The on-site scientific team provided preliminary descriptions of sedimentary textures, structures, colors, fossil content, identification of lithostratigraphic units (Andres, 1986; Benson, 1990), and lithologic contacts (Table T1; Figs. F2, F3, F4, F5, F6, F7, F8). Subsequent studies integrated preliminary descriptions with additional descriptions, biostratigraphy, biofacies studies, isotopic stratigraphy, and the gamma log. 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. For the nonmarine and nearshore sections (primarily the upper Miocene and younger section), lithofacies interpretations provide the primary means of recognizing unconformities and interpreting paleoenvironments.
Facies changes within onshore New Jersey Miocene sequences generally follow repetitive transgressive-regressive patterns (Owens and Sohl, 1969; Sugarman et al., 1993, 1995) that consist of (1) a basal, generally thin, transgressive quartz sand equivalent to the TST of Posamentier et al. (1988) and (2) a coarsening-upward succession of regressive medial silts and upper quartz sands equivalent to the HST of Posamentier et al. (1988). Miocene sections in Delaware lack the clear deltaic influence seen in coeval sections in New Jersey; however, they still comprise generally thin TSTs and thick HSTs. LSTs are usually absent in both coastal plains. Because the TSTs are thin, MFS are difficult to differentiate from unconformities. Both can be marked by shell beds and gamma ray peaks. Flooding surfaces (FSs), particularly MFSs, may be differentiated from sequence boundaries by the association of erosion and rip-up clasts at the latter, lithofacies successions, and benthic foraminiferal changes. The transgressive surface (TS), marking the top of the LST, represents a change from generally regressive to transgressive facies; because LSTs are generally absent, these surfaces are generally merged with the sequence boundaries (Fig. F3).
The overall association of facies suggests that most of the Bethany Beach section fits a wave-dominated shoreline model devised by Bernard et al. (1962) on Galveston Island and further developed by Harms et al. (1975, 1982), and McCubbin (1981). Whereas the New Jersey Miocene sections generally contain significant amounts of clay, plant debris, and micaceous sands consistent with a deltaic influence, the Bethany Beach section overall has less lignite, less mica in the sands, and fine-grained sediments dominated by silt rather than clay, reflecting movement of sediments by wave energy. The facies model used in this study recognizes the following environments (Fig. F2):
In these environments, small changes in sea level can produce dramatic shifts in sedimentary facies.
Benthic foraminiferal biofacies were used to recognize inner (0-30 m), middle (30-100 m), outer (100-200 m) neritic, and upper (200-600 m) bathyal paleodepths. The upper shoreface/foreshore, distal upper shoreface, and lower shoreface environments all lie within inner neritic depth ranges. The offshore environment encompasses middle neritic and deeper paleodepths.
Cumulative percent plots of the sediments in the cores were computed from samples washed for paleontological analysis (Table T2). Each sample was dried and weighed before washing, and the dry weight was used to compute the percentage of sand vs. silt and clay (Table T2). This differs from the method used in previous New Jersey Coastal Plain cores (Bass River, Island Beach, Atlantic City, and Cape May) in which the samples were not dried before washing. The sand fraction was dry sieved through a 250-µm sieve, and the fractions were weighed to obtain the percent of very fine and fine vs. medium and coarser sand. The sand fractions were examined using a microscope and a visual estimate was made of the relative percentages of quartz, glauconite, carbonate (foraminifers and other shells), mica, and other materials contained in the sample. The values for these sand components given in Table T2 are derived by dividing the visual estimate (a whole number percentage) by the weight percent sand and thus may not add up to 100%, due to rounding error.
Age: PleistoceneThe Omar Formation was erected by Jordan (1962) as a heterogeneous unit of gray quartz sands interbedded with clayey silts and silty clays that commonly contain abundant plant debris. At Bethany Beach (Fig. F4), sands and clays of the Omar Formation comprise the upper part of the section cored (5.0-52.9 ft; 1.52-16.12 m). We interpret these sands and clays as representing deposition in a complex of nearshore, lagoonal, and marsh environments.
Cross-bedded, fine to very coarse, moderate to poorly sorted sands at the top (5.0-5.35 and 5.7-7.2 ft; 1.52-1.63 and 1.74-2.19 m) and interbedded greenish gray, slightly sandy clays (5.35-5.7 ft; 1.76-1.87 m) are interpreted as back barrier tidal delta deposits. Interbedded sands and clays (7.2-31.5 ft; 2.19-9.61 m) are interpreted as lagoonal deposits, whereas organic-rich, in some cases peaty, clays (31.5-50.65 ft, 9.60-15.44 m) are interpreted as retrograding marsh deposits. The sands are generally fine- to very fine grained, silty in places, and moderately sorted, with scattered plant debris and some shells (including Mulina sp.); the interbedded clays are sticky, silty in places, and contain thin sand stringers with shell concentrations. The organic-rich clays are generally laminated (1- to 2-mm laminations). A sharp contact at 50.65 ft (15.44 m) is abrupt and irregular, with a weathering horizon at the top. This is interpreted as a sequence boundary, and thus the interval from 5.0-50.65 ft (1.52-15.44 m) comprises a single sequence tracing a transgression from estuarine to bay marsh to lagoonal to tidal delta environments (Fig. F4).
Radiocarbon accelerator mass spectrometer dates of >48.0 ± 10.5 ka at 40.9-40.95 ft (12.47-12.48 m) and 47.1 ± 1.2 ka at 50.4-50.5 ft (15.36-15.39 m) were measured at the Woods Hole Oceanographic NOAMS facility. These dates would reflect late Pleistocene (marine isotope Stage 3) deposition, if valid, but could also be "dead" radiocarbon ages. Samples examined for palynology between 8.0 and 24.5 ft (2.44 and 7.47 m) have a cool-climate assemblage, indicating a Pleistocene age. Pollen analysis of samples from lower in the Omar Formation, near the radiocarbon-dating samples (34.4, 40.9, and 45.8 ft; 10.49, 12.47, and 13.96 m), indicate a warmer climate. A sample from the Omar Formation near Bethany Beach was assigned to amino acid Zone IIa (75-130 ka) (Wehmiller in Groot et al., 1990), suggesting equivalence to marine isotope Stages (MISs) 5a-5e. Studies elsewhere in southern Delaware assign the Omar to amino acid Zones IIb (age undefined, somewhere between 130 and 200 ka) and IId (400-600 ka) (Wehmiller in Groot et al., 1990). Based on the Pleistocene pollen assemblages and the nearby aminostratigraphy, we interpret these as samples as beyond the range of radiocarbon dating and derived from radioactively "dead" carbon. Thus, we interpret the transgressive sequence from 5.0 to 50.65 ft (1.52 to 15.44 m) as late Pleistocene.
A strontium value of 0.709147 was obtained from a shell at 24.6 ft (7.50 m) (Table T3); this corresponds to an age of isotopic age of 1 ± 0.35 Ma (see "Strontium Isotope Chronostratigraphy"). Thus, Sr results suggest a middle-early Pleistocene age for this section. Further studies are needed to evaluate the validity of this measurement and the possibility that the shell was reworked from older strata.
A dark greenish gray (kaolinitic?) clay (50.65-51.6 ft; 15.44-15.73 m) becomes slightly sandy at its base (51.6-52.9 ft; 15.73-16.12 m). This interval appears to be bracketed by stratal surfaces interpreted tentatively as sequence boundaries (Fig. F4). The lower contact (52.9 ft; 16.12 m) is marked by a sharp change to laminated coarse sands. We tentatively interpret the environment of deposition as estuarine.
