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Detailed work in the southwest Piceance Creek basin and reconnaissance work elsewhere in the basin have shown the presence of a major regional unconformity between rocks of Late Cretaceous and Tertiary age. The time gap represented by this unconformity in the southwestern part of the basin, based on palynomorph studies, is from late Campanian or early Maestrichtian to late Paleocene time. The Cretaceous rocks beneath the unconformity consist of gray-weathering lenticular sandstone units as thick as 30 m and more, separated by gray claystone, gray mudstone, and thin coal beds. Lenses of small chert pebbles are scattered throughout the upper 410 m of Cretaceous strata underlying the unconformity, but they are most abundant in the uppermost 150 m. The uppermost 50 to 150 m of the section beneath the unconformity crop out in a distinctive white color. Petrographic and X-ray mineralogy studies at Hunter Canyon in the southwestern part of the basin show that the white color is due to a breakdown of feldspar and subsequent accumulation of kaolinite, and it probably marks a paleoweathering profile developed during the time interval represented by the unconformity. Paleosoils are locally preserved in the upper few meters as well. If the kaolinitic zone is a paleoweathering profile as we believe, however, then it is a secondary characteristic superimposed on whatever rocks were being subjected to surface weathering at that time. Rates of erosion during this time interval were probably not constant throughout the basin and consequently the presently preserved kaolinitic zone is probably not everywhere at the same stratigraphic level. This kaolinitic, sparsely conglomeratic zone has been called the Ohio Creek Conglomerate or the Ohio Creek Formation by previous workers. The presence of conglomerate and the white color, the two features used to define the formation, are independent of each other, however, and need not occur together. We recommend, therefore, that the Ohio Creek Formation be reduced in stratigraphic rank to member and redefined as the white-colored kaolinitic zone, which may or may not contain chert-pebble conglomerate, found at the top of the Hunter Canyon or Mesaverde Formation. 2 CONTRIBUTIONS TO STRATIGRAPHY

A study of the Cretaceous-Tertiary unconformity in the Piceance Creek basin, Colorado; the underlying Ohio Creek Formation (Upper Cretaceous) redefined as a member of the Hunter Canyon or Mesaverde Formation