Rocks of this unit were first described by Fahrig (1965) and Dressler (1974). Dressler (1979) then introduced the name Aulneau Formation to designate a sequence of sedimentary rocks and carbonatized pyroclastics outcropping east of Aulneau Lake and at Erich-Dimroth Lake (NTS sheet 24F02).

Aulneau Formation 1 (pPau1): Conglomerate Containing Dolomite Pebbles and Locally Gneiss, Fine Sandstone or Mafic Volcanic Rock Pebbles

The best outcrops of this unit are observed ~0.5 km east of Aulneau and Erich-Dimroth lakes (sheet 24F02). East of Aulneau Lake, the unit consists of clast-supported polymictic conglomerate composed of angular to subrounded dolomite pebbles and locally fine sandstone and mafic volcanic rock pebbles. The diameter of dolomite fragments varies from 0.2 to 30 cm, while sandstone and volcanic rock fragments reach 2 cm. The matrix is silty and composed of carbonate, quartz and feldspar (Dressler, 1979). In the Erich-Dimroth Lake area, conglomerate is polymictic and has a clast-supported, locally matrix-supported structure. It contains subrounded fragments of gneiss, dolomite, sandstone and volcanic rock up to 1 m in diameter, embedded in a well-foliated matrix of carbonate, chlorite and biotite (Dressler, 1979). According to Dressler (1979), some fragments display strong tectonic deformation.

This unit consists of grey, light grey or brownish, finely stratified quartzose or carbonatized sandstone and siltstone. Beds range in thickness from a few millimetres to ~5 cm and occur as an interbedded quartzose and carbonate-rich strips. Beds contain variable amounts of quartz in carbonate cement. Carbonate-rich beds commonly have a brownish colour due to the iron oxide dust they contain. In places, stratification is obscured by tectonic deformation (Dressler, 1979).

Aulneau Formation 3 (pPau3): Grey Mudstone, Locally with Sulfate Efflorescence

Mudstone varies in thickness from a few metres east of Aulneau Lake to >100m at Erich-Dimroth Lake. Mudstone is commonly grey and finely laminated. At Erich-Dimroth Lake, it has a whitish or yellowish patina due to efflorescence of sulfates (gypsum, hexahydrite) and iron hydroxides (Dressler, 1979).

Aulneau Formation 4 (pPau4): Grey, Buff or Brownish Dolomite with Rusty Brown Patina, Tuffaceous Dolomite

This unit consists of fine-grained or aphanitic grey, buff or brownish dolomite beds having a rusty brown altered patina. Dolomite changes laterally and vertically into tuff and dolomitic tuff (pPau5). In several places, Dressler (1979) notes that dolomite appears to contain plagioclase and quartz of detrital origin. Modal analyses of rocks indicate that they are composed of carbonates (53.6-73.2%), quartz and plagioclase (12.5-21.5%), and biotite (11-21.8%). Accessory minerals are muscovite, chlorite and opaque minerals (Dressler, 1979).

Aulneau Formation 5 (pPau5): Pyroclastic Rocks Generally Carbonatized

Pyroclastic rocks consist of tuff, dolomitic tuff, agglomerate and dolomitic agglomerate, metamorphosed to the greenschist facies. They are characterized by a high carbonate content that could reach >20%. Tuff and agglomerate depleted in carbonate are rarely observed. These rocks are greenish grey. Dolomitic tuff and dolomitic agglomerate are interstratified with dolomite, tuffaceous or not. They appear to pass laterally or vertically to dolomite (pPau4) in several places. Dolomitic tuff and agglomerate are greenish grey to brownish in fresh surface and yellowish rusty-brown in altered surface. They are composed of mafic volcanic debris of variable size and fine fragments of altered ferromagnesian minerals, floating in a matrix of carbonate, chlorite and variable amounts of biotite. Some fragments are completely replaced by dolomite, biotite and opaque minerals. Small grains of xenomorphic quartz and plagioclase are locally observed in the matrix (Dressler, 1979). A modal analysis of a sample of dolomitic tuff indicates that it is composed of 14.8% carbonate, 0.7% plagioclase and quartz, 49.6% biotite, 31.4% tremolite, 3.3% leucoxene, 0.2% apatite and traces of opaque minerals (Dressler, 1979). To the north, Dressler (1979) reports that dolomitic tuffs change to biotite-chlorite-carbonate schists.

Thickness and distribution

The Aulneau Formation belongs to the Gerido Lithotectonic Zone as defined by Clark and Wares (2004). It extends over a length of ~20 km in a NW-SE orientation, west of Le Moyne Lake (sheet 24F01). Its thickness varies from 500 m to 1000 m (Dressler, 1979).

Dating

None.