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The 2006 SHALDRIL II cruise was conducted in the northwestern Weddell Sea, with primary drilling targets in the James Ross Basin. A site drilled along the northern edge of the James Ross Basin sampled either latest Eocene or earliest Oligocene deposits, providing a lower chronostratigraphic benchmark for our seismic stratigraphic age model. Severe sea ice conditions forced abandonment of several of the James Ross Basin sites. Three alternate sites were drilled along the southern flank of the Joinville Plateau. Seismic data from the area show a thick, southward dipping stratigraphic succession with no conspicuous gaps. Three drill sites sampled this succession and recovered Oligocene, middle Miocene, and early Pliocene strata overlain by a thin drape of Pleistocene deposits. The PlioceneMiocene boundary appears to be represented by a disconformity within the cored interval. Otherwise, this is one of the most complete post-Eocene successions anywhere on Antarctica and its adjacent margins. Citation: Anderson, J.B., J. Wellner, S. Wise, S. Bohaty, P. Manley, T. Smith, F. Weaver, and D. Kulhanek (2007), Seismic and Chronostratigraphic Results from SHALDRIL II, Northwestern Weddell Sea, in Antarctica: A Keystone in a Changing World--Online Proceedings of the 10 ISAES X, edited by A.K. Cooper and C.R. Raymond et al., USGS Open-File Report 2007-1047, Short Research Paper 094, 4 p.; doi:10.3133/of2007-1047.srp094 Introduction Our understanding of Antarctica’s Cenozoic climate evolution and ice sheet development has increased slowly over the decades. The greatest hindrance to progress on this front has been a paucity of outcrops and long sediment cores that sample Cenozoic strata. All around the Antarctic continent, seaward dipping strata have been deeply eroded by the ice sheet when it advanced onto the continental shelf. Older strata occur just beneath the seafloor where they can be imaged using high-resolution seismic methods. When combined with chronostratigraphic data from drill core, it is possible to construct relatively detailed records of eustatic and climatic controls on continental shelf evolution, including a record of ice sheet expansion and decay over time. The concept behind SHALDRIL is to drill through the glacial section and sample older deposits. It is not intended to acquire very long core, but rather to rely on seismic stratigraphy as a framework for establishing the climatic and glacial history of the shelf and well-placed drill core to corroborate these interpretations and provide chronostratigraphic control. The success of this method is dependent upon having a mobile drilling platform, capable of operating in ice-covered waters, and a drilling system that can retrieve core in a matter of hours. A robust seismic stratigraphic framework enables selection of numerous sites, which increases the odds of being able to sample the desired stratigraphic targets where ice conditions allow. Another important aspect of SHALDRIL is that condensed sections (e.g., onlap of reflections) are targeted because they have the greatest likelihood of yielding suitable fossil material for biostratigraphic analysis. SHALDRIL II was a good test of this strategy because severe ice conditions prevented drilling any of the primary sites. Yet, each of the stratigraphic targets was sampled and each site yielded microfossils for age determination. SHALDRIL II was conducted in the northwestern Weddell Sea and the primary drilling targets were in the northern portion of the James Ross Basin, which is known to contain one of the thickest, most complete Cenozoic successions anywhere on Antarctica and its adjacent margins (Anderson, 1999). The drilling targeted latest Eocene through Pleistocene strata, based on a seismic stratigraphic age model for James Ross Basin. Seismic stratigraphy and age model for the |
