The unconformable character and paleogeographic significance of the Chazy-Black River group contact, Montréal area, southwestern Quebec

Abstract

An unconformity marks the Chazy-Black River group contact in southwestern Quebec. It is characterized by an eastward pinching out of the Pamelia Formation (Black River Group), borings, and truncation of the Chazy strata. Coarse-grained sandstone of the Pamelia Formation or Lowville Formation (where the former is missing) lies above the contact. Lithological differences between dolostone units in the Laval (Chazy Group) and Pamelia for- mations were recognized, allowing us to better position the contact. The Pamelia Formation (44 mthick) in eastern Ontario conformably overlies the Chazyan strata. From Montréal to Québec progressively younger Black River strata onlap older rocks. This onlapping is accompanied by substantial lithological changes where sandstone and different limestone lithofacies are preserved. These variations suggest a more limited accommodation space for the Black River strata in southwestern Quebec, and, therefore uplifting relative to eastern Ontario. This uplifting is probably due to old structural lineaments reactivation by the then ongoing Taconian Orogeny, and may indicate an incipient upthrow of the Beauharnois anticline.