Reports
Quaternary stratigraphy of the inner continental shelf of Maryland
1989, Toscano, M.A., Kerhin, R.T., York, L.L., Cronin, T.M., and Williams, S.J.
Report of Investigations 50
Abstract
Five distinct stratigraphic units are identified for the Maryland inner shelf from high-resolution seismicreflection profiles, vibracores, amino acid racemization age determinations and paleontology. The lowermost unit, Tl (Tertiary), lies below -21 meters (MSL) at the shoreface and is characterized by steep internal reflectors and extensive channeling near its top. T 1 is truncated by a persistent high amplitude, essentially horizontal reflector (horizon M I), which is coincident with the Tertiary/ Quaternary unconformity immediately onshore . M I likely represents the Illinoian glacial (stage 6) erosional surface. Overlying M I are approximately IO meters of units (Q2-Ql) of concordant strata with parallel to subparallel bedding . Horizon M2, a less prominent reflector, is associated with channeling and separates unit QI from Q2. Just above M2 are localized sands that range from thin (less than 1 meter) beds to remnant sand bodies on the same scale as modern shelf shoals. The 6-meter section above M2 consists of a dewatered, fossiliferous clayey silt (Q2), estimated from amino acid racemization data to be of last interglacial, oxygen-isotope stage 5 (128-75 ka) age. This mud sequence contains at least 4 distinct paleoclimatic zones as defined by ostracode assemblages, each representing particular paleocliminatic-oceanographic conditions. A warmer than present sandy zone (Zone 1) occurs below -22 meters MSL, a colder than present zone (Zone 2) occurs from -22 to -20 meters MSL, and a cool zone (Zone 3) occurs above -20 meters . A fourth zone (Zone 4), representing a warm interval, is tentatively identified above Zone 3; this zone appears to become more prominent southward in the study area. All ostracode assemblages from unit Q2 were representative of normal marine salinities, indicating an open shelf depositional environment. The Q2 mud is interpreted as a regressive, shallow shelf deposit of oxygen-isotope substages 5d-c, with the warmer climate sand below representing the transgressive portion of substage 5e.
Unit Q 1, below horizon M2, consists of shelly sands at its top meter and is interpreted to represent remnants of transgressive shelf sands 1 of pre-Illinoian (possibly oxygen-isotope stages 7 and/ or 9) age.
Incised into units Q2 and Q I are numerous paleochannels. Unit Q3 fills an extensive ancestral channel and tributary system, marked by reflectors M3 and M I, traceable to the present St. Martin River estuary, that was incised into the shelf during the most recent Pleistocene (Wisconsin) glaciation (oxygen-isotope stages 4, 3 and 2). Ironstained gravels and fluvial sandy silts barren of microfauna have been cored in this unit. The channels cut during the Wisconsin probably contain continuous fluvial-estuarine tidal channel fill sequences representing the oxygen-isotope stage 2-1 transition. The cored, upper portions of these channel fills contain radiocarbon-dated early Holocene (unit Q4) estuarine deposits. Transgressive, leading-edge coastal deposits (lowland swamp) of unit Q4 cut into and overlap portions of unit Q2, and have degrees of preservation below the shoreface ravinement unconformity that are demonstrably related to rates of sea level rise and pre-transgression topography. No Holocene back-barrier remnants were found within a distance of 40 km from shore or between depths of -12 to -22 meters MSL. Modern trailing-edge transgressive shelf shoals (unit Q5) discontinuously cap the sequence.
These data indicate that oxygen-isotope substages 5d-a represented a unique , relatively high, sea level stand of sufficient duration (40 ka) to deposit a thick regressive shelf mud. The Holocene transgression is represented only by remnants in truncated channels or by relict facies preserved far offshore due to initially high rates of sea level rise. From approximately 7000 yrs BP to the present the Holocene depositional record has been mostly obliterated by shoreface ravinement processes in connection with a slow rate of sea level rise.
Preservation of late Holocene transgressive deposits of barrier island systems seaward of barrier strandlines on the mid-Atlantic coastal plain should thus be rare and topographically confined. Remnants of trailing-edge shelf sands are most likely the only transgressive deposits preserved on the shelf during intervals of slow sea-level rise. Major preserved shelf sequences are more likely to be seaward thickening, finer grained sediments deposited during long, low energy, regressive phases of interglaciations. Consequently the mud sequence of unit Q2 is probably unique, at least in terms of the Pleistocene glacial/interglacial record on the mid-Atlantic continental shelf.
