Bradley Pillans

The Quaternary Period

The Quaternary Period, comprising the Holocene and Pleistocene Epochs, encompasses the last ~2.6 Ma during which time Earth’s climate was strongly influenced by bi- polar glaciation and the genus Homo first appeared and evolved. The base of the Quaternary System/Period and Pleistocene Series/Epoch is defined by the GSSP for the Gelasian Stage at Monte San Nicola section in Italy. The base of the Holocene Series/Epoch is defined at a depth of 1492.45 m in the NGRIP ice core from Greenland, with an age based on annual layer counting, of 11 700 years b2k (before AD2000), with a 2s uncertainty of 99 years; it is the first and only GSSP to be defined in an ice core.

The effects of secular disequilibrium on (U-Th)/He systematics and dating of Quaternary volcanic zircon and apatite

The (U–Th)/He dating method applied to U-rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions. However, in phases with crystallization ages less than ∼1 Ma, chemical fractionation within the ^(238)U decay series may modify the He ingrowth rate, causing He ages computed from the secular equilibrium age equation to be incorrect. The resulting systematic error depends on the [^(230)Th/^(238)U] activity ratio of the dated phase when it is erupted, and on the eruption age. Zircons, which exclude Th relative to U, will likely have secular equilibrium He ‘ages’ that underestimate the eruption age by up to a few tens of %, decreasing with increasing eruption age. Apatites tend to accommodate U and Th with little fractionation, so apatite secular equilibrium He ages will be nearly concordant with eruption age. If minerals are erupted immediately after crystallization, the disequilibrium effect can be reasonably accounted for based on Th/U systematics. However, crystals are likely to reside for unknown but potentially long periods in a magma chamber, such that the degree of secular disequilibrium will be reduced prior to the onset of He accumulation. (U–Th)/He analyses of co-genetic phases that fractionate the U/Th ratio differently, like apatite and zircon, can be used to better constrain eruption age, as well as to provide insights into magma chamber residence time. We illustrate this approach with (U–Th)/He analyses of zircons and apatites of the Pleistocene-age Rangitawa Tephra, New Zealand.

Luminescence chronology of loess‐paleosol sequences from Canterbury, South Island, New Zealand

Abstract The extensive Quaternary loess‐paleosol deposits of South Island, New Zealand, represent one of the major proxy records of paleoclimatic changes in the Southern Hemisphere. We attempted to produce the first numeric chronology of these subaerial sequences in the Canterbury region by using thermoluminescence and infrared‐stimulated luminescence dating methods. We examined five exposures: a 6 m thick section at Cust, north of Christchurch; two thicker (c. 14 m) sequences on Banks Peninsula (Barrys Bay and Onawe sites); farther south, a c. 12 m sequence in Timaru; and a c. 7 m sequence on the coast at the Normanby site near Timaru. Our results are largely based on single experiments per sample, and therefore provide imprecise ages for several of the older samples. The most satisfactory results are those from the youngest site (Cust), for which three samples were dated. Here, phases of maximum loess deposition are dated at 73 ± 13 ka (basal loess‐paleosol unit L3), 41 ± 5 ka (basal L2), and 27 ± 3 ka (basal LI). At Barrys Bay an age of 70 ± 15 ka was obtained in the basal LI, and at Timaru two separate samples in the base of LI also yielded ages of c. 70 ka, thus correlating the entire Cust loess sequence with the LI loess unit at these two other study sites. Only at Barrys Bay were ages (c. 130–250 ka) in stratigraphic order obtained for older samples (units L2 and deeper). At the other sites, some samples in the sub‐L1 units gave age reversals, and some (including the oldest sample at Barrys Bay) yielded poor precision (e.g., 20%). Units L2 at Timaru and Barrys Bay may correlate to all or part of MIS 6; however, the poor precision and some age reversals in other units at these sites and at Normanby and Onawe preclude any unambiguous correlations between sites or to the MIS time‐scale. Nevertheless, in the absence of any prior numeric ages, these first results serve as a basis for more precise future dating of these units. Although these reconnaissance dating results illustrate some of the problems for luminescence dating of such sequences in South Island, they do provide a beginning for a more accurate correlation of terrestrial and terrestrial‐marine sedimentary sequences in this part of the Southern Hemisphere.

