Clearing the Muddy Waters: Sedimentological Characterization and Comparison of Matrix-Rich Sandstones in the Neoproterozoic Windermere Turbidite System (British Columbia, Canada) and Ordovician Cloridorme Formation (Quebec, Canada)

Abstract

Although matrix-rich sandstones (MRS) i.e. sand rich strata with significant (>10%) matrix (mud & silt) content have become widely recognized in both the modern and ancient deep-marine sedimentary record, it is becoming increasingly evident that much work is needed to differentiate the various different kinds of MRS’ and link them to their unique modes of deposition. This study intends to elucidate the similarities and differences among MRS observed in slope and basin-floor deposits of the passive margin Neoproterozoic Windermere turbidite system and basin-floor deposits of the Taconic foreland-basin, Ordovician Cloridorme Formation (CF). In both areas, MRS occurs as flat-based, coarse-tail graded or massive, few cm- to several dm-thick, fine- to coarse-grained sandstone with 10 to 80% matrix and common mudstone clasts. Preliminary results show that the grain-size distribution of MRS are similar in both areas, but their matrix content varies. Based on matrix content MRS can be categorized into: F1 sandstone with 10-45% matrix content; F2a bipartite strata with a basal sandy (10-45% matrix) part overlain sharply by a planar-based muddier portion (20-80% matrix); F2b also a two-part bed, but with a sharp, irregular contact separating the basal sandy (10-45% matrix) and upper muddier (50%-80% matrix) portions and upward sand injection from the basal layer, and F3 sandstone with 50-80% matrix content. Typically, these facies are overlain by a fine-grained, well-sorted, thin-bedded traction structured unit and/or thin mudstone cap.

In both study areas F1, F2a, and F3 are common and form a depositional continuum that extends for a few to several 100s m laterally. This transition of facies is interpreted to reflect an evolution of flow structure controlled primarily by particle settling, namely sand, from mud-rich avulsion-related flows. Notably, F2a and F2b never stack nor lie adjacent to one another laterally in the CF. Also, F2b are absent in the Windermere. F2a are commonly intercalated with thin-bedded turbidites whereas F2b are mostly associated thick-bedded turbidites. Furthermore, F2b exhibits features similar to hybrid event beds (HEB), but the thinning of the basal sandy part with a corresponding thickening of the upper muddier part occurs over distances of only few m to 100s m. Accordingly, the interpretation of HEB deposited by longitudinal change from fully turbulent to cohesive behavior is not fully understood and requires further investigation.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018