Initial Results of ODP Leg 155 Drilling on Amazon Deep-Sea Fan: Implications for Depositional Processes and Distribution of Reservoir Sands in Mud-Rich Submarine Fans

R. D. Flood, D. J. W. Piper, J. E. Damuth

Ocean Drilling Program Leg 155 drilled 34 holes at 17 sites on the Amazon Deep-Sea Fan in April-May, 1994. The more than 4000 m of sediment recovered record at least a 250,000-year record of fan growth and major glacial-to-interglacial changes in sea-level and climate are reflected in the architecture and lithology of turbidite deposits. Fan sedimentation rates during periods of lowered sea-level, such as the last glacial, were extremely high (5-20 m/10³ yr); in contrast, rates during periods of high sea level, such as the Holocene interglacial, were much lower (^Lt1 m/10³ yr) because terrigeneous sediments were trapped on the shelf and could not reach the fan.

Our drilling strategy was to drill numerous, relatively short holes (ranging from 75 to 434 m deep) through various fan sub environments and sediment facies in carefully selected locations based on sedimentary models and seismic facies. Eight sites focused on the sedimentary facies and depositional record of the most recently developed channel-levee system from upper to lower fan. Three of these sites recovered sediments from the channel axis; thus, for the first time on a modern fan, we successfully recovered continuous sections up to 100 m thick of aggrading sandy channel fills, as well as sandy lobe deposits from the lower fan. Seven sites penetrated as deep as 434 m and sampled older buried channel-levee systems and interbedded mass-transport units to study fan chronology and lith stratigraphy; buried interglacial sections were recovered. Four sites on older, abandoned channel-levee systems penetrated 100-300 m and sampled high-resolution stratigraphic records of the last glacial cycle. Suites of wire-line logs were run at eight sites to compare log motifs with actual sedimentary facies.

Of particular interest are the nature and distribution of sandy facies within this mud-rich fan. Channel deposits contain turbidites and related gravity-controlled-flow deposits of fine-to-coarse sand up to several meters thick; however these deposits are laterally restricted to relatively narrow sinuous belts beneath the axes of channel-levee systems. In contrast, much more areally extensive, thick sandy deposits occur at the bases of many channel-levee systems and are marked by high-amplitude reflection packets (HARPs) on seismic profiles. HARP units contain beds up to several meters thick of fine sand to gravel deposited by turbidity currents and related gravity-controlled flows. Lower-fan depositional lobes apparently represent the most laterally extensive fine-to-coarse sand beds which are commonly in excess of 10 m in thickness. These initial observations indicate that mud-rich submarine fans may contain extensive reservoir-quality sands within specific fan sub environments.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995