ABSTRACT The Matanuska lowland north of Anchorage, Alaska, was episodically glaciated during the Pleistocene by the merged westward flow of the Matanuska and Knik glaciers. During the late Wisconsin glaciation, glacial Lake Atna filled the Copper River Basin, impounded by an ice dam blocking the Matanuska drainage divide at Tahneta Pass and the adjacent Squaw Creek headwaters and ice dams at other basin outlets, including the Susitna and Copper rivers. On the Matanuska lowland floor upvalley from the coalesced glacier’s late-Wisconsin terminus, a series of regularly spaced, symmetrical ridges with 0.9-km wavelengths and heights to 36 m are oriented normal to oblique to the valley and covered by smaller subparallel ridges with wavelengths typically ~80 m and amplitudes to 3 m. These and nearby drumlins, eskers, and moraines were previously interpreted to be glacial in origin. Borrow-pit exposures in the large ridges, however, show sorting and stratification, locally with foreset bedding. A decade ago we reinterpreted such observations as evidence of outburst flooding during glacial retreat, driven by water flushing from Lake Atna through breaches in the Tahneta Pass and Squaw Creek ice dam. In this view, the ridges once labeled Rogen and De Geer moraines were reinterpreted as two scales of fluvial dunes. New observations in the field and from meter-scale light detection and ranging (LiDAR) and interferometric synthetic aperture radar (IfSAR) digital elevation models, together with grain-size analyses and ground-penetrating radar profiles, provide further evidence that portions of the glacial landscape of the Matanuska lowlands were modified by megaflooding after the Last Glacial Maximum, and support the conclusion that the Knik Glacier was the last active glacier in the lowland.

ABSTRACT Upper Jurassic to Upper Cretaceous forearc basin strata exposed in south-central Alaska record variations in sedimentary basin development associated with late Mesozoic terrane accretion and translation of the Insular terranes (Alexander-Wrangellia-Peninsular) along the western North American margin. Upper Jurassic strata contain a dominant population of Late Jurassic (ca. 150 Ma) detrital zircons with juvenile to intermediate isotopic compositions (ε Hf[ t ] >5) that reflect sediment input from the adjacent oceanic Talkeetna arc, located to the north in a marine forearc basin. By Early Cretaceous time, Early–Late Jurassic (ca. 160 and 198 Ma) and minor Late Devonian–Permian detrital zircon grains that have juvenile to intermediate hafnium compositions (ε Hf[ t ] >5) indicate continued exhumation of the Talkeetna arc and a new sediment contribution from erosion of Insular terrane basement. Upper Cretaceous strata record final suturing of the terrane, resulting in an influx of sediment from the inboard Intermontane terrane, represented by Ordovician–Permian detrital zircon grains with a wide range of hafnium compositions (ε Hf[ t ] >–25). Late Cretaceous detrital zircon ages (ca. 90 and 70 Ma) in these strata reflect establishment of a coeval volcanic arc. Trends toward more-evolved hafnium isotopic data within our data record ~40 m.y. orogenic cycles of crustal thickening during the Late Triassic–Early Jurassic and Late Jurassic that can be linked to Cordilleran-scale deformational events. Overall, we prefer a model where the forearc basin in south-central Alaska, the part of the Intermontane terranes that is today located in northern British Columbia, Yukon, and eastern Alaska, and the part of the Chugach–Prince William terrane found on Kodiak Island were all juxtaposed by Late Cretaceous (Turonian) time.

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.