Late quaternary stratigraphy and neotectonics of Miramar Peninsula and Rongotai Isthmus, Wellington, New Zealand

Abstract

The area of study consists of 12 km2 in Wellington City's eastern suburbs. The landscape has undergone major changes due to urban development. Historical observations, and archival maps and photographs, are used to reconstruct the late Holocene landscape, allowing the recognition of geomorphic features. Beach ridges, inland wave cut cliffs, and marine terrace remnants are recognised, indicating uplift. Modern beach stratigraphy and sedimentary processes are examined, as an analogue to the deposition and lithology of ancient sediments in four major Holocene depositional areas, recognised in Lyall Bay, Evans Bay, Miramar Valley, and Seatoun. The effects of vertical tectonic movements (relative sea-level changes) of a few metres magnitude, on beach and nearshore sediments, is modelled in a series of tectonic stratigraphy models, which suggest that vertical tectonic movements result in preservation within Holocene marine sediments, of a series of paleoshorelines with distinct lithological characteristics. Mapping of the Quaternary stratigraphy is largely based on the examination of the subsurface. This was accomplished by the augering of 158 augerholes, the drilling of five drillholes, and the examination of drillholes drilled for previous studies. A stratigraphic nomenclature is developed based on the facies relationships identified from the tectonic stratigraphy models and geomorphology. Three periods of Quaternary vertical tectonic movements are recognised; (1) an early to mid Pleistocene uplift phase of at least 200 m, followed by (2) at least 65 m of subsidence which ceased by 120 ka, followed by (3) the initiation of the present uplift phase. Eight paleoshorelines are recognised and mapped in the area, representing a series of eight tectonic uplift events in the last 6500 years, and resulting in a total of 8 ± 1 m of uplift. The timing in cal years BP and vertical magnitude of the uplifts, numbered TU 8 - TU 1, is based on differences in paleoshoreline elevation, and 25 radiocarbon dates, mainly on incorporated shell material. These events are; TU 8 (6600 - 5730 years) <1.9 m, TU 7 (6600 - 5730 years) 1.1 m, TU 6 (5970 - 5730 years) <1.2 m, TU 5 (5400 - 4890 years) >0.6 m, TU 4 (3410 - 2740 years) 2 m, TU 3 (3410 - 2740 years) 1 m, T U2 (940 - 260 years) 0.8 - 0.95 m, TU 1 (1855 AD) 1.35 - 1.5 m These uplifts may correlate with known fault movements on the Wairarapa Fault (TU 8, 6, 4, and 1), Wellington Fault (TU 3), and newly recognised Point Gordon Fault (TU 2). A fault scarp on the newly recognised Point Gordon Fault, indicates a 1.7 m vertical offset, with a reverse up on the west sense of movement. Investigation of the historical and geological basis of the Hao-whenua earthquake of Maori tradition, suggests that it is likely to correlate with TU 2, dated between 1010 and 1690 AD. Uplift associated with this event, is indeed likely to have led to the closing of a channel between Lyall and Evans Bays and the joining of Miramar island to the mainland, as described in Maori tradition. The neotectonic record of the last 6500 years indicates a minimum average return time of 800 years for large uplift events, which are likely to be associated with earthquakes of magnitude 7 - 8 in the Wellington City area. However, this return time is of little practical use in assessing seismic hazard, as the uplift events are separated by two periods of inactivity of approximately 1500 years (c.5150 - 3100 cal years BP) and 2500 years (c.3100 - 600 cal years BP).