USGS Open-File Report 95-001

PIT OF DEATH CORE RECORD

The Pit of Death core is composed predominantly of massive mud (Figure 9, Appendix 4). A few thin laminated zones several inches thick are present but rare, and disrupted laminae of silt are common throughout. The mud color ranges from brownish hues (10YR, 7.5YR), to grayer or greener hues (2.5Y, 5Y). The contrasting hues give the mud the appearance of alternating between brown or pink and green or olive gray, although the color differences are subtle. The brown hues dominate. Near-vertical selenite veins cut the mud at all depths in the core.

Features referred to in the log as filled cracks are conspicuous in the Pit of Death core. The structures are vertical and planar in form, suggesting that they originated as cracks, but they are not flat-- they curve, pinch, and swell along their length. They vary in thickness from less than 1/2 inch to several inches, and in length from a few inches to several feet. Some cracks contain what appear to be angular to rounded clasts of the surrounding mud, although in general the grain size of the fill is similar to that of the matrix. The fill is different in color than the surrounding sediment, and is uniform in some but mixed in others. The cracks are interpreted as mud cracks that formed in partially dried mud, were filled with sediment reworked from the surface, and were deformed by compaction after burial. The upper boundaries of the filled cracks are diffuse in most cases, and it is not possible to identify a clearly defined surface of origin, suggesting that the episodes of drying that produced them were short-lived and were insufficient in length for soils or other indicators of disconformity to be formed. Filled cracks are present throughout the core, both in greenish mud and brown mud. Their positions are plotted in Figure 9.

Gravel and sand in the upper foot of the Pit of Death core were deposited in a Holocene barrier beach. The gravel disconformably overlies lacustrine mud and sand that contains the Bishop ash (0.759 Ma). Sand interbedded with the lacustrine mud above the Bishop is cross-bedded. Outcrops nearby expose lenses of gypsum-cemented, cross-bedded sand within the muddy lake beds. The sand lenses are elongate in the direction of the probable paleoshoreline (north-south), and probably were deposited as offshore bars in the shallow lake water.

The only other sand or gravel of significant thickness is near the bottom of the core. Pebbly mud in the interval between 428 and 431 ft overlies an interval of about 40 ft of sand and gravel, from which no core was recovered. From 472 ft to the base of the core at 512 ft, is a sequence of moderately cemented, poorly sorted, tuffaceous sand. Pumice lumps ranging from sand size to 2.5 inches in long dimension are common. Microprobe analyses of the glass indicate a likely correlation with a 6 Ma tephra previously collected near Adrian, Oregon (W.P. Nash, written communication, 1994). Therefore, the unrecovered sand and gravel at the base of the mud section is interpreted as marking an unconformity between the Miocene tuffaceous sand and the Plio-Pleistocene muddy lake beds. Paleomagnetic results suggest that the unconformity may date to younger than the Kaena subchron at approximately 3.1 Ma.

Pit of Death Stratigraphy. Br = Brunhes, J = Jaramillo, O = Olduvai, R = Réunion, X = Anomaly X.

Paleomagnetism.

The Bishop ash crops out in lacustrine clays along the northwestern shore of Sevier Lake (Oviatt, 1994) and occurs at a depth of about 13 ft in the Pit of Death core. Normal polarity directions are recorded in the uppermost part of the Pit of Death core to a depth of 58' where the Matuyama/Brunhes boundary is recorded (Cui et al., 1994). This suggests that sedimentation was extremely rapid between the time of the Matuyama/Brunhes polarity transition and Bishop ash time (about 65 cm/kyr). Unambiguous paleomagnetic directions were determined from 97% of the samples from the Pit of Death. These data reveal a straightforward magnetic polarity stratigraphy (Figure 10). Within the reversed polarity Matuyama Chron, we can identify the Jaramillo and Olduvai subchrons, as well as likely matches for the Réunion and X-anomaly cryptochrons although full normal polarity is not achieved in these two zones. In contrast to the Black Rock core, we are unable to identify the Cobb Mountain subchron in the Pit of Death core. The thick zone of normal polarity from 369 to 420 ft is interpreted to represent the upper part of the Gauss Chron. A sequence of gravels was encountered at depths of 420 - 470 ft and tephrochronology from below the gravels indicates that the lower lacustrine sequence below the gravels is of Miocene age. We cannot unambiguously correlate the short zone of normal polarity below the gravels with the geomagnetic polarity timescale, therefore we are unable to assign a precise age to the lowermost part of the core. Overall, however, the records from the Black Rock and Pit of Death cores reveal essentially coeval sedimentation histories.

Pit of Death inclination record.

View figure 11 or download the postscript file.

Pit of Death core depths vs. age.

View figure 12 or download the postscript file.

Comparison of the Black Rock and Pit of Death CMS records.

Greater paleomagnetic stability of samples from the Pit of Death core, relative to the Black Rock core, is likely to result from the consistently higher remanence intensities of the sediment from the Pit of Death core. Fine structures are preserved in the susceptibility record from the Pit of Death (Figure 9), which indicates that post-depositional dissolution has not affected the Pit of Death record as significantly as is the case for the Black Rock record. It is often possible to correlate susceptibility profiles from lake catchments with sediment derived from the same parent material (e.g. Verosub and Roberts, in press), however, the lack of similarity between the two profiles with respect to age (Fig. 12) suggests that reductive diagenesis has significantly obscured the magnetic signal from detrital material in the Black Rock core.

Pollen Analysis.

Ten samples from the Pit of Death were processed for pollen analysis (at the approximate depths of 20, 71, 93, 133, 175, 201, 223, 225, 381, 465 ft), and all samples were barren.

Ostracode Analysis.

Ten samples were examined from the Pit of Death Core, and the ostracodes present exhibit variable states of preservation and are composed of various mixtures of the three assemblages noted in the Black Rock ostracode discussion above. The mixing of apparently environmentally-incompatible taxa implies that material was reworked to this site from eroding lake sediments on the valley floor or margin.

Comparison of Black Rock and Pit of Death chronologies.

Fifteen samples from the Pit of Death were examined for diatom analysis (at the approximate depths of 18, 21, 68, 87, 101, 112, 122, 133, 162, 201, 259, 306, 354, 408, and 484 ft), and all samples were barren.

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