CATASTROPHIC SEDIMENTATION IN THE MID-MAASTRICHTIAN OF SOUTHERN COLORADO

The Raton Basin of SE Colorado represents one of a series of Rocky Mountain retro-arc foreland basins that were situated along the western margin of the Cretaceous Epeiric seaway. Famed for recording a thin (<1 cm) K-T bolide boundary layer, new sedimentological studies within the basin have revealed that major "catastrophic" events occurred lower in the stratigraphy during mid-Maastrichtian times.

The key horizon rests at the contact between the shallow marine Trinidad Sandstone and the non-marine coal-bearing Vermejo Formation. A number of unusual features in the basal Vermejo Fm including glass-clast conglomerates, soft-sediment bombs, mega-rippled bedforms, short-lived block faults, and a variety of huge mud-flake breccias associated with highly erosive basin-wide flood events, indicate a major depositional hiatus across the Trinidad-Vermejo boundary that is here interpreted as a regional erosional unconformity.

Conglomerates in the lowest part of the Vermejo Fm contain relatively fresh SiO2-rich (with minor Al2O3 and K2O) glass clasts, composed of well-sorted fragments of scattered angular sand grains and Fe-rich micro-spherules of uncertain origin. Collectively these phenomena may be related to a bolide impact or undocumented volcanogenic episode. The event resulted in deformation and catastrophic sedimentation including ‘tsunamis’ and major ‘alluvial’ flood events that operated independently from higher-order climatic, tectonic and sea level change.