Large-scale, latest Cretaceous uplift along the Northeast Pacific Rim: Evidence from sediment volume, sandstone petrography, and Nd isotope signatures of the Kodiak Formation, Kodiak Islands, Alaska

Abstract

The Chugach terrane largely comprises a voluminous flysch belt that extends for over 2000 km along the southern margin of Alaska. Sandstone petrography, paleocurrent patterns, lithofacies, and Nd isotopic characteristics of the flysch belt in Kodiak and adjacent islands constrain the predominant source terrane of the Chugach flysch as a rapidly uplifting recycled orogen with a lesser component of a magmatic arc source. The Kodiak Formation composes the portion of the Chugach flysch that crops out in Kodiak and adjacent islands. The Kodiak Formation was added to the North American continental margin by underplating in a subduction zone during latest Cretaceous time. It is a thick sequence of deep-water turbidites, most of which are coarse-grained, thickly bedded sandstone-shale sequences interleaved with units of thinly bedded sandstone and shale or nearly massive shale. Several occurrences of Inoceramus species, including new finds reported here, confirm that the Kodiak Formation was deposited during early Maastrichtian time, probably in less than four million years. The short absolute time period of deposition, coupled with estimates of the volume of the Chugach flysch, suggests that the original turbidite fan system represented by this unit was immense. Its depositional rate was probably within a factor of two of the rate for the Bengal-Nicobar fan system composed of erosional products of the Himalayan uplift. 118 absolute paleocurrent indicators show two components of sediment transport: a dominant component to the south, highly oblique to the northeast-trending structural grain, with a smaller component oriented north. Petrography of 23 sandstones spanning the width of the formation fall into two distinct categories, suggesting a mixture of provenances. The dominant provenance is a recycled orogen relatively high in quartz (Q 4 9 F 1 5 L 3 6 ). The lesser provenance is an uplifting magmatic arc rich in lithic volcaniclastic grains (Q 1 0 F 2 2 L 6 8 ). Nd isotopic signatures of volcaniclastic sandstones (e N d = +2 to +5) are significantly more radiogenic than those of quartzose sandstones (e N d = 0 to -2). Nd model ages suggest that the source terrane progressed from a recycled orogen, composed in part of Proterozoic materials, to a continental margin volcanic arc. The combination of sedimentological data indicates that the Kodiak Formation was deposited in a large submarine fan system near its source, filling the trench and spilling out onto the downgoing oceanic plate. The magnitude of uplift in the source region must have been large. Our data alone do not give a unique solution to the source terrane, but considering many aspects of the geology of the North American margin, the most likely candidate is the Coast Mountains belt of western British Columbia, including the Coast Plutonic Complex. This orogen satisfies constraints of sandstone composition, uplift history, and microplate reconstructions, and suggests a major convergent-margin event caused uplift of the westernmost margin of North America to begin during latest Cretaceous time. Deposition and accretion of the Chugach flysch may have occurred substantially south of its current latitude along a convergent margin with significant dextral shear.