A Petrologic and Diagenetic Study of the Onondaga Formation, New Jersey

Abstract

A petrologic investigation of the Onondaga Formation in New Jersey has never been conducted, although similar studies have taken place in the surrounding states of New York and Pennsylvania. Thirteen sample sites from the formation in New Jersey were analyzed using thin section point counting, insoluble residue analysis, X-ray Diffraction, and Scanning Electron Microscopy/Energy Dispersive X-ray microanalysis. These methods gave insight into the lithology, mineralogy, diagenesis, and depositional environment of the Onondaga Formation. Five main lithofacies were identified. These are two limestones including siliceous mudstones and siliceous mudstones with nodular chert, and three siliceous mudrocks including calcareous mudrocks, calcareous mudrocks with nodular chert, and cherty calcareous mudrocks. The mineralogy of these five lithofacies is roughly consistent amongst the sites. Carbonate minerals (calcite and dolomite), silica, alkali feldspars, and phyllosilicates are the main mineralogical groups with silica being by far the most abundant. Albite was the main alkali feldspar identified and illite, chlorite, and minor kaolinite, were the three main phyllosilicates. Carbonate mud and terrigenous sediments forming the Onondaga Formation were deposited in a carbonate ramp environment. After deposition, these sediments underwent burial and compaction, cementation, dissolution of fossils, chert nodule growth and dolomite formation at the expense of the carbonate mud, further burial and compaction in the form of fractures, stylolites, and pressure seams, and finally healing of these fractures by sparite. The source of silica for chert nodule growth consisted of sponge spicules, dissolution of terrigenous sediment, influx of silica rich fluids from meteoric waters, and minor volcanic ash dissolution. These sediments were formed close to their source of origin during the incipient Acadian Orogeny. The small amount of clay minerals present, with respect to silica, in the samples points to not much weathering of the newly formed mountains during sediment accumulation.