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Abstract: Diagenesis and Geochemistry of a Glen Rose Patch Reef Complex, Bandera County, Texas
Timothy J. Petta (1)
Rigid reef framework for the Pipe Creek patch reef complex (lower Glen Rose Formation) was produced by syngenetic rudist accretion, internal sedimentation, and submarine cementation of the reef frame and internal sediment. Clionid sponges bored the reef framework during accretion and produced cavities, calcareous silt, and peloids that can be easily mistakened for vadose features. Local pholad-bored surfaces developed on the reef crest(s) when vertical framework accretion exceeded local subsidence so that truncation and extensive bioerosion of the reef crest(s) occurred in the littoral zone. Small forereef beaches contain littoral cementation features. Submarine diagenesis of backreef beds included peloid induration and grain micritization.
Epigenetic diagenesis that affected the sequence at Pipe Creek is divided into three distinct phases: phase I -- marine connate -- closed, phase II -- early fresh water -- open, and phase III -- late fresh water -- open. During phase I, partial incongruent dissolution of magnesian calcite submarine cements and internal sediments in the caprinid reefs effectively raised the Mg/Ca ratio of the interstitial marine water. This water composition change stimulated dolomitization of clay-rich backreef lime muds by cloudy, 8 to 10µm anhedral to subhedral dolomite.
Fresh water began to displace marine connate water either during late Glen Rose or latest Fredericksburg time (phase II). The change from a closed marine to an open fresh-water system caused the final incongruent dissolution of magnesian calcite, partial dolomitization of the sediments by clear, 50 to 60µm, euhedral dolomite, inversion of some aragonitic mollusks to calcite, and conversion of lime mud to lime mudstone (micrite). As the water became progressively enriched in CO2, megascale dissolution of aragonitic allochems occurred. Moldic porosity developed during this phase has been preserved by the precipitation of intergranular equant sparry calcite. Clay-rich beds have recrystallized, indicating that clay materials have acted as nuclei for microspar and pseudospar. At the end of this phase, the rocks had been converted from predominately metastable (aragonite and magnesian calcite) to stable (calcite and dolomite) minerals.
Phase III is characterized by changes in rock fabric rather than mineralogy. Fractures and vugs that have developed through the Holocene epoch have been partially filled by bladed to micritic calcite cements that were precipitated in the meteoric phreatic and vadose zones.
Present-day values of some elements, notably Sr, in calcite cements and micrite are relatively low but do reflect original mineralogy. Higher Sr values within internal sediments (micrite), backreef micrite, and recrystallized mollusk fragments indicate an original high-Sr aragonite mineralogy. In addition, lower permeabilities of micrite prevented effective removal or flushing of Sr from the rocks by the modern ground-water system.
All the early fresh-water diagenetic features at Pipe Creek are thought to have evolved during burial. The small amount of diagenesis is attributed to subaerial exposure during deposition only affected perireef lime grainstones. Because of pervasive submarine diagenesis, reef beds appear to have much lower permeability than adjacent grainstones, although vuggy porosity is well-developed in the reefs. Many features in Cretaceous rudist reefs that have been attributed to syndepositional meteoric water diagenesis may have been developed during burial. Waters responsible for diagenesis may have flowed downdip from emergent land areas, or in an updip direction preceding or in conjunction with hydrocarbon migration.
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
(1) Shell Oil Company, Houston, Texas
Copyright © 1999 by The Gulf Coast Association of Geological Societies