Neoarchean coastal sedimentation in the Shebandowan Group, Northwestern Ontario

Abstract

The study interpreted depositional environments from sedimentological data present in metasedimentaiy rocks of the Neoarchean Shebandowan Group of the Wawa Subprovince. Outcrops in the study area contained sedimentary structures and bed sequences consistent with shallow water, coastal sedimentation, and represents an Important record of Archean depositional processes. Three depositional environments are represented In the rock record; tidal strandline, the shoreface, and the offshore. The tidal strandline was further divided Into the tidal flat and tidal channel sub-environments. The presence of these three environments provides unequivocal evidence for the existence of shallow-water shelves In the Archean; a period during which sedimentation was dominated by deposition in alluvial fan, fluvial environments and deep water settings. The three environments and associated sub-environments record processes reflective of differing current activity which controlled and Influenced deposition. The tidal environment was dominated by bidirectional tidal currents. Deposition In the shoreface was predominated by unidirectional wave-produced currents which overprinted prevailing tidal current activity, in the distal portions of the shoreface environment though, deposition was once again controlled by tidal currents. In the offshore, deposition was controlled by storm currents which generated distinctive beds of hummocky cross-stratification. The tidal environment Is composed of many sedimentary structures similar to those present In Phanerozoic and present-day tidal sequences. In the tidal flat sub-environment, vertical sequences of flaser, lenticular, wavy and coarsely interlayered bedding reflect current velocity fluctuations Intimately tied to spring - neap tidal cycles. The tidal channel sub-environment lacks many of the features characteristic of tidal channels described In the literature; such as extensive point bar development. Instead the tidal channels of the study area appear to represent sequences deposited In relatively straight channels. Migration of sandwaves and dune fields deposited the cross-stratified lithofacies of the shoreface environment. Similar to a high-energy non-barred coastline, the proximal portion of the shoreface lacks any evidence of beach development. Instead, the shoreface records a rapid and discontinuous transition from the tidal strandline environment. Hummocky cross-stratification (HCS), parallel laminated and massive sandstone beds as well as slltstone and mudstone beds typify the offshore environment. The HCS differs greatly In thickness and intemal structure from HCS described In the literature. The HCS In the study area reflects restricted and/or variable sediment supply and flow conditions. A paleotidal range was determined from the sediments of the tidal environment. The range Indicated a mesotidal environment and is comparable to Precambrian tidal ranges reported In the literature. Tidal rhythmites, present on the tidal flats, suggest a length of 26 days for the NeoArchean lunar month. Currents which deposited the tidal rhythmites produced both semidiumal and diurnal sediment sequences.