Geology and Paleoenvironments of the Northern Segment, Capitan Shelf, New Mexico and West Texas

Abstract

The sedimentation and structure of the Capitan shelf change along the basin-shelf margin of the Delaware Basin. In the southern Guadalupe Mountains a barrier “stratigraphic” reef is exposed with simple shelf folds that parallel the margin. This structural configuration changes abruptly near Dark Canyon to form the “northern segment”—a distinctive structural province. Major structures of the northern segment are (1) current-oriented mounds formed by shelf beds draped over biohermal cores that extend at approximately right angles to the shelf margin and owe their trend to Capitan marine currents; (2) a broad syncline trending northwest through part of Dark Canyon and a parallel anticline to the north; (3) arcuate folds that parallel the trend of the Capitan shelf margin and may result from subsidence of basement rocks; (4) shelf domes of irregular orientation, superimposed on the above-mentioned large structures (categories 1-3) and occurring at random in the shelf; and (5) structures associated with surface weathering and ground-water solution. Test drilling indicates that the lithology of the Capitan Limestone changes along the trend of the exposed part of the northern segment. The Capitan is mostly microcrystalline, fragmental, and detrital limestone; however, primarily detrital or recrystallized dolomite and dolomitic limestone occur between the current-oriented mounds in what are interpreted as ancient Capitan channels. Highly saline reflux water moving through the interpreted channels may have dolomitized the original lime mud and clastic sediments which had been eroded principally from nearby mounds. Sedimentary features of carbonate beds in two of the exposed shelf mounds indicate that they were subject, at times, to subaerial weathering, and that sediments of the mounds were deposited in part in an intertidal or supratidal environment. The tidal current ridges of the Great Bahama Bank near Andros Island have some similarities in shape and topographic setting to the current-oriented mounds of the Capitan shelf. Both the Bahama Bank and the Capitan features were formed on a shallow shelf adjacent to a deep sea floor, and their orientation was determined by prevailing currents moving across a shelf covered with shallow water. Shelf-ward from the tidal current ridges of the Bahama Banks are moundlike accumulations of sand, which are similar in shape to the Capitan shelf domes. The Bahama mounds and tidal current ridges were formed by marine-current deposition of oolites and carbonate detritus, whereas the Capitan current-oriented mounds and shelf domes were probably formed by marine current deposition of carbonate detritus and by organic biohermal growth. Knowledge of the structure and sedimentation of the northern segment helps explain some of the major environmental problems of Guadalupian paleogeology. The open lagoons between the current-oriented mounds connected the shelf to the basin and allowed unimpeded flow of water, including tidal currents, to and from the extensive Capitan shelf. Large flows of normal marine water moved through the lagoons onto the shelf where a high evaporation rate of water could occur in the areally extensive shelf seas. At the same time, reverse or reflux flow of highly mineralized brine was probably moving through the lower part of the lagoon and transported large amounts of quartzose sand and silt from the shelf into the basin. The reflux brine currents and similar marine currents may have formed base level for the areally extensive sandstone-siltstone beds that were laid down on the Capitan shelf and in the Delaware Basin.