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High-
By
Dominic A. Armitage1, David
T. McGee2, William R. Morris2,
and
David
J.W. Piper3
Search and Discovery Article #50086 (2008)
Posted August 1
*Adapted from oral presentation at AAPG Annual Convention,
1Geological
and Environmental Sciences,
2ConocoPhillips Co, Houston, TX.
3Geological Survey of
Analysis of sediment cores, 2D Huntec and 3D shallow
seismic-reflection data reveal two main canyon types: 1.) those that have
relatively broad, flat bottoms, which are probably formed by glacial outburst
floods with inner terraces likely to be formed by proglacial failures. These
canyons are principally erosional in their axes, their floors are dominated by
winnowed conglomerates and stiff Pleistocene muds with terraces recording recent
axis bypass; 2.) canyons that do not extend updip to the shelf margin but
terminate locally and appear to be created by retrogressive failure (modifying
aggradational deposits) and are draped by Holocene and Pleistocene muds. The
shallow 3D data reveals upper slope accommodation space created by large-scale
mass wasting events, reflecting a period of slope failure. These events are
succeeded by a complex history of deposition dominated by smaller-scale mass
transport deposits and canyon/channel overbank deposits. The slope failure and
associated deposits fundamentally setup the canyon configuration that is
observed on the modern seafloor. Two interrelated processes controlled canyon
development: 1.) the failure scarps resulting from the mass wasting event
created accommodation space available for canyon ridge aggradation and 2.) the
scarps captured subsequent sediment gravity flows necessary for their
construction. It is demonstrated that these scarps act as a precursor to canyon
development. Large slide blocks (up to ~2 km3) created topography on
the paleo-seafloor and were preferential sites for locally ponded deposition.
The canyon ridges internally record a complex history of overall aggradation via
sediment gravity flow deposits and degradation by erosive flows and slumping.
Isopach maps and reflection geometries of individual packages indicate offset
stacked overbank wedges in the construction of these ridges.
|
Slope basin formed as a result of major failure:
(A)
This sets up modern canyon configuration (seafloor). Canyon
locations are determined by the position of older slump scarps.
(B)
Mass failure created accommodation on the upper slope. Two types of
canyons on the slope:
(1)
Those that developed through ridge aggradation (connected to the
shelf). Canyon axes are predominantly non-depositional. Ridges
record a complex evolution of aggradation and degradation. These may
be composed of thick packages of very thinly bedded turbidites;
(2)
Those that initiate on the slope (not connected to the shelf) and
are probably formed by retrogressive failure. Fill is predominantly
overbank deposits from flows passing down surrounding canyons. |