Anatomy of a Carboniferous transgression: upper Tullig Cyclothem, County Clare, Ireland

Transgressive;Tullig;Cyclothem;Ireland;County Clare;Carboniferous;Namurian;Western Irish Namurian Basin;Clare Basin;Shannon Basin;Facies;Moore Bay;Flooding Surface;Central Clare Group

Obrock, Eric P. ; Best ; James L.

关键词: Transgressive;    Tullig;    Cyclothem;    Ireland;    County Clare;    Carboniferous;    Namurian;    Western Irish Namurian Basin;    Clare Basin;    Shannon Basin;    Facies;    Moore Bay;    Flooding Surface;    Central Clare Group;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/26012/Obrock_Eric.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Cyclothems record the sedimentary product of cyclicity in relative sea-level on either a regional or global scale, showing both regressive and transgressive phases.This study aims to contribute to our understanding of the sedimentary consequences of relative sea-level rise in a Carboniferous transgressive sequence in County Clare, western Ireland.The transgression in focus is at the top of the Tullig Cyclothem, and is exposed in several sections over a distance of 46 km. The aims of this thesis are to describe and interpret: 1) the detailed sedimentology and stratigraphy of the transgressive upper portion of the Tullig Cyclothem, 2) the nature (continuous or stepped) of, and controls upon, relative sea-level change, and 3) the paleoenvironmental transition from fluvio-deltaic to open marine conditions, and how this ties in to the overall evolution of the Western Irish Namurian Basin. These aims have been achieved by examination of all the outcrops of the transgressive upper Tullig cyclothem in County Clare, which show considerable lateral variation from north to south. One complete section of the transgressive Tullig is exposed at Trusklieve, where the sedimentary infill begins at the top of the fluvio-deltaic sandstone channels of the Tullig Sandstone.At this point, relative sea-level was at its minimum, and the beginning of the transgression is marked by symmetrical dunes, reworked dunes, and Zoophycos trace fossils.This surface is overlain by nearly 20 meters of unfossiliferous dark grey shales and siltstones, which are more organic rich and contain siderite nodules at their base, and are then topped by 4 meters of sandstone sheets, and finally a goniatite-rich fossil bed.This goniatite band marks the maximum relative sea-level within the Tullig cyclothem, and a fully marine environment.In terms of outcrop thickness, there is significant lateral variability between the nine outcrops studied in this thesis.The outcrops in the north (i.e. Furreera and Cliffs of Moher) are significantly thinner (less than 10 meters) than their southern counterparts, with the outcrop at Carrowmore Point being exceptionally thick (~50 meters).Additionally, the ratio of sandstone to siltstone also varies significantly between outcrops, and between one and three coarsening-upwards parasequences can be identified at these different locations.The outcrops that are centrally-located in the region (between Diamond Rocks and Carrowmore Point) possess the greatest thicknesses of sands and also up to three parasequences.At these localities, the presence of thick sandstones overlying shales and mudstones correlates strongly with the presence of mud diapirs, large-scale slumping and growth faults.The field study is used to propose a model for paleoenvironmental change through the upper transgressive portion of the Tullig Cyclothem: a transition from active fluvio-deltaic sands and coals of the upper Tullig Sandstone, followed by subaqueous delta plain muds and silts, progradational mouth bar sands, and finally the Reticuloceras stubblefieldi goniatite fossil bed.These changing paleoenvironments are interpreted in terms of the competition between relative sea-level change, which itself appears to be controlled by both eustatic sea-level change and local subsidence, and sediment supply, which is determined by both distance to source (as affected by relative sea-level) and local river channel avulsion and abandonment.