The geophysical environment in the general area of MD99-2209 is shown in Figures 4.1 (bathymetric contours), 4.2 (isopachs of Holocene sediments above the Cape Charles erosion surface (CCES)), and 4.3 (depth contours to the CCES). In general, biogenic methane bubbles become more widespread in the fresher, northern part of the Chesapeake, and even obscure the stratigraphy in areas outside paleochannels (Colman and Halka, 1989). West of the central bathymetric channel, the methane obscures the CCES in depths of only 5-10m, locally even less than 5 m, north of latitude 38° 52' N (Fig. 4.2). However, acoustic penetration is greater off Kent Island east of the channel, perhaps because of the higher salinity of north-flowing Atlantic water on the east side of the channel (higher salinity correlates with more abundant sulfate ion, which, when percolating into subbottom pore waters oxidizes methane, clearing some of the sediment of "acoustic turbidity"). The CCES reflector was detected up to ca. 10-15 mbsf on the few available profiles from this area (Fig. 4.2). Between about 38° 52' N and 38° 56' N (Fig. 4.3), the CCES can even be traced under the eastern side of the channel bottom, locally to depths of greater than 50mbsl, which approaches the thalweg depth of the paleo-Susquehanna (Cape Charles) paleochannel (Fig. 2.2).
Only one geophysical profile, an ORE (Ocean Research Equipment) Geopulse (0.3-5.0 kHz) and a 3.5 kHz profile collected simultaneously, constrain the acoustic stratigraphy near core 2209, as does the earlier core taken at the same site, RD-98-1. This geophysical profile crosses the core site on a NNE heading, from the modern bathymetric channel and up the relatively steep slope towards Kent Island (Figs. 4.1, 4.2, and 4.3). A 1.5 km long portion of this profile (the Geopulse record) is reproduced with a geological interpretation as Figure 4.4.
In May, 2000 NRL (Naval Research Laboratory) and MGS (Maryland Geological Survey) collaborated in several days of EdgeTech sidescan and chirp profile surveying in the area around MD99-2209. Analysis of these new data will be completed at a later time.
The 580 cm difference between penetration and recovery probably reflects some combination of sediment compaction in the core barrel, and penetration without recovery. (In the latter case the barrel probably filled up with 1720 cm sediment and then continued down another 580 cm, as shown in .Fig 4.4). The shipboard scientists considered sediment compaction, if any, to have been minor. However, gassy sediments cored in the Susquehanna River north of the MD core site were compacted in some cases by over 50% by the coring process (C. Larsen, personal communication, 2000). To cover the range of possibilities, we therefore place the lowest recovered sediments somewhere between 43 and 49 mbsl.
At a depth of 43-49 m below the modern sea surface (1720-2300 cm bsf), the core catcher must have nearly reached down to the coarse sediments presumed present at the paleochannel thalweg, whose depth is probably about 55-60 mbsl (Fig. 2.2). The deepest recovered sediment is a shelly sand, probably the upper part of fluvial estuarine (also called restricted-estuary) sediments, but possibly the base of these sediments. In the latter case, and under the no-compaction assumption of Fig. 4.4), the underlying 580 cm, penetrated but not recovered, are fluvial sands and gravels or/and swamp sediments whose ongoing decay could be contributing to the biogenic methane gas in the overlying section. However, if the 580 cm is entirely accounted for by sediment compaction, any fluvial and early restricted-estuary sediments must occur below 2300 cm bsf. MD99-2209 yielded a well-preserved, well-dated paleoenvironmental record back to ~7.6 ka (Colman and others, this volume). However, a hiatus and/or highly condensed section was recovered between depths of 810 cm (2344 yr BP) and 830 cm (5763 yr BP) in the core (Colman and others, this volume).
The "missing" sediments in the interval 810-830 cm bsf may have been swept away in one or more "freak" floods, eroded and/or not deposited during bottom current regimes different from today, or removed by submarine mass wasting. The latter alternative is least likely, as the core was taken near the bottom of the present, and probably also past channel. The condensed section might also be the result of temporally shifting depocenter geometry, as the paleochannel filled up with spatially overlapping submarine fans. In this case the recovered sediments represent two intervals of fan deposition, separated by a period when the site was nearly isolated, by the geometry of fans and feeding channels, from downslope sediment delivery. If this model is correct, the recovered sediments from MD99-2209 and RD-98-1 should reveal evidence of increased terrigenous components derived from adjacent Kent Island, compared to the condensed section, which should be richest in hemipelagic components.
