Magnetic Characteristics of Carboniferous Continental Depositional Systems: Implications for the Recognition of Depositional Hiatuses
Evans, Frank B.
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Quaternary magnetic studies have provided the conceptual framework to bridge magnetic studies into ancient systems. In cases where environmental materials have been subjected to diagenetic alteration two questions come to mind: 1) What part of the magnetic signal is preserved in the rocks; and 2) can the preserved signal be used to infer/identify magnetic patterns that are characteristic of the depositional, post-depositional, and/or diagenetic environment. Analyses of multi-parameter magnetic experiments conducted on upper Mississippian and lower Pennsylvanian continental successions reveal that distinct depositional, pedogenic, and diagenetic magnetic patterns can be separated and identified. Evidence for a primary depositional signal in several of the upper Mississippian lithofacies is identified by a detrital remanence component attributed to source-area-derived magnetite/titanomagnetite. Red and gray vertisols preserve a Mississippian pedogenic signal characterized by magnetic enrichment, depletion, and amalgamation patterns that are associated with the removal and transport of Fe-rich clays as well as vertical mixing by shrink-swell mechanisms. These well-developed vertisols are interpreted to reflect significant hiatuses in sedimentation associated with prolonged exposure on interfluve/floodplain surfaces that may correlative with incised valleys (lowstand surface of erosion). Similarly, in lower Pennsylvanian quartz arenite facies, early siderite cementation zones as well as conglomerate lags with distinctive magnetic characteristics are thought to reflect periods of prolonged exposure and to define unconformities within compound valley fills.
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