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