Geological Setting of DMNH

The sampled strata are from a small alluvial cone (DMNH 644) located on the west side of the Velvet Room in Porcupine Cave (figure 6.1). Systematic excavation identified a succession of well- to weakly bedded gravelly to silty unconsolidated strata approximately 3 m thick. The bedding is best developed within the upper meter of the succession, which is also the most fossiliferous.

DMNH 644, known as the DMNH Velvet Room excavation (and also as the Velvet Room Main Dig), was excavated between 1992 and 1997. A set of 26 grid squares 2 ft (~0.6 m) on a side were established by Donald L. Rasmussen prior to excavation. Over six years the grids were excavated by volunteers from the DMNH under the direction of D. L. Rasmussen, Robert G. Raynolds, and Louis H. Taylor. Each grid was excavated separately in a series of more than 30 levels. The levels were 2-4 cm thick in the upper 1.5 m of excavation and somewhat thicker in the rocky strata extending down to 3 m. The levels were assigned numbers, with level 1 being the youngest. During excavation, careful sketches were made of the vertical faces of unexcavated grids. Examination of the vertical faces revealed gentle depositional dips of up to 20° defining a small cone of alluvial debris. A systematic analysis of the excavated levels based on field notes, sketches, photographs, and measurements permitted the assignment of each level from each grid to one of 18 horizons designated alphabetically, with the youngest being A and the oldest being R. (See chapter 2 for more details about the excavation process.)

The sampled strata present several issues in terms of paleo-magnetic interpretation. To begin with, they are very poorly sorted and include grains up to granule and pebble in size. This poor sorting means that the measured vectors may be an artifact of the detrital orientation of large magnetic grains. This problem is probably not important for two reasons. First, most of the detrital grains are from the Paleozoic carbonates that constitute the cave walls, which generally lack a strong magnetic character. Second, the data demonstrate that individual samples, sites, and populations of samples exhibit relatively consistent primary and demagnetization behavior. This finding implies that random physical orientation of large detrital grains with respect to the primary rock magnetic field is a minor factor in the sample.

The strata are completely nonindurated. Although this characteristic facilitates the excavation of fossils, it makes paleo-magnetic collection and analysis difficult. We employed a variety of sampling and measurement techniques and a range of demagnetization spectra. The results are locally consistent and match across samples, sites, and methodologies. This finding implies that loose magnetic grains and sampling artifacts play a negligible role in the final results.

FIGURE 6.1 Map of the Velvet Room. The samples described in this chapter were obtained from DMNH 644. Other excavations in the Velvet Room are shown, as are hollow stalagmites (hs) and large rocks (r). Approximate dip of the roof rock is 35° west in the area indicated by the strike/dip symbol at the upper left of the diagram.

FIGURE 6.1 Map of the Velvet Room. The samples described in this chapter were obtained from DMNH 644. Other excavations in the Velvet Room are shown, as are hollow stalagmites (hs) and large rocks (r). Approximate dip of the roof rock is 35° west in the area indicated by the strike/dip symbol at the upper left of the diagram.

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