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 This volume is designed to showcase the geo-technical elements of oil and gas fields of the Cook Inlet Basin of southcentral Alaska. It contains 10 chapters written by 16 authors and co-authors who have extensive experience in the basin. All of the papers have been peer-reviewed. The first three chapters provide an introduction to exploration, stratigraphy, petroleum systems, seismic acquisition, and reservoirs of the basin. Following these are seven chapters that describe individual fields in detail. This volume is intended to serve as a key reference to the petroleum geology of the Cook Inlet Basin for a wide audience including oil and gas explorers, technical professionals, students and those seeking more information about the origin and habitat of oil and gas in the area.

A distinctive yet enigmatic suite of fault-bounded ultramafic massifs occurs within accretionary complex mélange of the McHugh Complex on the Kenai Peninsula of southern Alaska. The largest and most significant of these include Red Mountain and the Halibut Cove Complex, consisting of dunite and pyroxenite with chromite seams and lesser quantities of garnet pyroxenite and gabbro. Several different hypotheses have been advanced to explain their origin. Burns (1985) correlated these fault-bounded ultramafic massifs with others known as the Border Ranges Ultramafic-Mafic Complex. Other parts of the Border Ranges Ultramafic-Mafic Complex are located several hundred kilometers away along the Border Ranges fault, marking the boundary between the Chugach terrane and the Wrangellian composite terrane in the northern and eastern Chugach Mountains. Burns (1985) suggested that this entire group of ultramafic bodies represents the deep roots of the Talkeetna arc developed on the southern margin of Wrangellia during Early Jurassic–Cretaceous subduction. In this model, bodies such as Red Mountain represent klippen thrust hundreds of kilometers southward over the McHugh Complex and now preserved as erosional remnants. Bradley and Kusky (1992) suggested alternatively that the Kenai ultramafic massifs may represent segments of a thick oceanic plate offscraped during subduction, and therefore might represent ophiolitic, oceanic plateau, or immature island arc crust as opposed to the roots of the mature Talkeetna arc. In this scenario, the Kenai ultramafic massifs would be correlative with the McHugh Complex, not the Talkeetna arc. A third hypothesis is that the Border Ranges Ultramafic-Mafic Complex may represent forearc or suprasubduction zone ophiolites formed seaward of the Talkeetna arc during early stages of its evolution and incorporated into the accretionary wedge during subsequent accretion tectonics. The implications of which of these models is correct are large because the Talkeetna arc section is the world's premiere example of a complete exposed arc sequence, including the volcanic carapace through deep crustal levels. Many models for the composition and evolution of the crust rely on the interpretation that this is a coherent and cogenetic section of arc crust. We report six new U/Pb zircon ages that show that at least some of the deep ultra-mafic and mafic complexes of the Border Ranges Ultramafic-Mafic Complex are Triassic (227.7 ± 0.6 Ma; Norian) and significantly older than structurally overlying Jurassic rocks of the Talkeetna arc (201–181 Ma, continuing plutonism until 163 Ma) but the same age as the surrounding Triassic-Jurassic-Cretaceous McHugh Complex. New geochemical data that show that rocks of the Border Ranges Ultramafic-Mafic Complex have ophiolitic affinities, with Cr-chemistry further indicating that the complex's rocks formed in a suprasubduction zone ophiolite. Regional and detailed and field observations show that rocks of the complex are similar to and can be structurally restored with other fault-bounded units in the McHugh Complex mélange, and that a crude ophiolitic stratigraphy can be reconstructed through the Border Ranges Ultramafic-Mafic Complex and McHugh Complex. We suggest that the Border Ranges Ultramafic-Mafic Complex represents the forearc oceanic basement upon which the Talkeetna arc was subsequently built. The conclusion that the Border Ranges Ultramafic-Mafic Complex does not represent the base of the Talkeetna arc but instead contains remnants of a dismembered ophiolitic complex raises questions about the validity of mass balance calculations and bulk crustal compositions, as well as models of arc development used to understand the growth of continental crust.

Six samples collected from pre-, syn-, and post-Talkeetna arc units in south-central Alaska were dated using single-grain zircon LA-MC-ICP-MS geochronology to assess the age of arc volcanism and the presence and age of any inherited components in the arc. The oldest dated sample comes from a volcanic breccia at the base of the Talkeetna Formation on the Alaska Peninsula and indicates that initial arc volcanism began by 207 ± 5 Ma. A sedimentary rock overlying the volcanic section in the Talkeetna Mountains has a maximum depositional age of <167 Ma. This is in agreement with biochronologic ages for the top of the Talkeetna Formation, suggesting that the Talkeetna arc was active for ca. 40 m.y. Three samples from interplutonic screens and roof pendants in the Jurassic batholith on the Alaska Peninsula provide information about the tectonic setting of Talkeetna arc magmatism. All three samples contain Paleozoic to Proterozoic zircons and require that arc magmas on the Alaska Peninsula intruded into detritus that contained older continental zircons. This finding is distinct from observations from eastern exposures of the arc in the Chugach and Talkeetna Mountains, where there is only limited evidence for pre-Paleozoic zircons, and it suggests that there were along-strike variations in the tectonic setting of the arc.

