BEDROCK BIOEROSION BY INSECT LARVAE IN FRESHWATER AQUATIC SETTINGS: A NEGLECTED ASPECT OF BIOGEOMORPHOLOGY?

Observations along stream banks in the Alabama coastal plain indicate that burrowing (or boring?) by modern aquatic insect larvae in relatively poorly indurated bedrock may be locally extensive. As an example, flat-lying strata in the Eocene Tallahatta Formation along the Conecuh River in south Alabama are in places riddled with abandoned U-shaped burrows variably allied with the ichnotaxa Arenicolites, Diplocraterion and Rhizocorallium. These biogenic structures are provisionally attributed to mayfly nymphs that colonized bedrock surfaces during recent episodes of full river submergence. Burrow densities on some surfaces approach 30,000 U-tubes per square meter. Burrows are relatively uniform in dimension; burrow diameters and U-tube widths vary proportionally, ranging from 1 to 2 mm and 3 to 7 mm, respectively. Paired burrow apertures are crudely aligned with general stream-flow direction. Burrow orientations and depths vary as a function of lithologic heterogeneity in host strata, which comprise siliceous claystone and subordinate, thin (1 mm to 1 cm), silica-cemented, fine-grained quartz sandstone layers. Where emplaced in uniform claystone intervals, burrows are vertical and penetrate to depths of 1 to 1.5 cm. In contrast, where trace makers encountered and could not penetrate thin sandstones, U-tube axes are horizontal and extend for up to 2 cm along the bases of overlying claystone beds. Similar U- or pouch-shaped burrows have been noted by us and other authors in a variety of bedrock substrates, including weathered crystalline rocks, within other modern fluvial and lacustrine systems. Such freshwater aquatic bioerosion has received limited attention in biogeomorphologic studies yet it has implications for structural engineering and for paleoichnology, particularly studies of ancient continental firmgrounds. Further studies are required to assess (1) the affinities and spatial/temporal distributions of insect groups responsible for bedrock bioerosion; (2) the range of lithologies (and degrees of weathering thereof) that host these bioeroders; (3) relations among specific trace makers, environmental regimes, substrates, and trace morphology; and (4) the impacts that resulting biogenic structures have on erosion by physical processes.