The on-site scientific team provided preliminary descriptions of sedimentary texture, structure, color, fossil content, identification of lithostratigraphic units (NJGS Information Circular 1, 1990), and lithologic contacts (Table T1; Figs. F4, F5, F6, F7). Subsequent studies integrated preliminary descriptions with additional descriptions, biostratigraphy (nannoplankton and pollen), and the gamma ray 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 breaks. For the nonmarine and nearshore sections in the Potomac and Magothy Formations, lithofacies interpretations provide the primary means of recognizing unconformities and interpreting paleoenvironments; this information is supplemented by palynostratigraphy.
Cumulative percent plots of the sediments in the cores were computed from washed samples. Each sample was dried and weighed before washing, and the dry weight was used to compute the percentage of sand. 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.
New Jersey coastal plain strata were deposited on the trailing edge of a passively subsiding continental margin (e.g., Grow and Sheridan, 1988). Thermoflexural subsidence and sediment loading along a hinged margin has dominated the creation of accommodation space (e.g., Kominz et al., 1998). The depositional environment of the Lower Cretaceous to basal Upper Cretaceous (Barremian–lowermost Cenomanian) Potomac Formation was predominantly terrestrial, whereas the Upper Cretaceous to Miocene was primarily marine. Previous studies have interpreted the sediments of the Potomac Formation as having been deposited in a nearshore delta plain or coastal plain setting, and this deltaic influence continued into the marine sediments of the Late Cretaceous and Miocene (Owens et al., 1970).
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. The finer-grained units are in many intervals heavily overprinted by soil-forming processes. These range from light blue and gray gleyed soils indicating soil formation in reduced conditions to dark red (lateritic?) soils indicating oxidized conditions. These silty clays and clayey silts were probably deposited as heavily vegetated overbank deposits. Variations in color and downhole logs (Figs. F4, F5, F6, F7) are found on the 2- to 3-ft (60–90 cm) and 10-ft (3 m) scales; these cycles appear to reflect differences in evaporation and precipitation with red oxidized sediments (high gamma log values) alternating with gray or gleyed reduced sediments (low gamma log values). This cyclicity not only reflects regional climate changes, its regularity implies a global imprint on climate and precipitation (e.g., Milankovitch periodicities).
Owens et al. (1970) interpreted the facies of the Potomac Formation as being deposited in a delta plain and delta front. His facies model for the Potomac Formation interpreted sand facies as fluvial-dominated delta front deposits (Fig. F3B). We consider that the Potomac Formation was deltaically influenced but also propose that the depositional environment encompasses elements of an anastomosing river environment, (Fig. F3A) corresponding well with much of the observed Potomac facies (Figs. F3A, F5, F6, F7).
Anastomosed systems are streams that are divided into multiple coexistent channels that have stable islands or bars separating the channels (Smith and Smith, 1980). Bar stability is provided by either vegetation or fine-grained sediments or both working together (Makaske, 2001). Anastomosed systems differ from braided systems in that the channel and bar stability in the anastomosed system prevents the river from reworking sediments and organic-rich sediments. As a result, anastomosed systems are dominated by fine floodplain deposits and organic-rich sediments or coals. Channel sands in the anastomosed system are generally more confined and tend to aggrade (Makaske, 2001). Anastomosing rivers are believed to form by avulsion splitting the primary flow into two or more channels. Avulsion is facilitated by a low-gradient floodplain and by rapidly elevating baselevel control downstream (Smith and Smith, 1980). Rapid sea level rise in the Holocene is considered a factor in elevating base level, leading to anastomosis (Makaske, 2001), and such a model may be applicable to the mid-Cretaceous at Fort Mott. Because anastomosing rivers are aggradational features, their chances for preservation are good.
Major environments and sedimentary facies for the anastomosing river system were described by Smith and Smith (1980), and we follow their classification (Fig. F3A). Two major environments are found in an anastomosed system. The channel facies consists of gravel and coarse sand (Fig. F3A). In contrast, fine-grained overbank deposits compose the majority of the sediments (Fig. F3A). Levees flanking the rivers consist of sandy silt and silty sand containing roots. Low areas between levees have lakes and peat bogs or back swamps; they are differentiated based upon the frequency with which they receive water and their connection to the main channels. Lakes are intermittently supplied with fine sediment including laminated clay and silty clay with sparse organic matter. Peat bogs and back swamps have no direct connection to the main channels and may receive clastic sediments only during times of flood. Sediments are organic-rich (up to 98% organic matter) silty clay and clayey silt. Finally, crevasse splays are recognized as thin layers of sand and fine gravel rarely more than 40 cm thick (Smith and Smith, 1980).
An outstanding question is the extent to which the anastomosing environment includes the upper delta plain environments. Many modern workers now consider the channels in a deltaic environment to be fundamentally similar to anastomosing channels in a continental setting (see Makaske, 2001, for discussion). Thus, the contrast with the deltaic model outlined by Owens et al. (1970) is minor except for the interpretation of the sand bodies.
The anastomosing model (Fig. F3A) does not adequately explain the thick sand bodies at the base of the Potomac Formation Units 3 and 2 (290.65–363.6 ft [88.59–110.83 m] and 555.1–599.7 ft [169.19–182.79 m], respectively). These sand bodies appear to be laterally continuous and may extend from New Jersey into Delaware. They can be traced on gamma logs within New Jersey a distance of 24 mi (40 km). Such widespread sand sheets are difficult to explain using the anastomosing stream model. The thick widespread nature of sands at the base of Potomac Unit 3 have the geometry of a delta front shoreline sand, although no direct marine evidence has been uncovered within them. If this interpretation is correct, then at least for Potomac Unit 3 the sediments may represent a transgressive–regressive cycle, with delta front sands (Fig. F3B) being prograded over by an anastomosed river system or delta plain deposits, including lower delta plain interdistributary lakes, marshes and swamps, and upper delta plain deeply weathered soils.
Below a fill typified by coal (1.0–1.5 ft; 0.30–0.45 m), fine–medium laminated silty quartz sand (1.5–2.1 ft; 0.45–0.64 m) and heavily burrowed sandy silt (6.0–7.85 ft; 1.83–2.39 m) were deposited in estuarine environments (Fig. F4) similar to the adjacent Delaware River estuary. Fine sands with interbedded silty clays appear from 7.85 to 8.7 ft (2.39 to 2.65 m) and overlie a gravelly sand (9.9–11.1 ft; 3.02–3.38 m) that coarsens downsection to gravel (16.2–16.4 ft; 4.93–5.00 m). This pattern of gravel grading up to sand repeats at 16.4–19.6 ft (5.00–5.97 m) based on the gamma log and is interpreted as channels in an estuary. A soft twisted clay (20–20.9 ft; 6.10–6.37 m), reflecting some coring disturbance, overlies orange-colored slightly sandy silt (20.9–21.4 ft; 6.37–6.52 m), returning to gray silty clay (21.4–21.6 ft; 6.52–6.58 m). Silty clay (25.0–26.4 ft; 7.62–8.05 m) grades down to silty, slightly clayey, micaceous laminated sand that becomes coarser downsection from 26.95 to 27.7 ft (8.21 to 8.44 m) to a gravel channel fill at 27.5–27.7 ft (8.38–8.44 m). A sharp contact at 27.7 ft (8.44 m) separates the gravel from laminated silty clay of the Cretaceous below.
The complex juxtaposition of channel gravels, cross-bedded to laminated sands, sandy silts, and burrowed clays is typical of estuarine environments. The burrowed clays, silts, sands, and gravels between 1.5 and 27.7 ft (0.46 and 8.44 m) at Fort Mott, New Jersey, are interpreted as highstand "Delaware Bay" deposits. In New Jersey, these sediments are generally assigned to the Cape May Formation (Newell et al., 2000). Across the Delaware River, this unit would be assigned to the Lynch Heights or Scotts Corner Formation (Ramsey, 1993).
