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ABSTRACT

During late Cenozoic time, the San Joaquin basin was an inland sea that was connected to the Pacific Ocean through a relatively narrow strait west of Coalinga, California. The San Joaquin sea became progressively restricted in late Miocene and Pliocene time due to the northwestward migration of the Salinian block and uplift of the Temblor and Diablo Ranges, and by the beginning of Pleistocene time, the sea had completely withdrawn from the San Joaquin basin.

The upper Miocene and Pliocene (~7 to ~2.2 Ma) Etchegoin Group forms the basis for the interpretation of the late Neogene regressive history of the San Joaquin basin. The Etchegoin Group is comprised of the Jacalitos, Etchegoin, and San Joaquin Formations, that together attain a maximum thickness of 8100 ft (2470 m). These formations consist of siltstone, sandstone, conglomerate, and rare tuff beds that are arranged in the following facies associations: low-energy tide-dominated, high-energy tidedominated, low-energy shelf, and storm-dominated shelf facies associations. These facies associations include middle to outer shelf deposits, and tidal channel, tidal flat, intertidal shoal, and sand-wave deposits formed in estuarine environments.

The Jacalitos Formation (maximum thickness of 2477 ft; 755 m) is composed of sandstone that was derived dominantly from the Coast Ranges and a granitic terrane west of the San Andreas fault. The Jacalitos Formation records the shallowing of the San Joaquin basin that began during deposition of the underlying Reef Ridge Shale in the late Miocene. Marine invertebrate macrofaunal assemblages suggest that water depths during deposition of the upper part of the Jacalitos Formation reached a maximum of 121 ft (37 m).

The Etchegoin Formation (maximum thickness of 5485 ft; 1672 m) consists of sandstone that is rich in andesitic detritus, and was mainly derived from the Sierran magmatic arc, the Quien Sabe volcanics, and uplifted Franciscan-Coast Range source terranes. Deposition of this formation occurred largely in bay and estuarine environments.

Sandstone of the San Joaquin Formation (maximum thickness of 2232 ft; 680 m) was derived from the dissected Sierran magmatic arc, the Coast Ranges, and possibly the Gabilan Range west of the San Andreas fault. Brackish-water conditions periodically existed during deposition of the San Joaquin Formation and were associated with restriction of the San Joaquin sea.

Well-log cross sections of the lower Etchegoin Group that are correlated with measured surface sections are presented in order to illustrate typical well-log signatures of the Etchegoin Group and to show that the depositional environments and geometry of the sandstone beds interpreted from outcrop exposures are manifested in the subsurface. The Jacalitos Formation is characterized by relatively flat Spontaneous Potential (SP) curves, representing siltstone, and by serrated SP and resistivity curves, representing interbedded siltstone and sandstone. The Etchegoin Formation in the study area is marked by a sharp-based, fining-upward sandstone unit at its base, and is dominated by fining-upward sandstone units, interpreted as tidal channel deposits, and blocky to coarsening-upward sandstone units, interpreted as intertidal shoal and sand-wave deposits. The channels and shoals are generally elongate in the northwest-southeast direction. Many of the sandstone units are lenticular and encased in finer grained sedimentary deposits, suggesting that the Etchegoin Formation sandstone units could form good petroleum reservoirs.

The principal factor controlling the late Miocene and Pliocene sedimentary history of the San Joaquin basin was tectonism. Tectonic subsidence of the San Joaquin basin clearly kept pace with sedimentation in order to allow for the accumulation of up to 8100 ft (2470 m) of sediment deposited at intertidal to neritic water depths. During the middle to late Pliocene, rates of sediment accumulation exceeded rates of basin subsidence, leading to the regression of the San Joaquin sea.

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