Revised stratigraphy of the Blanchetown Clay, Murray Basin: age constraints on the evolution of paleo Lake Bungunnia

Paleo Lake Bungunnia covered more than 40 000 km2 of southern Australia during the Plio-Pleistocene, although the age and origin of the lake remain controversial. The Blanchetown Clay is the main depositional unit and outcrop at Nampoo Station in far-western New South Wales provides the most continuous lacustrine section preserved in the basin. Here the Blanchetown Clay represents the maximum lake fill and comprises: (i) a basal well-sorted sand with interbedded clay (Chowilla Sand), representing initial flooding at the time of lake formation; (ii) a thick sequence of green-grey clay comprised dominantly of kaolinite and illite, with the apparently cyclic occurrence of illite interpreted to represent cool and dry glacial climatic intervals; and (iii) a 2.6 m-thick sequence of finely laminated silt and silty clay, here defined as the Nampoo Member of the Blanchetown Clay. New magnetostratigraphic data constrain the age of the oldest lake sediments to be younger than 2.581 Ma (Matuyama–Gauss boundary) and probably as young as 2.4 Ma. This age is significantly younger than the age of 3.2 Ma previously suggested for lake formation. The youngest Blanchetown Clay is older than 0.781 Ma (Brunhes–Matuyama boundary) and probably as old as 1.2 Ma. The Nampoo Station section provides a framework for the construction of a regional Plio-Pleistocene stratigraphy in the Murray Basin.

Landscape and regolith features related to Miocene leucitite lava flows, El Capitan northeast of Cobar, New South Wales

Outcrops of leucitite lavas occur as scattered remnants up to 40 m thick in the El Capitan area, northeast of Cobar in western New South Wales. Two eruption sites have been located for these lavas. Preserved volcanic features indicate that the lavas were erupted on to a relatively low-relief, Early Miocene land surface, flowed along a shallow valley and underwent inflation. Geochemical analyses of the leucitites indicate only limited fractionation. Remnant outcrops of the leucitite lavas represent a very important time marker in the geomorphic history of the Cobar region, preserving evidence of both Early Miocene and post-Early Miocene landscape evolution and weathering conditions. A deep-weathering profile, similar to those common throughout the region and characterised by a ferruginous mottled zone and underlying bleached saprolite, is preserved beneath a dissected flow at one eruption site. Other deposits beneath the leucitite flows include baked soils, silcretes, and quartz-rich gravels and grits. Palaeomagnetic dating of the upper part of the deep-weathering profile indicates an Early to Middle Miocene age for hematite fixation. A new 40Ar/39Ar age on the volcanic plug at this site (17.14 ± 0.20 Ma, 2σ) refines the minimum eruption age for the leucitites and supports an Early Miocene age for ferruginisation of the deep-weathering profile. Topographic inversion of the basal contact of one of the leucitite flows indicates an average minimium erosion rate for the area of 1 m per million years. Weathering profiles on the leucitites are thin and lack significant ferruginisation or chemical leaching, indicating that post-Early Miocene weathering in the region has been very limited. These profiles also contain a significant aeolian component including abundant quartz dust.

Lake George revisited: New evidence for the origin and evolution of a large closed lake, Southern Tablelands, NSW, Australia

Lake George contains the longest continuous sedimentary record of any Australian lake basin, but previous age models are equivocal, particularly for the oldest (pre-Quaternary) part of the record. We have applied a combination of cosmogenic nuclide burial dating, magnetostratigraphy and biostratigraphy to determine the age of the basal (fluvial) unit in the basin, the Gearys Gap Formation. Within the differing resolutions achievable by the three dating techniques, our results show that (i) the Gearys Gap Formation, began accumulating at ca 4 Ma, in the early Pliocene (Zanclean), and (ii) deposition had ceased by ca 3 Ma, in the mid late Pliocene (Piacenzian). Whether the same age control provides an early Pliocene (Zanclean) age for the formation of the lake basin is uncertain. During the Piacenzian, the vegetation at the core site was a wetland community dominated by members of the coral fern family Gleicheniaceae, while the surrounding dryland vegetation was a mix of sclerophyll and temperate rainforest communities, with the latter including trees and shrubs now endemic to New Guinea–New Caledonia and Tasmania. Mean annual rainfall and temperatures are inferred to have been ∼2000–3000 mm, although probably not uniformly distributed throughout the year, and within the mesotherm range (>14°C <20°C), respectively. Unresolved issues are: (1) Does the basal gravel unit predate uplift of the Lake George Range and therefore provide evidence that one of the proposed paleo-spillways of Lake George, that above Gearys Gap, has been elevated up to 100–200 m by neotectonic activity over the past 4 million years? (2) Did a shallow to deepwater lake exist elsewhere in the lake basin during the Pliocene?

