Early-to-Late-Diagenetic Dolomitization of Platform Carbonates: Lower Ordovician Ellenburger Group, Permian Basin, West Texas and Southeastern New Mexico
AMTHOR, JOACHIM E., McGill University, Montreal, Quebec, Canada, and GERALD M. FRIEDMAN, Brooklyn College, City University of New York, NY, and Northeastern Science Foundation, Troy, NY
Pervasive early- to late-diagenetic dolomitization of Lower Ordovician Ellenburger carbonates in the deep Permian basin is recorded in core samples having present-day burial depths of 1.5 to 7.0 km. Fine-crystalline planar replacement dolomite formed during early diagenesis in a subtidal to peritidal setting under near-surface, low-temperature conditions, with Mg(2+) for dolomitization of precursor lime mud supplied by diffusion from overlying seawater. During intermediate burial (500-2000 m), medium- to coarse-crystalline planar-s dolomite replaced allochems and matrix, or occurred as void-filling. Planar-e dolomite precipitated along walls of pores and fractures, or formed porous mosaics of medium to coarse euhedral crystals. Nonplanar-a dolomite replaced a precursor limestone/dolos one only in zones that are characterized by original high porosity and permeability. During deep-burial (>2000 m), nonplanar dolomite cement (saddle dolomite), calcite, and authigenic quartz precipitated and occluded fractures and pore-space. The nonplanar dolomite cement is typically enriched in calcium (mean = 52.6 mole % CaCO(3)) and iron (mean = 6635 ppm). Burial-history and thermal maturation calculations suggest that deep-burial dolomite cementation occurred during the Late Pennsylvanian/Early Permian.
Inter- and intracrystalline dissolution surfaces are observed within the paragenetic sequence. Major truncation surfaces between early replacement dolomites and later void-filling dolomites, and between planar and nonplanar dolomite cements are evidence for dolomite dissolution. Deep-discharge of meteoric fluids as a result of frequent periods of karsting in overlying strata, and long-distance fluid migration during the Ouachita orogeny from foreland basins to the south are invoked for sources of undersaturated fluids causing dolomite dissolution and creating matrix-porosity in the deep subsurface. Dissolution of precursor dolomite may also provide a source of Mg(2+) for late-stage dolomite cements. Similar diagenetic relationships have been described from other deeply buried carbonat rocks elsewhere, indicating that trends and timing of dolomitization, dissolution and porosity formation, and cementation by late dolomite and calcite are intimately related to the evolution of sedimentary basins. The origin of massive dolostones such as the Ellenburger is best explained in the context of basin evolution, rather than by a single model of dolomite formation.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)