Early diagenetic origin and persistence of gamma-ray and magnetosusceptibility patterns in platform carbonates: comparison of Devonian and Quaternary sections

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• 作者：Hladil ; Jindrich ; Bosak ; Pavel ; Slavik ; Ladislav ; Carew ; James L. ; Mylroie ; John E. ; Gersl ; Milan
• 关键词：Gamma-ray spectrometry ; Magnetic susceptibility ; Carbonate platforms ; Devonian ; Quaternary
• 刊名：Physics and Chemistry of the Earth, Parts A/B/C
• 出版年：2003
• 出版时间：2003
• 年：2003
• 卷：28
• 期：16-19
• 页码：719-727
• 全文大小：330 K

文摘

Gamma-ray logs from boreholes in the Middle–Upper Devonian carbonate platform of Moravia display tripartite anomalies at locations, where lithological and biostratigraphic data suggest the occurrence of 4th order sedimentary cycle boundaries. Further, where sedimentary boundaries have been changed by later development of caves (usually phreatic caves changed to unroofed caves––erosion), the carbonate infillings in these corroded cycle boundaries are marked by another pattern that consists of a smooth symmetrical peak on gamma-ray activity in vertical section. The details procured using gamma-ray spectrometric and magnetosusceptibility methods suggest that the upper peak of the tripartite pattern corresponds solely to uranium concentration (flooding surface). The middle peak is marked by a thorium signal and a magnetosusceptibility response from paramagnetic minerals (paleosols). The lower peak corresponds to trapped uranium and microbial magnetite in cemented rock pores (originally dysoxic microenvironments in calcite). The boundaries marked with filled caves display only one broad and symmetrical uranium-related peak, and the thorium peak that is roughly similar to that seen at normal boundaries, but it is shifted slightly downward. At boundaries with caves the magnetosusceptibility peaks are shifted downward considerably, and may even occur within the underlying host rock. The question of whether these patterns are a primary imprint of early diagenetic influences or a much later redistribution that originated during pressure solution and cementation, was answered by study of Late Quaternary sections on San Salvador Island, Bahamas. This pragmatic test on young carbonate sediments confirmed the early origin and fixation of these geophysical patterns.