Kari N. Bassett

Climatic and tectonic controls on Jurassic intra-arc basins related to northward drift of North America

Upper Jurassic strike-slip intra-arc basins formed along the axis of earlier Lower to Middle Jurassic extensional intra-arc basins in Arizona. These strike-slip basins developed along the Sawmill Canyon fault zone, which may represent an inboard strand of the Mojave-Sonora megashear system that did not necessarily produce large-scale translations. Subsidence in the Lower to Middle Jurassic extensional arc was uniformly fast and continuous, whereas at least parts of the Upper Jurassic arc experienced rapidly alternating uplift and subsidence, producing numerous large-scale intrabasinal unconformities. Volcanism occurred only at releasing bends or stepovers in the Upper Jurassic arc, producing more episodic and localized eruptions than in the earlier extensional arc. Sediment sources in the Upper Jurassic strike-slip arc were also more localized, with restraining bends shedding sediment into nearby releasing bends. Normal fault scarps were rapidly buried by voluminous pyroclastic debris in the Lower to Middle Jurassic extensional arc, so epiclastic sedimentary deposits are rare, whereas pop-up structures in the Upper Jurassic strike-slip arc shed abundant epiclastic sediment into the basins. Three Upper Jurassic calderas formed along the Sawmill Canyon fault zone where strands of the fault progressively stepped westward in a releasing geometry relative to paleo-Pacifi c‐North America plate motion. We hypothesize that strike-slip basins in the Upper Jurassic arc formed in response to changing plate motions that induced northward drift of North America, causing sinistral deformation of the paleo-Pacifi c margin. Drift out of the northern horse latitudes into northern temperate latitudes brought about wetter climatic conditions, with eolianites replaced by fldebris-fl ow, and lacustrine sediments. “Dry” eruptions

Paleoliquefaction in Christchurch, New Zealand

Liquefaction during the 2010 moment magnitude (Mw) 7.1 Darfield earthquake and large aftershocks (known as the Canterbury earthquake sequence) caused severe damage to land and infrastructure in Christchurch, New Zealand. Liquefaction occurred at Mw-weighted peak ground accelerations (PGA7.5) as low as 0.06 g at highly susceptible sites. Trenching investigations conducted at two sites in eastern Christchurch enabled documentation of the geologic expressions of recurrent liquefaction and determination of whether evidence of pre–Canterbury earthquake sequence liquefaction is present. Excavation to water table depths (∼1–2 m below surface) across sand blow vents and fissures revealed multiple generations of Canterbury earthquake sequence liquefaction “feeder” dikes that crosscut Holocene-to-recent fluvial and anthropogenic stratigraphy. Canterbury earthquake sequence dikes crosscut and intrude oxidized and weathered dikes and sills at both sites that are interpreted as evidence of pre–Canterbury earthquake sequence liquefaction. Crosscutting relationships combined with 14C dating constrain the timing of the pre–Canterbury earthquake sequence liquefaction to post–A.D. 1660 to pre–ca. A.D. 1905 at one site, and post–A.D. 1415 to pre–ca. A.D. 1910 at another site. The PGA7.5 of five well-documented historical earthquakes that caused regional damage between 1869 and 1922 are approximated for the study sites using a New Zealand specific ground motion prediction equation. Only the June 1869 Mw ∼4.8 Christchurch earthquake produces a median modeled PGA7.5 that exceeds the PGA7.5 0.06 g threshold for liquefaction. Prehistoric earthquakes sourced from regional faults, including the 1717 Alpine fault Mw ∼7.9 ± 0.3 and ca. 500–600 yr B.P. Mw ≥ 7.1 Porters Pass fault earthquakes, provide additional potential paleoseismic sources for pre–Canterbury earthquake sequence liquefaction. The recognition of pre–Canterbury earthquake sequence liquefaction in late Holocene sediments is consistent with hazard model-based predicted return times of PGAs exceeding the liquefaction triggering threshold in Christchurch. Residential development in eastern Christchurch from ca. 1860 to 2005 occurred in areas where geologic evidence for pre–Canterbury earthquake sequence liquefaction was present, highlighting the potential of paleoliquefaction studies to predict locations of future liquefaction and to contribute to seismic hazard assessments and land-use planning.

