THE INTERNAL STRUCTURE OF SCROLLED FLOODPLAIN DEPOSITS BASED ON GROUND-PENETRATING RADAR, NORTH THOMPSON RIVER, BRITISH-COLUMBIA

A study of floodplain deposits using ground-penetrating radar (GPR) wa s conducted on a confined meander of the-North Thompson River near Kam loops, British Columbia. A survey grid consisting of 20 individual GPR profiles from 200 m to 900 m in length was constructed on a 0.9 km(2) section of the floodplain. All GPR profiles were collected using a pu lse EKKO(TM) IV GPR system with 100 or 200 MHz antennae, dependent on the thickness of floodplain sediments. Floodplain sediments consist of sand and gravel deposits approximately 7 to 14 m thick that overlie s ilts and clays interpreted to be glaciolacustrine in origin. Vibracore and auger data obtained at nine locations in the GPR survey grid show that changes from medium sand to gravel do not produce distinct radar reflections in GPR profiles but that the water table may locally prod uce a distinct, high-amplitude radar reflection. Area well-log data sh ow that rapid signal attenuation at depth in all GPR profiles coincide s with underlying silt and clay sediments. GPR profiles were, therefor e, useful in mapping 3-D variability in the thickness of floodplain de posits. All GPR profiles were divided into macroscale (> 150 m(2) in 2 -D cross-section) radar facies, delineated on the basis of the nature and orientation of reflections relative to surface scroll topography. Radar stratigraphy suggests that the floodplain is composed of a singl e, complex lateral accretion deposit with periodic channel fills locat ed between surface scroll ridges. Dip direction profiles, normal to su rface scroll ridges, exhibit mainly inclined stratification (IS) and/o r preserved ridge-and-swale (RAS) deposits dipping in the direction of floodplain accretion. Well-developed RAS architecture in 200 MHz GPR profiles displays mainly wavy reflections conforming with surface topo graphy and little inclined stratification. These reflectors are interp reted to be the result of processes of vertical accretion associated w ith floodplain formation. Preserved scroll ridge deposits exhibit asym metry at depth, dipping more steeply in the direction of floodplain ac cretion. Profiles of strike direction, parallel to surface scroll ridg es, show either parallel or slightly inclined (1-2 degrees) reflection s dipping in the downstream direction. (C) 1997 Elsevier Science B.V.