Sedimentologic and paleoceanographic implications of terrigenous deposits on the Maurice Ewing Bank, southwest Atlantic Ocean
MacDonald, Scott Edward
Anderson, John B.
Master of Arts
The Maurice Ewing Bank (MEB) provides a unique environment in which to study deep-sea clastic sedimentation as the observed sedimentary sequences are not readily explained by traditional depositional models. Grain size distributions of sediment samples are utilized to identify styles of sediment transport and current velocities at the benthic boundary layer. These data, along with physical oceanographic information, allow for examination of the relationship between sedimentation on the bank and circumpolar currents. Causal relationships between glacial episodes, current intensification, and bottom current scour were previously postulated. Results indicate that modern sediment distribution is controlled by both bottom and surface currents, and is modified by mass-flow processes. Indirect determinations of current velocities, using a curve relating sediment transport to current velocity, suggest a uniform energy regime of 8-12 cm/sec over the bank. These moderate velocities are supported by geostrophic velocity profiles. The results of this study also demonstrate that misinterpretations regarding current velocities may result when statistical parameters, such as mean grain size and sorting, are used as indicators of fluctuations in bottom current velocities. Similar conclusions can be drawn concerning the use of bottom photographs and nephelometer profiles to infer the relative strength of bottom energy regimes. Current velocities assigned to down-core samples indicate no major fluctuations in circumpolar current intensity since PIio-Pleistocene time. Similar results were obtained from examination of Argentine Basin cores. These results refute the previously inferred causal relationship between current intensification and glacial episodes. The identification of mass-flow deposits provides an alternate explanation for the depositional and erosional history of the MEB.