Canada, and Europe (Johnson et al.,
1985, in press). These Early–Middle Devonian deepening events have been utilized
along with other Devonian events by Johnson et al. (1985, 1989) to generate a thirdorder sea-level curve for the Devonian
(Fig. 12). The third-order sea-level fluctuations of the latest Early–Middle Devonian,
termed T-R cycles Ic, Id, Ie, and If by Johnson et al. (1985), correlate biostratigraphically with sequences 1, 2, 3, and 4, respectively; this intrabasinal and interbasinal
correlation implies a eustatic control on the
uppermost Lower–Middle Devonian sequences recognized in this study.
DEPOSITIONAL CONDITIONS
DURING PLATFORM DEVELOPMENT
Several lines of evidence indicate that
low-energy conditions prevailed across the
Middle Devonian platform. First, typical
high-energy deposits (peloid-ooid grainstones) are restricted to specific locations,
were deposited during limited time intervals, and compose ,20% of the Lower–
Middle Devonian succession. Second, stromatoporoid buildups are associated with
fine-grained rather than coarse-grained deposits, indicating that they were not waveresistant structures that modified surrounding depositional environments. Third,
.40% of the fourth- to fifth-order cycles
preserve upward-deepening facies trends at
their bases (transgressive-prone cycles), implying that high-frequency transgressions
were low enough in energy that transgressive deposits were not redistributed offshore
by wave or tidal processes. Fourth, transgressive-prone cycles lie directly above delicate, cycle-capping microkarst features that
show no evidence of abrasion or reworking
by shoreline currents and waves (see Elrick,
1995, for further discussion).
The interpreted low-energy conditions
may be explained by prevailing paleogeographic conditions. Middle Devonian paleogeographic reconstructions of Scotese and
McKerrow (1990), Witzke (1990), and Van
der Voo (1993, p. 263) place the western
United States between paleolatitude 108S
and 208S, with the paleoshoreline trending
roughly northeast-southwest (present coordinates) (Fig. 1). Although these low paleolatitudes were likely influenced by hurricanes, it is unlikely that westward- or
southwestward-moving, Southern Hemisphere hurricanes would have affected the
western sides of continents (Marsaglia and
410
Figure 9. Schematic depositional profiles of platform morphology during sequence 3.
Symbols as in Figure 7; TM 5 section TM. (A) During deposition of transgressive systems
tract 3/maximum flooding zone 3, a deep subtidal intraplatform basin developed along
former shallow platform. (B) During the early highstand systems tract 3, ooid shoal deposits prograded landward (east) and cyclic peritidal facies prograded seaward to infill the
intraplatform basin. (C) The loss of third-order accommodation during late highstand
systems tract 3 resulted in the progradation of cyclic peritidal and platform-margin facies
across the intraplatform basin and basin/slope deposits, respectively. Because of late highstand systems tract 3 progradation, the location of the slope break and the gradient along
the slope cannot be observed within the study area.
Klein, 1983). In addition, during Devonian
time, an eastward-migrating volcanic island
arc presumably lay some distance seaward
of western North America and collided with
the continent during the latest Devonian–
Early Mississippian Antler orogeny (Johnson and Pendergast, 1981; Speed and Sleep,
1982; Burchfiel and Royden, 1991). Prior to
collision, the island arc likely acted as a barrier to open-ocean storm waves and winds,
creating relatively low-energy conditions
along the western continental margin of
North America.
SEQUENCE-SCALE STACKING
PATTERNS
Fourth- to fifth-order or high-frequency
cycle stacking patterns (graphically illustrated using Fischer plots) have been used to
estimate changes in third-order accommodation space (Read and Goldhammer,
1988). The premise for the use of cycle
stacking patterns is that the combined effects of third-order through fifth-order sealevel rises generate successions of thickerthan-average cycles that exhibit evidence of
Geological Society of America Bulletin, April 1996
SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES
dominantly submergent conditions. The loss
of accommodation space during relative
third-order sea-level fall and superposed
high-frequency sea-level fall generates successions of thinner-than-average cycles that
are dominated by peritidal facies and exhibit
evidence of prolonged subaerial exposure.
