GROWTH MECHANISMS FOR CARBONATE CONCRETIONS IN THE WHEELER FORMATION, HOUSE RANGE, UTAH: IMPLICATIONS FOR PORE WATER BIOGEOCHEMISTRY

The extreme diversity in geochemical and petrographic data among mudrock-hosted carbonate concretions has led to the proposal of various different models to explain concretion growth and occurrence. It is generally agreed upon that most concretion formation is prompted by microbial oxidation of organics under anoxic conditions, but there are significant discrepancies between models with regards to spatial and temporal growth sequence. An understanding of the mechanisms and timing of concretionary growth can help to reconstruct a diagenetic history for the host rock. In this study, calcite concretions obtained from the Wheeler Formation in Western Utah were analyzed. The Wheeler Fm. is a middle Cambrian fossil lagerstätte, known for Burgess Shale-type preservation of nonmineralized organic carbon. Factors posited to be important in this type of preservation are anoxia and early occlusion of porosity/permeability through various mechanisms, including microbially instigated early diagenetic calcite cementation. Thus, implications that concretion formation could have for pore water chemistry and microbial ecology during diagenesis could provide insight into early diagenetic systems in the deposit. Bulk X-ray diffraction data, stable carbon and oxygen isotope geochemistry, polished slabs, thin sections and acetate peels were analyzed in order to elucidate mechanisms of concretion growth. The presence and distribution of pyrite indicates that the alkalinity required for carbonate cementation was most probably provided by sulfate respiration well into late stage diagenesis, but the d13C values indicate that the carbon mineralized by cementation was provided dominantly by detrital sources rather than organic matter. The data led to the proposal of three distinct growth mechanisms to explain the presence of three types of mineral distribution within the concretions. In all models the data seem to support multiple stages of growth, at least one of which was concentric. Petrographic relationships suggest that carbonate cementation occurred during very early digenesis near the sediment water interface and also during late stage digenesis after compaction, therefore permeability within the concretions could not have been completely occluded during early stage diagenesis.