Carbonate Margin, Slope, and Basin Facies of the Lisburne Group (Carboniferous-Permian) in Northern Alaska
The Lisburne Group (Carboniferous-Permian) consists of a carbonate platform that extends for >1000 km across northern Alaska, and diverse margin, slope, and basin facies derived from this platform that contain world-class deposits of Zn and Ba, notable phosphorites, and petroleum source rocks. Lithologic, paleontologic, isotopic, geochemical, and seismic data gathered in outcrop and subsurface studies of the Lisburne during the past 20 years allow us to delineate the distribution, composition, and age of these off-platform facies, and to better understand the physical and chemical conditions under which they formed.
The southern edge of the Lisburne platform changed from a gently sloping homoclinal ramp in the east to a tectonically complex, distally-steepened margin in the west that was partly bisected by the extensional Kuna Basin (~200 by 600 km). Carbonate turbidites, black shale, and radiolarian chert accumulated in this basin; turbidites were generated mainly during times of eustatic rise in the late Early and middle Late Mississippian. Interbedded black shale (up to 20 wt % Corg), granular and nodular phosphorite (up to 37 wt % P2O5), and fine-grained limestone rich in radiolarians and sponge spicules formed along basin margins during the middle Late Mississippian in response to a nutrient-rich, upwelling regime.
Geochemical and isotopic data indicate that suboxic, denitrifying conditions prevailed in the Kuna Basin and along its margins. High V/Mo, Cr/Mo, and Re/Mo ratios (all marine fractions, MF) and low MnO contents (<0.01 wt %) characterize Lisburne black shale. CrMF/VMF ratios of 0.8-1.9 and high δ15N (ave. 8.5 per mil) suggest moderate to strong denitrification.
Demise of the Lisburne platform was diachronous, and reflects tectonic, eustatic, and environmental factors. SW, SC (southcentral), and NW parts of the platform drowned during the Late Mississippian, coincident with Zn and Ba metallogenesis in the Kuna Basin and phosphogenesis along basin margins. This drowning was temporary except in the SW and likely due to nutrification consequent to upwelling and sea level-rise enhanced by regional extension, which allowed suboxic, denitrifying waters to form on platform margins. Final drowning in the SC area occurred in the Early Pennsylvanian and may also be linked to regional extension. In the NW, platform sedimentation persisted into the Permian; its demise here appears due to regional cooling and increased siliciclastic input.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California