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Published August 10, 1991 | Published
Journal Article Open

The Gravina Sequence: Remnants of a Mid-Mesozoic oceanic arc in southern southeast Alaska


Fragments of Upper Jurassic to Lower Cretaceous volcanic and basinal strata constitute the Gravina belt in southeast Alaska. In the Ketchikan area the Gravina belt is made up of two lithotectonic units. The lower unit consists of coarse marine pyroclastic and volcaniclastic strata, mafic flows, breccia, and fine-grained tuff which are locally intruded by hypabyssal bodies of diorite and quartz diorite. The volcanic rocks are characterized by tholeiitic arc basalts, lack felsic volcanic strata, and overlie Upper Triassic and older strata of the Alexander terrane. Augite and/or hornblende-bearing porphyritic rocks are common and locally intrude the Alexander terrane basement, where they are thought to represent the intrusive equivalents of lavas within the section. Age constraints for the volcanic unit, based on structural and stratigraphic relations with adjacent units, are late Middle to Late Jurassic. The Gravina belt upper unit consists of fine- to coarse-grained turbidites and related conglomeratic channel-fill deposits. The basinal rocks unconformably overlie Permian and Triassic rocks of the Taku terrane and remnants of the lower volcanic part of the Gravina sequence which overlie the Alexander terrane. The conglomerate units contain mostly volcanic and plutonic lithic clasts, some of which yield Pb-U zircon ages of 154–158 Ma. The predominance of pyroclastic deposits interbedded with massive flows, tuff, breccia, and argillaceous turbidites, and the lithologic and chemical composition of the volcanic rocks indicate a submarine volcanic arc setting for the Gravina sequence. The basinal pyroclastic rocks are inferred to have been shed from submarine stratovolcanos during the Late Jurassic. Epiclastic rocks were deposited as submarine fans, derived in part from erosion of a magmatic arc. The presence of fine-grained tuffaceous turbidites implies ongoing, but distant, volcanism. The pyroclastic and volcaniclastic rocks represent remnants of a Late Jurassic oceanic arc constructed on a composite basement consisting of the Alexander and Taku terranes. The strata accumulated in an intra-arc basin on the eastern edge of the Alexander terrane. The volcanic and basinal rocks were deformed during a major mid-Cretaceous intra-arc contractional event, in conjunction with the emplacement of a distinctly younger, arc-related plutonic suite.

Additional Information

© 1991 by the American Geophysical Union. Paper number 91JB00591. Received August 2, 1990; revised February 19, 1991; accepted February 23, 1991. Parts of this research were supported by National Science Foundation grants EAR 86-05386 and EAR 88- 034834 (to Saleeby). Additional support (to Rubin) was provided by a Geological Society of America Penrose Grant, a Sigma-Xi grant-in-aid, the U.S. Geological Survey, Alaska Branch, and by the U.S. Forest Service, Ketchikan Ranger District. Jeff Marshall provided excellent field assistance during the summer of 1987. We thank Fred Barker, Danel Cowan, Weecha Crawford, John Gatvet, George Gehrels, Linc Holllister Bill McClelland, Megran Miller, Jim Monger, and George Plafker for helpful discussions. George Plafker provided a very thorough and constructive review of the manuscript. Special thanks to Fred Barker (U.S. Geological Survey) for providing the geochemical analyses for the Gravina sequence volcanic rocks.

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