A middle Guadalupian organic buildup was cored in the North McElroy Unit (NMU #3713 well) in Upton County, West Texas. Fusulinid control indicates the buildup is equivalent to the Goat Seep reef that crops out in the Guadalupe Mountains.
The organic buildup at North McElroy consists of boundstones and associated flank grainstones. The reef biota is dominated by ramose and encrusting bryozoans, numerous types of calcareous sponges, the problematical encrusting organism Tubiphytes, encrusting algae, crinoids, brachiopods and trilobites. Intermixed with the biota are pockets and laminae of oolitic and pelletal grainstones and packstones. In addition to the organic framework (bindstone, bafflestone), an abundance of fibrous isopachous rim cements and crusts provided a stabilizing inorganic framework for the buildups; these submarine cements line cavities, fill cracks and fractures, and ornament the margins of numerous fossils.
Lateral equivalents to the organic buildup include fusulinid wackestones and grainstones of the open shelf, shallow shelf burrowed mudstones and wackestones, and bryozoan/sponge wackestones and packstones of the shelf margin. A source of ooids was somewhere nearby but its location has not been confirmed via coring. The spatial distribution of the sedimentary facies indicates a strong environmental change across a short distance in the basinward direction; perhaps this reflects deeper water with the buildup down-slope from the shelf margin.
Among the more significant aspects of diagenesis are mechanical degradation and biologic degradation (micritized rims), leaching (most of which was early and fabric-selective), gypsum-anhydrite transformations, cementation (both syndeposi-tionally and during burial), replacement of all calcite by dolomite, cementation and replacement by baroque dolomite, fracturing, and replacement of carbonate by anhydrite.
Figures & Tables
Carbonate buildups have long been a focus of intense geological study. An underlying reason is the importance of carbonate buildups as significant hydrocarbon reservoirs. This core workshop is intended to provide a “hands on” look at the subsurface geologic record created by carbonate buildups with emphasis on lithofacies, stratigraphy of buildups and their surrounding deposits, geometry, “reef”-building and sediment-producing organisms, and diagenesis and porosity evolution