BACKGROUND AND OBJECTIVES

The Bethany Beach borehole was the seventh continuously cored and logged onshore hole drilled as part of the Middle Atlantic Coastal Plain Drilling Project and the fourth drilled as part of ODP Leg 174AX. Drilling began in 1993 with ODP Leg 150 on the New Jersey (NJ) continental slope (Mountain, Miller, Blum, et al., 1994) as part of the New Jersey/Mid-Atlantic Sea-Level Transect (Miller and Mountain, 1994). The primary goal of the transect was to document the response of passive continental margin sedimentation to glacioeustatic changes during the Oligocene to Holocene "Icehouse World," a time when glacioeustasy was clearly operating (Miller and Mountain, 1994). During Leg 150, four sites were drilled on the NJ continental slope, providing a preliminary framework of sequence chronology for the Oligocene-Miocene of the region (Mountain, Miller, Blum, et al., 1994).

Concurrent with and subsequent to Leg 150, a complementary drilling program designated Leg 150X was undertaken to core coeval strata onshore in NJ. This drilling was designed not only to provide additional constraints on Oligocene-Holocene sequences but also to address an important goal not resolvable by shelf and slope drilling: to document the ages and nature of middle Eocene and older "Greenhouse" sequences, a time when mechanisms for sea level change are poorly understood (Miller et al., 1991). Sites were drilled at Island Beach (March-April 1993), Atlantic City (June-August 1993), and Cape May (March-April 1994) (Miller et al., 1994a, 1994b, 1996a; Miller, Newell, and Snyder, 1997). Together, Legs 150 and 150X were extremely successful in dating Eocene-Miocene sequences, correlating them to the ¹⁸O proxy for glacioeustasy, and causally relating sequence boundaries to glacioeustatic falls (Miller et al., 1996b, 1998a).

During Leg 174A, the Mid-Atlantic Transect was continued by drilling between previous slope and onshore sites, targeting the NJ continental shelf (Austin, Christie-Blick, Malone, et al., 1998). The Joint Oceanographic Institutes (JOI) planning committee endorsed a subsequent phase of onshore drilling as an ODP-related activity and designated the program ODP Leg 174AX. As part of this leg, sites were drilled at

1. Bass River, NJ (October-November 1996) (Miller et al., 1998b), targeting Upper Cretaceous to Paleocene strata poorly sampled during Leg 150X;
   
2. Ancora, NJ (July-August 1998) (Miller et al., Chap. 1, this volume), an updip, less deeply buried Cretaceous-Paleocene section complimentary to Bass River;
   
3. Ocean View, NJ (September-October 1999) (Miller et al., Chap. 2, this volume), targeting upper Miocene-middle Eocene sequences; and
   
4. Bethany Beach, Delaware (DE).

The Bethany Beach borehole extended the drilling transect along strike and down dip into the depocenter of the Salisbury Embayment (Fig. F1), with the goals of verifying the ages and regional significance of sequences, evaluating tectonic and sediment supply effects on sedimentation patterns, and testing models of sedimentation within sequences. The depocenter of the Salisbury Embayment extends into the central part of the Delmarva Peninsula, making Bethany Beach an ideal site to drill Miocene sequences where they reach their maximum regional thickness onshore and where the marine influence is greatest. The Bethany Beach borehole provides a more down-dip location for defining and dating sequences in the very thick Miocene section of the Salisbury Embayment. We were able to date 11-12 Miocene sequences at Bethany Beach ranging from 20.8 to 9.8 Ma. The well-dated sequence stratigraphic record from Bethany Beach provides material needed for future studies addressing the control of sea level, tectonics, and sediment supply on sequence stratigraphic architecture, in addition to local aquifer potential.

1. Sea Level. Previous drilling onshore in NJ has not provided a readily datable record of the late Miocene to Pliocene; hence, all attempts at reconstructing a sea level record younger than 12 Ma in NJ have been limited. The Delmarva region contains thicker, more marine, and more fossiliferous upper Miocene-Pliocene sections (e.g., Ward and Blackwelder, 1980; Olsson et al., 1987) than coeval estuarine to marginal marine strata in NJ. In addition, lower to middle Miocene sequences in near Bethany Beach are thicker than previously drilled sections in NJ and should represent deeper marine facies than the coeval NJ sections.
   

   Recovery of a thick, well-dated Miocene section at Bethany Beach provides material needed to derive a sea level estimate via backstripping that can be compared with backstripped records from other areas. Backstripping is a proven method for extracting amplitudes of global sea level from passive margin records (e.g., Steckler and Watts, 1978). One-dimensional backstripping progressively removes the effects of sediment loading (including the effects of compaction), eustasy, and paleowater depth from basin subsidence to obtain tectonic subsidence. By modeling thermal subsidence on a passive margin, the tectonic portion of subsidence can be assessed and a eustatic estimate obtained (Kominz et al., 1998). Backstripping requires relatively precise ages, paleodepths, and porosities of sediments, and these data are being collected from Bethany Beach. Backstripped estimates from Bethany Beach will be compared with eustatic estimates derived from NJ backstripped records (Kominz et al., 1998) and global ¹⁸O records (Miller et al., 1998a).
   

