Depositional environments of the eocene maniobra formation Northeastern Orocopia Mountains, Riverside County, Southern California

Abstract

Late-early and middle Eocene beds of the Maniobra Formation in the northeastern Orocopia Mountains, southern California are 1,460 m thick and consist predominantly of cobble-boulder conglomerate, mudstone, and sandstone. These beds lie unconformably above Cretaceous granite and quartz monzonite and unconformably below nonmarine beds of the Miocene Diligencia Formation. Four facies associations are recognized within the Maniobra Formation: 1) Shoreline, 2) Slope, 3) submarine canyon, and 4) middle-submarine fan. Beds comprising the shoreline association consist of nonmarine gruss and shallow-marine facies that lie unconformably above granitic basement. Gruss is poorly indurated, massive, coarse-grained weathered granite detritus. The shallow-marine facies includes thinly bedded, fine-grained sandstone and gravel deposits which are laminated, cross bedded, and contain marine burrows. Slope deposits consist predominantly of mudstone (facies G of Mutti and Ricci Lucchi, 1972) with conglomerate and sandstone channels (facies A). Many of the channel deposits show Bouma Ta and Tab intervals. Also present are chaotic beds (facies F) that include slumped beds, detached beds, pebbly mudstone beds, glide-blocks, and granitic rock-fall deposits. Arenaceous foraminifers present in these deposits indicate deposition at bathyal depth.

Submarine canyon facies are chiefly stacked channel complexes of cobble-boulder conglomerate (facies A) that incise into granitic basement and slope deposits. Channels have broadly concave-up bases, contain granitic clasts up to 5 m in diameter, and displaced shallowmarine fossils. These conglomerate channels grade vertically upsection into sandstone beds with repeated Ta and Tab Bouma sequences and mudstone interchannel deposits. These thinning- and fining-upward sequences are up to 25m thick. Interchannel facies consist either of thin-bedded turbidites with mostly Tbce intervals (facies D) or sandstone levee deposits. Levee deposits consist of laterally discontinuous sandstone beds, some with dune shaped tops, that interfinger abruptly with mudstone beds and conglomerate channels. Ophiomorpha and Thalassinoides burrows are common in interchannel deposits. Interchannel deposits also contain foraminifers indicative of bathyal depth. Middle-submarine fan channel deposits are laterally discontinuous, thick-bedded, amalgamated, channelized, coarse-grained sandstone and conglomerate beds (facies A and B). Many beds have erosional bases, rip-up clasts, and displaced shallow-marine fossils. Most sandstone beds are massive, though numerous beds with repeated Ta and Tab sequences do occur. These beds are arranged in thinning- and fining-upward sequences, averaging 5 m thick. Repeated thinning- and fining-upward sequences comprise sandstone packages that are up to 105m thick. Most of the middle-fan deposits consist of interchannel hemipelagic deposits with crevasse-splay channels locally. These mudstone beds yield foraminifers characteristic of middle bathyal or deeper water depths. Conglomerate analyses indicate that granite and quartz monzonite from the underlying basement were the chief source of sediment for the Maniobra Formation. A nearby metamorphic terrane also supplied a moderate amount of sediment. Submature arkose of the Maniobra Formation averages Q34 F65 L1 and Q34 P28 K38 , indicates a nearby granitic and metamorphic source terrane of high relief. Paleocurrent measurements, petrographic analyses, and facies distribution indicate southwest sediment transport from nearby granitic and metamorphic source terranes to the north and east.