A major lithologic change appears at an erosional surface at 27.7 ft (8.44 m), separating poorly sorted sands above (the Pleistocene–Holocene? Cape May Formation) from silty, micaceous, laminated Cretaceous clays below (Fig. F4). Clays from 27.7 to 32 ft (8.44 to 9.75 m) are weathered to a light gray from the typical dark greenish clays below 32 ft (9.75 m); they are slightly siltier in the top 0.5 ft (0.15 m). The color difference appears to be because of subaerial exposure of these marine clays. The clays continue to 35.9 ft (10.94 m), contain rare pyrite and flakes of plant debris, display faint silty laminae, are slightly micaceous, and are burrowed with 2- to 10-mm-diameter sand-filled burrows. The clays are siltier and slightly sandier near the base. A thin glauconite sand (35.9–36.1 ft; 10.94–11.00 m) is mixed with silty clay and concretions.
A distinct contact at 36.1 ft (11.00 m) separates glauconite sand above from quartz sand below and is interpreted as a prominent Upper Cretaceous sequence boundary separating a glauconitic clay from silty upper HST sand. The section from 27.1 to 36.1 ft (8.26 to 11.00 m) appears to represent a distinct sequence (Figs. F2, F4). It overlies sands assigned to the Englishtown Formation and clays assigned to the Woodbury Formation below, and thus could be the equivalent of the lower clayier facies of the "upper" Englishtown Formation, or a clayey, less glauconitic facies of the Marshalltown Formation.
Micaceous, clayey-silty fine to medium quartz sand with plant debris (36.1–38.7 ft; 11.00–11.80 m) composes the HST of a sequence that is assigned to the lower Englishtown Formation. Burrowed, micaceous, very silty sands to sandy silts (upper HST; 50–60 ft; 15.24–18.29 m) appear below an interval of poor recovery (38.7–50.0 ft; 11.80–15.24 m). These finer-grained sediments reflect a downward-fining succession. The boundary with the underlying Woodbury Formation is tentatively placed at the downhole appearance of >50% silt and clay (Fig. F4) at ~60 ft (18.29 m), although this contact is admittedly gradational and could be placed slightly higher upsection.
Micaceous silts and clays with sand laminae below 60 ft (18.29 m) are tentatively assigned to the Woodbury Formation, although previous mapping suggested that the Woodbury pinches out along strike north of Fort Mott (Owens et al., 1970). Silts and clays that we assign to the Woodbury Formation at Fort Mott may be equivalent to a finer-grained facies of the Englishtown Formation (Owens et al., 1970) (Fig. F4); this facies has been previously assigned to the Merchantville Formation in New Jersey at outcrop (Owens et al., 1970) (Fig. F4) and Delaware (Benson and Spoljaric, 1996). The assignment of this unit at Fort Mott to Zone CC19 (Fig. F4) agrees with correlations to the Woodbury sequence (Miller et al., 2004).
A trace of shells appears in the very micaceous, slightly clayey silts with burrowed sand laminae, between 60 and 63.6 ft (18.29 and 19.39 m). The sediments continue to fine downsection, with very clayey silts with scattered sand laminae (70–90 ft; 21.34–27.43 m) to predominantly silty clay with fewer sand laminae (below 90 ft; 27.43 m) and a trace of shell material; the mica also becomes finer grained downsection. The sediments are highly bioturbated, with burrows filled with very fine sand. Traces of glauconite appear at 94 ft (28.65 m) and become common (2%–5% of the core) below 99 ft (30.18 m). Glauconite sand increases at 100.7 ft (30.69 m), where there is concentration of gypsum on the core surface (100.5–100.6 ft; 30.63–30.66 m), an ammonite? fragment (100.7 ft; 30.69 m), and a minor gamma ray peak (Fig. F4); the contact with the underlying Merchantville Formation is gradational and a maximum flooding surface (MFS) is placed at this point. The Woodbury Formation at Fort Mott was deposited in a prodelta environment. Calcareous nannoplankton Zone CC19 (Campanian) was identified from the Woodbury Formation at 61.5, 71.1, and 73.6 ft (18.8, 21.67, and 22.43 m; see "Biostratigraphy").
The Merchantville Formation in the Fort Mott borehole is mainly a glauconitic, slightly quartzose silt to glauconite silty clay to glauconite-quartzose silty sand (Fig. F4). The section from 100.7 to 108 ft (30.69 to 32.92 m) consists of glauconitic-rich clay layers interbedded with less glauconitic clay. The unit is heavily burrowed with prominent large clay-lined burrows. Between 100.7 and 105 ft (30.69 and 32.00 m), large (5 mm–5 cm) indurated brown clay burrows/clay blebs (weathered shell material?) yield a mottled brown-green aspect to the core; less common, smaller brown burrows are found from 105 to 108 ft (32.00 to 32.92 m), and scattered, small brown burrows continue below this. Below 104.0 ft (31.70 m), the section consists of glauconitic, quartzose, very silty, slightly clayey sand with sandy burrow fills that continues to 113.3 ft (34.53 m); the section is heavily burrowed and has less glauconite, less clay, and fewer brown clay burrows than above. This suggests a general transgression from 104 to 113.3 ft (31.70 to 34.53 m), a condensed section from 104 to 100.7 ft (31.70 to 30.69 m), and an MFS at 100.7 ft (30.69 m). The Merchantville Formation at Fort Mott was deposited in neritic (middle?) environments. No age-diagnostic nannofossils or pollen were identified.
At 113.3 ft (34.53 m), there is a shift from glauconitic silt/silty sand above to glauconite-free silt below (Fig. F4). Glauconite is burrowed down to 114.25 ft (34.82 m), and it was difficult to pick out a distinct contact until the core was split. This silt is assigned to the Cheesequake Formation of Litwin et al. (1993) based on lithologic similarity and stratigraphic position between the Merchantville and Magothy Formations. The Cheesequake Formation consists of a single sequence containing a very micaceous, highly bioturbated "blue" silt; traces of glauconite appear downsection at 118.5 ft (36.12 m) and increase downsection to a highly burrowed, slightly glauconitic silt with clay-lined burrows at the base. Small granules and pebbles on the outside of the core appear to be washed downhole except for pebbles at the base of the succession below 119 ft (36.27 m). These pebbles were probably reworked from the Magothy Formation. (A shark tooth was found at 121.6 ft; 37.06 m.) The Cheesequake Formation at Fort Mott was deposited in neritic (inner–middle?) environments. It composes a truncated sequence with a thin (<5 ft; 1.52 m) TST consisting of glauconitic silt and a thicker (>7 ft; 2.13 m) lower HST consisting of silt; upper HST sands are absent at Fort Mott. In this region sands reported from the "lower Merchantville Formation" near Camden (Farlekas et al., 1976) are considered HST deposits. A chert pebble that blocked the core barrel at 124.8 ft (38.04 m) may mark the contact with the underlying Magothy Formation.
The Magothy Formation was first encountered at 130 ft (39.62 m) below a coring gap (124.8–130 ft; 38.04–39.62 m), where it consists of moderately well sorted, lignitic, medium quartz sand. We place the contact at 125 ft (38.10 m) based on the gamma and resistivity log signature (Fig. F4) that indicates sands below ~125 ft (38.10 m). Sands are broken by a thin gray clay (136.0–137.0 ft; 41.45–41.76 m) with thin sand interbeds and lignite beds (136.1–136.3 and 136.8–136.85 ft; 41.48–41.54 and 41.70–41.71 m). A pyrite nodule is present at 136.7 ft (41.67 m). Sands similar to above recur at 137.0–137.3 ft (41.76–41.85 m), with a dark gray clay layer from 137.3 to 137.9 ft (41.85 to 42.03 m), switching back to sands from 137.9 to 138.2 ft (42.03 to 42.12 m), a clay from 138.2 to 138.4 ft (42.12 to 42.18 m), a coring gap from 138.4 to 140 ft (42.18 to 42.67 m), interlaminated lignitic clay and fine–medium sand from 140 to 140.9 ft (42.67 to 42.95 m), and a granular medium–very coarse sand bed (140.9–141.1 ft; 42.95–43.01 m) with lignitic chunks at a distinct contact at 141.1 ft (43.01 m).
