In outcrop and in the subsurface, the unconsolidated sands, silts, and clays of the middle to upper Miocene Cohansey Formation are unconformably overlain by surficial units (Pleistocene Cape May, ?upper Miocene–Pliocene Pennsauken, ?upper Miocene Bridgeton, and ?upper Miocene Beacon Hill Formations) (Newell et al., 2000), and unconformably overlie the middle to lower Miocene Kirkwood Formation. Differentiation of Kirkwood and Cohansey Formation sands can be particularly difficult since both are nearshore deposits, though the latter generally consists of coarser grained, yellow sand vs. finer grained gray sand (Isphording and Lodding, 1969). In the deeper subsurface penetrated by Legs 150X and 174AX (e.g., Atlantic City and Ocean View) (Fig. F16), the first definite Kirkwood Formation is identified on the first downhole occurrence of brownish silty clay or silty micaceous gray sand. The upper limit of the Cohansey Formation and its lateral equivalents is even more enigmatic, particularly in the Cape May Peninsula. At the Cape May site, an estuarine unit of sand and sandy clay from 90 to 356.9 ft (27.43 to 108.78 m) was first thought to be ?Pleistocene and Pliocene age (Miller et al., 1994), though subsequent dinocyst studies show the entire unit is middle to upper Miocene (Zones DN7–DN9; >7.5 Ma) (de Verteuil, 1997). Owens et al. (1998) mapped this unit as a sand with woody clays in the Cape May Peninsula (his Tu unnamed unit) that may be laterally equivalent to the Cohansey Formation updip and along strike. This same unit of gray to black pebbly, clayey sand and woody clayey silt was mapped in the Cape May Peninsula by Newell et al. (2000) as an unnamed Tertiary unit (Tu); his sections indicate that the Tu is either laterally equivalent to or cuts into and removes the Cohansey Formation. We recognize this lignitic sand as a distinct formation and here designate these lithologically variable, dominantly estuarine sediments in the Cape May Peninsula as the Stone Harbor Formation.
We formally designate middle–upper Miocene sand and clay between 93.6 and 231.4 ft (28.53 and 70.53 m) at the Cape May Zoo corehole as the Stone Harbor Formation stratotype (Fig. AF1). Because the primary stratotype has relatively poor age control, a costratotype is designated at the Leg 150X Cape May site from 90 to 356.9 ft (27.43 to 108.78 m) because of its relatively precise dinocyst stratigraphy (de Vertueil, 1997). The name is derived from the Stone Harbor Country Club that is adjacent to the stratotype corehole, which is 7 km from the town of Stone Harbor, New Jersey. The name Cape May Formation is preempted by its use for the overlying Pleistocene–Holocene sands, whereas the use of other nearby towns for the name of the unit are either similarly preempted (e.g., Wildwood and Rio Grande) or would cause confusion (e.g., Cape May Zoo Formation, Cape May Courthouse Formation, and Middle Township Formation).
The Stone Harbor Formation at the Cape May Zoo corehole is 137.8 ft (42.0 m) thick and is primarily a sand, with the following facies:
Lignite is generally common throughout the formation, OHMs are present but not common, organic-rich beds are common, and mica is scarce. Sandier units are typically 30–40 ft thick (9.14–12.19 m) and alternate with the finer grained units that are typically 10–20 ft (3.05–6.10 m) thick. Environments of deposition are generally estuarine in the lower part and nearshore (including shoreface, lagoon, marsh, tidal channel, and backbarrier) to fluvial/estuarine in the upper part.
The Stone Harbor Formation in the Cape May corehole is 266.9 ft (81.35 m) thick and consists predominantly of sand, with the following facies in order of thickness:
Lignite is generally common and there are scattered granuliferous and pebbly beds. The environment of deposition is primarily estuarine.
The age of the Stone Harbor Formation is constrained primarily at the Cape May site, where it is assigned to Zones DN7 (322–357 ft; 98.15–108.81 m; ~12–12.5 Ma), DN8 (Sequences Ch3 [274–322 ft; 83.52–98.15 m] and Ch4 [210–274 ft; 64.01–83.52 m]; ~9–10.5 Ma), and DN8/9 (Sequences Ch5 [140–210 ft; 42.67–64.01 m] and Ch6 [90–140 ft; 27.43–42.67 m]; ~7.5–8.5 Ma). At the Cape May Zoo site, dinocysts constrain the lower sequence as upper Miocene.
Based on lithologic criteria the Stone Harbor Formation is readily differentiated from the Cohansey Formation by its more variable grain size (ranging from pebbles to clay) and facies, a greater abundance of lignite and organic-rich beds, and generally estuarine depositional environment. It is similar to the Cohansey Formation in containing some barrier and back-barrier environments and generally lacking calcareous fossils but locally containing dinoflagellate cysts. It is differentiated from the overlying Cape May Formation by its generally coarser, more variable, and more lignitic facies. As noted, it is possible that the Stone Harbor Formation is laterally equivalent to the Cohansey Formation, though the Stone Harbor Formation may in fact be cut into the Cohansey Formation (Newell et al., 2000). It appears that the Stone Harbor Formation is in part the same age as the Cohansey Formation (10–12.5 Ma), though it may be younger in the upper part (e.g., 7.7–8.0 Ma) than the Cohansey Formation.
There are possibly five sequences in the Stone Harbor Formation at the Cape May site (Sequences Ch2–Ch6) (de Verteuil, 1997), though the regional extent of these possible sequences has not been established. At Cape May Zoo, we identify two distinct sequences within the Stone Harbor Formation (93.6–180.4 and 180.4–231.4 ft [28.53–54.99 and 54.99–54.99 m]) and tentatively correlate them with de Verteuil's Sequences Ch3 and Ch4.
In the subsurface, the unit has been mapped across the Cape May Peninsula (Owens et al., 1998; Newell et al., 2000) and can be extended to at least the Leg 174AX Ocean View site (Fig. AF1). In the stratotype and Ocean View coreholes, the Stone Harbor Formation is overlain by the Cape May Formation and overlies nearshore sands assigned to the Cohansey Formation. The unit does not appear to outcrop.