Abstract

The Silurian–Lower Devonian sequence of the Gaspé Belt at the Laurentia margin south of the Québec reentrant and St. Lawrence promontory was deposited during the period between the two main orogenies that created the northern Appalachians: the Late Ordovician Taconian and the Middle Devonian Acadian orogenies. Although this sequence is traditionally considered to have been deposited during a period of quiescence between both orogenies, significant tectonic activity attributed to the Salinic disturbance began during late Llandoverian (Telychian) time and persisted until the Acadian orogeny. This tectonic activity has profoundly influenced the composition and distribution of the Silurian–earliest Devonian sedimentary facies.

The shelf and shelf edge history at the Laurentia margin along the Québec reentrant–St. Lawrence promontory is summarized according to four broad phases. Phase 1 is a Llandoverian–Wenlockian regressive phase (R1) related to post-Taconian successor basin filling, that culminated with extensive carbonate platform development. Phase 2 is a late Wenlockian–Ludlovian transgressive phase (T1). Phase 3 corresponds to a later Ludlovian–Pridolian second regressive phase (R2). Phases 2 and 3 were accompanied by extensional tectonics that produced shelf faulting and block tilting, on top of which block reefs and reef complexes settled and built a reef tract all the way along the Gaspé-Témiscouata shelf. Phase 4 is an Early Devonian phase of accelerated subsidence (transgression T2) affecting the northwestern part of the segment (Québec reentrant area), while the southeastern part (St. Lawrence promontory area) was already uplifted due to the ongoing collision between Laurentia and the western margin of Gondwana-related terranes to the south.

Composition and distribution of sedimentary facies were controlled by the interaction of tectonics, sediment influx, and sea-level fluctuations. Construction of a post-Taconian–pre-Acadian palinspastic map to plot facies has proven to be basic to obtaining a realistic picture of the paleogeography of the shelf and shelf edge of the Gaspé-Témiscouata segment at the margin of the Laurentia craton during the Silurian–earliest Devonian time interval.

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