ABSTRACT: Depositional Environment of the Upper Cretaceous Mokelumne River Formation, Sacramento Basin, California

Dane S. Johnson

The Upper Cretaceous Mokelumne River formation of the southern Sacramento basin is a fluvial-deltaic sequence deposited during the late Maestrichtian epoch. Quartzose-feldspathic sediments, sourced from the rising proto-Sierra Nevada terrain toward the east, were deposited within a fore-arc basin tectonostratigraphic setting. The Mokelumne River formation contains interbedded sands and shales in the northern area which grade into massive sands in the southern area, indicating a change in depositional environments. The upper contact of the unit is an unconformity which separates the Mesozoic and Cenozoic eras. The lower contact is an unconformity in the north which grades into a conformable contact with the underlying strata toward the south. The present study examines the depositional characteristics of the Mokelumne River formation in the northern area. Wireline log analysis indicates the presence of the following lower delta-plain deposits: distributary channels, natural levees, crevasse splays, and marshes with associated lignite beds. Subaqueous delta-plain deposits include the basal sand unit, locally termed the Bunker sand. The Bunker sand exhibits a coarsening-upward sequence on wireline logs and is interpreted to represent a prodelta distal bar. The sand-shale ratio decreases northward and westward and increases toward the south. The Modelumne River formation is productive in 29 fields and has produced in excess of 500 bcf of natural gas. Hydrocarbon accumulations are formed by updip fault truncation, stratigraphic traps, and updip erosional traps sealed by overlying clays.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990