Abstract

Virgilian cycles of the Holder Formation in the shelf area on the northeast side of the Pennsylvanian Oro Grande basin contrast lithologically with the cycles in equivalent basinal sediments cropping out to the south and west. Basin cycles begin with cross-bedded sandstone and grade upward through relatively thick shale to a dark tidal-flat lime mudstone and shale. Shelf cycles contain a variable sandstone and shale lower member, with local channel-fill conglomerate. This grades upward through normal marine limestone and shale to a massive capping bed of shoal-water lime grainstone of algal-foraminiferal lime mud mound facies.

The two sequences may be correlated by considering that sea level fluctuated repeatedly over the whole basin, and that major sedimentation alternated between the shelves and the basin. At periods of low stand, shelf areas were dry, sediment crossed the shelf, and terrigenous clastic materials began filling in the basin. Argillaceous sedimentation continued as sea level rose. The shelves were drowned, and deeper, turbid, and partly euxinic water in the basin made the basin inhospitable for organisms. Late in each cycle, clastic influx diminished, and thick shoal limestones formed in clear water on the shelves whereas, perhaps, little or no sedimentation took place in the basin. Regression regularly occurred during this period of carbonate shelf sedimentation. Limestone beds capping the cycles commonly possess leached or oxidized surfaces, indicating that their tops were exposed subaerially. Limestone was deposited in the basin only when sea level had dropped enough to turn its level floor into tidal flats and evaporitic ponds.

The basin-shelf margin area may be examined in detailed cross section on the western front of the northern Sacramento Mountains. These studies help document a sedimentational theory first published in 1958 by Van Siclen who pointed out that late Paleozoic carbonate shelf margins were built up around the east side of the Midland basin in north-central Texas, and that the basin was reciprocally filled with clastic sediments at times of lowered sea level. In the Sacramento-Oro Grande case periodic sea-level drops at a minimum of 100-150 feet can be estimated. Such fluctuations probably occurred here at least 20 times during latest Pennsylvanian time.

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