Facies Interpretation in Ordovician Carbonates of Southern Ontario Using K-Bentonites

Robert A. Trevail, Warren D. Huff, Dennis R. Kolata

The ability to interpret accurately paleogeographic and stratigraphic relationships existing in a sedimentary sequence can be significantly enhanced if one or more chronostratigraphic markers occur within the section under study. K-bentonite beds of the Ordovician Trenton and Black River Groups of southern Ontario represent the distal remnants of widespread airfall volcanic ash beds that were deposited in a marine environment on a slowly subsiding carbonate platform. Because deposition occurred essentially instantaneously in a geologic time frame, the beds make ideal chronostratigraphic markers, assuming they can be correlated on a regional basis.

Sixty-nine samples representing a number of K-bentonite beds were collected from 19 cores distributed across southern Ontario and analyzed by XRF, ICP/MS, and INAA for a specific set of major, minor, and trace elements. Stepwise discriminant analysis of the analytical results indicates that individual beds can be identified on the basis of chemical composition or fingerprint.

Two regional cross sections were constructed utilizing the most consistently identifiable K-bentonite beds. Facies relationships worked out on this basis indicate that Black River carbonates were deposited in water that gradually deepened to the southwest. Conversely, and probably due to tectonic forces associated with Taconic orogenesis, Trenton carbonate deposition occurred in relatively deeper water on a ramp dipping into the Appalachian basin.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.