Masses of microcrystalline quartz are abundant within several lithologies in the Barnett Shale (lower Mississippian) of central Texas. A typical quartz mass is elongated parallel to bedding and contains a central planar microporous structure that suggests collapse of a formerly hollow spheroidal or tubular body. An integrated imaging approach, utilizing transmitted polarized light microscopy, secondary and back-scattered electron imaging, cathodoluminescence imaging, and X-ray mapping reveals these quartz masses to be composed primarily of quartz-cemented silt-size detrital quartz with a minor admixture of detrital Ca-plagioclase and dolomite. Microcrystalline quartz-rich masses in the Barnett Shale are interpreted as agglutinated foraminifera that have been dramatically collapsed during compaction. Locally, a significant portion of the total detrital quartz resides within these biogenic accumulations. This study highlights the potential for using these imaging techniques to investigate agglutinated foraminifera in lithified materials that are not amenable to disaggregation and extraction of three-dimensional specimens. The combined imaging techniques provide an unambiguous view into aspects of skeletal ultrastructure, such as particle size and sorting, that cannot be readily obtained from either conventional light microscopy or SEM. These techniques reveal that agglutinated foraminifers are abundant in several lithologies of the Barnett Shale, pointing to the possibility that application in other organic-rich shales may reveal these organisms to be more widespread than previously recognized. Comparative observations in Devonian shales of the central eastern United States support this prediction.
Bibliographical noteFunding Information:
This study was supported by the Geology Foundation of the Jackson School of Geosciences, The University of Texas at Austin. We thank Michael Cameron of Devon Corporation for providing access to core samples. Work on Devonian black shales by JS is supported by the Petroleum Research Fund (ACS PRF #38523-AC8), and by an NSF equipment grant (EAR-0318769) for acquisition of an analytical SEM. Discussions with Joe Macquaker, Robert Folk, and Earle McBride supported our efforts to interpret petrographic observations in Barnett Shale.
- Agglutinated foraminifera
- Barnett Shale
- Black shale
- Quartz diagenesis
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