It is possible that new NRL and additional future detailed, high-resolution chirp profiles will reveal further evidence on the nature and geographic extent of the event(s) or depositional environment responsible for the hiatus/condensed section. However, the acoustic stratigraphy at the core site is obscured by methane bubbles (Fig. 4.4), and the solution to the problem of the hiatus may have to await additional deep coring. Reconnaisance 3.5 kHz and GEOPULSE records (Colman and Halka, 1989; Fig. 4.4) do not reveal a reflector or other structure that might correspond to the hiatus, in the interval between the bayfloor and the Cape Charles erosion surface, but this does not mean too much, given the wide track spacing and data resolution (Figs. 4.2 to 4.4). However, the seismoacoustic evidence (sidescan sonar and chirp profiles) from the area off Parkers Creek (Chapter 2) suggests that different bottom current regimes may have been responsible. For example, the area of Miocene bayfloor outcrop labeled "M" in Figure 2.4 apparently lacks the entire sediment record from the early Holocene transgression to the present. At MD99-2205 and 2206 we found the sediments directly above the Cape Charles erosion surface to be very young (~500 yr BP), indicating that sediments at this site were being eroded or not deposited almost into European settlement times (Chapter 2).
As one hypothesis for the hiatus at MD99-2209, we speculate that sea levels rose, and the bay deepened and widened, to some threshold before a strong tidal current regime was established, and which began to erode sediments and/or prevent sedimentation at the site. Tidal currents then weakened again as the channel north and south of Kent Island filled with sediment, isolating the basin. Additional coring, geophysical work, and numerical modeling of currents could test this model.
Seismic profile C9-1 (part of the Colman and Halka (1989) reconnaissance survey) shows the strong gas return from m 0 to m 550 (Fig. 4.4), a short transitional zone (m 550 to m 600), and then acoustically transparent sediments farther to the northeast, up the slope toward Kent Island. In acoustic expression, gas/no-gas boundaries are commonly too sharp to show a transition zone: In this case, profile C9-1 crosses the gas/no-gas boundary at a small angle (compare profile C5/6-1 (Figs. 2.4 and 2.5), allowing measurement of transition width. In addition, the strong gas-return multiple (Fig. 4.4) ends sharply at m 550, while the more indistinct direct returns continue another 50 m in the subbottom. We conclude that the actual width of the transition zone, measured perpendicular to the zone, must be less than 50 m. A narrow transition zone width (50 m) was also estimated for the gas/no-gas boundary investigated at MD99-2205 and 2206 (Chapter 2).
The main acoustic feature northeast of the gas return on profile C9-1 (Fig. 4.4) is a subbottom reflector identified (Colman and Halka, 1989) as the Cape Charles erosion surface (CCES), an unconformity which mainly records the progressive flooding and shoreline erosion of the Wisconsinan lower Susquehanna river system as a result of the late- and post-glacial sea level rise. However, in the main river paleochannel and in tributary paleochannels, the CCES is best defined by the contact between generally thin deposits of fluvial sand and gravel, and the underlying sediments, which may be older Quaternary paleochannel fills or Mesozoic to Tertiary strata, depending on the area. The CCES in the paleochannels could alternatively be defined as the contact between the last fluvial sediments and the first estuarine sediment, but this interface may not be an erosional unconformity, and furthermore probably has little or no seismic expression. In any case, because the fluvial sediments are probably thin, and for the most part are likely of high acoustic impedance, seismoacoustic methods may not be able to resolve them as a separate unit.
The transgression leading to formation of the modern Chesapeake Bay began as tidewater first entered north past the Virginia capes in latest Wisconsinan times. The transgression continues today, facilitated by a rising sea level everywhere, and active shoreline erosion over much of the tidewater area. The age of this developing CCES unconformity, and the age of the oldest sediments above it, thus ranges from latest Wisconsinan to zero. For example, along the Calvert Cliffs, Chesapeake Bay is encroaching, by shoreline erosion, upon Miocene sediments exposed in the cliffs. Modern beach sands with zero-age shells, together with reworked Miocene fossils, are being laid down on Miocene silty clay, thus continuing to extend the Cape Charles erosion surface outward and upward from the initial transgression as tidewater first eroded and overtopped the banks of the old channel, transforming the environment from fluvial to fluvial-estuarine (restricted estuary).
Over most of the Chesapeake Bay, modern sediments rapidly bury the modern CCES soon after it has formed by shoreline wave erosion. For this reason, we suppose that sediments directly overlying the older CCES under the Chesapeake Bay have an age close to the age of the unconformity -- i.e., these lowest sediments yield points on the sea level rise curve. Of course, care has to be taken in using the CCES to reconstruct the Chesapeake Bay sea level history. For example, along parts of channel margin off Parkers Creek (Chapter 2) and elsewhere, patches of exposed Miocene demonstrate ongoing erosion by tidal currents at modern depths of 16-22 mbsl.
