The Bethany Beach borehole was drilled in May and June 2000 as the seventh onshore site of the Coastal Plain Drilling Project and fourth site of Ocean Drilling Program (ODP) Leg 174AX, complementing shelf drilling during Leg 174A. Drilling at Bethany Beach targeted Miocene sequences at a point where they reach their maximum regional thickness onshore (i.e., the depocenter of the Salisbury Embayment). Recovery was very good (mean recovery = 80%), and a full suite of slimline logs was obtained from the surface to 205 ft and from the surface to total depth of 1470 ft (448.06 m) in mid-Oligocene sediments. A team of scientists from the Delaware Geological Survey (DGS), Rutgers University, the New Jersey Geological Survey (NJGS), and the U.S. Geological Survey (USGS) collaborated in drilling and stratigraphic studies of the borehole, which was funded by the National Science Foundation (NSF, Earth Science Division, Continental Dynamics Program), DGS, and USGS.
Sequence-bounding unconformities were identified on the basis of physical stratigraphy, including irregular contacts, reworking, bioturbation, major facies changes, gamma ray peaks, and paraconformities inferred from biostratigraphic and Sr isotopic breaks. Miocene sections in Delaware lack the clear deltaic influence seen in coeval sections in New Jersey; however, they still comprise generally thin transgressive systems tracts (TSTs) and thick highstand systems tracts (HSTs), with lowstand systems tracts (LSTs) generally absent. The overall association of facies suggests that most of the Bethany Beach Miocene section fits a wave-dominated shoreline model, with fluvial to upper estuarine, lower estuarine, upper shoreface/foreshore, distal upper shoreface, lower shoreface, and offshore (including inner and middle neritic) environments represented.
The Pleistocene (5-52.9 ft; 1.52-16.12 m) Omar Formation is interpreted to include two marginal-marine sequences. The upper sequence comprises a transgressive succession composed of marsh-lagoon-tidal delta sediments that probably correlates with marine isotopic Stage 5 (5.0-50.65 ft; 1.52-15.44 m). The lower sequence is a thin estuarine clay of uncertain age (50.65-52.9 ft; 15.44-16.12 m). The underlying Beaverdam Formation (52.9-117.5 ft; 16.12-35.81 m) consists of quartz sand, some gravelly sand, and subordinate silty clay, deposited primarily in fluvial and estuarine environments; the base of this formation at Bethany Beach represents estuarine(?) environments. The Beaverdam Formation contains two surfaces that may represent sequence-bounding unconformities or facies shifts in fluvial environments. The unit is poorly dated but apparently is Pliocene (possibly upper Miocene) as suggested by the presence of exotic pollen.
The informal Bethany formation (117.5-197.4 ft; 35.81-60.17 m) is characterized by interbedded sands and clays deposited in lower shoreface to estuarine environments. Pollen studies place this unit in the upper Miocene or Pliocene. Whereas the Bethany formation comprises one definite sequence at Bethany Beach, it can be subdivided by two additional surfaces (150.6 and 185.6 ft; 45.90 and 56.57 m), though the significance of these surfaces as unconformities vs. autocyclical facies shifts is unclear.
The informal Manokin formation is primarily an upper Miocene sand that can be divided into three sequences: (1) the upper sequence (197.4-294.1 ft; 60.17-89.64 m) is fine to medium sand deposited in lower shoreface or estuarine environments; (2) the medial sequence (294.1-374 ft; 89.64-114.0 m [N1]) coarsens upward from fine to medium sands, comprising a thick succession of regressive distal upper shoreface deposits; and (3) the lower sequence (374-452.45 ft; 114.0-137.91 m), which extends into the top of the underlying St. Marys Formation, coarsens upsection from offshore silty sands to lower shoreface sands. Transgressive systems tracts are very thin in these sequences.
