Download Early Pliocene volcanic ash rests on a polar desert pavement

Oligocene to lower Miocene San Gregorio Formation, Baja California
Sur, Mexico. Diatom Research, 1(2), 169-187.
Lohman, K., and G. Andrews. 1968. Late Eocene non-marine diatoms
from the Beaver Divide area, Fremont County, Wyoming. (U.S. Geological
Survey Professional Paper 593-E.) Washington, D.C.: U.S. Government Printing Office.
Scherer, R. 1989. Microfossil assemblages in "deforming till" from
Upstream B, West Antarctica: Implications for ice-stream flow models. Antarctic Journal of the U.S., 24(5).
Scherer, R., D. Harwood, S. Ishman, and P. Webb, 1988. Micropaleontological analyses of sediments from Crary Ice Rise. Antarctic
Journal of the U.S., 23(5), 34-36.
Schrader, H., and J. Fenner. 1976. Norwegian Sea Cenozoic diatom
biostratigraphy and taxonomy. initial Reports of the Deep Sea Drilling
Project, 38, 921-1,099.
Webb, P., D. Harwood, B. McKelvey, J. Mercer, and L. Stott. 1984.
Cenozoic marine sedimentation and ice volume variation on the East
Antarctic craton. Geology, 12, 287-291.
Early Pliocene volcanic ash
rests on a polar desert pavement
DAVID R. MARCHANT
Department of Geological Sciences
and
Institute for Quaternary Studies
University of Maine
Orono, Maine 04469-0110
DANIEL
R. Lux
Department of Geological Sciences
University of Maine
Orono, Maine 04469-0110
CARL C. SWISHER, III
Berkeley Geochronology Laboratory
Institute of Human Origins
Berkeley, California 94720
GEORGE
H. DENTON
Department of Geological Sciences
and
Institute for Quaternary Studies
University of Maine
Orono, Maine 04469-0110
Reconstructions of Pliocene climate based on the ecology of
marine diatoms and Nothofagus wood of assumed Pliocene age
within the Sirius formation suggest extensive ice-sheet collapse
accompanied by warm (2-5° C) marine seas in the interior of
East Antarctica (Harwood 1986; Webb et al. 1986) and the growth
of Nothofagus in the adjacent Transantarctic Mountains.
We report here an alternative climate reconstruction based
on isotopically dated volcanic deposits that overlie in situ polar
desert pavements. One such ash deposit of early Pliocene age
occurs in Arena Valley (77°51'S 161°E), Quartermain Mountains, Antarctica, and is described below.
A 30-centimeter-thick, light-gray to pale white, in situ volcanic ash surface deposit overlies a well-developed desert
pavement at 1,500 meters elevation in central Arena Valley
(figures 1 and 2). The buried pavement is composed of an
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Figure 1. Photograph of volcanic ash in west central Arena Valley.
The vertical face of the ash has been cut back to expose the underlying buried desert pavement. A highly weathered colluvial deposit (devoid of volcanic material) underlies the ash.
interlocking mosaic of closely spaced ventifacts of Ferrar Dolerite and Beacon Heights Orthoquartzite. The ventifacts are
commonly pitted and exhibit thick coatings of desert varnish.
The ash is overlain by a second desert pavement which is
identical in form, composition, and texture to the buried pavement (figure 1).
The age of the ash was determined by conventional argon40/argon-39 methods on bulk ash samples and by the laserfusion argon-40/argon-39 technique of dating single crystals.
Qualitative X-ray microprobe analyses indicated that the crystal fraction included anorthoclase, aegerine, subcalcic augite,
and magnetite. Anorthoclase was isolated using heavy liquids
and a Franz magnetic separator. Approximately 1,000 grams
of ash yielded about 1.0 grams of "pure" anorthoclase. Conventional argon-40/argon-39 incremental release heating of bulk
samples of anorthoclase using a Nuclide 6-60-SGA 1.25 mass
spectrometer yielded a plateau age of 4.69 ± 0.10 million years,
although the release spectrum was saddle-shaped. Lo Bello et
al. (1987) showed that such a release spectrum from volcanic
feldspars suggests xenocrystic contamination of the sample.