Age: Pliocene?The Beaverdam Formation consists primarily of white to buff to greenish gray quartz sand, some gravelly sand, and lesser light gray to greenish gray silty clay, deposited in fluvial and estuarine environments (Groot et al., 1990). It was originally described in Maryland (Rasmussen and Slaughter, 1955) and later recognized in Sussex County, DE, by Rasmussen at al. (1960) and Jordan (1962). Palynological studies place this unit in the Pliocene (Groot and Jordan, 1999).
The contact between the Omar and Beaverdam Formations was placed tentatively at 52.9 ft (16.12 m) at a change from clay above to coarse, poorly sorted sands below (Fig. F4). Sands from 52.9 to 70.0 ft (16.12 to 21.34 m) are generally homogenous, coarse to very coarse, and poorly sorted, with orange-tinted possible feldspar grains typical of the Beaverdam Formation (Ramsey, 1990), scattered cross laminae of organic-rich material, and scattered clay blebs. A laminated clay layer at 70.0-70.6 ft (21.34-21.52 m) overlies a slightly irregular contact at 70.6 ft (indicated with a ? unconformity on Fig. F4); however, there is little evidence of reworking other than clay blebs burrowed into the fine-medium sands below, and the significance of this contact is uncertain. Poorly sorted sands, including a bed of granular very coarse sands (73.4-73.5 ft; 22.37-22.40 m), continue down to a contact with a clay layer (86.3-86.85 ft; 26.30-26.47 m) that overlies gravelly coarse sand. The contact at 86.85 ft is associated with a large shift on the geophysical logs, suggesting that it may be a sequence-bounding unconformity; however, like the contact at 70.6 ft, this contact may reflect a change from sand to a clay layer in a fluvial system (Fig. F4). Sands above 86.3 ft (26.30 m) are cleaner/better sorted than below and may represent either fluvial or upper estuarine deposition; sands below 86.85 ft (26.47 m) are generally more poorly sorted, fine to coarse granular sands with scattered pebbles and appear to be fluvial in origin. Sands between 110.0 and 114.3 ft (33.53 and 34.84 m) are yellow (iron) stained. From 115 to 117.5 ft (35.05 to 35.81 m), the sands are fine to medium grained and better sorted than above, with ~3% opaque heavy mineral concentrated in cross laminae. The base of the sand contains granules and pebbles up to 15 mm diameter. These are tentatively interpreted as estuarine or possibly upper shoreface deposits. The boundary between the Beaverdam and Bethany Formations is placed at 117.5 ft at the contact between the gravelly sand and an underlying thin clay; a sequence boundary is also tentatively placed at that level. Thus, the sequence(s) from 52.9 to 117.5 ft (16.12 to 35.81 m) represent fluctuations between upper estuarine and fluvial conditions.
The Beaverdam Formation and the two underlying stratigraphic units, the Bethany formation and the Manokin formation, are all predominantly medium to very coarse sand. Because few clear lithologic breaks are present between 54.5 and 317 ft (16.61 and 96.62 m), lithostratigraphic subdivision of this section is uncertain, particularly the contact between the Beaverdam Formation and the Bethany formation. Stratigraphic subdivision is further complicated by the significant facies changes observed in these units based on comparison to geophysical logs of nearby wells; as little as 2 mi (3.2 km) away, an apparently equivalent section includes notably thicker fine-grained intervals. Sequence stratigraphic concepts are difficult to apply in this section, though several clay layers associated with erosional contacts serve as the basis for the preliminary sequence framework discussed herein. An additional complication for sequence definition and correlation of surfaces within the Beaverdam Formation is their common truncation by the erosional surface at the base of the overlying Pleistocene Omar Formation.
Age: late? Miocene to Pliocene?The Bethany formation is an informal local unit characterized by Andres (1986) as a lithologically complex unit of gray sand, interlayered gray, olive-gray, and blue-gray clay, and silt with common lignite, shells, and a "sawtooth" gamma log pattern. It includes two important local aquifers, the Pocomoke and Ocean City aquifers. The top of the Bethany formation is placed at 117.5 ft (35.81 m) where a gravelly sand interpreted as the basal Beaverdam Formation rests upon a thin clay placed in the Bethany formation (Fig. F4). The clay is 0.35 ft (0.11 m) thick, finely laminated, olive gray, and contains plant debris and lignite fragments. It is underlain by an interval of generally poorly sorted, granule- and pebble-bearing sands from 117.85 to 133.3 ft (35.92 to 40.63 m). These vary from lignitic silty, clayey fine sand to very poorly sorted, coarse, pebbly, silty, clayey sands. They represent deposition in an estuarine environment (probably fluvial-estuarine). Below this, sands generally fine downsection, with medium to coarse sands with abundant granules (135.0-139.1 ft; 41.15-42.40 m) passing into to moderately well sorted medium-coarse sand with less common granule zones and a few clay-filled burrows (140.0-150.6 ft; 42.67-45.90 m). We interpret this section as representing a downsection transition from lower estuarine to bay/back barrier, with the sequence shallowing upsection from 150.6 to 117.5 ft (45.90 to 35.81 m). A sharp burrowed contact at 150.6 ft (45.90 m) separates gray, fine, moderately well-sorted sands above from iron-stained, oxidized, heavily bioturbated, silty fine sands below. The contact is irregular, has burrowed clay blebs extending 0.2 ft (0.06 m) above the contract, and cemented burrows. It is interpreted as a sequence boundary and a subaerial surface of erosion separating shelf sands below from bay/back-barrier sands above.
Below this sequence boundary, from 150.6 to 162.25 ft (45.90 to 49.45 m), are bioturbated to very heavily bioturbated, silty, clayey, fine to medium sands with scattered clayey laminations broken by burrows (Fig. F4). These sands rapidly develop a 2- to 3-mm-thick rind of iron oxide when exposed to air. An irregular contact at 162.25-163.35 ft (49.45-49.79 m) separates these heavily bioturbated lower shoreface sands above from upper shoreface sands below. Although this could be a sequence boundary, it is more likely a facies change reflecting a flooding surface/parasequence boundary (Fig. F4). Below the contact, fine to medium sands are somewhat bioturbated, with hints of cross lamination interpreted as distal upper shoreface. Cross laminations with concentrations of opaque heavy minerals become more prominent below 173.0 ft (52.73 m); these sands also develop a thin limonitic coating after scraping. The clean, generally well-sorted, cross-laminated sands from 173.0 to 185.6 ft (52.73 to 56.57 m) are interpreted as deposits from foreshore/upper shoreface environments. Thus, the section from 150.6 to 185.6 ft (45.90 to 56.57 m) overall deepens upsection from foreshore/upper shoreface to lower shoreface. However, maximum bioturbation occurs around 163 ft (49.68 m); this may reflect a zone of maximum flooding, with TST below and a thin, clayey lower HST above.
A sharp lithologic contact at 185.6 ft (56.57 m) is tentatively interpreted as a sequence boundary. The contact consists of a thin gravel layer (185.59 ft; 56.57 m) underlying the foreshore sands, with a return to bioturbated, very fine to medium lower shoreface sands below. The contact may be unconformable or merely reflect an overstepping facies shift from lower shoreface to foreshore. This section grades downsection to a sandy clayey silt (190-195 ft; 57.91-59.44 m) to a sandy silty clay (195-197.4 ft; 59.44-60.17 m); we interpret this as a shallowing-upward trend within the inner neritic zone. Another possible sequence boundary at 197.4 ft (60.17 m) is associated with a sharp gamma log peak; this irregular surface overlies a white, possibly kaolinitic weathered clay zone (197.4-197.6 ft; 60.17-60.23 m) that is reworked as clay blebs (197.3 ft; 60.14 m) above. Very fine to fine silty sand underlies the clay (Fig. F4).
