SEISMIC REFRACTION SUBSURFACE IMAGING OF GLACIAL TILL THICKNESS FROM GRINNELL GLACIAL VALLEY - GLACIER NATIONAL PARK, MONTANA

Geologic mapping of Glacier National Park shows that glacial activity has created till cover with an estimated average thickness of 1 to 3 meters and a maximum of 30 meters (Carrara 1990). Our survey aims to better define these values by using the seismic refraction technique to determine the thickness of till in Grinnell Valley and to generate a consistent image of the bedrock/till interface. This data will not only provide insight into past erosional features and patterns of the glacier but also shed light on past glacier dynamics. The studied valley consists of three large glacial lakes (Swiftcurrent Lake, Lake Josephine, and Lake Grinnell) occupying most of the valley area. The study was made in a 1.5 km long forested area between Lake Grinnell and Lake Josephine. The bedrock of the valley is an argillite and siltite mudstone of the Appekunny Formation. This bedrock creates a clear increased velocity contrast from the glacial till and makes seismic refraction an ideal geophysical technique to define the bedrock/till boundary. Refraction data was obtained using a Geometrics® 24-channel geode seismograph model ES-3000, 6 pound weight drop, and twelve 14-Hz sensitive geophones spaced at 3 meters. Two 0.5 km cross-sections of the valley were made using a total of 14 separate seismic lines. Bedrock/till outcropping contacts were used as controlled starting points for the two surveys. Initial processing of first arrival refraction data shows a good contrast between the glacial tills and bedrock. Two fully interpreted seismic cross sections will be presented.