Of 14 fusulinid genera in the Upper Pennsylvanian-Lower Permian Bird Spring Formation, only Pseudoschwagerina? shows evidence of deformation caused by compaction. Most deformation occurred in the outermost whorl; the wall and septa were jumbled, and the

End_Page 530------------------------------

whorl collapsed. Only slight fracturing was observed in inner whorls. In places, the outer whorl was completely closed, the shell being flattened on top and bottom. In other specimens, chomata (internal structures) acted as a brace, preventing total flattening. At several places where the shell is particularly badly crushed, another fossil is in surface contact with the fusulinid. As compaction proceeded, grain-to-grain contacts (where stress was localized) changed to those along a surface; strain was realized by brittle failure of the fusulinid. No pressure solution took place. The major compressional stress, oriented normal to bedding, led to horizontal stresses away from the center of each fusulinid; extension cracks and buckling of the wall near the polar ends resulted. Most other fossils and the micrite matrix had sufficient strength to withstand later compaction, in part because of early submarine cementation. However, individuals belonging to this one fusulinid genus were selectively crushed during limestone compaction because of inherent weaknesses in the structural architecture of the shell. Pseudoschwagerina? was a large fusulinid with a thick, heavy wall; outer whorls were inflated, making it comparatively hollow. Septa which supported the wall were thin, straight (unfluted), and widely spaced.

End_of_Article - Last_Page 531------------