A Mineralogical Study of Some Lower Devonian (Helderberg) Rocks of the Central Hudson Valley, New York

Abstract

ABSTRACT The mineralogy of three lithologies (carbonate, argillaceous limestone, mudstone) has been characterized for formational units of the Helderberg outcrop belt between Ravena and Kingston, New York. In the mudstones, throughout the geologic time interval from Rondout through Becraft, the percentage range of clay minerals and calcite has remained relatively constant whereas there has beeen a continuous increase in quartz and decrease in dolomite. In the carbonate rocks, the quartz and clay mineral content is low and relatively constant; the carbonate mineralogy of the Manlius is more complex than that of other formations. The amount of each mineral in the argillaceous limestones is intermediate between the amounts of the mudstones and limestones. A clay mineral suite unvarying in type (illite and chlorite) and with little variation in relative amounts was available to Helderberg depositional environments. Quartz clasticity values suggest that detrital quartz in all lithologies at both Ravena and Kingston come from the same source. Textural information derived from thin section study and mineralogical data reinforce the ideas of earlier workers concerning the nature of depositional environments for Helderberg carbonate sediments. Mudstones formed in the same physical environments as the carbonate rocks with which they occur. A sensitive balance governed the change from mudstone to limestone sedimentation. The immediate control for this change existed in the region of sedimentation--not in the source area--although the data of this study does not generally permit this control to be identified. Trace element analysis suggests a continuity of New Scotland sedimentary conditions in time, as well as in space. Textures indicate that shortly after deposition of shallow water carbonate sediments, a process of leaching and infilling produced a coarse calcite mosaic in ooids and skeletal material. Concomitant was the increase in grain size of calcite micrite to a coarser spar (micrite enlargement). Dolomitization followed these changes. Grain mount studies show that authigenic quartz crystals formed at some unknown time during this post-depositional sequence of events.