ENIGMATIC STRUCTURES IN THE ESTUARINE INGERSOLL SHALE, CRETACEOUS EUTAW FORMATION, EASTERN ALABAMA: PYRITIZED MUCUS TRAILS PRODUCED IN A SOUPGROUND?

The Ingersoll shale, a compact estuarine lagerstätte in the Cretaceous Eutaw Formation of eastern Alabama, contains common yet enigmatic structures previously inferred to be of biogenic origin. Samples of the Ingersoll shale were examined macroscopically, in thin sections, under SEM, and via x-radiography in an effort to better characterize and interpret these structures. The structures in question are most readily observed on bedding-parallel surfaces with densities of 400-1400 specimens/m² (average ~800/m²). They typically are flat, short (6-27 mm), narrow (0.8 to 3.2 mm), horizontal, and unbranched, and commonly exhibit diffuse lateral margins. X-radiographs indicate straight to irregular paths and may indicate rare branching. In rare cases where the structures are oblique to subvertical, they are commonly ptygmatically deformed. The fills of these structures consist entirely of octahedral microcrystals of pyrite ranging from 10 to 30 um in diameter; framboidal arrangement of pyrite octahedra is not readily apparent. Based on all observations, these structures appear to be pyritized, discontinuous mucus or slime trails produced by polychaetes or other worm-like organisms that inhabited soupy, clay-rich, carbonaceous substrates beneath oxygenated marine waters. These structures are analogous to other pyritized burrows recognized in previous studies of marine black shales and other substrates.