HOLOCENE SEDIMENTOLOGICAL EVOLUTION OF THE MERRIMACK EMBAYMENT, WESTERN GULF OF MAINE

A geophysical and sedimentological study of the Merrimack Embayment in northern Massachusetts has added to our understanding of sediment deposition during deglaciation and shed light on the formation of the region's barrier island system. Previous investigators have shown that the Merrimack lowstand delta was deposited in a series of lobes at approximately 10.5 kya in 50 m of water and further speculated that the onshore reworking of the delta surface during the Holocene transgression may have accounted for barrier construction (Oldale et al, 1983; Edwards, 1985). New shallow seismic, multi-beam, and backscatter data along with bottom photographs and sediment samples have revealed the presence of widespread channel cut and fill structures landward of the delta that suggest braided streams carried sediment to the lowstand delta.

Bottom sediments indicate that surficial sediment ranges from silty sand at the site of the submerged delta to coarse sand and fine gravel in the innermost shelf (5-25 m depth). Fine grained sand (phi: +2) comprises low backscatter, cuspate ripples (wavelength: ~20cm) and coarse grained sand (phi: -1) makes up high backscatter, two-dimensional sandwaves (wavelength: >1m). Additionally, mud (phi: +5) often drapes bedrock outcrops on the sea floor.

Bottom samples and backscatter images indicate an expansive medium- to coarse- grained sandsheet centered off the mouth of the Merrimack River, which becomes diffuse in a southerly and offshore direction. In these regions, the sand sheet is broken into a series of linear to cuspate coarse-grained features surrounded by fine-grained, low backscatter sediments. Relatively large, coarse-grained linear features dominate the northern sector and are oriented in a NNE-SSW direction. These forms have a sharp edge with the surrounding fine-grained regions on their eastern side and a diffusive western edge, indicating possible movement in an ESE direction. Elsewhere, subordinate forms are oriented in ENE-WSW and ESE-WNW directions. The geometry and orientation of these positive bed features suggest that the sandsheet is being reworked in an offshore direction. One possible mechanism for this is water level set-up during northeast storms producing offshore currents and sediment transport.