ABSTRACT A robust set of modal composition data (238 samples) for Eocene to Pliocene sandstone from the Cook Inlet forearc basin of southern Alaska reveals strong temporal trends in composition, particularly in the abundance of volcanic lithic grains. Field and petrographic point-count data from the northwestern side of the basin indicate that the middle Eocene West Foreland Formation was strongly influenced by nearby volcanic activity. The middle Eocene to lower Miocene Hemlock Conglomerate and Oligocene to middle Miocene Tyonek Formation have a more mature quartzose composition with limited volcanic input. The middle to upper Miocene Beluga Formation includes abundant argillaceous sedimentary lithic grains and records an upward increase in volcanogenic material. The up-section increase in volcanic detritus continues into the upper Miocene to Pliocene Sterling Formation. These first-order observations are interpreted to primarily reflect the waxing and waning of nearby arc magmatism. Available U-Pb detrital zircon geochronologic data indicate a dramatic reduction in zircon abundance during the early Eocene, and again during the Oligocene to Miocene, suggesting the arc was nearly dormant during these intervals. The reduced arc flux may record events such as subduction of slab windows or material that resisted subduction. The earlier hiatus in volcanism began ca. 56 Ma and coincided with a widely accepted model of ridge subduction beneath south-central Alaska. The later hiatus (ca. 25–8 Ma) coincided with insertion of the leading edge of the Yakutat terrane beneath the North American continental margin, resulting in an Oligocene to Miocene episode of flat-slab subduction that extended farther to the southwest than the modern seismically imaged flat-slab region. The younger tectonic event coincided with development of some of the best petroleum reservoirs in Cook Inlet.

Abstract The Cook Inlet Basin is a northeast-trending collisional forearc basin that extends from Shelikof Strait northeastward to the east end of the Matanuska Valley. The basin is divided into three segments including, from northeast to southwest, the Matanuska Valley segment, Cook Inlet segment, and Cook-Shelikof segment. The matanuska Valley segment represents the collapsed onshore part of the basin. The Cook Inlet segment is a significant hydrocarbon province, with more than 1.3 billion barrels of oil and about 8 trillion cubic feet (TCF of gas produced since 1958. No commercial oil or gas production has been established in either the Cook-Shelikof or Matanuska segments of the basin.

Abstract The Albian-Cenomanian Nanushuk Formation is a thick regressive fluvialdeltaic-shelf succession that crops out in the foothills north of the Brooks Range and is present in the subsurface throughout most of the National Petroleum ReserveAlaska (NPRA). Three major facies assemblages are recognized in the Nanushuk Formation in outcrop at Rooftop Ridge and Ninuluk Bluff, and in the subsurface in the Grandstand, Square Lake 1, Wolf Creek 3, and Fish Creek 1 wells, including offshore, shoreface-delta-front, and bay fill assemblages. In the eastern NPRA and northern foothills belt, the shoreface-delta-front assemblage is the most common facies. Individual shoreface-delta-front deposits are arranged in coarsening-upward successions from 30 feet to over 100 feet thick that stack to form thicker regressive coarsening-upward successions bounded by distinct flooding surfaces. Sandstones of this assemblage are characterized by abundant storm wave-generated structures, suggesting that the shorezone in this area was significantly wave influenced, if not wave-dominated. Near the top of thicker regressive successions, shoreface-delta-front deposits lack appreciable mudstone and sandstones are amalgamated forming relatively thick bodies. Elements of the bay fill assemblage are present at the top of the Nanushuk Formation in outcrop at Ninuluk Bluff and in the subsurface in the Square Lake 1 well, where they are interpreted as an estuarine(?) complex. A possible sharp-based shoreface succession truncates the estuarine(?) complex at Ninuluk Bluff and is, itself, truncated by a major sequence-bounding unconformity at the base of the Seabee Formation (Turonian). This surface has also been recognized in the Square Lake 1 well and at Rooftop Ridge, demonstrating its regional extent. Reservoir potential is greatest in the shoreface-delta-front assemblage where constant wave activity has winnowed fine-grained material and reduced the volume of ductile framework grains. Stacked amalgamated shoreface-delta-front sandstones in the Nanushuk Formation may form thick potential reservoir units. Possible shoreward pinchout in backshore and bay fill mudstones may provide opportunities for stratigraphic traps in the shoreface-delta-front assemblage. Thick sandstone accumulations in estuarine settings, such as recognized at Ninuluk Bluff, may represent secondary reservoir targets.