A Study of a Namurian Crustacean-Bearing Shale From the Western Midland Valley of Scotland

Geology, Paleontology

Clark, Neil Donald Lewis

University:University of Glasgow

关键词: Geology, Paleontology;   

Others  :  http://theses.gla.ac.uk/77926/1/10999244.pdf

来源: University of Glasgow

【 摘 要 】

Crustaceans from a finely laminated shale above the Top Hosie Limestone (Limestone Coal Group) were discovered in 1981 by Mr S. P. Wood at Bearsden, near Glasgow. Spores common to the Bellispores nitidus-Reticulatisporites carnosus (NC) Zone, conodonts from the Kladognathus-Gnathodus girtyi simplex Zone, and goniatites of Crave noceras leion El Zone, suggest a Pendleian (Namurian) age for the shales. These shales form part of a newly defined formation, the Manse Burn Formation, for which the type locality is the Manse Burn, near Bearsden (NS529427329-NS53057325). The Manse Burn Formation has been further subdivided into six members on the basis of the fossil content and sedimentological characteristics of the shales, the Shrimp Member, the Posidonia Member, the Nodular Shale Member, the Platey Shale Member, the Betwixt Member, and the Lingula Member. The Shrimp Member of the Manse Burn Formation has been recognised at several other localities in the western Midland Valley of Scotland, the Hindog Glen (NS27905115), the Swinlees Glen (NS29415342), the Powgree Burn (NS33635219), Lochermill (NS24106472), the Red Cleugh Burn (NS65567846), the Burniebrae Burn (NS66037818), the Corrie Burn (NS68707876), and at East Kilbride (BGS bore hole data, filled quarries locations unknown). The Shrimp Member, which contains the greatest abundance of crustaceans, is restricted to a basin bound to the north by the Paisley Ruck and the Campsie Fault, to the south by the Dusk Water Fault, to the west by Arran, and to the east by the Kincardine Basin and the central Glasgow Basin. The areas to the north, west, and south are considered to be areas of positive relief during the deposition of the Manse Burn Formation. To the east the basin was open to either marine or fresh water influences dependant on seasonal increases in the freshwater input. The shales of the Manse Burn Formation were deposited in conditions which varied in both oxygenation and salinity conditions. The Shrimp Member was deposited in an environment which was seasonally influenced by marine or non-marine water sequentially, the Posidonia Member and the Platey Shale Member were deposited in more marine episodes with variable oxygen levels, the Nodular Shale Member and the Betwixt Member represent shales with the least marine character, and the Lingula Member has characteristics of both marine and non-marine influences and variable oxygenation levels. Towards the top of the Manse Burn Formation, the deposit becomes sandier with the onset of more terrestrial conditions. The Shales of the Manse Burn Formation appear to have been deposited in a low-energy back barrier lagoon with restricted marine access from the east. Many of the crustaceans have been preserved in francolite, which has prevented the normally rapid bacterial decay of the crustaceans, preserving the helicoidal ultrastructural detail of the cuticle, and possibly a representation of the original chemistry as well. The sodium concentration of the cuticle may provide a key to the life environment and salinity tolerances of the crustaceans. The crustaceans are also preserved as drusy calcite outlines, and calcareous microconcretions. Other modes of preservation include the rosette calcite structure of the myodocopids, pyrite replacement of the bivalves and some crustaceans, and the calcite recrystallization of bivalve shells. The crustaceans contained within these shales include, Tyrannophontes pattoni, Tealliocaris robusta, Crangopsis eskdalensis, Palaemysis dunlopi, Minicaris brandi, and Cyclus rankini. The first four of these, and Cyclus rankini, are members of the same palaeocommunity, which is here defined as the Western Midland Valley of Scotland Crustacean Palaeocommunity. Minicaris brandi is not considered to be part of the same palaeocommunity, as it is not preserved in the same manner as the other crustaceans, and is thought to have entered the back barrier lagoon from streams or rivers. Tyrannophontes is considered to be a stenohaline stomatopod adapted to normal marine conditions. Tealliocaris is a euryhaline decapod which appears to be best adapted to waters of higher salinity. Crangopsis and Palaemysis both appear to be euryhaline and adapted to cope with lower salinities as well as normal marine salinities. The distribution of the crustaceans in the back barrier lagoon suggests that the salinity decreases towards the west. Pseudotealliocaris etheridgei is synonymised with Tealliocaris etheridgn and Tealliocaris robusta, and doubt is cast on the validity of the genus Pseudotealliocaris. Waterstonella grantonensis is considered to be synonymous with Crangopsis eskdalensis, and specimens of the former differ only in their state of preservation.(Abstract shortened by ProQuest.).