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.
Description:
Includes bibliographical references (pages 99-112)
California State University, Northridge. Department of Geological Sciences.