GINA SEEGERS SZABLEWSKI, ERM-North Central, Inc., Milwaukee, Wisconsin 53202
JOHN L. ISBELL, Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201
Permian postglacial rocks exposed in the area between the Nimrod and Byrd Glaciers (figure 1) were studied during the 1993-1994 austral summer. Prior to this field season, upper Paleozoic rocks in the central Transantarctic Mountains were known to occur only in the area from just north of the Nimrod Glacier to the Ohio Range (Laird, Mansergh, and Chappell 1971; Elliot 1975; Barrett 1991; Collinson et al. 1994; figure 1). Upper Paleozoic rocks, including the postglacial shales and sandstones, are now known to extend to at least the Byrd Glacier (Isbell et al. 1994). The purpose of this article is to provide a brief description of the postglacial rocks in this area and to interpret their environment of deposition . Laird et al. (1971) first correlated postglacial rocks just north of the Nimrod Glacier with rocks of the Mackellar Formation in the Beardmore Glacier region. Because these rocks can be traced as far as the Wallabies Nunataks (figure 1), we apply the name "the Mackellar Formation" to all postglacial shale and sandstone successions between the Nimrod and Byrd Glaciers. In this area, rocks of the Mackellar Formation include all shale and thin fine- to medium-grained sandstone beds that occur above a sharp contact with diamictites of the underlying Pagoda Formation and below the gradational contact with thick medium-grained sandstones of the overlying Fairchild Formation (Isbell et al. 1994). Rocks of the Mackellar Formation range from 16 to 52 meters (m) thick, and the thickest rocks occur in the central portion of the study area near Chappell Nunataks (figure 1). The postglacial rocks thin toward the Byrd Glacier as well as toward the Ross Ice Shelf and the polar plateau.
In the study area, rocks of the Mackellar Formation occur in multiple coarsening-upward successions (CUSs). A complete CUS contains, in ascending order, the following lithofacies:
These lithofacies form one to three 16- to 30-m-thick CUSs.
The shale lithofacies at the base of the CUS is in sharp contact with either diamictite at the top of the Pagoda Formation or with medium-grained sandstone at the top of an underlying CUS (figure 2). These black to gray shales are 1 to 5 m thick and are laterally continuous across outcrop faces. Claystone occurs at the base of this lithofacies and grades progressively upward into alternating laminae of mudstone and siltstone and ultimately into alternating laminae of mudstone and very fine-grained sandstone.
The shale lithofacies grades upward into the 7.5- to 30-m-thick shale and interbedded very fine- to medium-grained sandstone lithofacies (figure 2). Sandstone layers in this lithofacies
Small load structures and small-scale overturned and folded laminae are also common. Individual beds, which thin and fine laterally, display a slight downward dip in a direction parallel to paleocurrent orientations. These beds grade laterally into finer grained deposits. Rare symmetrical ripples occur on the updip portions of the dipping sandstone beds. A low-diversity ichnofacies containing Isopodichnus occurs on shale and sandstone bedding planes.
The sandstone foreset lithofacies occurs at the top of the uppermost CUS (figure 2) and consists of 5- to 10-m-thick beds of medium-grained sandstone, which dip at 1° to 29° in a direction parallel to paleocurrent orientations. These beds grade downdip into fine- to very-fine-grained sandstones of the underlying shale and sandstone lithofacies. Sandstone foreset beds are overlain by medium-grained sandstones of the Fairchild Formation throughout the Nimrod-Byrd area.
Paleocurrent orientations and sandstone-shale ratios vary across the study area. Paleocurrent orientations directed obliquely toward the Ross Ice Shelf occur in rocks exposed in the Geologist Range, whereas in the Churchill Mountains and nearby Nunataks, directions oriented obliquely toward the polar plateau were recorded (figure 1). Regional paleocurrent orientations are toward the location of the present Weddell Sea. The highest sandstone-shale ratios occur near the Byrd Glacier and decrease toward the Nimrod Glacier.
In the Nimrod-Byrd area, rocks of the Mackellar Formation record flooding of the depositional basin following the demise of the upper Paleozoic ice sheet in Antarctica. The contact between the glacial rocks and the postglacial shales is a flooding surface that marks the change from glacial terrestrial to postglacial basinal conditions in central Transantarctic Mountains. The shale lithofacies represent distal fine-grained sedimentation far from the paleo-shoreline. The overlying CUSs then record multiple progradational events as deltas introduced coarser clastics into the basin. Sandstone laminae and beds within the CUS were deposited by underflow currents with finer grained, thinner units representing distal delta front sedimentation. Sandstone foreset beds represent the delta mouth bars of prograding Gilbert-type deltas. The abundance of underflow deposits, Gilbert deltas, and the presence of the trace fossil Isopodichnus suggest that the Mackellar basin was characterized by freshwater conditions in this area. Sandstones of the overlying Fairchild Formation record a change from deltaic to fluvial conditions within central Transantarctic Mountains.
Deposition of the Mackellar Rocks in the study area occurred within a basin that narrowed and shoaled toward the Byrd Glacier. Paleocurrent orientations and isopachs suggest basin margins located along the present polar plateau and along the Ross Sea side of the Churchill Mountains (figure 1). We thank Shawn Norman for his assistance in the field. Antarctic Support Associates, the U.S. Navy Squadron VXE-6, and the National Science Foundation provided logistic support in Antarctica. This research was supported by National Science Foundation grants OPP 92-96275 and OPP 94-19962.
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