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The Upper Cretaceous Gallup Sandstone of northwestern New Mexico, is a regressive, shallow-marine to alluvial sequence up to 140 ft thick. Vertical and lateral facies sequence and orientation of current- and wave-produced structures show that the shoreline advanced by episodes of delta progradation, followed by minor erosional transgression and subsequent seaward accretion of surf-zone and beach deposits. Each episode is thought to be a reaction to stream positions on a broad coastal plain. In a 200-sq mi area, 2 delta progradations and 3 strand plains are recognized, each with some important variations.
The deltaic deposits consist of: (1) distributary-channel sandstones, entrenched in older beach deposits; and (2) more widespread marine sandstones, thinning and grading seaward. The marine deltaic sandstones are in tabular beds deposited from short-duration currents; these beds vary in thickness and bioturbation, depending upon distance from dispersal centers. There is little evidence of sand transport or reworking by waves.
The strand-plain units consist of: (1) coarser sandstones with high-angle cross-strata in trough-shaped sets and minor interbeds of siltstone, overlain by (2) finer sandstones with low-angle cross-strata in wedge-shaped sets and local seaward-sloping heavy mineral placers. The coarser sandstones rest on a basal scour surface cut on older deltaic deposits, probably representing adjustment of profile with decrease in sand supply and increased effectiveness of wave action. Cross-strata dip directions record sand transport parallel with shore but in frequently reversing directions, suggesting the influence of surf generated by seasonal or more frequent weather changes. Upward gradation to sandstones with the characteristics of beach foreshore deposits indicates beach progradation.
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