PENNSYLVANIAN FLUVIAL CAVE SEDIMENTS, PHREATIC TUBES, AND BREAKOUT DOMES IN THE MISSISSIPPIAN LEADVILLE LIMESTONE, SW COLORADO, USA

The Mississippian Leadville Limestone was part of a widespread carbonate platform in western North America that served as host for world-class paleokarst development following late Mississippian eustatic sea level fall and subaerial exposure. In SW Colorado, the paleokarst has been fundamentally misunderstood as sinkholes (dolines) and terra rossa paleosols. Detailed stratigraphic and petrologic analysis shows instead: (1) creation of phreatic tubes that partially filled with fluvial cave sediments, (2) formation of breakout domes following collapse of the phreatic tubes (producing chaotic, mosaic, and crackle breccias), (3) infilling of the breccia matrix with flowstone, dripstone, and additional translocated sediments, and (4) creation of a new tier of phreatic tubes at a lower stratigraphic level. In other words, the sequence documents regional falling paleo-groundwater surfaces during the early Pennsylvanian, a time of significant paleoclimate change in western Pangaea.

The fluvial cave sediments are inundites (high-energy, ephemeral flood deposits) dominated by upper flow regime sedimentary structures. The most abundant facies are massive sandy siltstones with primary current lineation. Other facies include clast-supported pebble-cobble conglomerate, silty sandstone with climbing ripple lamination, siltstone-claystone rhythmites, and upper mudstone drapes with mudcracks. Occasionally these fluvial cave deposits are interbedded with debrites. Flows were sufficiently energetic to erode and transport speleothems as clasts, as well as transport limestone or chert cobbles and siltstone intraclasts.

The silty character of the fluvial cave sediments was inherited from the overlying loessite (eolian siltstones) of the early Pennsylvanian Molas Formation. Remobilization of the loess into the paleokarst system is consistent with the dominance of groundwater piping and sapping in modern loess deposits and derived soils. The Molas Formation is the oldest Paleozoic loessite found in North America, and its sediment source may have been expanding deserts (as part of the long-term late Paleozoic aridification trend in western North America), the initial phases of late Paleozoic alpine glaciation in the Ancestral Rocky Mountains, or both.