Glaciotectonic deformation associated with the Orient Point–Fishers Island moraine, westernmost Block Island Sound: further evidence of readvance of the Laurentide ice sheet

Abstract

High-resolution seismic-reflection profiles collected across pro-glacial outwash deposits adjacent to the circa 18 ka B.P. Orient Point-Fishers Island end moraine segment in westernmost Block Island Sound reveal extensive deformation. A rhythmic seismic facies indicates the host outwash deposits are composed of fine-grained glaciolacustrine sediments. The deformation is variably brittle and ductile, but predominantly compressive in nature. Brittle deformation includes reverse faults and thrust faults that strike parallel to the moraine, and thrust sheets that extend from beneath the moraine. Ductile deformation includes folded sediments that overlie undisturbed deposits, showing that they are not drape features. Other seismic evidence for compression along the ice front consists of undisturbed glaciolacustrine strata that dip back toward and underneath the moraine, and angular unconformities on the sea floor where deformed sediments extend above the surrounding undisturbed correlative strata. Together, these ice-marginal glaciotectonic features indicate that the Orient Point-Fishers Island moraine marks a significant readvance of the Laurentide ice sheet, consistent with existing knowledge for neighboring coeval moraines, and not simply a stillstand as previously reported.

Ductile and brittle styles of subglacial sediment deformation: An example from western Ireland

“…In practice, both ductile and brittle structures can be found within the same sediment units because they are both caused by an increase and then the progressive decrease of pore water pressure (e.g., Phillips et al, 2007Phillips et al, , 2013bLee and Phillips, 2008;Poppe et al, 2012;Neudorf et al, 2013). However, their precise relationships are uncertain.…”

Character, distribution, and ecological significance of storm wave-induced scour in Rhode Island Sound, USA

“…The RonkonkomaBlock Island-Nantucket moraine, which marks the maximum extent of the Laurentide Ice Sheet at around 25.5-28.1 ka (Peltier and Fairbanks 2006;Balco 2011), dislocated and thrust faulted the coastal plain and overlying deposits. The Point Judith-Buzzards Bay moraine, which partially underlies the study area, records a readvance at about 20.5-22.5 ka during the overall retreat of the Laurentide Ice Sheet (Balco 2011;Poppe et al 2012a). Where exposed on the seafloor, the submerged sections of the moraines beneath RIS are apparent from bands of gravelly sediment and submarine ridges capped by a lag deposit of gravel and boulders resulting from winnowing of the till and ice-proximal coarse-grained stratified deposits by marine processes ( Fig.…”