Use of Fine-Scale Stratigraphy and Chemostratigraphy to Evaluate
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J Paleolimnol (2010) 44:645–666 DOI 10.1007/s10933-010-9445-1 ORIGINALPAPER Use of fine-scale stratigraphy and chemostratigraphy to evaluate conditions of deposition and preservation of a Triassic Lagersta¨tte, south-central Virginia C. M. Liutkus • J. S. Beard • N. C. Fraser • P. C. Ragland Received: 13 March 2009 / Accepted: 19 May 2010 / Published online: 5 June 2010 Ó Springer Science+Business Media B.V. 2010 Abstract The rich, fossiliferous Triassic sediments surrounding lithologies reveal a change from silici- exposed in the Virginia Solite Quarry include a 34- clastic-dominated layers (Unit 1) to dolomite-silici- mm-thick ‘‘insect layer’’ that is notable for detailed clastic laminites above (Unit 2 and the insect layer), preservation of soft-bodied invertebrate and verte- separated by a boundary dolostone layer that is brate remains. We describe this unique Konservat- traceable for over 200 m. We interpret this sedimen- Lagersta¨tte and use sedimentologic and geochemical tary shift as the initial stages in the transgression of a analyses to interpret the environmental conditions shallow, saline, alkaline rift-basin lake over lake necessary to preserve such delicate fossils. This work margin deposits. The absence of bioturbation by is among the first attempts to apply detailed plants and benthic organisms, as well as a lack of geochemical/stratigraphic analysis to the study of predation on the insects, is not explained by signif- Lagersta¨tten and we report on a 332-mm-thick icant water depth, but is instead more reasonably section that includes the insect layer and the rocks considered a result of the chemistry of the water at immediately below and above it. Our analysis the lake margin, affected by groundwater seeps, successfully constrains various aspects of the depo- which provided F-, Mg-, and Ca-rich fluids. Although sitional and diagenetic history of the Lagersta¨tte and the initial conditions of preservation are remarkable, permits a detailed analysis of changing conditions it is equally impressive that the fossils survived prior to, during, and after deposition. Geochemical extensive diagenesis, e.g. dissolution of quartz and and sedimentologic analyses of the insect layer and coarsening of dolomite. Keywords Rift basin Lacustrine Á Á Newark Supergroup Insects P. C. Ragland—deceased. Á Á Danville-Dan River Basin C. M. Liutkus (&) Department of Geology, Appalachian State University, Boone, NC 28607, USA Introduction e-mail: [email protected] J. S. Beard N. C. Fraser P. C. Ragland Konservat-Lagersta¨tten—strata that preserve soft Virginia MuseumÁ of NaturalÁ History, Martinsville, parts and soft-bodied forms with exceptional fossil VA 24112, USA preservation—provide a unique picture of the entire N. C. Fraser spectrum of life in a fossil ecosystem. They are National Museums Scotland, Edinburgh EH1 1JF, UK among the most important paleontologic sites on 123 646 J Paleolimnol (2010) 44:645–666 Earth, and include the Burgess Shale, Solnhofen dipping border fault system, the Chatham fault zone. Limestone, Green River Formation, Barstow Forma- The basin is *170 km long and its width ranges from 3 tion, and others. Understanding the conditions under to 15 km (Fig. 1a). The 4,000 m of tilted basin fill (Dan which these and other fossil deposits form is crucial River Group) immediately overlie (unconformably) in gaining a detailed re-creation of past ecologic Proterozoic and Paleozoic igneous rocks, gneisses, and systems (Park and Downing 2001; Gierlowski- amphibolites, e.g. the Smith River Allochthon (Horton Kordesch and Park 2004; Fu¨rsich et al. 2007). The and McConnell 1991). Additionally, Paleozoic sedi- research herein aims to describe and understand the mentary rocks (including barite-bearing crystalline depositional and diagenetic environment of a Triassic limestone) are found to the north/northwest of the site Lagersta¨tte within the Danville-Dan River Basin of (Watson and Grasty 1915). The Virginia Solite Quarry south-central Virginia through detailed stratigraphic, is situated in the middle of the basin where it straddles paleontologic, and chemical analyses. By delineating the North Carolina-Virginia state line at Cascade, VA the conditions necessary for exquisite preservation of (Fig. 1a). The Solite sedimentary rocks are considered insect fossils at this site, we illustrate a depositional to be among the richest and most productive Triassic model that can be applied to other rift basin sites that terrestrial sequences in the world (Fraser et al. 1996). will aid in the location and interpretation of new non- The diversity of insects is particularly significant, but marine Lagersta¨tten. The conclusions presented here plants and vertebrates