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Ahead of Print Abstract
DOI:10.1306/10132525017
Rare earth elements in latest Famennian Woodford Shale phosphate nodules, Oklahoma, United States: Land-driven phosphorus overloading and evolving organic carbon preservation, southern Laurentia
Andrew Cullen1 and David Hull2
1 Independent Geologist, Norman, OK 73072
2 Practical Geoscience, Oklahoma City, OK 73116
Ahead of Print Abstract
The Upper Devonian-lower Carboniferous Woodford Shale, Oklahoma, United States, is a world-class oil-prone source rock with a thick (9.5m average) regionally extensive (>42,000 km2) (~16,000 mi2)phosphatic interval at the top of its upper member. Inductively coupled plasma mass spectrometry analyses of 27 phosphate nodules from nine upper Woodford outcrops defining a 120 km (75 mi) transect into increasing paleowater depths show that the concentrations of rare earth elements (REEs) and other “continental” trace elements have a strong negative correlation with distance from the paleoshoreline. Biomarker, palynology, and x-ray fluorescence elemental studies show the upper Woodford carries an increased terrestrial signal. A landward source for the REEs and phosphorus, rather than oceanic upwelling, is proposed. Chronostratigraphy places these nodules in a short-lived (1.9 m.y.) falling systems tract above a latest Famennian flooding surface (Dasberg event). The upper Woodford compacted sedimentation rate (15 cm/k.y.) is twice that of the lower and middle members. The upper Woodford has lower average total organic carbon than the middle and lower members and geochemical evidence indicates increased oxygenation with abundant radiolarian chert beds recording increased biologic activity. These data suggest a modal change in organic carbon preservation from slow accumulation in low oxygen conditions to rapid accumulation in more oxygenated waters. For the upper Woodford, a superestuarine circulation model should be considered with intense phosphogenesis recording a special set of circumstances. As latest Devonian sea-level fell and catchment areas expanded, increased nutrient delivery from erosion of soil and bedrock was amplified by wildfire-derived ash and organic material shed during the extinction of Late Devonian forests.
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