Stacking patterns of cycles composed wholly
of deeper subtidal facies should be used with
caution, because the thickness of these cycles is controlled by sediment accumulation
rates rather than sea-level– controlled accommodation space. Consequently, successions of thinner-than-average, deep subtidal– dominated cycles reflect slower
sedimentation rates in deeper water environments rather than a relative fall in thirdorder sea level.
Stacking patterns of third-order sequences can be used in a similar manner to
estimate changes in second-order (longterm) accommodation space. Intrasequence
facies distributions, facies-tract geometries,
platform morphology changes, and the nature of sequence boundaries are emphasized more than variations in sequence
thickness to estimate long-term accommodation changes. Similar to high-frequency
cycle stacking patterns, sequences capped by
highstand systems-tract peritidal facies reflect full aggradation of the platform to
long-term sea level, thus they provide a
more accurate estimate of long-term accommodation changes than do sequences
dominated by deep subtidal facies. The
comparison between basin/slope sequences
(dominated by deep subtidal facies) and
shallow platform sequences (dominated by
peritidal and shallow subtidal facies) provides a means of evaluating the relative contributions of third-order eustasy, subsidence, and sediment supply to second-order
accommodation trends.
The following discussion of second-order
accommodation changes is based on interpretations of comparisons between shallow
platform and basin/slope sequence stacking
patterns. During sequence 1, the evolution
from a homoclinal ramp (transgressive systems tract 1) to a distally steepened ramp
(highstand systems tract 1) reflects an increase in second-order accommodation
space as the former shallow-ramp region aggraded to form a nearly flat-topped morphology (keep-up sedimentation). Slope
steepening during sequence 1 is interpreted
to reflect differential sedimentation rates
along the shallow platform (relatively rapid)
versus the deeper platform region (relatively
slow). Increasing second-order accommoda-
tion space is also indicated by the development of a sequence boundary zone (indicating third-order sea-level fall was less than
tectonic subsidence across the study area)
rather than a single, regional unconformity.
During sequence 2, the generation of a
thick wedge of shallow subtidal facies during
late transgressive systems tract 2/early highstand systems tract 2, the development of an
aggradational platform-margin geometry,
and the development of a sequence boundary zone reflect continuing increases in second-order accommodation space. The aggradational facies patterns along the basin/
slope also support this interpretation
(Fig. 12).
The formation of an incipiently drowned,
intraplatform basin during transgressive systems tract 3 and maximum flooding zone 3
suggests a rapid and/or large magnitude increase in the rate of second-order accommodation gain along the shallow platform,
hence the interpreted inflection in the second-order accommodation curve, Figure 12.
However, the basin/slope region does not
record a similar magnitude or rate of accommodation gain. In fact, rock types and
facies trends of the basin/slope transgressive
systems tract 3 and early highstand systems
tract 3 are very similar to transgressive systems tract 2 and highstand systems tract 2,
despite the major morphologic and apparent accommodation differences between the
equivalent shallow-platform regions. The
difference between the apparent magnitude
or rate of third-order accommodation gain
along the shallow platform and basin/slope
during transgressive systems tract 3/maximum flooding zone 3 indicates that the accommodation gain was not solely the result
of third-order eustatic sea-level rise. The
greater apparent magnitude or rate gain
along the shallow platform suggests (1) differential subsidence rates existed between
the shallow platform and platform margin,
(2) apparent deepening was caused by sedimentation rates along the shallow platform
lagging behind platform-margin rates, (3)
the shallow platform was faulted and downdropped with respect to the platform margin, and/or (4) basin/slope environments
were insensitive recorders of third-order accommodation changes. Given the regional
extent of the intraplatform basin (.600 3
100 km; Fig. 1), the ability to correlate sequence 3 systems tracts across the platform,
and the lack of evidence of Middle Devonian tectonism in Nevada, it is unlikely that
fault-induced subsidence generated the intraplatform basin. Presently, it is not clear
which of the remaining three mechanisms,
in addition to third-order eustatic sea-level
rise, generated the incipient drowning event;
however, differential sedimentation rates
combined with sea-level rise have been invoked for other Paleozoic intrashelf basins
in North America (e.g., Markello and Read,
1982; Read, 1985).
