Paul Vella

Growth of contractional structures during the last 10 m.y. at the southern end of the emergent Hikurangi forearc basin, New Zealand

Abstract Growth histories of contractional structures at the southern end of New Zealands Hikurangi forearc basin have been analysed for the last c. 10 m.y. Growth data are from outcrop and seismic‐reflection profiles that contain syntectonic strata and angular unconformities, and from deformed fluvial terrace surfaces. Deformation is described for up to eight intervals of time, spanning c. 12 000 yr to 5 m.y., the ages of which were determined by biostratigraphy and tephrochronology. Reverse faults and related asymmetric folds, which strike parallel to the subduction margin and verge troughwards, experienced variable rates of shortening through time. The current period of deformation commenced at c. 1.8 Ma with displacement rates of c. 0.1–0.7 mm/yr on the main faults (i.e., Martinborough, Huangarua, and Mangaopari Faults). Before this time there were periods of accelerated deformation during the mid Pliocene (c. 3.4–2.4 Ma) and latest Miocene (c. 8.0–6.0 Ma). Therefore, shortening since 10 Ma accumulated mainly during three periods of 1–2 m.y., with structures active in the Quaternary forming in the late Miocene or earlier. Local intervals of accelerated deformation are coincident with the timing of intervals of uplift and faulting along much of the emergent forearc and cannot be attributed to local transfer of displacements between faults. Instead, these intervals of deformation appear to reflect regional changes in the kinematics of the upper plate. These changes could arise due to margin‐normal migration of strain to regions outside the forearc basin or may indicate temporal variations in the dynamics of subduction.

Paleomagnetism of the Khorat Group, Mesozoic, Northeast Thailand

Abstract The Khorat Group, middle or late Triassic to Cretaceous, consists of up to 5000 m of paralic and freshwater sedimentary deposits, mainly alluvial flood-plain red-beds. Paleomagnetic measurements tested using thermal and AF demagnetization by several authors in different laboratories are generally consistent. Selection of samples requires care because of deep weathering in the monsoon climate of Thailand. All authors have reported a clockwise rotation of the paleomagnetic declinations, which we estimate to be 37 ± 7 degrees, and which occured at some time in the last 100 My. Inclinations are not significantly different from those of the present day, indicating that Northeast Thailand (and the Indochina plate) have remained at nearly the same latitude since about late Triassic. Relative southward movement of the South China plate possibly resulted from dextral displacement along the Red River Fault. Relative southward movement of Eurasia involved either ocean consumption or continental crust shortening of 1000 km or more. Magnetostratigraphy suggests an unconformity, supported by stratigraphic evidence, between Sao Khua and Phu Phan formations in the upper part of the Khorat Group, and the unconformity is explained by a simple model involving the combined effect of subsidence at an exponentially decreasing rate, and Mesozoic eustatic sea-level changes.

Lithostratigraphic names, upper miocene to lower pleistocene, Northern Aorangi Range, Wairarapa

Abstract Twelve upper Cenozoic lithostratigraphic units at the northern end of the Aorangi Range are formally defined and their stratigraphic and interfacies relationships are illustrated and discussed. In order of increasing age they are: Pukenui Limestone (Lower Pleistocene); Onoke Group (uppermost Miocene, Pliocene, and basal Pleistocene) containing two main facies, (1) shallow-water bioclastic and chemical facies comprising Bull Creek Limestone, Haurangi Limestone, Makara Greensand, and Clay Creek Limestone, and (2) shallow- to deep-water inorganic clastic facies comprising Greycliffs Formation and Mangaopari Mudstone with Bridge Sandstone Member; Palliser Group (Upper Miocene) containing Bells Creek Mudstone and Sunnyside Conglomerate. All except Onoke Group and Bells Creek Mudstone are new names. Biostratigraphic zones represented in each formation and probable correlations with stages are mentioned but not exhaustively discussed.

Paleomagnetism of the Upper Cretaceous Mount Somers Volcanics, Canterbury, New Zealand

The Mount Somers Volcanics of mid Canterbury in eastern South Island are a high-K calcalkaline andesite–dacite–rhyolite suite erupted subaerially approximately 95 Ma ago during the long Late Cretaceous episode of normal geomagnetic field. Thermoremanent magnetic directions at 46 sites, corrected for post–eruption tectonic tilt, are stable and group well (a95= 3·8°, K = 31·7) with a very steep mean inclination (I = –85°, D = 354°). The resulting paleomagnetic South Pole position is at 52° S 174° E, indicating that Canterbury was within 10° of the South Pole 95 Ma ago, and that little finite rotation of Canterbury relative to the South Pole has occurred since. The Mount Somers Volcanics were erupted prior to the separation of eastern New Zealand from West Antarctica along the Pacific–Antarctic ridge. They provide a mid–Cretaceous pole position that can be compared with pole positions of similar age from Australia and with the polar–wander path previously reported for the Chatham Islands over the past 75 Ma....

