--> ABSTRACT: The Tale of Three Basins: Illite Age Analysis, Shale Diagenesis, and Interpretation of Diagenetic History, by J. Reed Glasmann; #91019 (1996)

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The Tale of Three Basins: Illite Age Analysis, Shale Diagenesis, and Interpretation of Diagenetic History

J. Reed Glasmann

The northern North Sea, North Slope, and Salton Trough basins are rifted basins with widely divergent depositional and provenance histories. Thick mudstone sequences in each of these basins exhibit classical smectite- to-illite alteration with increasing depth of burial; however, broad areas of the North Slope are characterized by fairly abrupt appearance of ordered illite/smectite (R1 I/S). Illite K-Ar ages of rift sequence shales are similar to those obtained from Ellesmerian sequence shales, but are adversely affected by contamination of Devonian-aged detrital illite. Illite Age Analysis (IAA) suggests that the mean age of illite diagenesis is 75-100 Ma in the vicinity of Prudhoe Bay field, much older than the mid Tertiary ages obtained from rift sequence shales in the West Staines area. The early Cretaceous illite ages of Prudhoe Bay rift shales are not predicted from stratigraphically-based diagenetic models. IAA corrections to measured ages assume a Devonian age for detrital illite in Ellesmerian sediments, but detrital ages for Torokian and younger sediments depend on the degree of unroofing of the Brookian metamorphic complex. Partial outgassing of Ellesmerian mica results in older detrital ages in Torokian shales, whereas detrital illite in Tertiary mudstones has an Aptian age. To correctly apply IAA to interpretation of shale diagenesis, detrital ages from each sample must be measured. Once detrital components are characterized, meaningful interpretation of diagenetic ages can proceed.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California