Controls on Microbial and Oolitic Carbonate Sedimentation and Stratigraphic Cyclicity Within a Mixed Carbonate-Siliciclastic System: Upper Cambrian Wilberns Formation, Llano Uplift, Mason County, Texas, USA

The upper Cambrian Wilberns Formation in central Texas records deposition on a low-gradient shelf within a mixed carbonate–siliciclastic tidal-flat system that changes offshore to subtidal shelf and open-marine oolitic skeletal shoals with large microbial mounds. Siliciclastic sediment is interpreted to have been delivered to the tidal flat by aeolian processes because of the narrow range in grain size and paucity of clay. Tidal influence is dominant as evidenced by reversing currents and desiccation on the tidal flat, and megaripples with reversing current indicators in offshore shoals. Intraclastic conglomerates were deposited in broad channels on the tidal flats during storm surges. Microbialite deposition is interpreted to be controlled by accommodation favouring amalgamated thin biostromes developed in the tidal flat vs. larger mounds with greater synoptic relief in the offshore, and current energy resulting in preferential elongation of offshore mounds in a NE–SW orientation. Intertidal mounds and biostromes grew in the presence of significant siliciclastic flux and trapped it within their structure, whereas offshore large buildups incorporated little siliciclastic component. Oolite and skeletal grainstone formed in tide agitated shoals associated with large subtidal microbial mounds. Storms extensively recycled and redistributed skeletal and oolitic sands from the offshore shoals across the shelf as thin sand sheets. Spatial mixing of siliciclastic and carbonate sediment occurred across the tidal flat and shelf. Low-frequency and intermediate-frequency stratigraphic cycles were driven by shifts in the shoreline and changes in rate of siliciclastic flux in response to relative sea-level fluctuation. Random facies stacking and the lack of metre-scale cyclicity are interpreted to reflect stratigraphic incompleteness and an episodic signal introduced by storms

Examining the Effects of Urbanization on Soil Chemistry Through Sr Isotopes in the Austin, TX Area

In urbanizing landscapes, soils may be altered by processes such as municipal water irrigation, which has implications for the study of urban environmental contaminants in soils and the streams to which the soil provide dissolved constituents. However, it is challenging to directly trace sources of dissolved ions to streams and the impacts on soils. We examine these changes by using Sr isotopes as natural tracers. In Austin, TX, municipal water has relatively high Sr isotope values (87Sr/86Sr of ~0.7089 to 0.7090, similar to the Colorado River source), whereas natural stream water has a lower 87Sr/86Sr value (87Sr/86Sr of ~0.7076, reflecting values for Cretaceous limestone bedrock). Streams from urbanized watersheds have intermediate Sr isotope values, indicating contributions to streams from municipal water, soils, or both. We evaluate the role of soils through isotope analysis of the exchangeable Sr in soils covering a range of soils types, soil horizons, and extents of urbanization. Results indicate that soils 87Sr/86Sr is not controlled by natural variations in soil type or horizon, but rather by the extent to which the soil is irrigated with municipal water. Irrigated soils 1) have higher 87Sr/86Sr (0.7086-0.7091) than unirrigated soils (0.7078- 0.7087), and 2) are within or slightly lower than the range of values for municipal water in the study area (0.7090-0.7091). This indicates that irrigation shifts soils towards municipal water values. Applying Sr isotopes in hydrologic systems may help trace urban environmental contaminants