Abstract

Unlike previous researchers who concentrated on carbonate strata in the Sacramento Mountains, New Mexico, we conducted facies analysis on sandstone units within the Upper Pennsylvanian Holder Formation (Virgilian). Detailed work on several exposures in Laborcita Canyon revealed that the sandstone bodies have flat bases and upward-coarsening grain size sequences and are composed of three main subfacies. The sandstone subfacies in ascending vertical order are 1) thin-bedded, fine-grained sandstone with parallel-laminae and hummocky stratification with graded "tempestite" sequences; 2) fine-grained sandstone similar to subfacies 1, but occurring within medium to thick-bedded, large-scale foresets up to 5 m high; and 3) medium-grained sandstone with multidirectional cross-stratification, ripples and abundant marine trace fossils. The sandstones were deposited in a storm-dominated, inner shelf environment, and the preserved depositional topography, lenticularity, distribution of sedimentary structures and paleocurrent relationships suggest that the Laborcita Canyon sandstones represent sand ridges. The sand ridges migrated and accreted seaward during storms that generated large waves (estimated wave heights = 1.25-6 m) and downwelling pressure gradient currents, which combined to transport sand efficiently offshore. Storm-flow paleocurrent indicators trend roughly normal to the regional paleoshoreline, suggesting that the ridges were deposited in a friction-dominated or transitional shelf dynamic zone rather than in a fully-developed geostrophic zone. Sand ridges on the U.S. Atlantic inner shelf are modern process analogues to the Laborcita Canyon sandstones because of the similarities in scale, internal bounding surfaces, grain-size sequences and storm processes that control deposition.

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