MAPPING THE OHIO CREEK CONGLOMERATE AT THE K-T BOUNDARY IN NORTHWEST COLORADO NEAR MEEKER, COLORADO

The Colorado Geological Survey recently completed a 3-quadrangle mapping program in the Meeker area of northwest Colorado that bridges 1:24,000-scale geologic quadrangle mapping from the Grand Hogback Monocline to the northern rim of Piceance Creek Basin. Along the west margin of the Grand Hogback is a distinctive 12m- to 24m-thick marker bed of cobbly-pebble conglomerate and pebbly conglomeritic sandstone. This unit's contacts are disconformable with the underlying Upper Cretaceous Mesaverde Group and conformable with Lower Paleocene strata above. Because of its distinctiveness and easily traced field exposures, we have retained the Ohio Creek Conglomerate name (Tweto, 1975) in the list of map units. However, there is some uncertainty in the stratigraphic hierarchy with this unit. Along the northern margin of the basin this rock package is mapped as a basal conglomerate of the Paleocene Fort Union Formation. Others have placed it as a member in the Mesaverde Group, or thickened it to over a hundred meters and call it the Ohio Creek Formation. South of Meeker, along the steeply dipping western edge of the Grand Hogback monocline, it was mapped as a Paleocene basal sandstone in the Wasatch Formation and, more recently, as an unnamed Paleocene conglomerate. Palynological analyses of samples collected by Newman (1966), O'Sullivan and Smith (1985), and for this mapping would indicates that the base of the Ohio Creek is Late Cretaceous while the conformable paludal mudstones, carbonaceous shales, and lignite directly above are Early Paleocene. The K/T boundary was not discerned in the field in what was likely a high-energy braidplain depositional environment of the unit. The basal conglomerate contains well-rounded pebble- to cobble-sized clasts that include chert, petrified wood, pink to red andesites, and rare reptilian bone fragments. Precambrian igneous and metamorphic rocks, silicified algal limestone, and arkosic quartzite were also noted. The latter lithologies suggest a syntectonic origin with a provenance in upthrown Precambrian and early Paleozoic rocks to the east to southeast during the early Laramide uplift of the Gore and Sawatch ranges.