SEDIMENTARY RECORD OF EXHUMATION ALONG A FOREARC BASIN-ACCRETIONARY PRISM BOUNDARY DURING RIDGE SUBDUCTION: PALEOGENE CHICKALOON FORMATION, NORTHERN CHUGACH MOUNTAINS, SOUTHERN ALASKA

Paleogene sedimentary strata exposed in the northern Chugach Mountains of south-central Alaska record forearc basin dynamics during a period of spreading ridge subduction. Measured stratigraphic sections spanning the Chickaloon Formation near Nelchina Glacier and Glacier Creek document >300 m thick successions of nonmarine conglomerate, sandstone, mudstone, and tuff. Amalgamated beds of lenticular sandstone and conglomerate record subaqueous deposition by streamflow and debris flow in shallow, low-sinuosity channels and bars. Subordinate carbonaceous mudstone indicates suspension fallout and pedogenesis in overbank and lacustrine environments. Strata at Glacier Creek comprise a northward fining succession of alluvial strata between two fault segments interpreted as syndepositional dextral faults. Forearc strata at Glacier Creek are faulted against northernmost prism strata. However, our mapping at Nelchina Glacier confirms an unconformable contact between the Chickaloon Fm. and Upper Cretaceous metasedimentary strata of the accretionary prism (the Valdez Group). Tuffs collected from both measured sections yield ca. 54 Ma U-Pb zircon ages. Compositional data document abundant argillite, mudstone, and chert clasts and subordinate mafic-intermediate volcanic-plutonic clasts. U-Pb geochronologic analyses of >800 detrital zircons reveal two different age spectra. The Chickaloon Fm. and older segments of the accretionary prism (the McHugh Complex) exhibit unimodal populations of Early Jurassic to Early Cretaceous detrital ages, whereas younger parts of the prism (the Valdez Group) yield a broader distribution of ages, including Precambrian to Late Cretaceous ages. Integration of our stratigraphic, geochronologic, and provenance data indicate that alluvial fan, fluvial, and lacustrine environments deposited sediment in strike-slip pull-apart basins along the southern margin of the forearc basin. These depositional systems transported clastic sediment chiefly from uplifted parts of the proximal accretionary prism (the McHugh complex). Our new data from the forearc basin support tectonic models the infer forearc uplift during subduction of progressively younger crust from ca. 75-60 Ma followed by subduction of a spreading center from ca. 60-50 Ma.