Abstract

The Lower Devonian Fortin Group, one of the major stratigraphic units in Connecticut Valley-Gaspe Synclinorium of the Gaspe Peninsula, and its stratigraphic equivalent, the Temiscouata Formation in northwestern New Brunswick and southeastern Quebec, are interpreted as a base-of-slope to deep-basin turbidite succession sandwiched between shallow-marine to nonmarine Siluro-Devonian sediments. The deep-water interpretation is based on a comparison with modern deep-sea turbidite depositional environments and the lower-slope to deep-basinal settings established by previous investigators for equivalent rocks in New England. The monotonous sequences of largely unfossiliferous, well-bedded dark slate, phyllite, and siltstone with intercalated sandstone packets of the Fortin Group and Temiscouata Formation in the Matapedia and Madawaska/St. John Valley regions, respectively, are interpreted as submarine overbank and channel-fill facies associations. Seven lithofacies have been distinguished: (1) Associations of massive sandstone beds up to 13 m thick and (2) associations of sandstone beds of intermediate (0.5-2.0 m) thickness forming isolated sandstone packets encased in slate and siltstone are the equivalent of modern deep channel-fill deposits. (3-5) Laterally continuous, largely nonbioturbated thin siltstone layers with climbing-ripple lamination (turbidite c division) and parallel lamination (turbidite d division) followed by slate (turbidite e division and hemipelagite) are analogous to muddy spill-over turbidites with interbedded hemipelagites on modern natural levees that accompany deep channels. Among the fine-grained sediments, a proximal-to-distal facies succession (with respect to presumed lateral distance from channels) consists of (3) siltstone-slate, (4) banded slate, and (5) laminated slate, if found in lateral succession. The same succession may reflect spill-over from progressively deeper channels. The channel/levee interpretation is supported by field observations of (i) a partly exposed channel wall, (ii) truncation of siltstone and slate layers at sandstone contacts, (iii) lag material at the base of massive sandstone layers, and (iv) the chevron pattern of nearly opposing paleocurrent directions in the spill-over facies. (6) Syndepositionally deformed and diamictite facies comprise fine-grained rocks of Facies 3-5. (7) Volumetrically subordinate conglomerates are the coarsest of the redeposited sediments. Siegenian (Praguian)-Emsian turbidite deposition occurred in a deep basin of the Connecticut Valley-Gaspe Trough that developed in response either to wrench tectonics or back-arc spreading related to a volcanic island arc (Piscataquis volcanic belt)/deep-sea trench system (Merrimack-Fredericton Trough).

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