The anatomy and interpretation of an Archean volcaniclastic sequence, Noranda region, Quebec

Abstract

Archean pyroclastic rocks at Reneault and D'Alembert in the Noranda region are composed of crystals and lithic, vitric and pumiceous fragments. The pyroclastic rocks contain two types of beds, whose primary structure sequences and other characteristics show that the deposits accumulated from different types of density flows. The first bed-type (A) is characterized by greater bed-thickness, reverse grading and the coarsest fragments in the deposit. Closest to source, the primary structure sequence in Type A beds is indicative of deposition from debris flows, but in more distal sections it suggests deposition from turbulent suspensions. The second bed-type (B) is characterized by a smaller fragment size, thinner beds, normal grading and the presence of parallel and (or) oblique stratifications. The primary structure sequence in Type B beds is similar to that observed in turbidites, which suggests deposition from turbulent suspensions of low density.Lateral and vertical variations of grain size, bed thickness and structure sequences indicate that the vent was south of Reneault and that the volcanic activity increased with time. Pillowed flows both underlie and overlie the pyroclastic units which suggests accumulation under water, but there is evidence that some eruptions were subaerial.