GROWTH FAULTS AND DEWATERING STRUCTURES IN PRODELTA SEDIMENTARY DEPOSITS: USING SEDIMENTARY STRUCTURES TO CONSTRAIN POST-FAILURE SEDIMENT TRANSPORT IN THE WESTERN IRISH NAMURIAN BASIN

The Western Irish Namurian (Carboniferous) Basin (WINB) in County Clare Ireland, contains coastal exposures of a variety of sedimentary environments ranging from fluvial deltaic deposits to deep water turbidites. Widespread growth faults are found within the relatively low-sloping prodelta deposits that characterize near shore marine facies of the Kilkee and Tullig formations. The failures are stratigraphically associated with thick channel sand bodies that produce soft sediment deformation of underlying fine grained (silt to sand) deposits. Fluid escape structures, including ball and pillow structures, as well as sand volcanoes and shale diapirs, generalize pre-failure conditions. Analyses of growth faults and dewatering structures characterize initial failure conditions and constrain post-failure sediment mobility. Field measurements of thickness, orientation, and throw of meter-scale growth faults were collected and coupled with the grain size of rock samples obtained from the water escape structures to characterize the rheology and yield strength of the sediments at conditions of failure. Evidence for syndepositional deformation suggests that the initial instability was caused by rapid sedimentation. Pressure conditions modeled under rapid sedimentation are compared to the characteristic yield strength of sediment in the WINB in order to predict post-failure sediment transport and depositional morphology. The outcrop-scale failures are then connected to post-failure transport processes of sediment from the shelf environment to the deep sea.