The St. Marys Formation (449.4-575.2 ft; 136.98-175.32 m) is a silty clay to clayey silt deposited in offshore inner to middle neritic (25-75 m) paleodepths. In addition to the sequence spanning the St. Marys and Manokin Formations, it can be broken into two distinct sequences (452.45-523.05 and 523.05-575.2 ft; 137.91-159.43 and 159.43-175.32 m) that were starved of sand input; lithostratigraphic successions show minimal changes, but benthic foraminiferal biofacies show evidence of moderate shallowing upsection.
The Choptank Formation (575.2-819.9 ft; 175.32-249.91 m) is a sandier unit than the immediately overlying and underlying formations and is characterized as interbedded fine to coarse sand, shell, silt, and some clay. At Bethany Beach, it can be divided into four sequences: (1) the upper sequence (575.2-649 ft; 137.91-197.82 m) is comprised of a regressive fine to coarse sand deposited in lower shoreface to upper shoreface environments; (2) the medial sequence (649-698.5 ft; 197.82-212.90 m) grades down from a granuliferous sand to a silt, representing a regression from lower shoreface to upper shoreface/estuarine environments; (3) the lower sequence (698.5-787.1 ft; 212.90-239.91 m) consists of a thick regressive HST in lower shoreface environments, a zone of maximum flooding in offshore environments, and a thick TST in upper to lower shoreface environments; and (4) the basal Choptank Formation (787.1-819.9 ft; 239.91-249.91 m) comprises the upper HST of a sequence spanning the Choptank/Calvert Formation boundary and includes sands of the locally important Milford aquifer. This uppermost Calvert-lowermost Choptank sequence shows a classic pattern of thin basal TST, thick medial lower HST silts, and upper HST sands at the top.
The Calvert Formation (819.9-1420 ft; 249.91-732.82 m) comprises interbedded silt, sand, and clay with common shells that can be broken into distinct sequences: (1) the lower part (819.9-897.7 ft; 249.91-273.62 m) of the sequence spanning the Choptank/Calvert Formation boundary includes the lower HST (819.9-887.7 ft; 249.91-270.57 m) and a thin TST (887.7-897.7 ft; 270.57-273.62 m); (2) the sequence from 897.7 to 981.3 ft (273.62 to 299.10 m) is composed of at least three shallowing-upward parasequences in offshore to upper shoreface environments; (3) the sequence from 981.3 to 1057.95 ft (299.10-322.46 m) comprises a classic thin lower shoreface-offshore TST and thick coarsening-upward HST representing deposition in lower to upper shoreface environments; (4) the sequence from 1057.95 to 1153 ft (322.46-351.43 m) sequence is predominantly a lower HST silt with a fine sand upper HST deposited in lower shoreface environments; and (5) a very thick sequence from 1153 to 1421.1 ft (351.43 to 433.15 m) is composed of thick coarse sands deposited in upper shoreface environments (upper HST), a medial silty sand to silt (lower HST) deposited primarily in offshore to lower shoreface environments, a very thin basal TST; the sandy upper HST portion of the sequence correlates to the Cheswold sand aquifer updip.
An unnamed lowermost Miocene glauconitic clay comprises the base of the sequence down to 1421.1 ft (433.15 m); below this, it may be divided into three thin sequences: an upper sequence from 1421.1 to 1430.5 ft (433.15 to 436.02 m), a middle sequence from 1430.5 to 1454.5 ft (436.02 to 443.33 m), and a lower sequence from 1454.5 to 1465.7 ft (443.33 to 446.75 m). An unnamed Oligocene foraminiferal clay was penetrated in the base of the borehole (1465.7-1467.95 ft; 446.75-447.43 m).
Our initial studies of the Bethany Beach borehole provide important findings in three areas of study:
The Bethany Beach borehole thus provides (1) excellent recovery, delineation, and dating of 11 or 12 lowermost upper Miocene to lower Miocene sequences; (2) constraints on the differential development of sequences during the Icehouse (glacioeustatic) world of the Miocene, due to processes of tectonics and sedimentation; and (3) new information on the hydrostratigraphy of important aquifers and confining units in southern Delaware.