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Sandstone provenance for Early Tertiary pull-apart basins in Southeast Alaska - Abstract
Upper Paleocene to Lower Miocene fluvial and marginal marine sedimentary rocks of the Kootznahoo Formation form discrete basins in southeast Alaska. Measured sections, fossils, radiometric ages, paleocurrent measurements, cobble counts, and sandstone point counts constrain the depositional history for basins studied on Admiralty, Kupreanof, and Kuiu Islands. Petrographic analyses indicate different provenance for individual basins and changing sedimentary sources through time.
Paleocene-Eocene conglomerate at the base of the Hamilton Bay-Big John Bay-Port Camden section on Kupreanof and Kuiu Islands contains dominantly metamorphic clasts. Modal data for interbedded sandstones indicate dissected arc and rifted continental margin provenance. Modal data for Eocene to Oligocene sandstones reflect increasing granitic sources and indicate basement uplift provenance. Detrital biotite from Oligocene sandstone yielded a 40Ar/39Ar age of 53.5 ± 0.6 Ma. The only possible granitic source rocks for biotite of this age are in the Coast Mountains; this origin is supported by paleocurrent data. This sandstone is intruded by 23 Ma gabbro and inferred to underlie basalt flows that have 40Ar/39Ar ages of 24.2 ± 0.4 Ma to 24.7 ± 0.5 Ma and a U-Pb age of 26.5 ± 0.3 Ma. Modal data for sandstones between flows show arc and rifted margin provenance.
The Little Pybus Bay section on Admiralty Island unconformably overlies Triassic argillite and its top includes a Late Eocene leaf fossil locality beneath a basalt flow dated at 25.4±1.3 Ma. Basal conglomerate contains dominantly argillite clasts, with quartz, schist, granite, chert, greenstone, and limestone clasts that resemble underlying Triassic and Paleozoic rocks. Granitic clasts have distinctive porphyritic textures resembling those of nearby plutons dated at about 90 Ma. Modal sandstone data indicate recycled lithic provenance. Conglomerate between flows in Little Pybus Bay and conglomerate from Murder Cove, conformably below basalt dated at 26.7 ± 1.0 Ma and on strike with an Oligocene leaf fossil locality, contain dominantly volcanic clasts. Conglomerate from Whitewater Bay, conformably below basalt dated at 30.3 ± 2.9 Ma, contains dominantly greywacke, argillite, and chert. Modal data for sandstones indicate recycled orogen provenance.
Breccia near the base of the Kootznahoo Inlet section on Admiralty Island includes clasts of graphitic phyllite, greenschist, marble, granite, and quartz. Conglomerate interbedded with bentonite that contains zircons with discordant U-Pb ages ranging from 120 to 58 Ma could be as old as Paleocene. Conglomerate at Late Eocene-Early Oligocene leaf fossil localities contains clasts of chert, mafic metavolcanic rock, greenstone, schist, phyllite, and rare marble. Modal sandstone data indicate dissected arc provenance. Conglomerate at Oligocene fossil localities contains predominantly black chert, argillite, and phyllite clasts, and minor quartz, greenstone, marble, and granite clasts that resemble underlying rock units. Modal sandstone data indicate dissected arc provenance. Oligocene volcanic rocks are notably absent. Conglomerate and sandstone near Early Miocene fossil localities are similarly dominated by black chert and argillite, with minor quartz, granite, greenstone, schist, and marble.
In summary, Paleocene-Eocene Kootznahoo Formation sandstones on Kupreanof Island had dominantly metamorphic sources and increasing granitic sources up section. Paleocene-Eocene sandstones on Admiralty Island had dominantly sedimentary clasts attributable to local sources; modal data for these sandstones indicate recycled orogenic provenance, which supports structural control for basin formation. Detrital biotite ages, granitic clasts, basement uplift provenance, and paleocurrent data for Early Oligocene sandstones on Kupreanof Island suggest Coast Mountains uplift exposed granitic basement by 25 Ma. The Tertiary basins were well-established before a widespread volcanic event dominated sedimentation in all of the basins except the Kootznahoo Inlet basin between 24 and 30 Ma. The location of volcanic centers is clearly related to structures that control the location of the pull-apart basins.
Acknowledgments and Associated Footnotes
2 P. J. Haeussler: USGS 4200 University Dr., Anchorage, AK 99508
3 T. S. White: USGS 4200 University Dr., Anchorage, AK 99508
4 P. W. Layer: USGS 4200 University Dr., Anchorage, AK 99508
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