Relative sea-level rise and the development of valley-fill and shallow-water sequences in Nantucket Sound, Massachusetts

Abstract

Valley-fill sequences offshore southern Cape Cod, Massachusetts, record Late Pleistocene and Holocene subaerial and submarine sedimentation in a post-glacial setting. Complications arise because some of the lows that contain valley-fill sediments represent variations in glacial drift thickness, whereas others reflect channel incision into glacial sediments and subsequent infilling. The valley-fill preserved in topographic lows in the presently submerged glacial terrain is composed of Late Pleistocene glaciolacustrine silts and clays covered with Holocene sand units. In contrast, the valley-fill sequence preserved in incised valleys, which are thought to have been caused by groundwater seeps in the Late Pleistocene, comprises a transgressive-systems tract composed of a consistent sequence of coastal embayment and nearshore marine facies that succeeded one another in response to Holocene relative sea-level rise. This transgressive valley-fill sequence started to accumulate approximately 7600 BP and reflects the submergence and consequent onshore migration of coastal embayment and shoreline environments along Late Pleistocene spring-sapping valleys. The Holocene transgressive-systems tract that rests unconformably atop the lake deposits is limited to the upper meter of the sequence. The absence of coastal embayment and other paralic facies in these topographic lows suggests that the infilled lows did not accommodate protected coastal embayments, indicating that they were most likely filled by glacial processes prior to the Holocene transgression. The occurrence of both topographic-low and incised-valley fill sequences in Nantucket Sound reveals the depositional complexity of the deposits associated with glacial recession in the region.