Porcupine Cave Interior

The record of cave formation inside Porcupine Cave mirrors some of the complexity deduced from the regional geological history. Based on cross-cutting relationships, the earliest features in the cave are elongated veins and fissure fills of strongly altered dolomite with intense limonitic weathering. These features probably derive from the Paleozoic karst surface that developed on the Manitou Dolomite during subaerial exposure prior to accumulation of the Harding Quartzite. Analysis of the ridge adjacent to the cave reveals a series of fissures and weathered zones deep in the Manitou that all emanate from this unconformity (figure 5.4). The modern entrance to the cave is located at the base of an adit cut into one of these mineralized fissures by unsuccessful prospectors.

Much of the cave is composed of rooms cluttered by extensive rock falls in which slabs of roof rock have collapsed to mantle the floor. In areas where the cave has not collapsed, speleothems provide evidence that the cave was once wet. Carbonate dissolution and precipitation actively sculpted the interior of the cave.

Two styles of speleothem growth are evident (figure 5.6). An early conformable phase of calcite encrustations, which drape

FIGURE 5.6 Speleothems in the Gypsum Room. Note the multiple episodes of speleogenesis, each associated with a characteristic style of carbonate precipitation.
FIGURE 5.7 Hollow stalagmite in the Velvet Room. Note that the stalagmite is positioned directly below a cupola. Although this area of the cave is completely dry today, the evidence indicates past episodes of calcium carbonate precipitation and dissolution.

and mantle the cave surfaces, suggests deposition below water. A superimposed deposit of horizontally bedded calcite- and gypsum-rich interlayered material suggests deposition in a cave partially filled by water.

In many parts of the cave where the ceiling has been stable, a series of cupolas developed. These hemispherical features are often over a meter in diameter and appear to be incised perpendicular to the inclined strata. The cupolas formed from dissolution of carbonate by waters that condensed on the cave ceiling. Directly beneath the cupolas occur a variety of poorly defined and corroded stalagmites (figure 5.7). The stalagmites tell an interesting story. In addition to being generally degraded, they are hollow and dissected. They grew at a time when slowly dripping water focused by the cupolas was supersaturated with calcium carbonate dissolved from the cave roof. At a later time, conditions changed, the falling fluids became corrosive (undersaturated in calcium carbonate), and the stalagmites were eviscerated.

Careful analysis of the strata bearing the Plio-Pleistocene fossils indicates that they were deposited under varying degrees of wetness in the cave. In the Velvet Room, for example, the well-bedded DMNH excavation (DMNH 644) suggests sorting by water of particles on a low-relief debris cone. Although water may not have flowed over the alluvial cone, it is clear that depositional conditions varied sufficiently abruptly to produce stratification. The same is evident in the area where the camel bones were discovered in Tobacco Road. In the Pit (CM 1925), alternating muddy nodules and massive silts have been interpreted to represent alternating wet and dry conditions within the cave (Barnosky and Rasmussen, 1988). Other localities like Mark's Sink (DMNH 1349) are more chaotic and are suggestive of cave fill by slow collapse of roof materials and gradual, random aggradation of dry debris.

The variable style of filling has had a dramatic impact on the nature of the faunal record. Several small excavated sites in the cave have little or no stratigraphic context. Sites exca vated in the Gypsum Room and the Badger Room are examples of this. In other cases, such as Mark's Sink (DMNH 1349), significant excavations have been made in materials that are very poorly bedded to nonbedded in character. In this case, the superposition of fossil material is known but the relative ages are still unclear.

In some places in the cave, accumulation of sediment took place in a sufficiently systematic pattern to impose visible stratification and preserve stratified fauna. The DMNH Velvet Room excavation (DMNH 644), the Pit (CM 1925), and the CM Velvet Room excavation (CM 1927) are all examples of this style of deposition.

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