Stratigraphy, Depositional Rates, and Other DSDP Leg 96 Conclusions: Mississippi Fan: ABSTRACT

Abstract

The Quaternary Mississippi fan consists of at least seven seismically mappable fan lobes. Eight sites were drilled into the youngest fan lobe during DSDP Leg 96. The Holocene (Ericson Zone Z) is capped by a marly foraminiferal ooze. Assuming an age of 12,000 yr for the Holocene/Pleistocene boundary, a minimum accumulation rate of 3-30 cm/1,000 yr (1-12 in./1,000 yr) is computed for the Holocene. The youngest fan lobe was deposited during the upper part of Ericson Zone Y (late Wisconsin glacial, 12,000-85,000 y.B.P.). Deposition rates for the Y Zone are extremely high for the middle fan sites, averaging 12 m/1,000 yr (39 ft/1,000 yr). Lower fan accumulation rates are 7 m/1,000 yr (23 ft/1,000 yr) for the channel sites and 6 m/1,000 yr (20 ft/1,000 yr) for the channel-mouth depositional lobes. These rates are not corrected for compaction. Foraminifera are scarce; the occurrence of shallow-water benthic species indicates a displaced inner and middle neritic origin for the sediments. Drilling on the youngest fan lobe shows that most of the silts and sands were transported through the upper and middle fan channel onto the lower fan, producing a 6 to 10-km (4 to 6-mi) wide, 135-m (443-ft) thick aggradational channel deposit. Much of the fine-grained sediment spilled out of the channel onto the adjacent overbank areas, constructing a broad marginal plain. The channels on the lower fan are smaller and shift position frequently. Channel-mouth depositional lobes are constructed at the end of the active channels and are composed of more than 50% net sand. End_of_Article - Last_Page 463------------