Eocene underplating along the Kodiak Shelf, Alaska: Implications and regional correlations

Abstract

Structural geology and geophysical data from the Kodiak Shelf suggest that the Mesozoic rocks exposed on the shelf are structurally underlain (at about 12 km depth) by several kms of Eocene age strata. Kinematic data from the Late Cretaceous to Paleocene Ghost Rocks Formation indicate that this formation and probably all of the Kodiak Islands, were uplifted vertically to nearly their present elevations. Landward tilting and imbrication are not indicated. The age of uplift is indicated by a regional, angular unconformity of Early Eocene to Early Oligocene age that separates deep‐sea rocks from shallow water to non‐marine rocks. The uplift of the accretionary prism is believed to have been caused by underplating of an Eocene sedimentary sequence because (1) a band of seismic reflections that occur 12 to 20 km beneath the shelf is interpreted as the top of the underplated material and (2) an obductively offscraped sequence of Eocene deep‐sea rocks crops out on the seaward side of the Kodiak Shelf, suggesting that a thick trench‐fill sequence may have been present prior to uplift of the prism. The underplated material is interpreted to be part of either a previously unrecognized turbidite fan of Early Eocene age or a proximal equivalent of the Zodiac fan of Late Eocene to Early Oligocene age. Other possible on‐land remnants of the underplated material may be present in Prince William Sound (the Montague belt), the Gulf of Alaska (lower sections of the Yakutat block) and in the Coast Ranges of Oregon and Washington. The large volume of underplated material beneath the Kodiak shelf suggests that underplating may be the dominant process in the growth of convergent margins.