ENVIRONMENTAL FACTORS INFLUENCING THE PALEOECOLOGY OF THE KEECHIE CREEK SHALE IN MINERAL WELLS PARK, TEXAS

The Keechie Creek Shale of North Central Texas represents a marine shelf environment during the Upper Pennsylvanian. Cyclothems are repetitive stratigraphic sequences that are unique to the Pennsylvanian and earliest Permian periods. We can detect variations in oceanographic environments through geochemistry.

Nine 4L bulk sediment samples were collected every 0.4m from a 3.2m section at Mineral Wells Fossil Park. Samples were disaggregated, fossils (>2mm) were sorted, and identified, and the two most abundant brachiopod genera, Crurithyris and Neochonetes were counted. Geochemical analysis, via handheld XRF, was performed on sediment from each sample. Brachiopod abundance increases upsection, with total specimen counts rising from 110 at 0.0m to 1203 at 1.2m, then slightly decreasing to 935 at 1.6m.

Sample 1.2m, the sample with the highest brachiopod abundance, had the lowest calcium values. This may suggest slightly more siliciclastic facies, representing rapid depositional events or minor transgressive lags. Linear regression models show a positive relationship between manganese values and Crurithyris abundance (p=0.01 and r²=0.57). Manganese indicates that there is oxygenated iron in the environment. Both Crurithyris and manganese peaked in sample 1.2, dropped, then peaked again in sample 2.8. Vanadium is used as a proxy for weathering and vanadium: aluminum ratio is a proxy for oxygen. High V:Al was found in the lower stratigraphic samples and had a negative relationship with total fossil abundance. Higher V: Al ratios corresponds with lower Neochonetes abundance (linear regression, p=0.03 and r²=0.46), suggesting this taxon could handle lower oxygen.

Lower oxygen occurs in deep ocean environments, whereas higher oxygen reflects shallower environments in the photic zone. From the data, we interpret environmental change in two separate shallowing upward parasequence events. Sea level decreased from 0.0m to 1.2m, resulting in increasingly oxic conditions. Sea level rose in 1.6 and anoxia returned, then the pattern repeated from 2.0m to 2.8m. The oxygen content in turn affected the fossil abundances: Crurithyris excelled in more oxic environments while Neochonetes persisted in lower-oxygen conditions.