Evolution of Glacially Derived Freshwater and Overpressure in the Massachusetts Shelf: An Integration of Geophysical and Numerical Methods

Abstract

The continental shelf offshore Massachusetts, USA experienced repeated glaciations throughout the late Pleistocene that emplaced freshwater and generated overpressure in the shelf sediments that still remains offshore. To show this, I processed and interpreted high-resolution, multi-channel seismic data that was collected offshore Massachusetts to infer the glacial history and to incorporate the glacial history into numerical modeling. Interpretations of the seismic data reveal the shelf stratigraphy and the location of a late Pleistocene (Marine Oxygen Isotope Stage 12) ice sheet. The ice sheet extended 100 km farther onto the shelf compared to the Laurentide ice sheet during the Last Glacial Maximum (LGM). It also contained an ice stream that was likely sourced from the Gulf of Maine. I show that the late Pleistocene ice sheet influenced the shelf hydrogeology by generating overpressure and emplacing freshwater into the shelf sediments. Overpressure is modeled in 1D from high-resolution, full-waveform inversion p-wave velocities obtained from the seismic data and from a finite-difference fluid flow model that accounts for sedimentation and ice sheet loading. The results demonstrate how loading from the late Pleistocene ice sheet caused focused fluid flow that created localized zones of overpressure nearly 1-2 MPa in offshore sediments. Freshwater emplacement into shelf sediments is estimated with a finite-element, variable-density model of fluid flow and heat and solute transport that accounts for ice-sheet loading and sea-level change. The model helps explain how the late Pleistocene ice sheet emplaced nearly 100 km3 of freshwater into the sediments. Our results thus integrate seismic interpretations of ice sheet history with numerical techniques of fluid flow modeling to show how the past glacial history influenced the present freshwater distribution.