Regional setting

The Cow Branch Formation is exposed in the Dan River-Danville Basin,^[2][4] a narrow half-graben which extends across the border of Virginia and North Carolina in the eastern United States. The basin has also been termed the Danville Basin^[12] (emphasizing the northern portion in Virginia) or the Dan River Basin^{[13][1][9][5]} (emphasizing the southern portion, in North Carolina). It is one of many Triassic-Jurassic rift basins stretching from northeast to southwest in eastern North America, collectively described as the Newark Supergroup.^[5]

The Cow Branch Formation was initially distinguished by Meyertons (1963), working in the Virginian portion of the basin. He considered it to be a member of the Leaksville Formation, a name which encompassed almost all Triassic sediment in the basin.^[12] Thayer (1970), working in North Carolina, split up the Leaksville Formation and raised its members to formation status within the Dan River Group.^[13] The Cow Branch Formation was divided into upper and lower members, though the lower member has subsequently been renamed to the Walnut Cove Formation.^[5] Recent revisions place the Cow Branch Formation above the Dry Fork Formation and below the Stoneville Formation.^[5]

The type section of the Cow Branch Formation was a former roadcut along Virginia Route 856, in Pittsylvania County southeast of Cascade.^[13] A new lectostratotype was proposed in 2015: a large stone quarry extending across the state line by the Dan River near Eden, North Carolina.^[5] This quarry, commonly known as the Solite Quarry, is technically a cluster of three quarry pits, one in Pittsylvania County, Virginia and two in Rockingham County, North Carolina. The site is home to the most extensive and fossiliferous exposures of the formation. Exceptionally-preserved fossils were first reported from the site in 1978, and collection has continued to the present.^[1][9][4]

Sedimentology and paleoenvironment

The sediments of the Cow Branch Formation are dark grey to black in color and generally fine-grained. Blocky mudstones and thinly-laminated shale are the most common lithologies. The formation is thickest and most fine-grained at the state line, approximately in the middle of its exposed area. Here, the formation is about 1,900 metres (6,200 ft) thick. Coarser sediments such as dark grey sandstone are more prevalent to the southeast and northwest, though periodic black mudstone beds are still frequently encountered. Color is the most useful metric for distinguishing the Cow Branch Formation in the field, as red and purple sediments are practically absent, unlike the Dry Fork and Stoneville formations.^[5]

The Cow Branch Formation represents a lacustrine (lake) system in a warm tropical climate, only around 2°^[14] to 4°^[15] north of the equator. Deposition preceded at an estimated rate of around 46.3 cm/kyr.^[15] Bioturbation is almost completely absent, indicating that the lake bed was uninhabited by burrowing animals. Insect-bearing fossil layers were likely completely freshwater while the fish-bearing layers may have been somewhat saltier. As in modern rift lakes, high water levels could have initiated brine seeps along the edge of the basin, adding sodium into the lake system. Quartz is conspicuously absent even from the siliciclastic layers, having been replaced with albite (high-sodium feldspar) through diagenetic processes.^[3][4]

The high frequency of dolomite in the formation indicates that the lake was strongly alkaline, with its water saturated with magnesium supplied from older carbonate rocks in the area.^[3][4] The lack of bioturbation, mudcracks, or root casts has traditionally been taken as evidence that the waters were deep enough to be continually stratified, with the hypolimnion (deepest portion) completely lacking oxygen.^[1][2][8] An alternative hypothesis suggests that the lake was rather shallow, albeit still deep enough to have been permanent during the formation's deposition. This is supported by the abundance of dolomite, a mineral which forms most easily in salty shallow-water environments. In addition, the insect-bearing layers nearly lack organic carbon, suggesting that the lakebed was fully oxygenated even at its deepest extent.^[3][4] The lake sediments have a high concentration of fluorine, a fact which may help to resolve the near-absence of bioturbators. As with excessive salinity, excessive fluorine can be toxic for fully aquatic organisms (including bioturbators and freshwater plants), but air-breathing insects can persevere and thrive close to the shoreline.^[3][4]

Reptiles

Fish

Undescribed pholidophorids and holosteans were present.^[7][2] A relatively large freshwater shark tooth is also known from the formation.^[8]

Arthropods

"Conchostracans" (clam shrimp) from the formation have typically been assigned to the genera Cyzicus and Palaeolimnadia,^[1][2] though they may instead be species of Eustheria.^[5] Apart from arthropods, other invertebrate fossils from the Cow Branch Formation include (uncommon) Scoyenia burrows and indeterminate unionid bivalves.^[7]

The most abundant insects are aquatic hemipterans (78% of specimens identifiable to order), with beetles in a distant second place (10%).^[23] Most hemipteran and beetle fossils have yet to be assigned to the species level. In terms of named diversity, dipterans (flies) make up the bulk of the assemblage, despite representing only 1.5% of insect fossils from the Solite Quarry.^[4] Within the shale layers bearing insect fossils, aquatic insects are most abundant in the early part of a layer, while terrestrial insects maintain a low but steady presence through the entire layer.^[3]

97 different morphotypes (forms) of beetles occur in the Solite Quarry. In terms of size distribution, the beetle fauna is similar to modern ecosystems, with just over half of all morphotypes between 2 and 4 mm in length. Most morphotypes are rare, with 84% known from a single specimen. The two major exceptions are Leehermania and Holcoptera, both of which are known from numerous specimens (about 77% of all beetle fossils from the site). Few morphotypes are shared between Solite and Triassic-Jurassic insect faunas. Archostematan beetles are rare at Solite despite making up the vast majority of Triassic-Jurassic beetle diversity, as indicated by other sites.^[23]

Plants

Plant fossils are abundant. The most common examples are conifer foliage, followed by bennettitales and ginkgophytes. Ferns and sphenophytes are also present, though less common. Two endemic forms, the seed taxon Edenia villisperma and the leaf taxon Pannaulika triassica, have been compared to angiosperms (flowering plants) in their structure, though they likely are unrelated to true angiosperms.^[4]