TURBIDITE CHANNEL OVERBANK DEPOSITION IN A LOWER DEVONIAN OROGENIC SHALE BASIN, FORTIN GROUP OF GASPE PENINSULA, NORTHERN APPALACHIANS, CANADA

The Lower Devonian Fortin Group, one of the major stratigraphic units in Connecticut Valley-Gaspe Synclinorium of the Gaspe Peninsula, and i ts stratigraphic equivalent, the Temiscouata Formation in northwestern New Brunswick and southeastern Quebec, are interpreted as a base-of-s lope to deep-basin turbidite succession sandwiched between shallow-mar ine to nonmarine Siluro-Devonian sediments. The deep-water interpretat ion is based on a comparison with modern deep-sea turbidite deposition al environments and the lower-slope to deep-basinal settings establish ed by previous investigators for equivalent rocks in New England. The monotonous sequences of largely unfossiliferous, well-bedded dark slat e, 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 ov erbank and channel-fill facies associations. Seven lithofacies have be en 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 l amination (turbidite d division) followed by slate (turbidite e divisi on and hemipelagite) are analogous to muddy spill-over turbidites with interbedded hemipelagites on modern natural levees that accompany dee p channels. Among the fine-grained sediments, a proximal-to-distal fac ies succession (with respect to presumed lateral distance from channel s) consists of (3) siltstone-slate, (4) banded slate, and (5) laminate d slate, if found in lateral succession. The same succession may refle ct spill-over from progressively deeper channels. The channel/levee in terpretation is supported by field observations of (i) a partly expose d channel wall, (ii) truncation of siltstone and slate layers at sands tone contacts, (iii) lag material at the base of massive sandstone lay ers, and (iv) the chevron pattern of nearly opposing paleocurrent dire ctions in the spill-over facies. (6) Syndepositionally deformed and di amictite facies comprise fine-grained rocks of Facies 3-5. (7) Volumet rically subordinate conglomerates are the coarsest of the redeposited sediments. Siegenian (Praguian)-Emsian turbidite deposition occurred i n a deep basin of the Connecticut Valley-Gaspe Trough that developed i n response either to wrench tectonics or back-are spreading related to a volcanic island are (Piscataquis volcanic belt)/deep-sea trench sys tem (Merrimack-Frederiction Trough).