Chapter 4: Stratigraphy and sedimentology of the Whiteout Conglomerate; An upper Paleozoic glacigenic unit, Ellsworth Mountains, West Antarctica
Published:January 01, 1992
Charles L. Matsch, Richard W. Ojakangas, 1992. "Chapter 4: Stratigraphy and sedimentology of the Whiteout Conglomerate; An upper Paleozoic glacigenic unit, Ellsworth Mountains, West Antarctica", Geology and Paleontology of the Ellsworth Mountains, West Antarctica, G. F. Webers, C. Craddock, J. F. Splettstoesser
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The upper Paleozoic Whiteout Conglomerate, situated conformably upon the Crashsite Group and overlain conformably by the Polarstar Formation, may represent the entire period of Gondwanaland glaciation in West Antarctica. The formation, mainly massive diamictites, makes up about 1,000 m of a 13,000-m-thick total stratigraphic column in the Ellsworth Mountains.
In the northern Sentinel Range, the Whiteout Conglomerate contains as many as six dark gray to black, thick, massive-bedded diamictites that are separated by recessive layers of laminated to thin-bedded shale, mudstone, and sandstone as thick as 15 m. Clasts greater than 5 mm in diameter compose 3 to 10 percent of most diamictites. Quartzite (66 percent) is the dominant clast type, with granite (17 percent), shale-argillite (8 percent), and carbonate (5 percent) composing most of the remainder. Some clasts are faceted and striated.
A truncated Whiteout section of about 250 m in the Meyer Hills to the south contains thinner diamictites with more variable rock types and a stratigraphic sequence punctuated by striated boulder pavements and thin laminated beds of shale, sandstone, and pebble lags. Of the pebbles in the diamictites, 47 to 86 percent are quartzites. Limestone and granite each compose about 20 percent. Conspicuous changes in pebble compositions occur across contacts marked by boulder pavements.
Petrographically, the diamictites are 62 percent silt and clay matrix with 38 percent sand grains and granules. The sand-sized grains have an average quartz/feldspar/lithic (Q:F:L:) ratio of 75:9:16; the studied specimens displayed no significant petrographic variations between the northern Sentinel Range and the Meyer Hills. However, there is a difference in megaclast compositions; in the northern Sentinel Range, shale-argillite and vein quartz are abundant, but they are absent in the Meyer Hills where carbonate is abundant. Till pellets, products of ablation on glaciers and icebergs, are common in bedded units and rare in the diamictites.
The Crashsite Group quartzite and Whiteout Conglomerate are dominated by quartz and quartz-rich lithic fragments, whereas the overlying Polarstar Formation contains abundant volcanogenic detritus, reflecting a dramatic change in source area. Contact zones with both enclosing formations contain scattered oversized clasts.
Striated boulder pavements in the Meyer Hills generally indicate paleo-ice flow to the present northwest. With the Ellsworth block restored to its probable late Paleozoic position via a 90° clockwise rotation, the glacier source area was the East Antarctica landmass now situated south of the Ellsworth Mountains. Sedimentologic similarities with formations in the Pensacola Mountains to the south indicate that the Ellsworth Mountains have moved northward at least 500 km from a hypothetical former position just east of the Pensacolas.
Our glacial model suggests that ice sheets generated the glacial sediments of the Whiteout Conglomerate under a polar climatic regime. The Meyer Hills sequence represents deposition directly by glaciers and also by floating ice and currents in a zone of fluctuating grounding line. Deeper water and sub-ice shelf conditions, i.e., glacial-marine, are postulated for the thick sequence of the northern Sentinel Range. There, periodic retreat of the ice shelf margin was accompanied by iceberg rafting and bottom current activity, probably in response to climatic warming.