Abstract

Unusually shaped concretions are occasionally dredged on the north-central and northeast margin of Georges Bank. In their centers are molds and casts of molluscan fossils of Miocene age. The fauna is probably correlative with fauna of the Chesapeake Group of Maryland and suggests deposition in a shallow open sea with cool temperate water. The mineralogical and textural composition of the sandy mudstone and muddy sandstone surrounding the fossils is similar to that of some Tertiary Atlantic Coastal Plain units including Miocene deposits. The framework consists of angular quartz of fine-sand grade, feldspar, lithic grains, phosphorite, glauconite, mica, plant fibers, and dark organic grains. Ovules, probably fecal pellets, and patches of matrix reworked by organisms are common. Grains float in a matrix paste of microcrystalline calcite, clay minerals, phosphate, ferruginous cement, or mixtures thereof. The concretions are early diagenetic in origin and are concentrated at present as a lag pavement in areas near outcrops of Miocene deposits on the top of Georges Bank.

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