Abstract

The Pershing district of the southern Humboldt Range, northwestern Nevada, contains a large, Upper Triassic submarine olistostrome containing carbonate clasts from pebble size to 700 m in length that are supported mostly by a lime-mud matrix. The olistostrome comprises debris flows separated by intervals of hemipelagic sedimentation, turbidity flows, and density-modified grain flows. The olistostrome body is triangular-prism shaped with a flat top and keel-like base. It lies in a thick sequence of fine-grained siliciclastic rock. The lower submarine gravity flows were deposited in a pre-existing trough at a lower slope or base-of-slope. The highest gravity flow filled the trough and spilled over its sides. The source of olistostrome clasts and matrix was a carbonate platform (Dun Glen Formation) to the northeast that formed on the surface of a pre-existing shallow-marine delta. The olistostrome and older shelf-basin transitional rocks of the Humboldt Range indicate the existence of a Middle and Upper Triassic shelf-basin transition with an arcuate trace that is convex to the southwest at the southern Humboldt Range. This bend in the Middle to Late Triassic margin of western North America probably existed since at least Middle Triassic and did not result from later tectonics. The bend was apparently a controlling factor in the location and orientation of Jurassic contractile deformation.

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