NUCLEATION OF GROWTH FAULTS ALONG DEWATERING STRUCTURES WITHIN THE NAMURIAN TULLIG FORMATION; KILLARD BAY, WESTERN COUNTY CLARE, IRELAND

Coastal exposures of the Namurian Tullig Formation at Killard Bay provide new insight into the development of growth faults. Here, exposures of a transgressive sequence in the Tullig Fm include a thinly bedded siderite-rich muddy siltstone that thickens and coarsens up section to a very-fine grained sandstone that is marked by wave ripples. Overall, these strata represent the transition from a distal shelf environment to a northeastwardly prograding mouth bar succession. The Tullig Fm at Killard is deformed by a series of syn-depositional (primary) and post-lithification (tectonic) structures. Syn-depositional structures include soft-sediment deformation, conjugate fracture sets, and growth faulting within the sandstone units higher in the exposed section. These strata were later involved in a shallowly NE-plunging Variscan syncline.

Of particular interest are the primary structures, which record the nucleation and evolution of a growth fault array along fluid-escape structures. Soft-sediment deformation associated with fluid-escape (e.g., ball-and-pillow structures) is widespread in thicker (>0.5 m) sandstone beds that are stratigraphically higher in the exposed sequence. Approximately east-west trending sets of conjugate fractures are common in relatively thinner (0.1-0.2 m) bedded sequences of siltstone and very fine sandstone that directly overly zones of soft-sediment deformation. The presence of fluidized sandstone and a normal sense of shear along fracture zones suggests the conjugate fractures provided conduits for fluid escape and partially accommodated the resulting volume loss. In general, north-dipping fractures accommodate relatively more slip than adjacent south-dipping fractures. Within a distance of 15-20m in the study area, the amount of slip on individual north-dipping fractures increases northwards from centimeter to meter-scale as fractures begin to cut down-section into underlying sandstone beds and become listric normal faults. Farther north of this transition, these listric normal faults form an array of growth faults. Total slip on the growth faults ranges from approximately 2-10 m. Preferential thickening of strata in the hanging wall of these growth faults suggests that deposition was contemporaneous with active deposition.