212-8 Alaskan Oil in Canadian Gravel: Reconstructing Permo-Triassic Rivers and their catchments in the supergiant Prudhoe Bay oil field, Alaska

Session: Reconstructing Earth Surface Processes in Orogenic Systems

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The Permo-Triassic Ivishak Formation in the supergiant Prudhoe Bay Field lies on the Arctic Alaska Block that has experienced significant rotation since the Late Permian, and the provenance and scale of formative rivers are thus in question. Was the Ivishak fed by the continental-scale Trans-Laurentian River that drained Pangea, or something smaller? The Ivishak consists of several hundred meters of sandstone, conglomerate and lesser mudstone that  conformably overlies the marine Kavik Shale. Cores and well logs show the thickest unbroken fining-upward facies successions are about 12 m, placing an upper limit on maximum thickness of the largest formative channels and inferred flow depths. Despite gravel-bed rivers being dominant higher in the Ivishak, shoreline facies are gravel free, indicating that the gravel-sand transition lay upstream of the shoreline. Observations of bedload conglomerates with a maximum pebble diameter of 2 cm, suggest bed shear stress on the order of 14-20 Pascals and critical shear velocity on the order of 0.12-0.14 m/s. Integration of bankfull shear stress estimates, flow depths, grain size and sedimentary structures suggest that the gravelly Ivishak rivers  had a mean bankfull flow depth on the order of 6 m and flowed at a peak velocity of about 1.3 m/s over a slope on the order of 2 x 10^-4. Backwater length is on the order of 10 kilometers. Empirical estimates suggest channel widths between 100-500 m and estimates of bankfull discharge range from 500-2600 m³/s. Power-functions suggest the maximum catchment areas for Ivishak rivers were on the order of 100 km², far too small to reflect an origin from the Trans-Laurentian Rivers. Detrital zircon data in Ivishak outcrops show local sources, versus derivation from the Canadian Shield, and indicate sediment was derived from the adjacent Sverdrup basin in Canada. The paleohydraulic analysis is supportive of moderate-scale steep-gradient rivers and is consistent with local derivation from a relatively small source-to-sink system, despite the fact that continental-scale river systems were well developed in the Permo-Triassic. The detailed analysis shows how paleohydraulics and detrital zircon analysis are integrated to constraint the scale of a deep-time source to sink system.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6959

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