are also well represented. are also important for the ongoing study of these The Solite Quarry comprises three separate quar- Triassic rift-basin depositional environments because ries, designated A, B, and C (Fraser and Grimaldi the data appear to indicate a strikingly different 2003; Fig. 1b). Together, they expose several hun- depositional environment (shallow littoral) than has dred meters of the largely lacustrine upper member of been suggested previously for dark fine-grained, the Cow Branch Formation (Meyertons 1963; Olsen laminated lacustrine sediments. et al. 1991; Kent and Olsen 1997). Although previ- ously considered Carnian (Olsen et al. 1989), fol- Geologic and geographic setting lowing the work of Muttoni et al. (2004), a Norian age is now preferred. The Cow Branch Formation The Dan River-Danville basin is part of the Mesozoic represents the lacustrine, middle member of the Dan rift system in eastern North America that formed River Group, which begins and ends with fluvially- during the breakup of the supercontinent Pangaea derived siliciclastics of the Pine Hall and Stoneville during the Triassic (Olsen et al. 1989; Schlische 2003). Formations, respectively (Olsen et al. 1991). Within The basin is an elongate half-graben with a southeast- the Cow Branch Formation, bedded lacustrine shales, a b To Cascade Cascade N Creek A SOLITE QUARRIES VA NC B C Clay pit To Eden 0.5km Fig. 1 a Location of the Solite quarry within the Danville-Dan River Basin. b Map showing the location of Quarries A, B, and C within the Solite Quarry’s boundary. Modified from Fraser and Grimaldi (2003) 123 J Paleolimnol (2010) 44:645–666 647 mudstones, and sandstones preserve Van Houten including coleopterans (beetles), thysanopterans cyclicity, thought to reflect changes in lake level and (thrips), blattoids (cockroaches), and many dipterans precipitation due to Milankovitch orbital changes that (true flies) (Fraser et al. 1996; Blagoderov et al. 2007; affect solar radiation. Fig. 3). The insect bed is also characterized by dense In various Newark Basin exposures, Olsen and accumulations of conchostracan valves that sometimes Johansson (1994) suggest that these cycles reflect obscure details of the insects. A variety of macrophyte conditions ranging from complete desiccation to a plant fossils, including fern, cycad, and conifer leaf lake 200 m or more in depth. Olsen (1986) divided fragments, are preserved throughout the insect bed, but each Van Houten cycle into three divisions based there is no evidence of bioturbation such as root traces, upon lake depth: Division 1, red to purple root-marked burrows, etc. A number of specimens of the small sandstones produced by lake transgression; Division aquatic tetrapod Tanytrachelos ahynis were also 2, black, laminated mudrocks with fish remains recovered from the insect layer. These are invariably representing a lake highstand interval; and Division fully articulated and preserve elements of soft tissue 3, gray to red bioturbated siliciclastics developed including skin and myotome muscle blocks (Fraser during lake regression and a subsequent low stand. At et al. 1996). Fish are exceptionally rare and only a the Solite Quarry, seventeen repetitive sedimentary single specimen was taken from the insect layer during sequences, Van Houten cycles, are exposed in Quarry the intensive collecting of the past 12 years. B and each is *10 m thick (Olsen 1986). Olsen and Johansson (1994) noted that there is an unusually fossiliferous cycle near the top of Quarry Methods B, designated cycle 2 in Fig. 2 by Olsen (1979), where soft-bodied insects are preserved in the basal A newly excavated, unweathered, and continuous portion of a Division 2 sequence. They interpret these 332-mm-thick section was sampled in 1998 from the particular layers as reflecting deposits of a deep, north side of Quarry B (Fig. 2). The base of the stratified lake, possibly the deepest lake sediments in section lies within a carbonaceous siltstone immedi- the cycle. The lack of bioturbation and exceptional ately subjacent to the insect layer (Fig. 2). The top of preservation of vertebrate and invertebrate fossils the measured section is the base of a zone of intense supposedly reflect an anoxic bottom environment local syn- to post-depositional deformation that (Olsen and Johansson 1994). This fossiliferous sec- appears to be confined to a single bed or series of tion was termed the ‘‘insect layer’’ by Fraser et al. beds \10 cm thick. Nevertheless, it is obvious that (1996). Recent work (Fraser and Grimaldi 2003; the continuity of the section above this layer, Grimaldi et al. 2004, 2005) confirmed that the particularly at the scale of our study, is compromised preservation of insects and vertebrate soft parts is by the deformation. Samples for this study were largely confined to the laminated