Geology and landslides of the eastern Te Aute district, southern Hawkes Bay

Abstract

A succession of Upper Cretaceous to Quaternary sedimentary strata of the East Coast Deformed Belt is subdivided into 12 formations. Three unconformities and two stratigraphic breaks of uncertain character are recognised. Upper Cretaceous sedimentation is typified by an influx of clastic detritus. A progressive fining upward sequence is represented by Paleocene and Eocene formations. Total stratigraphic thickness is small, reflecting a tectonically quiescent period and environment of deposition which was probably deepwater, continental slope. The East Coast Deformed Belt, a tectonically active and structurally complex zone since Oligocene times is interpreted as the accretionary sediment prism in the arc-trench gap of the obliquely convergent plate margin. The Waimarama-Mangakuri Coastal High and Elsthorpe Anticline are located over a deep-seated thrust zone. Both are characterised by complexly folded and thrust-faulted Upper Cretaceous to Miocene sequences. Thrusts commonly are accompanied by mélange and crush zones. The 'coastal high' developed during Upper Oligocene and Miocene times. The Elsthorpe Anticline formed as an offshoot from this during Upper Pliocene-Lower Pleistocene times. Deformation has occurred progressively since Oligocene times. A sporadically preserved early phase, recognised in east-west trending structures and inverted sequences, followed by imbricate thrust faulting along the dominant northeast-southwest structural grain are present within the 'coastal high'. The narrow elongate thrust zones are separated by broad, shallow synclinal basins in which thick flysch sequences accumulated unconformably on older strata during Neogene times. These basin sequences have, in part, subsequently been incorporated into the imbricate accretionary margin. Little evidence exists to suggest that major transcurrent faulting has been important in coastal Southern Hawkes Bay. Mount Kahuranaki, a limestone klippe, is located to the east of a change in axial trend of the Elsthorpe Anticline, and is genetically related to this. It formed as the result of a gravity glide from the anticlinal crest onto the eastern flank. A large-scale Regional-Slump which developed during Quaternary time involves the progressive eastward downfaulting and rotation of large blocks of land extending over 20 km in length, incorporating much of the Waimarama-Mangakuri Coastal High. Only the head of the slump is exposed onland. Its extent offshore to the east is unknown. Large 'parasitic' landslides involve the crown of the western (main scarp) margin of the slump. Soft rock lithologies encountered are typical of the Belt, and with bedrock structure are important controlling factors on slope failure. The main associations recognised between failure type, lithology and structure are: a) Earthflows, debris-flow slides, mudflows, and creep on Upper Cretaceous and Lower Tertiary alternating sandstone and carbonaceous mudstone, glauconitic sandstone and bentonitic mudstone. Complex folding, intense fracturing, shearing and development of crush or mélange zones on thrust faults has reduced much o f the soft rock to 'soil'. b) Rotational slide-earthflow, regolith slides, clayflows on massive mudstones and thinly bedded mudstones-sandstones of Middle Tertiary age. These sequences are tightly folded and thrust faulted. c) Large planar slides (block and wedge glides) occur in a Middle to Upper Miocene flysch succession which is gently folded and moderately faulted. The slides occur on bedding planes in conjunction with steeply dipping primary fractures. d) Sheet and regolith slides are widespread irrespective of lithology. Important controls on precipitation and near-surface groundwater conditions are exerted by the moderate to steep relief and the limestone-capped Maraetotara Plateau. Permeability contrasts both within bedrock and soil masses are recognised as primary factors influencing slope failure. Severe earthquakes and regional uplift are common and widespread in the tectonically active East Coast Deformed Belt (eg Napier, 1931). Lowering o f base levels in conjunction with regional (tectonic) uplift is related to successive generations of slope failures. Landslides belong to an old (relict) or a new (rejuvenated) landscape, each of which is composed of several erosion surfaces. Slope failure susceptibility is assessed in terms of bedrock and soil failures. Five critical control parameters (based on landform and bedrock characteristics) are given susceptibility ratings, which are related to a rating scale of slope failure susceptibility.