Deep-Water Sedimentary Structures, Pliocene Pico Formation, Santa Paula Creek, Ventura Basin, California: ABSTRACT

The Pliocene Pico Formation, according to paleogeographic and paleoecologic interpretations, was deposited in marine waters at least 300 m. deep. Sedimentation of mud, sand, and some gravel was largely the result of bottom-following underflows generally travelling west. Resulting sedimentary structures--some viewed in stilled-stages of development--are: stratification with eroded and deformed contacts, internal stratification, graded bedding, small-scale cross-stratification, disturbed bedding, fossils with preferred orientation, imbricated clasts and shells, ripple marks, flame structures, pull-aparts, load pockets, load waves, and many others. Ideal graded bedding is generally rare, but most sandstones display grading superposed on other structures such as internal lamination. Thin but persistent strata with signature sedimentary structures imply infilling on a nearly horizontal sea floor by bottom-contact currents tending to level the accretional surface. Eroded and deformed contacts at the base of beds imply vigorous current impact and drag. Larger disruptions such as deformed or disturbed zones, several beds thick, may result from current drag rather than from gravity-induced downslope slumping. Accordingly, some penecontemporaneous folds are less reliable indicators End_Page 522------------------------------ of paleoslopes than current-induced structures such as cross-stratification. Underflows, acquiring energy through flow down the trough margin, probably debouched from submarine canyons; many flows were ephemeral but others deposited relatively continuously for longer periods. Most underflows possessed sufficient energy to move a tractional load, and the stronger ones vigorously eroded and disturbed the sea floor. Complexity in form and genesis of the sedimentary structures dictates a comparable complexity in the depositing current. End_of_Article - Last_Page 523------------