Paper No. 6
Presentation Time: 2:35 PM
ICHNOFABRICS GENERATED BY INSECTS IN HOLOCENE CARBONATE EOLIANITES OF THE BAHAMAS: CHARACTERISTICS AND IMPLICATIONS
Trace fossils can be common and diverse in late Holocene carbonate eolianites of the Bahamas. Two large, complex trace fossils, informally referred to as cluster burrows and stellate burrows and interpreted as originating from burrowing activites of insects, can impart distinctive ichnofabrics to these rocks. Stellate burrows are abundant in windward sea-cliff exposures of eolianite on Moriah Harbour Cay, Exumas, facing Exuma Sound. These cliffs consist entirely of limestones (age < 1,800 ybp) assigned to the Hanna Bay Member, Rice Bay Formation and comprising three lithofacies representing foreshore, backshore, and dune paleoenvironments. Upper foreshore and backshore beds contain well-preserved specimens of Psilonichnus upsilon, burrows of the ghost crab Ocypode quadrata. These burrows typically are not sufficiently closely spaced to generate an ichnofabric, but their presence identifies landward extent of the backshore to dune paleoenvironment. The overlying eolianite succession exhibits lee and stoss surfaces and other physical sedimentary structures characteristic of coastal dunes. The most striking characteristic of these eolianites is the abundant presence of large and complex stellate burrows, characterized by a central shaft of 3 to 5 cm diameter with numerous oblique, smaller branching-upward shafts (~1 cm diameter) with smooth, unlined walls commonly forming gentle arcs toward their distal ends. Parallel central shafts commonly join at the base, forming distinctive U-shaped structures that reach total heights of >1 m. Other examples exhibit irregular arrangement of shafts that merge to form exceptionally large and architecturally complex structures. The result is a distinctive ichnofabric that reaches a 3- to 3.5-level on the Droser-Bottjer Ichnofabric Index scale. The presence of stellate burrows can result in a significant increase in the porosity and permeability of host eolianite deposits. Previously, stellate burrows have been assigned to the ichnogenus Cellicalichnus, with origin attributed to burrowing and brooding (calichnia structure) of halictid (sweat) bees. Sweat bees are common today in the Bahamas, and the stellate trace fossils are similar to previously described forms of Cellicalichnus. Nonetheless, important differences remain to be resolved.