REFINING THE STRATIGRAPHY OF COMPLEX GLACIAL AND POSTGLACIAL DEPOSITS USING 3D GPR AT HADDAM MEADOWS STATE PARK, HADDAM CONNECTICUT

Haddam Meadows State Park, located on the banks of the Connecticut River, near Haddam, Connecticut, has been used by the USGS to teach geophysical methods to students for over 30 years. As part of the annual Geophysical Field Experience held in the spring of 2015, students collected three-dimensional (3D) ground-penetrating radar (GPR) data over a 200 x 100 meter (m) grid adjacent to the Connecticut River. GPR data were collected with a 1-m line spacing using a high-resolution radar system equipped with a broadband 160-Megahertz center-frequency antenna. Although the overall stratigraphy of the site is well understood from previous studies, the high-resolution GPR data reveal previously unknown details.

PC-based data processing and volumetric imaging software was used to analyze the 3D GPR data. The data reveal three stratigraphic facies within the roughly 10-m vertical section of sediment imaged by the GPR: (1) an upper 2-3-m thick layer of Holocene-age, floodplain sand deposits; (2) a 3-m thick layer of early-postglacial sand and gravel stream-terrace deposits (transported by waters spilling from glacial Lake Hitchcock), and (3) an underlying thick section of dipping subaqueous glacial lacustrine beds. The clay-rich lacustrine sediments strongly attenuated the GPR signal, preventing deeper penetration into underlying sediments. The 3D GPR data provide insight and details about small-scale structures underlying the site such as (1) complex internal bedding; (2) the geometry of a paleochannel incising the terrace deposits and infilled by Holocene sediment; and (3) the geomorphic surface of the stream terrace deposits which the GPR data show dip toward the river. These results demonstrate the value of 3D GPR and modern imaging software to delineate and refine complex glacial stratigraphy to depths of 10-15 m in the glaciated northeast U.S.