The very thin Magothy Formation (16.1 ft; 4.91 m) here appears to compose one sequence (Magothy II) (Miller et al., 2004) deposited in fluvial (lower delta plain?) or upper estuarine environments. The Raritan Formation and its downdip equivalent, the Bass River Formation, are cut out at Fort Mott, with the Magothy Formation directly overlying the Potomac Formation. The Magothy Formation at Fort Mott is assigned to pollen Zone V of Christopher (1982), which is generally correlated with the upper Turonian–Santonian; the absence of stratigraphically higher advanced Normapolles suggests late Turonian.
The Potomac Formation can be lithostratigraphically subdivided into at least three distinct units based on successions of medium to fine quartz sands overlain by fine-grained units (clay and silty clay). The highest, Potomac Unit 3, is well defined at its base (363.6 ft; 110.83 m) by the contact of the "middle aquifer" sands with an underlying confining unit (Figs. F2, F5). The base of the middle unit, Potomac Unit 2, could be placed at the base of the "lower aquifer" (599.7 ft; 182.79 m) or slightly lower at the base of an unnamed sand (644.1 ft; 196.32 m). The lowest, Potomac Unit 1, also has a medium to fine quartz sand that represents the lowest section recovered at Fort Mott.
The Potomac Formation elsewhere in the Atlantic coastal plain is biostratigraphically subdivided using palynology into Zone I (?Barremian–Aptian), Zone II (Albian), and Zone III (Albian–lower Cenomanian). At Fort Mott, there is a general correspondence of the lithostratigraphic zones defined in this study to the biostratigraphic zones of the same number. The possible exception is the placement of the base of Unit 2 and with respect to the base of Zone II. The base of lithostratigraphic Unit 2 is most readily placed using lithostratigraphic criteria at the base of the distinct "lower aquifer" sands (599.7 ft; 182.79 m). However, pollen suggestive of Subzone IIB (see "Pollen" in "Biostratigraphy") is found below the "lower aquifer" sands to 641.1 ft (195.41 m). Based on this pollen evidence, the boundary between Units 1 and 2 is placed at a lithologic break at 644.1 ft (196.32 m), where the lithology shifts from sandy overbank deposits above to paleosols below.
The top of the Potomac Formation was placed at 141.1 ft (43.01 m) at an abrupt contact between gray granular lignitic sands of the Magothy Formation and underlying very fine to fine, silty, slightly micaceous sands. There is little evidence of reworking at the contact, although the basal Magothy sands appear to be a lag deposit and irregular laminations in the basal Magothy Formation appear to result from bioturbation. The contact shows a minor increase in gamma log values at 141 ft (42.98 m), with the largest increase present at ~148 ft (45.11 m) (Fig. F5). At Fort Mott, clays and silty clays with subordinate silts, clayey silts, and very fine sands dominate the Potomac Formation. The clays are generally gray with red mottling. The general environment of deposition is interpreted as an anastomosing river system (Fig. F3A) with various subenvironments represented, including levee, lake, swamp, crevasse splay, and fluvial channel deposits (in order of decreasing abundance). The thicker sand deposits may also have formed in a delta front environment; based on their thickness (~60 ft or 18.29 m) and lateral extent (over 24 mi or 40 km), these deposits may represent distributary-mouth bar deposits. If this interpretation is correct, the overall facies model may be reinterpreted as fluvial-deltaic environments, with delta front deposits underlying lower and upper delta plain deposits (Fig. F3B).
The top of the Potomac Formation at Fort Mott consists of laminated sands (141.1–148 ft; 43.01–45.11 m). The section from 141.1 to 141.4 ft (43.01 to 43.10 m) consists of whitish tan, very fine grained, slightly micaceous, laminated sand, whereas the section between 141.4 and 141.6 ft (43.10 and 43.16 m) consists of interlaminated clayey silty red and white laminated sands. Laminated fine sands from 141.6 to 143.3 ft (43.16 to 43.68 m) are slightly coarser and cleaner than above, with clay rip-up clasts appearing at 143.0–143.3 ft (43.59–43.68 m). Whitish tan, very fine grained, slightly micaceous, laminated sands (145.0–148.0 ft; 44.20–45.11 m) return below a coring gap (143.3–145.0 ft; 43.68–44.20 m), with thin reddish clay laminae (145–146 ft; 44.20–44.50 m) and cross-bedded laminae in rusty sand (146–146.3 ft; 44.50–44.58 m); below 145.6 ft (44.38 m), the sand becomes slightly siltier and sphaerosiderite nodules appear, perhaps reworked from below. From 146.3 to 147.5 ft (44.58 to 44.96 m), the section consists of tan, reddish, and gray sand that has an increasing abundance of sphaerosiderite nodules downsection to 147.8 ft (45.05 m); the sphaerosiderite nodules are fused into larger concretions at 147.4 ft (44.93 m). From 147.8 to 148.0 ft (45.05 to 45.11 m), sphaerosiderite nodules decrease and clay increases down to a clay layer at 148.0 ft. This contact has small concretions, thin bands of rusty stain, and is associated with a major gamma ray decrease upsection (Fig. F5).
The sequence and environmental significance of these laminated sands is unclear: (1) it is unclear if this sand is a local overbank environment (e.g., crevasse splay), a local fluvial meandering sand, or a regionally significant sand; and (2) although the sands are bracketed by abrupt erosional contacts, the lower surface may be a local erosional feature of a fluvial/alluvial plain environment, whereas the upper surface is clearly the contact with the Magothy Formation and has regional significance. The interval at 125–148 ft (38.10–45.11 m) is correlative with the upper aquifer within the PRM aquifer system (Fig. F2). This aquifer, a major water producer to the north–northeast in Salem and Gloucester Counties, is a minor aquifer here, as well as to the south. For example, wells at Fort Mott and adjacent Finns Point are screened in the aquifer below (291.4–310.6 ft; 88.82–94.67 m), within the Potomac Formation (middle aquifer of the PRM aquifer system).
Slightly silty clay with hard concretions present from 148 to 149 ft (45.11 to 45.42 m), and 150 to 153 ft (45.72 to 46.63 m) is mostly light gray with mottling of greenish gray and dusky red. A gray clay (153–153.5 ft; 46.63–46.79 m) and very fine sand with orange mottling (153.5–154.3 ft; 46.79–47.03 m) overlie a thicker interval of interlaminated silty clay and very fine sand with scattered cross laminations (154.3–164.2 ft; 47.03–50.05 m) and indications of deformed laminae (soft sediment deformation?) (155–158 ft; 47.24–48.16 m). The section at 157–158 ft (47.85–48.16 m) is darker gray than above or below with scattered lignite and may be more organic rich. The mottled clays and interlaminated clay-sands were deposited in subaerial overbank and subaqueous overbank (e.g., swamp) environments, respectively.
