SILICA DIAGENESIS IN THE LOWER DEVONIAN HELDERBERG GROUP OF NEW YORK

The Kalkberg Formation of the Helderberg Group was deposited in the Appalachian foreland basin (Lower Devonian). It is a mixed carbonate and siliciclastic unit with abundant pyrite and minor chert and bears local evidence of the Bald Hills volcanics (K-bentonite ash layers). Silicification occurs as chert, fossiliferous chert, and silica replacement of fossils representing both early and late stage silica diagenesis. Silica replacement of fossils occurs both within beds and as a weathering process on fracture solution surfaces. Joint faces have a pattern of differential weathering in which carbonate-rich matrix recedes to reveal silicified fossils and fine-grained siliciclastic-rich lithologies weather out in relief and bear calcitic fossils and molds. Infrequently, brachiopods in siliciclastic-rich matrix are have incomplete replacement by silica and often characterized by the presence of pronounced beekite rings. Brachiopods, crinoids, and gastropods are commonly silicified on joint surfaces, while bryozoans fail to silicify. Synsedimentary chert nodules preserve a diverse assemblage of bryozoan, echinoderm, and brachiopod bioclasts.

Matrix lithology has been suggested as a factor in selective silicification in cases of early diagenesis, but is generally not considered in late stage silica diagenesis selectivity. Similarly, early silicification of fossils has been linked to the presence and decay of organic matter and the original mineralogy of the organism. In late stage silicification, patterns of selectivity are evident (preference for carbonate lithology and preference for brachiopods), but selectivity is poorly constrained. Acid etched samples were compared to thin sections to determine the diagenetic history of silicification and differentiate early from late stage silica diagenesis.