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Latest Cretaceous basin formation within the
Salinian terrane of west-central California
Deveric Cardona
December 16, 2015
Introduction
Little is known about the detached crutstal block branded the Salinian
terrane. A terrane consisting of Cretaceous plutonic and older high-grade
metamorphic rocks overlain by Upper Cretaceous to Quaternary sedimentary strata.
However this study is meant to bring the pieces together and reconstruct the
Salinian history. This studies main focus investigates on the Upper Cretaceous
sedimentary succession. This is because tertiary tectonic overprints and poor rock
exposures have obscured the structures that accommodated late Cretaceous
deformation. Therefore sedimentology analyses of different areas will be used to
determine the location and depositional environment of the Salinian terrane. This
modern terrane is bounded by the San Andreas Fault to the east, the Nacimiento
Fault to the west and the big Pine Fault to the south (Figure. 1). Within the terrane
there are abrupt terminations and sharp contrasts of basement rock types along
faults, indicating early amalgamation of different basement types; these all
correspond to a composite terrane.
Tectonics and Faulting
The Salinian terrane is scattered around southern California coastal
ranges and is offset in many places; this can only be connected with faulting and in
turn tectonics. The San Andreas has right lateral motion and has offset a crustal
piece of the Salinian Terrane by 300-330 km. However new statistics suggests the
Salinian strata has traveled thousands, rather than hundreds, of kilometers from its
origin. To emphasize the hypothesis of a long travelling Salinian Terrane, samples
were gathered for paleomagnetic analysis. This paleomagnetic study showed
clockwise motion of the formation as well as large northward displacement.
Additionally faulting in this area was caused by tectonism occurring in the Neogene
period. Although the length of offset is still under scrutiny the dextral natured faults
of the California region have offset much of the Salinian rocks. This period is
indicative of a transform boundary and right lateral motion between the North
American and Pacific plates. There are strike slip faults trending northwest to the
southeast that can be correlated with the transform boundary plate motion.
Furthermore the overall tectonic setting of the Salinian terrane was highly unstable
which caused both rapid uplift and subsidence. I will discuss the relevance of these
phenomena later on in the paper.
Stratigraphic Characteristics
At main study areas of Santa Margarita Lake, Pozo Grade, Lake Nacimiento
and Junipero Serra Peak, stratigraphic columns were built for further analysis of the
Salinian terrane. The Salinian strata were deposited in the Maastrichtian time,
although late Campanian ages have also been discovered, however these times are
inconclusive due to the lack of marker beds such as tuffs or basalt flows. The
Salinian Upper cretaceous strata are mainly coarse grain sediments deposited by
high-density flows. However the clast types change due to an unroofing of the
Sierra Nevadas, which gradually changed the composition of the clasts being
deposited. The strata of the Salinian show characteristics of way marine facies
(features of a depositional environment) grading into non-marine facies, suggesting
deposition within fan-delta and sub-marine fan complexes. Ancient flow indicators
known as paleocurrents, hint that the offshore direction was toward the southwest,
meaning the shoreline trend was most likely northwest to southeast. The sediments
were being deposited on continental crust and due to data gathered, the basin never
reached 2,500 meters in depth. The Upper cretaceous deposits in this region were
more proximal than distal.
Santa Margarita Lake
West of the La Panza range lies the Santa Margarita Lake where a sequence of
around 4 km overlie a quartz-monzonitic and granodioritic basement. These rocks
are formed deep within the earth and are rich in silica. This sequence is abundant in
fossils ranging from rare Paleogene sporomorphs and Late Cretaceous to Paleogene
foraminifers. The Sequence consists of ranging sediment sizes and varying
depositional flows. The lowest seven hundred meters of this sequence were
deposited through mass flows and gravity flows. The depositional environment of
the lowest section was most likely a proximal gravelly cone that draped a fault
controlled, basin-margin slope. Above the gravel cone lies a slope channel complex
that consists of slope channel fill deposits. These fills consist of sandstone,
conglomerates and interchannel mudstones associated with turbidites and
calcareous concretions. Upsection, the slope deposits grade into nearshore
deposits. Here we see tractive flows depositing sandstones and conglomerates
showing many sedimentary structures. These sediments are being deposited in a
shallow marine environment. Continuing up the strata we transition into braided
stream conglomerate beds that formed subareal. Even further upsection we again
see marine rocks showing shelf facies. As stated earlier this region is victim to
unstable tectonic settings explaining the sporadic transitions into marine and nonmarine deposits.
Pozo Grade
In the La Panza Range along Pozo Grade road, a thin sequence (around 2 km)
of the Upper Cretaceous strata is exposed. This sequence was deposited by a
transgressive fan delta system and the beds on the base of the sequence overly a
plutonic basement. This section is correlated with debris and high-density
turbulent flows depositing on the steep alluvial fan part of the fan delta. This
section generally progresses into the delta plain and eventually the transition zone,
where stream gravels flowed into a shallow marine environment.
