Late-Mississippian Paleokarst Features and Related Porosity Creation and Destruction in the Aspen, Colorado Area

R. Mark Maslyn
Consultant, Littleton, CO

Open space porosity within paleokarst features in the Mississippian Leadville Formation occurs in the Aspen, Colorado area. Three episodes of solutional porosity generation and one episode of porosity destruction are recognized. The first two porosity enhancement stages are paleokarst related. Porosity destruction accompanied burial and infilling of the paleokarst by overlying sediments. During burial, solutional modification of the paleokarst features by basinal brines occurred.

The first paleokarst episode occurred after deposition of the basal Red Cliff, its subsequent dolomitization, and subaerial exposure. Solutional features developed on and within the Red Cliff including a rubble breccia at the upper contact. Following deposition of the overlying Castle Butte Limestone member, the Leadville Formation was again subaerially exposed. This second paleokarst episode resulted in more extensive paleokarst development including caves, sinkholes, solutionally enlarged joints and a rubble breccia along the upper Castle Butte surface. During this paleokarst episode, caves developed in the earlier paleokarst breccia zone between the Red Cliff and Castle Butte members. Locally significant erosion is also shown by thickness variations of the Leadville Formation. In one case, the formation is removed along with an additional 30 meters of underlying sediments, allowing Pennsylvanian age shales to be deposited on Ordovician age sediments.

Following the paleokarst formation, the area was submerged and the Leadville Formation overlain by interbedded Pennsylvanian black shales and carbonates. The openings of many of the paleokarst features were filled by black carbonaceous shale. During burial, the remaining porosity in the paleokarst features helped channel basinal brines that locally enlarged paleokarst features producing caves and cave roof brecciation.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado