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Cohen, H.A., 1992

Stratigraphic, sedimentologic, and provenance constraints on evolution of the Gravina Belt, northern southeast Alaska

Bibliographic Reference

Cohen, H.A., 1992, Stratigraphic, sedimentologic, and provenance constraints on evolution of the Gravina Belt, northern southeast Alaska: Princeton, New Jersey, Princeton University, Ph.D. dissertation, 275 p., illust., maps.


Lithofacies associations and successions of the Jura-Cretaceous Gravina belt on Admiralty, Douglas, and northern Kupreanof Islands (southeast Alaska) are dominated by deep-marine slope, inner-fan, and middle-fan depositional environments. Non-channelized outer-fan facies are exposed over broad areas on eastern Kupreanof and Mitkof Islands. Four turbidite systems are recognized: a Late Jurassic slope wedge (southeastern Admiralty Island), a Cretaceous, elongate submarine fan system (Admiralty Island to Kupreanof and Mitkof Islands), and smaller-scale system(s), ?entirely of Cretaceous age (Seymour Canal to Douglas Island). Sandstones of the Seymour Canal Formation (Gravina belt) exhibit framework modes typical of detritus shed from undissected arc sources. Sandstones of channelized conglomeratic facies are slightly enriched in quartz, sedimentary, metasedimentary, and plutonic detritus relative to thin-bedded turbidite sandstones; they record local unroofing of the underlying Alexander terrane, mostly during Early Cretaceous time. Biotite and hornblende grains extracted from five sandstones and dated with the 40Ar/39Ar laser microprobe yield Jura-Cretaceous ages ranging from 91.1 +/- 5.3 Ma to 157.6 +/- 3.6 Ma. These confirm that the primary source of detritus was contemporaneous volcanism in the adjacent Gravina arc. Volcanism and sedimentation were active until, and likely after, the Albian onset of compressional deformation of Gravina belt units in southern southeast Alaska. No unequivocal provenance ties to coeval sedimentary basins of the cordillera are found in any sandstone or conglomerate detritus. Sandstones of the Gravina belt do not yield a clear signal of rift-related uplift or silicic volcanism commonly associated with initiation of marginal basins. The Gravina belt overlies the inboard margin of the Alexander terrane and preserves a geologic record of this microcontinental boundary from Late Jurassic through mid-Cretaceous time. Models for initiation and evolution of the Gravina basin must take into account its isolation from North American sediment sources, volcaniclastic composition, lack of petrologic or lithologic evidence for rift-related sedimentation or sea-floor spreading, and lack of similarity to well-documented ancient marginal basins. Primary control on sedimentary petrology and facies was volcanism and uplift in the adjacent Gravina arc and Alexander terrane, consistent with models of the Gravina belt as an intraoceanic arc basin.

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