Coso Formation
Coso Formation
Pliocene to late Miocene
Consists of alluvial and lacustrine sediments and lesser interbedded pyroclastic material intermittently blanketing and onlapping the northwestern to northeastern flanks of the Coso Range in the southern Owens Valley. Fluvial packages, consisting of fanglomerates, are generally coarsest at higher elevations proximal to the source, with grain size decreasing distally and grading into and interfingering with lacustrine strata in the adjacent lowlands (Power, 1958). Fanglomerates consist of crudely bedded and poorly sorted near-source boulder conglomerates rapidly grading to moderately well-sorted pebble conglomerate and coarse sandstone; composition is dominantly arkosic, consisting of materials derived from granitic uplands, with a subordinate fraction of intermixed predominantly rhyolitic volcanic materials (especially pyroclastic), which increases in abundance more distally (Power, 1958). Lacustrine beds consist of thin-bedded and commonly tuffaceous silt and fine-grained sand, comprised of reworked volcanic and lesser granitic materials (Power, 1958). Fluvial and lacustrine strata are intercalated with typically massive and unwelded vitric-ash, crystal-ash, and pumice-lapilli tuff beds containing granitic and metamorphic rock fragments (Power, 1958).
Hosts Blancan and Hemphillian mammalian fauna (e.g., Bacon and others, 1982). Maximum age >~6 Ma based on K-Ar ages on volcanic domes intruding lower strata; minimum age >2.5 Ma based on K-Ar ages of interbedded to strictly overlying volcanic units (Bacon and others, 1982; map unit cov). Exposed stratigraphic thickness locally exceeds 200 m but base is not exposed; unconformably overlies basement and is unconformably overlain by Pleistocene volcanic rocks (map unit covy); stratigraphic sequence is continuous (conformable) in basin lowlands but strata in upland areas were deposited more episodically with intervening disconformities (Power, 1958).