USING MICRO-GRAVITY TECHNIQUES TO MAP ALLUVIUM THICKNESS AND PLEISTOCENE LOCATION OF THE WEST BRANCH OF THE SUSQUEHANNA RIVER NEAR MUNCY, PA

Laurentide glaciation during the early Pleistocene dammed the northeast-flowing West Branch of the Susquehanna River (WBSR), scouring bedrock and creating 100-km-long glacial Lake Lesley near Muncy, PA. Subsequent paleo-outwash floods and modern fluvial processes have deposited as much as 30 m of alluvium in this area, but little is known about the valley fill architecture. This project determined if variations in the thickness of the valley fill were detectable as deviations in the gravitational field using micro-gravity techniques.

A LaCoste and Romberg Gravitron unit was used to collect gravitational field readings at 50 locations over 5 transects across the Muncy Creek and WBSR valleys, with at least two gravity base stations per transect. Latitude, longitude and ground surface elevation at each location were measured using an OPUS corrected Trimble RTK-GPS unit. The gravimeter was returned to a base station within every two hours and a looping procedure was used to maximize confidence in the gravity measurements. A two minute calibration reading at each station was used to minimize any tares in the data.

The Gravitron digitally recorded finite impulse response filtered gravity measurements every 20 seconds at each station. A measurement period of 15 minutes was used for each base station occupation and a minimum of 5 minutes at all other locations. Remeasurment were utilized at if drift or other externalities affected readings. Average, median, standard deviation and 95% confidence interval were calculated for each station. Tidal, drift, latitude, free-air, bouguer and terrain corrections were then applied.

The results show that gravity field decreases as alluvium thickness increases across the axes of the WBSR and Muncy Creek valleys. However, the location of the gravity low does not correspond with the present-day location of the WBSR, but is believed to reveal the Pleistocene location of the river and possible glacial ice dam. A local gravity deficit of approximately 1 mGal indicates that the thickness of alluvium may be a much as 50 m for a density contrast of 0.4 g/cm³. Well logs from the area are being used to refine the joint-inversion gravitational models. The result is a glacio-fluvial isopach map of the river valley and determination of the density contrast between the alluvium and the underlying bedrock.