GLACIOTECTONIC DEFORMATION ASSOCIATED WITH THE ORIENT POINT – FISHERS ISLAND MORAINE, WESTERNMOST BLOCK ISLAND SOUND

Late Wisconsinan end moraines are prominent physiographic features across southern New England. They form linear ridges of composed of stratified and unstratified drift that stretch for tens of kilometers, comprise much of the coastal islands, and are exposed as bouldery lag deposits on the intervening sea floor. Although there is general agreement that the Late Wisconsinan terminal moraine was largely emplaced by glaciotectonic processes, there is much less agreement on the modes of formation for the coastal recessional moraines. Some authors interpret them to be products of stagnation-zone retreat, others contend that they are of glaciotectonic origin.

Chirp high-resolution seismic-reflection profiles were acquired aboard the NOAA Ship Thomas Jefferson during hydrographic survey H11250 in westernmost Block Island Sound. These profiles, collected across pro-glacial outwash deposits adjacent to the 18-19 kyBP Orient Point – Fishers Island end moraine segment, reveal extensive deformation. A rhythmic seismic signature indicates the host outwash deposits are composed of fine-grained glaciolacustrine sediments. The deformation is variably brittle and ductile, but is always compressional in nature. Brittle deformation includes numerous thrust faults whose strikes roughly parallel that of the moraine. These faults, which extend over 1.3 km in front of the moraine, are limited to the upper 21 m of section and have vertical and horizontal offsets that exceed 7 m and 35 m, respectively. Ductile deformation includes folded sediments that overlie relatively flat-lying, undisturbed deposits, showing that they are not draped features. Other seismic evidence for compression along the ice front consists of outwash strata that dip toward the moraine and angular unconformities on the sea floor that suggest the deformed sediments originally extended above surrounding undisturbed correlative strata. Together these ice-marginal glaciotectonic features suggest that the Orient Point – Fishers Island recessional end moraine marks a significant readvance of the Laurentide ice sheet.