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Pontax Formation
Stratigraphic label: [narc]pnt
Map symbol: nApnt

First published: 14 November 2017
Last modified:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Translation of original French

 

 

Informal subdivision(s)
Numbering does not necessarily reflect the stratigraphic position.
 
nApnt2 Wacke with conglomerate horizons
nApnt1 Conglomerate with wacke horizons
 
Author:Bandyayera and Daoudene, in preparation
Age:Neoarchean
Reference section:None
Type area:NW portion of Boisrobert Lake area (NTS sheet 32N11)
Geological province:Superior Province
Geological subdivision:La Grande Subprovince
Lithology:Sedimentary rocks
Type:Lithostratigraphic
Rank:Formation
Status:Formal
Use:Active

 

Related unit(s)

 

  • Eastmain Group

     

    • Auclair Formation
    • Low Formation
    • Clarkie Formation
    • Anaconda Formation
    • Wabamisk Formation
    • Komo Formation
    • Kasak Formation
    • Bernou Formation
    • Pontax Formation
    • Anatacau-Pivert Formation
    • Pilipas Formation
    • Natel Formation
    • Kauputauch Formation
 

 

 

Background

Volcano-sedimentary rocks of the Boisrobert region (NTS sheet 32N11) were all assigned to the Anatacau-Pivert Formation during compilation of the La Grande Subprovince in SIGÉOM, based on the work of Franconi (1978) and Moukhsil (2000). New geophysical data (D’Amours, 2011), as well as results of the 2016 geological survey (Bandyayera and Daoudene, in preparation), helped delimit and better define the sedimentary sequence of this area, formally referred to as the Pontax Formation.

 

Description

The Pontax Formation is introduced to describe a sedimentary sequence of conglomerates and wackes located at contact between the Lower Eastmain River Volcanic Band (LERVB) to the north, the Champion Lake terrain to the south and west, both belonging to the La Grande Subprovince, and the Rupert Complex of the Nemiscau Subprovince to the est. The Pontax Formation unconformably lie on amphibolitized basalts of the Anatacau-Pivert Formation (unit nAnp1). Some of the Pontax Formation outcrops show quartz veins that cut basalt and end sharply in contact with sediments, which shows that they predate deposition of the sedimentary sequence. Sediment stratification is also oblique to bedding observed in basalts. The Pontax Formation was not included into the metasedimentary Nemiscau Subprovince due to its close association with the LERVB, preservation of primary structures and low-grade metamorphism. It consists of two units: 1) conglomerate with wacke horizons (nApnt1), and 2) wacke with conglomerate horizons (nApnt2).

 

Pontax Formation 1 (nApny1): Conglomerate with Wacke Horizons

Unit nApnt1 consists of a package dominated by conglomerates locally associated with wacke horizons. Conglomerate is composed of subrounded, joint or floating monogenetic to polygenetic fragments. These centimetric to decimentric fragments make up up 80% of the rock. Their composition varies from felsic to intermediate (tonalite, granodiorite, diorite). Conglomerate also contains fine-grained mafic (5%) or felsic (10%) fragments that are likely derived from erosion of the underlying volcanic sequence. The matrix is medium grained, rich in biotite (30%) and contains up to 20% garnet. Garnet is present in both the matrix and fragments.

Polygenetic conglomerate contains bedded wacke horizons consisting of alternating centimetric to metric beds that are more or less rich in cordierite and garnet porphyroblasts. This alternation likely reflects variations in the primary composition of the protolith composed of sand beds and more aluminous clay beds. Locally, preferential elongation of porphyroblasts marks a mineral and stretch lineation.

In places, unit nApnt1 contains metric to decametric horizons of monogenetic conglomerate interstratified with centimetric to decimetric wacke beds. Monogenetic conglomerate contains more than 50% dark grey, subrounded, slightly stretched and flattened felsic clasts, ranging in diameter from 5 mm to 3 cm. The matrix is similar to biotite paragneiss (25-35%) and garnet paragneiss (15%). Wacke beds are fine to medium grained, foliated and resemble the conglomerate’s matrix.

In the NW corner of the Boisrobert region (sheet 32N11), north of the Pontax River, there are several horizons of wacke and conglomeratic wacke interstratified with conglomerate beds. Conglomeratic wacke horizons have well-preserved primary bedding and reverse sorting indicating stratigraphic polarity of the sedimentary sequence to the SE. In this area, wacke horizons contain in several places metric horizons rich in cordierite (30%) and garnet (20%). These horizons are in contact with heavily altered zones characterized by many metasomatic zoned veins.

 

Pontax Formation 2 (nApny2): Wacke with Conglomerate Horizons

Unit nApnt2 contains, in places, decametric horizons of metasediments rich in cordierite (2-15%) and garnet (2-20%) poikiloblast and, more rarely, sillimanite (5-10%). The distribution of cordierite in some beds probably reflects variations in composition associated with bedding in the protolith, whereas garnet is everywhere. Cordierite grains are deformed, flattened and partially transformed to pinnite. Sillimanite occurs as prismatic rods or brown fibre clusters. Primary stratification is locally preserved in certain wacke horizons that have cross-bedding or parallel lamination.

Under the microscope, wacke contains quartz (25-40%), plagioclase (5-15%), K-feldspar (5-15%), dark to brownish orange biotite (5-25%), chlorite (2-10%), muscovite (2-15%), sericite (5-20%), garnet (up to 20%), cordierite (0-20%), epidote (0-5%), sillimanite (2-5%) and tourmaline (2%). Accessory minerals are brown tourmaline (dravite), hornblende, apatite, titanite, carbonates, zircon and monazite. 

 

Thickness and distribution

The Pontax Formation outcrops NW of the Boisrobert Lake area (sheet 32N11); its E-W lateral extension is more than 20 km long and varies in thickness from 1 to 6 km.

 

Dating

None. 

Stratigraphic Relationship(s)

The Pontax Formation unconformably lies on amphibolitized basalts of the Anatacau-Pivert Formation. It is cut by late intrusive phases of the Champion Complex. 

Paleontology

Does not apply.

References

Author(s)TitleYear of PublicationHyperlink (EXAMINE or Other)
BANDYAYERA, D. – DAOUDENE, Y.Géologie de la région du lac Nemiscau, secteur ouest de la rivière Rupert (SNRC 32N06, 32N07 et 32N11). Ministère de l’Énergie et des Ressources naturelles, Québec.In preparation
FRANCONI, A.La bande volcanosédimentaire de la rivière Eastmain inférieure (ouest de la longitude 76° 15’). Ministère des Richesses naturelles, Québec; DPV 574, 186 pages et 2 plans.1978DPV 574
MOUKHSIL, A.Géologie de la région des lacs Pivert, Anatacau, Kauputauchechun et Wapamisk (SNRC 33C01, 33C02, 33C07, 33C08). Ministère des Ressources naturelles, Québec; RG 2000-04, 49 pages et 4 plans.2000RG 2000-04
D’AMOURS, I.Synthèse des levés magnétiques de la Baie-James. Ministère des Ressources naturelles et de la Faune, Québec; DP 2011-08, 5 pages, 2 plans.2011DP 2011-08

 

 

4 février 2019