Upper Permian magnetic stratigraphy of the lower Beaufort Group, Karoo Basin

Lanci, L.; Tohver, Eric; Wilson, Anne B.; Flint, Stephen S.

doi:10.1016/j.epsl.2013.05.017

Cited by 89 publications

(107 citation statements)

References 24 publications

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“…This contrasts with an estimated duration of 3 Myr for the overlying Dicynodon AZ alone [27]. The duration of the Tapinocephalus AZ is more difficult to estimate as although U-Pb zircon ages (in situ secondary ion mass spectrometry) ranging from 264 to 268 Ma have been obtained from the lower Abrahamskraal Formation [24], these have uncertainties on the order of 2-6 Myr and are not biostratigraphically well constrained. The biochronology of the Permian Beaufort Group would therefore benefit most from more precise age constraints on the Eodicynodon AZ and the very lowest Beaufort Group strata.…”

Section: Discussionmentioning

confidence: 90%

When and how did the terrestrial mid-Permian mass extinction occur? Evidence from the tetrapod record of the Karoo Basin, South Africa

et al. 2015

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A mid-Permian (Guadalupian epoch) extinction event at approximately 260 Ma has been mooted for two decades. This is based primarily on invertebrate biostratigraphy of Guadalupian -Lopingian marine carbonate platforms in southern China, which are temporally constrained by correlation to the associated Emeishan Large Igneous Province (LIP). Despite attempts to identify a similar biodiversity crisis in the terrestrial realm, the low resolution of mid-Permian tetrapod biostratigraphy and a lack of robust geochronological constraints have until now hampered both the correlation and quantification of terrestrial extinctions. Here we present an extensive compilation of tetrapod-stratigraphic data analysed by the constrained optimization (CONOP) algorithm that reveals a significant extinction event among tetrapods within the lower Beaufort Group of the Karoo Basin, South Africa, in the latest Capitanian. Our fossil dataset reveals a 74 -80% loss of generic richness between the upper Tapinocephalus Assemblage Zone (AZ) and the mid-Pristerognathus AZ that is temporally constrained by a U-Pb zircon date (CA-TIMS method) of 260.259 + 0.081 Ma from a tuff near the top of the Tapinocephalus AZ. This strengthens the biochronology of the Permian Beaufort Group and supports the existence of a mid-Permian mass extinction event on land near the end of the Guadalupian. Our results permit a temporal association between the extinction of dinocephalian therapsids and the LIP volcanism at Emeishan, as well as the marine end-Guadalupian extinctions.

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Section: Discussionmentioning

confidence: 90%

When and how did the terrestrial mid-Permian mass extinction occur? Evidence from the tetrapod record of the Karoo Basin, South Africa

et al. 2015

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“…This approach differs from the method used by Lanci et al (2013), where interpreted ash ages were calculated from all zircons between 300 Ma and 250 Ma and included all ages within a dispersed range of up to 20 million years. Results from previous geochronology studies on tuffs within the Karoo Supergroup (Coney et al 2007;Fildani et al 2007, 2009; Lanci et al 2013;Rubidge et al 2013), tuffs within correlative basins (Rocha-Campos et al 2006Spalletti et al 2008;López-Gamundi et al 2013;Domeier et al 2011), and Permo-Triassic crustal plutonic rocks (Pankhurst et al 2014) in South America contained populations of inherited grains. Based on this known inheritance, indiscriminately including these older zircons in weighted age calculations is not a favourable approach to obtain reasonable maximum depositional ages for the Karoo tuffs.…”

Section: U-pb Results and Interpretationmentioning

confidence: 99%

“…Five samples were collected from the Tanqua depocentre in the western Karoo Basin and were integrated with previously published results from Fildani et al (2007Fildani et al ( , 2009) and Lanci et al (2013). Four new samples were collected from the southwestern Laingsburg depocentre to augment previously published data in the Laingsburg area (Fildani et al 2007(Fildani et al , 2009).…”

Section: U-pb Methods and Analysesmentioning

confidence: 99%

“…Therefore, the traditional assumption that zircon grains within tuffs are dominantly co-eruptive is at times flawed. Within the Karoo Supergroup of South Africa, multiple U-Pb zircon studies have resolved complex spectra of ages (Coney et al 2007;Fildani et al 2007Fildani et al , 2009Lanci et al 2013;Rubidge et al 2013), leading to conflicting age interpretations for the absolute, maximum depositional age of the strata. This problem makes the Karoo strata ideal to test commonly held assumptions that zircon within volcanic tuffs is dominantly co-eruptive and that these ages represent the depositional age of sedimentary strata.…”