Lake Nacimiento
Upper Cretaceous strata are rarely exposed around Lake Nacimiento due to
heavy vegetation cover and its location in the Hunter-Ligget Military Reservation.
This sequence shows sediments from bottom to top as shallow-marine deposits to
slope mudstone and submarine turbidites. Having these deposits stacked from a
shallow marine environment to a deeper submarine turbidite, indicates an overall
transgression or sea level rise. The foraminiferal assemblages around lake
Nacimiento also show an increase in water depth because the forams tend to live in
certain water depths; this is consistent with a transgression. The sequence analyzed
in the Hunter-Ligget Military Reservation consisted mainly of sediment-gravity-flow
deposits. Some of these sediments consist of a pebbly mudstone overlain by
granular massive turbidites and conglomerate beds that have a lenticular geometry,
which are most likely the slope-channel fill deposits.
Junipero Serra Peak
West of Junipero Serra Peak there is a poorly exposed outcrop containing
mollusks and foraminifers with ages ranging from Late Cretaceous to Paleocene.
This outcrop belt has submarine canyon-fill deposits to the south overlain by
Paleocene to Eocene strata and further north only Paleocene to Eocene strata
overlying basement rock. This thinning out of the submarine canyon fill indicates a
progressive onlap (sediments being deposited on a plane). The stratigraphic
column taken in this section shows mudstones underlying shallow-marine strata
with sediment-gravity-flow deposits on top.
Throughout the sequences observed, a conglomerate bed can be found in
each. These beds consist of clast types ranging from basaltic to rhyolitic volcanic
rocks, gabbroic to granitic plutonic rocks and low- to high-grade metamorphic
rocks. These conglomerate beds helped in connecting separated stratigraphic
sections and assessing the provenance and its evolution.
Sequence Correlations
The correlations and similarities in each sequence are as follows: gently
dipping outcrops to the southeast, similar clast types in the conglomerate beds,
palecurrent directions and facies indicating Late Cretaceous sediment deposition to
the south and transgressive sequences overlying regressive sequences. Due to lack
of biostratigraphic information, marker beds and laterally persistent stratigraphic
units, the correlations observed have been hindered. However after restoration of
the land that has been obstructed by faults, many of the outcrops line up correctly
and accommodate the proposed offset.
Basin Model
Instead of the previous interpretation of the Salinian terrane containing
deposits of separate transform-margin basins, this study has shown the Salinian
terrane is more consistent with one continuous basin. This basin contains sediment
fills of a fan-delta/ submarine-fan system that sat along the margin of a topographic
high, bounded by normal faults. This can be proven by the correlating lithologic
facies of each sequence and the paleomagnetic data. After restoration of the Salinian
terrane, the observed paleocurrents suggest deposition occurring south to
southwest with a northern provenance, most likely the Sierra Nevadas. This would
indicate a coastline trending northwest to southeast. This coastline was controlled
by normal faulting of blocks, which caused uplifting of basement rock to the
northeast as well as creating a rapidly subsiding basement to the southwest. The
tectonic setting was very unstable giving sporadic changes in sedimentary facies as
well as clast types. The evolution of this Basin can be best seen in the stratigraphic
column of the Santa Margarita Lake which shows fan-deltaic progradation into deep
water, followed by marine transgression. The entire Salinian basin had continuous
deposition that accumulated around 12 km of burial depth.
Conclusion
The Salinian terrane was deposited in a northwest to southeast trending
basin, bounded by normal faulting. Due to an unstable tectonic setting the basin
experienced rapid as well as episodic subsidence. The sequences of this basin
consist of coarse grain facies of a fan-delta/ submarine-fan system, where gravity
flows deposited seaward of the marine to non-marine transition zone. The
correlating conglomerate clasts, lithologic facies, vertical successions and
connecting dips of the Salinian terrane sequences indicate a continuous basin. This
basin however has been rotated clockwise and offset by the tectonic transform
boundary motion of the Pacific and North American plates. Due to the dextral
motion of the San Andreas Fault and Neogene tectonism the Salinian terrane is now
offset and uplifted in many areas giving us the wide spread outcrops we see today.
Figure. 1
References
Grove, K., 1993 Latest Cretaceous basin formation within the Salinian terrane of
west-central California: Geological Society of America Bulletin V. 105. P.447-463
Grove, K., 1986, Depositional environments of Upper Cretaceous and lower Tertiary
strata near Nacimiento Lake, central California Coast Ranges, in Grove, K., and
Graham, S., eds., Geology of Upper Cretaceous and lower Tertiary rocks near Lake
Nacimiento, California: Society of Economic Paleontologist and Mineralogist, Pacific
Section, p. 1-16