Because of the significant facies changes in the Beaverdam-Bethany-Manokin interval, the upper and lower boundaries of the Bethany formation are considered time-transgressive, occurring at significantly different levels in nearby wells. Where prominent stratigraphic surfaces are present in the upper part of the Bethany Beach borehole, it is not always clear whether they should be interpreted as regional unconformities or as local cut-and-fill structures related to facies changes. Considering the estuarine to nearshore environment in this interval and difficulty in correlating to nearby wells, many of these surfaces (e.g., 185.6 and 197.4 ft; 56.57 and 60.17 m) may simply be normal erosional surfaces that reflect facies changes. Nevertheless, the stacking pattern associated with the 197.4-ft (60.17 m) surface, from progradational below to retrogradational above, suggests that this surface may be a regional unconformity. The surface at 150.6 ft (45.90 m) appears to be consistently recognizable in the area and thus is interpreted as a sequence boundary.
Age: late MioceneThe transition from the Bethany formation to the upper part of the informal Manokin formation is picked at 197.4 ft (60.17 m) where the section changes from interbedded sands and silts/clays above to an overall sand section below (Fig. F4). Andres (1986) established the Manokin formation as an informal local stratigraphic unit characterized predominately by gray sand, with some beds of gravel, and local clayey/silty, lignitic, and shelly beds. The Manokin formation is sandier than the Bethany formation and generally consists of a lower, upward-coarsening progradational section and an upper aggradational or mixed section with facies indicative of higher-energy depositional environments (Andres, 1986). We found that the progradational and aggradational sections comprise distinct sequences.
The boundary between the Bethany formation and the Manokin formation coincides with a tentative sequence boundary at 197.4 ft (60.17 m) (Fig. F4). The section immediately under this boundary, from 197.4 to 205.1 ft (60.17 to 62.51 m), is enigmatic. This section consists of heavily bioturbated silty fine sand, similar to that noted between 185.6 and 190 ft (56.57 and 57.91 m), that is interpreted as a lower shoreface deposit. Within this unit, a lignitic sand (203.0-203.25 ft; 61.87-61.95 m) is stained brown throughout and shows aspects of a weathered soil, though there are not obvious roots. At 205.1 ft (62.51 m), the section changes to lignitic, bioturbated nearshore sand. We interpret this as a minor flooding surface; alternatively, we could have placed a sequence boundary at this level rather than at 197.4 ft (60.17 m).
The section from 205.1 to 294.1 ft (62.51 to 89.64 m) consists of bioturbated, cross-laminated, silty fine to medium sands with laminations of lignite and opaque heavy minerals (Fig. F4). This interval is interpreted as representing upper shoreface/foreshore to lower estuarine environments. Burrows are up to 0.2 ft (6 cm) long. The amount of lignitic material and plant fragments is greater above 232 ft (70.71 m). The sands display several fining-upward successions (e.g., 223-225, 229-231.5, and 292-294 ft; 67.97-68.58, 69.80-70.56, and 89.00-89.61 m); poor recovery precludes further details. The lowermost succession grades down to a gravelly sand (293.8-294.0 ft; 89.55-89.61 m) that overlies a thin clay (294.0-294.1 ft; 89.61-89.64 m) and a medium sand below 294.1 ft (89.64 m). A sequence boundary is placed at 294.1 ft (89.64 m) at the contact between the clay and the sand.
Beneath the 294.1 ft (89.64 m) sequence boundary, the Manokin formation consists of predominantly medium sand to 318 ft (96.93 m) (Fig. F5). These sands are homogenous because of heavy bioturbation but are laminated in places. They include trace amounts of opaque heavy minerals and lignite and thin interbeds of silty organic-rich clays (305.5-305.65, 317.25-317.55, and 318.0-318.5 ft; 93.12-93.16, 96.70-96.79, and 96.93-97.08 m). The sands between 318.35 and 369.3 ft (97.03 and 112.56 m) are generally well-sorted fine to medium sand without obvious shell fragments. The section thus coarsens upsection and is interpreted as the HST. These sands are considered to reflect distal upper shoreface environments rather than upper shoreface based on the infrequent cross beds and concentrations of opaque heavy minerals. In places they develop limonitic rinds when exposed to air. The mud fraction increases slightly beginning at 370 ft (112.78 m), and the formation is slightly silty sand to 373.3 ft (113.78 m).
There is a major lithologic change in an interval of no recovery between 373.3 and 375.0 ft (113.78 and 114.30 m). The section changes from a silty sand above this surface to a shelly, glauconitic, granule-bearing sand below that continues to 403.3 ft (122.93 m). We tentatively place a sequence boundary at a gamma log kick at 374.0 ft (114.00 m) in the interval of no recovery and interpret the silty sands from 370 to 373.3 ft (112.78 to 113.78 m) as the TST of the overlying sequence (Fig. F5). Alternatively, the succession from 294.1 to 373.3 ft (89.64 to 113.78 m) could represent a very thick HST. Local well log correlations indicate that the 374.0-ft (114.00 m) surface is correlatable, supporting its interpretation as a sequence boundary. However, the facies shift from lower shoreface below to distal upper shoreface above could be explained simply as a facies shift due to shallowing upsection.
Mollusk shells become very common under this tentative sequence boundary. The shells range from large whole clams (Mercenaria) to coarse shell hash. Though glauconite appears in trace amounts in washed residues above this point (such as at 350 ft; 106.68 m), only below this level is it clearly present in visual inspection of the core. Phosphate also appears at this level and continues to 390 ft (118.87 m). The interval from the 374-ft (114.00 m) sequence boundary to 403.3 ft (122.93 m) is interpreted as representing deposition in lower shoreface environments.
The section from 405 to 449.4 ft (123.44 to 136.98 m) is composed of silty, shelly, homogeneous, heavily bioturbated sands with traces of glauconite (Fig. F5). The section becomes increasingly finer grained downward, with thinner shells and fewer quartz granules than in the overlying shelly sand. The abundance of shells further decreases below 415 ft (126.49 m), and the shells become thinner-walled and smaller. Lignite and mica are present in trace amounts in this interval, and the lignite becomes more common and coarser (up to 1.5 cm) downsection. At 438 ft (133.50 m) the amount of silt in the core increases, and below 445 ft (135.67 m) the sediments are dominantly silt. The base of this interval from 449.1 ft to 449.4 ft (136.89 to 136.98 m) includes clays burrowed up from the underlying interval. Overall, the sediments from 405.0 to 449.4 ft (123.44 to 136.98 m) indicate quiet offshore/middle neritic environments that reflect deeper paleodepths than the overlying interval. Thus, the section from 449.4 to 374.0 ft (136.98 to 114.00 m) is interpreted as an upward-shoaling HST succession.
Age: late MioceneThe St. Marys Formation was established in Maryland by Shattuck (1902) and first recognized in Delaware by Rasmussen et al. (1960) as an important Miocene aquitard in Sussex County. It is characterized by glauconitic laminated to burrowed sandy and silty clays (Andres, 1986; Ramsey, 1997) and is an overall finer-grained unit than the Manokin. In many places, it appears to gradually coarsen upward into the Manokin formation (Andres, 1986). In this borehole (Fig. F5), the formation boundary is a distinct contact at 449.4 ft (136.98 m) where a bioturbated surface separates very fine sandy silt of the basal Manokin formation from the slightly silty clay of the St. Marys Formation. Burrows are up to 1 cm (0.03 ft) wide and extend down from the contact to 450.25 ft (137.24 m) and up to 449.1 ft (136.89 m) in the overlying sequence. This contact could be interpreted as a sequence boundary or MFS. We favor the latter because there is an erosional surface slightly lower in the section, at 452.45 ft (137.91 m), associated with a distinct gamma log kick that appears to be the sequence boundary. This possible sequence boundary at 452.45 ft (187.91 m) separates slightly glauconitic (with medium-coarse glauconite) shelly clay above from laminated brown clay below. From the contact at 452.45 ft (137.91 m), brown clays are burrowed up to 451 ft (137.46 m) and glauconitic clays are burrowed down to 454 ft (138.38 m).
