*Adapted from oral presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23

¹Geological and Environmental Sciences, Stanford University, Stanford, CA. ([email protected])

²ConocoPhillips Co, Houston, TX.

³Geological Survey of Canada (Atlantic), Dartmouth, NS, Canada.

Abstract

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.

uAbstract uFigures uConclusions     uAbstract uFigures uConclusions

Selected Figures   Location of the 3D volume, Southwest Grand Banks Slope. East-west seismic profile illustrating basin-forming failure. Time-structure map on top of mass transport deposits (MTDs). Isochron map of the interval between MTD top surface and seafloor.  Internal ridge growth: canyon ridges evolve partly through aggradation (and subsequent erosion) of wedging packages. Evidence for Recent bypass? Sediment cores in transect across central canyon show thin Holocene on levees and Pleistocene on canyon floor. Conclusions 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. Return to top