Only two studies were conducted by U.S. workers in the last four years using magnetic fabrics in sediments and sedimentary rocks to document bottom current flow. This area will probably see growth in the next quadrennium since it has the potential for environmental applications, not only in marine sediments [ Ellwood, 1993], but in lacustrine sediments, and in ancient settings [ Schieber and Ellwood, 1993].
Ellwood [1993] used AMS in a comprehensive rock magnetic study of the depositional and erosional patterns which formed the giant mudwaves (ripples) found in the Argentine basin of the South Atlantic Ocean. These large features (135 m amplitude, 5 km wavelength) formed in response to currents associated with Antarctic Bottom Water flow and were sampled in various locations by box and piston cores. Magnetic fabric measurements (AMS) indicate prolate or nearly neutral fabrics on the downstream side of the mudwaves, suggesting erosion, whereas oblate fabrics typical of deposition occur on the upstream side of the mudwaves. Larger magnetic grain sizes at one mudwave compared to another may indicate either higher current velocities or greater proximity to the sediment source. Finally, the greater magnetic mineral concentrations, determined by bulk susceptibilities, in the mudwaves which lie in the center of the basin compared to sediments from the margins of the basin suggest that the mudwaves act as sediment sinks. Changes in magnetic properties (magnetic grain size, concentration) downcore indicate a major change in bottom water flow in the Argentine basin in the past. Studies like this indicate the important contribution that magnetic fabric studies can make to understanding present-day environmental systems.
AMS fabric measurements can also be used to reconstruct and study ancient environmental systems. The Mid-Proterozoic sedimentary rocks of the Newland Formation in Montana were the focus of Schieber and Ellwood's [1993] AMS study of paleoenvironmental conditions in the basin. They found oblate magnetic fabrics with vertical minimum axes in the Newland Formation shales which they interpret to be primary depositional, rather than tectonic, magnetic fabrics. Horizontal magnetic lineations are then used to indicate paleocurrent flow in the sedimentary basin. The magnetic fabric data shows consistent geographic patterns of flow azimuths which agree with other non-magnetic paleoflow indicators. Imbrication of the maximum susceptibility axes can also be used to indicate flow directions. The consistency of paleocurrent flow patterns throughout the lower part of the stratigraphic section indicate relatively long term stability of currents in the basin. From the paleocurrent patterns it is evident that the extent of the basin was much greater during the lower Newland than its present erosional outline. A significant change in flow patterns between the lower and upper part of the Newland Formation suggest a major change in basin configuration. In the upper part of the Newland Formation the correlation between the azimuth of current flow with respect to the ancient shoreline (parallel or perpendicular) and the sediment grain size supports a new depositional model for storm deposits [ Duke et al., 1991].