"Cherty" stringers in the Barnett Shale are agglutinated foraminifera

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.