Sharon D. Woods¹, R. Douglas Elmore¹, M. H. Engel¹
(1) University of Oklahoma, Norman, OK

Abstract: Paleomagnetic dating of the smectite-to-illite conversion: Testing the hypothesis

Development of a method to date the smectite-to-illite transformation could have important implications for hydrocarbon exploration. This method is based on isolation of the chemical remanent magnetization (CRM) that resides in authigenic magnetite which forms during the transformation. Results from Jurassic sediment on Skye, Scotland, suggest a relationship between clay diagenesis and acquisition of a CRM. Previous diagenetic studies indicate that Jurassic siltstones, mudstones, and some limestones in north Skye contain abundant detrital smectite, whereas the clays in the same age sediment in south Skye have been altered to illite. The clay diagenesis has been attributed to heating by hydrothermal fluids driven by Tertiary igneous activity. The magnetization in the unaltered sediment in north Skye is weak and the specimens do not contain a stable magnetization. In contrast, most south Skye specimens contain a component removed between 200-350°C that is either southerly and up or northerly and down. An antipodal component is removed between 350°-580°C. The intermediate and high temperature components have directions that are similar to primary directions in the Tertiary igneous rocks. Rock magnetic analysis reveals that intermediate temperature component resides in pyrrhotite and it is interpreted to be related to the hydrothermal heating. The high temperature component resides in magnetite and it is interpreted to be a CRM related to the conversion of smectite-to-illite. The presence-absence test and the timing of remagnetization are consistent with the hypothesized connection between magnetite authigenesis and the smectite-to-illite conversion.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana