2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 23
Presentation Time: 1:30 PM-5:30 PM


FAGNAN, Brian A., 607 1/2 Mt. Rushmore Rd, Apt. 4, Custer, SD 57730, fagnan@hotmail.com

Jewel Cave National Monument, South Dakota was mapped geologically and fracture trends determined for comparison with cave passageway trends allowing for an interpretation of the development of Jewel Cave. Jewel Cave is a multi-layered cave system with four levels and 125+ miles of passageways in the Mississippian Pahasapa Limestone.

The major lithologic units in the study area are the Pahasapa Limestone and the overlying Minnelusa Formation. The Pahasapa Limestone forms major cliffs in the study area because of its bedding thickness, homogeneity, well developed vertical jointing and resistance to physical weathering. The Pennsylvanian-Permian Minnelusa Formation consists of limestone, dolomite, sandstone, shale and evaporite units. Facies changes occur on a local to regional scale.

Normal faults, including the Jewel Cave Fault Zone, trend east-west and are displaced down to the south from three to ~450 feet. Most faults are located outside the boundaries of the cave with only one fault cross-cutting the cave. Cave passages show little to no offset with the cross-cutting faults, but faults are associated with small tight passages, between larger rooms because of infilling of the fault zone due to cave breakdown over time, indicating faults predate cave development.

Three hundred and fifty four surface fractures show a dominant trend of N85E with minor orientations at N15W and N35W. Two hundred and seven fractures in the cave show a dominant orientation of N75E with a minor trend at N25W. A comparison of all the trends of all fractures and cave passages suggests a difference of 15° for these trends. At a scale of individual passages, however, cave fractures parallel cave passages. Cave passageways are interpreted to have formed via dissolution from paleo-hydraulic gradients in the direction of the cave fractures.

Anticlines and synclines indicated by the developed structure contour map pre-date the Jewel Cave Fault Zone that cross cuts the folds. Cave passages are better developed in the limbs of folds. Syncline and anticline axial areas inhibit the growth of cave passages, which is also valid for anticlinal and synclinal hinges on monoclines.