Recognizing Estuarine and Tidal Creek Sandbars by Biogenic Sedimentary Structures: ABSTRACT

Abstract

Estuarine and tidal creek sandbars bear many similarities to fluvial channel sandbars in their physical sedimentary structures, but are considerably different with respect to biogenic sedimentary structure. Consideration of these biogenic structures with current-produced physical sedimentary structures and with facies geometry produces unique, readily recognizable, paleoenvironmental indicators. The abundant burrowing fauna collectively found in estuarine and tidal-creek bars represents species which are individually characteristic or very common in other intertidal environments, such as beach (burrowing shrimp, Callianassa major), tidal flat (burrowing shrimp, Callianassa atlantica, and polychaete worms Onuphis and Diapatria), marsh (fiddler crab, Uca), estuarine channels (burrowing shrimp, Upogebia), and sand flats (acorn worm, Balanaglossis). Other biogenic structures found in tidal creek bars include tracks, trails, and markings produced by more typical subtidal organisms such as the sand-collar snail, Polinicies; hermit crab, Clibanarius; blue crab, Callinectus; mantis shrimp, Squilla; and feeding depressions made by rays. Two subenvironments are found on most of the tidal creek sandbars studied and each contains a characteristic suite of biogenic and physical sedimentary structures. The channel side of the bar consists predominantly of sand and preserves a record of megaripples and small current ripples. Associated with these structures are burrows of Onuphis, Callianassa major, Callianassa atlantica, and Uca. Energy is less and muddy sand accumulates on the side of the bar away from the channel current. Ripple laminae generally are not developed here and bioturbated sediments comprise the principal structures. A great density of burrows is found in this subenvironment, and Upogebia, Diapatria, and the razor clam, Tagelus, are typical burrowing forms. End_of_Article - Last_Page 731------------