Petrofacies and Depositional Environments of Upper Jurassic Naknek Formation, Lower Cook Inlet, Alaska: ABSTRACT

Abstract

More than 3,000 m of conglomerate, sandstone, and siltstone were deposited in fluvial, shelf, slope, and basin-floor environments in an Upper Jurassic fore-arc basin on the Alaska Peninsula. Distance from uplands to basin floor was short and sediment supply locally concentrated, resulting in narrow facies belts and abrupt facies changes. Three successively younger depositional packages are exposed from north to south: (1) deep-water proximal turbidites from the Iniskin-Tuxedni area to Contact Point; (2) nonmarine sandstones northwest of Akumwarvik Bay; and (3) shallow-shelf sandstones east of Akumwarvik Bay. Three distinctive and successively younger sandstone petrofacies are recognized from north to south, but these petrofacies do not correlate exactly with the three depositional packages. (1) Naknek sandstones in the Iniskin-Tuxedni area contain abundant plagioclase (typically replaced by zeolites), volcanic rock fragments, reddish hornblende, with a notable lack of quartz and K-feldspar. (2) Naknek sandstones south of Iniskin Peninsula and north of Akumwarvik Bay contain quartz, K-feldspar, and metamorphic rock fragments, with a decrease in volcanic rock fragments. (3) Naknek sandstones east of Akumwarvik Bay have more quartz, K-feldspar, and metamorphic rock fragments than older Naknek sandstones End_Page 909------------------------------ and are better sorted and rounded due to abrasion on a shallow shelf. Depositional environments did not control sandstone composition. Distinctive Naknek petrofacies are due to (1) erosion of Lower Jurassic volcanics followed by unroofing of Lower to Middle Jurassic diorite and granodiorite plutons; and (2) variations of provenance in separate drainage basins through time. End_of_Article - Last_Page 910------------