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USGS Open-File Report 95-001


Localities and Setting

Two cores were collected from Millard County in western Utah in the summer of 1993 (Fig. 1). The Black Rock core site is on top of an early Pleistocene basalt flow east of the abandoned rail siding at Black Rock, Utah (NW 1/4, Section 23, T24S, R10W; 4930 ft [1503 m] elevation; 38°42'55"N, 112°56'59"W). The Pit of Death core site lies on the northwest margin of the presently dry Sevier Lake Basin (NE 1/4, Section 32, T20S, T12W; 4535 ft [1383 m] elevation; 39°02'02"N, 113°13'00"W). The Bishop Ash (~759 ka) is present within the surficial sediments at both sites, and as discussed below, both sediment cores provide continuous coverage back to the middle Pliocene. The regional climate today is arid, with hot dry summers and a dominance of winter precipitation. Although no weather stations occur near the core sites, shadscale-dominated vegetation, such as that at the core sites, typically grows in climates with ~3.1 to ~5.7 inches (79 to 145 mm) of mean annual precipitation (Billings, 1949). The mean annual precipitation at the Black Rock site probably approaches the higher figure, whereas that at the more xeric Pit of Death site probably falls closer to the lower figure.

View figure 1.

Locations of sites discussed in text. The shaded areas represent elevations above 6000 ft (1829 m). Woodland and forest vegetation generally occurs above this elevation, steppe and desert below.

Dry steppe vegetation is present today at the Black Rock Site, with shadscale (Atriplex confertifolia) and greasewood (Sarcobatus vermiculatus) being the dominant plants, with snakeweed, Indian ricegrass, and horsebrush (Gutierrezia sarothrae, Oryzopsis hymenoides, and Tetradymia sp.) occurring as common elements. Cheatgrass, rabbitbrush, and mormon-tea (Bromus tectorum, Chrysothamnus nauseosus, and Ephedra nevadensis) are present but less abundant, and sheepbane and tumbleweed (Halogeton and Salsola) dominate disturbed areas around the site. Big sagebrush (Artemisia tridentata) occurs as a very rare plant within approximately a kilometer of the site; it becomes more abundant on sites several kilometers to the east at slightly higher elevations. The vegetation at the Pit of Death site is similar to that at Black Rock, but is more sparse, reflecting more arid conditions. Mormon-tea is dominant at the Pit of Death, with shadscale, horsebrush, greasewood, and other Chenopodiaceae present as common elements in the vegetation.

The regional vegetation of the eastern Great Basin is characterized by a general elevational progression of zones following the gradient from hot-dry environments on the lowest valley floors to cool-moist environments at high elevations. Sparse shadscale and greasewood steppe associations (such as that at the Pit of Death site) occur in the hottest and driest locales, giving way to more dense and slightly more diverse steppe communities on the upper bajadas. Sagebrush intermingles with these xeric elements on the upper bajadas and lower mountain slopes, and then continues as an understory element in higher elevation woodland and forest associations. Utah juniper (Juniperus osteosperma) is generally the lowest occurring tree in the region, and it forms pygmy-conifer woodlands with pinyon pine (Pinus monophylla) on the lower to middle mountain slopes. Ponderosa pine (Pinus ponderosa), fir/Douglas fir (Abies/Pseudotsuga) , and aspen (Populus tremuloides) forest assemblages occur with increasing elevation. The arid mountain ranges of westernmost Utah generally support limber pine / bristlecone (Pinus flexilis / P. longaeva) subalpine forests at high elevations, whereas the more massive Wasatch Range captures more precipitation and hosts spruce (Picea) forests at high elevations. The Wasatch Range also hosts oak- (Quercus-) dominated mountain brush communities that are not present in the more arid Great Basin ranges. Regional studies of the modern pollen rain (e.g. Davis, 1984; Thompson, 1992) indicate that the major vegetation assemblages of the Great Basin are reflected in the pollen rain.


Core collection and curation. The Black Rock and Pit of Death sediment cores were drilled in July, August, and September of 1993 with a Portadrill 524-3A rotary drilling rig that took a 3 " (7.62 cm) diameter core. The cores are archived at the USGS Core Repository on the Denver Federal Center. The coring equipment and original records are in English units, and to maintain continuity with those records, core depths are reported here in English units.

Stratigraphy and sedimentology. Core sediments were logged in the field, and the stratigraphy and sedimentology was studied in greater detail in the laboratory by Oviatt and Kelsey (Black Rock) and Oviatt and Bracht (Pit of Death). These researchers have analyzed samples for carbonate, sand, and mud contents at approximately one-foot (30 cm) intervals throughout both cores. X-ray analysis is on-going for carbonate minerals from representative samples from both cores.

