DEPOSITIONAL ENVIRONMENTS AND PRELIMINARY SEQUENCE STRATIGRAPHY OF THE LEADVILLE LIMESTONE, SW COLORADO

The sequence stratigraphy of the Mississippian (Osagean/Tournaisian) Leadville Limestone in southwest Colorado has received little attention despite exceptional outcrop and core data, and economic importance. Here we propose a preliminary sequence stratigraphic framework for the southern San Juan Mountains and the eastern portion of the Paradox basin. Sequences appear to have thickened (Cappa and Rice, 1996) and prograded to the northwest into the Antler foreland basin. Two sequences form 35m of shallow water facies separated by clastics and exposure in an up-dip location near Rockwood, Colorado. Fifty miles west, in a mid-shelf position in the McElmo Dome CO² field, the Leadville has at least four sequences represented in core (Kinder Mogan YA-1 well), a higher facies diversity, and it almost doubles in thickness (65 m). In both locations, the red and/or green siltstones, and quartz-lithic arenites, and breccias mark exposure surfaces associated with sequence boundaries. These fracture and paleokarst-filling clastics percolated well below the exposure surfaces. At the top of the Leadville, deep red siltstones, fine grained sandstones of the Pennsylvanian Molas Formation infiltrate down to 18m (60ft) below the eroded top of the formation.

In the updip location near Rockwood, the lower sequence is dominated by dolomitic mudstones and boundstones with crinkly laminations, rip-up clasts, gypsum molds, and ostracods. Sequence 2 contains four parasequences that range from wackestones to rugose crinoidal packstones and ooid crinoidal grainstones with bi-directional current ripples, indicating shallow subtidal to intertidal. The upper parasequences are dominated by shoal facies, suggesting limited accommodation space and progradation. In the McElmo Dome Field, the lower two sequences range from subtidal to supratidal, with dolomitized subtidal bryozoan wackestones. Biomoldic, intercrystalline, vuggy, and fracture porosity give this zone the highest porosity and permeability. The upper sequences contain grain-dominated facies similar to the upper sequence at Rockwood. Ongoing research into the regional variation in facies, sequence geometries, diagenesis, and fracture mechanics, will yield a deeper understanding of the evolution of the Leadville Limestone and reservoir quality predictability.