Paleomagnetic evidence for periods of intense oxidative weathering, McKinnons mine, Cobar, New South Wales

The McKinnons gold mine, ∼35 km southwest of Cobar in New South Wales is a non-operational opencut mine with extensive regolith profile development down to a depth of ∼80 m. Twenty-three oriented paleomagnetic samples, from saprolite at different levels through the profile, yielded two well-defined paleomagnetic poles, one of Paleocene age (60 ± 10 Ma), and the other of Miocene age (15 ± 4 Ma). These reflect two periods of remagnetisation associated with deep, intense, oxidative weathering under humid conditions. They provide important time constraints for the history of climate change, weathering and landscape evolution of the Cobar region.

Magnetic properties and particle-size analysis of dust-storm samples collected in Lanzhou and Sydney

Measurements of magnetic properties and particle size were carried out for dust-storm samples collected from Sydney (Australia) on 23 September 2009 and from Lanzhou (China) on April 2010. The results show that the magnetic mineral content of the Lanzhou sample is much higher than that of the Sydney sample. Magnetite, maghemite and possibly hematite occur in the Lanzhou sample. In addition to these minerals, there is also goethite in the Sydney sample. The magnetic grainsize of the Sydney sample is finer than that of the Lanzhou sample. These magnetic property differences are probably related to the source materials and anthropogenic particles. As to particle-size distribution, the Sydney sample shows a wide and flat curve with four peaks, while the Lanzhou sample displays a narrow curve with three peaks. The multi-peak particle-size distribution curves for the dust-storm samples indicate that there are multiple dust sources. Furthermore, a comparison of particle size between the Lanzhou storm sample and Jiuzhoutai loess/paleosol samples was also carried out. The results show that an increase in the 1–10 µm component and decrease in the 10–20 µm component of the loess/paleosol samples are mainly caused by pedogenic process (i.e. physical and chemical weathering of unstable minerals), while the increase in the 0.02–1 µm component is principally attributed to the formation of new minerals and the weathering of unstable minerals. The similarity of particle-size distribution curves between dust-storm and loess/paleosol samples implies that modern storm events are a good analogue for eolian processes operating in the Quaternary Period when loess/paleosol layers formed.

QUATERNARY STRATIGRAPHY | Overview

The general scope of stratigraphy includes all rocks, both consolidated and unconsolidated, and their organization into distinctive stratigraphic units based on their properties. Stratigraphic correlation is the process by which stratigraphic units in different places are shown to be similar in character and/or stratigraphic position. Correlation may be made on the basis of lithology, fossil content, age, or any other property, as demonstrated by the correlation of the Plio–Pleistocene boundary from its Global Stratotype Section and Point in Italy to other sections around the world. Correlation on the basis of age is dependent on evaluation of dating uncertainties and limitations. Although Quaternary stratigraphy was originally founded on the study of continental deposits, the oxygen isotope stratigraphy of deep-sea cores is the template against which almost all other stratigraphic records are now compared. Astronomical calibration provides an accurate and precise chronology of the marine isotope record. Examples of marine–terrestrial correlation are given from Wanganui Basin, New Zealand, and elsewhere in Australasia, as well as from China and western Europe.

Combining geochemistry and geochronology of transported regolith to reveal bedrock-hosted mineralization in the arid east Wongatha area of south central Western Australia

Metal anomalies in transported regolith that overlie bedrock-hosted mineralization indicate that a component of mineralization (the exogenic component) can migrate through regolith. In the east Wongatha area of Western Australia, the exogenic component in the fine fraction of sandplain deposits is spatially linked to known and/or inferred bedrock-hosted Au mineralization. In three regolith profiles, the concentration of Au in aqua regia and deionized water Au are correlated, and in two of the profiles Au varies in concentration independent of changes in regolith composition. In one profile, the Au concentration in chemically-mature regolith dominated by quartz sand decreases from 31 ppb at c. 180 cm depth to 7.3 ppb at 15 cm. Optically stimulated luminescence (OSL) ages of stratigraphically-controlled regolith samples ranging from 166.9 ± 46.6 ka to 5.4 ± 1.1 ka show a strong correlation with depth (r2 = 0.99) over a three-metre interval, indicating a sandplain accumulation rate of c. 17 mm/1000 years. The decrease in Au concentration in east Wongatha regolith can be related to the migration rate of the exogenic component and the rate of sandplain accumulation. Supplementary material: Screening and analytical data for the < 50-µm fraction of regolith from three profile sites in the east Wongatha area are available at https://doi.org/10.6084/m9.figshare.c.4074413