BIOEROSION OF A BEDROCK STREAM BED BY MODERN CHIRONOMID AND TRICHOPTERAN LARVAE (CONECUH RIVER, ALABAMA): ICHNOTAXONOMIC, ICHNOFACIES, AND GEOMORPHOLOGIC IMPLICATIONS (Invited Presentation)

Eocene siliceous claystone and sandstone locally forming the bed of the Conecuh River in south Alabama are densely riddled by burrows produced by modern chironomid (midge) and trichopteran (caddisfly) larvae. Chironomid larvae produce simple, diminutive U-shaped burrows oriented normal to claystone substrate surfaces. In contrast, in both claystone and sandstone, trichopteran larvae produce larger, vertical, irregularly Y-shaped structures consisting of an upper U-form burrow extended upward by agglutinated chimneys and one or two subvertical shafts that extend downward from the U-form base. Both structure types exhibit striated burrow margins and linings that in the U-forms include a thickened interlimb septum constructed of silk and manipulated sediment. Spatial distribution of burrow types is controlled by differences in substrate texture and induration, proclivities of tracemakers to establish themselves in transiently available substrates, and other poorly understood factors. Chironomid and trichopteran burrows generally resemble the ichnogenera Arenicolites and Polykladichnus, respectively. However, establishing ichnotaxonomic equivalence is problematic given uncertainties regarding potential modes of preservation of the modern burrows and presumed differences in modes of production of U-shaped burrow components in soft vs. firm substrates. The latter differences have broader implications for inferred tracemaker behaviors and ichnotaxonomy of other U-form structures (e.g., Rhizocorallium) emplaced in firmgrounds. Ichnofabrics produced by modern insect larvae in these fluvial firmgrounds are similar to those of modern marginal marine firmgrounds and closely match those considered diagnostic of the Glossifungites ichnofacies in marine strata. Hence, the definition of the Glossifungites ichnofacies should be expanded to include comparable ichnofossil assemblages preserved in continental deposits. Prolific burrowing by insect larvae (and other invertebrates) likely has a significant impact on rates of erosion of bedrock stream channels. Understanding such impacts will require further biogeomorphologic studies that explore biological affinities and environmental requirements of bedrock bioeroders in freshwater aquatic settings.