Late Pleistocene stratigraphy of Upper Cape Cod and Nantucket Sound, Massachusetts

Abstract Soon after the Laurentide ice sheet reached its maximum southern extent at Martha's Vineyard and Nantucket islands 21,000–20,000 yr ago, a system of subglacial tunnel valleys, extending from southern Cape Cod (the Upper Cape) to the ice margin, were eroded into the fine-grained sediments resting on basement. We infer that these fine-grained sediments may represent Illinoian proglacial lake deposits that were fluvially eroded prior to late Wisconsin. During the Wisconsin glacial retreat, a proglacial lake formed in Nantucket Sound and sediments ranging from clay to fine sand were deposited. Compaction of the clays in the sound suggests that these sediments may have experienced glaciotectonic deformation resulting from minor advances during the overall glacial retreat. When the ice margin reached the south shore of the Upper Cape 19,000 yr ago it temporarily halted and a poorly sorted mixture of gravel, sand, and mud (ice-contact deposits) was emplaced along the jagged ice front on the north shore of the proglacial lake in Nantucket Sound. As the ice re-initiated its northward retreat another lake was formed in the Upper Cape. This lake may have separated from the one in Nantucket Sound by the ice-contact facies ridge. As the lake in the Upper Cape grew northward, keeping pace with the retreating ice front, very fine sands, silty sands and sandy silts were emplaced over the Illinoian lacustrine clays. When the ice front reached a sharp change in basement slope in central Upper Cape the ice fragmented and large ice blocks were left behind. A mixed unit of sands interbedded with silt and clay was deposited in front of the ice and between the ice blocks. The mixed unit prograded southward into the proglacial lake as a series of overlapping deltas with the distal end of the basin being a site of finer sediment accumulation. With continued northward retreat of the ice front, another proglacial lake formed north of the mixed unit that was dammed to the south by the stranded ice blocks and the mixed unit. Eventually the water level in this basin rose sufficiently that one lake extended from the glacial front to the ice-contact facies ridge along the south shore of the Upper Cape. Soon thereafter, the ridge along the south shore of the Upper Cape was breached and the lake in the Upper Cape joined the glacial lake in Nantucket Sound. Linkage of the lake in the Upper Cape with the Nantucket Sound lake was soon followed by the collapse of the southern dam of the lake in the sound. This event resulted in erosion of lake sediments in the Upper Cape and Nantucket Sound, with erosion in the Upper Cape being so pronounced as to exhume the Illinoian lake sediments. A braided fluvial system deposited outwash sands and gravels on the exposed erosional surface. Construction of these outwash plains stopped about 17,000 yr ago and they underwent fluvial erosion. Lows in this fluvial terrain became sites of local lacustrine deposition. Soon thereafter, outwash deposition was re-initiated, burying the previously exposed sediments. After the outwash plains were constructed, groundwater seeps, associated with a proglacial lake in Cape Cod Bay to the north, eroded a series of parallel linear valleys on the outwash plains. The last Wisconsin event in the Upper Cape was the formation of a kettle lake system when the buried ice blocks melted. The lakes became sites of deltaic deposition during a moister period 11,000–8000 yr ago when the water level in these lakes was higher than now. The present configuration of Nantucket Sound and the coastal zone is the result of deposition and erosion during and after the Holocene transgression.