The NSS Bulletin - ISSN 1090-6924
Volume 51 Number 2: 72-99 - December 1989

A publication of the National Speleological Society

Geologic History of the Black Hills Caves, South Dakota
Arther N. Palmer and Margaret V. Palmer

The origin of caves in the Black Hills is intimately linked to the diagenetic and erosional history of the Pahasapa Limestone. Beds of gypsum and anhydrite were deposited contemporaneously with the limestone, particularly in the middle dolomitic beds. Mobilization, hydration, recrystallization, reduction, and solution of the sulfates caused fracturing and brecciation of the surrounding rock in irregular zones, some nearly vertical and others sub-concordant with the bedding. Most of the remaining sulfates were replaced by calcite and quartz, producing competent calcite veins surrounded by crumbly quartz-rich bedrock. A late Mississippian karst surface developed on the Pahasapa, and caves formed in the mixing zone between fresh water and underlying saline water. The caves consisted mainly of irregular vaulted rooms, although some were vertical fissures that connected with the overlying karst surface. Surface depressions, as well as most of the caves, were filled with clay and quartz sand early in the Pennsylvanian Period. The present caves were formed during the Tertiary Period, probably by artesian groundwater, but there is also evidence for cave origin by rising thermal water. Solution may have been enhanced by mixing with low-CO2 water infiltrating under closed vadose conditions through the sandstone caprock. The Tertiary caves intersect both the ancient sulfate zones and the paleokarst. Many of the upper levels are exhumed and enlarged paleo-caves, and most lower-level passages follow former sulfate zones. Much of the present cave pattern is therefore inherited from Mississippian features. The network outline of the caves is closely related to the stresses within the uplifting Black Hills during the late Cretaceous and early Tertiary Laramide orogeny. Yet the basic outlines of the caves were determined long before, during the Mississippian Period. Apparently many of the present fracture zones were formed by minor tectonic unrest at that time and were merely reactivated during the Laramide.

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