ENIGMATIC LATE CRETACEOUS TECTONIC SUBSIDENCE OF THE COOK INLET FOREARC BASIN DURING MAJOR SUBDUCTION ACCRETION, SOUTHERN ALASKA

Forearc basins are dynamically linked to subduction zone processes. In particular, phases of significant basin subsidence are generally ascribed to episodes of tectonic erosion of the upper plate. However, new data from the Late Cretaceous of southern Alaska appear to illustrate an example of rapid tectonic subsidence in the forearc that was coeval with tectonic accretion of the Valdez Group along the margin, rather than tectonic erosion.

We examined the ~1100 m thick Upper Campanian to Lower Maastrichtian Kaguyak Formation at its type locality on the upper Alaska Peninsula. The lower part is dominantly fine-grained with a diverse and abundant trace fossil assemblage, including Helminthopsis, Phycosiphon, Schaubcylindrichnus, Terebellina, Teichichnus, and Thalassinoides. Primary sedimentary structures are rare, likely reflecting deposition below wave base and thorough disruption of lamination by bioturbation. We interpret this part of the unit as offshore transition to shelfal.

In contrast, the upper part of the section is dominated by well-bedded, rhythmic alternations of siltstone and very fine- to medium-grained sandstone. Trace fossils and bioturbation are rare and the totality of sedimentary facies is consistent with deposition via sediment gravity flows, ranging from high density flows to more dilute, turbulent flows. Based on the sedimentary facies, the lack of wave-generated structures, and the dearth of bioturbation, we interpret the upper half of the Kaguyak as deposited in deep water, likely in a base of slope to basin floor setting.

The origin of this episode of deepening is unclear, although integration of our data with published information from the Matanuska Valley and lower Alaska Peninsula suggest that abrupt and significant subsidence is widely recorded along the entire length of the forearc basin at this time. Part of the subsidence may reflect thermal effects from the coeval arc, and/or a flexural signal from the structural thickening of the accretionary prism. Alternatively, the increase in accommodation may be related to tectonism further inboard, including the Cretaceous docking of the Wrangellia composite terrane with the ancestral margin of Alaska and associated changes in the regional plate convergence accommodated at the outboard subduction zone.