BIOEROSION OF A BEDROCK STREAMBED BY MODERN INSECT LARVAE, CONECUH RIVER, SOUTH ALABAMA

Studies along the Conecuh River, Alabama exemplify the impact that insect larvae may have on bioerosion of freshwater aquatic bedrock substrates. In stretches south of Point A Dam, the river is characterized by riffles reflecting differential erosion of flat-lying siliceous claystone and sandstone of the Eocene Tallahatta Formation. Transects made during periods of low discharge reveal a complex pattern of stream-bed bioerosion, mainly by midge and caddisfly larvae.

Midge larvae are limited to claystone, including claystone clasts. Their U-shaped burrows, occurring in densities up to 30,000/square meter, are vertical to subvertical, 2-7 mm wide, and 1-1.5 cm long. Marginal tubes, 0.5-2.0 mm in diameter, exhibit organic linings and fine scratch traces. As viewed on bedding surfaces, one of the paired apertures is larger and funnel-shaped. Narrow spaces between parallel to subparallel U-tube limbs contain a septum of manipulated sediment or an organic lining. Where larvae could not penetrate sandstones at depth, U-tube axes turn horizontal along bases of claystone beds. Midge burrows are variably allied with the ichnotaxa Arenicolites, Diplocraterion, and Rhizocorallium.

Caddisfly larvae penetrate claystone and sandstone. Their biogenic structures, which locally occur in high densities particularly where flow is rapid and turbulent, are manifest by single or paired, 3-to-5-mm diameter agglutinated chimneys that extend 1-2 cm above the substrate and, in some cases, downward into upper parts of associated burrows. Most burrows are inverted Y-shaped structures formed by 4-6-mm wide tubes of disparate length. Long branches (up to 4 cm) are subvertical and variably curved. Shorter (1-2 cm) side branches are gently curved and oblique. Walls of branches exhibit scratch traces. Lithic partitions between branches commonly are extended upward by thin silk nets. Relations of these caddisfly structures to known ichnotaxa are unclear.

Further studies are required to assess (1) affinities of bedrock bioeroding insect groups in freshwater aquatic settings; (2) range of host lithologies; (3) relations among trace makers, environmental regimes, substrates, and trace morphology; and (4) impacts that resulting biogenic structures have on erosion by physical processes.