NJ Coastal Plain
NJ Coastal Plain
Marine conditions overprinted by a delta on the New Jersey coastal plain returned during the Santonian and continued through the Campanian. This switch is believed to represent increased accommodation due to relative sea-level rise. The Cheesequake Formation is a poorly sampled thin sequence. In outcrop, the formation is a silt and is interpreted as a middle neritic deposit.
A major mid-Turonian eustatic lowering is associated with the sequence boundary between the Bass River/Raritan and the Magothy sequences. Middle neritic sediments that dominated the Bass River sequences were replaced by sediments representing upper delta plain, lower delta plain and delta front environments during Magothy time. In general, in cores Magothy sediments are more terrestrial (upper delta plain) to the south and more marine (lower delta plain) to the north. The Magothy Formation in outcrop consists of sands and silty clays deposited in complex non-marine to marginal marine environments (Owens and Gohn, 1985). Regionally, this unit thickens to over 300 m toward the Long Island Platform and thins toward Delaware where it all but disappears (Perry et al., 1975; Olsson et al., 1988).
In outcrop the Magothy Formation is divided into six members.
| 1) The Sayreville Sand contains bioturbated and cross bedded, poorly sorted, sands and organic-rich clays interpreted as an estuarine deposit. |
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| 2) The Old Bridge Sand sand is a granuliferous medium to very coarse sand with fining upward packages representing channel sediments deposited in a nearshore fluvial delta. |
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| 3) The South Amboy Fire Clay is a yellow, white, and red mottled clay containing sphaerosiderite representing paleosols deposited in an upper delta plain environment. |
| 4) The Amboy Stoneware Clay consists of tan clays with thin sand beds and laminae representing soils deposited in a delta plain setting. Other facies may include: fining-upward successions of interbedded and interlaminated cross bedded, micaceous, fine sand and clay beds probably representing a migrating levee complex; interlaminated gray and reddish brown clay, organic-rich clay beds, and micaceous sand deposited in lower delta plain overbank swamps; and organic-rich clays with interlaminated sands representing lower delta plain overbank deposits including cut-off channels and oxbow lakes. |
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5) The Morgan Beds generally consist of cross bedded sand with intermittent mud beds deposited in marsh, tidal channel, and tidal channel/bay environments within a delta front to lower delta plain. |
6) The Cliffwood Beds are cross-bedded to burrowed, fine sand and interbedded organic-rich, sand deposited in deltaic, lower delta plain to upper delta front environments. A lower unit consists of thicker bedded, cleaner, heavily burrowed, medium sands representing a proximal upper shoreface deposit and faintly laminated, organic-rich, slightly silty clay interpreted as a bay/lagoon deposit. |
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A regional unconformity (Owens & Gohn, 1985) separates the Potomac Formation from the Raritan and Bass River Formations (Cenomanian-lower Turonian). This unconformity represents a shift from predominantly terrestrial sediments to marine sediments but with evidence for the nearby presence of a delta. The Raritan is found up dip and the Bass River clay is found in down dip boreholes. The Bass River Formation contains abundant marine fossils which distinguishes it from the Raritan. The Woodbridge Clay was extensively mined up until the late 20th century and used in brickmaking and pottery.
Woodbridge Clay Member of the Raritan Formation
The oldest sediments (Barremian to earliest Cenomanian age, ~120-98 Ma) identified in the coastal plain are assigned to the Potomac Formation. Most of the Potomac Formation sands are interpreted to represent a variety of riverine environments including anastomosed rivers in an upper delta plain (Sugarman et al., 2004), meandering and braided rivers. Some of the Potomac sediments may have been deposited in nearshore environments. Fine-to-coarse quartz sand that is locally lignitic and micaceous is generally interpreted to represent channel sands. However, a few thick sand bodies (generally 15-30 m; representing a small percentage of the total thickness of the Potomac) appear to be regional and could represent nearshore marginal-marine or delta-front environments. These sands lack the carbonate fossils (mollusks and foraminifers) that would be expected in a marine environment, but they also contain little evidence of channels in the form of fining upward successions that might be expected of a fluvial environment. In addition to river deposits, the Potomac is well know for its floodplain deposits including its deeply weathered and very colorful clay/silt paleosols.
Potomac Formation clays and silts generally are mottled red and gray and represent ancient soil deposits.
Potomac mottled silts & clays in Delaware courtesy of Pete McLaughlin, Delaware Geological Survey