A thick interval of interfluvial mottled clay paleosols appears below the interlaminated clay and sand. Whitish gray to light gray silty clay with clayey silt laminae and red mottles are present from 164.2 to 256.8 ft (50.05 to 78.27 m). The silty clay contains dusky red zones with concretions (172.6, 173.5, and 175.0 ft; 52.61, 52.88, and 53.34 m), which include features resembling root traces and drab halos around root traces (164–172.6 ft; 49.99–52.61 m; spanning a coring gap from 167–170 ft; 50.9–51.82 m). From 180 to 182 ft (54.86 to 55.47 m), the light gray silty clay contains thin olive and red bands and sphaerosiderite nodules with reddish rinds (?hematite). From 182 to 195.7 ft (55.47 to 52.61 m), the whitish gray silty clay is banded with hematitic red silty clays with sphaerosiderite nodules and a few root traces (190–191, 192.5–193.5, and 195–195.3 ft; 57.91–58.22, 58.67–58.98, and 59.44–59.53 m). The section at 195.7–197.7 ft (59.65–60.26 m) is a mostly red and olive-brown mottled silty clay, whereas that at 197.7–200.2 ft (60.26–61.02 m) is a banded light gray and olive-gray silty clay. Mottled red and gray very fine sandy silt with sphaerosiderite nodules with reddish rinds (201.8–203 ft; 61.51–61.87 m) breaks an otherwise monotonous series of clays. The interval from 203.8 to 215.2 ft (62.12 to 65.59 m) is a banded to mottled red (orange and dusky) and white silty clay with sphaerosiderite nodules. Red silty clay with thin whitish bands (215.3–217.2, 219.4–220.0, and 223–223.7 ft; 65.62–66.20, 66.87–67.06, and 67.97–68.18 m) alternate with the banded to mottled red (orange and dusky) and white silty clay with sphaerosiderite nodules. At 223.7 ft (68.18 m), there is a surface overlying intensely mottled whitish silty clay that shows signs of breccia-like fabric (223.7–225.6 ft; 68.18–68.76 m). Red and white mottled silty clays that persist from 225.6 to 248.2 ft (68.76 to 75.65 m) can be split into more intensively mottled redder intervals (227.0–230.3, 234.0–236.5, and 247.7–248.2 ft; 69.19–70.20, 71.32–72.09, and 75.50–75.65 m), whiter intervals with sphaerosiderite nodules (230.3–234 and 241.5–245.5 ft; 70.20–71.32 and 73.61–74.83 m), and mottled gray and red intervals (236.5–241.5 and 245.5–247.7 ft; 72.09–73.61 and 74.83–75.50 m). A contact at 248.2 ft (75.65 m) separates red clay from brown, white, and red mottled silty clay where the mottles extend over 0.5 ft (0.15 m; 248.2–250.2 ft; 75.65–76.26 m). There is transition from 250.2 to 256.8 ft (76.26 to 78.27 m) that changes downsection from alternating light gray and olive-gray mottled silty clay with sphaerosiderite nodules to darker gray silty clay (254–256.0 ft; 77.42–78.03 m) to organic-rich (lignitic) clayey silt (256.0–256.8 ft; 78.03–78.27 m) to a contact at 256.8 ft (78.27 m). This darkening downsection reflects a change in environmental oxidation from mottled paleosols above to swampy subaqueous environments below.
A thin break of very fine to fine sandy clayey silt with common lignite and disseminated pyrite (256.8–259.9 ft; 78.27–79.22 m) underlies the paleosols and overlies an erosional contact with another interval of paleosols at 259.9 ft (79.22 m). Rusty mottling at 258.0–259.0 ft (78.64–78.94 m) and rare reddish clay mottles at 259.3 ft (79.03 m) document incipient soil formation in this predominantly subaqueous environment. Otherwise, the section from 259 to 259.9 ft (78.94 to 79.22 m) is gray with abundant charcoal (the charcoal may document dry season forest fires). There are granules at the irregular contact at 259.9 ft (79.22 m). This section represents deposition in a swampy floodplain environment and a possible rise/fall of baselevel.
Mottled paleosol clays return from 259.9 to 274.8 ft (79.22 to 83.76 m). From 259.9 to 262 ft (79.22 to 79.86 m), oxidized sediments reappear, characterized by predominantly light gray to light olive-tan silty clay with minor red mottling and sphaerosiderite nodules with hematite rinds. The interval at 262–265 ft (79.86–80.77 m) is predominantly red mottled silty clay with light gray to light olive-tan mottles, decreasing sphaerosiderite nodules, and increasing dusky red mottling downsection. From 265 to 270 ft (80.77 to 82.30 m), the lithology returns to light gray to light olive-tan clay with common dusky red mottles, whereas that at 270–274.8 ft (82.30–83.76 m) returns to red silty clay with extensive light gray to light olive mottles. The mottles cross cut, suggesting multiple generations of formation; some follow cracks whereas other halos are probably root traces. This lithology continues from 280 to 280.8 ft (85.34 to 85.59 m) below a coring gap (274.8–280 ft; 83.76–85.34 m). The section from 280.8 to 281 ft (85.59 to 85.65 m) is a light gray silty clay, whereas that at 281–283.3 ft (85.65–86.35 m) is medium gray silty clay with reddish mottles near the bottom. The section below 282.8 ft (86.20 m) is a slightly sandy clay. Below a coring gap (283.3–290 ft; 86.35–88.39 m) are gray silts with very fine to fine sand and common sphaerosiderite nodules with hematite rinds (290–290.65 ft; 88.39–88.59 m) that overlie a distinctive bright red slightly sandy clayey silt (290.65–290.95 ft; 88.59–88.68 m). The clays from 259.9 to 290.95 (79.22 to 88.68 m) represent terrestrial soils developed on an overbank.
A shift in sediment type to a predominantly sand section with a few mud breaks characterizes the core from 290.65 to 363.6 ft (88.59 to 110.83 m). Sands and silty sands are present from 290.65 to 296.7 ft (88.59 to 90.43 m). These whitish, very fine "sugar" sands include siltier zones (290.95–291.1 and 293.6–293.7 ft; 88.68–88.73 and 89.49–89.52 m), with silt lamina, mica, disseminated sphaerosiderite nodules, and opaque heavy mineral laminae common throughout. The sands at 293.65–296.7 ft (89.50–90.43 m) are fine–medium grained, have less silt, are laminated to cross laminated in places (including opaque mineral lamination), and are slightly brownish (because of intrusion of drilling mud). A very coarse sand with gravel appears below a sharp contact at 296.7 ft (90.43 m). The gravel is coarsest from 297.0 to 297.3 ft (90.53 to 90.62 m), with grains up to 1 cm in diameter. The change from this gravelly sand to progressively finer clasts up to 290 ft (88.39 m) appears to represent a fining-upward fluvial succession. This gravelly sand is underlain by a structureless medium to coarse quartz sand at 297.3–298.5 ft (90.62–90.98 m).
The underlying section consists of several fining-upward fluvial successions from pebbly granule beds to fine–medium sands: 300–300.4, 300.4–302.5, 302.5–306.7, and 306.7–310.6 ft (91.44–91.56, 91.56–92.20, 92.20–93.48, and 93.48–94.67 m). Opaque black mineral laminae are present in the sands. It is not clear if these sands are meandering, anastomosing, or delta front; the repetitive reasonably thick (2–4 ft; 0.61–1.22 m) successions of granules to fine sand with inclined laminations indicative of trough cross beds may have been deposited in anastomosing (or possibly meandering) environments. The base of the sand–granule succession is at 310.6 ft (94.67 m) where clays return.
The sand from 290.65 to 310.6 ft (88.59 to 94.67 m) composes an aquifer screened at nearby Finns Point. This is the "upper sand" of the middle aquifer of the PRM aquifer system; this "upper middle" sand can be identified at Artificial Island near Salem, New Jersey (well 33–401; 10 mi [16 km] to the south) (Cauller et al., 1999), at Clayton, New Jersey (24 mi [40 km] to the east) (Owens et al., 1998), and Delaware City, Delaware (3 mi [5 km] to the southwest) (Benson and McLaughlin, 2001; McLaughlin and Benson, 2002) and is thus laterally persistent. It is absent at New Castle, Delaware (3 mi [5 km] to the north) (Benson and McLaughlin, 2001; McLaughlin and Benson, 2002). This lateral persistence suggests that the sands may be part of a larger deltaic system and possibly a delta front sand.
