SUMMARY AND FUTURE WORK

Drilling at Fort Mott was successful in providing data to evaluate local and regional aquifers. It also was an important site for understanding the regional and global significance of the Aptian–lowermost Cenomanian (~120–98 Ma) Potomac Formation (possibly Barremian in part). This unit can comprise more than one half of the entire thickness of the coastal plain section; for example, at Cape May it is nearly 1100 m of a total coastal plain section of 1900 m (Olsson et al., 1988). Understanding age, porosity, and thickness of the Potomac Formation is important for improving backstripping of the Upper Cretaceous and younger section for sea level studies. This constituted the primary impetus for including this site as part of Leg 174AX, which has as a major objective of obtaining a sea level record by backstripping Cretaceous sections at Ancora, Bass River, Millville, and Sea Girt. However, drilling at Fort Mott also yielded surprising new insights into the depositional history of the coastal plain and potentially provides a key record for deciphering the climate and chronology of deposition of the mid-Cretaceous on this margin.

The Potomac Formation has been previously interpreted as a fluvial unit deposited in a delta plain or coastal plain setting. Our examination of the Potomac Formation at Fort Mott yields the following observations:

1. Potomac Formation sediments in the Fort Mott borehole and elsewhere are predominantly fine grained (clays, silty clays, and clayey silts), with a few critical sand bodies. Many of the finer-grained intervals are heavily overprinted by soil-forming processes. These range from light blue and gray gleyed soils formed in poorly drained, reducing conditions to dark red soils indicating highly oxidized conditions. These silty clays and clayey silts were probably deposited as heavily vegetated levee deposits. Variations in color appear to reflect differences in evaporation and precipitation. The general depositional environment of the Potomac Formation best fits an anastomosing river system characterized by extremely low topographic gradients. Lakes are a common subenvironment of such systems, and we interpret several organic-rich fine-grained intervals as lacustrine deposits, some as oxbow lakes and some as larger alluvial plain lakes.
2. Sand bodies at the bases of Potomac Units 3 (lowermost Cenomanian) and 2 (Albian) appear laterally continuous in southernmost New Jersey; these sands may be present on top of previously unrecognized sequence boundaries. These sequence boundaries potentially reflect baselevel (in this case related to sea level) lowerings. Two other sands in Unit 1 are also candidate sequence boundaries.
3. These thick, apparently continuous sands have the geometries of delta front shoreline sands, although no direct marine evidence has been uncovered within them. If this interpretation is correct, then these sands may constitute the transgressive systems tracts and the anastomosing river facies model represents the highstand systems tracts. Thus, the basal delta front sands were prograded over by delta plain deposits, including lower delta plain interdistributary lakes, marshes, and swamps and upper delta plain deeply weathered soils.
4. Cyclicity is evident in both the color and downhole logs (Figs. F4, F5, F6, F7). In particular, cyclicity is noted on the 2- to 3-ft (60–90 cm) and 10-ft (3 m) scales. Red oxidized sediments (high gamma log values) alternate with gray or gleyed reduced sediments (low gamma log values), providing colorful bar codes for spectral analysis. This cyclicity not only reflects regional climate changes, but its regularity implies a global imprint on climate and precipitation (e.g., Milankovitch periodicities). Future work is needed to test the assumption that these cycles represent precession and short eccentricity cycles, with hints of the long eccentricity cycle.
5. Identification of precessional and eccentricity cycles may, in the future, provide a potential means to parse time in these largely nonmarine units. Pollen biostratigraphy only allows coarse correlation to the stage level, suggesting that the age of these sediments are is ~98–120 Ma. Further investigation may allow us to provide an astronomical correlation for these sediments.

Recognizing that possibly global processes of eustatic change (e.g., the sequence boundaries) and evaporation–precipitation cycles with possible Milankovtich periodicities sheds new light onto these otherwise monotonous soils and sands. Like the Newark Basin Upper Triassic–Lower Jurassic redbeds, the stratigraphic record should reveal entirely new stories by integrating continuous coring with new tools and new insights.