ABSTRACT Five slump faults along the rim of the Markley Gorge are identified in this paper; it is likely that many others exist. Slump faults have not been identified along the rims of the other gorges in the Sacramento Valley but their presence can not be ruled out.

Abstract The productive Upper Cretaceous-Eocene section is very complex. The principal tectonic factor is the progressive southward tilt of the Great Valley of California, broken by the transverse Stockton fault, which constitutes the southern boundary of the Sacramento Valley. This tilt is reflected in a general southerly thickening and shaling out of the section. Another factor is progressive uplift of the west side, which gradually caused isolation of the Sacramento basin. The axis of the valley is near the west side. The third major factor is differential movements within the valley, which resulted in three sub-basins somewhat different in sedimentation conditions—Rio Vista, Colusa, and Chico. The Diablo uplift in the southwest is a homocline complicated by faulting and folding. The main source of sediments was at the east, but additional sediments came from the rising west side. The sediments were reworked and redeposited by vigorous currents in an intricate and commonly unpredictable pattern determined in part by local structures. The major Midland fault zone runs west of north, from the south end of the valley to Winters and possibly farther north. It diminishes above the Domengine Sandstone. A peculiar feature of the valley is the presence of erosional channels—Princeton gorge in the north near the west side (Capay fill in Upper Cretaceous rocks); Markley gorge (Sacramento Canyon; Markley fill in contact with progressively older Eocene-Cretaceous beds); and the newly discovered Brentwood gorge in the Diablo slope. There is considerable controversy about the nature of Meganos “shaleways” east of the Midland fault and north and south of the River Island area. The writer's contention is that these are remnants of the original Rio Vista basin on both sides of a Meganos delta. The other view ascribes their origin to density currents, surface erosion, and other factors.

Abstract The geologic description of Maine Prairie gas field is based primarily on detailed analysis and correlation of electric logs by the writer. The Maine Prairie gas field is a multizone dry- gas field consisting of five separate pools. The principal, or Wineman pool, which constitutes the northern two-thirds of the field, is the result of a combination trap and is unusual in that updip closure is provided by the impermeable Markley gorge, a channel-like feature of late Eocene age, traceable on the northeast and southwest for many miles. The Maine Prairie gas field includes approximately 2,100 acres, and estimated ultimate recoverable reserves are approximately 75,000 million cu ft, according to the California Division of Oil and Gas. The nearly pure methane gas produced has a heating value of approximately 1,000 Btu/cu ft and is sold exclusively to Pacific Gas and Electric Company.

ABSTRACT Upper Cretaceous and Paleogene strata in the forearc Sacramento Basin have been affected by fault activity associated with a north-northwest trending syndepositional growth fault system. This system, the Midland Fault System, is the dominant structural feature in the southern Sacramento Basin. Syndepositional movement along the Midland Fault System has resulted in a thickening of Upper Cretaceous and Paleogene strata along the western (downthrown) side of the growth faults, and the formation of domal structures on the downthrown side of growth faults at the Bunker, Lindsey Slough, and Rio Vista gas fields. Late Paleocene-early Eocene growth fault activity resulted in the formation of the domal structure at the Rio Vista Gas Field, as evidenced by the thinning of the Anderson Sand and Meganos Shale near the crest of this structure. The relatively constant thickness of the McCormick Sand and Martinez Shale west of the Midland Fault, and the increased rate of post-Martinez Paleogene deposition stepping westward across the fault system, indicates a Late Paleocene through Eocene age for most Midland Fault System activity in the Rio Vista area. Early Eocene movement along the Midland Fault System has resulted in the removal of the Hamilton Sand, Meganos Shale, Anderson Sand, Martinez Shale, and the upper portion of the McCormick Sand east of the fault system, with only the lower portion of the McCormick Sand remaining east of the fault system.

ABSTRACT A prominent erosion and fill feature has been observed in the subsurface of the Sacramento Valley in Northern California. The feature, which has been termed a “gorge”, extends for approximately 65 km along a narrow, sinuous trend that is generally parallel to the course of the Sacramento River. As much as 600 m of Upper Cretaceous deltaic sediments have been removed by erosion and the trough later filled with sediments that have been correlated with the Eocene B-2 through B-4 Zones of Laiming (1940 , 1943 ). It is suggested that submarine erosion was initiated by the uplift of adjacent land areas with consequent rejuvenation of the stream. Filling of the trough was started as the sediments built up on the basin floor and the marginal land mass was reduced by erosion. Foraminifera, as well as certain textural features of the sediments, indicate that the entire sequence was laid down in a marine environment. There is no evidence in the fill or in the underlying beds that subaerial processes were involved. This erosional feature is important for hydrocarbon exploration in that natural gas is present in sediments within the “gorge” as well as being trapped in Cretaceous sediments truncated by the “gorge”.

ABSTRACT The Markley Canyon is a large, ancient erosional feature in the Sacramento Valley which has truncated Eocene through Cretaceous sediments. Erosion of the canyon was post-middle Eocene CP14a (and probably CP14b) Subzone based on calcareous nannofossils, post-A-1 Zone based on benthic foraminifera, and occurred primarily during the late-Eocene (CP15 Zone) early to early late Refugian benthic foraminiferal Stage. Filling of the canyon began in the earliest Oligocene CP16a Subzone, latest Refugian Stage, and continued into the early Oligocene upper CP16 Zone, early Zemorrian Stage. Samples studied from the lower part of the Markley Canyon Fill and from other widespread areas in California demonstrate that the late Refugian, Uvigerina vicksburgensis benthic foraminiferal Zone is in part correlative to the early Oligocene Coccolithus subdistichus (CP16a) calcareous nannofossil Subzone, with no evidence of diachroneity.