From 310.6 to 320.3 ft (94.67 to 97.63 m), the core consists of micaceous clayey silts characterized by an alternation between light bluish gray, gray with purple mottles (including banding and halos around root traces), and gray with red clayey mottles. The environment is interpreted as floodplain/overbank altered by soil-formation processes. The micaceous silts probably reflect provenance, with a source from the nearby Piedmont.
The section recovered from 320.4 to 337.9 ft (97.66 to 102.99 m) is predominantly sand. The sands fine upsection from slightly micaceous medium–coarse at the base (337.9 ft; 102.99 m) to micaceous, silty, very fine sand with laminations of opaque heavy minerals at the top. A thin clay interval indicated on the gamma log was poorly recovered (328–330 ft; 99.97–100.58 m), although the section at 327.8–328 ft (99.91–99.97 m) is slightly clayey silty sand. Silty clay appears at 337.9 ft (102.99 m) as shown by higher gamma log values (Fig. F5). Between 337.9 and 352.15 ft (102.99 and 107.34 m), there are interbedded fine sands and sandy clayey silts. The sand beds are very fine to medium grained, between 0.4 and 1.1 ft (12 and 34 cm) thick, and in many places are invaded by red drilling mud. The silts commonly have interlaminated clays and are light greenish gray.
The section is again dominated by sand from 352.15 to 363.6 ft (107.34 to 110.83 m). The sands range from medium to fine grained, have a few slightly silty zones, and display clay rip-up clasts (355–355.5 and 361–361.5 ft; 108.20–108.36 and 110.03–110.19 m) and, in places, show fining-upward packages. The coarser sands are red owing to drilling mud invasion. An irregular cemented sand appears at 355.9 ft (108.48 m). The base of the section consists of crumbly granular sands (362.8–363.6 ft; 110.58–110.83 m), iron cemented intervals (363.3–363.4 ft; 110.73–110.76 m), and charcoal (363.5–363.6 ft; 110.79–110.83 m). They are tentatively interpreted as meandering river deposits, partly based on the lack of any evidence of marine deposition, although it is possible they are delta front. The contact at the base of the sands at 363.6 ft (110.83 m) may be a sequence boundary.
The sands from 320.4 to 363.6 ft (97.66 to 110.83 m) compose the "middle" (320.4–337.9 ft; 97.66–102.99 m) and "lower" (352.15–365 ft; 107.34–111.25 m) part of the middle aquifer of the PRM aquifer system. Elsewhere (Owens et al., 1998), this aquifer has been assigned to pollen Zone III (Cenomanian) (Benson and McLaughlin, 2001) and the confining unit below has been assigned to Zone II (Albian). Most of Potomac Unit 3 is barren of pollen at Fort Mott (Fig. F5).
The interval at 363.6–599.7 ft (110.83–182.79 m) is assigned to the Potomac Unit 2 (Figs. F6, F7). From 363.6 to 417.7 ft (110.83 to 127.31 m), the interval is a gray silty clay, frequently mottled red, with common root structures throughout; these are interpreted as paleosols. Four lithologic types are recognized in this interval based on the color and nature of mottling:
A transition to coarser, less oxidized facies appears below 417.7 ft (127.31 m). At 417.7–419 ft (127.31–127.71 m), the silty clays become tannish gray with increasing amounts of silt. Interbeds of very silty, very fine sands appear at 419 ft (127.71 m), and the interlaminated silty clays/clayey silts grade down to predominantly silty, very fine sands at 420 ft (128.02 m). The gamma log shows low values from 417 to 438 ft (127.10 to 133.50 m), consistent with the higher sand content, although this interval is too silty to compose an aquifer. At 424–426 ft (129.24–129.84 m) the sands gradually coarsen downsection from very silty, very fine sand to silty fine sand to clean fine sand. The sand from 425.5 to 426 ft (129.69 to 129.84 m) contains reddish silty and organic-rich laminae with hints of cross bedding. From 426.0 to 426.3 ft (129.84 to 129.94 m) the fine sand is siltier. The interval at 430–430.6 ft (131.06–131.25 m) is a light gray clayey sandy silt with common small charcoal fragments. A charcoal-rich bed (430.6–431.1 ft; 131.25–131.40 m) consists of a clayey, sandy silt matrix and charcoal up to 10 mm, a pyrite nodule (431.0 ft; 131.37 m), and almost pure charcoal at the base. Below this is a bed (431.1–431.35 ft; 131.40–131.48 m) with interlaminated clayey silt and charcoal flake layers. The interval at 431.35–432.1 ft (131.48–131.70 m) consists of two charcoal clast layers (431.35–431.45 and 431.55–431.65 ft; 131.48–131.51 and 131.54–131.57 m) within a zone dominated by clay rip-up clasts that include fragments of hematite(?), cemented and pyritic concretions, and charcoal, as well as common very fine mica. The features evident from 425.5 to 432.1 ft (129.69 to 131.70 m) indicate subaqueous overbank deposition within the floodplain and, hence, more humid conditions in contrast to soil/subaerial deposition in the section above. A surface in the core at 432.1 ft (131.70 m) separates grayish brown lignite-rich clay above from laminated sandy silt and sand below.
The section at 432.1–438.0 ft (131.70–133.50 m) consists of medium–dark gray silty clay with a few interlaminated sands. Mica and charcoal are common in both the silts and sands, with charcoal concentrations at 433.5, 433.7, and 434.8 ft (132.13, 132.19, and 132.53 m) and a zone at 436.5–438 ft (133.05–133.50 m). In a few places there are reddish concretions and red clays (432.3 and 437.15 ft; 131.77 and 133.24 m), with other scattered red clay beds. These interlaminated silts and sands probably represent cutoff channel/oxbow lake sedimentation adjacent to a soil-forming floodplain.
More uniform finer-grained sediments return below a coring gap (438–440 ft; 133.50–134.11 m) associated with a gamma kick (439 ft; 133.81 m). The section from 440 to 445 ft (134.11 to 135.64 m) consists of fairly homogeneous light gray slightly clayey silt with common sphaerosiderite nodules. This section probably represents an incipient soil on the levee/overbank. Below this, the section becomes more laminated, more micaceous, darker gray slightly clayey silt with scattered dark (?organic rich) laminae (445–447.5 ft; 135.64–136.40 m). In situ sphaerosiderite nodules are lacking from 445 to 446.3 ft (135.64 to 136.03 m). This section probably represents more subaqueous influence than above. Darker micaceous clayey silts with common scattered very fine charcoal fragments and scattered very fine sand beds, sand-filled burrows or root traces, and very dark gray clay (447.5–452 ft; 136.40–137.77 m) appear to represent deposition in a fluvial-lacustrine environment (?oxbow lake). The unit below this (452–458.15 ft; 137.77–139.64 m) is similar, although slightly darker and lacking sand; it appears to be a lacustrine deposit.
An interesting contact is present at 458.8 ft (139.84 m). From 458.15 to 458.8 ft (139.64 to 139.84 m), the lithology consists of an organic-rich clayey silt with graded, abundant dispersed charcoal (ranging in size from silt sized to 2 mm diameter) beds at its base. A yellowish tan clayey sandy silt below the contact (458.8–458.95 ft; 139.84–139.89 m) appears to represent subaerial weathering of the underlying subaqueous silts (458.95–469.05 ft; 139.89–142.97 m). The contact at 458.8 ft (139.84 m) may represent a transition from a desiccated lake/overbank deposit to a deeper-water lake above (i.e., a change to wetter conditions), although in marine facies this would be interpreted as a sequence boundary.
Complex gray silt and clay facies deposited in subaqueous floodbasins appear below the 458.8 ft (139.84 m) contact: light gray clayey silts (458.95–459.05 ft; 139.89–139.92 m), laminated clays with clay rip-up clasts and scattered charcoals (459.05–459.4 ft; 139.92–140.03 m), and dark grayish brown clayey silt with common dispersed charcoal fragments (459.4–460.0 ft; 140.03–140.21 m). The section from 460 to 461 ft (140.21 to 140.51 m) is a clay that has been disturbed by coring. Interbedded gray clayey silt to silty clay with charcoal zones (461–466 ft; 140.51–142.04 m) include clay rip-up clasts (e.g., 461.5 ft; 140.67 m).
