--> Spatial and Temporal Evolution of Matrix-Poor to Matrix-Rich Sandstones in the Ordovician Cloridorme Formation, Quebec, Canada: A Detailed Reassessment of Greywackes After Half a Century

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Spatial and Temporal Evolution of Matrix-Poor to Matrix-Rich Sandstones in the Ordovician Cloridorme Formation, Quebec, Canada: A Detailed Reassessment of Greywackes After Half a Century

Abstract

Deep-marine sandstones with significant (>20%) detrital mud and silt matrix content have been variously termed slurry beds, transitional flow deposits and hybrid event beds, however, details about the spatial and temporal evolution of these enigmatic rocks remain poorly understood. Exceptional wave-cut exposures in the Ordovician Cloridorme Formation, Quebec provide an unparalleled opportunity to document the lateral and vertical changes of similarly matrix-rich but also associated matrix-poor basin-floor sandstone.

Matrix-poor sandstone occurs as thick- to medium bedded, fine- to coarse-grained, scour-based, <20% matrix content, normally graded sandstone, whereas matrix-rich sandstone (MRS) occurs as flat-based, massive to coarse-tail graded, medium- to thin-bedded, fine- to coarse-grained sandstone with 20-80% matrix and common mudstone clasts. Based on matrix content percent, MRS can be subdivided into: clayey sandstone (20-50%); bipartite bed with a basal sandy (20-60%) part overlain sharply by a planar- to irregular-based muddier portion (40-80%); and sandy claystone (50-80%).

These strata form a horizontal depositional continuum consisting of matrix-poor sandstone to clayey sandstone to bipartite bed and then sandy claystone over a distance of a few to several 100s m, which then is draped everywhere by a thin-bedded turbidite and/or silty mudstone cap. Vertically, similar facies preferentially overlie one another and form packages 2-9 beds thick. These consistent vertical and horizontal changes in lithology suggest regularity in the spatial and temporal character and pattern of sedimentation, which here is interpreted to be a consequence of particle settling in a rapidly but systematically evolving negligibly-sheared sand-mud suspension.

At an even larger scale, packages stack vertically to form 1-7 m-thick units that consist of interbedded matrix-poor and matrix-rich sandstone or alternatively only matrix-rich sandstone. These units are typically overlain by 2-9 m thick sand-rich basin floor splays, and only uncommonly, are intercalated with fine-grained, thin-bedded turbidite deposits. The systematic arrangement of these units is interpreted to reflect deposition along the margins of a turbulent jet flow related to upflow channel avulsion and the initiation of the local sediment-transport system.