MICROBIAL MAT FACIES AND ICHNOCOENOSES IN THE LOWER CRETACEOUS (ALBIAN) DAKOTA GROUP, DENVER, COLORADO, USA

Microbially induced sedimentary structures (MISS) and trace fossils provide insight into micro-scale facies changes and physicochemical properties in paralic – nearshore marine environments. Well-exposed surfaces containing MISS and “biofilm catenae” in the Lower Cretaceous (Albian) Upper Muddy Formation of the Dakota Group record lateral and seasonal variations in microbial mat types with overprinting by invertebrate traces and megafaunal tracks. When microbial mats are present, they aid in the preservation of well-defined megafaunal trackways, epibenthic, and endobenthic invertebrate traces by enhancing bedding plane cementation. Microbial mats and biofilms control trace distribution in intertidal settings by protecting interstitial fauna from subaerial exposure. Burrowing is dominated by deposit and filter feeding behaviors between the sediment surface and a microbial mat where bacteria-coated sand grains also provide a consistent food source. Examples from the Muddy Formation record similar associations within and below fossilized microbial mats. Traces associated with epibenthic mats and biofilms are dominated by grazing traces and vertical burrows indicative of the Skolithos Ichnofacies. Trace fossils underneath epibenthic mats are represented by vertical and horizontal deposit and filter feeding behaviors indicative of a mixed Skolithos-Cruziana Ichnofacies. This study follows a process-ichnologic framework to evaluate the abundance, size, and distribution of invertebrate traces associated with, or overprinting, MISS to better interpret physicochemical paleoenvironmental conditions and seasonal fluctuations recorded in the Muddy Formation. Special attention is applied to surfaces containing both minuscule traces (ichnogenetic stages) and MISS suggesting strong seasonal indicators (i.e. “tattered” mats) as proxy for reproduction seasonality for potential tracemakers in the rock record. Since trace fossils and MISS are used to better understand the significance of surfaces in a sequence stratigraphic context and enhance paleoenvironmental reconstructions of facies associations, this study may secondarily assist the petroleum industry in better predictions of sandbody-shale abundance, distribution of facies, and geometries of architectural elements.