The Douglas Lake Member is a geologic unit of member rank of the Lenoir Limestone that overlies the Mascot Dolomite and underlies typical nodular member of the Lenoir Limestone in

Douglas Lake Douglas Lake, also called Douglas Reservoir, is a reservoir created by an impoundment of the French Broad River in Eastern Tennessee. This lake is located only a few miles from the Pigeon Forge/ Gatlinburg area, and also the Great Smoky Mountain ...

, Tennessee, region. It fills depressions that are part of a regional unconformity at the base of Middle Ordovician strata, locally the Lenoir Limestone, that separates them from the underlying Lower Ordovician strata, locally the Knox Group.U.S. Geological Survey, 2020
Geologic Unit: Douglas Lake
Walker, K.R., Steinhauff, D.M., and Roberson, K.E., 1992. ''Uppermost Knox Group, the Knox unconformity, the Middle Ordovician transition from shallow shelf to deeper basin at Dandridge, Tennessee'', In Driese, S.G., and others, eds., ''Paleosols, paleoweathering surfaces, and sequence boundaries'', ''University of Tennessee, Department of Geological Sciences Studies in Geology'', no. 21, p. 13–18.

Nomenclature

The type locality of the Douglas Lake Member lies on the north shore of Douglas Lake, northeast of Douglas Dam, Jefferson County. It was named by Josiah Bridge for Douglas Lake, Tennessee.Bridge, J., 1955. ''Disconformity between Lower and Middle Ordovician Series at Douglas Lake, Tennessee.'' ''Geological Society of America Bulletin'', 66(6), pp.725-730.

Lithology

The Douglas Lake Member is composed of a diverse set of sedimentary rocks, including rubble conglomerate,

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...

conglomerate, and black dolomite. In outcrops along the north shore of Lake Douglas, the chert conglomerate overlies the rubble conglomerate and both of which fill prehistoric

sinkhole A sinkhole is a depression or hole in the ground caused by some form of collapse of the surface layer. The term is sometimes used to refer to doline, enclosed depressions that are also known as shakeholes, and to openings where surface water ...

s, called '' paleokarst'', developed in upper surface of the Mascot Dolomite. Up to of black, fine-grained, thick-bedded dolomite fill two large, prehistoric sinkholes and grade upward almost imperceptibly into Lenoir Limestone. Such paleokarst depression fills, which are part of a regional unconformity, are common throughout this region. They are generally included in the Lenoir Limestone as the Douglas Lake Member.Carpenter, R.H., Fagan, J.M. and Wedow, H., 1971. ''Evidence on the age of barite, zinc, and iron mineralization in the lower Paleozoic rocks of east Tennessee.'' ''Economic Geology'', 66(5), pp.792-798. At

Douglas Dam Douglas Dam is a hydroelectric dam on the French Broad River in Sevier County, Tennessee, in the southeastern United States. The dam is operated by the Tennessee Valley Authority (TVA), which built the dam in record time in the early 1940s to mee ...

, the Douglas Lake Member consists of three-part sequence of

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to r ...

, conglomerate and shale and / or shaly dolomite that fill one of these ancient sinkholes developed in the Mascot Dolomite of the Knox Group. This prehistoric sinkhole varies between in with and is at least deep. The upper unit consisted of about of thin-bedded, slabby reworked volcanic ash and impure dolomite with green shale partings. The upper unit overlaid a middle unit, which consisted of varve-like graded beds with concentrations of organic matter at their top and interlayered with recurrent layers and lenses of conglomerate and breccia. It is this unit from which all of the fossils were recovered. The lowest unit consisted of the lowest member is composed of of massive, blocky, fine-grained, pyroclastics without laminations. This exposure of the Douglas Lake Member was named the 33 beds in 1944.Laurence, R.A., 1944. ''An early Ordovician sinkhole deposit of volcanic ash and fossiliferous sediments in east Tennessee''. ''The Journal of Geology'', 52(4), pp.235-249. In 1955, the 33 beds were assigned to the Douglas Lake Member of the Lenoir Limestone. The 33 beds were completely excavated for construction of Douglas Dam by the

Tennessee Valley Authority The Tennessee Valley Authority (TVA) is a federally owned electric utility corporation in the United States. TVA's service area covers all of Tennessee, portions of Alabama, Mississippi, and Kentucky, and small areas of Georgia, North Carolin ...

in 1942, but yielded articulated arthropod fossils including ''

Chasmataspis ''Chasmataspis'' is a genus of chasmataspidid, a group of extinct aquatic chelicerate arthropods. It was found in the Early Ordovician deposits of Tennessee, United States. Morphology Just like other chasmataspidids, the body of ''Chasmataspis ...

'' and '' Douglasocaris''. This kind of preservation by volcanic ash is paralleled by Silurian

Herefordshire Lagerstätte Coalbrookdale Formation, earlier known as Wenlock Shale or Wenlock Shale Formation and also referred to as Herefordshire Lagerstätte in palaeontology, is a fossil-rich deposit (''Konservat-Lagerstätte'') in Powys and Herefordshire at the Engla ...

Fossils

File:20200606 Chasmataspis laurencii.png, link=, Reconstruction of

chasmataspidid Chasmataspidids, sometime referred to as chasmataspids, are a group of extinct chelicerate arthropods that form the Order (biology), order Chasmataspidida. Chasmataspidids are probably related to horseshoe crabs (Xiphosura) and/or sea scorpions (E ...

