Bed Thickness Patterns and Facies of Amazon Fan Turbidite Successions from Electrical Resistivity Images Calibrated to Cores
Carlos Pirmez and Richard N. Hiscott
Amazon Fan grew through the stacking and overlapping of channel-levee depositional units, composed of overall fining- and thinning-upward turbidite successions. The basal portion of the channel-levee systems consists of high- amplitude reflections (HARPs) that were interpreted prior to drilling to represent flows resulting from channel avulsion. These acoustic units appear as high velocity, high bulk density, and low gamma-ray, low resistivity intervals on wireline logs. Cores confirm the presence of thick beds of unconsolidated sand, but recovery was poor in these intervals. A complete facies description was made possible through a combination of borehole electrical resistivity images and other logs calibrated to cores where available.
The HARP units are composed of predominantly sandy facies including massive sand beds up to 10 m thick, often containing clay clasts and evidence of soft sediment deformation, and interbedded sandy and muddy turbidites. Mud clasts are often indurated and can be recognized in the logs by an increase in resistivity and gamma-ray response. The presence of mud clasts may reflect upstream erosion of earlier channel-levee deposits as seafloor gradients adjust to changing flow characteristics. The onset of channel avulsion is marked by an abrupt boundary between muddy, often bioturbated turbidites below and sand beds above, organized in packets with no apparent bed thickness trend. Groups of sand beds, apparently organized in thickening-upward cycles of the order of 5-10 m thick are interpre ed to originate from flows at the distal end of channels. Sand beds tend to be thicker on the lower portion of the fan where flows are less constrained by topographic barriers.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California