SUBBOTTOM STRATIGRAPHY OF LAKES IN NEW HAMPSHIRE: CLINOFORMS, UNCONFORMITIES, SAND DEPOSITS, SLUMPS, AND TURBIDITES INTERPRETED FROM GROUND-PENETRATING RADAR PROFILES

Mid basin lake cores obtained in sediments under deeper water may contain unconformities, unexpected sand deposits, and inconsistent histories of accumulation rates when several cores are obtained, as first discussed in the classic 1982 study of Davis and Ford at Mirror Lake in Thornton, New Hampshire. One certain explanation is the action of turbidity currents, which exist in lakes of all sizes, but seismic and acoustic imaging to identify their evidence can be defeated by gaseous organics and scattering from coarse sediments. In New Hampshire however, most lakes are electrically resistive, and allow exceptional subbottom penetration of ground-penetrating radar (GPR) signals. Consequently these lakes can be surveyed with GPR to find where an ideal site might exist and the cause of anomalous deposits. I discuss select GPR profiles from Squam, Mirror, Profile and Loon Lakes, Joe English and Cherry Ponds, and Quincy Bog, the sediments beneath which include gyttja, glaciofluvial silts, sands and gravels, and till. I imaged these sediments at subbottom depths of 6 to more than 30 m and at a variety of pulse bandwidths. I interpret bright, layered horizons as sandy end members of turbidites, single horizons leading to stacked beds as evidence of turbidity currents that failed to erode or deposit, and vertical strings of diffractions as evidence of meandering turbidites. Abrupt changes in signal intensity within stacks show the historical transition from sandy to silty sediments, while the signal wavelet phase shows that these horizons represent thin layers of relatively higher density due to relatively higher mineral content. Creation and erosion of deltaic clinoforms, and of along-shore river channels are likely to have been caused by high energy turbidity currents. Shore-to-shore continuity of horizons suggests storms mainly caused these apparently episodic pulses of deposition.