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.
Description:
Includes bibliographical references (pages 65-73)
California State University, Northridge. Department of Geological Sciences.