Between 461 and 508.15 ft (140.51 and 154.88 m) the cores are predominantly dark gray silty clay with dispersed fine plant debris, interbedded silty very fine sands, and scattered charcoal fragments. Special features include: (1) thin sand laminae; (2) slightly sandy, heavily bioturbated sandy zones; and (3) cemented red lithified concentrations (Table T2). These red zones and concretions appear to be plinthites (cemented concretions) in an overbank/desiccated lake setting. Pyrite concretions appear at 463.8 ft (141.37 m).
Color varies in the clayey beds because of changing oxidation. From 461 to 471 ft (140.51 to 143.56 m), they are light to medium gray; from 471 to 471.5 ft (143.56 to 143.71 m), a lavender hue is present; at 471.5–475 ft (143.71–144.78 m), the clays are light to medium gray; and brownish gray hues become common from 475 to 480 ft (144.78 to 146.30 m). Color laminations of gray, light brown, and red dominate the interval from 480 to 498.4 ft (146.30 to 151.91 m). The color changes at 498.4 ft (151.91 m) to a darker distinctly laminated dark gray with olive-gray bands. From 500.0 to 507.6 ft (152.40 to 154.72 m), the section is interlaminated gray and tan. The color boundaries tend to be more gradational than sharp.
These fine-grained sediments appear to have been deposited in small lakes in a fluvial setting. The paucity of sand and lack of abundant organic matter/pyrite argues against cutoff channel/oxbow lake subenvironments and points toward larger floodplain standing lakes. Although the burrowed sands are suggestive of estuarine environments, occasional soil concretions indicate subaerial exposure, consistent with ephemeral lakes. There are hints of cyclicity on a small (2–3 ft; 60–90 cm) scale in the gamma logs and cores and ~10-ft (3.05 m) cyclicity on the gamma logs.
The section returns to red and light gray silts and clays at 508.25 ft (154.91 m) at a sharp contact. An indurated yellowish to blackish clayey silty sand (508.2–508.25 ft; 154.90–154.95 m) overlies the irregular contact; this indurated interval is overlain by blebs of red clay (508.15–508.2 ft; 154.88–154.90 m), apparently reworked from below, and a return to blackish clayey silty sand (507.6–508.15 ft; 154.72–154.88 m). This contact could either be a sequence boundary or a facies shift from soils in an alluvial floodplain to a lake in the floodplain. There is a change in gamma log signature at ~508.25 ft (154.95 m) from smaller, thinner variations above to thicker, large-scale variations below.
Mottled clays and silts deposited as soils appear below the 508.25 ft (154.95 m) surface and continue to a surface at 555.1 ft (167.98 m). The section at 508.25–508.55 ft (154.95–155.01 m) is banded red and light olive-gray clay with sparse sphaerosiderite nodules overlying a coring gap. The section below this (510–512 ft; 155.45–156.06 m) consists of a light gray, slightly micaceous silty clay with common sphaerosiderite nodules and red mottles following what appear to be large root traces. The interval from 512 to 515 ft (156.06 to 156.97 m) is a light gray, very silty micaceous clay with small circular sand zones that may be root fills and common sphaerosiderite nodules with hematite rinds that increase in abundance downsection.
The section is silty overall down to 515 ft (156.97 m), predominantly clay from 517 to 520 ft (157.58 to 158.50 m), and returning again to silt at 520–529.9 ft (158.50–161.51 m). From 515.0 to 519.5 ft (156.97 to 158.34 m) the section is a mottled, orange-red, light olive-gray silty clay. The section at 519.15–520.35 ft (158.24–158.60 m) is an olive-gray silty clay with small reddish brown mottles and a trace of mica. From 520.35 to 523.5 ft (158.60 to 159.56 m) the section consists of light olive-gray, slightly micaceous clayey silt with 1-cm-thick red mottles and abundant small sphaerosiderite nodules (very fine sand size). The section from 523.5 to 529.3 ft (159.56 to 161.33 m) consists of a return to light gray, very silty micaceous clay with common sphaerosiderite nodules and small cracks with black (?manganese) infill. The interval at 529.3–530.9 ft (161.33–161.82 m) is light olive-gray and brown mottled clay with sphaerosiderite nodules transitioning downsection to 538.3 ft (164.07 m) to mostly brown, mottled, slightly silty, slightly micaceous clays with some reddish and grayish mottles and scattered sphaerosiderite nodules. From 538.3 to 546.0 ft (164.07 to 166.42 m) the section returns to a light olive-gray clay with orange-red and dusky red mottles and sphaerosiderite nodules, abundant in places. The mottles appear to follow traces of large roots. The section at 546.0–550.4 ft (166.42–167.76 m) is light gray to light brownish gray very silty clay with brown mottles. The section from 550.4 to 550.5 ft (167.76 to 167.79 m) is a light gray to olive-green banded silty clay. The interval between 550.5 and 551.1 ft (167.79 and 167.98 m) is a brown and light gray silty clay. At 551.1–555.2 ft (167.98–169.22 m) is a light olive-gray silty clay with light brown mottles and thin sand. A sharp contact at 555.1 ft (169.19 m) separates silty clay above from very charcoal rich, medium to dark gray clay below; reworked charcoal laminae continue up above the contact to 555.0 ft (169.16 m). The underlying clay continues with rare sand laminae to 558 ft (170.08 m) and is interpreted as a lacustrine deposit. The contact at 555.1 ft (167.98 m) is marked by a gamma log kick (Fig. F6) and could be either a sequence boundary or a shift to paleosols above from lacustrine deposits below.
From 558 to 560.3 ft (170.08 to 170.78 m), the clay becomes increasingly sandy downsection. The section between 560.3 and 560.8 ft (170.78 and 170.93 m) is interbedded fine–very fine sand and sandy dark gray clay with common charcoal. The underlying bed (560.8–563.7 ft; 170.93–171.82 m) is very fine micaceous sand with scattered thin interlaminated clay layers and abundant charcoal. At 563.7–563.8 ft (171.82–171.85 m) is a thin silty clayey very fine micaceous sand. Sands from 560.3 to 563.7 ft (170.78 to 171.82 m) appear to have too much interlaminated clay and charcoal to compose a good aquifer. There is a coring gap at 563.8–570 ft (171.85–173.74 m).
Sands continue from 570 to 596.8 ft (173.74 to 181.90 m), constituting a potential aquifer. The sands from 570 to 580.5 ft (173.74 to 176.94 m) are generally very fine to fine grained, lignitic, and slightly silty, with thin clay interbeds at 574.0–574.9, 577.1–580.5, and 580.9–581.1 ft (174.96–175.23, 175.90–176.94, and 177.06–177.12 m). Interbedded sands and clays continue below this: very fine carbonaceous sands (581.1–581.8 ft; 177.12–177.33 m), a silty clay layer (581.8–582 ft; 177.33–177.39 m), and a very fine sand with scattered carbonaceous material (582–582.3 ft; 177.39–177.49 m). Below a coring gap (582.3–590 ft; 177.49–179.83 m), the sediments coarsen downward to 593.3 ft (180.84 m) from fine sand with scattered charcoal fragments (up to 4 cm long) to granuliferous sand. An irregular contact at 593.3 ft (180.84 m) with a rip-up clast of underlying material appears to be the base of a channel. Thus, the section from 581.1 to 593.3 ft (177.12 to 180.84 m) appears to be fluvial channel fill; however, a marine channel interpretation cannot be precluded. The section at 593.3–596.0 ft (180.84–181.66 m) is very charcoal rich (fragments from 2–3 mm to 6 cm across), micaceous, very fine sand with cross beds and soft-sediment deformation features probably deposited in a fluvial overbank/crevasse splay setting. Interbedded fine–very fine sands and clays with abundant charcoal compose the section from 596.0 to 599.7 ft (181.66 to 182.79 m): a clayey sandy silt with soft-sediment deformation features and abundant charcoal fragments (596.8–598.4 ft; 181.90–182.39 m), a fine–very fine sand with charcoal (598.4–599.0 ft; 182.39–182.58 m), a sandy charcoal bed (599.0–599.2 ft; 182.58–182.64 m), a clay (599.2–599.4 ft; 182.64–182.70 m), and a fine–very fine sand (599.4–599.7 ft; 182.70–182.79 m). These are also tentatively interpreted as crevasse splay environments.
