Abstract

An integrated bio- and allostratigraphic study of uppermost Turonian and Lower Coniacian strata at Pueblo, Colorado, and coeval strata 1650 km to the NW at Horseshoe Dam, Alberta, has shown that physical disconformities, related to relative changes in sea level, can be recognized at each locality. Stratigraphic control provided by the succession of inoceramid bivalves suggests that at least two disconformities are synchronous, implying some degree of eustatic control on their formation. The new study shows that the Fort Hays Limestone Member of the Niobrara Formation, exposed near Pueblo, Colorado, contains a major faunal gap embracing the zones of Cremnoceramus waltersdorfensis hannovrensis and Cremnoceramus crassus inconstans; this faunal gap is also recognized over large areas of the U.S. western interior. The faunal gap was formerly explained in terms of biogeographic provincialism. However, study of an expanded proximal foredeep section of the Upper Turonian to Lower Coniacian Cardium Formation at Horseshoe Dam, in southern Alberta, revealed the presence of both C. waltersdorfensis hannovrensis and C. crassus inconstans zones. These zones are overlain, at Cardium transgressive surface E7, by strata with Cremnoceramus crassus crassus. Depositional successions and transgressive surfaces in the Horseshoe Dam section were traced, using wire-line logs, to coeval strata exposed at Deer Creek, Montana, 365 km to the SE. There, Cremnoceramus deformis erectus and Cremnoceramus waltersdorfensis waltersdorfensis occur above Cardium erosion surface E5.5 and mark the base of the Coniacian. The discovery of this succession of Lower Coniacian inoceramid faunas indicates that the North American record can be directly interpreted in terms of the standard European zonation. At Pueblo, the stratigraphic gap corresponding to the missing C. waltersdorfensis hannovrensis and C. crassus inconstans zones is represented by a shell hash at the top of bed 37 of the Fort Hays Limestone; the top of this bed is correlated with the E7 surface at the top of the Cardium Formation. A second hiatus is recognized in the Fort Hays Limestone at a hardground at the top of bed 29; the inoceramid succession suggests that this surface correlates with a latest Turonian lowstand unconformity represented by the Cardium E5.5 surface in Alberta. The transgressive-regressive successions and their associated inoceramid faunas recorded by the E5.5 and E7 surfaces in Alberta, and equivalent surfaces in the Fort Hays Limestone in Colorado are comparable to the transgressive-regressive facies relationships and the inoceramid faunal succession documented in central Europe; this similarity implies an element of eustatic control on these relative sea-level changes.

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