ABSTRACT: Ground Penetrating Radar: High-Resolution Structure of Deltas and Spits in Large Lakes

SMITH, DERALD G., and HARRY M. JOL, University of Calgary, Calgary, Alberta, Canada

Ground penetrating radar (GPR) field experiments were conducted on Holocene and late Pleistocene lacustrine coastal features (Athabasca, Lesser Slave, Erie, Ontario, Bonneville, glacial Lake Inveremere, and glacial Lake McConnell). GPR profiles provide high resolution (50 cm), continuous reflections of facies and succession thicknesses, and orientation of major sedimentary structures.

A pulseEKKOTM IV GPR system was used with 50 and 100 MHz antennae at 1 m steps for all reflection surveys. The profiles were processed and plotted (wiggle trace format) using pulseEKKOTM IV software. Near-surface velocity measurements were calculated from common midpoint surveys. GPR transects were carried out on level surfaces (beaches) or were corrected for irregular topography. In our experience, GPR was found to be most effective (resolution and depth of penetration) in dry and/or wet (freshwater), clean (no clay), sandy, and gravelly environments.

Three deltaic types have been identified based on differences of radar facies (patterns) and associated depositional processes: (1) fan-foreset, (2) wave-influenced, and (3) braid. Fan-foreset deltas have basal horizontal reflectors overlain by steeply inclined (25 degrees) reflections. Wave-influenced deltas have basal horizontal reflections overlain by low-angle inclined (2-10 degrees) reflections and capped by discontinuous, wavy reflections. Braid deltas are dominated by continuous to semicontinuous, wavy reflections overlying basal continuous horizontal reflections.

Spits are dominated by inclined (2-25 degrees) reflections interpreted as foreset beds in distal spit water depths to 20 m. The foresets are inferred to be deposited by underwater sediment avalanching and wave processes. Individual, long, continuous sigmoidal reflections are interpreted as major geomorphic event boundaries, deposited by rare high magnitude storm waves and/or bottom currents. Data and interpretation generally agree with the proposed depositional models. Loss of signal return from below 13 m at Sandy Point spit, Lake Athabasca, is attributed to a lithofacies change from sand to finer grained sediment.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)