Sedimentology and stratigraphy of the northern and central metasedimentary belts in the Beardmore-Geraldton area of northern Ontario

Abstract

The northern and central metasedimentary belts (NMB, CMB) are east-trending, regional (formation- to group-scale) iithostratigraphic units within the Beardmore-Geraldton Archean terrane of northern Ontario's Superior Province. Gravelly braided rivers deposited the lithofacies assemblage of clast-supported polymict conglomerate and interbedded sandstone that comprises most of the NMB. Felsic volcanic pebbles and cobbles are the most abundant clast lithology. The CMB contains several east-trendinq lithofacies assemblages that together form a generally northward coarsening sequence. The southern CMB is composed of mudstone, iron formation and sandstone with common graded beds. This fine-grained assemblage is paralleled to the north by a horizon of rhythmically bedded and cross-bedded sandstone units, and a heterogeneous, conglomerate-rich assemblage. The northern CMB strata are mostly a conglomeratic assemblage similar in form (lithofacies) and composition (provenance) to the NMB. Some fine-grained units (mudstone. iron formation, graded sandstone) of relatively minor regional extent are present near the north margin of the CMB. The CMB's southern (lower) fine-grained subaqueous facies are apparently capped at different sites by: 1) rhythmic delta front couplets; 2) sandy braided river deposits with very rare associated intertidal strata; and 3) a conglomeratic submarine fan or fan-delta front resedimented assemblage. The northern (upper) CMB is a gravelly braided river deposit, with minor aquabasinal facies. The CMB is probably a 1-2 km thick structurally modified homoclinal sequence. Relative positions of its depositional paleoenvironments, as deduced from lithofacies assemblages, suggest that the epiclastic portion of the CMB is the record of a dominantly coarsening-upward, subaqueous to subaerial trend that was produced by a prograding clastic system(s), likely a number of fan-deltas. The extreme eastern part of the CMB is composed of generally oligomict, coarse (conglomeratic) felsic pyroclastic and/or reworked volcaniclastic facies which are probably subaerial deposits. Similar clast compositions and several sedimentoiogical criteria, including average maximum deformed clast size, strongly suggest that highly proximal fluvial facies in the NMB and more distal fluvial and aquabasinal facies in the CMB were originally part of a continuous coarse clastic wedge or sheet.