HISCOTT, R.N., Earth Sci. Dept., Memorial Univ., St. John's, Newfoundland, Canada A1B 3X5, rhiscott@kean.ucs.mun.ca; PIRMEZ, C., Borehole Research Group, Lamont-Doherty Earth Obs., Palisades, NY 10964, pirmez@ldeo.columbia.edu; FLOOD, R.D., Marine Sci. Res. Center, State Univ. New York, Stony Brook, NY 11794-5000, rflood@ccmail.sunysb.edu.

Most reviews suggest submarine fans span a spectrum from small, sand-rich, radial, "low efficiency" fans (e.g., California borderland fans) to large, mud-rich, elongate, "high-efficiency" fans dominated by channel-levee complexes. The latter rest on oceanic crust, are fed by major rivers draining collisional orogens, and include Mississippi, Bengal, Indus and Amazon fans. Amazon Fan was drilled at 17 sites during ODP Leg 155. One objective was to characterize high-amplitude reflection packets (HARP's) which underlie groups of channel-levees in seismic profiles. The HARP's have broadly sheet-like geometry, consist of stacked low- relief lenses (?channels), and have thicknesses at ODP sites of about 100 m.

Coring of HARP's at Sites 931, 935, 936, 944 and 946 yielded 25% recovery, mainly fine to pebbly sand with local clay clasts, or thinly bedded silt-mud turbidites; gamma-ray logs indicate abundant unrecovered sand. Microresistivity images (Formation MicroScanner tool) and conventional wire-line logs permit construction of long bed- by-bed sections in which thick beds of sand and pebbly sand cluster into packets 5-20 m thick (exceptionally 40 m thick), reminiscent of sandstone packets which characterize flysch in orogenic belts. Abrupt appearance of sand corresponds with upsystem avulsion and partial flushing of channel segments undergoing adjustments to new gradients above the avulsion site.

The view that sand-rich and mixed sand-mud turbidite successions best match the architecture of small, California borderland fans is challenged by Leg 155 results. Large elongate fans may also contain thick, widespread sandy units formed by avulsion episodes which punctuate major phases of fan growth.