Use of fine-scale stratigraphy and chemostratigraphy to evaluate conditions of deposition and preservation of a Triassic lagerstà¤tte, south-central Virginia

Fraser, Nicholas C; Liutkus, C M; Beard, J S; Ragland, P C

Use of fine-scale stratigraphy and chemostratigraphy to evaluate conditions of deposition and preservation of a Triassic lagerstà¤tte, south-central Virginia

Resource type

Journal article

Creator (person)

Fraser, Nicholas C

ORCID

Liutkus, C M

Beard, J S

Ragland, P C

Date published

2010

Abstract

The rich, fossiliferous Triassic sediments exposed in the Virginia Solite Quarry include a 34-mm-thick “insect layer” that is notable for detailed preservation of soft-bodied invertebrate and vertebrate remains. We describe this unique Konservat-Lagerstà¤tte and use sedimentologic and geochemical analyses to interpret the environmental conditions necessary to preserve such delicate fossils. This work is among the first attempts to apply detailed geochemical/stratigraphic analysis to the study of Lagerstà¤tten and we report on a 332-mm-thick section that includes the insect layer and the rocks immediately below and above it. Our analysis successfully constrains various aspects of the depositional and diagenetic history of the Lagerstà¤tte and permits a detailed analysis of changing conditions prior to, during, and after deposition. Geochemical and sedimentologic analyses of the insect layer and surrounding lithologies reveal a change from siliciclastic-dominated layers (Unit 1) to dolomite-siliciclastic laminites above (Unit 2 and the insect layer), separated by a boundary dolostone layer that is traceable for over 200 m. We interpret this sedimentary shift as the initial stages in the transgression of a shallow, saline, alkaline rift-basin lake over lake margin deposits. The absence of bioturbation by plants and benthic organisms, as well as a lack of predation on the insects, is not explained by significant water depth, but is instead more reasonably considered a result of the chemistry of the water at the lake margin, affected by groundwater seeps, which provided F-, Mg-, and Ca-rich fluids. Although the initial conditions of preservation are remarkable, it is equally impressive that the fossils survived extensive diagenesis, e.g. dissolution of quartz and coarsening of dolomite.