2. Tectonics. Understanding stratigraphic contrasts between Bethany Beach and the areas previously drilled in NJ is critical for assessing tectonic effects. The thickest Cenozoic section in the region is present in the Salisbury Embayment just south of Bethany Beach near the Delaware/Maryland border (Fig. F1B) (Olsson et al., 1988). Whereas NJ is technically part of the same basin as the Salisbury Embayment, some differential movement between the two regions is required to explain the stratigraphic contrasts (Benson, 1994). For example, the differences between thick marine upper Neogene sequences in Delmarva and coeval thin nonmarine sequences in NJ require excess accommodation space in the Delmarva region (Owens and Gohn, 1985). Backstripping of records from New Jersey well sites and Bethany Beach will provide a baseline for comparison of sections between the two regions and evaluation of tectonic influences vs. sea level changes on sequence expression.
   
3. Sequence Architecture and Sediment Supply. Comparison of Miocene sequences from Bethany Beach with those in NJ will allow evaluation of facies models and the effects of differences in sediment supply and sedimentary environment on sequence architecture. Miocene sequences of New Jersey reflect a strong deltaic influence on sedimentation that is lacking at Bethany Beach, and Miocene sedimentation rates are higher than those sampled in NJ. Despite these differences, drilling at Bethany Beach demonstrates that both areas display similar sequence architecture. Sequence boundaries are typically coincident with transgressive surfaces and are expressed as the flooding of marine clays over deltaic sands. As a result, LSTs are largely absent and TSTs are present at the base of nearly all sequences but are thin. Sequences are predominantly regressive HST successions. Nevertheless, this study has revealed important differences in sequence stratigraphic architecture between these two areas. For example, in NJ HSTs consist of a lower prodelta silty clay and an upper delta front/nearshore sandy unit vs. typical medial offshore silts and upper shoreface sands in Delaware. An understanding of similarities and differences in the expression of sequences between NJ and Bethany Beach will help differentiate the influences of sediment supply and tectonics on sequence stratigraphy.
   

   Comparison of NJ boreholes and the Bethany Beach site will also provide a north-south transect that will allow an evaluation of the effects of regional vs. local sediment supply and regional climate change on sedimentation in this region. Poag and Sevon (1989) documented that offshore depocenters have remained near their present locations during the Cenozoic, implying that there was no major shift in the number of major riverine systems; however, stream capture and avulsion have strongly influenced the local position of fluvial systems. Poag and Sevon (1989) and Pazzaglia (1993) ascribed the Oligocene to Miocene margin transformation to changes in sediment supply linked to hinterland uplift (central Appalachian). They emphasized that the largest increase in sedimentation from the continental shelf to the rise occurred in the middle Miocene, although it is clear that sediment supply increased in the NJ region by the Oligocene (Miller et al., 1997) or even the late Eocene (Pekar et al., 2000). The ascription of progradation to hinterland tectonics requires that sediment input increased overall across the region at about the same time. Drilling at Bethany Beach will determine if the timing of increased sediment supply in this region was the same as in NJ.
   

4. Local Lithostratigraphy and Aquifer Stratigraphy. Drilling at Bethany Beach was impelled by another major objective: to provide a more complete understanding of the geological history of the sediments underlying southern Delaware, with a special attention to the aquifers of this rapidly growing area. The Delaware Geological Survey partially funded direct drilling costs for the Bethany Beach borehole with these goals in mind. The lithostratigraphy of the near-surface units of coastal Sussex County, Delaware, is complicated by significant lateral facies changes. Because previous studies have been based principally on geophysical log data, the Bethany Beach hole will provide valuable core data to better understand the depositional history of the area. The site will provide a valuable stratigraphic reference section for the Neogene of the Delaware Coastal Plain and help formalize the stratigraphic nomenclature of the shallow section. In addition, it will allow us to establish a sequence stratigraphic framework that can be compared with that of adjacent states. In particular, the findings should improve our understanding of the hydrogeology of the locally important Pocomoke and Manokin aquifers, especially in delineating the distribution of fresh- and saline-water zones deeper in the subsurface.

Comparison of the Bethany Beach borehole with previous drilling results from ODP Legs 150, 150X, 174A, and 174AX provides a means of evaluating global, regional, and local controls on the stratigraphic record. Cognizant of this, onshore drilling of the Leg 150X and 174AX boreholes was sponsored by the National Science Foundation, Earth Science Division, Continental Dynamics and Ocean Drilling Programs, the New Jersey Geological Survey, and the Delaware Geological Survey.