CHEMICAL MICROSTRATIGRAPHY OF THE TRIASSIC "INSECT LAYER" IN THE DANVILLE BASIN

The 30 mm thick "insect layer," exposed in the Danville basin near Cascade Virginia, is a globally important fossil locality. It is notable for exquisitely detailed preservation of soft-bodied invertebrates including beetles, water bugs, flies, grasshoppers, and spiders. We have conducted a continuous chemo-stratigraphic analysis of a 332 mm thick section that includes the insect layer and the rocks immediately below and above it. We have tentatively identified 3 chemically and lithologically distinct units within the analyzed section. Beneath the insect layer is a relatively massive, carbonaceous, micaceous, carbonate-poor siltstone (Unit 1). The base of Unit 2 is marked by an irregular, but continuous 1-3mm thick bed of fine-grained ferroan dolomite that is immediately overlain by the insect layer. Unit 2 is finely laminated and contains about 30% shale with the remainder clay-rich or carbonaceous dolostone. The top of the unit is marked by an exceptionally carbonate-poor shale. Unit 3, the top 160 mm of the section, consists largely of clay-poor, carbonaceous dolostone and dolomitic siltstone and sandstone. Although laminated in part, this unit also contains graded beds up to 3 cm thick. Chemistry generally reflects lithology, with high Ca and Sr in carbonate rich rocks, high Si and Al in clastic units, and high alkalis in clay-rich and micaceous units. Both S and organic C increase nearly monotonically throughout the section. The insect layer marks the first occurrence of substantial carbonate in the section. It also has the lowest organic C and lowest S of any analyzed sample and has among the highest Fe and Mg - higher than many rocks containing substantially more modal dolomite. Our observations suggest an increasingly deep and anoxic depositional environment. The insect layer may reflect an environment in transition from ephemeral lake or bacterially matted lake margin to standing water marked by periodic carbonate precipitation. High CO₂ in the lake sediment may have rendered it toxic to scavengers and other benthic organisms, either directly or by promoting the growth of toxic algae.