APOGEE-PERIGEE CYCLES PRESERVED IN THE FLUVIO-ESTUARINE TRANSITION IN TURNAGAIN ARM, ALASKA; IMPLICATIONS FOR ANCIENT TIDAL RHYTHMITES

Lunar apogee-perigee-influenced sedimentation has been interpreted in many ancient tidal rhythmites. Although there are many ancient examples, few modern examples of near-complete apogee-perigee cycles have been recorded in modern sediments. In the summer of 2003 and 2004, sequential thickening and thinning of laminae bundles representing apogee-perigee cycles were noted on a tidal flat at the confluence of Glacier Creek and the Turnagain Arm estuary in Alaska. Turnagain Arm is a macrotidal estuary with semidiurnal tides and a spring tidal range of more than 9 m. Glacier Creek is a small, transverse drainage, which drains a glacial valley developed above a Pleistocene alluvial fan. At the confluence of the creek and estuary is a broad reentrant in the shoreline, and the flats accrete above a gravel bed within the reentrant. The flats are bordered landward by a sedge marsh and seaward by the fluvial channel as it enters the estuary. The flats themselves are dissected by a series of small drainages, which drain the marsh at low tide and become flood tidal channels during rising tide. Mutually evasive currents are developed during flood between these channels and fluvial discharge in Glacier Creek.

Trenches were made across the flat in the summer of 2003 and 2004. In 2004, flags and washers were set across the flat. This recorded the semidiurnal tidal sedimentation at different locations across the flats during part of a neap-spring cycle. Profiles from trenches in 2003 and 2004 were significantly different, which indicates that the 2003 flats were completely eroded prior to 2004 sedimentation. The 2004 flat preserved three and a half months of near-continuous sedimentation, which corresponds closely to the timing of spring thaw in the estuary. The creek and estuary are frozen during winter months. Presumably, the previous year's flat was eroded during a late-season flood in 2003, or from winter freeze and ice rafting. The accommodation space created by removal of the previous year's flat allowed for rapid vertical accretion of tidal sediments, which may also be enhanced at this location by the rotational current pattern created during flood tides. A sheltered position within the reentrant, along the margin of the fluvial channel, protects the flats from erosion by currents in the main tidal channel.