Abstract

The Robb lake lead-zinc deposit occurs within a brecciated Silurian-Devonian dolostone succession of transgressive-progradational sabkha rhythmites. The rhythmites are characterized by light gray color, white sparry dolomite cemented bird's-eyes, dolomite pseudomorphs of gypsum, and zebroid textures, which are diagnostic evaporitic cementation structures. The rhythmites were identified by logging color and lithologic variations observed in diamond drill core. The light gray tops of rhythmites are attributed to penecontemporaneous oxidation during sabkha progradation; the darker gray bases of rhythmites are attributed to penecontemporaneous reduction during marine transgression. Recording the color variation encountered in drill cores, even of highly brecciated intervals, ermits stratigraphic correlation based on key beds and rhythmites. Relative sag of groups of rhythmites from the brecciated areas demonstrates the origin of breccias by collapse. Correlation also suggests that manto-like breccias formed after gentle tilting of the host succession. Tilting probably resulted from differential consolidation of an underlying shale-carbonate facies front. The orientation of the manto-like breccias is interpreted to have been controlled by infiltration of migrating formational fluids along horizontal planes within the tilted evaporitic succession. Well-defined rhythmites in the upper half of the succession average 2.8 m in thickness with a standard deviation of 1.8 m (N = 1,295). Poorly defined rhythmites in the highly brecciated lower half of the succession have an average thickness of 4.4 m with a standard deviation of 3.7 m (N = 791) proving that even monotonously repetitive and brecciated strata can be differentiated by careful attention to thickness data. End_of_Article - Last_Page 599------------

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