Masters Thesis

Stratigraphy of early to middle(?) Triassic marine to continental rocks, Southern Inyo Mountains, California

Early Triassic marine to continental strata exposed along the west flank of the southern Inyo Mountains contain a record of transition from a tectonically quiescent marine environment to a tectonically active, terrestrial, Andean-type, volcanic-plutonic arc environment. Study of these transitional rocks reveals the following stratigraphic units (from lower to upper}: generally structureless blue-gray limestone (Bls); a heterogeneous sequence of limestone and finegrained sandstone and siltstone (Hm); yellow-orange, limonite-bearing, very fine silty marble (Ym); grayish-red and grayish-green sandstone, and siltstone, claystone, and minor conglomerate (Rm); red-brown, tabular-bedded sandstone and conglomerate (Rc); grayish-green, fine-to coarse-grained, generally poorly sorted, predominantly structureless sandstone and minor sandy conglomerate (Gc); pale-olive, calcitic claystone and minor tabular-bedded, pebble and granule conglomerate (Osh); yellowish-gray calcareous chert (Wc); grayish-red, well-sorted, fine-grained sandstone (Rss); light-brownish-gray, sandy pebble conglomerate and boulder conglomerate (Vcg); grayish-red and pale-red, well sorted, silty, fine-grained sandstone and conglomerate (Rgss); and volcanic flow rock or sill rock (Volc). The heterogeneous marine rocks (Hm) together with the thick-bedded limestone (Bls) are interpreted to be shallow marine deposits formed in a quiet-water, tidally influenced coastal area that was subject to the passage of storm waves and mild wave currents. The contact between these marine rocks and the overlying continental deposits is conformable in the south and unconformable in the north. The limonite-bearing marble (Ym) is interpreted to be a supratidal sabkha deposit. These deposits interfinger with midfan to fan-base clastic rocks of the Rm and Rc lithosomes which compose an alluvial fan assemblage named the Cerro Gordo fan. Rm is interpreted to be sheetflood deposits formed mostly at the fan-base. Conglomeratic rocks of the Rc lithosome are interpreted to be braided-stream and debris-flood deposits, generally more proximal than Rm, formed mostly on the midfan. These deposits do not contain volcanic clasts and therefore probably predate volcanic eruption in the area. Lithosomes above Rc and Rm alluvial fan deposits contain volcanic debris and therefore are interpreted to post-date volcanic eruption. These deposits are units Gc, Osh, Rss, Wc, and Vcg, and represent continued deposition on the Cerro Gordo fan following volcanic eruption. Gc deposits are volcanic rich and are interpreted to represent debris flood and waning-stage flow deposits formed in a reducing environment. The Osh lithosome is interpreted as fine-grained over- bank deposits silting up an abandoned Gc channel. Rss deposits are interpreted as midfan to fan-base interdistributary deposits formed in very shallow, braided channels as low longitudinal bars, channel fill, and as sheetflood deposits. The Vcg unit probably was deposited in a confined channel which filled with boulder conglomerate during a storm event and then with braided-stream deposits during lower stage flow. (See more in text)

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