PYRITE OOIDS AS A RELIC FEATURE OF INTERMITTENT SEA LEVEL DROP AND SHALLOW WATER CONDITIONS IN DEVONIAN BLACK SHALES

Devonian black shales in Tennessee and Indiana contain unusual beds of pyrite ooids. Textural and geochemical studies reveal that the latter originated as chamositic iron ooids, and were replaced by pyrite during early diagenesis. Although the pyrite is very fine grained and faithfully mimics the laminated texture of the precursor grain, instances of partial replacement and inclusions of silicate minerals nonetheless reveal the secondary nature of the pyrite. Crushed pyritic cortexes in partially replaced grains that were compacted and deformed during burial, as well as sulfur isotope data, indicate that pyrite replacement occurred early in burial history. Pyrite ooid beds occur above erosion surfaces that were most likely produced by a lowering of sea level, and have been interpreted as sequence boundaries because of their large lateral extent. Realization that these pyrite ooid beds represent replacement of pre-existing beds of chamositic iron ooids, suggests an oxygenated water column at the time of their formation and wave interaction with seafloor sediments. This interpretation agrees with earlier work that stipulates that erosion surfaces in Devonian black shales of the eastern US reflect lowering of sea level that allowed wave reworking and erosion of earlier deposited black shales. An interval of chamositic iron ooids in the Upper Devonian (Late Fammenian) of Iowa falls within the same time bracket (conodont data) as a pyrite ooid bed in the Chattanooga Shale of Tennessee. The wide spatial separation of these apparently simultaneous Fe-ooid occurrences is suggestive of eustatic lowering of sea level in the late Fammenian. Further careful conodont dating of proximal chamositic ooid beds and distal pyrite ooid occurrences may help to confirm eustatic origin for other shallowing episodes in the Devonian inland sea, and help to isolate eustatic shallowing events from those forced by local tectonics and sedimentation. This should in turn allow precise correlation of the Devonian succession of the eastern US with the global record of Devonian sea level variations.