Abstract

The Roberts Mountains allochthon in the northern Independence Range, Nevada, includes a 1.9-km-thick section of Ordovician rocks which were deposited west of the North American continental shelf. Lower Ordovician (upper Arenig) graptolitic mud-stone, distal turbidites, and chert were deposited in a continental-rise setting. Overlying Middle Ordovician (probably Llanvirn) basalts represent a seamount sequence. Clastsupported basaltic breccia and bedded volcaniclastics represent a debris apron deposited on the seamount flank, and carbonate deposition on a higher summit is indicated by common limestone clasts with a shallow-marine fauna. Seamount subsidence is represented by the stratigraphically highest volcanics, which consist of nonvesicular pillow basalt of deep-marine origin. Seamount deposits were overlapped in Middle Ordovician (Late Llanvirn to Llandeilo) time by quartz-sand turbidites and mudstone of possible lower-fan origin. These deposits are overlain conformably by Upper Ordovician (lower Caradoc), thick-bedded, gravity-displaced, quartz sandstones, which represent the middle to upper part of a large submarine fan. Cutoff of sand supply in the Late Ordovician (middle Caradoc) resulted in deposition of mudstone and chest over fan surfaces. Ordovician rocks of the Roberts Mountains allochthon represent a continental-rise setting in which sea-mount construction occurred during fan-related turbidite deposition. A grossly similar type of margin exists today in the Grand Banks area of the North Atlantic.

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