The section below the sequence boundary, from 452.45 to 515.6 ft (137.91 to 157.15 m), is a uniform, generally laminated clay to silty clay with scattered shells (mostly gastropods) and numerous shell concretions developed around burrows. Concretions are Fe and S rich, weathering to Fe- and S-stained rinds on the core near the concretions. Scattered very fine glauconite and traces of mica are present in the clay to silty clay. Burrows break up the laminations, especially between 452.45 and 454.0 ft (137.91 and 138.38 m); below this laminations are better preserved. The depositional environment of this section is not clear; the dominance of Pseudononion pizarrensis in the foraminiferal fauna (paleodepth = 25-50 m; see "Biostratigraphy") favors a starved outer inner to inner middle neritic environment. The dominance of gastropods, the numerous concretions, and the laminations suggest a low-oxygen benthic environment. The absence of sands could be attributed either to erosional truncation of the sandy part of an upper HST or to sediment starvation. We favor the latter interpretation based on the upsection decrease in gamma log values, suggesting shallowing upsection, and on the lack of evidence for a major erosional truncation (e.g., rip-up clasts and hardgrounds).
A minor concentration of glauconite and shells at 479.15 ft (146.04 m) is associated with a minor gamma ray log peak. We do not interpret this as a sequence boundary but instead as a parasequence boundary associated with a minor flooding surface. A similar gamma log peak at 457-458 ft (139.29-139.60 m) could be interpreted as another flooding surface; it is expressed as a minor increase in glauconite in the core. Uniform, generally laminated clay continues to 515.5 ft (157.12 m). A poorly recovered indurated interval (515.5-516.2 ft; 157.12-157.34 m) with carbonate cement and scattered phosphate? grains may mark the MFS within this sequence. Below this, a silty glauconite sand (520.0-523.05 ft; 158.50-159.43 m) represents a TST deposited in middle neritic environments. Large shells are present at 522-522.7 ft (159.11-159.32 m).
A distinct sequence boundary is placed at 523.05 ft (159.43 m) (Fig. F5) at the top of an indurated zone (523.05-523.8 ft; 159.43-159.65 m). Glauconite sand is extensively burrowed down into underlying clays to 524.2 ft (159.78 m), obscuring the contact. Glauconite sand-filled burrows extend down to 524.6 ft (159.90 m), with a few glauconite blebs burrowed down to 526 ft (160.32 m). The lithology below the contact is laminated, slightly silty clay with a few nodules and is slightly lighter gray (and probably less organic rich) than the clays in the overlying sequence. This is similarly interpreted as representing a starved low-oxygen middle neritic offshore environment.
Laminated clays continue down to 545.5 ft (166.27 m), with an increasingly silty section of silty clays and clayey silts down to 553.5 ft (168.71 m). Glauconite content increases from ~1%-2% at the top of the silty interval to 2%-3% at the bottom (Fig. F5). The silts are underlain by a glauconite sand from 553.5 to 557.5 ft (168.71 to 169.93 m), which is in turn underlain by a stiff slightly silty micaceous clay that continues downward to 570.23 ft (173.81 m). The contact between the sand and the clay at 557.5 ft (169.93 m) is extensively burrowed, with large burrows extending 1.35 ft (0.41 m) down into the clay. Burrows are present throughout the underlying clay and generally increase in abundance downward. It is shelly and laminated in places and becomes harder downward. We interpret this clay interval as an offshore deposit representing outer inner to middle neritic paleodepths.
The nature of the surface at 557.5 ft (169.93 m) is not clear. Although it looks typical of many sequence boundaries in the coastal plain with a basal glauconite bed, the facies stacking pattern under it from 557.5 to 570.23 ft (169.83 to 173.81 m) is clearly transgressive, deepening upsection from upper shoreface to distal? upper shoreface to outer inner neritic or middle neritic offshore environments (Fig. F5). Thus, we interpret the surface at 557.5 ft (169.93 m) as a MFS, with the TST below. The MFSs identified thus far in the Bethany Beach borehole show much greater evidence for erosion than MFSs in New Jersey.
At 570.23 ft (173.81 m), the clays grade abruptly down to a muddy homogeneous sand with faint laminations and scattered chalky shell fragments with increasing bioturbation toward the top (Fig. F5). The sand is interpreted as a lower shoreface deposit. From 572.6 to 573.8 ft (174.53 to 174.89 m), the lithology changes to a shell bed with a poorly sorted quartz sand matrix. From 573.8 to 575.2 ft (174.89 to 175.32 m) is a cemented mollusk-rich sandstone with large shells (including Pecten) and a poorly sorted quartz sand matrix deposited in upper shoreface environments overlying a fine-medium slightly clayey sand (580-581 ft; 176.78-177.09 m) with common shell fragments deposited in upper shoreface environments. We interpret a sequence boundary below the sandstone at 575.2 ft (175.32 m), associated with a sharp gamma ray peak in an unrecovered interval (575.2-580 ft; 175.32-176.78 m).
Age: middle-late MioceneThe Choptank Formation was defined by Shattuck (1902) in Maryland as a division of the Chesapeake Group based on lithology and fossil content. Rasmussen et al. (1960) first applied the name Choptank Formation in Delaware to the part of the Miocene that includes important aquifers near the town of Milford. It is a sandier unit than either the overlying St. Marys Formation or the underlying Calvert Formation (Ramsey, 1997) and is characterized as interbedded fine to coarse sand, shell, silt, and some clay (Benson, 1990).
The contact between the St. Marys and Choptank Formations is placed at 575.2 ft (175.32 m) at the top of an unrecovered interval (575.2-580 ft; 175.32-176.78 m) across which the section changes from predominantly silty to significantly sandy (Fig. F6). From 580 to 581 ft (176.78 to 177.09 m) is a thin bed of fine to medium sand. This is underlain by a gravelly silty sand from 581 to 584 ft (177.09 to 178.00 m) that was deposited in upper shoreface to lower estuarine environments. Sediments from 584 to 593 ft (178.00 to 180.75 m) are laminated sand and sandy clayey silt with common shells and bioturbation disrupting laminae. These are interpreted as distal upper shoreface deposits, from a slightly deeper environment than the overlying gravelly sand. This is underlain by an interval of heavily bioturbated sands from 593 to 606.75 ft (180.75 to 184.94 m) that contains scattered clay laminae, discrete clay-lined burrows, rare cross laminae, and shell fragments. These sands reflect further deepening downsection from distal upper shoreface to lower shoreface. Overall, the entire interval from 581 to 606.75 ft (177.09 to 184.94 m) appears to comprise a shallowing-upward succession. This succession is underlain by a shell bed at 606.75 ft (184.94 m). The gamma log kick evident at this contact can be traced in southern Delaware, suggesting that it may be a sequence boundary (Fig. F6). However, there is little evidence in the Bethany Beach borehole for erosion at this level, and thus this surface may instead represent a flooding surface at the top of a parasequence that can be traced regionally.
The shell bed at the top of this succession (606.75-607.4 ft; 184.94-185.14 m) has abundant fragments of thick shells in a medium sand matrix and is underlain by medium sands down to 620 ft (188.98 m). Fine to very coarse sands predominate from 620 to 630 ft (188.98 to 192.02 m), passing back to medium sands from 630 to 635.5 ft (192.02 to 193.70 m). Shells are less common below the shell bed at the top, but fragments become more common toward the bottom of this interval. The sands from 606.75 to 616.15 ft (184.94 to 187.80 m) are clean and interpreted as upper shoreface deposits; sands below, from 616.15 to 635.5 ft (187.80 to 193.70 m), contain thin silt laminae and were deposited in slightly deeper distal upper shoreface environments (Fig. F6).