At the northeast end of the reproduced seismic profile (Fig. 4.4), the CCES reflector is about 2.5 mbsf (14 mbsl), but is difficult to distinguish in the strong, reverberant bottom return. As the CCES return is followed SSW in the direction of MD99-2209, the reflector becomes clear at about m 1225 (3 mbsf, 15 mbsl), and remains continuous about as far down the channel slope as m 800 (6.5 mbsf, 22 mbsl). Still farther the CCES reflector dives further below the floor of the bay, but is discontinuous, finally disappearing from view under the gas/no-gas transition zone about m 550 (Fig. 4.4). At that point the CCES is at 12 mbsf (31 mbsl). The CCES dips more steeply toward the channel axis than does the present bathymetric channel, as is also true in the area of MD99-2204 and 2207 (Vogt and others, Chapter 3, this volume).
The gradual southwestward thickening of the Holocene cover probably reflects a combination of more rapid sedimentation in deeper water, and the successively greater age of the basal sediments as the channel axis is approached. If we assume the Fairbanks (1989) sea level rise curve approximates the Chesapeake's sea level history, the CCES is ~1.5 ka older at 31 mbsl than it is at 14 mbsl in Figure 4.4. If sedimentation rates were the same at the two sites, the 10 m excess sediment at the 31 mbsl site would have had to have been deposited at the high rate of ~6.7 mm/yr. Because generally higher sedimentation rates prevail in deeper parts of the Chesapeake, we suppose that the actual rate must have been less than this value. The above alternatives are readily tested by additional cores.
MD99-2209 is located roughly 500 m south-southwest of the last discernable reflection from the CCES. It is tempting to project the CCES below the gassy zone in a smooth curve, descending a further 5 m to the shelly sand recovered at the bottom of the core at 17.2 mbsf (43 mbsl) if no compaction occurred in the core, and up to 23 mbsf (49 mbsl) if the entire difference between recovery and penetration reflects compaction.
In the absence of gas, the shelly sand, unless extremely thin, should appear as a prominent reflector in the chirp profiles. Unless detailed surveying can discover acoustically transparent "windows" in the channel bottom, only additional cores can test this extrapolation of the CCES into the shelly sand. In any case, a sea level 43-49 m below present is on the one hand 13-19 m shallower than the bottom of the paleochannel (62 mbsl) determined from coring at the site of the Chesapeake Bay bridge ~10 km north of MD99-2209 (Fig. 4.2), and on the other hand, based on the Fairbanks (1989) curve, dates from at least 9.4 to 9.6 ka, ~ 2 ka older than the sediments at the bottom of the core (We assume one 9220 yr BP shell from 1694 cm bsf (Colman and others this volume) is reworked). The actual situation probably lies between the two following "bounding scenarios" :
Additional deep coring and seismoacoustic profiling is mandatory to determine this earliest history- the "restricted estuary" phase- of the Chesapeake Bay in this area.
Colman, S.M., Halka, J.P., Hobbs, C.H., Mixon, R.B., and Foster, D.S., 1990, Ancient channels of the Susquehanna River beneath Chesapeake Bay and the Delmarva Peninsula: Bulletin of the Geological Society of America, v. 102, p. 1268-1279.
Colman, S.M., Halka, J.P., and Hobbs, C.H., III, 1992, Patterns and rates of sediment accumulation in the Chesapeake Bay during the Holocene rise in sea level, in Fletcher, C. and Wehmiller, J.F., eds., Quaternary coasts of the United States - marine and lacustrine systems: Society of Economic Paleontologists and Mineralogists Special Publication No. 48, p. 101-111.
Ellison, R.L., and Nichols, M.M., 1976, Modern and Holocene foraminifera in the Chesapeake Bay region: Int. Symp. on Benthonic Foraminifera of Continental Margins, Part A., Ecology and Biology: Maritime Sediments, Spec. Publication 1, p. 131-151.
Fairbanks, R.G., 1989, A 17,000-year glacio-eustatic sea level record - influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation: Nature, v. 342, p. 637-642.
Hagen, R.A., and Vogt, P.R., 1999, Seasonal variability of shallow biogenic gas in Chesapeake Bay: Marine Geology, v. 158, p. 75-88.
U.S. Department of Interior, U.S. Geological Survey
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