A 790-m thick vertical section of lower middle Eocene sandstone represents a regressive deltaic sequence that grades upward from delta front through lower delta plain into upper delta plain. The sandstone crops out as a narrow strip of overturned east-west strata and south-plunging open folds within the Topatopa Mountains, 16 km north of the town of Piru, California.

The sandstone gradationally overlies prodelta/transition-zone siltstone of uppermost "Capay stage" (early Eocene) and underlies, with a 15° angular unconformity, Eocene to Oligocene nonmarine boulder conglomerate of the Sespe Formation.

The lower 95 m of the sandstone is characterized by an upward-coarsening sequence, fine at the base to medium at the top, laminated to bioturbated sandstone. Bioturbation is as high as 75% with Ophiomorpha burrows common. Shallow marine mollusks diagnostic of the "Domengine stage" (late early through early middle Eocene) occur within lenses of medium sandstone. These rocks were deposited in a shoreface environment on a delta front.

Above the delta-front deposits are about 100 m of coarse sandstone. Structureless and bioturbated sandstone is dominant, but herringbone cross-bedding, planar cross-bedding 10-150 cm high, planar lamination, and scour-and-fill structures are common. Structures are interpreted as megaripples, sand waves, and channels in a tidal sand-flat environment. Coal lenses higher in the section are interpreted to be marsh deposits. Bioturbation can be up to 80% and Ostrea occurs locally. This tidal flat differs from most cited modern examples because it lacks a mud component or associated mud-flat deposits. The tidal sand-flat and marsh deposits were formed within a lower delta plain. Overlying these rocks is a 205-m interval of interfingering tidal-flat and braided-river deposits.

The remaining 390 m is comprised mostly of a sequence of fining-upward cycles overlain by an interval of structureless sandstone. Fining-upward sequences consist of trough cross-bedded, very coarse sandstone at the base grading into red and green mudstone at the top. Environmental interpretations are diagonal bars for the sandstone and abandoned channels for the mudstone. The structureless sandstone locally contains scour-and-fill structures and gravel lenses. No fossils were found. This uppermost 390-m thick section was deposited by a braided-river system within an upper delta plain. About 1.5 km to the west, these braided-river deposits thin to about 140 m, and the interval of interfingering tidal-flat and braided-river deposits thickens to 380 m.

The shoreface, tidal-flat, and braided-river deposits within these Eocene beds are similar to those found on the modern Frazier River deltaic system in Vancouver, British Columbia.

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