Transgressive systems tract 3 and early
highstand systems tract 3 along the basin/
slope are very similar in lithology and facies
patterns to transgressive systems tract 2 and
early highstand systems tract 2; this is true
despite the differences in shallow platform
morphologies between the two sequences.
This suggests that depositional processes
along the shallow platform were decoupled
with the basin/slope. This decoupling is also
indicated by the fact that in each of the basin/slope sequences, there is no evidence of
nondeposition or slow deposition (condensed intervals, hardgrounds, or other features) that would correspond to times of
subaerial exposure on the shallow platform.
In other words, the majority of the flattopped shallow platform was exposed during many high-frequency sea-level falls/lowstands, thus was unable to provide sediment
to slope and basin regions; contributions
from pelagic calcareous microfossils is not
considered because they did not evolve until
the Mesozoic. The lack of evidence of nondeposition or slow deposition in basinal deposits implies that some of the deposits were
derived locally from the continuously submerged slope and/or were derived from
along-slope sediment sources via geostrophic or contour currents. Relatively high accumulation rates for basinal facies is indicated by the fact that sequences 2, 3, and 4
are thicker along the basin/slope than along
the shallow platform. If sedimentation rates
were significantly slower along the basin/
slope, in part resulting from slow deposition
or nondeposition during high-frequency exposure events, then basinward-thinning geometries or basin starvation would be observed (e.g., Upper Permian Capitan–Bell
Canyon complex of west Texas; Garber et
al., 1989).
Rapid shallowing during highstand systems tract 3, development of a flat-topped
platform, and the concurrent seaward progradation of the platform margin over the
former basin/slope (catch-up style deposition) signal a decrease in the rate of secondorder accommodation gain (Fig. 12). Again,
the formation of a sequence boundary zone
indicates that relative third-order sea-level
Geological Society of America Bulletin, April 1996
411
M. ELRICK
fall rates were less than tectonic subsidence
rates across the study area.
A decrease in the rate of second-order
accommodation gain during sequence 4 development (Fig. 12) is reflected by the decrease in sequence thickness relative to underlying sequences, development of a thin
transgressive systems tract and maximum
flooding zone (,5 m thick) along the basin,
and seaward progradation of the platform
margin. The rate decrease was not great
enough, however, to generate a type 1 sequence-bounding unconformity. Although
not described in this study, the extent and
apparent magnitude of deepening recorded
in the overlying sequence along the basin/
slope and shallow platform (Fox Mountain
member of Simonson Dolomite) indicate a
significant increase in the rate of secondorder accommodation gain related to the
Taghanic onlap (Fig. 12; Johnson, 1970).
