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Browsing by Author "Rosen, Lawrence Collinger"

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    The Sedimentology and Petrology of the Early Proterozoic Gowganda Formation around Gowganda - Elk Lake, Ontario, Canada
    (1985-04) Rosen, Lawrence Collinger
    Evidence for glacial-marine sedimentation in the Gowganda Formation (Huronian Supergroup) in the Gowganda - Elk Lake area of northeastern Ontario changes the previously known regional distribution of continental glacial and glacial-marine sedimentary facies. Lindsey (1969) interpreted the Gowganda to be a marine deposit in the south (Espanola-Whitefish Falls area) and a continental deposit in the north (Cobalt area) which includes the area of this study. In the study area, the Gowganda rests unconformably upon the Archean basement. The subhorizontal formation is from 0 to >1600 m thick (based on drill hole data) and no complete sections are present. It is a heterogeneous assemblage of diamictite, orthoconglomerate, graywacke, arkose, siltstone, argillite, dropstone laminite, and sedimentary breccia. Lateral and vertical facies changes can be rapid; however, the graywacke-argillite association is abundant, widespread, and constitutes the dominant facies. Diamictite is a prominent lithology and varies from massive to weakly stratified with clasts up to 1.2 m in apparent diameter; units vary from a decimeter to as much as 20 m in thickness. Contacts with other units can be sharp, irregular, or gradational. Diamictite is restricted to the lower two-thirds of the formation. Orthoconglomerate and massive to thick bedded arkose, occasionally cross-bedded, are most common at the base of the section and again near the top. Interbedded graywacke, siltstone, and argillite commonly constitute fining-upward sequences from 2 cm to 4 m thick in the lower portion of the section and both fining-upward and coarsening-upward sequences in the upper part of the section. Internal stratification includes graded beds, ripple marks, laminations, and convolute bedding. In addition, features such as ripped-up clasts, flame structures, and other loading phenomena are present. Thinly laminated siltstone and argillite sequences with dropstones as large as 30 cm are present in the lower two-thirds of the section. They are more common in the central and western portions of the study area. Dropstone sequences are as much as 40 m thick, while individual units vary from 1 to 5 m thick. Sedimentary breccias and units with soft-sediment deformation structures are fairly widespread but constitute a minor part of the section; units vary from 0.5 to approximately 4 m thick. Paleocurrents in the Gowganda and overlying Lorrain Formation have vector means of 143° and 168°, respectively. The low variance (2718) in the Lorrain suggests deposition in a fluvial environment. Detritus composing the Gowganda Formation was derived from a granite-greenstone terrane which underlies the formation and crops out locally in the study area and to the north. The chemical index of alteration (CIA) suggests that climatic conditions were initially moderate, deteriorated to frigid conditions, and eventually returned to a more moderate climate. Gowganda rocks are typically Na-rich and probably reflect the source area. The diamictites are depleted in Fe and Mn relative to crustal abundances and this suggests that the diamictites were deposited in a marine environment. The depositional model for glacial-marine sedimentation in the Gowganda Format ion is based on Antarctic models and involves an ice shelf. Proximal deposits include massive and stratified diamictites. Intermediate and distal deposits contain dropstone laminites. Meltwater is minimal or absent. In this model, weakly stratified diamictite and thicker, massive diamictite are interpreted to be the products of rapid fallout of abundant debris beneath an ice shelf. The generally thinner, massive diamictite is attributed to submarine gravity flow mechanisms. The fining-upward and coarsening-upward sequences reflect the changing conditions within the basin as increases and decreases in both energy and sediment supply occurred, possibly due to fluctuations in the position of the ice margin. The graywackes resulted from the interaction of various types of sediment gravity flows, dominantly turbidity currents with subordinate fluidized/liquidized flows and possibly grain flows. The more well-sorted sandstone, siltstone, and argillite may represent more normal marine sedimentation. Breccias and deformed units represent either resedimentation from unstable depositional sites or tectonic instability. The vertical distribution of diamictite and dropstone facies suggests two major ice advances occurred, separated by normal marine deposition. Amelioration of the climate in late Gowganda time is indicated by the lack of glacial features in the rocks. Sedimentation appears to have been largely marginal marine to deltaic; a transition to fluvial deposition occurs at the top of the section and is evident in the overlying Lorrain Formation.