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 fans

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 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 tha t periodically supply sand to interchannel depressions following avulsions also need to be considered. an and e all of and the ural illed annts ll as y a nd sediere , et At sevient sinrage 935, etraits, he INTRODUCTION The Amazon Fan is a large cone-shaped deposit of terrigenous sediments derived from the Amazon River drainage basin. The fan succession reaches a thickness of ~7 km near the northeast Brazil shelf break. Its age is middle Miocene and younger (Castro et al., 1978). The surface of the fan is covered by a distributary network of meandering channels that has been mapped with SeaBeam swath bathymetry and GLORIA side-scan sonar (Damuth et al., 1988; Flood et al., 1991). High-resolution seismic data show that these meandering channels are bounded by high natural levees and are perched atop their own overbank deposits (i.e., the channel-levees and often the channel talweg are elevated above the surrounding fan surface; Pirmez and Flood, 1995). Channel-levees result from the vertical and lateral expansion of turbidity currents traveling through the meandering channels (Hiscott et al., this volume). The aggrading channel and overbank deposits often bifurcate. Such bifurcation is caused by levee breaching and avulsion, leaving one branch of the channel inactive and initiating the formation of a new channel downslope. As a result, the fan builds laterally and vertically through the stacking and overlapping of these lens-shaped, channel-overbank deposits. 1Flood, R.D., Piper, D.J.W., Klaus, A., and Peterson, L.C. (Eds.), 1997. Proc. ODP, Sci. Results, 155: College Station, TX (Ocean Drilling Program). 2Lamont-Doherty Earth Observatory, Borehole Research Group, Palisades, NY 10964, U.S.A. PIRMEZ@LDEO.columbia.edu 3Earth Sciences Department, Memorial University of Newfoundland, St. Joh A1B 3X5, Canada. 4University of Hawaii-SOEST, Honolulu, HI 96822-2219, U.S.A. Previous Chapter Table of C The channel patterns on the Amazon Fan are seen on other socalled mud-rich fans such as the Indus Fan (McHargue, 1991; Droz and Bellaiche, 1991), Mississippi Fan (Weimer, 1990), Rhône F (Droz and Bellaiche, 1985), Magdalena Fan (Pirmez et al., 1990), Zaire Fan (Droz et al., 1996). Although these submarine fans hav been acoustically imaged to varying degrees of detail, sampling the fan sediments generally has been limited to the upper 10 m, the lithofacies and/or sedimentary processes associated with acoustic units observed on seismic reflection data are conject (e.g., Weimer, 1990; Manley and Flood, 1988). Prior to Ocean Dr ing Program (ODP) Leg 155, only the Mississippi Fan was drill with the specific objective of deep sampling of submarine fan ch nel-levee deposits, which allowed characterization of the sedime associated with the youngest channel system on that fan, as we other seismic units (Bouma, Coleman, Meyer, et al., 1986). Onl limited logging program was available on the Mississippi Fan, a the coring techniques employed produced limited recovery and verely disturbed the original structures of the unconsolidated se ments at some sites. More recently, during Leg 116, turbidites w drilled at the distal edge of the Indus Fan (Cochran, Stow, Auroux al., 1989). During Leg 155, 34 holes were drilled at 17 sites (Fig. 1A). several sites, the uppermost ~100 m of the succession was cored eral times with the advanced piston corer (APC) to ensure suffic sediment for detailed shore-based studies. If one ignores all but a gle penetration of this upper part of the sediment column, the ave core recovery at the five sites considered in this paper (Sites 931, 936, 944, and 946), from the seafloor to the depth of greatest pen tion, was 65%. Core recovery was high in mud-rich levee depos particularly from 0 to 100 meters below seafloor (mbsf) where t n’s,