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A Shallow Epeiric Sea Interpretation for an Offshore Middle Devonian Black Shale Facies in Eastern North America
Most recent papers dealing with the low diversity black shale facies within epeiric sea environments of the Devonian Appalachian region have used a deep, stratified basin patterned after the present day Black Sea. A facies scheme is thus tied directly to depth, with shallow (less than 50 m), highly diverse faunas in grey shales are considered aerobic; intermediate (50-150 m) infauna dominated, lower diversity faunas in dark grey shales are dysaerobic; and deep (greater than 150 m) virtually barren, laminated black shale environments are anaerobic. As a result of tying relative depth to facies and faunal patterns, attempts are being made to work out transgressive and regressive patterns on both a local and regional scale.
An alternative proposal is that black shale facies formed in an aerated, shallow, muddy epeiric sea, in which faunal and facies distribution result from deposition within a bathymetrically subdued setting similar to modern continental shelves. Faunal diversity is thus more dependent on infaunal turbidity and fluidity, storm winnowing and re-suspension, than on paleobathymetry and oxygen budget. Accordingly the virtually barren, black, laminated shales were deposited in widespread, relatively shallow conditions during periods of varying salinity, and in the Upper Devonian are separated by extensive flood and storm deposits previously attributed to turbidites. Widespread and thin Middle Devonian calcareous and noncalcareous intervals with high diversity faunas dominated by filter feeders and micro-carnivores were laid down during periods of low re-suspension, due to the absence of a highly mobile infauna, and during times of low sedimentation rates. High re-suspension and turbidity from an active infauna are responsible for the low diversity epifaunas in the non-laminated shales.
Depth criteria established within the imperfectly laminated black shale facies of the Middle Devonian Ledyard Member of the Ludlowville Formation, New York are presented here. These include sedimentary features attributable to processes below wave base, but above storm wave base, and benthic faunas restricted to an effective photic zone. Critical comments on previous interpretations regarding several Upper Devonian black shales are also included.
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