Abstract: Diagenesis and Geochemistry of Glen Rose Patch-Reef Complex--Bandera County, Texas

Timothy J. Petta

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 mistaken for vadose features. Local pholad-bored surfaces developed on the reef crests when vertical framework accretion exceeded local subsidence, so that truncation and extensive bioerosion of the reef crests occurred in the littoral zone. Small fore-reef beaches contain littoral cementation features. Submarine diagenesis of back-reef beds included peloid induration and grain micritization.

Epigenetic diagenesis that affected the sequence at Pipe Creek is divided into three distinct phases: Phase 1, marine connate, closed; Phase 2, early fresh water, open; and Phase 3, late fresh water, open. During phase 1, partial incongruent dissolution of magnesian-calcite submarine cements and internal sediments in the caprinid reefs effectively raised the magnesium/calcium ratio of the interstitial marine water. This water-composition change stimulated dolomitization of clay-rich back-reef carbonate muds by cloudy, 8 to 10µ anhedral to subhedral dolomite.

Fresh water began to displace marine connate water either during upper Glen Rose or uppermost Fredericksburg deposition (phase 2). The change from a closed marine to an open freshwater system caused the final incongruent dissolution of magnesian calcite, partial dolomitization of the sediments by clear, 50 to 60µ, euhedral dolomite, inversion of some aragonitic mollusks to calcite, and conversion of carbonate mud to carbonate mudstone (micrite). As the water became progressively enriched in carbon dioxide, mega-scale dissolution of aragonitic allochems occurred. Moldic porosity which 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 predominantly metastable (aragonite and magnesian calcite) to stable (calcite and dolomite) minerals.

Phase 3 is characterized by changes in rock fabric rather than by mineralogy. Fractures and vugs that developed during the Holocene Epoch have been partially filled by bladed to micritic calcite cements that were precipitated in the meteoric phreatic and vadose zones.

Present values of some elements, notably strontium, in calcite cements and micrite are relatively low but do reflect original mineralogy. Higher strontium values within internal sediments (micrite), back-reef micrite, and recrystallized mollusk fragments indicate an original high-strontium aragonite mineralogy. In addition, lower permeabilities of micrite prevented effective removal or "flushing" of strontium from the rocks by the modern groundwater system.

All the early freshwater diagenetic features at Pipe Creek are thought to have evolved during burial. The small amount of diagenesis attributed to subaerial exposure during deposition only affected peri-reef carbonate 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.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas