Transgression and regression in the Sacramento Basin during Late Cretaceous and Paleogene highstands and lowstands with implications for the history of the Sierra Nevada, California: Part 2—Paleogene

Between 27 and 30 third-order Paleocene and Eocene eustatic cycles of falling sea level are recorded on various published global sea-level charts. Several were of sufficient magnitude to cause erosion in the Sacramento Basin, California, and create regional unconformities. Four are major erosion surfaces that floor fluvial valleys that cut down a few hundred meters into the Cretaceous delta plain. The others are low-angle disconformities that record more moderate erosional events. The unconformities divide the strata into seven stratigraphic sequences.

The valleys trend west or southwest to a deep graben that formed in the southwestern corner of the basin when crustal extension caused subsidence of the subduction complex that had flanked the basin during the Cretaceous. Normal faults in the Kirby Hill fault system became active and created a horst block that separated the graben into the shallow Rio Vista basin on the east from the deeper Suisun basin to the west. North-south sediment transport in the Cretaceous was replaced by northeast-southwest transport, a new shelf edge formed along the horst block, and Paleocene rivers cut submarine canyons through the shelf edge, feeding submarine fans into the Suisun basin.

Planktic Foraminifera and calcareous nannoplankton in valley deposits indicate that valley filling took place in the middle Paleocene (Selandian Stage), late Paleocene (Thanetian Stage), early Eocene (Ypresian Stage), and latest Eocene−early Oligocene (Rupelian Stage) during planktic foraminiferal and nannoplankton zones P3 (CP3), P5−P6a (CP9a), P7 (CP9b−CP10), and P18 (CP16−CP17). This implies that incision occurred in CP2−early CP3 (62−61 Ma), CP8b (56−55 Ma), and CP15 (38−37 Ma), when some sea-level charts record major sea-level falls of >250 ft (75 m). Downcutting in the Sierra Nevada likely occurred during those events. Sea level began rising after these lowstands and reached highstands in the latest Thanetian and Ypresian, when the valleys and the eroded delta plain were buried by transgressive marine shale and shoreline sand bodies were plastered against the Midland Fault scarp in the Rio Vista graben.

Less than 200 ft (60 m) of strata were removed during early and late Paleocene (Danian and Thanetian Stages) and early Eocene (early and late Ypresian Stages) erosion. These events took place during medium sea-level falls of between 80 and 250 ft (25−75 m). They were succeeded by widespread transgressions to or east of the present boundary between the basin and Sierra Nevada when sea level rose. For example, the disconformity between the middle Ypresian Capay Shale and late Ypresian Domengine Formation formed when a river flowing from the Sierra eroded 100−125 ft (30−40 m) into the Capay. During the Capay transgression in CP10−CP11 (53−50 Ma), the crystalline bedrock in the Sierra was undergoing deep chemical weathering, creating a thick kaolinitic-quartzose lateritic soil that was eroded when sea level began falling in CP12a between 50 and 49 Ma.

Fault displacement on the eastern margin of the Rio Vista graben accelerated during the middle Eocene, causing the shelf edge to backstep to the Midland Fault. No new submarine canyon was cut through the shelf, but arkosic sediment was shed from an uplift southwest of the basin during CP14a and was deposited in a submarine fan that spread eastward through the Suisun graben into the Rio Vista graben. It was dammed against normal faults in the Denverton Creek and Midland fault systems and subsequently buried by a delta that prograded to the southwest from an andesitic source northeast of the Sierra Nevada. This river cut a deep valley through its delta in the late Eocene and filled it in the early Oligocene.

Another unconformity formed in the middle Oligocene after nannoplankton zone CP19. It is overlain by Miocene alluvial fans along the eastern edge of the basin and by channel deposits in the Sierra Nevada. These rivers carried rhyolitic silt, sand, and gravel from calderas in central Nevada but did not reach the Rio Vista graben. This unconformity may be partly due to sea-level fall at ca. 30 Ma, but the development of coarse-grained alluvial fans implies steeper stream gradients and major uplift east of the basin after the Eocene.