Oligocene paleogeography of New Zealand: maximum marine transgression

This Special Issue of the New Zealand Journal of Geology and Geophysics is part of the on-going debate as to whether all of the continent of Zealandia was completely below sea level sometime during...

Timing of normal faulting in the Waikato Coal Measures, New Zealand, and its implications for coal‐seam geometry

Abstract Coal seams within the Waikato Coal Measures can vary significantly (>50%) in thickness over short lateral distances (<500 m). Normal faulting during coal‐measure deposition could produce the observed variations in seam thicknesses, but the timing of faulting has not been determined precisely. To constrain the timing of faulting, and its potential impact on coal seam geometry, we have analysed displacements for nine normal faults striking both NNW to N and NNE to NE. Displacements were derived from drillholes, mine workings, and seismic reflection lines. The data indicate that normal faulting occurred after peat accumulation in the coal measures, in some cases perhaps commencing in the latest Eocene to early Oligocene (i.e., c. 33–36 Ma). Normal faulting had little or no impact on the accumulation of peat in the Waikato Coal Measures. Variations in coal‐seam thickness were controlled by basement paleotopography and fluctuations in sediment supply.

The geological setting of the Darfield and Christchurch earthquakes

Abstract The 2010–2011 Canterbury earthquake sequence occurred near the southeastern margin of Neogene deformation associated with the Australia–Pacific plate boundary. Basement comprises indurated rocks of the Torlesse Composite Terrane, of Permian to Early Cretaceous age, overlain by 1–2 km of less-indurated Cretaceous–Neogene rocks and unconsolidated Quaternary sediments. Proximity to the subduction interface between Gondwana and the paleo-Pacific Ocean produced a Mesozoic-age structural grain in the basement rocks, aligned broadly east–west in the Canterbury to Chatham Rise areas. These structures provided an inherited weakness that was likely reactivated by present-day stress. Mid- to Late Cretaceous extension, marked by localised fault-bounded grabens, was followed by deposition of a Late Cretaceous to Paleogene passive-margin transgressive sedimentary sheet and minor intraplate basaltic volcanics. Mid-Cenozoic inception of the modern Australia–Pacific plate boundary heralded deposition of a regressive succession of Neogene sediments and further episodes of volcanism, most notably constructing the Late Miocene Banks Peninsula intraplate volcanoes. The east- to northeast-striking faults associated with the Darfield and Christchurch earthquakes are probably aligned with the Mesozoic structural grain within the Torlesse basement rocks.

Pahau Terrane type locality: Fan delta in an accretionary prism trench‐slope basin

Abstract The sediments of the Pahau Terrane, part of the Torlesse Superterrane, have never been adequately described or formally denned, although an unofficial type area in the Pahau River exists. This research formalises the type locality and interprets the sedimentology and tectonic setting of the deposits. Four well‐defined lithofacies are able to be distinguished in the area: mudstone, interbedded mudstone‐sandstone, sandstone, and conglomerate. We propose the Lochiel Formation to encompass these lithofacies and the Mt Saul Member for the conglomeratic bodies found within the Lochiel Formation. The Pahau River Group is to encompass the Lochiel Formation and future formations defined within the Cretaceous Torlesse Supergroup and is envisioned to extend over a large part of the Pahau Terrane. A shallow‐water fan delta model is proposed for the Lochiel Formation in the type area, based primarily on coarsening and thickening upwards sequences, mixed terrestrial and marine depositional setting indicators such as rootlets and dinoflagellates, and conglomeratic bodies with deposits indicative of both bedload and debris flow transport. A fan‐delta model is chosen over a coarse‐grained delta as local relief had to be high enough to produce debris flows in the delta plain. The presence of rootlets directly under a debris flow deposit indicates a subaerial environment. Paleocurrent indicators from 14 locations suggest a paleoflow direction towards the east‐northeast corrected for regional rotation to east‐southeast. The rare occurrence of bi‐directional and wave ripples suggests that these deposits were not greatly influenced by tidal currents or wave reworking, but were dominated by fluvial unidirectional currents. The tectonic setting for the Pahau Terrane is interpreted as distal forearc to accretionary prism. QFL analysis for the Lochiel Formation suggests a transitional arc provenance, similar to other analyses in the Pahau Terrane. Conglomerate clast rounding and the wide variety of clast compositions indicate a large catchment area, suggesting that the relief needed to produce the debris flows was provided locally by a large fault scarp. This was likely to have been a thrust fault with the Pahau River fan delta deposited in a trench‐slope basin on top of the accretionary prism.

