Abstract

The 1,700 m.y. Ortega Group in northern New Mexico is a sequence of quartz arenites and subordinate mudstones in excess of 1,000 m. Sedimentation took place in diverse shallow-shelf environments under the influence of tidal, wave, and storm processes. Time-velocity asymmetry of tidal flow on the inner shelf produced large-scale trough cross beds and tabular-planar cross beds with multiple reactivation surfaces. In contrast, tabular cosets structured internally by small-scale trough cross beds and herringbone cross beds resulted from symmetrical tidal flow on the inner shelf. Fair-weather waves reworked the upper surface of tidal sand bodies on the inner shelf during quiet periods within the tidal rhythm. During storms, sand was entrained into the water column on the proximal inner shelf leaving winnowed pebble lags at the top of tidal sand bodies. Sand transported by storm-induced gradient currents was deposited on the outer shelf as discrete lobes. Progradation of sand lobes produced 1-11-m-thick genetic packages which display an upward thickening of sandstone beds and are capped by cosets of tabular, planar-laminated sandstones. Storm sands were reworked by wave processes on the proximal outer shelf, whereas storm-deposited sands on the distal outer shelf remained unmodified. No evidence of tidal processes exists on the outer shelf. Distribution of facies and paleocurrent patterns indicates that the shelf sloped to the south and experienced an overall transgression which culminated in the drowning of the outer shelf with onlap of black, basinal muds. Absence of shallow-water turbidites below storm wave base reflects low gradients on the shelf. Vertical transition from tide- to storm-dominated sedimentation in the Ortega Group does not represent a change in depositional style with time on the shelf. Rather, all facies coexisted across the shelf, with tidal processes prevailing on the shallow inner shelf and storm processes on the outer shelf. The German Bight in the North Sea and parts of the Bering Sea off Alaska are Holocene counterparts for the depositional framework envisaged for the Ortega Group; in both, proximal, tide-dominated environments pass distally into deeper-water, storm-dominated environments.

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