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Regional Sequence Stratigraphy and Thickness Patterns of the Mississippian Leadville Limestone, Southwestern San Juan Mountains, Colorado


The Mississippian Leadville Limestone is important economically due to the abundance of oil and CO2 in Southwest Colorado. The objectives of the research are to correlate facies and thickness variations at a regional scale, providing a sequence stratigraphic framework for permeability and porosity variation of the Leadville Limestone in Southwest CO. In order to do this, thirteen stratigraphic sections have been measured, sampled (at meter scale) and correlated. Within these sections, seven facies build into two unconformity bounded sequences within the Leadville Limestone. The lower sequence boundary atop the Devonian Ouray Limestone is marked by karst collapse, caliche, and auto-brecciation. The middle sequence is bounded by an undulatory surface with well-rounded medium grain quartz arenite, interpreted as eolian. This 5-40 cm zone extends across the entire 105 kilometers extent of outcrops. The upper sequence boundary is karsted and underlies the Pennsylvanian Molas Shale, which in some sections penetrates to within 2 meters of the base of the Leadville Limestone. The northern-most section in Ouray, CO is 61 meters thick, but 105 kilometers farther south, the Leadville Limestone thins to 34 meters just north of Durango, CO. Variations in the thickness of stratigraphic sections have been caused by karsting during an unconformity between the Early Mississippian (Osagean) and the lower Molas Formation. Dominantly shallow water facies indicate that accommodation space was limited near Durango where total thickness is 34 meters. Facies range from intratidal to supratidal. In contrast, 80 kilometers downdip the 64 meter thick Leadville Limestone in core from well YA-1 from McElmo Dome Field, Southwest CO, displays evidence of deeper water bryozoan wackestone facies with more accommodation space. However, in Ouray, CO the thickness (61 meters) is similar to YA-1 but is dominated by shallow water facies. This could be indicative of slower subsidence producing keep up cycles, while a greater subsidence to sediment supply relationship produced an equally thick section with catch up cycles (e.g. Soreghan and Dickenson, 1994) in YA-1. Later during the Mississippian, progradation into the basin and eustatic sea level fall resulted in the final unconformity of the upper Leadville Limestone.