Sedimentology of an Archean submarine channel-fill deposit in the Abitibi greenstone belt of Canada

Abstract

The Archean La Bruère Formation, which forms part of the Timiskaming Group of the Abitibi greenstone belt of Rouyn–Noranda, comprises a conglomerate–sandstone assemblage at least 360 m thick. The upward variations in grain size and con glomerate/sandstone ratio permit subdivision of the formation into three conglomeratic and two sandy members. Member 1 is composed essentially of conglomerate; members 2 and 4 consist of stacked composite beds of conglomerate and sandstone; member 3 is a sequence of cross-bedded sandstone. The formation is capped by a turbidite sequence (member 5).Three bed types (facies) were identified in the La Bruère Formation: two are restricted to the conglomerate; the third, to the upper member. Facies A, is characterized by matrix-supported, poorly sorted gravel beds, massive or reversely graded. In facies B, generally restricted to member 4, the conglomerate is commonly clast supported and transitional with the overlying sandstone bed, forming couplets that show primary structure sequences consisting, from base to top, of (i) normally graded gravel, (ii) plane-bedded sandstone, and (iii) cross-bedded sandstone. These sequences are similar to, although coarser and thicker than, some sequences found in turbidites. Facies C, observed only in member 5, is characterized by classic turbidites.In the conglomerate members of the formation, beds of facies A show no correlation between bed thickness and maximum grain size, whereas those of facies B show a fairly good correlation. The absence of correlation could result from accumulation in a channel rather than on an alluvial fan, but it may also be due to the nature of the flows responsible for the deposit and (or) the grain availability at the source.The characteristics of the three facies suggest that the La Bruère gravel accumulated from laminar and turbulent mass flows in a subaqueous channel, below storm wavebase and therefore in a relatively deep basin.