Paleocurrent Patterns in Upper Cretaceous Beds of Marsh Creek Formation of Northern Diablo Range, California: ABSTRACT

Abstract

Cretaceous turbidite beds that crop out along the northeastern flank of the Diablo Range between Altamont Pass and Mount Diablo exhibit numerous paleocurrent features. More than 400 pieces of paleocurrent data from sole marks, sand grain and conglomerate clast fabrics, carbonaceous fragment orientation, and parting lineations collected from outcrops within a 300-sq-mi area were used to deduce that the trend of the ancient currents which deposited these beds was from northwest to southeast. The majority of the beds studied dip homoclinally northeast into the San Joaquin Valley. Therefore, the paleocurrent analysis was dominantly two dimensional for each time-stratigraphic interval in the section. The sequence studied is more than 20,000 ft thick and ranges in age from Maastrichtian to Turonian, but a few localities may be as old as Aptian. Dating of the beds was based mainly on microfossils, but a few ammonites and other diagnostic Upper Cretaceous megafossils also were found. A change in thickness of the sequence from about 10,000 ft to more than 20,000 ft takes place along the strike of the beds from near Mount Diablo in the northwest corner of the area studied to Altamont Pass in the southeastern part of the area. Current trends observed near the base of the section show little deviation from those noted higher up and, therefore, the pattern of current flow must have remained static in this area for the duration of Late Cretaceous time. Deviations from the general paleocurrent pattern were observed in some of the paleocurrent features studied, but the number of these deviations is minor compared with the majority of the data collected. End_of_Article - Last_Page 561------------