Depositional Environment and Age of Upper Cretaceous Strata in the Santa Monica Mountains, Los Angeles County, California

S. A. Shapiro¹, J. M. Alderson², and A. E. Fritsche^1
1Dept. of Geological Sciences, California State Univ, Northridge, Northridge, CA
²Los Angeles, CA

Upper Cretaceous strata in the Santa Monica Mountains of Los Angeles County, California are assigned to the Trabuco and Tuna Canyon Formations. The Trabuco Formation, which overlies the Upper Jurassic Santa Monica Formation in angular unconformity, consists mostly of pink, cobble to boulder conglomerate, with some red and gray, arkosic sandstone and red mudstone. The coarseness of the clasts, lack of fossils, and pink color indicate a rather steeply sloping alluvial fan environment dominated by braided streams.

The Tuna Canyon Formation, which conformably(?) overlies the Trabuco, consists of grayish-orange, structureless or inverse-tonormal- graded, mostly matrix-supported, cobble conglomerate; grayish-orange, laminated, graded, or crudely cross bedded, micaceous and arkosic sandstone; and yellow-brown to black, laminated shale that in some places has been contorted and intruded by sandstone dikes and sills. Features indicative of turbidite deposition are absent. The most common ammonites found are Scaphites cf. S. planus, Otoscaphites puerculus, Baculites spp., Prionocyclus cf. P. cobbani, and Metaplacenticeras spp. Other common fossils include the bivalves Glycymeris veatchii, Pterotrigonia evansana, and Inoceramus subundatus, the gastropod Turritella chicoensis pescaderoensis, some foraminifers, and dispersed plant fragments. These fossils indicate an age of late Turonian through late Campanian for the Tuna Canyon Formation. The sedimentary characteristics indicate that the lower part of the formation was deposited on a steeply sloping fan delta that was fed by channelized debris flows. On the shallower parts of the fan delta, between the active channels, wave-dominated shoreface sand deposits were formed. Between the active channels in deeper water, hemipelagic mud and plant material were deposited, which were later subjected to slumping events due to the steepness of the slope. The upper parts of the formation represent more normal shoreface deposits that accumulated after a cessation of debris flow deposition. The fan delta model for the lower part of the Tuna Canyon Formation is consistent with a steep Andean-like coast that deepened rapidly toward a trench.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California