SECULAR CHANGES IN CENOZOIC ARC MAGMATISM AND THEIR INFLUENCE ON FOREARC BASIN SANDSTONE COMPOSITION AND RESERVOIR QUALITY IN COOK INLET BASIN, SOUTHERN ALASKA

Oil and gas production from the Cook Inlet region is of major importance to the economy and energy landscape in southern Alaska. Reservoir quality in Cenozoic nonmarine sandstones is variable and controlled primarily by original detrital grain composition and burial history. Recent studies have established a robust data set of modal compositional data from Eocene through Pliocene sandstone from the Cook Inlet forearc basin (see Helmold et al., 2013; doi:10.14509/25035). There is a clear signal indicating strong temporal trends in composition, particularly in the abundance of volcanic lithic grains. The relative contribution from igneous sources exerts a first order control on the subsequent resistance to compaction and susceptibility to diagenetic alteration, particularly by zeolites.

Field and petrographic point count data from the northwestern side of the basin indicate that the Eocene West Foreland Formation was strongly influenced by nearby volcanic activity, with a likely source region, at least in part, near the Merrill Pass pluton in the western Alaska Range. The Oligocene Hemlock Conglomerate and Miocene Tyonek Formation have a much more mature, quartzose composition, with limited volcanic input. The middle and upper Miocene Beluga Formation includes abundant argillaceous sedimentary lithic grains and records an upward increase in volcanic lithic material. The up-section increase in volcanic detritus continues into the upper Miocene and Pliocene Sterling Formation.

These first-order observations are interpreted to primarily reflect the waxing and waning of nearby arc magmatism. Recent improvements in the precision and abundance of geochronologic information confirms that the western Alaska Range arc was nearly dormant from ~25 – 5 Ma, which coincides with the development of the best reservoir quality in Cook Inlet. The reduced arc flux may record the insertion of the leading edge of the Yakutat terrane, potentially resulting in an Oligocene – Miocene episode of flat slab subduction that extended further west than the modern seismically imaged flat slab region. This shallowly dipping subduction may also have promoted increased coupling and tectonic erosion of the upper plate in southern Alaska, contributing to the remarkable amount of Neogene tectonic subsidence in the forearc basin.