Paleoenvironmental and tectonic changes across the Cretaceous/Tertiary boundary at Tora, southeast Wairarapa, New Zealand: A link between Marlborough and Hawke's Bay

Abstract The Late Cretaceous‐Paleocene succession exposed on the Tora coast, near the southeastern tip of the North Island, is distinguished by an unusual lithofacies of the Whangai Formation, and by an apparently unique formation, Manurewa Formation, which spans the Cretaceous/Tertiary (K/T) boundary. The Late Cretaceous siliceous Whangai Formation at Tora includes zones of slumps and olistostromes, containing megaclasts of limestone up to 3 m long. The olistostromal deposits suggest steep submarine topography with a high rate of erosion, and imply tectonic activity. The common occurrence of hummocky cross‐stratification suggests deposition in shelf depths above storm wave base. The sharply overlying Manurewa Formation is interpreted as the infill of a major shallow channel complex, perhaps >9 km wide and spanning the K/T boundary in time. The older of two channelled units is of latest Cretaceous (latest Haumurian/late Maastrichtian) age, and consists of bioturbated alternating thin sandstone and mudstone with thin conglomerate lenses and limestone beds. It is likely to have been deposited in a low‐energy environment, probably deeper than that of the Whangai. The younger channel system, of early Paleocene (early Teurian) age, erodes into the older in the northeast, and into the underlying Whangai Formation in the southwest. Basal deposits consist predominantly of medium to coarse, thick‐bedded, glauconitic sandstone, with local low‐angle cross‐stratification and microflora typical of low salinity conditions, suggesting deposition in shallow shelf depths. These deposits contain olistrostromes with megaclasts up to 1 m long of limestone and rarer dark grey siltstone or very fine sandstone clasts typical of Whangai Formation. The inclusion of megaclasts of Whangai Formation indicates that local emergence and erosion of older strata was occurring. Deposits grade upward into well‐sorted bioturbated sandstones of the Awhea Formation, with prominent low‐angle cross‐stratification, interpreted as very shallow marine, probably nearshore deposits. The channel system represented by the Manurewa Formation records an initial relative sea‐level rise, followed by an abrupt sea‐level fall at, or close to, the K/T boundary. New Zealand was in a passive margin tectonic setting at the time, but the widespread presence of olistostromes, some including clasts derived from older strata, suggest that local tectonic activity and uplift was occurring. The effects may have been enhanced by a climatic shift in storm tracks and intensity in the latest Cretaceous, which is supported by the evidence of strong wave activity. By contrast, to the south in Marlborough, the K/T boundary succession is commonly characterised by an apparently conformable lithologic change from limestone to chert, although with local hiatus. To the north, in southern Hawkes Bay, the coeval succession is characterised by a disconformity separating greensand from underlying light grey, slightly calcareous mudstone of the Whangai Formation. The Tora sequence may provide the link between two distinctly different lithologic successions.

