A Formation of Late Miocene and Early Pliocene Age on the North Slope of the Santa Susana Mountains, California: ABSTRACT

Abstract

It is proposed to recognize a new formation for coarse- and fine-grained sedimentary rocks of late Miocene and early Pliocene age on the north slope of the Santa Susana Mountains in the east-central part of the Ventura basin. These rocks were included by Kew (1924) in the Modelo formation. They overlie and intertongue with shale of late Miocene age in the Modelo formation, and are overlain by the Pico formation of Pliocene age. The formation is well exposed near Tapo Canyon in the type region straddling the boundary between Los Angeles and Ventura counties. Two main types of sedimentary rocks are represented in the new formation: generally light-colored sandstones and conglomerates, and brown-weathering shales and mudstones. Marked lateral changes in lithology are common. The coarser-grained rocks are lenticular, but the shales and mudstones are fairly persistent. In the type region both the lower and upper boundaries are apparently gradational. In any one section the first thick-bedded pebbly sandstone lens above the shales and thin-bedded sandstones of the Modelo marks the base of the new formation. The top is drawn at the base of the first soft olive-gray siltstone containing rusty concretions, a type of lithology characteristic of the Pico formation. Along the north limb of the Pico anticline the formation has the following exposed thickness: East Canyon, 1,825 feet; Wiley Canyon, 1,750 feet; Towsley Canyon, 2,200 feet; Pico Canyon, 3,000 feet. At Tapo Canyon the formation is 4,350 feet thick. Vertical grading is present on two scales. Units tens of feet thick consist of conglomerate at the base, changing upward to thick-bedded sandstone, then alternating sandstone and mudstone, and finally mudstone at the top. Individual beds within these large units commonly are graded. This grading, the abundance of extremely angular blocks of mudstone as clasts in the conglomerates, and the presence of lump structures throughout the formation suggest the possibility that submarine slumps and turbid currents were important agents in the deposition of the coarser-grained rocks. End_of_Article - Last_Page 2631------------