Abstract
Understanding how past coastal systems have evolved is critical to predicting future coastal change. Using over 12,000 trackline kilometers of recently collected, co-located multi-channel boomer, sparker and chirp seismic reflection profile data integrated with previously collected borehole and vibracore data, we define the upper (< 115 m below mean lower low water) seismic stratigraphic framework offshore of the Delmarva Peninsula, USA. Twelve seismic units and 11 regionally extensive unconformities (U1-U11) were mapped over 5900 km2 of North America's Mid-Atlantic continental shelf. We interpret U3, U7, U9, U11 as transgressive ravinement surfaces, while U1,2,4,5,6,8,10 are subaerial unconformities illustrating distinct periods of lower sea-level. Based on areal distribution, stratigraphic relationships and dating results (Carbon 14 and amino acid racemization estimates) from earlier vibracore and borehole studies, we interpret the infilled channels as late Neogene and Quaternary courses of the Susquehanna, Potomac, Rappahannock, York, James rivers and tributaries, and a broad flood plain. These findings indicate that the region's geologic framework is more complex than previously thought and that Pleistocene paleochannels are abundant in the Mid-Atlantic. This study synthesizes and correlates the findings of other Atlantic Margin studies and establishes a large-scale Quaternary framework that enables more detailed stratigraphic analysis in the future. Such work has implications for inner continental shelf systems tract evolution, the relationship between antecedent geology and modern coastal systems, assessments of eustacy, glacial isostatic adjustment, and other processes and forcings that play a role in passive margin evolution.
Highlights
Seismic stratigraphic analysis provides a framework for understanding the depositional and erosional history of an area and defines the antecedent geology over which modern coastal processes act (Vail and Mitchum Jr., 1977; Belknap and Kraft, 1985)
Because Unconformity 1 (U1), Unconformity 2 (U2), Unconformity 4 (U4), U5, Unconformity 6 (U6), Unconformity 8 (U8), Unconformity 10 (U10) are broad U-shaped structures that incise underlying strata we interpret these as subaerial unconformities formed from fluvial erosion as sealevel retreated during regression (Vail and Mitchum Jr., 1977; Catuneanu et al, 2009)
We identify 12 system tracts bounded by 11 regional unconformities
Summary
Seismic stratigraphic analysis provides a framework for understanding the depositional and erosional history of an area and defines the antecedent geology over which modern coastal processes act (Vail and Mitchum Jr., 1977; Belknap and Kraft, 1985). During sea-level lowstands large rivers continued across many continental shelves, likely to the present-day shelf edges (Twichell et al, 1977; Tesson et al, 2015; Yoo et al, 2017) Where present, such remnant channels and their associated fill sequences can form dominant components of an area's geologic framework and may exert controls on modern shorelines (e.g., Foyle and Oertel, 1997; Posamentier, 2001; Mallinson et al, 2005; Baldwin et al, 2006; Green, 2009; Mallinson et al, 2010a; Thieler et al, 2014; Zhuo et al, 2015).
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