Stratigraphic controls on pool formation in a domed bog inferred from ground penetrating radar (GPR)

Ground penetrating radar (GPR) and terrain conductivity measurements reveal a close correlation between the location of water-filled pools and stratigraphic/lithologic features in Caribou Bog, a 2200-hectare peatland in central Maine. The geophysical profiles, supported by coring, define the general peatland stratigraphy as till underlain by glacio-marine sediment overlain by organic-rich lake sediment transitioning into terrestrial peat. GPR measurements show that open pools coincide with abrupt thinning of lake sediment and glaciomarine clay such that elevated mineral soil surfaces are in direct contact with the base of terrestrial peat. The width of these features, the GPR reflection patterns, the anomalously low terrain conductivity and the core samples obtained support the interpretation of these elevated mineral surfaces as the extension of buried eskers that outcrop on the periphery of Caribou bog. The geophysical results indicate that two parallel esker ridges are orientated N-S and dip towards the south. We suggest that the distribution of open pools is related to the distribution of depressions and beads associated with a beaded esker deposit, the pools possibly being maintained by local groundwater flow cells associated with the esker. GPR and EM31 surveys across Sphagnum pools (defined here as Sphagnum filled wet depressions) suggest a continuous sequence of underlying lake sediment and glacio-marine deposits devoid of elevated mineral surfaces. We suggest that Sphagnum pools developed after peatland formation (secondary features) due to changes in water table, peat accumulation and/or climatic controls.