Provenance analysis of the Paparoa and Brunner Coal Measures using integrated SEM-cathodoluminescence and optical microscopy

Abstract An integrated approach using grain‐by‐grain comparison of SEM‐cathodoluminescence (CL) andpetrographic characteristics was applied to investigate the provenance of fluvial/lacustrine quartz‐rich lithic arenites deposited in the Late Cretaceous transtensional Paparoa Basin in New Zealand. Results show most quartz in the Paparoa Basin was derived from a relatively low grade metamorphic source, most likely the underlying metasedimentary Greenland Group. This interpretation is supported by the presence of sedimentary lithics and Greenland Group conglomerate clasts in the Paparoa members. The eastern part of the basin is characterised by the addition of detrital chlorite and muscovite, some clearly replacing potassium feldspar in retrograde metamorphic lithoclasts, the lack of plutonic quartz, and an increase in polycrystalline metamorphic quartz. All this indicates an increase in metamorphic grade of the source area to the east upward through the stratigraphic section followed by a decrease. Such a change in grain type most likely reflects the migration of the basin past the exposed metamorphic rocks of the Paparoa Metamorphic Core Complex to the northeast of the strike‐slip Paparoa Tectonic Zone. On the west side of the basin, c. 15% of quartz grains are also derived from a plutonic source, probably the Barrytown Granite pluton or similar intrusions. Our provenance information provides the first concrete evidence for sinistral strike‐slip fault movement as predicted by tectonic reconstructions.

Reconsidering basin geometries of the West Coast: the influence of the Paparoa Core Complex on Oligocene Rift Systems

Oligocene and Miocene strata of the Nile Group are dominated by neritic cool-water carbonates formed within the Paparoa Trough along the western flank of the Challenger Rift System. In outcrop west of the Paparoa Range, the Nile Group contains three formations, the Waitakere Limestone, dominated by photozoan floatstone and grainstone facies, the Tiropahi Limestone, containing both impure heterozoan packstones and wackestones, and the Potikohua Limestone, consisting entirely of skeletal heterozoan grainstones. These correlative formations represent lateral depositional facies interpreted to record a submarine depression bounded by a northern irregular palaeohigh and a southern open marine plateau. It is proposed that these variations are the product of reactivation of structures associated with the underlying Paparoa Core Complex superimposed on the NE–SW rift axis emphasized within current basin models. This interpretation is consistent with exposures elsewhere in the West Coast region and highlights the need for palaeogeographic models of South Island to take older, rift-related basement structures into account.

The effect of volcanism on cool-water carbonate facies during maximum inundation of Zealandia in the Waitaki–Oamaru region

The paleogeography of the Waitaki–Oamaru region during the Oligocene–Miocene maximum inundation was defined by a volcanic paleohigh producing a rimmed cool-water carbonate shelf geometry. Basaltic surtseyan style cones became the setting for a productive cool-water carbonate factory isolated from terrigenous input. To the west, impure wackestones and calcareous siltstones contain terrigenous material derived from low relief landmasses farther to the west. A lowstand following the cessation of volcanism caused the paleohigh to become subaerially exposed, forming an extensive dissolution surface in the east correlative with submarine firmgrounds to the west. Stronger currents from the south and significant storm events sweeping over the high reworked carbonate and glauconitic sediment and deposited it in channels to the north. Carbonate deposition west of the paleohigh filled in the deeper part of the basin eventually resulting in a wider, shallower and more regular shelf environment. Depositional environments of the Waitaki–Oamaru region during the Waitakian (Late Oligocene–Early Miocene) were overall shallower than during the earlier Whaingaroan (Early Oligocene) Stage when a more complex paleotopography existed. This is unlike elsewhere in New Zealand where maximum depth was reached during the Waitakian Stage. Thus, the existence of an isolated, submerged paleohigh in a cool-water carbonate basin can have a significant effect on the evolution of that basin, stimulating carbonate factories to develop where they might otherwise not.