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Study of several Pennsylvanian-Permian shelf sandstones in the northern Sacramento Mountains, New Mexico, suggests that siliciclastic shelf-bars were migrating on a high-energy shelf adjacent to the Pedernal uplift. These shelf-bars had sufficient relief, 2 to 6 m (6.5 to 20 ft), to provide the clear-water, agitated conditions requisite to carbonate grain-shoal development in areas of low clastic influx. The carbonate and siliclastic cycles can be
explained by a relatively simple progradational cyclic model without invoking major sea level fluctuations.
A typical cycle contains 3 major components: (1) shelf-bar stones; and (3) interbar siltstones. Shelf-bar sandstones, 1 to 6 m (3 to 20 ft) thick, are typically overlain by carbonate grainstone/packstone facies 1.5 to 6 m (5 to 20 ft) thick, which are grainstone/packstone facies, 1 to 6 m (3 to 20 ft) thick, which are in turn overlain by siltstones, 1 to 6 m (3 to 20 ft) thick, thus completing the cycle, 4 to 18 m (13 to 59 ft) total thickness.
Shelf-bar sandstones in the Holder Formation (Virgilian) are lenticular concavo-convex sand bodies, 2.5 to 6 m (8 to 20 ft) thick, with flat depositional bases. Average grain-size (4^phgr - 0^phgr) and maximum clast size (12 mm, 0.5 in.) increases upward. Seaward dipping (W - NW) accretion foresets, up to 0.6 m (2 ft) thick, and bimodal trough cross-stratification, 10 to 30 cm (4 to 12 in.) thick, characterize lower sandstone facies. Hummocky cross stratification, 7 to 9 cm (2.8 to 3.5 in.) thick, 1.2 to 2.5 m (3.9 to 8 ft) wavelength, is common along accretion foreset surfaces and the tops of many of these sets are rippled and burrowed. Syndepositional deformation structures are common. Missourian and Virgilian shelf-bar sandstones were rapidly deposited by storm-induced currents and were modified by fair-weather tidal oscillatory flow and biological processes.
In contrast with Virgilian shelf-bar sandstones, Wolfcampian bars are thinner, 1.5 to 2 m (5 to 6.5 ft) thick, coarser grained, and locally have erosional bases. Bimodal through cross-stratification is well-developed at the expense of seaward dipping accretion foresets which if present, are poorly developed. Hummocky cross-stratification and burrows are rarely present, whereas ripples and syndepositional deformation structures are common. Average grain size coarsens-upward crudely. These Wolfcampian shelf-bars were deposited in shallow water subject to strong tidal currents, which winnowed finer sediments, produced distinct bimodality in sedimentary structures, and prevented the bars from achieving significant depositional relief.
Carbonate units, 1 to 5 m (3 to 16 ft) thick, decrease in micrite and increase in abundance of allochems upward in the vertical sequence. Sedimentary structures are similar to those of the associated siliciclastic shelf-bars, indicating a change in sediment-supply without significant change in hydrodynamic conditions. Carbonate units are thinner, 1 to 2 m (3 to 6.5 ft), become less important volumetrically in Wolfcampian sediments as compared with those in Virgilian rocks.
The spectrum of high energy siliciclastic shelf-bar sandstones, and the presence of fan-deltaic and fluvial deposits in overlying sediments (Wolfcampian) suggests that the shelf adjacent to the Pedernal uplift continuously prograded into the Orogrande basin. Shifting clastic influx along depositional strike caused drastic local changes in shoreline configuration, longshore currents, and subsequent clastic and carbonate sediment distribution patterns.
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