Characterising sand distribution within fault zones that cut interbedded sandstone-shale sequences

Many previous studies have highlighted the complex fault rock distributions within brittle faults that cut clastic sedimentary strata. This presentation investigates possible controls on the distribution and development of sand-rich fault rocks within layer-bound, post-depositional normal faults that cut an interbedded sandstone-shale sequence exposed on the foreshore at Lamberton, SE Scotland. The key assumption underlying our study is that at least some of the complexity in fault rock distribution arises from the interaction and eventual coalescence of multiple fault strands (e.g. Childs et al. 1996). Analysis of closely spaced, small-displacement faults, such as those exposed at Lamberton, may therefore provide insights into the initial stages of fault rock development.

The faults were sampled along two sub-horizontal sandstone beds and are regularly spaced (coefficient of variation, Cv < 1), with a mean trace spacing of ca. 30 cm and a mean throw of ca. 4 cm. Qualitative inspection of the fault pattern reveals that many of the sub-parallel faults interact across relay zones that display a wide range of ramp dips and aspect ratios. These observations are consistent with the order of magnitude variation in horizontal displacement gradients measured at fault tips, between 0.026-0.26. Many of the faults appear to be linked, giving rise to splays and/or fault-bound sandstone lenses. With increasing displacement, the former are likely to be bypassed by slip on the main fault and will be preserved as a damage zone; the latter will become incorporated into the fault zone. We suggest that small (cm-scale) sandstone lenses preserved along fault traces initially developed as relay zones. With continuing displacement, larger relays will become incorporated into the fault zone giving rise to thicker, more laterally continuous sand lenses. A key control on fault zone width is therefore the distance over which faults are able to interact elastically.