Douglas Lake Member

The type locality of the Douglas Lake Member lies on the north shore of Douglas Lake, 2.4 kilometers (1.5 mi) northeast of Douglas Dam, Jefferson County. It was named by Josiah Bridge for Douglas Lake, Tennessee.[1][2]

The Douglas Lake Member is composed of a diverse set of sedimentary rocks, including rubble conglomerate, chert 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 sinkholes, called paleokarst, developed in upper surface of the Mascot Dolomite. Up to 10 meters (33 ft) of black, fine-grained, thick-bedded dolomite fill two large, prehistoric sinkholes and grade upward almost imperceptibly into Lenoir Limestone.[1] Such paleokarst depression fills, which are part of a regional uncomformity, are common throughout this region. They are generally included in the Lenoir Limestone as the Douglas Lake Member.[5]

At Douglas Dam, the Douglas Lake Member consists of three-part sequence of volcanic ash, 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 18 to 27 meters (59 to 89 ft) in with and is at least 45 meters (148 ft) deep.[2][4] The upper unit consisted of about 11 meters (36 ft) of thin-bedded, slabby reworked volcanic ash and impure dolomite with green shale partings. The upper unit overlied a middle unit, which conisted 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 22 meters (72 ft) of massive, blocky, fine-grained, pyroclastics without laminations.[6] This exposure of the Douglas Lake Member was named the 33 beds in 1944.[7] In 1955, the 33 beds were assigned to the Douglas Lake Member of the Lenoir Limestone.[1][4] The 33 beds were completely excavated for construction of Douglas Dam by the Tennessee Valley Authority in 1942, but yielded articulated arthropod fossils including Chasmataspis and Douglasocaris.[6] Fossil plant compressions from this unit reveal the variety of non-vascular plants which lived on land before Silurian evolution of vascular land plants.[3]

At Douglas Dam, the Douglas Lake Member has long been known for its articulated xiphosuran or eurypterid Chasmataspsis laurenci and phyllocarid Douglasiocaris collinsi.[6] As the Douglas Dam Lagerstatte, these fossils are associated with compression fossils of a variety of non-vascular land plants, including hornwort Casterlorum crispum, liverwort Cestites mirabilis, balloonwort Janegraya sibylla, peat moss Dollyphyton boucotii. and harsh moss Edwardsiphyton ovatum. Also found were remains of the mycorrhizal fungus Palaeoglomus strotheri and the likely lichen Prototaxites honeggeri. The fossils are compressions of original carapaces of the arthropods and the organic carbon of the plant fossils.[3]

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 sequences. 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 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.[8] Therefore, the Douglas Lake Member and the fossils it contains are younger than the underlying Floian strata and older then the late Darriwilian limestones of the Lenoir Limestone.[3]

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.[7][3][9] Although the biota it strkingly terrestrial, Caster and Brooks[6] 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.

• List of fossiliferous stratigraphic units in Tennessee

• Stamm, N., 2020. USGS Geologic Names Committee Archives (maintained ca. 1900-1990), National Geologic Map Database.