Except for incipient drowning during transgressive systems tract 3 and maximum flooding zone 3, sediment accumulation rates along
the Middle Devonian shallow ramps/platforms kept pace with fifth- through secondorder accommodation gains, thus generating aggradational to slightly progradational
sequence-scale stacking patterns or keepup style deposition. These sequence-scale
stacking patterns and the development of
sequence boundary zones rather than sequence-bounding unconformities reflect the
effects of second-order accommodation gains
related to the lower part of the Kaskaskia
sequence of Sloss (1963) (Fig. 12). These
long-term, keep-up style sedimentation patterns are similar to those observed in many
Cambrian–Ordovician and Mesozoic rimmedshelf systems (e.g., Read, 1989; examples in
Simo et al., 1993). However, the Middle Devonian deposits lack features that characterize rimmed shelves such as platform-rimming reef complexes or grainstone shoals,
and gradients seaward of the platform margins are not steep enough to generate thick
accumulations of coarse-sediment gravityflow deposits. The aggraded nature of the
Middle Devonian shallow ramp/platform
deposits suggests that fourth- to fifth-order
sea-level oscillations were of low magnitude
(,10 m), which allowed sediment accumulation rates to keep pace with the combined
high-frequency and third-order sea-level
rise rates. These low-magnitude, fourth- to
fifth-order sea-level oscillations support interpretations for Middle Devonian greenhouse climate conditions that are characterized by globally warm temperatures, high
mean ocean temperature, and as a result,
412
Figure 10. (A) Cross section of sequence 4. Symbols as in Figure 3. Note that sequence
4 is relatively thin, deepening during transgressive systems tract 1 is minor, and facies
along the shallow platform are dominated by peritidal deposits. (B) Partial stratigraphic
columns illustrating details utilized to interpret systems tracts and boundaries. Symbols
as in Figure 4B. Scale in meters.
minor continental ice sheet development
(Fischer, 1981; Frakes et al., 1992; Wright,
1992) (see discussion in Elrick, 1995).
Low-magnitude or slow rise/fall rates of
third-order sea-level oscillations are also
suggested from the Middle Devonian deposits because each sequence boundary is gra-
dational over meters to tens of meters, indicating tectonic subsidence rates were
greater than sea-level fall rates, and fourthto fifth-order cycles are absent in intraplatform basin, slope, and basinal facies. If
third-order magnitudes or rise/fall rates
were greater, then during third-order low-
Geological Society of America Bulletin, April 1996
Figure 10. (Continued).
Geological Society of America Bulletin, April 1996
413
M. ELRICK
Figure 11. Schematic depositional profiles of platform morphology during sequence 4.
Symbols as in Figure 7; TM 5 section TM. (A) During deposition of late transgressive
systems tract 4/maximum flooding zone 4, a gentle dipping ramp with a distally steepened
edge developed between sections NA and TM. Landward of section TM, a wide region of
shallow subtidal and peritidal facies was deposited, indicating the platform was relatively
flat topped. (B) Seaward progradation of cyclic peritidal facies and peloid-ooid grainstones
during late highstand systems tract 3 resulted in the generation of a flat-topped platform
across the entire study area. Because of late highstand systems tract 3 progradation, the
location of the slope break and the gradient along the slope cannot be observed within the
study area.
stands, superposed high-frequency oscillations would have affected sedimentation
patterns in more of the deeper water environments. For example, along the intraplatform basin during high-frequency sea-level
fall and lowstand, peritidal facies would
have prograded over intrashelf basin facies
to generate meter-scale peritidal cycles,
while along the slope, platform-margin
grainstones would have prograded over basin and slope facies to generate meter-scale,
grainstone-capped cycles.
CONCLUSIONS
(1) Uppermost Lower–Middle Devonian
carbonates (270 – 400 m thick) of the eastern
Great Basin were deposited along a westward-thickening, carbonate platform. Six regional facies representing peritidal, shallow
subtidal, stromatoporoid buildup, deep subtidal, slope, and basinal environments are
recognized. Four third-order, transgressive-
414
regressive sequences ('1.5–2.5 m.y. durations) are identified based on shallowing
and deepening patterns in regional facies,
intensity and stratigraphic distribution of
subaerial exposure features, and stacking
patterns of fourth- to fifth-order, upwardshallowing cycles.
(2) Depositional sequences are 30 –180 m
thick and can be correlated across the entire
platform-to-basin transition. Transgressive
systems tracts along the basin/slope are
characterized by upward-deepening successions of proximal through distal turbidites
overlain by fine-grained hemipelagic deposits (sequences 2, 3, and 4). Transgressive systems tracts along the shallow platform are
composed of successions of peritidal cycles
overlain by subtidal cycles, successions of
thicker-than-average peritidal cycles, or by
noncyclic deep subtidal facies. Along the
shallow platform, maximum flooding zones
are defined by succession of peritidal cycles
dominated by subtidal units, intervals of
noncyclic deep subtidal facies, or distinct
deeper subtidal units within a single cycle.
