57 Advances and Applications in Geochronology for Interpreting Stratigraphic and Basin Records, Part II

This session welcomes submissions related to applications of geochronology (e.g., U-Pb, 40Ar/39Ar) in stratigraphic research, including topics such as eruption and maximum depositional age estimation, provenance analysis, numerical modeling of geochronological data, and multi-disciplinary chronostratigraphic characterization.

Sunday, 19 October 2025
1:30 PM - 5:30 PM
HBGCC, 304C

Defining the Timing of the Ordovician Sediment Deposition in Southern Quebec and Ontario (Canada) Using K-bentonite Tephrochronology

Paper number: 57-3
Program: Technical Sessions
Time: 2:10 PM – 2:25 PM

Authors

Abstract

The St. Lawrence Platform in southern Quebec is a series of Cambro-Ordovician carbonate and siliciclastic units and several interbedded volcanic ash layers. In the Montreal area, the Ordovician Trenton Group is composed of 200-250m of well-bedded and fossiliferous marine limestone. The stratigraphic sequence consists of the Mile-End, Deschambault, Montréal and Tétreauville formations that grades from calcarenite, to massive crystalline limestones and finally to alternating micritic limestones and shale layers. The Neuville formation of Québec city and the Lindsay Formation of south-eastern Ontario are comparable to the micritic limestones of the Tétreauville Formation. The Cambro-Ordovician stratigraphic sequence records the opening of a large ocean basin in response to extensional tectonics post Grenville orogeny, and its closing during the Appalachian orogeny. Understanding the complex tectonomagmatic and stratigraphic evolution of the St. Lawrence platform hinges on precise temporal constraints for the deposition of the Cambro-Ordovician sedimentary sequences. 

Several weathered volcanic ash beds, known as K-bentonites, are interbedded in this sedimentary sequence. These ashes contain relatively pristine primary zircon crystals that can be used to provide precise ages to help determine the timing of sediment deposition. Eight bentonite beds have been sampled in the Trenton Group within Montreal Island, whereas two beds were collected in north of Québec City and eleven beds in south-eastern Ontario.

U-Pb dating has been carried out on zircon crystals by LA-ICP-MS and high-precision CA-ID-TIMS techniques. The LA-ICP-MS data (320 dated grains) from the Montréal bentonites show that most grain rims record Sandbian to Katian (458.2-445.2Ma) Ordovician ages. Some inherited ages grouped around 998-1202Ma suggesting contamination by Laurentian basement rocks (Grenville orogeny) in the magmatic source region. Preliminary CA-ID-TIMS data show significant variations in zircon ages within individual samples, suggesting abundant autocrystic recycling of zircon, or the presence of inherited xenocrystic cores. Nonetheless, the youngest ages obtained for many individual samples agree with the stratigraphic superposition of these bentonites and possibly approximate their eruption ages. It provides some degree of confidence that our data can be used to estimate the timing of ash emplacement and sediment deposition. The current dataset suggests a preliminary sedimentation rate of 2cm/1000 years consistent with this type of sediments. Other samples from Montreal will be used to refine the ages and sedimentation rates throughout the stratigraphic column.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413238

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