ON THE ORIGIN AND SIGNIFICANCE OF PYRITE BEDS IN DEVONIAN BLACK SHALES FROM THE EASTERN US

Devonian black shales of the eastern US contain distinct pyrite-enriched beds. Ranging in thickness from millimeters to centimeters, these pyritic beds represent an extreme case of iron enrichment in these shales. Viewed in detail, the bulk of the pyrite is contained in irregularly shaped aggregates that consist of pyrite framboids in a matrix of submicron size pyrite crystals. These polyframboidal aggregates can constitute up to 50 percent of the rock volume and result in whole rock iron contents that represent an up to tenfold increase over the under and overlying black shales. Polyframboidal aggregates formed initially as early diagenetic fills of algal cysts, such as Tasmanites. Reworking of these cysts by bottom currents separated the cyst fills from the enclosing organic membrane and led to the observed polyframboids. The polyframboid clusters behaved like soft grains during transport, and were deformed and squeezed during compaction. They were apparently held together by a soft, gelatin-like organic matrix, that is preserved as kerogenous matter in interstitial spaces between framboids and pyrite crystallites. Infill of algal cysts by framboids implies very small sedimentation rates, and concentration into distinct pyritic beds reflects removal of finer sediment constituents due to reworking by bottom currents. In a sequence stratigraphic context, beds of polyframboidal pyrite mark extreme sediment starvation and maximum flooding surfaces. They occur for example prominently at the level of the Protosalvinia (Foerstia) interval. Because rapid weathering of fine grained pyrite renders these kinds of beds unrecognizable in outcrop they have not been reported previously. In fresh sample material or drill core, however, they are easily recognized due to their high pyrite content.