Continuity of shale-smears in poorly lithified strata

Fault-seal algorithms were primarily developed from outcrops where clay-rich fault-rock is mainly derived from shale beds. We analyse the continuity of shale-smears and consider their impact on fault seal for interbedded sand-shale sequences in New Zealand. Our data are from six coastal outcrops of poorly lithified strata (burial depths ∼≤1.5 km) displaced by a random sample of 194 small normal faults with displacements of 0.02–1.4 m. Each fault is 100% exposed in cross section and displaces a single shale bed by more than the bed thickness. The faulted shale beds display a range of geometries from no smear, to discontinuous smear and continuous smear. The relative frequencies of these three smear types vary between the six outcrop localities, with up to 60% of shale beds at each locality showing no smear and discontinuous smears typically covering <50% of the fault-trace length between shale-bed cutoffs. First-order changes in shale-smear continuity between sample localities reflect differences in shale composition and competence, while locally the geometries of individual smears can be controlled by the number and displacements of slip surfaces within fault-zones. The absence of shale-smear on many beds decreases fault-seal potential and could be accounted for in shale-smear algorithms. Fault-seal algorithms were primarily developed from outcrops where clay-rich fault-rock is mainly derived from shale beds. We analyse shale smear (and non smear) from six outcrops to determine how common shale smear is and what factors control it. Below we list the highlights. • Faulted shale beds display a range of geometries from no smear, to discontinuous smear and continuous smear. • The relative frequencies of these three smear types vary between the six outcrop localities, with up to 60% of shale beds at each locality showing no smear and discontinuous smears typically covering <50% of the fault-trace length between shale-bed cutoffs. • First-order changes in shale-smear continuity between sample localities reflect differences in shale composition and competence, and fault-zone architecture. • The absence of shale-smear on many beds decreases fault-seal potential and could be accounted for in shale-smear algorithms.