The sands and interbedded clays from 560.3 to 599.7 ft (170.78 to 182.79 m) are assigned to pollen Zone IIB (Albian) (556.2 and 577 ft; 169.53 and 175.87 m) (see "Biostratigraphy") or Zone IIB/I (Aptian or older) (599 ft; 182.58 m).
A sharp irregular contact at 599.7 ft (182.79 m) separates sand above from paleosols below. The contact may be a sequence boundary. The section below includes a silty clay (599.7–600.0 ft; 182.79–182.88 m), sandy silt (below a coring gap; 600–605 ft; 182.88–184.40 m), with scattered sphaerosiderite nodules with hematite rinds (605–606.2 ft; 184.40–184.77 m), and gray and heavily mottled, red, slightly silty clay with abundant sphaerosiderite nodules representing interfluvial paleosols (606.2–615.4 ft; 184.77–187.57 m) (Fig. F7). These clays become slightly sandy and less mottled below 613.5 ft (186.99 m). A contact at 615.4 ft (187.57 m) separates the light gray sandy clay with sphaerosiderite nodules above from dark gray (?organic rich) slightly silty clay below.
The gray clays (615.4–620.1 ft; 187.57–189.01 m) contain small charcoal fragments, lack sphaerosiderite nodules, and have thin micaceous sand interbeds below 617 ft (188.06 m). They are interpreted as lacustrine. This facies continues to 620.1 ft (189.01 m), where there is a return to lighter gray mottled silty clays.
Different clays appear from 620.1 to 628.1 ft (189.01 to 191.44 m). At 620.1–625.7 ft (189.01–190.71 m) the section contains variably light to medium gray sandy clayey silts with olive mottling and evidence of bioturbation in places. Between 621.7 and 622.5 ft (189.49 and 189.74 m) this mottled interval is relatively hard, with obvious root casts and/or desiccation cracks; below this the mottling is less intense and bioturbation is more obvious. This passes downward into a slightly darker, laminated, medium gray sandy clayey silt with scattered charcoal fragments at 625.7–626.4 ft (190.71–190.93 m). Interbedded dark and light gray clay and sandy clay is present from 626.4 to 628.1 ft (190.93 to 191.44 m), with a coring gap to 630 ft (192.02 m). The environment of deposition of the section from 620.1 to 628.1 ft (189.01 to 191.44 m) is uncertain; the facies are consistent with cutoff channel/floodplain lakes, although slight mottling of the section indicates incipient soil formation.
Sands and silts predominate from 630.0 to 644.1 ft (196.32 to 198.99 m). Silty sand with white rip-up clasts (630.0–630.3 ft; 192.02–192.12 m) overlies an erosional contact (630.3 ft; 192.12 m). Interbedded micaceous, gray sandy silt alternates with burrowed silty fine sand to 637.6 ft (194.34 m); sphaerosiderite nodules appear sporadically from 630.5 to 632.0 ft (192.18 to 192.63 m), again suggesting incipient soil formation on a fluvial (proximal levee?)-lacustrine (cutoff channel?) environment. Bedding is obscured above 632 ft (192.63 m) and laminated below. Another sharp, irregular contact at 637.6 ft (194.34 m) separates the silty sand above from clayey fine sand below. Soft, silty, very fine sand (637.6–638.2 ft; 194.34–194.52 m) passes downsection to a series of fining-upward successions of medium sand to fine sand to clay with variably abundant charcoal, possibly representing overbank deposits, that are present to 644.1 ft (196.32 m).
As discussed above, the lithologic assignment of the section from 599.7 to 644.1 ft (182.79 to 196.32 m) is problematic. It may constitute a distinct sequence (Subunit 2a or 1b), be part of the larger Unit 2 sequence, or be part of the Unit 1 sequence. Based on the distinct facies successions from sand to clay upsection, we favor this being a distinct sequence.
From 644.1 ft to 649.2 ft (196.32 to 197.88 m) the section consists of an extremely hard, clayey, light gray silt with an interval of burrowed silty sand (648.3–648.8 ft; 197.60–197.75 m). At 649.2–652.7 ft (197.88–198.94 m) is a succession that passes from mottled orange and light gray slightly sandy, slightly clayey silt downsection to darker grayish brown slightly sandy clayey silt with abundant sandy burrows/rootlet fills; this appears to be the same lithology decreasingly affected by soil processes downsection. Sporadic sphaerosiderite nodules appear from 649 to 651 ft (197.82 to 198.42 m). The environment appears to be fluvial overbank swamp with common organic matter overprinted by soil processes exemplified by root casts.
Below 652.7 ft (198.94 m) the section returns to a heavily mottled character, which persists through 710.6 ft (216.59 m). The section alternates between clay and silt and between zones of extensive mottling and grayer gleyed zones. The mottling below 652.7 ft (198.94 m) becomes particularly intense compared to upper sections of the borehole, suggesting intense soil weathering under tropical–subtropical conditions of varying seasonal precipitation. The section between 652.7 and 652.85 ft (198.94 and 198.99 m) consists of light gray silty clay above a coring gap (652.85–658.8 ft; 198.99–200.80 m). From 658.8 to 660 ft (200.80 to 201.17 m), the lithology is light gray clay with sphaerosiderite nodules and red mottling and banding. The mottled zones are commonly characterized by intensely mottled and cracked red, brown, and olive fine-grained lithologies with scattered concretions. The interval at 660–665 ft (201.17–202.69 m) is predominantly clay with sphaerosiderite nodules; the mottles are red and appear rootlike and commonly become narrower downward. From 665 to 676 ft (202.69 to 206.04 m), the section is siltier with fine micaceous sand at 670.1–671 ft (204.25–204.52 m) and contains sphaerosiderite nodules altered to hematite throughout; gamma log values are distinctly lower in this section. From 670.1 to 673.6 ft (204.25 to 205.31 m) the core is mostly orange-red with olive, orange, and gray mottles with small (up to 5 mm) dusky red concretions (hematite cemented sand). The interval at 673.6–676.1 ft (205.31–206.08 m) is slightly sandy to clayey light gray micaceous silt with common sphaerosiderite nodules with hematite rinds and small hematite concretions. A dramatic change appears at an uneven contact at 676.1 ft (206.08 m) to a deeply mottled red to dusky red to olive to light gray extensively cracked, slightly silty clay that continues to 685.2 ft (208.85 m) with a few zones of small (up to 1 cm) hematite concretions. From 685.2 to 690.2 ft (208.85 to 210.37 m), large orange to olive mottles are present on light gray to pinkish clay with little silt and small hematite concretions (up to 2 mm) concentrated in the orange-olive mottles. Deeply mottled, red, intensely cracked clay continues at 690.2–703.9 ft (210.37–214.55 m) with scattered sphaerosiderite nodules, common small hematite concretions in deeper red zones, and orange to olive mottles from 703 to 703.9 ft (214.27 to 214.55 m). An abrupt change to light pinkish purple clayey, slightly sandy, micaceous silt is present at 703.9 ft (214.55 m) with scattered hematite grains after sphaerosiderite. Light pinkish purple clay continues to 705.55 ft (215.05 m), where the section changes to light gray slightly clayey micaceous silt (705.55–710.6 ft; 215.05–216.59 m) that becomes increasing sandy downsection with a few red and olive mottles and sphaerosiderite nodules. The bottom of this section (710.3–710.6 ft; 216.50–216.59 m) has abundant small holes filled with very fine sands that are probably root casts.
