Abstract: Changes in Miocene Paleobathymetry of California Continental Borderland North of 32°

R. E. Arnal, J. G. Vedder

Dart and gravity cores from the banks and ridges of the California continental borderland were processed to obtain microfossils. Benthic foraminiferal assemblages were used as depth and time indicators to establish seven paleobathymetric facies diagnostic of lagoonal to abyssal depths (0 to > 2,500 m).

Early Miocene assemblages suggest lower bathyal and abyssal depths. In middle Miocene time, the eastern flank of the Santa Rosa Cortes Ridge separated two regions. On the east, the paleobathymetry generally was midbathyal except near San Clemente Island where neritic conditions existed. On the west, greater relief prevailed with one physiographic high southwest of San Nicolas Island, and another south of Tanner basin. South of the Santa Rosa Cortes Ridge midbathyal to abyssal depths prevailed. The late Miocene bathymetry was similar to that of the middle Miocene east of the Santa Rosa Cortes Ridge, with the exception of abyssal depths directly west of Santa Barbara Island. The physiographic high southwest of San Nicolas Island persisted into early Mohnian time but may have extended fa ther northward.

A detailed examination of the paleobathymetry for the middle and late Miocene reveals: (1) rapid changes in depth over short distances similar to modern bathymetric gradients for the California continental borderland; (2) general increase in water depth from early Mohnian to late Mohnian and Delmontian time; (3) continuation of the deepening trend in the outer part of the region investigated as indicated by early Pliocene abyssal assemblages.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA