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Clay minerals from a complete reference section of the nonmarine Upper Cretaceous (Campanian) Judith River Formation (JRF; Western Interior Basin, north-central Montana) were semi-quantitatively characterized by way of X-ray diffraction (XRD) to determine whether or not their trends indicate the presence of a third-order sequence boundary (01) marked by a macroscopic shift from sandstone-to mudstone-dominated facies and a sudden shift in well-log properties. The technique of clay mineral semi-quantification used here involved oriented specimens of clay-sized extracts and the normalized relative intensity ratio method. Analyses of four known mixtures composed of Source Clays from the Clay Minerals Society suggest that the uncertainty of the semi-quantitative results presented is ±6.3 Wt.%.
The four clay mineral groups that comprise the JRF clay mineral suite are kaolin, smectite, illitic material, and chlorite. Kaolin is interpreted to be primarily of an early diagenetic origin and is most abundant in sandstone lithofacies whereas the other three clay mineral groups are largely detrital in nature and are most abundant in mudstone lithofacies.
The results of a non-parametric statistical test including all samples point out that there is a significant decrease in the average relative abundance of kaolin upward through Dl and an increase in that of smectite at the 95% confidence level. An identical test concerning just sandstone bodies indicates that the average relative abundance of kaolin is greater in sandstones below Dl than those above. Since abundant kaolin and smectite are restricted to sandstones and mudstones, respectively, the results of the former test simply reflect the fact that the JRF is dominated by sandstones below D1 and mudstones above. The reasoning for the decrease in the average relative abundance of kaolin within sandstones alone is explained here as a consequence of the higher rates of deposition naboveD1ashigherrates of deposition remove sediment from the zone of meteoric water flushing and thus early diagenetic processes more rapidly. Regardless of the exact explanations, the semi-quantitative clay mineral trends of the JRF indicate the presence of discontinuity D1 and a change in accommodation space.
The results of this study fall in line with the only predictive model concerning clay mineralogy in a sequence stratigraphic framework which states that the abundance of early diagenetic clay minerals (i.e., kaolin) should decrease across sequence stratigraphic surfaces. However, this study alone cannot validate that prediction and future work is needed to determine the utility of semi-quantitative clay mineral trends for identifying sequence boundaries within the JRF and other formations.
Nelson, Kenneth, "Clay Minerology Across a Sequence Boundary in the Upper Cretaceous (Campanian) Judith River Formation, North-Central Montana" (2009). Geology Honors Projects. 5.
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