2. SANDY TURBIDITE SUCCESSIONS AT THE BASE OF CHANNEL-LEVEE SYSTEMS OF THE AMAZON FAN REVEALED BY FMS LOGS AND CORES: UNRAVELING THE FACIES ARCHITECTURE OF LARGE SUBMARINE FANS1Carlos Pirmez,2 Richard N. Hiscott,3 and John D. Kronen, Jr.4 |
ABSTRACTContinuous 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 than ~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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