Relative Sea - Level Rise and the Development of Channel - Fill and Shallow - Water Sequences on Cape Cod , Massachusetts

Channel-fill sediments located in shallow-water off the south shore of Cape Cod, Massachusetts, provide a record of the late-Pleistocene and Holocene geological evolution in a post-glacial setting. Though conventionally difficult to sample adequately and anticipated to have low preservation potential, channel-fill sequences record in some detail differing relative sea-level and sedimentation processes. Two distinct channel-fill sequences record differing sequence stratigraphies, and hence different origins and postglacial histories. These sequences have accumulated in channels eroded into two different late-Pleistocene glacial units. The first fill-type was encountered in channels on the upper portions of the channel network in northern half of the study site. Channels in this portion of the channel system were incised into the late-Pleistocene glacial outwash substrate by spring sapping (Uchupi and Oldale, 1994). The channel-fill sequences are comprised of a transgressive systems tract composed of a consistent sequence of coastal embayment and shoreline facies that have succeeded one another in response to Holocene relative sea-level rise. As relative sea-level flooded these paleo-channels, marsh environments were established in response to rising relative sea-level. With continued sea-level rise, the marsh environments migrated farther up channel. The exposed paleo-channels continued to flood, accommodating quiet water coastal embayments, likely protected from wave action by barrier beaches located more seaward. As relative sea-level rise continued, the coastline was driven landward over regions within the paleo-channels that formerly accommodated marsh and embayment sedimentation. The landward migration of the coastline was indicated by beach and barrier facies that covered the fine grained coastal embayment sediments. With further relative sea-level rise, beach and barrier settings were eroded as the shoreface migrated farther landward and nearshore marine deposition by wave and tidal flows ensued. Sedimentary environments similar to those recorded in the channels are found in modern coastal embayments on the south shore of Cape Cod. The second channel-fill type, which forms part of the southern and western portion of the channel network is more difficult to relate to the previously described sequence. The channels that contain fill were not adequately defined in this survey but were probably incised during the late-Pleistocene in response to ice melting and retreat. The sediments that make up this channel-fill are composed mainly of late-Pleistocene glaciolacustrine silts and clays. Sediments that make up the Holocene transgressive systems tract are limited to the upper meter of this channel sequence. They are composed of two sand units that reflect Holocene beach and nearshore sedimentation. The absence of coastal embayment and other paralic facies from the systems tract suggests that these channels did not accommodate protected embayments or that these sediments were not well preserved during the submergence of this region. Changes in the channel orientation or in the rate of relative sea-level rise may have contributed to this difference in sediment fill. Thesis supervisor: Dr. David G. Aubrey Title: Senior Scientist, Department of Geology and Geophysics