Stratigraphic analysis of a sediment pond within the New England Mud Patch: New constraints from high-resolution chirp acoustic reflection data

摘要

The New England Mud Patch (NEMP) is an anomaly on the Atlantic coast of North America. This similar to 13,000 km(2) area, located south of Cape Cod between the similar to 60 m and 160 m isobaths, is a region of active fine-grained deposition on a shelf that is predominantly non-depositional or erosional. Prior studies theorized that NEMP sediments are derived from fines winnowed from Georges Bank, transported westward by coastal currents, and then settled in more quiescent conditions. A chirp seismic reflection (2015) and coring (2016) survey of part of the NEMP was conducted in support of a planned acoustic experiment for the Office of Naval Research. The survey focused on a similar to 30 km (E-W) by similar to 8 km (N-S) region between the 70 m and 85 m isobaths, encompassing a sediment "pond" > 12 m thick. The dense (250 m) survey lines allow a pseudo-3D stratigraphic interpretation. The sediment pond occupies an embayment eroded into pre-LGM substrate sediments, perhaps by glacial outwash. Marine sands beneath the mud are organized into oblique sand ridge morphology. The sense of obliquity, morphologic asymmetry, and internal dipping reflectors indicate that the sand ridges formed under east-directed transport. However, as evidenced by westward-prograding depocenters and onlap, NEMP deposition occurred under west-directed transport, consistent with modern shelf conditions. The onset of fine-grained deposition was therefore contemporaneous with a significant shift in the hydrologic regime. Muds deposited immediately above the sand ridges include a significant sand component, whose modal grain size is identical to that of the sand ridges. This admixture may indicate that, during the early stages of mud deposition, the tops of the sand ridges remained unburied and exposed to reworking. Episodic, storm-driven transport of sand to the muddy deposits may account for some of the sand component of NEMP sediments.