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The depositional history of the Sespe Formation was studied using sedimentary facies analysis, clast counts, paleocurrent and clast morphological measurements, and petrographic methods. Sedimentation is interpreted to have occurred mainly as part of two depositional sequences in a coastal-braid-plain forearc-basin setting. Both sequences are present in the Santa Ynez Mountains, whereas only the upper sequence is recognized in the eastern Santa Maria basin. The lower sequence is part of a late middle to late Eocene sedimentary offlap attributed to a high input of terrigenous sediment derived mostly from the Mojave Desert region. This sequence is capped by an intraformational erosional unconformity; much or all of the lower Oligocene section is inferred to be missing. The upper sequence is part of a late Oligocene to early Miocene sedimentary onlap and includes sediment derived from both Franciscan Complex and Mojave Desert sources. It probably represents basin backfilling in response to eustatic sea level rise. The intraformational erosional unconformity and associated upsection change to a Franciscan provenance were not necessarily created entirely by tectonism (Ynezan orogeny), as previously supposed. Regional relations suggest that the effects of a middle Oligocene eustatic sea level drop were also important. Eustatic effects may account for an upsection paleohydrological change from braided to meandering fluvial deposition in the upper depositional sequence. The observed sediment dispersal pattern is inconsistent with active faulting during sedimentation; the Ynezan orogeny can be explained by anticlinal warping of the San Rafael uplift. More recent horizontal offsets along the Santa Ynez and Little Pine Faults are suggested by the juxtaposition of contrasting lithofacies and sediment-source mismatches, but the sense, timing, and magnitude of such movements cannot be evaluated further without additional subsurface information.


Environmental Sciences | Geology