A heterolithic interval appears at 710.6–722.8 ft (216.59–220.31 m) (Fig. F7). From 710.6 to 712.9 ft (216.59 to 217.29 m) the lithology is heterogeneous, with mottled red and gray very clayey sandy silt and blebs of other lithologies (silty clay and very sandy clayey silt) lacking sphaerosiderite nodules; this facies may be colluvium. Below a coring gap (712.9–720 ft; 216.59–219.46 m), is a heterolithic bed (720–722 ft; 219.46–220.07 m) with mottled micaceous sandy clayey silt with clasts up to 7 mm that is more deeply cracked and redder at the top. At 722 ft (220.07 m), the section becomes sandy, with very silty micaceous sand to 722.8 ft (220.31 m) followed by a coring gap (722.8–725 ft; 220.31–220.98 m). The sand is poorly sorted, fines upward from medium to very fine grained, and is marked at its base by a few angular pebbles up to 1 cm in diameter.
The section from 725 to 732.2 ft (220.98 to 223.17 m) consists of spectacularly mottled, poorly sorted sandy silt. The section has large (1–5 cm) light gray mottles on a deep red base lithology. The light gray mottles frequently have olive rims from 1 to 10 mm wide; at the core of some of the light gray mottles are areas of weak red color. The mottling may represent alteration in a rooted soil zone affected by an alternation of oxidizing and gleying (reducing) conditions because of water table rise and fall over a century- to millennial-scale time interval.
An interval of gray poorly sorted silty sand appears from 732.2 to 734.4 ft (223.17 to 223.85 m) with scattered sphaerosiderite nodules and hematite concretions. This is underlain by another extensively mottled zone between 734.4 and 738.3 ft (223.85 and 225.03 m), but this zone is different than that at 725–732.2 ft (220.98–223.17 m). It is mostly clay to silty clay, dominated by deep red colors and olive-yellow patches with isolated light gray zones. The dusky red clay areas are discrete and appear to be individual clay pieces that may have physically moved into this deposit by soil or slope processes (e.g., colluvium). A very hard silt/siltstone appears at 738.3–738.6 ft (225.03–225.13 m). This indurated zone is underlain by a very soft interval of interlaminated whitish sand and clay from 738.6 to 739.1 ft (225.13 to 225.28 m), probably representing overbank deposition.
Laminated deep red clays are present from 739.1 to 743.9 ft (225.28 to 226.59 m) with scattered white mottles and subordinate interbedded micaceous white clayey silts. These laminated clays and silts were apparently deposited under subaqueous conditions (?floodplain).
Below a coring gap (743.9–748 ft; 226.59–227.99 m) is an intensely mottled, variably micaceous deep red and pinkish white sandy, slightly clayey silt (748–752.8 ft; 227.99–229.45 m) with dusky red irregular concretions and some olive mottles and light olive-brown mottling predominating in a slightly sandier zone (e.g., 751.5–752 ft; 229.06–229.21 m). The lower part of the section (752–752.8 ft; 229.21–229.45 m) has extensive "wormy" mottling and cracks.
A major lithologic change appears in a coring gap (752.8–753.5 ft; 229.45–229.67 m). The section at 753.5–758.15 ft (229.67–231.08 m) is a predominantly medium–dark gray zone with interbedded silty fine–coarse sand, silty clay, sandy silts with scattered lignite, mica, sand-filled root casts or burrows in the lower part, and soft-sediment deformation (756–756.5 ft; 230.43–230.58 m). This section may represent a wet fluvial overbank environment (perhaps swamp subenvironment).
Clays and clayey silts return at 760–765.6 ft (231.65–233.35 m). Brown silty clay and clayey silt (760–762.9 ft; 231.65–232.53 m) and mottled red orangish tan and gray slightly silty clay with cracks and sphaerosiderite nodules (762.9–"765.05" ft; 232.53–"233.19" m) represent intense paleosol formation. Dark gray silty clay with charcoal (765.05–765.6 ft; 233.19–233.35 m) represents a wet floodplain environment.
A coarse-grained unit found from 765.6 to 769.2 ft (233.35 to 234.45 m) is expressed as a distinct low on the gamma log. Mottled tan and olive sandy silt with organic flecks (765.6–767 ft; 233.35–233.78 m) overlies a very sandy silt to very silty sand (767–769.2 ft; 233.78–234.45 m) that is predominantly fine to medium grained. The sand is mottled red and olive near the top to light gray near the base and appears to be coarser near the base. The silts and sands may represent overbank deposits altered into soil near the top.
An abrupt change is present at 769.2 ft (234.45 m) to mottled olive-yellow and light gray hard, waxy clayey silt (769.2–771.6 ft; 234.45–235.18 m) with cracks and scattered red mottles that represents another paleosol. It passes downward into a sandy silt at 771.6 ft (235.18 m) and to a poorly sorted fine–coarse silty sand at 772.3 ft (235.40 m) that again probably represents an overbank deposit.
Below a coring gap (772.5–780 ft; 235.46–237.74 m), slightly silty clay returns (780–789 ft; 237.74–240.49 m) as a deep red mottled interval. The upper part (780–783 ft; 237.74–238.66 m) has abundant small mottles of various shades of red. Tan and light gray mottles appear at 783 ft (238.66 m). Deep dusky red and black-red mottles appear at 785.5 ft (239.42 m) (manganese-iron oxide?). The section becomes increasingly indurated and slightly sandy downsection. Olive-colored mottles contain scattered small (<1 mm) hematite nodules, probably after sphaerosiderite. The sediments represent multiple generations of paleosols as indicated by multiple generations of mottling with different-colored rims and cores.
The section from 789 to 790.5 ft (240.49 to 240.94 m) is a fining-upward succession from a fine- to medium-grained quartz with coarse-grained sphaerosiderite nodules to a silty medium- to fine-grained sand to slightly sandy silt. It probably represents a crevasse splay deposit within the overbank.
Red mottled clay with root casts (790.5–791.5 ft; 240.94–241.25 m) grades down to into a light gray sphaerosiderite nodule-rich slightly micaceous silt (791.8 ft; 241.34 m) that coarsens downward to a silty sand (795 ft; 243.32 m) and a slightly silty sand (797 ft; 242.93 m) to a medium–coarse sand (798.05 ft; 243.25 m). Mottled material at the top is a deep red with small, "wormy," whitish mottles. The sand is micaceous and in the basal part contains very angular quartz grains that suggest proximity to source. The entire succession probably represents a channel-fill deposit.
Another cycle from 798.05 to 799.5 ft (243.25 to 243.69 m) consists of a fining-upward succession from slightly micaceous fine–medium sand to a clayier, siltier, mostly fine-grained sand. Below a coring gap (799.5–805 ft; 243.69–245.36 m), a dark gray lignitic micaceous clayey silt (805–808.65 ft; 245.36–246.48 m) abruptly overlies a silty clay. The silt probably represents a swamp environment.
From 808.65 to 808.9 ft (246.48 to 246.55 m) the section coarsens downsection from a soft, light medium gray, slightly silty clay that overlies a very hard, medium light gray, slightly sandy clayey silt (808.9–811 ft; 246.55–247.19 m) to sandy silt (811–812 ft; 247.19–247.50 m) to whiteish gray slightly silty medium sand (813–814 ft; 247.80–248.11 m). The section becomes progressively darker and purplish upsection above the bottom of the hole (814 ft; 248.11 m). Thus, from the bottom of the hole to 808.65 ft (246.48 m) the cores trace a fining-upward succession deposited as a channel fill.