Abstract:
Package-3 unconformities identified by Bart and Anderson (1995) were mapped to evaluate their glacial unconformity interpretation using a regional grid of single channel seismic data from the Antarctic Peninsula outer continental shelf. Detailed correlations show many crosscutting relationships within Package 3 such that the unconformities and units they bound are not regionally continuous across the margin. Comparison of Package-3 unconformities with the modern bathymetry reveals that cross-shelf trough and bank morphology is absent, which indicates that ice streams probably did not exist on the shelf during Package-3 time. On the continental shelf, only the middle part of Package 3 is sampled at ODP Leg 178 sites (1097 and 1103 on the southern and northern sectors of the margin, respectively.) Although mapping results do not uniquely require/preclude ice sheet grounding, taken together, the diamict lithology and crosscutting supports the interpretation of waxing and waning of grounded ice on the outer continental shelf. Chronostratigraphic resolution at Leg 178 sites on the continental shelf is too coarse to allow unconformity-to-unconformity correlations between Package-3 strata sampled at the two shelf drill sites. At most, fifteen (15) Package-3 glacial unconformities are found at any one location, and this represents the most conservative estimate of the maximum number of grounding events from the perspective of regional shelf stratigraphy. Basinward of the continental shelf, coeval section sampled on continental-rise drift 6 at ODP site 1095 exhibits distinct meter-scale bioturbation events, capping thick (up to 20 m) unbioturbated section, interpreted to result from >30 glacial episodes. The large difference between the minimum estimate of shelf grounding events (15) and the number of glacial cycles deduced from drift lithology (30) suggests either 1) amalgamation of shelf glacial unconformities, or 2) lithologic alternations on the rise reflect phenomena unrelated to grounding events. Chlorite-smectite covariance at drift 6, interpreted to represent approximately 16 glacial episodes, shows much better correspondence to the minimum number of Package-3 glacial unconformities. However, the poor covariance between the two drift-derived proxies (bioturbation and clay mineralogy) demonstrates that one or both may be recording something other than grounding events on the adjacent shelf.