PYRITE OOIDS IN LATE DEVONIAN BLACK SHALES OF THE EASTERN US: RELICT FEATURES OF INTERMITTENT IRONSTONE FORMATION

The Late Devonian black shales of the eastern US have a complex depositional history. Detailed sedimentologic/stratigraphic studies have shown that wave reworking of the seabed was a common occurrence. Laterally extensive erosion surfaces, recognized within the Chattanooga Shale, form the basis for a sequence stratigraphic reevaluation of the Late Devonian succession. They are interpreted to reflect intermittent lowering of sea level that brought black shales deposited over the Cincinnati Arch within the reach of wave reworking. Lag deposits that characterize these erosion surfaces show variable composition and contain quartz sand, bone fragments, and pyrite grains. In several places near the Cincinnati Arch these lags also contain pyritic ooids that may form beds as much as 15 cm in thickness. Examining these unusual sediment grains via reflected light petrography, microprobe, and ion probe, shows that they are of secondary origin and formed by diagenetic replacement of chamosite ooids. Thus, pyrite ooid beds can be considered relic features of oolitic ironstones and by extension should have formed in shallow, wave agitated water. By comparison with other oolitic ironstones, they should have formed during the hiatus between the end of a regression/sea level drop and the onset of the next sea level rise. Pyrite ooid beds furnish direct evidence of significant sea level drops during the accumulation of the Late Devonian black shales in eastern North America. In the context of known Mid to Late Devonian ironstone occurrences around the margins of the Appalachian basin and on the Transcontinental Arch, it may be possible to tie occurrences on the Cincinnati Arch to the latter. This would allow an integration of distal and proximal sequences and refinement of sequence stratigraphic models.