38. BIOSTRATIGRAPHY AND SEDIMENTATION RATES OF THE AMAZON FAN1Naja Mikkelsen,2 Mark Maslin,3 Jacques Giraudeau,4 and William Showers5 |
ABSTRACTAlthough the Amazon Fan is a complex stratigraphic sequence of mass-transport deposits (MTDs), levee complexes, and hemipelagic oozes, the interpretation of micropaleontological and isotopic data from the Amazon Fan has provided excellent results. Compared to other mega deep-sea fans, the Amazon Fan is highly structured, thus providing an unusual opportunity to investigate the Quaternary evolution of a deep-sea fan. Biostratigraphic work has been carried out on two of the main sedimentological fan units: (1) the upper Pleistocene/ Holocene sediments that cap the entire fan area and are equivalent in age or younger than the paleomagnetic Lake Mungo Excursion of 32 ka; and (2) the interglacial deep-carbonate units that underlie the MTDs and are younger than the Pseudoemiliania lacunosa event of 475 ka. Age assignments are primarily based on biostratigraphic analyses of calcareous nannofossils and planktonic foraminifers. Nannofossils are common only in the Holocene calcareous muds and in the deep carbonate units. Foraminifers are common in the same units, but are also present in the glacial levee muds underlying the Holocene calcareous clays. Additional age constraints are provided by interpretation of the paleomagnetic record and accelerator mass-spectrometry 14C dates. Stable oxygen- and carbon-isotope data are used to further refine the biostratigraphic and environmental interpretations. Biostratigraphy of the cores retrieved during Leg 155 has provided an overall stratigraphic framework for understanding the Quaternary evolution of the Amazon Fan. Biostratigraphic and paleomagnetic data are combined to provide estimates of sedimentation rates, ranging from 5 cm/k.y. during interglacial periods, to over 5000 cm/k.y. during glacial periods. A combination of biostratigraphy, seismic stratigraphy, magnetostratigraphy, and sedimentation rate constraints was used to date the top of both the near-surface MTDs and the deep MTDs. These data suggest that the deep MTDs were last active at ~33 ka and 45 ka, whereas the near-surface MTDs were active during Termination I (11-14 ka). |
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