Depositional environments of the Avenal Sandstone of Reef Ridge, Central California

Abstract

The late early through early medial Eocene age Avenal Sandstone crops out along the west side of San Joaquin Valley, near Avenal, California. The formation has a maximum thickness of 130 m and rests unconformably on the Upper Cretaceous Panoche Formation. Northwest of Garza Peak, the Avenal grades vertically upward through fluvial-deltaic, sand-flat tidal-channel, sand-flat, and upper shoreface deposits. Fluvial-deltaic deposits are channelized, matrix-supported, pebble conglomerate, which is interbedded with lenses of coarse-grained sandstone and which locally contains megafossils. The sand-flat tidal-channel deposits consist of tidal-channel lag deposits, which are pebble-conglomerate stringers, and tidal-channel sandstone-fill deposits, which are dominated by bidirectional tabular and trough cross-bedded, parallel-laminated, ripple-laminated, and convolute-bedded, well sorted, fine-grained sandstone, that locally contains beds with grading. Sand-flat deposits are characterized by Ophiomorpha-burrowed (up to 50%), moderately to well sorted, fine-grained sandstone. Upper shoreface deposits are fine-grained sandstone defined by an increase in burrowing to between 50% and 90%. Southeast of Garza Peak, the Avenal consists of lower shoreface deposits, which are structureless (90% to 100% bioturbated), silty to fine-grained sandstone with scattered molluscan- and discocyclinid-bearing lenses. Sedimentary structures are rare. Degree of bioturbation increases upsection. Megafossils, including leaf debris and wood fragments, suggest deposition in a nearshore-marine environment, and the mollusks and discocyclinids are indicative of warm temperate to subtropical waters. Following uplift and erosion of Cretaceous strata, an eastward-sloping tidal-dominated delta and sand flat formed north of Garza Peak, while to the south a headland existed. Continued transgression brought lower shoreface sedimentation to the whole area. Absence of transition and locally shoreface deposits to the north indicates that the area was slightly uplifted and eroded during the early medial Eocene before being covered by the bathyal shale of the Kreyenhagen Formation.