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.

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Abstract Permian deposits of the Junggar and Turpan-Hamibasins of the Xinjiang Uygur Autonomous Region of northwest China preserve some of the thickest and mostareally extensive lake strata on Earth. In the south Junggar depocenter, these nonmarine deposits are up to 5 kmthick and organic-rich facies rank among the thickestand richest petroleum source rocks in the world (Grahamet al., 1990; Lawrence, 1990; Demaison and Huizinga, 1991;Carroll et al., 1992).In addition, Permianlacustrine deposits are estimated to span 1000km along strike, indicating that widespread lakes represented amajor paleogeographic feature of central Asia (Figure 1).Unfortunately, the remote location of these deposits has hindered detailed studies, and the western literature contains only sparse reference to this important record of continental sedimentation. The purpose of this paperis to briefly review the Permian nonmarine stratigraphyand report on recent field-based studies documentingthe Permian lacustrine stratigraphy exposed along the north and south flanks of the Bogda Shan (Figure 2).