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Continuous bed-by-bed descriptions of thick, sand-rich turbidite successions drilled at five sites on the Amazon Fan have been prepared using a combination of Formation MicroScanner (FMS) and geophysical wireline logs. FMS images enable a complete lithologic section to be derived at Sites 931, 935, 936, 944, and 946 over intervals of poor core recovery up to several tens of meters thick, characterized as high-amplitude reflection packets (HARPs) in the seismic data. On the middle fan, HARP deposition coincides in time with the initiation of a new channel segment after channel bifurcation. Toward the lower fan, HARP units tend to stack directly on top of each other, as overbank deposits thin downfan; these HARP units probably contain deposits formed at the mouths of channels.The turbidite successions within the HARP intervals include sand bodies 5-25 m thick formed of sand beds 0.1-4 m thick. These sand bodies correlate with episodes of channel bifurcation on the middle fan. Most beds thicker than ~1 m contain mud clasts, interpreted to result from upslope levee erosion and channel entrenchment after channel bifurcation. On the lower fan, bed clusters and sets of amalgamated beds form sand bodies as thick as 50 m, with individual beds often exceeding 3 m in thickness and containing abundant mud clasts. Most bed clusters show no apparent trends in bed thickness, although a few clusters show clear thickening-upward trends interpreted to represent channel mouth progradation.Other studies suggest that, for turbidites, the number of beds whose thickness is greater than T is proportional to T -B , where B is a positive exponent. The scaling exponent required to fit the Amazon Fan bed-thickness distributions is different for beds thinner and thicker than about 0.35 m. This difference is attributed to either selective removal, by erosion, of beds thinner thañ 0.35 m, or greater confinement, by seafloor morphology, of flows that deposited such thin beds on the fan surface. The flows that deposited beds thicker than 0.35 m in HARP units were apparently able to freely spread in the interlevee area.The so-called mud-rich Amazon Fan contains thick sheet-like units indistinguishable in facies and sand content from many ancient mud/sand-rich successions described by field geologists. Although it remains true that small fans at convergent and strike-slip plate boundaries are potentially good analogs for such ancient deposits, larger channel-levee-dominated systems that periodically supply sand to interchannel depressions following avulsions also need to be considered.

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Three sites were drilled in the Izu-Bonin forearc basin during Ocean Drilling Program (ODP) Leg 126. High-quality formation microscanner (FMS) data from two of the sites provide images of part of a thick, volcaniclastic, middle to upper Oligocene, basin-plain turbidite succession. The FMS images were used to construct bed-by-bed sedimentary sections for the depth intervals 2232-2441 m below rig floor (mbrf) in Hole 792E, and 4023^330 mbrf in Hole 793B. Beds vary in thickness from those that are: near or below the resolution of the FMS tool (2.5 cm) to those that are 10-15 m thick. The bed thicknesses are distributed according to a power law with an exponent of about 1.0. There are no obvious upward thickening or thinning sequences in the bed-by-bed sections. Spaced packets of thick and very thick beds may be a response to (1) low stands of global sea level, particularly at 30 Ma, (2) periods of increased tectonic uplift, or (3) periods of more intense volcanism.Graded sandstones, most pebbly sandstones, and graded to graded-stratified conglomerates were deposited by turbidity currents. The very thick, mainly structureless beds of sandstone, pebbly sandstone, and pebble conglomerate are interpreted as sandy debris-flow deposits. Many of the sediment gravity flows may have been triggered by earthquakes. Long recurrence intervals of 0.3-1 m.y. for the very thickest beds are consistent with triggering by large-magnitude earthquakes (M = 9) with epicenters approximately 10-50 km away from large, unstable accumulations of volcaniclastic sand and ash on the flanks of arc volcanoes.Paleocurrents were obtained from the grain fabric of six thicker sandstone beds, and ripple migration directions in about 40 thinner beds; orientations were constrained by the FMS images. The data from ripples are very scattered and cannot be used to specify source positions. They do, however, indicate that the paleoenvironment was a basin plain where weaker currents were free to follow a broad range of flow paths. The data from sandstone fabric are more reliable and indicate that turbidity currents flowed toward 150° during the time period from 28.9 to 27.3 Ma. This direction is essentially along the axis of the forearc basin, from north to south, with a small component of flow away from the western margin of the basin.

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