Abstract

In southwestern Colorado, paleokarst of the Mississippian Leadville Limestone contains fluvial cave sediments, debrites, and fissure-fill sediments. The fluvial cave sediments form fining-upward sequences consisting of: 1) clast-supported, pebble-cobble conglomerate composed of limestone and chert clasts and siltstone intraclasts (lithofacies Gm), 2) massive sandy siltstone which may show primary current lineation (lithofacies SSm), 3) laminated siltstone-mudstone rhythmites (lithofacies Fl), and 4) massive silty mudstones (lithofacies Fm). The sequences commonly terminate with an upper mudcracked claystone drape (lithofacies Fcd). Some silty sandstone with climbing ripple lamination (lithofacies Sr) is observed. The fluvial cave sediments are interpreted as ephemeral, high-energy, flood-flow deposits (inundites) that flowed through open-framework phreatic tubes. The inundites are interbedded with debrites, speleothems, and karst-collapse breccias. The fluvial sediments are texturally and compositionally identical to the overlying Pennsylvanian Molas Formation, which has recently been reinterpreted as a loessite (eolian siltstone).

The allochthonous paleocave sediments are interbedded with parautochthonous cave sediments (chaotic, mosaic, and crackle breccias) and with autochthonous cave sediments (speleothems). Observed speleothems include flowstone, dripstone, and cave pearls. Some of the inundite events were sufficiently energetic to erode and transport fragments of speleothems.

Exposures of Leadville paleokarst demonstrate that: 1) phreatic tubes formed by subsurface erosion below the water-table; 2) the tubes were partially to completely filled by fluvial cave sediments; 3) individual phreatic tubes subsequently collapsed to form breakout domes (upward-widening domal structures made from karst breccias); and 4) the breakout domes were subsequently infilled by translocated loess infilling fissures and breccia matrix, speleothem formation, and multiple episodes of cementation. A vertical sequence of phreatic tubes and superimposed breakout domes suggests that regional paleo-groundwater surfaces were falling during the early Pennsylvanian. This may have been controlled by the downcutting of adjacent solution valleys and/or by paleoclimate change.