GeoResources - Rapid Environmental Changes on a Carbonate platform - High-Resolution Analysis of a Transgressive System Tract based on Palynofacies and Geochemistry


	Rapid Environmental Changes on a Carbonate platform - High-Resolution Analysis of a Transgressive System Tract based on Palynofacies and Geochemistry
	Abstract





	Scientists Hartmut Jäger, GeoResources and University of Heidelberg Geoff Clayton, Trinity College Dublin Robbie Goodhue, Trinity College Dublin

	Abstract The Aran islands are made of Upper Visean limestones of typical carbonate platform facies. Several hundred meters of platform limestones represent a very stable palaeoenvironment of several million years duration, from the Asbian to the Brigantian. Within the thick limestone succession, a thin shaly interval was observed, providing evidence of significant palaeoenvironmental changes on the platform. The limited thickness (0,2 - 1,6 m) and sharp lithological boundaries of the shaly interlayer suggest a very short-lived and rapid palaeoenvironmental change on the carbonate platform. Two different lithostratigraphic units can be distinguished within this interval. The lower part (maximum thickness >1m) consists of medium grey mudstones with abundant pyrite nodules, showing strong lateral variation in thickness and even pinching out in places. The upper part consists of very dark grey shales, only 0,2-0,3 m thick but maintaining its thickness along the whole length of the accesible section. The obvious differences in lithology suggest that there is a clear change of palaeoenvironmental conditions even within this brief event. To obtain more information on these significant environmental changes, detailed palynological and geochemical studies were undertaken, including palynofacies analysis, clay mineralogy analysis, TOC and CNS and C-isotopes. Miospore assemblages from samples towards the top of this interval enabled precise dating. First results suggest a change from restricted marine conditions almost without terrestrial input at the bottom of the unit, to shallow marine conditions with high terrestrial input at the top. The lower part of the shaly interval represents deposition within locally restricted minor depressions on the platform, isolated from both terrestrial input and from open marine environments. The upper part represents widespread deposition on the platform with substantial terrestrial input. This scenario can be explained by a drop of sea-level, leading to isolated ponds of sea-water on the platform cut off from any terrestrial input during the lowstand system tract (lst). During the following rise of sea-level, the platform was again widely flooded and large amounts of terrestrial organic matter were deposited on the platform. This transgressive system tract (tst) can be divided in 4 different phases showing the different levels of sea-level rise and connection to the hinterland. The very dark grey shales of the upper part of the interval seem to represent the maximum flooding surface (mfs), followed by a huge succession of carbonate platform deposits representing the highstand system tract (hst). Although the shaly unit is only a very minor part of the platform succession, it records crucial information on sea-level changes, which are of great importance in elucidating the paleoenvironmental evolution of the platform.

	Presentations & Publications XII. International Palynological Congress, Bonn, 2008

	Funding Organizations GeoResources