Miocene reversal of bottom water flow along the Pacific margin of the Antarctic Peninsula: stratigraphic evidence from a contourite sedimentary tail

A Fossil Mounded Sedimentary Body (MB) has been identified in the sedimentary record on the central continental rise west of Adelaide Island, on the Antarctic Peninsula Pacific margin. The growth patterns of the MB are defined through a detailed regional stratigraphic analysis using multichannel seismic reflection profiles. The MB has an elongated NE trend. It overlaps and continues to the NE of an extensive cluster of seamounts, and it developed between two non-depositional troughs. Nine seismic units have been identified: Unit 9 (the pre-MB stage), Unit 8 (MB growth stage), Units 7 and 6 (MB maintenance stage), Units 5 and 4 (transitional stage), and Units 3, 2 and 1 (inactive stage). We interpret the MB as a patch drift plastered against the NE, lee side of an obstacle, as a long Contourite Sedimentary Tail (CST), within a deep current that flowed northeastward. This segment of the rise is, however, affected at present by a SW-flowing branch of the Lower Circumpolar Deep Water (LCDW) from the Weddell Sea. The depositional patterns of the MB growth and maintenance stages, which are attributed an early Miocene age on the basis of regional correlation of MCS profiles with DSDP Site 325 and ODP Leg 178 drill sites, provide the first evidence that bottom currents on the central continental rise flowed towards the NE at that time, probably as part of the Lower Circumpolar Deep Water (LCDW) of the Antarctic Circumpolar Current (ACC). We suggest that significant palaeocirculation and palaeoceanographic changes occurred in this area, and probably more widely, during the middle Miocene or at the Miocene/Pliocene boundary. Although these results do not modify the regional stratigraphy of the major sediment drifts found on the continental rise of the Antarctic Peninsula's Pacific margin, they do indicate that the bottom current regime controlling the development of contourite deposits may have changed over time and also that more than one water mass has probably affected their distribution.

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