Abstract

The Taconic foreland basin resulted from a collision between the North American craton and the Ammonoosuc arc. The basin is positioned between a broad carbonate shelf on the west and the clastic arc terrane. In the downslope direction, basin deposits changed from distal shelf carbonates (Trenton Limestone), to coeval interbedded hemipelagic black shales and calcilutites on the slope (Dolgeville Formation and Utica Shale), to silty shales, siltstones, and sandstones laid down by turbidites on the basin floor (Snake Hill Shale). Transport direction was to the east on the slope, and axial (north-northeast–south-southwest) on the basin floor. Clay particles were derived from the volcanic source terrane. The facies gradually shifted west laterally; the length of time span investigated is about 6 m.y.

From the distribution of organic carbon and the concentration of benthic epifauna and infauna, it can be inferred that conditions were aerobic on the shelf (> 1 ml/L O2), anaerobic on the slope (< 0.4 ml/L O2), and dysaerobic on the basin floor (< 1 ml/L O2). Through time, four long-term anaerobic and dysaerobic cycles are revealed, lasting between 500,000 and 1,000,000 yr. Anaerobic cycles are characterized by over 50% higher organic carbon values, lack of infaunal burrowing traces, and a highly impoverished benthic epifauna. Dysaerobic cycles are marked by lower organic carbon contents, sporadic burrowing traces, and a slightly more diverse and abundant benthic epifauna. This cyclicity was most likely caused by changes in the density stratification within the water column, possibly related to climatic changes. The longest anaerobic cycle occurred during the transgressive phase that led to widespread deposition of black shale over the carbonate platform.

Anoxic conditions in the Taconic foreland basin may have been influenced by the prevailing global oceanographic conditions during the Middle Ordovician.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.