Petrology and potential tectonic significance of a K-bentonite in a Taconian shale basin (eastern Ontario, Canada), northern Appalachians

A 6 cm thick K-bentonite, herein defined as the Russell Bed, occurs in an Upper Ordovician deep-basin shale succession in eastern Ontario, Canada, forming part of the distal Taconic foreland in eastern North America. The bed lies within the pygmaeus graptolite Biozone, which is about 451 to 452 Ma i...

Full description

Saved in:
Bibliographic Details
Published in:Geological magazine 2005-03, Vol.142 (2), p.145-158
Main Authors: SHARMA, SAJAL, DIX, GEORGE R., VILLENEUVE, MIKE
Format: Article
Language:English
Subjects:
Appalachians
Bentonite
biozones
Canada
clastic rocks
Clay
Crystals
Deep water
depositional environment
Eastern Canada
eastern Ontario
flysch
Geochemistry
Geology
Graptolithina
Illite
K-bentonite
mineral composition
Mineralogy
North America
Northern Appalachians
Ontario
Ordovician
Original Article
Paleoceanography
paleogeography
Paleozoic
Petrology
Plate tectonics
Russell Bed
sed rocks, sediments
Sedimentary petrology
sedimentary rocks
shale
Shales
Silurian
Species extinction
Structural geology
Taconic Basin
Taconic Orogeny
tectonics
Upper Ordovician
Citations: Items that cite this one
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:A 6 cm thick K-bentonite, herein defined as the Russell Bed, occurs in an Upper Ordovician deep-basin shale succession in eastern Ontario, Canada, forming part of the distal Taconic foreland in eastern North America. The bed lies within the pygmaeus graptolite Biozone, which is about 451 to 452 Ma in age. Although some bentonites are reported from this interval in eastern North America, we are reporting the first set of compositional data for a bentonite of this age. Gamma-log correlation identifies a potential minimum distribution area of 90%)–smectite (I/S) structure with about 7.5% K2O%. Age-dating by Ar–Ar analysis of the phlogopite crystals yielded a younger than expected (440–445 Ma) age. This difference, along with evidence of localized chloritization of phlogopite, likely reflects known post-Ordovician hydrothermal activity within the basin. On the basis of several geochemical proxies, the magmatic source of the Russell K-bentonite falls within the trachyandesite field and was Ba-enriched. Comparison of geochemistry and mineralogy with older, Middle to Late Ordovician and younger Early Silurian K-bentonites within the Taconic orogen along eastern Laurentia and Baltica reveals that the potential source magma for the Russell Bed was more mafic, more alkaline, and less fractionated than sources typical of older (platform) bentonites. Instead, it is more similar to the younger Llandovery bentonites of Scandinavia and Scotland. It remains uncertain if it signals local or regional compositional change in volcanic source in the northern Appalach
ISSN:0016-7568
1469-5081
DOI:10.1017/S001675680400041X