COMPARISON BETWEEN GEOCHRONOLOGIC AND GEOCHEMICAL PROVENANCE MODELS FOR NEOGENE COOK INLET BASIN STRATA, SOUTH-CENTRAL ALASKA

The provenance of most basin systems today is interpreted based on detrital zircon geochronology, which has all but replaced the use of geochemical approaches that can sometimes better constrain and effectively complement provenance information inferred from isotopic approaches. Here we compare mudstone and sandstone whole rock major and trace element geochemistry of the Late Oligocene-Middle Miocene Tyonek, Late Miocene Beluga and Pliocene Sterling formations in the Cook Inlet Basin, Alaska, with existing detrital zircon U-Pb geochronology to better elucidate the provenance. Zircons are abundant in some lithologies but deficient in others so detrital zircon U-Pb geochronology only detects certain components in a source region. Previous detrital zircon data demonstrate that zircon-bearing source areas for the Cenozoic strata vary dependent upon which margin of the basin they are exposed.

Based on detrital geochronology, the Tyonek, Beluga, and Sterling formations exposed along the eastern margin of the basin primarily derived sediment from the adjacent accretionary prism and accreted juvenile (mafic) arc rocks along the eastern margin. The Beluga and Sterling formations exposed along the western margin also match this eastern provenance. In contrast, the Tyonek formation exposed along the western margin reflects a primarily granitic sediment source to the north. Our new geochemical data from all the formations suggest a mostly juvenile to intermediate sediment source and evidence for sediment recycling from accretionary prism strata is not observed. Discrimination diagrams based on trace elements consistently show a trend from mafic to andesitic sources for the Late Cenozoic strata but rarely reach felsic compositions demonstrating the loss of the granitic signature in the mudstone.

This study demonstrates that combining geochronologic and geochemical data from sedimentary rocks can help to better resolve provenance and provide insight into non-zircon-bearing source areas. Furthermore, geochemical approaches can be applied to both sand- and mud-sized fractions, further expanding our understanding of sediment flux and potential source rocks in a basin.