|Reference section:||Approximately 1 km northeast of Mont-Vidéo Hill, sheet 32C05-200-0202|
|Type area:||Central-east portion of sheet 32C05-200-0202|
|Geological province:||Superior Province|
|Geological subdivision:||Abitibi Subprovince|
Table des matières
Sedimentary rocks of this formation were first identified by James and Mawdsley (1929), who associated them with “Keewatin or Timiskaming” type units. Subsequently, Tremblay (1946, 1947a, 1947b, 1950a, 1950b) assigned these units to the “Kewagama Group”. Dawson (1966) subsequently used the same nomenclature as Tremblay. These sedimentary rocks were described by Jones (1964) in the northwest quarter of the Fiedmont Township. This author provides a comprehensive description of these sedimentary rock units (grauwacke, granitoid pebble polygenetic conglomerate, biotite schist, quartzite), but does not provide a formal formation name at that time. He only calls them “Fiedmont Sedimentary Zone” and interprets them as contemporary to volcanic rocks that surround them. The presence of these sedimentary rocks is also mentioned in Imreh (1979).
The formal name « Fiedmont Formation » appears for the first time in the cartographic synthesis work of Imreh (1984). The name and the names of its subdivisions also appear in the mapping of sheet 32C05-200-0201 by Imreh (1991). The author then reports the presence of sedimentary rocks (grauwacke, conglomerate) located at the interface or within volcanic units of the Héva-Nord Formation and in the upper part of the Dubuisson Formation. Following this logic, the name « Fiedmont Formation » is also used by Beullac (1983), one of the collaborators of the Imreh team, to describe the eastward continuity of these sedimentary units in sheet 32C05-200-0202.
The Fiedmont Formation includes conglomerate, conglomeratic sandstone, grauwacke and biotite schist units. Stratification is characterized by alternating massive grauwacke beds, 2 to 5 cm thick, and fine-grained schistose beds, 7 to 10 cm thick, all forming a SW polarity homoclinal sequence. In general, stratification is observed relatively well but with unequal quality, in part because of increasing deformation on the edges of this formation. Stratification has an orientation ranging from E-W to NE-SW, with a 30 to 60° inclined dip to the SW. It should be noted that these sequences are overturned.
Lithologies have a relatively well-developed schistosity, particularly on the edge of the La Corne batholith. Schistosity is generally oriented 110° with a 50° dip northward. It curves further towards the SE on the eastern edge of sheet 32C05-200-0201. It is further marked by preferential orientation of biotite and sericite flakes.
Two informal units are recognized in the Fiedmont Formation: nAfd2 and nAfd3. Unit Afd1 does not exist.
Sandstone beds range from massive to well laminated, medium to fine grained. They are dark grey, with shades ranging from grey to brown in altered surface. The majority of these rocks are composed of quartz and feldspar, with interstitial biotite, sericite and chlorite. Cross-beds are observed in these grauwackes, indicating a stratigraphic summit towards the SW. Such structures are representative of a shallow fluviatile deposition environment.
Conglomerate beds are located in the central and eastern portions of sheet 32C05-200-0201. This unit consists of blocks, stones and pebbles of mafic and felsic volcanic rocks, biotite quartz diorite, foliated biotite tonalite and quartz vein fragments. These various fragments are coated with a sandy matrix. They are rounded, elliptical, subangular or angular and can reach 20 to 30 cm in diameter. Conglomerate and grauwacke beds can reach an average thickness of a few tens of centimetres.
The Fiedmont sedimentary rock strip occupies a unique structural position, tightly confined by regional faults on its northern (Aiguebelle Fault), eastern and southern sides. It continues for approximately 18 km east-west with an average thickness of about 1 km.
The particular facies associations and polygenetic nature of conglomerates suggest a deposition environment commonly associated with Timiskaming age basins. However, an ongoing U-Pb dating on zircons from a sandy facies gave a preliminary age of 2717 Ma (J. David, personal communication, 2017), comparable to that of the Lanaudière Formation. Further mapping and geochronology work will be required on this formation to clarify these ambiguities.
|Isotopic System||Mineral||Crystallization Age (Ma)||Reference(s)|
|U-Pb||Zircon||2717||David, personal communication, 2017|
This strip of sedimentary rock, of east-west general orientation, is bordered on its north side by the Aiguebelle Fault. The Fiedmont Formation curves around the La Corne Batholith and continues southeast in a north-south direction.
Does not apply.