A paleontologic and petrographic analysis of claystone of the upper Miocene Castaic formation leads to the following conclusions: the eastern Ventura basin, California, was a partially landlocked bay with restricted circulation in late Upper Miocene time. Organic material was deposited slowly in shallow depressions on a clay bottom. A dwarf Foraminifera fauna evolved. Radiolaria, diatoms, and pelagic Foraminifera were occasionally swept in from the open sea. After burial and completion of pyrite filling of the diatoms and radiolaria, their siliceous frustules and tests were dissolved leaving internal molds. Partial coalification resulted in a low-grade coallike material in thin seams in the marine claystone matrix.