Aquatic insect larval constructions in tropical limestone deposits (tufa) : preservation of depositional environments

Tufas are known worldwide as useful archives of palaeoenvironmental information. Recognition of structures within tufas that preserve depositional environments will ensure maximum information is obtained from fossil tufa sequences. Here we document distinctive characteristics that allow recognition of aquatic insect larval constructions within tufa deposits and the depositional environments under which each develops. In tufas from the Gregory River (NW Qld, Australia), fixed-dwelling tubes built by larval midges (Diptera: Chironomidae) represent tufa deposition under flowing or standing water conditions. In thin sections of chironomid tufa, the tubes appear as rings approximately 600 μm in diameter composed of micrite and organic matter. A combination of chironomid tufa and larval caddis-fly (Trichoptera: Hydropsychidae) fixed retreats and nets is a common tufa dam fabric, and indicates deposition in flowing water. The hydropsychid retreats are preserved as 2 mm diameter rings of calcite arranged in a lacework pattern. Tufas formed under spray hydraulic conditions on pool banks near waterfalls contain marquees built between cyanobacteria colonies by larval moths (Lepidoptera: Pyralidae). In thin section, these marquees appear as a calcite encrusted silken sheet up to 5 mm long.8 page(s