Sedimentology, Petrography, and Depositional Environment of Dore Sediments Above the Helen-Eleanor Iron Range, Wawa, Ontario

Abstract

<p>Archean sediments of the Dore group, located in the Wawa greenstone belt, were studied. The sediments are stratigraphically above the carbonate facies Helen-Eleanor section of the Michipicoten Iron Formation. A mafic volcanic unit, 90 m thick, lies between the iron range and the sediments.</p> <p> Four main facies have been identified in the first cycle of clastic sedimentation above the mafic flow rocks, a 170 m break in stratigraphy separates the volcanics from the first facies. The basal sedimentary facies in an unstratified and poorly sorted granule-cobble conglomerate, 200 m thick, interpreted as an alluvial mass flow deposit. Above the conglomerate, there is a 20 m break in the statigraphic column. The second facies, 220 m to 415 m thick, consists of laminated (0.1-2 cm) argilites and siltstones, together with massive, thick-bedded (8-10 cm) greywackes. The argillites and siltstones are interpreted as interchannel deposits on an upper submarine fan, and the greywackes are explained as in-channel turbidity current deposition. The third facies, 750 m to 1300 m thick, consists of normally graded greywackes (15-20 cm thick beds) interbedded with sharp based siltstones; respective interpretations are overbanked and in-channel deposition on the middle fan. Above the siltstones, there is a 100 m break in the stratigraphic sequence. The fourth facies is a subarkosic sandstone, 1400 m to 1625 m thick, which is characterized by alternating cycles of ripples, megaripples and parallel lamination--suggesting deposition by an ephermal or braided river. The sedimentary cycle is terminated by the intervention of the thick gabbroic sill.</p> <p>The thick-bedded greywackes of the second facies contain significant amounts of authigenic talc, magnesite, ferroan dolimte and chlorite; they are not rich in detrital quartz (up to 33%) and feldspar (less than 1%). The scarcity of detrital minerals does not strictly comply with the definition of greywacke. However, based on the Mg-Fe character of the authigenic assemblage, the original or detrital minerals are thought to have been ferromegnesian.</p>