Highstand systems tracts along the basin/
slope are composed of fine-grained hemipelagic deposits overlain by distal through
proximal turbidites, which may be capped
by platform-margin grainstones. Along the
shallow platform, highstand systems tracts
are well defined by upward-shallowing successions of cyclic shallow subtidal through
peritidal facies.
Sequence boundary zones (2–16 m thick)
along the shallow platform are composed of
stacked, exposure-capped peritidal and subtidal cycles that exhibit upsection increases
in the proportion of tidal-flat subfacies and
concurrent increases in the intensity of cycle-capping subaerial exposure features.
These cycle-capping exposure features indicate that the shallow platform was affected
by multiple episodes of subaerial exposure
rather than a single, long-lived exposure
event. Sequence boundary zones along the
basin/slope (6 –20 m thick) are composed of
upward-shallowing successions of proximal
turbidites or platform-margin grainstone
shoal deposits, which are immediately overlain by basinal facies. The lack of exposure
features in these sequence boundary zones
indicates that third- through fifth-order sealevel fall rates were less than tectonic subsidence rates along the basin/slope.
(3) High-resolution conodont and brachiopod biostratigraphy permits correlation
across contrasting facies belts of the basin/
slope and platform margin, while stratal
stacking patterns and exposure features enable correlation across the rest of the
shallow platform where biostratigraphically
diagnostic fossils are absent. The four sequences are biostratigraphically correlated
with previously dated, uppermost Lower–
Middle Devonian transgressive-regressive
sequences in the western and eastern
United States, western Canada, and Europe, indicating they are eustatic in origin.
(4) Subsequence-scale correlations across
the study area indicate that platform morphologies alternated between distally steepened ramps during transgressive systems
tract development and flat-topped platforms during highstand systems tract development. Except for incipient drowning
during transgressive systems tract 3 and
maximum flooding zone 3, the sequences
stack in an aggradational to slightly progradational pattern (keep-up style sedimentation) and are bounded by sequence
boundary zones rather than unconformities,
suggesting that greenhouse climate modes
Geological Society of America Bulletin, April 1996
SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES
Figure 12. Devonian third-order, eustatic sea-level curve (or more correctly termed, a relative paleobathymetric curve) modified from
Johnson et al. (1985). The second-order accommodation curve represents the envelope of third-order highstand positions and reflects the
increase in accommodation associated with the lower part of the Kaskaskia sequence of Sloss (1963). Note that incipient drowning
associated with sequence 3 is interpreted as the second-order maximum flooding zone. Third-order segments labeled Ia–IIf are previously
recognized deepening events (T-R cycles of Johnson et al., 1985, in press) in the western, midwestern, and eastern United States, western
Canada, and Europe. Biostratigraphic and sequence stratigraphic correlations recognized in this study indicate that sequences 1, 2, 3,
and 4 correlate with T-R cycles Ic, Ib, Ie, and If, respectively.
and second-order accommodation gains related to the lower part of the Kaskaskia sequence controlled sequence-scale stacking
patterns.
ACKNOWLEDGMENTS
Funding for this project was provided by
the American Chemical Society (PRF
#25593-G8) and a Research Allocations
Committee grant from the University of
New Mexico. Field assistance was gratefully
received from Tom Oesleby, Chris Andronicus, Bob Goldhammer, Gabriella Savarese,
Sue Rotto, Katerina Petronotis, and Mark
Boslough. My gratitude goes to the late J. G.
‘‘Jess’’ Johnson for our invaluable discussions about the Devonian stratigraphy/biostratigraphy of the Great Basin. The manuscript benefited from reviews by Todd
LaMaskin, Katie Giles, Brian Pratt, Carl
Drummond, Charles Kahle, Jim Markello,
Steve Reid, and Steve Dorobek.
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Geological Society of America Bulletin, April 1996