Michiel O. de Kock

Timing and tempo of the Great Oxidation Event

Significance We present U-Pb ages for the extensive Ongeluk large igneous province, a large-scale magmatic event that took place near the equator in the Paleoproterozoic Transvaal basin of southern Africa at ca. 2,426 Ma. This magmatism also dates the oldest Paleoproterozoic global glaciation and the onset of significant atmospheric oxygenation. This result forces a significant reinterpretation of the iconic Transvaal basin stratigraphy and implies that the oxygenation involved several oscillations in oxygen levels across 10−5 present atmospheric levels before the irreversible oxygenation of the atmosphere. Data also indicate that the Paleoproterozoic glaciations and oxygenation were ushered in by assembly of a large continental mass, extensive magmatism, and continental migration to near-equatorial latitudes, mirroring a similar chain of events in the Neoproterozoic. The first significant buildup in atmospheric oxygen, the Great Oxidation Event (GOE), began in the early Paleoproterozoic in association with global glaciations and continued until the end of the Lomagundi carbon isotope excursion ca. 2,060 Ma. The exact timing of and relationships among these events are debated because of poor age constraints and contradictory stratigraphic correlations. Here, we show that the first Paleoproterozoic global glaciation and the onset of the GOE occurred between ca. 2,460 and 2,426 Ma, ∼100 My earlier than previously estimated, based on an age of 2,426 ± 3 Ma for Ongeluk Formation magmatism from the Kaapvaal Craton of southern Africa. This age helps define a key paleomagnetic pole that positions the Kaapvaal Craton at equatorial latitudes of 11° ± 6° at this time. Furthermore, the rise of atmospheric oxygen was not monotonic, but was instead characterized by oscillations, which together with climatic instabilities may have continued over the next ∼200 My until ≤2,250–2,240 Ma. Ongeluk Formation volcanism at ca. 2,426 Ma was part of a large igneous province (LIP) and represents a waning stage in the emplacement of several temporally discrete LIPs across a large low-latitude continental landmass. These LIPs played critical, albeit complex, roles in the rise of oxygen and in both initiating and terminating global glaciations. This series of events invites comparison with the Neoproterozoic oxygen increase and Sturtian Snowball Earth glaciation, which accompanied emplacement of LIPs across supercontinent Rodinia, also positioned at low latitude.

New U–Pb geochronologic and palaeomagnetic constraints on the late Palaeoproterozoic Hartley magmatic event: evidence for a potential large igneous province in the Kaapvaal Craton during Kalahari assembly, South Africa

Abstract The volcanic Hartley Formation (part of the Olifantshoek Supergroup, which is dominated by red bed successions) in South Africa recorded depositional and tectonic conditions along the western Kaapvaal Craton during the late Palaeoproterozoic. It formed in association with red bed deposition elsewhere in the cratonic hinterland and along the craton’s northern margin. However, the exact correlation of the Olifantshoek Supergroup with these other red-bed successions is hindered by poor geochronological constraints. Herein, we refine the age and palaeopole of the Hartley Formation, and provide geochronological constraints for large-scale 1.93–1.91 Ga bimodal magmatism on the Kaapvaal Craton (herein named the Hartley large igneous province). We present new age constraints for the mafic and felsic phases of this event at 1923 ± 6 Ma and 1920 ± 4 Ma, respectively, which includes the first reported age dating of the Tsineng Dyke Swarm that has been linked to Hartley volcanism. A mean 1.93–1.91 Ga palaeomagnetic pole for the Hartley large igneous province at 22.7°N, 328.6°E with A95 = 11.7° represents a significant improvement on a previously published virtual geomagnetic pole. This improved pole is used to refine the late Palaeoproterozoic apparent polar wander path of the Kaapvaal Craton. This can assist in correlation of red-bed successions in southern Africa.

Palaeomagnetism of Mesoproterozoic limestone and shale successions of some Purana basins in southern India

The ‘Purana’ basins were long considered Neoproterozoic basins until geochronology and palaeomagnestism showed parts of the Chattisgarth and lower Vindhyan basins to be a billion years older. Historically, the successions in the Chattisgarth Basin are correlated with similar successions in the Pranhita–Godavari and Indravati basins. In India, differentiating between early–late Mesoproterozoic rocks and those spanning the Mesoproterozoic–Neoproterozoic boundary is possible by comparing magnetic declination and inclination; palaeomagnetism is therefore a very useful correlation tool. Here we report a new Stenian-aged palaeopole (50.1°N, 67.4°E, radius of cone of 95% confidence A 95 = 12.4°, precision K = 30.1) from carbonate and shale successions of the Pranhita–Godavari and Chattisgarth basins (the C+/– magnetization). In addition, an early diagenetic remagnetization (component A) was identified. No primary or early diagenetic magnetizations were identified from the Indravati Basin. Here, as well as in stratigraphically higher parts of the other two successions, widespread younger magnetic overprints were identified (B+ and B– magnetic components). Our C+/– palaeopole is constrained by palaeomagnetic stability field tests, is different from known 1.4 Ga and 1.0 Ga Indian palaeopoles, but similar to a 1.19 Ga palaeopole. Penganga Group (Pranhita–Godavari Basin) deposition was probably initiated at around 1.2 Ga. A similar palaeomagnetic signature confirms its correlation with the Raipur Group (Chattisgarth Basin), of which the deposition spans most of the Stenian period ( c . 1.2–1.0 Ga). Sedimentation in these groups began significantly later than c . 1.4 and c . 1.6 Ga, as suggested by ages reported from below the Raipur and Penganga groups, respectively.

