CARBONATE MARGIN, SLOPE, AND BASIN FACIES OF THE LISBURNE GROUP
(CARBONIFEROUS–PERMIAN) IN NORTHERN ALASKA
JULIE A. DUMOULIN
US Geological Survey, 4210 University Drive, Anchorage, Alaska 99508, USA
CRAIG A. JOHNSON
US Geological Survey, Box 25046, MS 963, Denver, Colorado 80225, USA
JOHN F. SLACK
US Geological Survey, National Center, MS 954, Reston, Virginia 20192, USA
KENNETH J. BIRD
US Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA (Retired)
MICHAEL T. WHALEN
Department of Geology and Geophysics, University of Alaska, PO Box 757320, Fairbanks, Alaska 99775-7320, USA
THOMAS E. MOORE
US Geological Survey, MS 901, 345 Middlefield Road, Menlo Park, California 94025, USA
ANITA G. HARRIS
US Geological Survey, National Center, Reston, Virginia 20192, USA (Retired)
PAUL B. O’SULLIVAN
Apatite to Zircon, Inc., 1075 Matson Road, Viola, Idaho 83872-9709, USA
ABSTRACT: 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 that contain world-class deposits of Zn and Ba, notable phosphorites, and petroleum source rocks. Lithologic, paleontologic, isotopic, geochemical, and seismic data gathered from outcrop and subsurface studies during the past 20 years allow us to delineate the distribution, composition, and age of the 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 mudrocks, 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 mudrocks (up to 20 wt% total organic carbon), 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.
Detrital zircons from a turbidite sample in the western Kuna Basin have mainly Neoproterozoic through early Paleozoic U-Pb ages (~900–400 Ma), with subordinate populations of Mesoproterozoic and late Paleoproterozoic grains. This age distribution is similar to that found in slightly older rocks along the northern and western margins of the basin. It also resembles age distributions reported from Carboniferous and older strata elsewhere in northwestern Alaska and on Wrangel Island.
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 mudrocks. Low CrMF/VMF ratios (mostly 0.8–4.0) suggest moderately to strongly denitrifying conditions in suboxic bottom waters during siliciclastic and phosphorite sedimentation. Elevated to high Mo contents (31–135 ppm) in some samples are consistent with seasonal to intermittent sulfidic conditions in bottom waters, developed mainly along the basin margin. High d15N values (6–12‰) imply that the waters supplying nutrients to primary producers in the photic zone had a history of denitrification either in the water column or in underlying sediments.
Demise of the Lisburne platform was diachronous and reflects tectonic, eustatic, and environmental drivers. Southwestern, south-central, and northwestern parts of the platform drowned during the Late Mississippian, coincident with Zn and Ba metallogenesis within the Kuna Basin and phosphogenesis along basin margins. This drowning was temporary (except in the southwest) and likely due to eutrophication associated with upwelling and sea-level rise enhanced by regional extension, which allowed suboxic, denitrifying waters to form on platform margins. Final drowning in the south-central area occurred in the Early Pennsylvanian and also may have been linked to regional extension. In the northwest, platform sedimentation persisted into the Permian; its demise there appears to have been due to increased siliciclastic input. Climatic cooling may have produced additional stress on parts of the Lisburne platform biota during Pennsylvanian and Permian times.
KEY WORDS: carbonates, black mudrocks, phosphorites, upwelling, Alaska
How to cite this article
Dumoulin JA, Johnson CA, Slack JF, Bird KJ, Whalen MT, Moore TE, Harris AG, O’Sullivan PB. 2013. Carbonate margin, slope, and basin facies of the Lisburne Group (Carboniferous–Permian) in northern Alaska. In Verwer K, Playton TE, Harris PM (Editors). Deposits, Architecture, and Controls of Carbonate Margin, Slope, and Basinal Settings, Special Publication 105: SEPM (Society for Sedimentary Geology), Tulsa, OK. doi: 10.2110/sepmsp.105.02.