Volcanic and granitic rocks of the Jack River igneous field were erupted and emplaced in the suture zone between the accreted Wrangellia composite terrane and the former margin of southern Alaska. The volcanic rocks unconformably overlie Jurassic-Cretaceous shale and sandstone of the Kahiltna assemblage and include 100–300 m of basalt, basaltic andesite, and andesite lava flows overlain by a rhyolite unit that consists of over 900 m of lava flows and pyroclastic deposits. Seven basaltic and rhyolite lava samples yield 40 Ar/ 39 Ar ages ranging from 56.0 ± 0.3 to 49.5 ± 0.3 Ma. Two granitic samples yield 40 Ar/ 39 Ar ages of 54.6 ± 0.4 and 62.7 ± 0.4 Ma. These age dates indicate that the onset of Jack River magmatism at ca. 62.7 Ma coincided with the terminal phase of terrane accretion and continued after accretion to at least 49.5 Ma. The volcanic rocks range between tholeiitic and high-K calc-alkaline series and show a bimodal distribution with respect to silica (dacite is absent). The Jack River basalts are tholeiitic, have rare earth element and high field strength element ratios that are in the range between Pacific enriched mid-ocean-ridge basalts and Hawaiian ocean-island basalts (e.g., La/Yb = 5.0–8.4; Nb/Zr = 0.07–0.11), and have a within-plate geochemistry (e.g., Ti/V >50; high Zr/Y). All of the Jack River volcanic rocks exhibit some degree of enrichment in large ion lithophile and/or fluid mobile elements (e.g., Cs, Ba, Th, U, K, and Pb), although the basalts have low ratios between large ion lithophile and high field strength elements (e.g., Ba/Nb as low as 32.7 and Pb/Nb of 0.28–0.35). The granitic rocks (granites to granodiorites) are strongly depleted in the heavy rare earth elements, and most samples exhibit characteristics of adakites (e.g., Al 2 O 3 >15 weight %, Yb = 0.6–1.2 ppm, Y = 5.5–12.5 ppm, and Sr/Y = 20.4–66.2). The Jack River basalts were derived from partial melts of a mantle source that was more enriched than depleted mid-ocean-ridge basalt mantle and that ranged toward an enriched mantle (EM-I-type) composition.The basalts then evolved by assimilation and fractional crystallization to form intermediate magmas. Rhyolite magmas were formed later as anatectic melts of upper crustal argillaceous rocks (Kahiltna assemblage), resulting in the bimodal volcanism. The granitic adakite magmas may have formed by melting of garnet-bearing metamorphosed sedimentary rocks (meta-Kahiltna assemblage) that formed lower crustal rocks in the suture zone. Although the Jack River igneous rocks do exhibit some arc-like geochemical characteristics (e.g., elevated large ion lithophile elements), they differ from calc-alkaline arc rocks in that (1) they are a bimodal volcanic suite; (2) the rhyolites are not comagmatic with the basaltic and intermediate rocks; (3) the basalts and andesites have higher TiO 2 (>1.5 weight %) than is typical for arc basalts and andesites; (4) the basalts do not exhibit depletion of high field strength elements (e.g., Ta and Nb) with respect to large ion lithophile elements; (5) the basalts have an intraplate geochemical affinity; and (6) adakites are present. These characteristics show that the geochemistry of postcollisional suture zone magmatism can be transitional between calc-alkaline arc and intraplate magmatism. The Jack River volcanic field is deformed into a broad, northeast-trending syncline, which is crosscut by small-scale brittle faults that include northwest- and west-trending normal-slip and oblique-slip faults, and a southeast-dipping reverse fault that places Kahiltna assemblage rocks over the Jack River volcanic rocks. The pattern of Jack River deformation is consistent with right-lateral simple shear along the Denali fault system and indicates an episode of post-49.5 Ma strike-slip along the McKinley strand of the Denali fault. The Jack River rocks, therefore, record the magmatic response to terrane accretion and the kinematics of margin-parallel transport of an accreted terrane assemblage after it was sutured to the continental margin.