Digital reconstruction of the mandible of an adult Lesothosaurus diagnosticus with insight into the tooth replacement process and diet

Fragmentary caudal ends of the left and right mandible assigned to Lesothosaurus diagnosticus, an early ornithischian, was recently discovered in the continental red bed succession of the upper Elliot Formation (Lower Jurassic) at Likhoele Mountain (Mafeteng District) in Lesotho. Using micro-CT scanning, this mandible could be digitally reconstructed in 3D. The replacement teeth within the better preserved (left) dentary were visualised. The computed tomography dataset suggests asynchronous tooth replacement in an individual identified as an adult on the basis of bone histology. Clear evidence for systematic wear facets created by attrition is lacking. The two most heavily worn teeth are only apically truncated. Our observations of this specimen as well as others do not support the high level of dental wear expected from the semi-arid palaeoenvironment in which Lesothosaurus diagnosticus lived. Accordingly, a facultative omnivorous lifestyle, where seasonality determined the availability, quality, and abundance of food is suggested. This would have allowed for adaptability to episodes of increased environmental stress.

The Timing of the Palaeoproterozoic Great Oxidation Event using Dykes, Sills and Bolcanics of the Ongeluk Large Igneous Province, Kaapvaal Craton

The timing of the Palaeoproterozoic Great Oxidation Event using Dykes, Sills and Volcanics of the Ongeluk Large Igneous Province, Kaapvaal Craton

Constraining the Timing of the Molopo Farms Complex Emplacement and Provenance of Its Country Rock

layered, mafic-ultramafic intrusion straddling the southern border of Botswana with South Africa. It does not outcrop due to Cenozoic cover, but is believed to intrude the sedimentary succession of the Neoarchean to Paleoproterozoic Transvaal Supergroup. This is based on numerous intersections in exploration drillcore. The emplacement of the MFC is currently poorly constrained by an unpublished Rb-Sr date of 2044 ± 24 Ma. It is however, widely considered to be related to the Bushveld Large Igneous Province or LIP. Here the U-Pb zircon provenance of sedimentary country rock to the MFC as well as U-Pb baddeleyite geochronology of the maficultramafic rocks of the MFC are reported. Samples for this study originate from eight boreholes from both the southern and northern limbs of the MFC. The youngest concordant U-Pb ages of detrital zircon grain populations within 6 analysed samples are dominated by ages between 2018 ± 39 Ma and 2276 ± 19 Ma. These do not allow for conclusively distinguishing between characteristic provenance of either the Olifantshoek Group or the Transvaal Supergroup. U-Pb ID-TIMS dating of four fractions of baddeleyite yield a free regression upper intercept age of 2052±16 Ma, with a negative lower intercept and relatively high MSWD of 4. However, one of the analyses resulted in a younger Pb/Pb date compared to other fractions. A weighted mean of all the fractions is 2052±4 Ma, while the weighted mean of the three oldest fractions is 2055±3 Ma, which illustrates this problem. Rejecting the youngest analysis gives a free regression upper intercept date of 2060±6 Ma. The lower intercept points toward discordance associated with the Karoo LIP, and lowers the MSWD to 0.36. Upon forcing regression through 180 Ma, an upper intercept date of 2057±3 Ma, with a MSWD of 0.38, is achieved. This is interpreted as representing the emplacement age for the MFC, and is within error of the age of the Bushveld Complex (i.e., 2054.4 ± 1.3 Ma). Given the baddeleyite constraints it becomes clear that the sedimentary country rock of the MFC represents the Transvaal Supergroup. What remains unresolved is the possible presence of younger mafic intrusions (e.g., dolerite sills) in the area that are currently wrongfully identified as MFC – as suggested by preliminary geochemical data. Some of these intrusions may likely intrude into sedimentary successions younger than the Transvaal Supergroup, which would explain the presence of younger U-Pb detrital zircon age populations in some of our samples. Michiel O. de Kock, Livhuwani Ravhura, Clarisa Vorster, Nicolas J. Beukes, and Ashley P. Gumsley, 2016. Constraining the Timing of the Molopo Farms Complex Emplacement and Provenance of Its Country Rock. Acta Geologica Sinica (English Edition), 90 (supp.1): 78.