Bioerosion of a modern bedrock stream bed by insect larvae (Conecuh River, Alabama): Implications for ichnotaxonomy, continental ichnofacies, and biogeomorphology

摘要

Observations along the Conecuh River, southern Alabama, demonstrate the impact that insect larvae can have on bioerosion of freshwater aquatic bedrock substrates. Eocene siliceous claystones and sandstones exposed along the river are locally densely riddled by modern burrows produced by chironomid (midge) and trichopteran (caddisfly) larvae. Chironomid larvae produce diminutive U-shaped burrows (U width = 2-7 mm, U length = 1-4 cm, burrow diameter = 0.5-2.0 mm) generally oriented normal to bedrock surfaces. Trichopteran larvae generally produce somewhat larger (burrow depth = 2-4 cm, burrow diameter = 4-6 mm), vertical, crudely Y-shaped burrows consisting of an upper U-tube, which is extended above the substrate by agglutinated chimneys, and one or two basal subvertical shafts. Both burrow types exhibit faint scratch traces on exposed burrow margins and silk-and-sediment linings that include a thicker constructed wall or septum in interlimb areas of their U-tubes. Distribution of the two structure types is controlled by bedrock texture and induration, tracemaker propensity to occupy transiently available substrates, and other as yet unidentified factors.