EVIDENCE FOR A GLACIOTECTONIC ORIGIN OF THE ORIENT POINT – FISHERS ISLAND MORAINE, WESTERNMOST BLOCK ISLAND SOUND

Character and distribution of pro-glacial drift and glaciotectonic deformation features, as revealed by Chirp and Boomer seismic-reflection profiles, provide evidence that the 18-21 kyBP Orient Point–Fishers Island moraine formed during a significant readvance of the Laurentide ice sheet. Stratigraphic units interpreted to be drift provide evidence of this readvance and include: 1) Fine-grained glaciolacustrine deposits in close proximity to the moraine, rather than coarse-grained outwash typical of stalled ice-front positions. 2) Undisturbed pro-glacial stratified drift within the study area is either flat-lying or dips gently toward the moraine rather than prograding away, suggesting that the moraine in its present position was not the source. 3) Deformed strata are underlain by similar, but relatively flat-lying beds that show no deformation, indicating that departures from the horizontal in the upper part of the section are not simply drape features. 4) The moraine overlies glaciolacustrine drift, an architecture that can be explained only by glacial readvance. Glaciotectonic deformation features in the pro-glacial drift also provide evidence of a readvance and include: 1) Numerous thrust faults that strike roughly parallel to the moraine, indicating that compression was perpendicular to the ice front. 2) Large-scale thrust sheets extend from beneath the moraine, revealing that construction was by thrusting and stacking of pro-glacial deposits. 3) Where faults intersect the seafloor, displaced strata have positive relief in relation to adjacent coeval drift surfaces, suggesting thrusting. 4) Folding is widespread, closely associated with the faulting, and consists of anticlines, synclines, drag, and recumbent folds. Fold axes and fault strikes have similar orientations. 5) Both faulting and folding progressively increase in magnitude and frequency toward the moraine. 6) Normal faults, grabens, collapse structures, and other features typical of tensional deformation are absent. Strike-slip and oblique-slip faults are confined to shear zones between adjacent thrust-fault blocks.