MICROSCOPIC LIFE UNDERFOOT OF DINOSAURS: MICROBIAL MAT FACIES IN THE CRETACEOUS SOUTH PLATTE FORMATION, DAKOTA GROUP, COLORADO, USA

The South Platte Formation of the Dakota Group records an ancient coastal zone of the Late Cretaceous Western Interior Seaway. Extensive sandstone bedding planes of the South Platte Formation are spectacularly exposed at Dinosaur Ridge in Morrison, Colorado. The rock succession records extended tidal flats that eventually became cut by tidal channels; later-on swampy forested and deltaic settings became established. Although the exposures are best known for their iconic dinosaur trackways, they also contain structures produced by microbial mats. MacKenzie (1972) interpreted enigmatic ‘patchy ripples’, as being influenced by algal mats deposited in an intertidal sand flat. We expand on his work suggesting that the section at Dinosaur Ridge contains up to three distinct microbial lithofacies, including: (i) intertidal strata characterized by ‘multidirected ripple marks’ and ‘microbial mat chips’; (ii) supratidal strata, which display ‘erosional remnants and pockets’, ‘microbial mat chips’ and ‘roll-ups’; and (iii) rock beds including tidal channels that once cut into the supratidal deposits, with rare specimens of ‘roll-ups’ and ‘microbial mat chips’. Overall the studied succession is thought to have been deposited in a semi-tropical coastal setting. The microbial facies support previous interpretations that the depositional environments first experienced periods of prolonged aridity that later were succeeded by increasingly wet conditions.