4. TURBIDITY-CURRENT OVERSPILL FROM THE AMAZON CHANNEL: TEXTURE OF THE SILT/ SAND LOAD, PALEOFLOW FROM ANISOTROPY OF MAGNETIC SUSCEPTIBILITY AND IMPLICATIONS FOR FLOW PROCESSES¹

Richard N. Hiscott,² Frank R. Hall,³ and Carlos Pirmez⁴

ABSTRACT

On the Amazon Fan, the meandering Amazon Channel is flanked by levees tens of meters to >100 m high. Grain-size characteristics of the thicker and coarser grained spillover turbidites recovered by coring of the levees can be used to infer the nature of the suspended load and velocity of Pleistocene turbidity currents that transited the channel. Magnetic-mineral alignments in these turbidites provide estimates of flow directions of the overspilling currents. Together, these data augment our understanding of the development and maintenance of large submarine channels. The grain size of spillover turbidites from 10 to 50 m below the seafloor at Ocean Drilling Program Sites 939, 940, 934, 936, 944, and 946 was determined using a Sedigraph 5100 particle-size analyzer. The magnetic alignments were determined by measuring anisotropy of magnetic susceptibility using a Kappabridge KLY-2 susceptibility meter.

From the upper to the lower fan, the median size increases and the levee height decreases. Paleoflow during overspill was at a high angle to levee crests, but with considerable dispersion. Paleoflow data can only be properly interpreted in conjunction with information on the local channel shape and the position of low points, or saddles, in the adjacent levee crest. Comparison of (1) the texture of spillover turbidites with (2) grain sizes of sand in the talweg of the Amazon Channel, (3) independent velocity estimates based on differential levee heights, and (4) suspension theory, leads to the conclusion that a single type of mixed-load turbidity current could have transported very coarse sand as bedload along the talweg and contributed to levee growth by spilling a suspension of mainly silt and mud from the flow top. While transiting the Amazon Channel, such turbidity currents were constantly entraining seawater, but were losing a greater volume of dilute suspension from the flow top through overspill. As a result, there is a thirtyfold decrease in channel cross-sectional area from the upper fan to the lower fan. On the lower fan, with levee heights less than 25 m, even some of the sand load from the lower part of turbidity currents was lost to overspill.

¹Flood, R.D., Piper, D.J.W., Klaus, A., and Peterson, L.C. (Eds.), Proc. ODP, Sci. Results, 155: College Station, TX (Ocean Drilling Program).
²Earth Sciences Department, Memorial University of Newfoundland, St. John’s, Newfoundland, A1B 3X5, Canada. rhiscott@kean.ucs.mun.ca
³College of Marine Studies, University of Delaware, Newark, DE 19716, U.S.A.
⁴Lamont-Doherty Earth Observatory, Borehole Research Group, Palisades, NY 10964, U.S.A.