Clay Mineral Assemblages as Low Grade Metamorphic Geothermometers: Application to the Thrust Faulted Disturbed Belt of Montana, U.S.A.
Janet Hoffman, John Hower, 1979. "Clay Mineral Assemblages as Low Grade Metamorphic Geothermometers: Application to the Thrust Faulted Disturbed Belt of Montana, U.S.A.", Aspects of Diagenesis, Peter A. Scholle, Paul R. Schluger
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Clay mineral assemblages in sedimentary rocks can be indicative of the maximum temperature (below 300° C) and metamorphic grade to which these rocks have been subjected. The chemical composition of the rock and pore fluid, along with the detrital mineralogy are also influential in determining the mineral assemblages that form at higher temperatures. Clay mineral assemblages are also dependent on reaction time at low temperatures. The most useful minerals which can be used as geothermometers are as follows: (1) for shales: illite/smectites, illite, and chlorite; (2) for sandstones: chlorite, chlorite/smectite (corrensite) and dickite; and (3) for volcanics and volcanogenic rocks: chlorite/smectites and zeolites. In pelitic sediments the conversion of smectite to illite and its subsequent recrystallization to 2M muscovite allow a detailed temperature zonation of these rocks into a sequence of temperature ranges.
These temperature (grade) sequences are applied to the disturbed belt of Montana. Mesozoic strata involved in Laramide thrust faulting events have mineral assemblages, principally the mixed-layer clays and zeolites, which reflect a low grade metamorphic environment and indicate metamorphic temperatures between 100° and 200° C. These metamorphic mineral assemblages are displayed by strata as lithologically diverse as shales, sandstones, volcanogenic rocks, and bentonites. Field evidence indicates that these strata were never sufficiently buried during sinking of the sedimentary basin to produce the observed mineralogic and chemical changes. Comparison of stratigraphically equivalent sediments on the adjacent undisturbed Sweetgrass Arch show mineral assemblages indicative of low temperature (<<100° C) conditions. Heating to a degree higher than that predicted from normal geothermal gradients is inferred to be caused by burial beneath thrust plates. This is suggested as the mechanism that generated the temperatures indicated by observed mineral assemblages in the disturbed belt.
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There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.