Jones Mountains, Antarctica: Evidence for Tertiary glaciation revisited

The Jones Mountains (lat. 73°30’ S, long. 94° W) were discovered in January, 1960. In November of the same year, a University of Minnesota party led by Cam Craddock initiated studies of the geology of this previously unknown small group of mountains. The rocks exposed in this east-west trending group consist of a Mesozoic basement complex of granites in fault contact with younger felsic rocks. Both of these are cut by basalt dikes, which in turn are cut by felsic dikes. This sequence of rocks is truncated by an erosion surface that is essentially horizontal. Overlying the unconformity is a sequence of Tertiary volcanoclastics about 500 m thick. The lower 10-20 m part of this sequence is made up of rocks unknown higher in the sequence. These rocks are of some importance as they contain erratic clasts in the form of pebbles, cobbles, and boulders, striated and faceted, of rock types unknown locally. A second lithology in these lower rocks is a tillite (diamictite) with a matrix of palagonite and exotic erratic clasts. The surface and the rocks immediately above it were the focus of additional studies in 1964 and 1968-69. The surface, now known to be exposed along the front of the mountains for about 33 km, is planed and polished and is marked by striations, grooves, and chattermarks. Ice-flow directions from these markings indicate of movement from south to north. The surface itself suggests a glacial erosion origin and the overlying rocks suggest the interaction of volcanic eruptions and ice. Field work supported by laboratory analysis confirms the conclusion that the sequence represents the result of subglacial eruptions. Recent Ar/Ar incremental heating analysis of one basaltic sample provides a robust and precise age of 7.63 ± 0.11 Ma, interpreted as an accurate measurement of eruption age, thereby supporting previous interpretations based on K-Ar results of Jones Mountains volcanism at approximately 7 Ma. Other samples yielded low-precision, disturbed age spectra and isochrons indicating significant excess Ar. When first reported in 1965, the Jones Mountains represented the first documented case for Tertiary glaciations in Antarctica. The Byrd Land Coast and other areas in Antarctica are known to contain similar sequences, and Tertiary glaciation is now generally accepted.