Extent and Nature of Ross Sea Unconformity in the Western Ross Sea, Antarctica

Seismic-reflection data collected in the Ross Sea over the last decade have shown local occurrences of angular uncon­ formities in the upper 3 0 0 m of the sedimentary section (Houtz and Meijer, 1 9 7 0 ; Houtz and Davey, 1 9 7 3 ; Wong and Chris-¬ toffel, 1 9 8 1 ; Davey and Christoffel, 1 9 8 4 ; Cooper, Davey, and Behrendt, this volume). During Leg 2 8 of the Deep-Sea Drilling Project (DSDP) , lithologic and paleontologic changes that occur in core samples at a subbottom depth of 2 0 m at sites 2 7 0 and 2 7 2 were interpreted as evidence of a discon¬ formity of Pliocene age. The disconformity in the core was correlated with an angular unconformity observed on the seismic-reflection profiles at those sites (Hayes et al., 1 9 7 5 ) . The local angular unconformities seen on isolated seismic lines have been assumed to be contemporaneous with the angular unconformity at the DSDP sites and, thus, representative of a single erosional surface of regional extent. The postulated regional angular unconformity is informally called the Ross Sea unconformity (RSU) and is believed to record the climax advance of the West Antarctic Ice Sheet (WAIS) . The timing of this event is uncertain (Kellogg et al., 1 9 7 9 a , 1979b ; Sav­ age and Ciesielski, 1 9 8 3 ) ; the most definitive statement that can be made is that the WAIS began to advance sometime after 10 Ma. Paleontologic data from cores in the Ross Sea suggest the possibility of at least one regional disconformity that is younger than the RSU (Fillon, 1 9 7 2 , 1 9 7 5 ; Savage and Cie­ sielski, 1 9 8 3 ) . The timing and, indeed, the existence of this disconformity is controversial (Kellogg et al., 1 9 7 9 a , 1979b ; Savage and Ciesielski, 1 9 8 3 ) . This disconformity has not been reported on seismic-reflection data. The extensive and interconnecting seismic-reflection profiles in the eastern Ross Sea (Houtz and Davey, 1 9 7 3 ) make it possible to correlate unconformities and to tie these events with the unconformities at the DSDP drillsites (Hayes and Davey, 1 9 7 5 ) . Seismic coverage in the western Ross Sea, however, is generally sparse regionally or restricted to small areas, as for example McMurdo Sound and Terra Nova Bay. Although trackline coverage in local areas was adequate to correlate unconformities within the area and, in some cases, to correlate events between areas (e.g., Wong and Christoffel, 1 9 8 1 ; Davey and Christoffel, 1 9 8 4 ) , there was inadequate data to tie these local areas to the eastern Ross Sea . Consequently, pre­ vious investigators were unable to make line-to-line correla­ tions of angular unconformities observed in the eastern Ross Sea with those observed in the western Ross Sea and to show that a given unconformity was, in fact, contemporaneous. Dur­ ing cruise L2-84-AN of the R / V S.P. Lee, we collected a regionally extensive network of interconnecting seismic¬ reflection profiles in the western Ross Sea (Fig. 1) . The pur­ pose of this paper is to trace the RSU around the western Ross Sea and to correlate the unconformity on our seismic-reflection profiles with the RSU defined at DSDP site 2 7 3 , thereby tying our regional grid of tracklines with those in the eastern Ross Sea .