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.
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.
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).
Download a postscript file containing these six figures.
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
Methods
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.
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"
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.
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