'' File:Douglasocaris part.jpg, link=, Fossil of crustacean '' Douglasocaris'' At Douglas Dam, the Douglas Lake Member has long been known for its articulated

Chasmataspidid Chasmataspidids, sometime referred to as chasmataspids, are a group of extinct chelicerate arthropods that form the Order (biology), order Chasmataspidida. Chasmataspidids are probably related to horseshoe crabs (Xiphosura) and/or sea scorpions (E ...

''Chasmataspsis laurenci'' and

phyllocarid Phyllocarida is a subclass of crustaceans, comprising the extant order Leptostraca and the extinct orders Hymenostraca and Archaeostraca. This clade of marine crustaceans diversified extensively during the Ordovician The Ordovician ( ) is a ...

''Douglasocaris collinsi''. In addition, enigmatic fossil ''Cestites mirabilis'' is known, originally interpreted as

ctenophore Ctenophora (; : ctenophore ) is a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and they ar ...

. In 2000,

Gregory Retallack Gregory John Retallack (born 8 November 1951) is an Australian paleontologist, geologist, and author who specializes in the study of fossil soils ( paleopedology). His research has examined the fossil record of soils though major events in Earth ...

, who is famous for paleobotany research and interpretation of

Ediacaran Biota The Ediacaran (; formerly Vendian) biota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period (). These were enigmatic tubular and frond-shaped, mostly sessile, organis ...

as terrestrial environment, considered this site as terrestrial environment with plant fossils. In 2019, he claimed

compression fossils A compression fossil is a fossil preserved in sedimentary rock that has undergone physical compression. While it is uncommon to find animals preserved as good compression fossils, it is very common to find plants preserved this way. The reason fo ...

of a variety of non-vascular land plants, described four taxa of plants, two taxa of fungi and reinterpreted ''Cestites'' as a

liverwort Liverworts are a group of non-vascular land plants forming the division Marchantiophyta (). They may also be referred to as hepatics. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry ...

. He claimed that the fossils are compressions of original carapaces of the arthropods and the organic carbon of the plant fossils. While this is accepted in a few studies and review of his book, in 2022 researchers such as

paleobotanist Paleobotany or palaeobotany, also known as paleophytology, is the branch of botany dealing with the recovery and identification of plant fossils from geological contexts, and their use for the biological reconstruction of past environments (pale ...

Dianne Edwards Professor Dianne Edwards CBE, FRS, FRSE, FLS, FLSW (born 1942) is a palaeobotanist, who studies the colonisation of land by plants, and early land plant interactions. Early life Edwards was born in Swansea, South Wales, and spent much of he ...

who studied about Paleozoic plants referred his study and commented "When diagnostic features are absent, such fragmentary organic materials can be misinterpreted, leading to implausible attributions". This is agreed in later review as well, considered to lack sufficient characters to be unequivocally assigned to land plants.

Age

The paleokarst depressions occupied by the Douglas Lake Member are part of the Knox Unconformity that separates the Lower Ordovician Knox Group from the Middle Ordovician Chickamauga Group. The Lenoir Limestone is the basal unit of the Chickamauga Group in the Douglas Lake region. In this region, the Knox Unconformity is a highly irregular surface, which appears to represent karst terrain that formed during a 12- to 13-million-year-long period of subaerial exposure that forms a hiatus in sedimentary record between the Sauk and

Tippecanoe sequence The Tippecanoe sequence was the cratonic sequence or the marine transgression following the Sauk sequence; it extended from roughly the Middle Ordovician to the Early Devonian. The Tippecanoe is bound by two Unconformities, at the base by the Knox ...

s. This unconformity represents periods of falling relative sea level starting in latter part of the Floian Stage, which halted the accumulation marine sediments of the Knox Group and exposed the region to terrestrial erosion and karstification. It is not until late in the

Darriwilian Stage The Darriwilian is the upper stage of the Middle Ordovician. It is preceded by the Dapingian and succeeded by the Upper Ordovician Sandbian Stage. The lower boundary of the Darriwilian is defined as the first appearance of the graptolite species ' ...

that rises in relative sea level drowned the Douglas Lake region and initiated the accumulation of the marine sediments that now comprise the Lenoir Limestone.Morgan, William A., 2012, ''Sequence stratigraphy of the great American carbonate bank'', In J. R. Derby, R. D. Fritz, S. A. Longacre, W. A. Morgan, and C. A. Sternbach, eds., ''The great American carbonate bank: The geology and economic resources of the Cambrian — Ordovician Sauk megasequence of Laurentia. American Association of Petroleum Geologists Memoir'' 98, pp. 37–79. Therefore, the Douglas Lake Member and the fossils it contains are younger than the underlying Floian strata and older than the late Darriwilian limestones of the Lenoir Limestone.

Depositional environment

At Douglas Dam, the Douglas Lake Member is argued to have accumulated within a within a cenote at a time of lowered sealevel and, based on paleogeographic reconstructions, to have been many kilometers from the sea.Gray, J. and Boucot, A.J., 1993. ''Early Silurian nonmarine animal remains and the nature of the early continental ecosystem''. ''Acta Palaeontologica Polonica'', 38(3-4), pp.303-328. Caster and Brooks differently interprets the environment of deposition at Douglas Dam as being a marine submarine spring fed by an underground channel or system of underground channels from the nearby land. Steinhauff and Walker suggested that a

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...

al marginal marine setting is the most likely.

References

{{Reflist

External links

* Stamm, N., 2020
USGS Geologic Names Committee Archives (maintained ca. 1900-1990)
Ordovician Tennessee Ordovician System of North America Paleozoic southern paleotemperate deposits Darriwilian Shale formations of the United States Lagerstätten Conglomerate formations of the United States Dolomite formations of the United States Geological members