THE ELLISDALE MEMBER OF THE LOWER MARSHALLTOWN FORMATION – EVIDENCE FOR A FRESHWATER TIDAL ESTUARY IN THE LATE CRETACEOUS OF CENTRAL NEW JERSEY

The vertebrate fauna and geology of the Ellisdale Fossil Site in Monmouth County, New Jersey has been the subject of intensive studies since its discovery in 1980. The Ellisdale site occurs within the basal portion of the Marshalltown Formation, of the Late Cretaceous Matawan Group of New Jersey. Vertebrate fossils are concentrated with rip-up clasts near the base of a flaser-bedded clay sequence in a lag deposit consisting of siderite pebbles, poorly graded sand, and lignite. Remains of animals from at least four paleoenvironments are represented at the Ellisdale Site: marine, lagoonal/backbay, estuarine/freshwater, and terrestrial, suggesting an allogenic model where pre-burial transport, mixing and concentration of the fossils took place prior to their deposition in the pebble layer.

A recent palynological study has determined that the silty, flaser bedded clays found above and below the pebble layer span the entire Middle Campanian (76.4 – 80.8 ma), and that the sediments were laid down in a fresh water environment of deposition. The presence of well-preserved amphibian remains also supports the idea that the environment was fresh water as amphibians are salt-intolerant. Study of the larger clasts has shown chaotic orientation, indicative of a tidal depositional regime.

Throughout most of its outcrop zone, the Englishtown/Marshalltown contact is present as an unconformity at the top of the Englishtown Fm., with a basal transgressive lag above the unconformity transitioning immediately into the glauconitic marine sand facies of the Marshalltown Formation. The flaser-bedded clays and sands observed at Ellisdale were previously considered part of the Englishtown Formation, but sedimentological analysis has demonstrated that this stratum is trangressive and is more appropriately placed in the Marshalltown. Thus, we propose the recognition of an Ellisdale Member of the Marshalltown Formation present in the central portion of the NJ strike zone. Structural models suggest the Ellisdale Member may represent the delta of a significant Late Cretaceous river that lay between the northern flank of the Southern New Jersey structural high and the southern edge of the Raritan Embayment in a low spot.