Anatomy of the marginal zone of the Waulsortian Carbonate bank (mudmound) complex, North Co. Galway

Abstract:Two closely-spaced drill-cores through the marginal zone of the Waulsortian Complex in north Co. Galway have been studied petrographically to help identify factors that limited spread of the banks and led to their demise. The Waulsortian limestones found there have textural and structural characters and skeletal-grain content of Waulsortian Phase C, indicating a relatively shallow-water environment but below the depths colonised by calcareous chlorophyte algae. Bank depositional structures of both flat-lying and clinoform type are present. Four associated limestone facies are distinguished: off-bank, precursor (beneath and around the banks), cap (directly overlying banks) and cover facies. Spatial relationships between these are used to construct a depositional model. Five developmental stages are recognised through the sequence and used to correlate the two sections. The distribution of biostratigraphically significant foraminifera supports the sedimentological correlation. Throughout the sequence there was unusual physical disturbance of the sediments by water movement. There is evidence of progressively increasing damage to the polymud-forming processes. Formation of cryptofibrous cements and micritisation were affected at a late stage. Interference in development of precursor muds, probably more vulnerable than the banks, could have ‘killed’ the Complex by halting bank-initiation. The timing of these changes correlates with a sea-level fall identified elsewhere in Ireland and abroad.