Therefore, the age determined by bulk analysis of hand-picked
feldspars probably represents only a maximum age of the ash.
To obtain a more accurate age for the ash, individual anorthoclase crystals were dated using the argon-40/argon-39 laserfusion technique. Results indicated that the ash was composed
of at least two distinct populations. The younger population
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I-, ;
Figure 2. Scanning electron microscope image of glass shards within the ash. Scale bar is 0.1 millimeter.
was by far the largest and yielded an age of 4.474 ± 0.032
million years. This is our most accurate date for the eruption
and subsequent deposition of the ash.
The polar desert pavement preserved beneath the early Pliocene ash-fall deposit has the following paleoclimatic implications:
• It suggests that a dry, polar climate existed in Arena Valley
prior to 4.474 ± 0.032 million years. Nothofagus cannot exist
under such a climate (Sakai 1981).
• It demonstrates that Arena Valley was not filled with ice
from local valley glaciers or the east antarctic ice sheet at
this time.
• Because Arena Valley is a tributary to Taylor Valley, (which
opens to McMurdo Sound), the volcanic ash suggests that
tectonic uplift of Arena Valley is restricted to less than 1,500
meters within the last 4.474 million years.
• It suggests that the general morphology of Arena Valley
antedates 4.474 ± 0.032 million years.
• It demonstrates that the massive ice sheet overridings postulated by Denton et al. (1984) antedate 4.474 ± 0.032 million
years.
1989 REVIEW
We thank David P. West, Jr., at the University of Maine
Geochronology Lab for initial dating of the ash from bulk anorthoclase samples. W.C. McIntosh provided figure 2. Martin
Yates assisted in the microprobe analyses of the crystal com ponent of the ash, and S.C. Wilson assisted in the field.
This work was supported by National Science Foundation
grant number DPP 861-3842.
References
Denton, G.H., M.L. Prentice, D.E. Kellogg, and T.B. Kellogg. 1984.
Late Tertiary history of the Antarctic Ice Sheet: Evidence from the
Dry Valleys. Geology, 12, 263-267.
Harwood, D.M. 1986. Recycled siliceous microfossils from the Sirius
Formation. Antarctic Journal of the U.S., 21(5), 101-103.
Lo Bello, Ph., C. Feraud, G.M. Hall, D. York, P Lavina, and M. Bernat.
1987. 40Ar/39 Ar step-heating and laser fusion dating of Quaternary
pumice from Neschers Massif, Central France: The defeat of xenocrystic contamination. Chemical Geology (isotope geoscience section),
66, 61-71.
59
Sakai, A. 1981. Freezing resistance of trees of the south temperate
zone, especially subalpine species of Australia. Ecological Society of
America, 62, 563-570.
Webb, P. N., D. M. Harwood, B. McKelvey, M.G.C. Mabin, and J.H.
Mercer. 1986. Late Cenozoic tectonic and glacial history of the Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 99-100.
The origin of isolated gravel ripples
in the western Asgard Range,
Antarctica
Measurements made across ripples document the strongly
asymmetric cross sections (figure). The actual crest is not a
line but a diffuse area up to 20 centimeters wide. Two indices
used to describe the form and shape of ripples were calculated.
Average values of the ripple index (RI L/H) and the Ripple
Symmetry Index (RSI = SL/LL) also appear in table 1. Average
values for symmetry indexes range from slightly asymmetric
to very asymmetric. Perfectly symmetric ripples have values
of RSI = 1. The asymmetry is the opposite of most wind and
water current ripples which typically have values of RSI > 2
(Tanner 1967).