Between 635.5 and 645 ft (193.70 and 196.60 m), the section is silt interbedded with sand and scattered shells and becomes progressively silty downsection (Fig. F6). These deposits represent lower shoreface environments that progressively shallow upsection. Below the silt is shelly, silty, phosphatic sand (645-647.5 ft; 196.60-197.36 m) with a shell bed between 647.5 ft and 648.3 ft (197.36 and 197.60 m). The environment of deposition of the shell bed is uncertain; it could be an upper shoreface environment, implying deepening above it, or a mid-shelf condensed section (Kidwell, 1984), implying shallowing upsection. We consider it likely that a sequence boundary is present in the interval of no recovery between 648.3 and 649 ft (197.51 and 197.82 m); we place the boundary at 649 ft (197.82 m) associated with a minor gamma log peak. This placement implies that the shell bed represents part of the transgression above the sequence boundary.
Granuliferous silty sand with common phosphate and shell fragments is present from 649 to 662.5 ft (197.82 to 201.93 m) below the sequence boundary. These sands differ those above by the presence of shell fragments instead of whole shells and by their poorly sorted rather than moderately sorted character. An irregularly cemented zone is present from 656 to 661.5 ft (199.95 to 201.63 m), and a hard indurated layer is present from 661.5 to 662.5 ft (201.63 to 201.93 m). This interval represents deposition in upper shoreface to estuarine environments, reflecting shallower depths and higher energy than above.
Below a coring gap from 662.5 to 670 ft (201.93 to 204.22 m), the core is composed of finer sediments than above, with laminated sands and silts (670-686 ft; 204.22-209.09 m) fining downward to silt (686.3-687.0 ft; 209.18-209.40 m). Drilling disturbance obscures the section from 687.0 to 690 ft (209.40 to 210.31 m). The interval from 670 to 677.5 ft (204.22 to 206.50 m) reflects deposition in the upper shoreface (Fig. F6). From 677.5 to 690 ft (206.50 to 210.31 m), the section is interpreted as distal upper shoreface transitioning downsection to lower shoreface silts. Below that, the section is again sandy and consists of shelly medium sands to sandy shell layers that contain abundant phosphatized shell fragments between 690 and 694.1 ft (210.31 and 211.56 m) and are hard to weakly cemented in places. They represent deposition in lower shoreface environments.
Below these cemented sands, a facies change occurs in an unrecovered interval (694.1-700 ft; 211.56-213.36 m) associated with a minor gamma log increase and a major break in Sr isotopic ages (see "Strontium Isotope Chronostratigraphy"). A sequence boundary is placed at 698.5 ft (212.90 m). The section immediately under this inferred sequence boundary (700-703.2 ft; 213.36-214.34 m) is composed of shelly, heavily bioturbated, medium to fine silty sands. The environments above and below this inferred sequence boundary are interpreted as lower shoreface, but the facies under it differ in the greater degree of burrowing, the less cohesive texture, and the more common occurrence of glauconite and shells. The glauconite grains are generally the same size as or smaller than the quartz grains, possibly indicating that the glauconite grains are reworked. The sediments contain abundant sulfur, and many grains have a limonitic coating that may reflect subaerial exposure.
Under the burrowed bed at the top of the sequence the sands fine slightly to fine-grained and burrowing is less common (710-719.45 ft; 216.41-219.29 m). Indistinct silt laminae are present, but the finer material is mostly mixed with the sand by bioturbation. Between 720 and 750.3 ft (219.46 and 228.69 m) the lithology becomes finer, changing to fine to very fine slightly shelly burrowed sands interlaminated with silts (Fig. F6). This fining downsection represents a deepening downsection within lower shoreface (700-750.3 ft; 213.36-228.69 m) environments. Heavily burrowed, slightly clayey, less shelly silt predominates from 750.3 to 765 ft (228.69 to 233.17 m). The shift to silts at 750.3 ft (228.69 m) is abrupt and represents a change to offshore environments. Sands are burrowed down in the silt to 751.5 ft (229.06 m). Pseudononion pizarrensis dominates a generally low-diversity biofacies in most of this interval (721, 726, 751, 756, and 766 ft; 219.76, 221.28, 228.90, 230.43, and 233.48 m), indicating inner neritic (~30 m) paleodepths. A gamma log peak between 753 and 755 ft (229.51 and 230.12 m) is inferred as the MFS. The identification of the MFS is supported by recovery of an especially diverse foraminiferal fauna at 753.9 ft (229.79 m), suggestive of outer inner neritic to inner middle neritic paleodepths (~50 m).
Fine to very fine sands are interlaminated with silts from 765 to 765.9 ft (233.17 to 233.45 m). Shelly, extensively bioturbated fine to very fine sands predominate from 765.9 to 779.1 ft (233.45 to 237.50 m), becoming increasing shelly and sandy downsection. The section from 765 to 779.1 ft (233.17 to 237.50 m) was deposited in lower shoreface environments that shallow downsection. Interbedded cemented sands, sandy shell hash, and silty sands with scattered shells from 780 to 785.3 ft (237.74 to 239.36 m) were deposited in distal upper shoreface environments. Shells are concentrated in layers, and below 766 ft (233.48 m) some are dark and possibly phosphatized. Thin (0.2 ft; 6 cm) indurated zones are found between 776.5 and 792 ft (236.68 and 241.40 m). The section from 776.5 to 785.3 ft (236.68 to 239.36 m) is silty vs. cleaner sand below. From 785.3 to 787.1 ft (239.36 to 239.91 m), a silty, rubbly, shelly sand with fragments of cemented sandstone and sand steinkerns was deposited in upper shoreface environments. Thus, the section from 755 to 787.1 ft (230.12 to 239.91 m) deepens upsection, comprising the TST.
Clean, proximal upper shoreface/foreshore sands underlie a surface at 787.1 ft (239.91 m). We place a sequence boundary at 787.1 ft (239.91 m) based on (1) a change in stacking pattern from progradation/aggradational below to retrogradational above and (2) a sharp gamma log kick. The physical expression of the sequence boundary in the core is poorly expressed because the upper shoreface/foreshore sands are reworked/ripped up into upper shoreface environments (Fig. F3).
Clean, well sorted fine-medium sands with scattered silt and heavy mineral laminae (some cross laminae) continue from the sequence boundary at 787.1 to 804.55 ft (239.91 to 245.23 m), with interbedded indurated zones including a sandy molluscan limestone (792-793.5 ft; 241.40-241.86 m) (Fig. F6). A calcite-cemented shelly sandstone and interbedded fine sands was poorly recovered (805-812.5 ft; 245.36-247.65 m). It overlies a fine to medium quartz sand with shell debris (812.5-814.9 ft; 247.65-248.38 m). Below a coring gap (814.9-819 ft; 248.38-249.63 m), a calcareous sandstone (819-819.9 ft; 249.63-249.91 m) overlays a sandy, clayey silt (819.9-888.7 ft; 249.91-270.88 m). Silty sands above 819.9 ft (249.91 m) were deposited in lower shoreface environments as part of the upper HST; the silts below represent offshore environments and comprise the lower HST. The sands from 787.5 to 819.9 ft (240.03 to 249.91 m) correlate updip with the local Milford aquifer. The base of this sandy interval marks the base of the Choptank Formation and is underlain by siltier facies of the Calvert Formation.