Surficial geological sequence, Black Island and Brown Peninsula, McMurdo Sound, Antarctica

Abstract On Black Island and Brown Peninsula a sequence of moraine deposits and topographic benches is interpreted as reflecting four cycles of alternating climate. The previously described glacio-marine Scallop Hill Formation is related to the second of the climatic cycles, the Taylor Formation to the fourth. Erratic boulders of marine Tertiary sediments previously reported from McMurdo Sound are confined to the moraine of the third cycle. Benches may have been formed by earlier ice shelves like the present Ross Ice Shelf, and have been raised by progressive differential uplift.

Eocene and Oligocene Sedimentary Cycles in New Zealand

Abstract Neritic deposits of Eocene and Oligocene age, partly underlain by coal measures, occur over large areas of south-west Auckland, west Nelson and north Westland, and north-east Otago (Waitaki). The well described deposits of south-west Auckland and Waitaki consist of sedimentary units bounded by unconformities and each containing distinctive fossils. Each sedimentary unit reflects an increase, each unconformity a decrease, in depth of sea, and the successions represent cycles of deposition and non-deposition or erosion. The succession of cycles in Waitaki matches that in south-west Auckland. The early Tertiary of Nelson and Westland has not been fully described but seems to contain comparable sedimentary cycles. The changes in sea depth could have been caused either by essentially uniform vertical tectonic oscillations of virtually the whole of New Zealand, or by eustatic sea-level fluctuations. If eustatic sea-level fluctuations occurred they should have affected strata of the same age and facies ...

The Permian at Parapara Peak, North-west Nelson

Abstract Permian strata, previously mapped as Cambrian, are described from Parapara Peak, west of Takaka, North-west Nelson. The name Parapara Group is proposed for these rocks, with the following four formations in ascending order: Pupu Conglomerate, Flowers Formation, Pariwhakaoho Formation, and Walker Quartzite. The strata differ considerably from the Permian elsewhere in New Zealand in their lack of lavas and tuffs, and lack of volcanic detritus, in their predominantly arkosic nature, and in their fauna, which closely resembles that of south-east Australia, especially Tasmania.

Coiling Ratios of Neogloboquadrina Pachyderma (Ehrenberg): Variations in Different Size Fractions

In surface sediments of the subantarctic and cooler subtropical areas of the southeastern Indian Ocean, coiling ratios of Neogloboquadrina pachyderma are more dextral in coarser size fractions than in finer size fractions. A sample from near lat 45°S. contained 14 percent (95 percent confidence limits 8 to 22 percent) dextral tests in the 0.124 to 0.175-mm fraction, and 67 percent (57 to 76 percent) in the size fraction >0.175 mm. The difference decreases toward the north where dextral coiling eventually becomes dominant in all size fractions, and toward the south where sinistral coiling becomes dominant in all size fractions. This phenomenon is thought to be due to the existence of at least two races or taxa in the range of forms usually classed as N. pachyderma . To these, the names N. pachyderma pachyderma (Ehrenberg) and N. pachyderma incompta (Cifelli) are applied provisionally. Whether the taxonomic interpretation is correct or not, the empirically determined differences in coiling ratios of the different size fractions provide a previously unsuspected latitude-dependent variable in planktonic foraminiferal faunas. When applied to the analysis of fossil samples, they enhance the value of coiling ratios as paleotemperature indices and can be used to detect north-south movements of ocean surface hydrologic features.

Upper Eocene-lower Oligocene benthonic foraminifera, Port Elizabeth and Cape Foulwind, New Zealand

Abstract At Port Elizabeth, type section of the Runangan Stage and of part of the Kaiatan Stage, five biostratigraphic zones based on benthonic smaller foraminifera are recognised between the lowest exposed Kaiatan strata and the Whaingaroan strata below the Cobden Limestone. The fauna gradually changes from an inner shelf type at the base of the section to an outer shelf type higher up. At Cape Foulwind only the two upper zones are represented (Runangan-Whaingaroan Stages), and the fauna remains an inner shelf type throughout the section. The climate of the region seems to have been mostly warm-temperate, except for a brief subtropical period in late Kaiatan and early Runangan times.

Foraminifera from upper miocene turbidites, Wairarapa, New Zealand

Abstract Abundant and well preserved Foraminifera in turbidites of Upper Miocene (Kapitean) age, at Cleland Creek, were compared with Foraminifera in four different depth biofacies of about the same age. The turbidites were deposited in depths certainly greater than 2,000 ft, and probably between 4,000 and 6,000 ft, and were derived from all shallower depths up to about 400 ft or less. Fragile shells and large shells are less common in turbidites than in non-turbidites, and many shells are considered to have been destroyed during transport. The basal layer of each turbidite rhythm is considered to consist of “slumped” neritic sediment with little intermixed deep-water sediment, the intermediate layer to have been deposited by a swift turbidity current, and the upper layer to have been deposited from suspension after the turbidity current ceased flowing. No trace of autochthonous sediment was found between turbidite rhythms.