REWORKING OF A SUBMERGED FLUVIODELTAIC SEQUENCE IN THE MERRIMACK EMBAYMENT, WESTERN GULF OF MAINE

Recent shallow seismic, multi-beam, backscatter, and bottom sediment data shed new light on the emplacement of the fluviodeltaic sequence deposited by the Merrimack River during the late-Pleistocene lowstand (12 kya; at present depth: -45m) and its subsequent surficial reworking through the Holocene transgression to the present time. Seismic data reveal the presence of widespread channel cut and fill structures landward of the delta suggesting deposition by braided streams. Truncations of these features indicate that these sediments were probably reworked and sorted by wave and inner-shelf processes during the Holocene transgression and may have contributed sand to developing barriers that presently border the Merrimack Embayment.

Bottom samples indicate that surficial sediment ranges from silty sand at the submerged delta to coarse sand and fine gravel in the innermost shelf (depth = 10-30m). Coarse-grained sand (average: 0 phi) comprises symmetrical sandwaves (wavelength: >1m) and forms an expansive featureless sand sheet centered off the Merrimack River. Very fine sand to silty sand defines the paleo-delta front and the region seaward of the delta. Fine-grained sand (average: +2.5 phi) discontinuously overlies a coarser-grained sand sheet in many locations and forms long wavelength (10's to 100's of meters), low amplitude (1-2 m), asymmetrical bedforms that are preferentially oriented offshore. These are believed to be moribund features that are activated by northward flowing bottom currents induced by water level set-up in the Cape Ann / Ipswitch “bowl” during northeast storms.

Present sedimentological trends suggest that although the braided stream deposits may have contributed sediment to the onshore barriers during the Holocene transgression, there is little to no sand contributed from the inner shelf today, nor is it expected in the future regime of accelerated sea-level rise.