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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