32 Using Ground-Penetrating Radar to Investigate Near-Surface Geology and Sedimentary Records of Environmental Change

Tuesday, 24 March 2015
8:00 AM - 12:00 PM
Jefferson Room

USING GROUND PENETRATING RADAR TO IDENTIFY ICE-DAMMED DEPOSITS AND RECONSTRUCT A DEGLACIAL HISTORY FOR ANDOVER LAKE, CT

Paper number: 32-8
Program: Theme Sessions
Time: 10:15 AM – 10:35 AM

Authors

Abstract

Shallow lakes can provide useful GPR windows for imaging sub-lake records of local deglaciation. Here we report on ≈17 km of 50, 100, and 200 MHz GPR transects and 3D visualizations beneath Andover Lake, a 63 ha dammed (1927) water body in Eastern CT. Radar interpretations are constrained by 11 percussion and 8 vibracore, soundings, and bedrock outcrop. Four sub-lake radar facies occur: (R1) strong paired reflectors indicative of bedrock; (R2) largely opaque, irregular reflectors associated with boulder-rich till; (R3) low amplitude, laterally continuous, and layered soft sediment (gyttja) that drape other facies; and (R4) variably transparent, channeled, or laterally continuous reflectors associated with stratified sand/gravel/cobble deposits. 3D radar for a 50 x 80 m area in a bay and a 27 x 46 m area at a beach resolve architecture of R4 deposits in the southern end of the lake. Bay radar reveals 3-5 m of wavy to chaotic reflectors with numerous point reflectors likely caused by cobbles (in vibracores) and/or boulders visible on the lake bed. In contrast, 3D radar from the beach identify 9 m of stratified fine sand (R4) above bedrock. Subfacies ≈1.5 to 3 m thick have parallel and laterally continuous to chaotic and channelized reflectors. Vibracore reveal rhythmically bedded silt and fine sands with crossbeds, ripples and silt caps all indicative of rapid deltaic sedimentation in standing water.

Importantly, thick sandy facies (R4) are only present in the SE end of the lake, whereas boulder-till facies (R2) dominate the NW. The abrupt R2-R4 boundary identifies the edge of a previously north-draining, ice-dammed pond. This boundary together with positions of nearby published ice margins and 1m2 bare earth LIDAR topography suggests that ice retreated across Andover Lake ponded water enabling deltaic deposition at the beach. Meltwater from this ice-dammed pond initially drained northward over a bedrock sill, contributing to the formation of a mapped gravel delta in nearby Hop River valley. As ice retreated out of the Andover Lake basin runoff shifted to the present lake’s outlet channel draining the ice pond. 14-C dates for peat recovered immediately above ice-dammed deltaic deposits at the beach indicate that vegetation colonized the drained lake bed by 12,130-11,929 calendar years BC at the latest.