This work has provided new information on depositional environment and climate during deposition of the Asbian and Brigantian succession on the North Wales carbonate platform. Outcrops of the Loggerheads Limestone Formation (LLF) and the Cefn Mawr Limestone Formation (CMLF) were studied, principally at Cefn Mawr quarry (National Grid Reference SJ200635) and Llangollen (National Grid Reference SJ230435). Two 125 metre cores from Cefn Mawr quarry, one in the LLF and other 50/50 in the LLF and CMLF were made available by Hanson Cement, the quarry operator. Chemical analysis of the mudrock content of limestone units, not previously undertaken, has demonstrated a step increase at the LLF-CMLF boundary, interpreted as a switch to a more humid regime. Study of mudrock partings and their relationship with limestone units strongly suggests that the carbonate factory was shut down and restarted without change in water depth and that the partings are 'event deposits', of indeterminate duration, related to periods of wet climate. Petrological study of a bituminous coal bearing succession on an exposure surface in the mid Asbian has revealed alternating coal laminae of different microlithotypes, reflecting peat formation in an environment which switched regularly from marine influenced to meteoric dominated, possibly in response to a monsoonal climate. Palynological study of the same succession has revealed that the spores were characteristic of forest flora, which required permanently wet conditions for their survival. The presence of calcrete on palaeokarst surfaces below coal bearing successions indicates that climate was semi-arid prior to the commencement of peat formation. Coal petrology and palynology of coal and associated mudrocks has provided data on climate which is not available from study of the limestone succession alone.
|Date of Award||1 Aug 2021|
- The University of Manchester
|Supervisor||John Nudds (Supervisor) & Catherine Hollis (Supervisor)|