Age: early-middle MioceneThe Calvert Formation was defined by Shattuck (1902) in Maryland as a division of the Chesapeake Group based on lithology and fossil content and was first recognized in Delaware by Rasmussen et al. (1960). It is characterized by interbedded silt, sand, and clay with common shells (Benson, 1990; Ramsey, 1997) and is overall finer grained than the overlying Choptank Formation. The Choptank/Calvert boundary is picked in the borehole at 819.9 ft (249.91 m) at the change from the predominantly sandy section to predominantly silt (Fig. F6).
Sandy, clayey, laminated (including cross laminations) to bioturbated silt with scattered to rare shells characterizes the upper Calvert Formation from 819.9 to 887.7 ft (249.91 to 270.57 m). This interval was deposited in inner neritic environments and comprises the lower part of the sequence identified in the lower part of the Choptank Formation. The section becomes finer downward in this interval, with sand decreasing from ~40% of the section to ~10% at 880 ft (268.22 m). The silts are indurated from 851.65 to 853.75 ft (259.58 to 260.22 m). The MFS is interpreted at 887.7 ft (270.57 m) at a burrowed contact. Below the MFS from 887.7 to 888.7 ft (270.57 to 270.88 m) and coring gap, 889-889.45 ft (270.97-271.10 m), the amount of shell and sand increases as the section becomes a darker, sandier shelly silt. Heavily bioturbated fine silty sand with rare to scattered shells is present from 889.45 to 897.7 ft (271.10 to 273.62 m). A heavily burrowed contact (897.7-898.0 ft; 273.67-273.71 m) separates silty fine sand above from medium sand below, with silty fine sand burrowed down to 899.2 ft (274.08 m). The contact is interpreted as a sequence boundary and is tentatively placed at 897.7 ft (273.62 m) at the level of a sharp gamma ray peak (Figs. F3, F6).
Cemented and unconsolidated sands characterize the upper HST of the underlying sequence (897.7-926.6 ft; 273.62-282.43 m) (Fig. F7). A number of calcite-cemented layers were encountered below 900 ft (274.32 m): 901.5-902.8, 911.35-914, 922.2-923.05, 924.1-926 (irregular), and 931.9-932.4 ft (274.78-275.17, 277.78-278.59, 281.09-281.35, and 281.67-282.24 m). The sands contain shells and shell fragments, with the indurated zones containing the most shell material. The sands were deposited in distal upper shoreface environments.
Clayey silty sands from 926.6 to 932.4 ft (282.43 to 284.20 m) contain thin-walled shells deposited in lower shoreface environments. Below a coring gap (932.4-940 ft; 284.07-286.51 m) is an interval of clayey sand representing distal upper shoreface environments (940.5-940.9 ft; 286.66-286.79 m). This is underlain by more proximal upper shoreface deposits that include silty sand with abundant shell debris (940.9-942.5 ft; 286.79-287.27 m) and granuliferous silty shelly sand (942.5-947 ft; 287.27-288.65 m), both of which include barnacles and bone phosphate. A large clay-infilled burrow is present at the contact between clayey sand and the shelly sand (940.9 ft; 286.79 m); the contact is interpreted as a flooding surface.
Another calcite-cemented sandstone was encountered from 945 to 945.3 ft (288.04 to 288.13 m), with partly cemented sand continuing to 947 ft (288.65 m). These are interpreted as upper shoreface deposits (Fig. F7). They are underlain from 947 to 950.5 ft (288.65 to 289.71 m) by silt with common interbedded shelly sands that represent distal lower shoreface to offshore environments. A contact at 950.5 ft (289.71 m) marks another FS where these lower shoreface silts overlie upper shoreface sands. From 950.5 to 953.5 (289.71 to 290.63 m), the section is silty sand with granule-sized fragments of thick shells and phosphate. This is underlain by clayey sand (953.5-954.9 ft; 290.63-291.05 m), with a thin interbedded shell hash (954-954.15; 290.78-290.82 m), and a cemented shell hash with sand (954.9-956.25 ft; 291.05-291.47 m), also interpreted as promixal upper shoreface deposits. The section fines downward. Silty sand from 956.25 to 958.45 ft (291.47 to 292.14 m) represents distal upper shoreface environments. From 960 to 964 ft (292.61 to 293.83 m), the section is laminated to slightly cross laminated sandy silt deposited in inner neritic/offshore/distal lower shoreface environments (Pseudononion pizarrensis biofacies; ~25-50 m paleodepth). Below 964 ft (293.83 m), the section becomes sandier and a secondary gamma log peak at 964 ft (293.83 m) is interpreted as an MFS. The underlying sediments are silty sands (964-967 ft; 293.83-294.74 m), passing to mottled tan and blue-gray clayey silts (967 ft; 294.74 m), and then to a sulfur-bearing silty interval (968-970 ft; 295.05-295.66 m). The environment of this section is enigmatic. It could be interpreted as representing lagoonal environments, based on the presence of sulfur, or as lower shoreface to offshore, based on stratigraphic succession and the interbedding of sand and silts. The presence of marine to marginal marine dinoflagellates supports an interpretation of lower shoreface to offshore environments (see "Biostratigraphy").
Very fine to fine sands dominate from 970 to 977.4 ft (295.66 to 297.91 m), with clayey and silty sand in the upper part (970-971.1 ft; 295.66-295.99 m) and silty sand below (Fig. F7). There are a few laminations in this generally well-bioturbated interval. These sands represent lower shoreface environments. They are underlain by slightly silty fine to medium sands with shells and shell fragments (980-981.3 ft; 298.70-299.10 m) that also represent lower shoreface environments. These silty sands pass downward into a calcite-cemented sand layer with shells (981.3-981.5 ft; 299.10-299.16 m) and a sandy shell hash (981.5-983.8; 299.16-299.86 m) deposited in upper shoreface environments. Between 983.8 and 995.1 ft (299.86 and 303.31 m), cemented shelly sands (983.8-985.0, 986.7-988.0, 991.65-992.3, and 994.0-995.1 ft; 299.86-300.23, 300.75-301.14, 302.25-302.45, and 302.97-303.31 m) alternate with shell hash facies (985-986.7, 988.0-991.65, and 992.3-994 ft; 300.23-300.75, 301.14-302.25, and 302.45-302.97 m). A shelly, slightly silty sand from 995.1 to 996.5 ft (303.31 to 303.73 m) has clay laminations. We interpret the facies from 996.5 to 981.3 ft (303.73 to 299.10 m) to represent upper shoreface environments, with a clear overstepping to distal lower shoreface environments at 981.3 ft (299.10 m).
It is clear that a sequence boundary should be picked somewhere in the turnaround of stacking patterns between the progradational section below 1000 ft (304.80 m) and the clear deepening above 970 ft (295.66 m). Based on deepening of paleoenvironments, a slight increase in gamma log values, and a clear decrease in resistivity values, we prefer to place a sequence boundary at 981.3 ft (299.10 m). The overlying section from 981.3 to 964 ft (299.10 to 293.83 m) would represent a TST succession deepening from lower shoreface to offshore environments. The section under the sequence boundary from 981.3 to 1000 ft (299.10 to 304.8 m) was deposited in upper shoreface environments as part of the HST of the underlying sequence.
The section from 1000 to 1050.85 ft (304.8 to 320.30 m) generally shows a fining and deepening trend downsection (Fig. F7). Well-sorted, clean medium-fine sand with scattered coarse shell fragments and interspersed clayey silt laminae (1000-1007 ft; 304.8-306.93 m) grades down to relatively homogeneous, predominantly fine silty sand with shell fragments and common thin clayey silt laminae (1007-1021 ft; 306.93-311.20 m). Heavily bioturbated, predominantly fine sands with numerous clay/silt laminations (1021-1046 ft; 311.20-318.82 m) are more heavily bioturbated in the upper part and more laminated in the lower, with a general increase in grain size upsection. These silty sands represent proximal lower shoreface environments. Under this, a shelly fine to very fine lower shoreface silty sand (1046-1048 ft; 318.82-319.43 m) overlies clayey silt with scattered sand laminae, scattered thin shell laminae, and scattered bioturbation (1048.0-1050.6 ft; 319.43-320.22 m) deposited in offshore environments.