Paleomagnetism. Oriented paleomagnetic samples were taken by carving pedestals into the cores and slipping plastic boxes (2.5 x 2.5 x 1.5 cm) over the pedestals. In general, samples were collected at 1 m (3.3 ft) intervals throughout the length of both cores. For paleomagnetic studies, 307 and 139 samples were analyzed from the Black Rock and Pit of Death cores respectively. The samples were stepwise demagnetized by either alternating field (AF) or thermal methods. AF demagnetization was generally carried out at successive peak fields of 0, 10, 20, 25, 40, and 60 mT, and in some cases, additional intermediate steps were included at peak fields of 5, 15, 30, and 50 mT. Thermal demagnetization was carried out at successive steps of 20, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, and 600°C. Magnetic susceptibility was measured at each thermal demagnetization level to monitor for thermal alteration. Remanence measurements were made on a 2G Enterprises cryogenic magnetometer. AF demagnetization was carried out with a Schonstedt AF demagnetizer and thermal demagnetization was carried out with a Schonstedt thermal demagnetizing unit. Magnetic susceptibility was measured with a Bartington Instruments magnetic susceptibility meter.

Characteristic remanence directions were determined from stably magnetized samples by linear regression fits to the demagnetization data. Three types of demagnetization behavior are evident and are designated as types A, B, and C. Type A behavior is displayed by demagnetization data that decay to the origin of vector component plots with little deviation from linearity (Figure 2a, d). Type B behavior is displayed by samples that yield more scattered demagnetization data, nevertheless, the direction of magnetization can be unambiguously determined (Figure 2b, e). Type C behavior is displayed by samples for which no stable direction of remanence, or even an indication of polarity, can be determined (Figure 2c, f). In general, the data quality from the Pit of Death core are of substantially higher quality than those from the Black Rock core. Of the 139 samples analyzed from the Pit of Death core, 81%, 16%, and 3% of the samples displayed behavior of types A, B, and C, respectively. Of the 307 samples analyzed from the Black Rock core, 17%, 54%, and 29% of the samples displayed behavior of types A, B, and C,respectively. The Black Rock and Pit of Death cores were azimuthally unoriented. The paleomagnetic polarity was therefore determined solely from inclination data. All ages of paleomagnetic datums follow those of Cande and Kent (1992).

Figures 2a - 2f; Vector component diagrams illustrating representative demagnetization behavior for types A (a and d), B (b and e), and C (c and f), as described in the text. Sample names and depths of samples in core are shown for each sample. Open (closed) symbols represent projections onto the vertical (horizontal) plane. Decay of remanence intensity with stepwise demagnetization is shown on the lower right hand side of each plot. Samples measured at 0 to 60 mT levels were demagnetized by AF methods and samples measured at 0 to 500 or 600°C were demagnetized by thermal methods.

View figure 2a. sample BRU108, depth = 331'0"

View figure 2b. sample BRU145, depth = 447'11"

View figure 2c. sample BRU10, depth = 55'11"

View figure 2d. sample POD7, depth = 28'3"

View figure 2e. sample POD38, depth = 88'10"

View figure 2f. sample POD67, depth = 324'9"

Download a postscript file containing these six figures.

Magnetic susceptibility. The magnetic susceptibility of the Black Rock and Pit of Death core sediments was measured by analyzing selected depth intervals with a Sapphire whole-core pass-through susceptometer. For susceptibility studies, 766 levels were measured from the Black Rock core and 411 from the Pit of Death core.

Palynology. Sediment samples from the Black Rock and Pit of Death cores were processed with chemical reagents (HCl, HF, heavy liquids) to remove unwanted mineral materials. The sample residues were analyzed under 400X to 1000X magnification, and a minimum of 300 terrestrial pollen grains were counted from each sample (except for samples above ~150 ft in the Black Rock core, where pollen concentrations were so low that only 200 grain counts were possible). Of the 159 samples processed from the Black Rock site, 142 contained sufficient pollen for analysis. Ten samples were processed from the Pit of Death core, and all were barren of pollen.

Plant macrofossil analysis. Seeds and leaf fragments and Ruppia and other plants are present throughout most of the Black Rock core. The occurrences of these plant macrofossils has been recorded during the sediment descriptions and by examination of slides prepared for ostracode analysis (see below).

Ostracode Analysis. Sediment samples for ostracode analysis were split into two fractions, a larger fraction for isotopic analysis of ostracode shells (~15 g) and a smaller fraction for ostracode counts (~5 g). The samples were subjected to a freeze/thaw process to disaggregate clay particles and then washed with hot water over a 100 mm mesh screen. Approximately 920 samples have been prepared from the Black Rock core, and 10 from the Pit of Death Core.

Diatom Analysis. Preliminary analysis of diatoms from the two cores were conducted using water mounts of unprocessed sediment smears. Twenty-four samples have been analyzed from the Black Rock core and 10 from the Pit of Death core.

Continue to Black Rock Core Record

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