About This Item
Share This Item
More than 20 terrigenous sand beds, 1-9 m thick, were cored at 2 DSDP sites in the lower Mississippi Fan, in 3,300 m of water, 600 km from the shelf edge. The shallowest sand beds occur 15 and 23 m below the sea floor, and constitute 50% of the recovered cores.
These beds were sampled at 20-30 cm intervals for size analysis. Sand content averages 80% with 10-15% variation within a bed. Mean and modal sand is fine grained and averages 2.75-3.0 phi. Maximum sand size ranges up to coarse sand, and within some beds, it varies between 0.5 and 2.0 phi. Grading is poorly developed on the scale of 1 m and larger. There is no consistent grading trend, and in some intervals, mean and maximum sand size show opposite trends. These data confirm that, like many ancient fans, the lower Mississippi Fan was a major site of sand deposition, but unlike them, consistent coarsening-upward sequences are not defined.
The poorly developed grading, variations in grading trends, and changes in sand percentages result from sand deposition within a broad, shallow, meandering or braided channel in which sands are reworked. The many separate beds are probably the result of changes in channel position that occurred on the average of once every 7,000 yr. Sand transport to the lower fan ended at least 2,000 yr before Holocene time. These newly discovered sand bodies, interlayered with finer clastic, potentially organic-rich material, suggest that stratigraphic reservoirs may be present in deep water, lower fan environments.
End_of_Article - Last_Page 294------------