Extensive drilling in the southern Ventura basin in the last decade has provided useful data about Pliocene basin floor topography. The pre-basinal Miocene Modelo Formation, mainly siliceous shale, limestone, and organic shale with a bathyal microfauna, underwent considerable submarine tilting and faulting, which produced a surface of high relief upon which the deep-sea Plio-Pleistocene Pico Formation was deposited. Among the land forms on this surface was a seaknoll at South Mountain.

The seaknoll is characterized by relatively uneroded fault scarps with slopes exceeding 40 degrees, a veneer of glauconite sandstone locally containing a talus of Modelo limestone fragments from the scarps, and, near the seaknoll summit, a small biostrome in a shelly glauconite sandstone matrix. The steep fault scarps at South Mountain, near the center of the basin, contrast with the bevelled fault scarps at nearby Berylwood anticline and the Oxnard Plain, near the southern edge of the Pliocene basin and on the site of pre-Pico submarine slope eroded in the Modelo Formation.

The Pico Formation has onlapped and buried the seaknoll and the submarine slope. The Pico contains graded sandstones interbedded with siltstones containing indigenous bathyal and displaced neritic microfaunas. The sandstone beds lens out and the enclosing siltstones thin in the direction of the seaknoll, suggesting that the seaknoll underwent less subsidence than the surrounding sea floor during its burial by Pico sediments.

Because the Berylwood-Oxnard submarine slope was in the path of turbidity currents carrying sediment from subaerially eroded highlands on the south and east, it underwent submarine erosion. The seaknoll, because it was separated from the subaerial highlands by deep-sea channels, was relatively unaffected by turbidity current scour. Both submarine slope and seaknoll remained below sea level until buried.

Erosional submarine unconformities of the Berylwood and Oxnard Plain submarine slope type may be relatively common in diastrophic basins where rapid sedimentation and strong deformation are closely related. Above these unconformities, the immediately overlying sediments are deep-sea rather than shallow-water. Because certain deep-sea currents can erode the sea floor if given enough time, erosional unconformities cannot be used as evidence for subaerial erosion without additional supporting data, nor can onlapping depositional surfaces be equated with former sea levels.