MICROBIAL MATS IN SILICICLSATIC TIDAL FLATS OF THE CRETACEOUS DAKOTA GROUP AT DINOSAUR RIDGE, COLORADO, USA (Invited Presentation)

Microbial mats are highly abundant in siliciclastic shallow-marine settings, where they form characteristic sedimentary structures. The structures (called microbially induced sedimentary structures – MISS) differ significantly from the morphologies of common microbialites in precipitated lithologies such as carbonate rocks. MISS are prevalent in the Upper Cretaceous South Platte Formation, exposed at Dinosaur Ridge close to Morrison, Colorado, USA. The MISS are associated with trace fossils formerly caused by mangrove forest, ‘dinosaurs’ and invertebrates typical of shore deposits of the Western Interior Seaway. The exposure at Dinosaur Ridge includes a supratidal facies that displays dm-scale ‘erosional remnants and pockets’, ‘tattered’ bed surfaces, and abundant ferruginous ‘mat chips’. An ‘over-flip structure’ is present in one of the tidal channels. Upper intertidal facies contain ‘multidirected ripple marks’. Based on studies of modern analogues and on microfacies analyses of thin-sections, we conclude that ancient microbial mats once colonized the sandy deposits. Three generations of erosional remnants and pockets may have originated by ornithomid and ornithopod dinosaurs crossing the mat-covered tidal flats.