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The 1,700 million year old Ortega Group in northern New Mexico accumulated in diverse shallow shelf environments under the influence of tidal, wave, and storm processes. Tidal and fair-weather wave reworking dominated the inner shelf but a significant storm overprint is indicated by offshore-directed trough cross-stratification, and winnowed lags and scour channels at the top of tabular units. Storm-surge currents supplied sand to the outer, mud dominated shelf where deposition occurred predominantly under flat-bed conditions. Amalgamated, upward-thickening depositional units of horizontally stratified sandstone comprise 1 to 7 m (3 to 23 ft) thick genetic packages. Based on their position in the progradational shelf sequence, these sandstones are inferred to have accumul ted in proximal reaches of the outer shelf. The upper parts of individual 2 to 25 cm (.78 to 9.8 in.) thick depositional units are commonly defined by inter-laminated siltstone and mudstone, and the thinner basal sandstones frequently have wave-rippled tops. Scour channels are often present at the top of the sandstone packages. The sandstone:mudstone ratio decreases outward on the shelf with discrete, 2 to 5 cm (.78 to 1.9 in.) thick, horizontally-stratified sandstone beds and rare hummocky cross-stratified beds passing distally into mm-thick horizontally stratified sandstones. Associated lenticular sandstones are exclusively wave rippled. The preponderance of horizontal stratification in outer shelf sandstones coupled with the resemblance of individual depositional units to b-d turbidit beds suggests suspension fallout under conditions of high but waning bed shear. Such conditions may have been related to unidirectional storm surge currents or oscillatory storm waves; the paucity of hummocky cross-stratification may favor the former process. Wave-rippled sandstones developed through fair-weather reworking of the storm-deposited sandstones. In the absence of bioturbation, Precambrian shelf sequences provide an excellent opportunity for studying outer shelf depositional facies and processes.
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