Department of Geoscience

Here we describe an individual gomphothere specimen recovered from Esmeralda County, Nevada. It was recovered from an area known as "The Sump," just 18 miles north of Dyer, NV, currently dominated by badland topography. Remnants of a paleo-forest are visible in the highlands of the site, while the central basin exposes mudrocks and sandstones, interfingered in places by several chalky, volcaniclastic layers. The sediments denote a past marsh environment, an ideal feeding ground for browsers of C 3 plant material, such as trees and shrubs. A slot canyon feeds the inactive channel where the discovery was made.

Investigations are currently underway to evaluate the impact of potentially adverse conditions (e.g. volcanism, faulting, seismicity) on the waste-isolation capability of the proposed nuclear waste repository at Yucca Mountain, Nevada, USA. This paper is the first in a series that will examine the probability of disruption of the Yucca Mountain site by volcanic eruption. In it, we discuss three estimating techniques for determining the recurrence rate of volcanic eruption (λ), an important parameter in the Poisson probability model. The first method is based on the number of events occurring over a certain observation period, the second is based on repose times, and the final is based on magma volume. All three require knowledge of the total number of eruptions in the Yucca Mountain area during the observation period (E). Following this discussion we then propose an estimate of E which takes into account the possibility of polygenetic and polycyclic volcanism at all the volcanic centers near the Yucca Mountain site.

Martian rampart crater rock fragmentation shock-tube experiments ejecta fluidization rapid decompression grain size analysis Crater and ejecta morphology provide insight into the composition and structure of the target material. Fluidized ejecta surrounding Martian rampart craters are thought to result from the addition of water to the ejecta during impact into a water-rich (ice or liquid) regolith. Here we test experimentally an alternate hypothesis. We propose that the decompression of a rock-water mixture across the water vaporization curve during the excavation stage of impact cratering results in an increased proportion of fines in the ejecta. This enables the ejecta to flow with little or no liquid water present. To test this hypothesis, fragmentation experiments were conducted on sandstone (28 vol% open porosity) from the northern Eldorado Mountains, Nevada, using a shock-tube apparatus at the LMU Munich, Germany. Rock samples with 0-92% of their open pore space filled with water were pressurized to 15 MPa at 177 • C or 300 • C and rapidly decompressed. As the water vaporization curve is crossed, the water in the pore space rapidly flashes to steam causing, together with the expanding gas in the water-free pore space, the sample to fragment. The presence of water has a significant effect on the grain size distribution and grain shape of the fragmented rock samples. In comparison with (dry) control samples, samples with water with 15-50% open pore space exhibit much smaller grain sizes. The predominant grain shape of dry as well as partially water-saturated samples is bladed, reflecting fracturing parallel to the decompression front. Samples with > 80% water in open pore space had an increase in fines and larger particles but less intermediate sized particles. Fragments from experiments with > 80% water in open pore space also displayed a more equant grain shape, indicating that the decompression of water caused fracturing independent of the orientation of the decompression front. These results may provide insight into the morphology of Martian rampart craters. We propose here that even relatively low water contents in the target (∼16%) may be sufficient to produce a significant increased proportion of fines allowing the ejecta to flow with little or no water present.