THE CHIGNIK FORMATION, ANIAKCHAK NATIONAL MONUMENT & PRESERVE, ALASKA: A LATE CRETACEOUS (CAMPANIAN-MAASTRICHTIAN) ESTUARY-FILL

The Chignik Fm. is a cyclic succession of sedimentary rocks deposited at a relatively high latitude under Late Cretaceous greenhouse conditions. A ~300 m succession exposed along the coast of Aniakchak Bay represents shallow marine to near shore marine environments in the lower part and continental alluvial/coastal plain environments in the upper part. Nine facies associations represent shallow marine deposits, tidal flats, large and small fluvial channels, crevasse splays, backswamps, tidal marshes, floodplains, paleosols, and storm deposits. The basal shallow marine part of the section consists of offshore shales and siltstones with thin sandstones interbeds that coarsen-upward into bioturbated cross-bedded sandstones. Interbedded sandstones and siltstones, double mud drapes and flaser, wavy, and lenticular bedding represent tidal flats. Layers of broken bivalve shells in non-preferred orientations are interpreted to represent wave/storm action along the coast. Tidal channels are characterized by burrowed, cross-bedded and ripple cross-laminated fining-upward sandstones. Fluvial deposits are represented by fining-upward, cross-bedded and ripple cross-laminated sandstones with erosional lower contacts. Mudstones rich in organic matter and thin coal beds represent floodplains, backswamps, and tidal marsh deposits. Blocky mudstones with siderite nodules, root traces, organic matter, plant fragments, and bioturbation are interpreted as paleosols. In places standing tree trunks are present at the top of paleosols. The entire section contains a rich fossil floral and faunal record. Currently over 75 dinosaur track sites have been documented including hadrosaurids, ankylosaurs, non-avian theropods, and avian theropods. A large multi-story fluvial channel (~30 m) overlying shallow marine deposits is interpreted as an incised valley. The valley fill is overlain by coastal/tidal flat deposits that transition upward into continental fluvial depositional environments. The entire section is interpreted as a transgressive-regressive succession, consistent with a tide-dominated estuary fill. The modern Aniakchak river entering Aniakchak Bay provides an excellent modern analogue.