FALLEN ARCHES: MYTHS CONCERNING THE CAMBRIAN-ORDOVICIAN PALEOGEOGRAPHY OF WESTERN NORTH AMERICA

Sedimentologic, biostratigraphic, and stable isotope data from numerous measured sections reveal a complex architecture for lower Paleozoic strata in the Colorado Rocky Mountains. Deposition of the quartzite-dominated Sawatch Formation blanketed a Precambrian erosion surface in the early Late Cambrian, eliminating inherited topography. Post-Sawatch deposition produced the Dotsero Formation, a regionally extensive carbonate- and shale-rich succession that records blanket-like deposition with only minor facies changes across the state. A widespread meter-scale stromatolitic biostrome, the Clinetop Bed, caps the Dotsero in most areas. A late Cambrian (Eoconodontus conodont Zone) erosional episode removed the Clinetop and an additional 8-10 m of upper Dotsero from just east of the Homestake shear zone (HSZ) in the Sawatch Range eastward to the Mosquito Range. The overlying Manitou Formation differs in member stratigraphy on the east versus west sides of the state. This contrast was previously interpreted as the result of deposition on either side of a basement high within the Colorado "Sag", a region of breaching across the Transcontinental Arch. Our new biostratigraphic data show that the northwestern members of the Manitou Formation are older than the members exposed in the southeastern part of the state, with little or no overlap in age between the two areas. Older Manitou strata were eroded in the southeast as a result of regional uplift that occurred during deposition of the Rossodus manitouensis conodont Zone. Younger Manitou strata that onlap this unconformity are preserved throughout the southeastern part of the state but presumably were removed in northwestern Colorado due to up-to-northwest motion on the HSZ and erosion associated with younger unconformities. With recognition of the unconformities within the Eoconodontus and R. manitouensis Zones and the age contrast between eastern and western Manitou strata, there remains no evidence for existence of the Colorado Sag and northeast trending high within the arch that isolated depositional sub-basins on either side. Similarly there is no evidence that emergent highlands of the Transcontinental Arch existed to the south (the Uncompaghre and Sierra Grande) or to the north (Souxia) during deposition of the Dotsero Formation in the latest Cambrian.