A contact at 1050.6 ft (320.22 m) separates silts with sand laminae above from very fine to fine sand with scattered shell debris below (1050.6-1050.8 ft; 320.22-320.28 m). The surface at 1050.6 ft (320.22 m) is a FS, perhaps the MFS, and thus the overlying silts (1048-1050.6 ft; 319.43-320.22 m) would represent the lower HST and the sands above 1048 ft (319.43 m) the upper HST. Below the FS is a hard, slightly glauconitic, slightly shelly, silty fine sand (1050.8-1052.6 ft; 320.28-320.83 m). The sand grades to a hard, mottled, burrowed silt with scattered shells (1052.6-1054.5 ft; 320.83-321.41 m), a hard clayey silt with faint laminae (1054.5-"1055.45" ft; 321.41-"321.70" m), and a silty sand (1055-1056.35 ft; 321.56-321.98 m). This is underlain by a dolomite-cemented, silty, very fine sand (1057-1057.8 ft; 322.17-322.42 m) and a light-colored slightly sandy silt (1057.8-1057.95 ft; 322.42-322.46 m). The interval from 1050.8 to 1057.95 ft (320.28 to 322.46 m) appears to represent deposition in a distal lower shoreface setting. This interval comprises the TST of the sequence.
A distinct contact at 1057.95 ft (322.46 m) is heavily bioturbated and separates the indurated sands above from distinctly different mottled burrowed facies below (Fig. F7). A silty clay bed just below this surface (1057.95-1058.05 ft; 322.46-322.49 m) grades down to an underlying mottled yellow and gray clayey silty very fine sand with burrows surrounded by iron staining (1058.05-1058.2 ft; 322.49-322.54 m). This is underlain by a mottled silty very fine sand with 2%-3% glauconite (1058.2-1058.75 ft; 322.54-322.71 m) that weathers to a mottled rusty brown with gypsum crystals on the surface, followed by more mottled yellow and gray clayey silty very fine sand (1058.75-1059 ft; 322.71-322.78 m). Together, these features suggest that this interval (1057.95-1059 ft; 322.46-322.78 m) was deposited in distal lower shoreface environments, with the mottled appearance between 1058.05 and 1059.0 ft (322.49 and 322.78 m) as evidence for extended submarine or subaerial exposure.
We interpret the surface at 1057.95 ft (322.46 m) as a sequence boundary (Fig. F3). The sharp contact marks a stacking pattern change from regressive distal lower shoreface below to transgressive lower shoreface sands and offshore silts above. This is supported by the evidence of extended exposure just under this surface from 1057.95 and 1059 ft (322.46 and 322.78 m).
Burrowed silty very fine sand with scattered shell fragments (1059-1064 ft; 322.78-324.31 m) grades down to a sandy silt (1065-1109 ft; 324.61-338.02 m) (Fig. F7). These sands comprise the upper HST of the sequence. The section from 1077 to 1109 ft (328.27 to 338.02 m) consists of a laminated dark gray very fine sandy clayey silt. The fine sand decreases downsection, with small shell fragments increasing below 1086 ft (331.01 m) from rare to somewhat common. The section from 1109 to 1112.45 ft (338.02 to 339.07 m) becomes slightly sandy and shelly. Laminated silts with very fine sand containing scattered shell fragments and some foraminifers continue from 1112.45 to 1143.7 ft (339.07 to 348.60 m). Shells become rare toward the base of this section. Clay content increases downsection, becoming laminated clayey silt from 1143.7 to 1152 ft (348.60 to 351.13 m). At 1152 ft (351.13 m), there is a gradational change to a glauconitic, shelly, silty sand down to 1152.55 ft (351.30 m). Below an unrecovered interval (1152.55-1153.5 ft; 351.30-351.59 m) is a medium to coarse quartz sand with some granules that contains large broken shell fragments in the upper 0.5 ft (0.15 m) (1153.5-1154 ft; 351.58-351.74 m). The major shift in lithofacies between cores from silty glauconitic sand above to coarse quartz sand below associated with a large gamma ray log kick suggests that the contact was not gradational but abrupt and in the unrecovered interval (1152.55-1153.5 ft; 351.30-351.58 m). We interpret this shift as a sequence boundary (1153 ft; 351.43 m) separating TST silts above from HST sands below (Fig. F7).
A thick sand interval from 1153.5 to 1227 ft (351.58 to 373.99 m) is interpreted as a HST succession (Fig. F8). The uppermost part (1153.5-1170.4 ft; 351.58-356.74 m) consists of clayey, silty, coarse, heavily bioturbated quartz sands. Below this (1170.4-1184.9 ft; 356.74-361.16 m), clayey, silty sands are interbedded with cleaner, though poorly sorted, heavily bioturbated dry sands. These upper HST sands (1153-1184.9; 351.43-361.16 m) were deposited in upper shoreface environments. Silt increases from 1184.9 to 1187.5 ft (361.16 to 361.95 m), reflecting a slightly deeper water environment (proximal lower shoreface). Below an interval of no recovery (1187.5-1196.7 ft; 361.95-364.75 m), silty mottled medium sands with scattered flat to inclined silty clay laminae (1196.7-1201 ft; 364.75-366.06 m) also represent proximal lower shoreface environments.
Mottled, heavily bioturbated olive-gray very silty/clayey sands between 1210 and 1213.4 ft (368.81 and 369.84 m) contain thin (~0.2 ft; 6 cm) silt interbeds (Fig. F8). The section is siltier and somewhat clayey from 1213.4 to 1216.5 ft (369.84 to 370.79 m). The sands from 1210 to 1213.4 ft (368.81 to 369.84 m) were deposited in proximal lower shoreface environments, whereas the siltier section from 1213.4 to 1216.5 ft (369.84 to 370.79 m) was deposited in distal lower shoreface environments. The sands above 1216.5 ft (370.79 m) comprise the Cheswold aquifer farther north in Delaware. An interesting interval is present between 1216.5 and 1217.0 ft (370.79 and 370.94 m), where there is mixture of light-colored silts from above the contact (1216.5 ft; 370.79 m) and darker silts from below the contact. Clasts in this interval could be either indurated rip-up clasts or burrows. In either case, it is clear that this surface is one of marine omission and heavy bioturbation. We interpret this surface as a FS (parasequence boundary), though not the MFS of the sequence between 1153 and 1421.14 ft (351.43 and 401.56 m).
Homogenous olive-gray very muddy sands extend from 1217 to 1225.7 ft (370.94 to 373.59 m) and were deposited in distal lower shoreface environments (Fig. F8). Grain size gradually decreases in this interval, with scattered shells in the lower part. Heavily bioturbated, sparsely shelly sandy silts with traces of glauconite throughout predominate from 1225.7 to 1343.7 ft (373.59 to 409.56 m), marking the top of offshore deposits representing the lower HST. The silt is sandier near the top (up to ~40% sand), with fine sand generally decreasing downsection to ~1315 ft (400.81 m), reflecting shallowing upsection in offshore environments. The silt is broken by a number of interesting horizons. A ~1-ft-thick (0.30 m thick) calcite-cemented indurated zone (1237.2-1238.15 ft; 377.10-377.39 m) is composed of sandy silt similar to the unconsolidated material above and below. A 0.1-ft (3 cm) sand bed was recovered from 1272.25 to 1272.35 ft (387.78 to 387.81 m), and very thin sand stringers are present below 1280 ft (390.14 m). An interval of very abundant shells is present between 1291 and 1293.7 ft (393.50 and 394.32 m), with the main clayey shell bed present between 1292 and 1293.2 ft (393.80 and 394.17 m). A surface at 1317.45 ft (401.56 m) is present just below a 0.15-ft-thick (4.5 cm thick) cemented sand layer that is overlain by a condensed interval between 1315.8 and 1317.3 ft (401.06 and 401.51 m). The condensed interval is marked by abundant shells and phosphatized burrows. Dark olive-gray silts return below the sharp contact at 1317.45 ft (401.56 m). This contact may be either a FS of a thick sequence between 1153 and 1421.3 ft (351.43 and 433.21 m) or a higher-order sequence boundary.
