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The sub-Sirius Group erosion surface is well developed and exposed on nunataks in the upper Shackleton Glacier region where a terrain of Beacon Supergroup and Ferrar Dolerite rocks have been deeply dissected prior to deposition of Sirius Group strata (Webb et al., Antarctic Journal, in this issue, 1996b,c). Roberts Massif provides one of the best examples of this relict landscape known from the Transantarctic Mountains and so was examined in detail during the 1995-1996 field season.
Roberts Massif, approximately 20 kilometers (km) by 20 km, is a deeply dissected and largely ice-free nunatak bordered by the inland ice plateau to the south and by the Zanneveld and Shackleton Glaciers to the east and west, respectively. Total topographic relief is a little over 700 meters (m), and Misery Peak is the highest point at about 2,723 meters. The nunatak consists of extensively faulted Beacon Supergroup and Ferrar Dolerite; the former is well exposed in the walls of deeply dissected glacial valleys, and the latter makes up extensive upland plateaus (about 2,400 m) and lowland valley floor platforms (about 2,000 m). Sirius Group sediments overlie these older rocks via a rugged disconformity, which displays buttress unconformity relationships where successions abut near-vertical paleovalley walls. Where Sirius successions occupy paleovalley floor settings, however, the disconformity surface exhibits a gently undulating relief. A pale green coloration or staining of Beacon Supergroup and Ferrar Dolerite cliff surfaces is apparent in the central region of Roberts Massif, indicating that Sirius Group strata once abutted these surfaces. These glaciated valley walls are also part of the sub-Sirius erosion surface. Deeply weathered surfaces, joints, and cracks in these older rocks have been infiltrated by remobilized Sirius Group diamictite matrices. Thick Sirius Group successions formerly blanketed much of this high-relief terrain.
A significant geomorphic feature exposed in the northern lowland region of Roberts Massif is a widespread sub-Sirius abraded pavement with an area of at least 25 square kilometers, developed on gently dipping dolerite sills. This erosion surface is one of the most extensive glacial paleosurfaces yet reported in Antarctica. The most striking aspect of the abraded pavement at Roberts Massif is the extensive development of ridges and grooves. On a scale of a few kilometers, they trend northward from near the foot of the steep scarp that stands above the northern lowland. The ridges and grooves do not appear to be controlled by either the bedrock structure or composition because the joint pattern in the dolerite is irregular and few joints are parallel to the grooves. In cross section, the pavement comprises open concave depressions and convex ridges with amplitudes of 2-3 m and wavelengths of 5-10 m. Ridges and grooves are continuous for 50-100 m, then either die out or bifurcate. The smooth abraded surface of these forms is disrupted by joints and fractures where angular blocks have become detached. Whereas vertical joint cracks measure up to several centimeters across and their interior rock walls are weathered, the later-developed sub-Sirius Group erosion surface is fresh and polished. We suggest that a phase of deep weathering preceded glacial erosion and deposition of the Sirius Group.
Sets of striations are well preserved on the ridge and groove pavement where it is protected beneath the Sirius Group strata. The main set of striations are aligned parallel to the grooves and are consistent in orientation to within 20°, i.e., north. A northerly paleoice movement direction is indicated by rare crescentic gouges and friction cracks on the ridge forms. In a few places, sets of striations with transverse orientation intersect the northerly set. These do not appear to extend beneath the Sirius Group sediments. Because a set of crescentic friction cracks was also observed on top of a thin overlying remnant of the Sirius diamictite, it is inferred that the transverse striations were produced by younger Pleistocene ice, which moved across the northern lowland during expansion of a pre-cursor of the Zanneveld and other glaciers.
What was the relationship between the Sirius Group diamictite and the underlying grooved pavement? Two-dimensional clast fabrics were measured in sediments occurring within 10 centimeters of the contact. The fabric derived is typical of a lodgement till, having a northerly preferred orientation, within 10° of that of the grooves. This finding possibly suggests that the deposition of the diamictite took place soon after the pavement was formed. Mayewski (1975) and Mayewski and Goldthwait (1985) coupled the formation of the basal erosion surface and the deposition of the overlying Sirius Group as part of a single major event, their Queen Maud Glaciation. Temporal relationships between the erosion surface and the diamictite, however, cannot be ascertained at this time.
The present lowland topography does not appear to be compatible with the ice-flow patterns derived from grooves and fabrics. One would normally expect such irregular topography to result in local variations of ice-flow directions, but this is not the case. Additionally, the pavement occurs at different levels across Roberts Massif, suggesting that the landscape was disrupted by faulting after its formation. This argument is supported by stratigraphic displacements within the Sirius Group itself. We consider that the topography during the erosion of the pavement and deposition of the Sirius Group was subdued and that the present-day high-relief terrain developed subsequently.
As defined by Sugden and John (1976, p. 194), heavily grooved surfaces of consistent orientation are generally associated with regions of areal scouring. The sub-Sirius Group erosion surface exposed at Roberts Massif is similar to the megagroove glacial topography at Kelleys Island in western Lake Erie, a feature ascribed to the passage of ice streams in a state of compressional flow (Goldthwait 1979). The orientation of grooves and fabrics described above suggests that ancestral Roberts Massif was overridden by a glacier or large ice stream flowing from the south. This ice movement does not necessarily indicate that the protoantarctic ice sheet was thicker than that of today, but rather that the Transantarctic Mountains in this area were probably less of a barrier to ice flow than is presently the case. Subsequent uplift and glacial downcutting led to the present glacial valley entrenchment of major outlet glaciers, such as the Shackleton and Beardmore.
This work was supported by National Science Foundation grants OPP 94-19056 (Peter Webb) and OPP 91-58075 (David Harwood). We thank Derek Fabel and John de Vries for assistance during our field activities.
References
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