Floral succession and isotopic diagenesis of the Anahuac Formation at Damon Mound, Texas

Abstract

The Anahuac Formation at Damon Mound, Texas includes a fault block of the Heterostegina limestone (Oligocene), raised to the surface by Cenozoic salt tectonism. Four cores drilled into the reef complex by Dresser Minerals (29-77, 19-77, 19- A-77, and L-5) have been studied for calcareous algal succession paleoecological interpretation and isotopic diagenesis. The Damon Mound reef complex is an ancient analog to present Gulf Coast bathymetric highs. The reef column represents a sequence beginning with deep water substrata colonized by Porites douvillei shoaling upward into a massive reef boundstone. The column evidences numerous attempts at early colonization and emergence from a nepheloid layer. The succession of calcareous algae suggests a number of characteristic assemblages reflecting general paleoecological zonation and depth correlation between the four Dresser cores. Morphological adaptations suggest the presence of increasing herbivore pressure with the advancement of the reef complex and decreasing terrigenous sedimentation. Fifty-six isotopic analyses were performed at spaced intervals along the coralline algal bearing regions of the Dresser cores. Intergranular whole-rock drillings were used to minimize vital effects and focus the study on diagenesis. The isotopic data from the Heterostegina limestone exhibited a narrow range of values from -5.15 to - 2.04%o (O︢.030%o) for 8180 and -4.85 to +1.36%o (O︢.025 %o) for 813C. The isotopic composition indicates a common marine limestone that has undergone deep-burial diagenesis with Oligocene formation waters in a closed isotopic reservoir. Hydrocarbons infiltrated the fault block, cores 29-77 and L-5, during late stage diagenesis evidencing a breach of the closed reservoir; however, the effect was limited to late- forming cements filling voids and fractures.