Observing Late Pleistocene Glacial Tectonics using Geophysical Techniques

Dan Davis, Stony Brook University

Glacial tectonic contractional belts display most aspects of thin-skinned tectonics, but at a scale intermediate between those of natural orogenic wedges and laboratory analog models. On Long Island, NY, the Ronkonkoma and Harbor Hills terminal moraines each include a range of push-moraine processes. Sediments exposed by shoreline erosion and by human excavation show small (cm-m) and medium (10s of m) scale folds and faults. In some cases, the prevalence of gravels in fault zones indicates the inversion of the typical competence relations (clays stronger and coarser materials weaker) that is often characteristic of permafrost conditions.

We augment our direct observations of sediments through the use of GPR (ground-penetrating radar) and, to a lesser degree, resistivity measurements. Using GPR there is an intrinsic tradeoff between resolution and depth of useful signal penetration. High frequency (500 MHz) antennas can typically resolve objects as small as 5-10 cm, but to a depth of only 4-8m, depending upon the properties of the sediments. Low frequency (50-100 MHz) antennas, however, can penetrate as deeply as 20-40m, but with a resolution typically in the range of 30-70 cm.

One interesting structure that we have imaged is a prominent hill on the campus of Stony Brook University. The hill, known local at "The Wall", is draped with a capping diamict and an internal structure consisting of subhorizontal nearly undeformed sedimentary layers that appear to be outwash sequences deposited while the glacier was still to the north. We interpret The Wall to be the autochthonous footwall of a frontal and lateral lamp, and we believe the capping diamict to be a basal till deposited as the glacier stepped up over the now-relict hill. The allocthonous sediments and ice were then transported southward to where the ice melted and the sediments were deposited.

We have carried out extensive surveys in the Hither Hills area of the South Fork of Long Island, 14 km west of the eastern end of the island at Montauk. Hither Hills is a remarkable 4 km by 2 km region of regularly spaced washboard hills cored by folds beneath which we have imaged a detachment surface that is exposed along the eroded shoreline as a till. A range of evidence indicates that his was not a single contractional wedge but, instead, was probably a series of folds formed by semi-periodic re-advances during a time of overall glacial retreat.


Cutting edge research in structural geology geophysics geochemistry and tectonics