Structure and stratigraphy of the Marietta District, Excelsior mountains, west-central Nevada
Bowen, Corey Scott
Oldow, John S.
Master of Arts
A thick sequence of upper Paleozoic and Mesozoic rocks are exposed in the Marietta district of the central Excelsior Mountains, westcentral Nevada. Rocks are divided into allochthonous and autochthonous terranes by the westernmost known extent of the Luning thrust, a regionally extensive thrust surface which juxtaposes pre-Tertiary layered rocks of disparate depositional and deformational histories in the Mina-Hawthorne region of west-central Nevada. The Luning allochthon in the Excelsior Mountains is an imbricate pile of thrust slices each of which is composed of a single formation or single lithologic assemblage. Lithologic units exposed in the allochthon are: (1) the Permian Black Dyke Formation of volcanic island-arc affinity consisting of extrusive and intrusive rocks of mafic to intermediate composition and volcanogenic sedimentary rocks; (2) the Jurassic-Cretaceous (?) Dunlap Formation, consisting of coarse, continentally derived conglomerate and mature quartz-arenite with interbedded shallow marine to intertidal mudstone, siltstone, and carbonates; and (3) rocks of the Marietta assemblage, possibly of Jurassic or Cretaceous age, consisting of mudstone and siltstone with intercalated conglomerate lenses, volcaniclastic sandstone, and minor volcanic flows. The autochthon in the Excelsior Mountains is composed of the Jurassic-Cretaceous (?) Dunlap Formation, compositionally similar to the Dunlap Formation in the overlying allochthon, which overlies the Permian Mina Formation with angular unconformity. The Mina Formation is an interbedded succession of pelite, bedded chert and volcanogenic turbidites of deep marine origin. Two episodes of thrusting and two phases of folding are recognized in the area studied. Folds within the allochthon and the autochthon occur as two superimposed fold sets designated P1 and P2 in order of decreasing age. First phase folds (P1) are northeasterly trending, upright tight to isoclinal major and minor folds with axial plane cleavage. Second phase folds (P2) are northwesterly trending, overturned, open to close minor folds. Three thrust nappes are recognized. Rocks within the nappes are folded in both fold sets and in one case a nappe is folded in second generation (P2) folds. In all other cases, basal thrusts of nappes truncate first and second phase folds indicating displacement after folding. Pre-Tertiary layered rocks in the Marietta district are intruded by Cretaceous granitic rocks on the west and are uncomfortably overlain by Tertiary volcanic rocks on the north. Several Cenozoic high-angle faults cut through the study area.