SEDIMENTOLOGICAL EVIDENCE FROM THE MAASTRICTIAN KODIAK FORMATION (SOUTHERN ALASKA) FOR LARGE-SCALE UPLIFT ALONG THE NORTHEAST PACIFIC RIM RELATED TO TRANSPRESSIONAL TECTONICS

The Kodiak Formation is a large sequence of accreted turbidites in southern Alaska. Inoceramid fossils found within the unit, including five new localities reported here, confirm that the Kodiak Formation was deposited in less than four, and perhaps less than two, million years.  The short time period of deposition and immense volume of the formation suggest a depositional rate on the same order as the Bengal-Nicobar fan complex derived from the Himalayas.  118 absolute paleocurrent indicators show a dominant component of sediment transport to the south, oblique to the northeast-trending structural grain, and a smaller component oriented north. Sandstone compositions (n=23) 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 (Q₄₉F₁₅L₃₆).  The lesser provenance is an uplifting magmatic arc rich in lithic volcaniclastic grains (Q₁₀F₂₂L₆₈).  Nd isotopic signatures of volcaniclastic sandstones (e_Nd=+2 to +5) are significantly more radiogenic than those of quartzose sandstones (e_Nd =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 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 transpressional 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.