Section: Introductionmentioning

confidence: 99%

“…To circumvent limited biostratigraphic age controls with large uncertainties for the depositional age of the Karoo strata, recent studies have attempted to constrain the absolute age of the vertebrate fossil-defined PermianTriassic boundary within the Karoo Supergroup strata using U-Pb geochronology on zircon from interbedded volcanic tuffs (Coney et al 2007;Fildani et al 2007Fildani et al , 2009), similar to the approach used to define the age of the GSSP Permian-Triassic boundary from tuffs within fossiliferous marine strata in southern China (252.28 ± 0.08 Ma; Shen et al 2011). Fourteen volcanicash zircon U-Pb ages from the Beaufort Group in the central and southwestern Karoo Basin have yielded wholly Permian maximum depositional ages ranging from 266 to 252 Ma (Coney et al 2007;Rubidge et al 2013;Lanci et al 2013). Zircons from 16 tuff samples from the underlying Ecca Group exposed in the southwestern Karoo Basin contain 12 Triassic and 193 Permian grains and yield maximum depositional ages ranging from 275 to 250 Ma (Fildani et al 2007(Fildani et al , 2009.…”

Section: Introductionmentioning

confidence: 99%

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U-PB zircon tuff geochronology from the Karoo Basin, South Africa: implications of zircon recycling on stratigraphic age controls

McKay

Weislogel

Fildani³

et al. 2015

International Geology Review

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Along the >650 km long southern margin of the Karoo Basin in South Africa, we traversed four evenly spaced stratigraphic transects and collected 22 samples of volcanic, air-fall tuffs thought to be distal deposits derived from the Permian-Triassic Southern Gondwanan volcanic arc. We present 469 new U-Pb zircon ages determined by sensitive high-resolution ion microprobe reverse geometry (SHRIMP-RG) at the Stanford-USGS Microanalytical Center in order to constrain the maximum depositional ages for the southern Karoo Basin strata. Weighted means of these youngest coherent zircon populations were selected to maximize the number of analyses while minimizing the mean square weighted deviation (MSWD) to increase the robustness and decrease the influence of Pb-loss and inheritance in determining the maximum depositional age. Maximum depositional ages for the marine Ecca Group range from 250 to 274 Ma, whereas in the conformably overlying terrestrial Beaufort Group maximum depositional ages ranged from 257 to 452 Ma. Across the southern Karoo Basin, the Ecca Group tuffs produce maximum depositional ages that young upward; however, the Beaufort Group tuffs yield maximum depositional ages that are geochronologically out of sequence. Furthermore, maximum depositional ages of the Beaufort Group tuffs are consistently older than ash ages within the underlying marine strata. Our results are supported by previously published U-Pb tuff zircon geochronology in the Karoo Basin and demonstrate that the presence of out-of-sequence, older tuff ages are repeatable in Beaufort Group tuffs along the southern margin of the basin. We propose that tuffs in the Karoo Basin are correlative with tuffs in southern South America, and that the age spectra of these tuffs were influenced by magmatic crustal recycling. We use these data to highlight the complexity of U-Pb zircon datasets from tuffs, address the use of U-Pb zircon ages to provide absolute age controls, and discuss the implications of these new age controls on the Permian-Triassic Karoo strata.

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Magnetostratigraphic constraints on the age of the lower Beaufort Group, western Karoo basin, South Africa, and a critical analysis of existing U‐Pb geochronological data

Tohver

Lanci

Wilson

et al. 2015

Geochem Geophys Geosyst

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We studied three partially overlapping sections with a composite thickness of ∼600 m in the upper Permian fluvial siltstones and fine‐grained sandstones of the Abrahamskraal Formation, the basal unit of the Beaufort Group, in the Karoo Basin of Western Cape Province, South Africa. Paleomagnetic analysis reveals three components of Natural Remanent Magnetization (NRM). Heating to ∼180°C removes a remanent magnetization parallel to the present‐day field, which is interpreted as a viscous overprint. An intermediate unblocking temperature component is removed by heating to 450°C; this direction is always of normal polarity and is identical to a regional overprint imparted during the Early Jurassic emplacement of the Karoo Large Igneous Province. A high‐temperature component isolated above 450°C is of dual polarity and is interpreted as primary on the basis of a positive reversals test. The virtual geomagnetic pole position for the Abrahamskraal Formation computed from the average high‐temperature characteristic remanent magnetization direction is in agreement with the late Permian directions for stable Gondwana and with previous results from the lowermost Abrahamskraal Formation and Waterford Formation at the Ouberg Pass section. The predominantly normal polarity of this magnetization is in agreement with either a middle‐late Lopingian age (ca. 254–256 Ma) or a late Guadalupian age (ca. 262 Ma) according to the global geomagnetic polarity time scale. We integrate these new results with existing magnetostratigraphic, biostratigraphic, and geochronological results from the Karoo Basin, with particular emphasis on the controversy over zircon age data reported from the underlying Ecca Group.

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Upper Permian magnetic stratigraphy of the lower Beaufort Group, Karoo Basin

Cited by 89 publications

References 24 publications

When and how did the terrestrial mid-Permian mass extinction occur? Evidence from the tetrapod record of the Karoo Basin, South Africa

When and how did the terrestrial mid-Permian mass extinction occur? Evidence from the tetrapod record of the Karoo Basin, South Africa

U-PB zircon tuff geochronology from the Karoo Basin, South Africa: implications of zircon recycling on stratigraphic age controls

Magnetostratigraphic constraints on the age of the lower Beaufort Group, western Karoo basin, South Africa, and a critical analysis of existing U‐Pb geochronological data

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