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Chapter 12: Lithostratigraphy
Law of Superposition
In any succession of strata, not disturbed or overturned since
deposited, younger rocks lie above older rocks.
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Stratotype: a designated type unit or section, consisting of readily
accessible rocks, where possible, in natural outcrops, excavations,
mines, or bore holes.
Lithosome: masses of rock of essentially uniform character and
having intertonguing relationships with adjacent masses of different
lithology.
Formation: a lithologically distinctive stratigraphic unit that is
large enough in scale to be mappable at the surface or traceable in
the subsurface.
Member: smaller stratigraphic units within a formation.
Bed: the smallest stratigraphic unit and found within a member.
Group: composed of formations with some kind of stratigraphic
unity.
Supergroups: a combination of groups.
Stratigraphic relations
Contacts: planar or irregular surfaces that separate different
lithologic units.
Conformity: a surface that separates younger strata from older rocks
but along which there is no physical evidence of nondeposition.
Hiatus: a break or interruption in the continuity of of the geologic
record
Unconformity: a surface of erosion or nondeposition separating
younger strata from older rocks, that represents a significant hiatus.
Abrupt contact: directly separates beds of distinctively different
lithologies
Diastem: short hiatus in deposition with little or no erosion before
deposition is resumed.
Gradational contact: the change from one lithology to another
reflects gradual changes in the depositional conditions over time.
Vertical and lateral contacts
Conformable contact types
Progressive gradual contact: where one lithology grades into
another by progressive, more or less uniform changes in grain size,
mineral composition, or other physical characteristics.
Intercalated contacts: gradational contacts that occur because of an
increasing number of thin interbeds or another lithology that appear
upward in the section.
Pinch-out: a lateral change in lithology accompanied by
progressive thinning of units to extinction.
Intertonguing: lateral splitting of a lithologic unit into many thin
units that pinch-out independently.
Progressive lateral gradation: where one lithology grades onto
another laterally by more or less uniform changes in grain size,
mineral composition, or other physical characteristics.
Abrupt contact
Intercalated contact
Pinch-out
Unconformable contact types
Angular unconformity: younger sediments
rest upon the eroded surface of tilted or
folded rocks.
Disconformity: contact between younger
and older beds is marked by a visible,
irregular or uneven erosional surface.
Paraconformity: beds above and below the
unconformity contact are parallel with no
discernable erosion, but whose ages are
vastly different.
Nonconformity: an unconformity
developed between sedimentary rock and
older igneous or massive metamorphic rock
that has been eroded prior to being covered
by sediments.
Angular unconformity in the Grand Canyon
Cyclic Successions
Autocyclic successions:
controlled by processes that take
place within the basin itself, and
their beds show only limited
stratigraphic continuity.
Allocyclic successions: caused
mainly by variations external to
the depositional basin. Allocyclic
successions may extend over
great distances and perhaps even
from one basin to another.
Type
(order)
Other terms
Duration
m.y.
Probable cause
____
200-400
Major eustatic cycles caused
by formation and breakup of
supercontinents
10-100
Eustatic cycles induced by
volume changes in global
mid-ocean spreading ridge
system
1-10
Possibly produced by ridge
changes and continental ice
growth and decay
Fourth Cyclothem (Wanless and
Weller, 1932); major cycle
(Heckel, 1986)
0.2-0.5
Milankovitch glacioeustatic
cycles, astronomical forcing
Fifth
0.01-0.2
Milankovitch glacioeustatic
cycles, astronomical forcing
First
Second Super cycle (Vail,
Mitchum, and Thompson,
1977b); sequence (Sloss,
1963)
Third
Mesothem (Ramsbottom,
1979); mega-cyclothem
(Heckel, 1986)
Minor cycle (Heckel,
1986)
5th order
3rd & 4th order
1st & 2nd order
1st and 2nd order global sea-level cycles
Estimated mean global temperature curve for
Phanerzoic time and corresponding climate modes.
A Review of the Milankovitch cycles
Simplified, schematic representation of lithofacies
Indicating both vertical and lateral changes
Walther’s Law: The various deposits of the same facies-area
and similarly the sum of the rocks of different facies-area are
formed beside each other in space, though in a cross-section
we see them lying on top of each other…..huh?
Simply stated….those facies seen in vertical section can be
superimposed onto environment that today would have them
side by side….huh?
Transgression: the movement of a shoreline in a landward
direction.
Regression: the movement of a shoreline in a seaward
direction.
Walther’s Law illustrated by the growth of a
delta through time.
Transgression followed by regression tends to produce a wedge of
sediments in which deeper water sediments are deposited on top of
shallower water sediments in the basal part of the wedge, and
shallower water sediments are deposited on top of deeper water
sediments in the top part of the wedge.
Eustatic sea-level changes:
changes in sea level that are
worldwide and that affect sea
level on all continents
essentially simultaneously.
Relative sea-level changes:
changes of sea level that affect
only local areas. Relative sealevel changes may involve
some global eustatic change
but are also affected by local
tectonic uplift or downwarping
of the basin floor and sediment
aggradation.
Review of Lithostratigraphic terminology
Correlation of Lithostratigraphic Units
Lithocorrelation: links units of similar lithology and stratigraphic
position.
Biocorrelation: expresses similarity of fossil content and
biostratigraphic position
Chronocorrelation: expresses correspondence in age and
chronostratigraphic position.
apparent
actual
Chronocorrelation problem in the Grand Canyon
Grand Canyon
Stratigraphy
Goosenecks of the San Juan River, Utah allow
exposure of Permian-Pennsylvanian strata. The river
flows for five miles while progressing linearly only a
mile toward the Colorado River.
Lithologic similarity: established on the basis of a variety of rock
properties including gross lithology (i.e., sandstone, shale, or
limestone), color, heavy mineral assemblages or other distinctive
mineral assemblages, primary sedimentary structures such as
bedding and cross-lamination, and even thickness and weathering
characteristics.
Key bed or Marker bed: an easily recognized and relatively
ubiquitous bed that cannot be confused with any other bed, (i.e., a
thin ash bed among sedimentary beds.) Key/Marker beds are used
to help correlate beds that lie immediately above and below the
key bed.
Triassic and Jurassic formations of the Colorado Plateau
Correlation by Instrumental Well logs
Well logs are simply curves sketched on paper charts that are
produced from data obtained from measurements in well bores.
These traces record variations in such rock properties as electrical
resistivity, transmissibility of sound waves, or adsorption and
emission of nuclear radiation in the rocks surrounding a borehole.
Most of the rock properties measured by well logs are closely
related to lithology and thus assist in lithocorrelation.
Sonde: an instrument lowered to the bottom of a well bore that
measures a specific rock property. The sonde is capable of
measuring all of the beds it passes on the way to the bottom of the
bore.
Tension
measurement
Control and
recording equipment
Depth
measurement
Winch
Wirelines
Sonde
Well log types
Electric log: records the resistivity of the rock units as the sonde
passes up the bore hole in contact with the wall of the hole. **A
marine shale whose pore spaces are filled with saline formation
water will have a much lower electrical resistivity than a porous
sandstone filled with oil or gas.
Gamma ray log: measures the natural gamma radiation in rock
units.
Sonic log: measures the velocity with which a sound signal passes
through rock units—helps to determine porosity.
Formation density log: provides information about porosity and
lithology (including geochemical, formation microscanner and
magnetic susceptibility logs.)
An example of a sonic
and gamma ray log.
Correlation by use of well logs
Structural and Isopach map