Facies architecture and grounding-line fan processes of morainal banks during the deglaciation of Coastal Maine

Abstract

Submarine jet processes are inferred from glaciomarine facies exposed in gravel pits in southwestern coastal Maine. The geometry and distribution of fan facies depend on (1) the proximity to the grounding line and stream effluxes, (2) the angle of jet trajectory from glacier conduits, and (3) sediment remobilization processes. Syndepositional glaciotectonic deformation further augments morainal bank formation through pushing and squeezing of fan sediments at the grounding line. Complex process-sediment interactions along the temperate tidewater margin of the Laurentide ice sheet are recorded in the facies architecture of sedimentary sequences in morainal banks. Morainal bank accumulation was governed by rapid fluvial deposition, and outwash jet dynamics that controlled the spatial distribution of sedimentary facies. Near the grounding line, meltwater dynamics change rapidly where sediment-laden fresh water is discharged at or near the sea floor. Turbulent mixing with ambient sea water and discharge fluctuations cause rapid changes in flow competence, accompanied by the dumping of bed-load gravel and sand at the apices of grounding-line fans in the morainal banks. Jet trajectory relative to the morainal-bank slope determines whether a jet will make contact with the morainal bank. Gravel and sand remain in bed-load traction as long as the jet is in contact with the morainal bank. Bed-load dumping and settling of the coarse suspended load occurs where buoyant forces in the jet overcome momentum forces or where the jet is deflected by the morainal bank. Below these buoyant jets, backflow eddies can produce currents that drive ripple migration toward the grounding line. Abrupt facies changes record fluctuations in jet discharge and trajectory and sediment redistribution by sediment gravity flows initiated near the grounding line.