Lacustrine sequence stratigraphy of the Greymouth Rift Basin , New Zealand : tectonic evolution and distribution of source rock facies

Lacustrine sequence stratigraphy provides a useful framework to understand the primary controlling factors of sedimentation in terrestrial rift basins as well as for considering the type and distribution of organic-rich facies. Sequence stratigraphic analysis on detailed sedimentary facies in the Greymouth Rift Basin, New Zealand improves correlation and can be used to predict the lateral extent and variability of source rock lithologies. The geometry indicates alluvial fans/fan deltas along the steep faults to the northwest, lakes alternating with meandering rivers and raised mires along the basin axis, and low gradient meandering river fans to the northand south-east hinge side of the basin. Four different potential source rock facies have been identified and correlated: lacustrine, marshy shoreline, raised mires, and meandering fluvial abandoned channels. Lowstand Systems Tracts correspond to axial meandering fluvial systems with scattered dirty coals and organic rich mudstones from abandoned channels. Transgressive Systems Tracts start with raised mire complexes subsequently overlain by marshy shoreline facies. Maximum flooding of lakes during Highstand Systems Tracts correlate thick organic rich lacustrine mudstones across the basin. The tectonic-sedimentary models suggest the Greymouth basin evolved from small sub-basins that widened and deepened through time via small-displacement normal fault segments which became progressively more connected until they formed a major border fault. This fault gave rise to the large half graben basin and was responsible for the lateral and vertical facies changes.