Below the surface at 1317.45 ft (401.56 m), sandy silts predominate and the section shoals downward from offshore to distal lower shoreface. The section from 1317.45 to 1320.3 ft (401.56 to 402.43 m) is heavily bioturbated, irregularly indurated silt with thin interlaminated sand. Dolomite-cemented, slightly shelly siltstone is present between 1320.3 and 1320.8 ft (402.43 and 402.58 m). Laminated sandy silts (1320.8-1322.9 ft; 402.58-403.22 m) overlie heavily bioturbated, slightly micaceous sandy silts with a few thin sand laminae and rare shell fragments (1322.9-1343.75 ft; 402.95-409.58 m). Burrows in this section tend to have sandy fill. Silts below ~1320 ft (402.34 m) appear to be slightly richer in fine-grained glauconite (Fig. F8). The interval from 1343.75 to 1355 ft (409.58 to 413.00 m) consists of bioturbated homogeneous sandy silt to silty sand. From 1355 to 1400 ft (413.00 to 426.72 m), laminae are better preserved in silts with interlaminated very fine sands that display low-angle cross laminations and truncated lamination. We interpret the section from 1317.45 to 1343 ft (401.56 to 409.35 m) as representing offshore environments and the section from 1343 to 1393 ft (409.35 to 424.59 m) as distal lower shoreface environments, though the transition between the two is gradual. The section from 1393 to 1400 ft (409.35 to 426.72 m) is interpreted as offshore.
The section from 1400 to 1420 ft (426.72 to 432.82 m) deepens downsection (Fig. F8). Homogeneous, faintly laminated, micaceous, slightly sandy silt (1400-1410 ft; 426.72-429.77 m) overlies silts with common (every 0.5-1.5 ft; 0.15-0.45 m) clay laminae, some sandy laminae, and more abundant glauconitic burrows (1410-1420 ft; 429.77-432.82 m). These sediments represent offshore environments. Foraminifers in a sample at 1411.0 ft (430.07 m) suggest depths from deep inner neritic to middle neritic (~30-50 m). High values are evident on the gamma log at the base of this unit between 1418 and 1420 ft (432.21 and 432.82 m), reflecting the increasing clay and glauconite comment. The MFS is interpreted to be somewhere in this interval, probably associated with a secondary gamma log peak at 1418 ft (432.21 m), supporting the interpretation of the surface at 1317.45 ft (401.56 m) as a FS rather than a MFS.
We place the base of the Calvert Formation at the top of the glauconitic sandy silt at 1420 ft (432.82 m) (Fig. F8). As recognized in Delaware, the Calvert Formation is a unit of silts and sands that is finer grained and less shelly than the overlying Choptank Formation. Generally, the Calvert Formation has no more than trace glauconite. Where more abundant glauconite is found in the lower Miocene section, it has been separated as different units, such as the "Unnamed Glauconitic Sand" and "Unnamed Glauconitic Silt" of Benson (1990). The glauconitic sandy silt at 1420 ft (432.82 m) at Bethany Beach provides a distinct lithologic and gamma log break and thus a reasonable place to locate the formational boundary. It is also worth noting that the lower part of the Calvert Formation at this site (from ~1290 to 1420 ft; 393.19 to 432.82 m) appears to correlate to the significantly more glauconitic facies of the "Unnamed Glauconitic Sand" of Benson (1990) in the Oh25-02 well, which is ~10 mi north near Lewes, DE.
Age: early MioceneThis unnamed lithologic unit consists of clayey glauconitic sands, clayey glauconitic silts, clay, and glauconitic clay (Fig. F8). The top of this unit is a thin, glauconitic, heavily bioturbated silt from 1420 ft (432.82 m) to a sequence boundary at 1421.1 ft (433.15 m). Burrows containing glauconitic clay extend ~1 ft (0.30 m) below the boundary. Below the sequence boundary at 1421.1 ft (433.15 m) is an olive, faintly laminated clay unit that extends to 1428.3 ft (435.35 m). These are offshore deposits; foraminifers at 1422.0 ft (433.54 m) suggest water depths of ~50 m. A contact zone between 1429.0 and 1431.25 ft (435.56 and 436.25 m) is disturbed by drilling in the upper 0.7 ft (0.21 m); the lithology of the zone is indurated clayey glauconitic sand. The glauconite sand appears in situ down to 1430.5 ft (436.02 m), with reworked clay above and piped-down glauconite sand below. We interpret the contact at 1430.5 ft (436.02 m) as a lower Miocene sequence boundary (Fig. F3). The thin sequence from 1421.1 to 1430.5 ft (433.15 to 436.02 m) thus consists of a thin glauconite TST and a silty lower HST; the upper HST sands appear to be truncated/absent.
The sequence boundary at 1430.5 ft (436.02 m) and underlying reworked contact zone (down to 1431.25 ft; 436.25 m) overlie a hard burrowed silty clay with disseminated shells and shell debris (1431.25-1447.35 ft; 436.25-441.15 m). This also represents offshore deposition, with foraminifers suggesting deepening downward from middle neritic depths (50-75 m) near the top of the sequence to deeper middle neritic or even outer neritic depths (80-100 m) in the lower part. Glauconite is burrowed down into this unit to 1432.4 ft (436.60 m) and reappears in trace abundances in laminated clayey silts (1432.4-1453 ft; 436.60-442.87 m), increasing downsection as the section changes to clayey glauconitic sands (1453-1454.5 ft; 442.87-443.33 m). The clayey glauconitic sands contain weathered brown glauconite/goethite, typical of Oligocene sediments in New Jersey (e.g., Miller et al., Chap. 2, this volume) and may represent part of a thin TST. We tentatively place a MFS at a secondary gamma log peak at 1445 ft (440.43 m).
A burrowed contact associated with a major gamma log peak is interpreted as a sequence boundary at 1454.5 ft (443.33 m) (Fig. F8). Clayey glauconite sand overlies glauconitic clay (1454.9-1457 ft; 443.45-444.09 m) at this surface, with large burrows of glauconite sand continuing down into the clay. The clay grades down into a clayey glauconite sand at 1457.0 ft (444.09 m) with "tricolored" (green, black, and brown) glauconite grains. Brown glauconite grains become less prominent or disappear and the glauconite becomes fine to medium grained below 1461.8 ft (445.56 m). The clays at the top of the sequence are interpreted as the HST and the underlying clayey glauconitic sand as the TST, making the contact at 1457.0 ft (444.09 m) the MFS. The base of the sequence is marked by a prominent sequence boundary at 1465.7 ft (446.75 m) that separates black glauconite sand above from a dark olive-gray foraminiferal clay below.
Age: OligoceneThe section at the bottom of the hole (1465.7-1467.95 ft; 446.75-447.43 m) consists of a thinly laminated dark olive-gray foraminiferal clay (Fig. F8). The clay was deposited in an offshore setting, with foraminifers suggestive of an outer neritic paleoenvironment. It is lithologically similar to the Absecon Inlet Formation in New Jersey (upper Eocene) (Browning et al., 1997). Foraminiferal and Sr isotopic results indicate it is lower upper Oligocene.