AN ANCIENT EXAMPLE OF FLUVIAL CAVE SEDIMENT DERIVED FROM DUST (EOLIAN SILT) INFILTRATION, MISSISSIPPIAN LEADVILLE LIMESTONE, SOUTHWESTERN COLORADO, U.S.A

Silt-rich grain size distributions are geologically rare and typically eolian. Such sediments/rocks are called dust/dustites in a general case, or loess/loessite if derived from glacial deposits. Both types require a source area of silt-sized materials, transport mechanisms (prevailing winds of sufficient energy) and depositional mechanisms (such as trapping in the lee of topographic obstacles or adhesion to wet or vegetated surfaces).

This study evaluates a third type of silt-rich geological deposit, paleo-cave sediments derived from mixtures of dust and cave sediments. Cave sediments can be autochthonous (speleothems), parautochthonous (karst breccias), or allochthonous (fluvial cave sediments). The provenance of fluvial cave sediments is the overlying landscape, and they are introduced to the karst system by flood events.

The Mississippian Leadville Limestone (SW Colorado) was subject to karst processes following Late Mississippian eustatic sea-level fall, including formation of phreatic tubes, tower karst (kegelkarst), solution valleys (poljes), sinkholes (dolines), solution-enhanced joints (grikes), surficial flutes (rillenkarren), solution pans (kamenitzas), and breakout domes containing mosaic and crackle breccias. Flowstone, dripstone, and cave pearls are interbedded with karst breccias and fluvial cave sediments in the Leadville Limestone.

The overlying Pennsylvanian Molas Formation is an eolian siltstone (dustite) with sediment sources from the peri-Gondwanan and Grenville rocks of eastern North America. Evidence that the fluvial cave sediments in the Leadville Limestone are derived from this dustite include compositional and textural matches, especially grain size distribution trends vertically downward from the former landscape surface. Evidence suggests episodic resedimentation processes, for example phreatic tubes with up to eight successive debrites or inundites interbedded with speleothems, or grikes infilled by up to six jointites with superimposed paleosols.

Late Cenozoic cave sediments are increasingly utilized as archives of geologic change. The role of dust, including its inherited properties from a distant source area, land-atmosphere transfer processes, deposition, and resedimentation in the karst system, remain promising areas for research.