Small-scale faulting and fault sealing in Taratu formation, coastal Otago

Matthews, Abby.

Small-scale faulting of Taratu Formation, exposed along the coast of Otago in southeast New Zealand, can be -seen in isolated outcrops of conjugate normal, conjugate thrust and reverse faults. Many of these structures are comparable to deformation bands described by other authors (Aydin and Johnson, 1983; Antonellini and Aydin, 1994; Joude et al., 2002), where displacement has been accumulated through development and growth of deformation band zones. Through the mapping of vertical sectj.ons, a chronology of faulting has been established, with extensional normal faults offset by thrust and reverse faults. Orientation of regional structures is mirrored extensively by the smaller scale faults, reflecting the tectonic stress field acting at the time of their formation. Normal and thrust faults are therefore inferred to coincide with periods of basin extension and tectonic inversion, respectively. Analysis of extension fractures within the coal (known as cleats) showed that major fractures strike normal to local faults. Both the fractures and faults have formed in response to the prevailing stresses, and at some point there has been a change in the direction of least horizontal stress, along which these structures have formed. As the normal faults are inferred to be synsedimentary, the cleats are postulated as having formed in response to positive tectonic inversion rather than basin extension. The nature of movement (oblique shearing) along the low-angle thrusts has been determined by the orientation of striated material along the fault planes. In contrast, the normal faulting involves almost pure dip slip. Geochemical analysis of a large reverse fault near Wangaloa shows that fluids are highly mobile within the fault zone, elevating levels of clay minerals and certain trace metals. Where the smaller faults offset coal beds within the quartz conglomerates, sandstones and siltstones, smearing of coal and cataclasis has occurred along the fault plane. Reduction of porosity along these faults may provide sites for fault sealing and the subsequent restriction of permeability. Subsequently, while the large-scale fault provides a pathway for fluid migration, small-scale faults are found to act as barriers to fluid flow.

63, [11] leaves : ill. (some col., 1 folded) : 30 cm.

2003Matthews