GROUND-PENETRATING RADAR AND CORING USED TO STUDY THE LARGE-SCALE STRUCTURE OF POINT-BAR DEPOSITS IN 3 DIMENSIONS

Detailed three-dimensional (3-D) observations of sandy point-bar depos its from the River South Esk in Scotland were made using very closely spaced (metres) vibracores and ground-penetrating radar (GPR) profiles . In order to explain the origin of the observed patterns of depositio n, use was made of previous studies of channel geometry, flow and sedi ment transport. In addition, the mode and nature of channel migration and point-bar accretion were determined using published maps, aerial p hotographs and detailed topographic surveys. Point-bar deposits accumu lated in response to channel-bend expansion and downstream migration, resulting in preservation of sequences that fine upwards and downstrea m. Lower-bar deposits are mainly very-coarse to coarse sands with medi um-scale trough cross-strata overlying basal gravels: associated radar facies are generally low-amplitude, relatively discontinuous inclined reflectors. Upper-bar deposits are mainly fine to medium sands with m edium- and small-scale cross-strata and vegetation-rich layers: associ ated radar facies are generally moderate- to high-amplitude, laterally continuous, inclined reflectors. Large-scale inclined stratasets seen in GPR profiles resulted from episodic point-bar accretion. Abrupt la teral changes in inclination of these stratasets, and preservation of distinct unit bars (bar heads, scroll bars), lower-bar platforms and i nner-bank swale fills, record discrete episodes of erosion and deposit ion associated with floods with recurrence intervals of decades to cen turies. Such detailed 3-D description and interpretation of these larg e-scale features of point-bar deposits was only possible through the u se of GPR profiles tied closely to cores, and through the availability of much previously collected information on channel geometry, water f low, sediment transport, erosion and deposition.