Three named stratigraphic units that are wholly or in part of Pliocene age form discontinuous outcrops around the southeast margin of the Los Angeles basin: the Fernando Formation (lower and upper Pliocene), the Capistrano Formation (upper Miocene and lower Pliocene), and the Niguel Formation (upper Pliocene).

The Fernando Formation is exposed in the Puente Hills, in the Northwestern Santa Ana Mountains, and at Upper Newport Bay; it consists of two stratigraphic units that typically are separated by an unconformity. The thickness of the formation ranges from nearly 6,000 ft in the western Puente Hills to about 1,300 ft at Newport. The common lithologic types in the upper unit are siltstone, sandstone, and conglomerate; the lower unit is predominantly siltstone. Gaps in the faunal record preclude accurate zonation, but much of the provincial Pliocene is represented by a composite section. Using a twofold chronology, the lower unit at most localities is early Pliocene in age, and the upper unit, late Pliocene, although a precise time boundary is difficult to define on the basis of mollusks. M gafaunas from the northeast part of the basin characteristically indicate inner sublittoral environments, whereas those nearer the present coast, which contain large displaced assemblages, suggest deposition in outer sublittoral to upper bathyal depths. Basinward, the Fernando Formation grades into subsurface strata that commonly are called Repetto and Pico Formations.

The Capistrano Formation is exposed at Newport Bay and in the Capistrano syncline. Most of the formation at Newport is composed of mudstone, but near Dana Point it consists of a radiolarian mudstone facies; a deep-sea, fan-valley, coarse-grained facies; and a foraminiferal mudstone facies. In the vicinity of San Juan Capistrano, the type area, the formation is about 2,100 ft thick and is composed chiefly of mudstone. Between El Toro and Arroyo Trabuco, the Oso Member, a large lenticular sandstone unit as much as 1,400 ft thick, constitutes most of the formation. The lower part of the Capistrano Formation and the Oso Member are assigned a late Miocene age; the upper part in the San Juan Capistrano-Dana Point area is early Pliocene and locally separated from the lower part by an unconfo mity. At Newport Bay the upper part is eroded, and the formation is entirely late Miocene in age. Small assemblages of mollusks from the upper part of the formation near Dana Point are displaced, and their species content suggests deposition in the upper bathyal zone. The lower part (Miocene) of the Capistrano Formation probably correlates with the Malaga Mudstone Member of the Monterey Shale and with the upper part of the Puente Formation; the upper part (Pliocene), with the lower unit of the Fernando Formation at Newport Bay and in the Puente Hills.

The Niguel Formation caps most of the low hills between El Toro and San Juan Capistrano. Much of the formation at the northernmost and westernmost outcrops may be nonmarine, and the uppermost beds in the type area 5 mi north of San Juan Capistrano may be nonmarine. The chief constituents of the formation are sandstone, conglomerate, and siltstone. Conglomeratic beds at the base are deeply channeled into older rocks. A thickness of 350 ft is estimated for the formation in the type area. Mollusk assemblages contain many species diagnostic of a Pliocene age, and the composite fauna is provisionally assigned to the latter half of the epoch. Mixed depth assemblages are common, but the localities in the inland area contain an abundance of shallow, warm-water species, whereas those nearer th coast contain some extant species that range into upper bathyal depths. The composite megafauna is strikingly similar to those from the San Diego Formation at Pacific Beach and from the upper member of the Fernando Formation in the eastern Puente Hills; it closely resembles those from the upper part of the Pico Formation in the eastern Ventura basin and from the Careaga Sandstone in the Santa Maria district.

End_of_Article - Last_Page 438------------