Interpretation of sea floor geologic units for Buzzards Bay, Massachusetts (BuzzardsBay_surfgeol, polygon shapefile; Geographic WGS 84)

Three major seismic stratigraphic units, Proterozoic bedrock (Z), glacial drift (Qd), and postglacial deposits (Qfe/Qmn), are separated by two regional unconformities (reflectors) defined by reflections U1, Ur (fig. 13). In addition, glacial drift (Qd) is divided into subunits based on seismic facies and Qfe/Qmn is divided into subunits by a regional unconformity defined by the reflection at Ut. Z, inferred to be undifferentiated Proterozoic bedrock based on what has been mapped onshore (Zen and others, 1984), was the deepest unit, and where the upper surface was iinterpreted as an erosional unconformity, where the reflection at U1 is highly irregular and diffracted. U1 is overlain by glacial till and ice-contact deposits (Qdt or Qdm) or stratified drift (Qdl and Qdf). Z only occurs at the sea floor outside of the seismic survey area. Qdt includes Pleistocene glacial till and ice contact deposits and Qdm includes till and ice contact deposits of the Buzzards Bay moraine. Irregular reflections and hyperbolic diffractions within Qdt and Qdm and where these units intersect the sea floor, were interpreted as acoustic signatures of boulders. Pleistocene stratified glacial drift includes the units Qdl, Pleistocence glaciolacustine and Qdf, glaciofluvial that were deposited by glacial meltwater. Qdf typically overlies Qdl and in places a local glaciofluvial erosional unconformity at the base of Qdf dissects Qdl. The discontinuous and undulation reflections within Qdf are cut-and-fill features that have eroded Qdl in places. Ur is the late Wisconsinan to early Holocene regressive unconformity, which was initially formed by proglacial lake drainage and by ice-distal meltwater fluvial erosion and was later modified by continued subaerial fluvial erosion until the early Holocene. This postglacial drainage surface shows a single main channel originating from points at the head of the bay and associated tributary channels. Towards the mouth of the bay, this channel appears to bifurcate, having branches that flowed through breaches in the complex topography formed by glacial drift. Ur is a composite unconformity because it merges with Ut primarily at the interfluves Qfe was deposited within fluvial and later in estuarine environments with the onset of the marine transgression. Qfe fills the fluvial valleys incised by Ur into the surface of Pleistocene glacial deposits. Qfe is thickest in the thalwegs of the fluvial cut valleys and is thin to absent on interfluves. The maximum thickness of Qfe could not be determined where biogenic gas within Qfe attenuates the seismic reflections. Ut , a continuous and flat horizon was interpreted to be a transgressive erosional unconformity formed by waves based erosion during the Holocene transgression. Ut is not well defined above Qfe within the fluvial valleys, with some exceptions (fig. 16), and therefore the thickness and distribution of Qfe was not mapped. Data quality and resolution were limiting factors, but also because much the seismic signal within and just above Qfe and the reflection at Ut were attenuated by gas. Ur and where Ur merges with Ut separates glacial from postglacial units. This unconformity marks the surface of glacial deposits. Qmn is a post-transgressive Holocene nearshore marine unit overlying Ut and older stratigraphic units (figs. 14-16). Qmn deposits are predominantly muddy within the deeper areas of the basin and are sandier close to shore or where paleoshorelines existed. We mapped the total thickness and distribution of postglacial sediment (Qfe and Qmn), which includes fluvial, estuarine, and marine deposits and are bounded by the Ur/Ut composite unconformity and the sea floor. The thickness ranges from 0 to 21 meters. The thicknesses of dredge spoil deposits, which range from 0 to 4 meters thick, were not included in the total postglacial sediment thickness. All thicknesses were calculated using a seismic velocity of 1,500 m/s. The postglacial sediment thickness (fig. 19), including thickness of dredge spoil, was subtracted from a regional swath-bathymetry DEM to produce a structure surface of the top of glacial deposits relative to NAVD 88 (fig. 18).