SEQUENCE STRATIGRAPHY AND FACIES OF THE MISSISSIPPIAN LEADVILLE LIMESTONE, SOUTHWESTERN SAN JUAN MOUNTAINS, COLORADO

Despite their economic importance, the surface exposures of the Mississippian Leadville Limestone in Southwestern Colorado have not been viewed in a sequence stratigraphic context. Based on observations from twelve measured stratigraphic sections, we recognize two sequences with seven distinct facies.

The boundary between the Devonian Ouray Limestone and the Mississippian Leadville Limestone is the base of the first sequence, marked by a variegated siltstone and a caliche encrusted, auto brecciated conglomerate. The mid-Leadville sequence boundary has an undulatory surface partially filled with a quartz arenite. This surface can be traced in outcrop over 65 km in southwestern Colorado from north of Hermosa to Ouray. The upper sequence boundary, the top of the Leadville Limestone, is extensively karsted and later in-filled with red siltstones and sandstones of the Pennsylvanian Molas Formation.

The seven facies include a range from dolomitic mudstones to peloidal grainstones. Bioturbated peloidal packstones, containing silt and fine grain sand, are inferred to be deposited near storm weather wave base. Interbedded microbial boundstones and finely crystalline dolomitic mudstone facies contain microbial laminations, rare stromatolites, mudcracks, and soft sediment deformation. This supratidal facies was likely deposited in a sabkah or tidal flat. The medium grained, well rounded quartz arenites and sand-rich packstone facies may be marginal marine to eolian.

There are three variations of peloidal grainstones. Compacted peloidal grainstones with ostracods and discontinuous lenses of black chert may be indicative of a restricted marine lagoon environment. Uncompacted peloidal grainstones contain planar lamination, plane tabular cross bedding, and chert. This was deposited in an intermittently exposed shallow beach/tidal flat setting. A third variety includes peloidal grainstones containing crinoids and other skeletal fragments. This indicates deposition in an open marine environment above wave base. Crinoidal grainstones contain rugose corals, brachiopods, plane tabular cross bedding, and grey chert. This facies was deposited in and open marine shoal environment.