Abstract

The Lower Cretaceous (Edwards) Mulberry Canyon Section is located in Taylor County, Texas and consists of two tidal flat-capped parasequences that contain three stages of dolomite. Of the three stages of dolomite present, previous work has established the probable environment and conditions for the formation of the first stage of dolomite; however, the origin for the other two stages has not been conclusively established. The first stage is fine-grained (<10μm), matrix-selective, often-laminated dolomite that has replaced the micrite in unfossiliferous mudstones (tidal flats). The second stage is fine-grained (<10μm), matrix-selective dolomite that has replaced the micrite in fossiliferous subtidal wackestones/packstones. The third stage is coarse-grained (10-240μm) dolomite that is filling or partially filling fossil-moldic porosity along with equant calcite in the fossiliferous subtidal wackestones/packstones.

A hypersaline penecontemporaneous origin for the dolomitized tidal flats is suggested by the presence of mudcracks, tepee structures, possible algal laminations, modified cubic pyrite crystals, their association with the Kirschberg Evaporite, and the geochemical data. Values for δ¹⁸O of both tidal flats (δ¹⁸O=-0.9 to -0.2±LPDB, δ¹³C=0.6 to 1.9±LPDB) have a more hypersaline signature when compared to Lower Cretaceous marine (δ¹⁸O=-4.4±LPDB, δ¹³C=2.4±LPDB) and meteoric (δ¹⁸O= -8.8±LPDB, δ¹³C=1.1±LPDB) values determined in this study. Values for δ¹³C of both tidal flats (δ¹³C=0.6 to 1.9±LPDB) vary in a possible mixing fractionation trend suggesting (along with the presence of modified pyrite) bacterial processes such as methanogenesis and/or sulfate reduction in the tidal flats.

The tidal flat hypersaline brines sank, moved seaward (seepage reflux), and mixed with marine water. This mixed hypersaline and marine fluid was responsible for selective dolomitization of the underlying subtidal wackestones/packstones matrix. Oxygen isotope values (δ¹⁸O=-3.7 to -0.9±LPDB) of the dolomitized subtidal wackestone/packstone matrix below the upper tidal flat have a marine signature, suggesting a marine influence in this stage of dolomite.

Coarse-grained dolomite filling or partially filling moldic porosity exhibits marine to meteoric isotopic values (δ¹⁸O=-7.2 to -4.7±LPDB, δ¹³C=-0.3 to +0.5±LPDB). In addition to isotopic evidence, the coarse-grained dolomite exhibits similar concentrations of Mn, Fe, and Sr (Mn=124ppm, Fe=1098ppm, and Sr=276ppm) to the meteoric values (Mn=117ppm, Fe=863ppm, and Sr=192ppm) suggesting that mixed waters (marine and meteoric; Dorag dolomitization) were responsible for this last stage of dolomite.