The palynology and stratigraphy of the Cambrian Nolichucky Shale and associated formations at Thorn Hill, Tennessee, USA

A detailed palynological and stratigraphic study was undertaken on the Cambrian Nolichucky Shale, Maynardville Limestone and Copper Ridge Dolomite (Conasauga Group), as exposed at Thorn Hill, northeastern Tennessee, USA. The majority of palynological samples show low abundance and diversity. Fifty two acritarch taxa (fifteen assigned to species; twenty six left in open nomenclature; eleven new forms), two problematica and four invertebrate components are described. Based upon trilobite biostratigraphy and stable carbon isotope chemostratigraphy, the stratigraphic position of the Cambrian Series 3/Furongian (Marjuman/Steptoean; Aphelaspis Zone/Crepicephalus Zone) boundary is broadly indentified within the uppermost section of the Nolichucky Shale. The first record of SPICE identified using organic residues is also reported, the onset of which is coincident with the Aphelaspis Zone/Crepicephalus Zone boundary). Recovered acritarchs have little immediate, comparative, stratigraphic value but some forms (Acritarch sp. 1 and Acritarch sp. 2) may have future potential use as biostratigraphic markers for shallow marine upper Cambrian Series 3 units. Palynofacies data and sedimentology indicate that the acritarch abundance peak in the Nolichucky Shale may represent maximum water depth within a transgressive/regressive cycle. Based on TEM analysis of vesicle wall ultrastructure, Peteinosphaeridium? sp. 1, Peteinosphaeridium? sp. 2, and Acritarch sp. 2 are interpreted as belonging to the Class Chlorophyceae. The vesicle wall ultrastructure of Acritarch sp. 1 is trilaminar, consisting of a central alveolar layer bounded by two, thin, electron dense layers and is comparable to that of diapause egg cases of extant branchiopods. Based upon wall ultrastructure, vesicle size and surface ornament, recalcitrant wall biochemistry and evidence of co-occurrence of branchiopod-type crustaceans, Acritarch sp. 1 is interpreted as being the egg case of a Cambrian branchiopod-type crustacean, and as such represents the earliest evidence for diapause-type dormancy in the fossil record.