--> Sedimentological and Ichnological Characteristics of the Inner Estuary, Fluvio-Tidal Transition and Riverine Reach of the Ogeechee River Estuary, GA, USA

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Sedimentological and Ichnological Characteristics of the Inner Estuary, Fluvio-Tidal Transition and Riverine Reach of the Ogeechee River Estuary, GA, USA

Abstract

Recognition of differences in the sedimentological character of sediments deposited within the tidally influenced inner estuary and fluvio-tidal transition zone can significantly aid in subsurface facies identification. The Ogeechee River Estuary serves as an excellent modern analogue of a sand-dominated micro- to mesotidal estuary. Sedimentological and ichnological characteristics and facies distribution of sand- and granule-rich deposits are presented. This research project is focused on the inner estuary, fluvio-tidal transition and fluvial part of the Ogeechee River Estuary, Georgia, USA. The inner part of the estuary is characterized by oligohaline salinities (3-4 ppt), large meander belts with point bars covered by sinuous dunes and superimposed linguoid current ripples. Medium-to fine-grained sand and finely dispersed organic debris accumulate on bar surfaces. Vague surficial tracks and trails, produced by amphipods and insects, are present. Large- and small-scale trough cross-stratification with preserved tidal reversals, flaser bedding, mud rip-up clasts, and “cryptically” bioturbated sediment fabric likely passes into the rock record. The apparent “cryptic bioturbation” seems to reflect the activity of meiofauna or alternatively may have been caused by destruction of primary sedimentary structures by air bubbles. The fluvio-tidal transition is typified by freshwater to oligohaline salinities (0-3 ppt) and fluvially dominated point bars with tidal influence. Point bars are formed of coarse- to very coarse-grained sand and abundant organic debris. Reworked dunes with reactivation surfaces and superimposed shallow rhomboidal ripples will be preserved in the rock record as ebb-dominated, large- and small-scale trough cross-stratification, and planar horizontal lamination. Fine-grained material and mud clasts are sourced locally from the surrounding marshes and swampy forests. Tracks of birds, trails of snails and dragonfly larvae, and burrows by blue crabs are observed on the point bars. Fiddler crab burrows and tunnels occur within firm substrates. The fluvial reach of the Ogeechee River Estuary exhibits extensive point bars occupying up to 80% of the river floor, consisting of coarse- to medium-grained sand with granules. It is characterized by freshwater salinities (0-1 ppt). The underwater point bars are covered by ebb-oriented sinuous dunes and superimposed current ripples. Fiddler crab burrows occur within the muddy firm ground.