Abstract
At least four Pleistocene uplifted marine terraces occur in the north Taranaki area. These are the NT1, NT2, NT3, and an older terrace formation (informally named 'Urenui upland terrace surface'). The NT1 Terrace, correlated with ∂¹⁸O Substage 5a (c. 80ka B.P.), is restricted to the Turangi Road-Motunui coastal area. An average uplift rate of 0.29m.ka⁻¹ is inferred from the NT1 strandline altitude at Turangi Road. The NT2 Terrace, correlated with ∂¹⁸O Substage 5e (c. 120ka B.P.), is the dominant terrace in north Taranaki occurring along the entire length of the field area (Motunui- Awakino River mouth). The NT2 Terrace is the sole terrace formation north of White Cliffs and is clearly delineated by a linearly concordant abandoned sea cliff. The average uplift rate for the last 120ka was inferred from the NT2 strandline altitude at several localities: At Tongaporutu river mouth 0.27m.ka⁻¹. 10km further north at Mohakatino an uplift rate of 0.20m.ka⁻¹ was found. About 3km north at Mokau River mouth uplift rates of c. 0.15m.ka⁻¹ and 0.17m.ka⁻¹ were respectively calculated for the north and south sides. A clear trend of constant tectonic uplift rates between Motunui and Tongaporutu and linearly decreasing uplift rates north of Tongaporutu toward the Awakino River was thus found. The NT3 Terrace, correlated with ∂¹⁸O Substage 7a (c. 210ka B.P.), wedges out just north of Urenui (near Okoki Pa). The 'Urenui upland terrace surface' occurs above the NT3 Terrace in the Urenui-Onaero area and probably encompasses more than one terrace formation, but these were not differentiated. The completely dissected North Taranaki Surface (NTS) occurs inland of the marine terraces. The altitude of the envelope of ridge-crest concordance (c. 250m above MSL) is not representative of total tectonic uplift as it has been erosionally degraded by an unpredictable amount.
Non-marine terraces, sometimes looking remarkably like marine terraces are common throughout the north Taranaki landscape. Four criteria were used to distinguish them from non-marine terraces. (1) Absence of marine coverbeds. (2) Excessive shore-parallel and shore-normal tilt. (3) Lack of terrace-surface dissection compared with that expected of a marine terrace of similar altitude. (4) Parallelism of the coverbed/sedimentary rock contact with bedding in the North Taranaki Basin rocks. The non-marine terraces and related landforms result from the structural and lithological configuration of gently southwest dipping sandstones-mudstones of North Taranaki Basin sedimentary rocks. Two models providing a mechanism of topographic exploitation are presented. One involves ground water sapping, the other, earthquake induced planar sliding. The latter is favoured.
A 2- 4 m rhyolitic deposit occurs in the Mokau area above the NT2 Terrace. The informally named Mokau rhyolitic deposit consists of highly weathered, devitrified matrix crystal-rich in sodic-andesine plagioclase (high temperature form) and β-quartz. A common highly weathered vermicular-form phyllosilicate was identified as original biotite. Thickness and bedding characteristics infer a distal ignimbrite style emplacement. Age is estimated to be at least 0.5Ma. The Mokau rhyolitic deposit is tentatively correlated with either early Whakamaru Group ignimbrite/s or the Rocky Hill Ignimbrite.
A Holocene notch feature associated with holes much like those seen formed in the modern intertidal zone by an isopod crustacean ‘S. quoyanum’ was studied at Aria Beach, Mokau. It was concluded that the notch was a 'structural notch' and hence not directly related to paleo-sealevel. The holes are not considered to be of intertidal origin, but their genesis remains ambiguous.
A multifractal analysis of topography based on the model of Chase (1992) was proposed but not applied to real data.