AGGLUTINATED FORAMINIFERA IN DEVONIAN BLACK SHALES OF THE EASTERN US: IMPLICATIONS FOR PALEOENVIRONMENTAL RECONSTRUCTIONS

Late Devonian black shales of the Illinois and Appalachian Basins contain lenticular bodies of fine-crystalline silica that in thin section range from 50 - 200 μm in length and are approximately 10 – 20 μm thick. They apparently underwent soft sediment deformation during compaction and have a granular-porous appearance in polished section. Color SEM-CL (cathodoluminescence) shows them to consist largely of micron-sized detrital quartz that “floats” in a matrix of diagenetic silica. The narrow size distribution of quartz grains within these silica bodies is in stark contrast to the much coarser (5 – 50 μm) and poorly sorted quartz silt seen in the surrounding shale. The unusual sorting, medial divisions, and occasional oval-shaped bodies with hollow interiors allow identification as collapsed test of agglutinated foraminifera.

In modern oxygen-stressed environments (e.g. Santa Barbara Basin), benthic forams of this type will not thrive once the redox interface rises above the sediment surface for extended time periods. Because these agglutinated foraminiferal remains occur throughout the Devonian black shale succession, they suggest that there were no time periods during which persistent bottom water anoxia developed. Although the latter has been proposed on the basis of various geochemical proxies (e.g. DOP, Mo/Al, U/Th, etc.), it is incompatible with the presence of agglutinated foraminifera, evidence of crypto-bioturbation, and distribution patterns of pyrite framboid diameters. This points to a persistent dilemma with empirically derived geochemical proxies – they are only as “good” as prior assumptions about the prevailing paleoenvironment.

Even in the shale package that is considered to have had the most anoxic paleoenvironment, the Cleveland Shale, remains of agglutinated foraminifera are readily found in thin sections and positively identified with Color SEM-CL. This suggests that when looked at with the right methodology and equipment, many other black shales in the rock record will likely reveal the presence of benthic agglutinates, casting doubt on earlier interpretations of anoxic bottom waters during their deposition.