The surfaces of the gravel ripples consist primarily of weathered sandstone. Individual clasts commonly have thick quartz
weathering rinds and a desert varnish composed of a reddish
silicious crust (Weed and Ackert 1986). Clasts of sandstone
and dolerite up to 15 centimeters in diameter commonly occur;
some clasts are much larger. Frost cracks commonly occur on
the lee sides of ripples. A concentration of well-sorted gravel
up to 2 centimeters in diameter occurs on the lee side of the
ripple crest on many ripples. A poor to fairly well-developed
slip face sometimes occurs within this material on the lee side
edge. The pavement is crudely sorted. The largest clasts commonly occur on the stoss side of the ripples and typically rest
directly on the bedrock at the toe of the ripple. Other clasts
on the stoss side are commonly setting or leaning on one
another with no matrix material between them. The average
clast size on the surface of the lee side decreases toward the
heel of the ripple (figure). Lithologic, shape and textural data
on samples of gravel collected from ripple surfaces appear in
table 2.
Excavations through the ripples show that the surface pavement overlies a thin, sandy, pebbly diamicton. A layer of rotted
bedrock up to several centimeters thick commonly occurs between the bedrock and the overlying diamicton. Bedrock structures such as worm tubes and bedding are sometimes preserved
within this layer. The layer pinches out at the toe and heel of
the ripple. The ripples are generally less than 30 centimeters
thick at the crest. Although excavations were situated to avoid
visible frost cracks, sand wedge structures (Berg and Black
1966) occurred in many excavations (figure).
The pebbles and small cobbles within the diamicton are similar in lithology, size, and surface texture to those on the surface of the ripple. The weathered clasts are supported by a
matrix of sand and fine gravel. Individual sand grains have a
reddish stain. The coarse sand and fine gravel is composed
largely of dolerite grus. Six samples were analyzed to determine the grain-size distribution of the matrix material. The
results appear in table 3. The samples are gravelly muddy
sands and gravelly sands. The frequency distributions are bimodal. The primary peak includes at 1.5 4 is inherited from
the Beacon Sandstones (Barrett 1972). The secondary peak occurs at -0.5 . The samples are well sorted and slightly fineskewed. There is virtually no mud in the samples.
ROBERT P. ACKERT, JR.
Department of Geological Sciences
University of Maine
Orono, Maine 04469
Fields of isolated gravel ripples occur throughout the uplands of the Dry Valleys. The ripples were reported in Denton
et al. (1984) as part of a system of features indicative of subglacial sheet flow of meltwater beneath an ice sheet which overrode the Transantarctic Mountains. As part of a project
designed to test the Denton et al. (1984) hypothesis of ice sheet
overriding, detailed field studies were made in Njord Valley
(77°36'S 1617E) in the western Asgard Range. Njord Valley
is an ice-free, north-facing, hanging valley which overlooks
the Dais in upper Wright Valley. The valley is eroded into
sandstones of the Beacon Super Group; surrounding heights
are capped by Ferrar Dolerites (McKelvey and Webb 1962).
Within the valley, a complete set of features reported by Denton et al. (1984) are preserved. Among the features studied
were several well-developed fields of isolated gravel ripples.
Fieldwork was conducted during the 1983-1986 field seasons.
Preliminary results are presented here.
The ripples are conspicuous features due to their size and
to the contrast between the reddish, weathered sandstone gravel
composing the bulk of the ripples and the light, unvarnished
sandstone bedrock exposed between them. From a distance,
the asymmetric cross section of the ripples is readily apparent.
The ripples have short, steep slopes facing up-valley and longer,
gentler slopes on the down-valley sides. For purposes of discussion, the up-valley side is assumed to be the stoss side and
the down-valley side the lee side. Although the fields of gravel
ripples occur in topographic lows such as troughs or basins
on valley floors, individual ripples tend to occur on local topographic highs. The figure shows a schematic cross-section of
a typical gravel ripple.
Table 1 summarizes data which describe the size and form
of the ripples. The largest ripples are up to 0.5 meters high, 8
meters wide, and 100 meters long. The ripple crests are generally perpendicular to the valley axis, sinuous, and in a few
cases bifurcated. The distance between crests is relatively constant within a given area. As the sinuosity decreases, the wavelength decreases. Typically, the width of the area of exposed
bedrock between the ripples is several times greater than the
width of the ripples.
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