The timing of the Cape Orogeny: New 40 Ar/ 39 Ar age constraints on deformation and cooling of the Cape Fold Belt, South Africa

Hansma, Jeroen; Tohver, Eric; Schrank, Christoph; Jourdan, Fred; Adams, David

doi:10.1016/j.gr.2015.02.005

Cited by 60 publications

(40 citation statements)

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“…New 40 Ar/ 39 Ar data reported by Hansma et al . [] from shear zones along strike from Laingsburg indicate that this section of the fold‐thrust belt was active between 260 and 275 Ma. This range of deformation ages provides a minimum constraint of ca.…”

Section: Discussionmentioning

confidence: 99%

“…In the late Paleozoic, deformation and orogenic uplift along this margin has been linked by some authors to sedimentary deposition into the retro-arc foreland basins, i.e., the Karoo Supergroup of the Karoo Basin [Lock, 1978;Winter, 1984;de Wit and Ransome, 1992]. The age of deformation in the Western Cape Fold Belt is constrained by new 40 Ar/ 39 Ar muscovite ages of 275-260 Ma [Hansma et al, 2015]. These new age data provide a firm basis for tectonostratigraphic correlation with the foreland basin deposits, calling for better age resolution of the Karoo Supergroup itself.…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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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“…10 Here the basin is bound by a narrow zone of deformation known as the Cape Fold Belt (CFB). 13 KLIP intrusions and deformation associated with the CFB distinguishes the Main Karoo basin from other well-known shale gas basins in the world.…”

Section: Introductionmentioning

confidence: 95%

Deflating the shale gas potential of South Africa’s Main Karoo basin

Kock¹,

Beukes²,

Adeniyi³

et al. 2017

S. Afr. J. Sci.

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The Main Karoo basin has been identified as a potential source of shale gas (i.e. natural gas that can be extracted via the process of hydraulic stimulation or ‘fracking’). Current resource estimates of 0.4–11x109 m3 (13–390 Tcf) are speculatively based on carbonaceous shale thickness, area, depth, thermal maturity and, most of all, the total organic carbon content of specifically the Ecca Group’s Whitehill Formation with a thickness of more than 30 m. These estimates were made without any measurements on the actual available gas content of the shale. Such measurements were recently conducted on samples from two boreholes and are reported here. These measurements indicate that there is little to no desorbed and residual gas, despite high total organic carbon values. In addition, vitrinite reflectance and illite crystallinity of unweathered shale material reveal the Ecca Group to be metamorphosed and overmature. Organic carbon in the shale is largely unbound to hydrogen, and little hydrocarbon generation potential remains. These findings led to the conclusion that the lowest of the existing resource estimates, namely 0.4x109 m3 (13 Tcf), may be the most realistic. However, such low estimates still represent a large resource with developmental potential for the South African petroleum industry. To be economically viable, the resource would be required to be confined to a small, well-delineated ‘sweet spot’ area in the vast southern area of the basin. It is acknowledged that the drill cores we investigated fall outside of currently identified sweet spots and these areas should be targets for further scientific drilling projects.

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“…275-260 Ma, and the second at ca. 255-245 Ma (Hansma et al, 2016). The southern Cape Fold Belt is associated with low temperature metamorphism and deformation folds that decrease in amplitude northward toward the Karoo foreland basin (Paton et al, 2006).…”

Section: Geologic Backgroundmentioning

confidence: 99%

Refining the chronostratigraphy of the Karoo Basin, South Africa: magnetostratigraphic constraints support an early Permian age for the Ecca Group

Belica¹,

Tohver²,

Poyatos‐Moré

et al. 2017

Geophysical Journal International

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Summary:The Beaufort Group of the Karoo Basin, South Africa provides an important chrono-and biostratigraphic record of vertebrate turnovers that have been attributed to the End-Permian mass extinction events at ca. 252 Ma and ca. 260 Ma. However, an unresolved controversy exists over the age of the Beaufort Group due to a large dataset of published U-Pb SHRIMP zircon results that indicate a ca. 274-250 Ma age range for deposition of the underlying Ecca Group. This age range requires the application of a highly diachronous sedimentation model to the Karoo Basin stratigraphy and is not supported by published paleontologic and palynologic data. This study tested the strength of these U-Pb isotopic datasets using a magnetostratigraphic approach. Here we present a composite ~1500 m section through a large part of the Ecca Group from the Tanqua depocentre, located in the southwestern segment of the Karoo Basin. After the removal of two normal polarity overprints, a likely primary magnetic signal was isolated at temperatures above 450°C. This section is restricted to a reverse polarity, indicating that it formed during the Kiaman Reverse Superchron (ca. 318-265 Ma), a distinctive magnetostratigraphic marker for Early−Middle Permian rocks. The Ecca Group has a corresponding paleomagnetic pole at 40.8°S, 77.4°E (A95 = 5.5°). UPb SHRIMP ages on zircons are presented here for comparison with prior isotopic studies of the Ecca Group. A weighted mean U-Pb age of 269.5 ± 1.2 Ma was determined from a volcanic ash bed located in the uppermost Tierberg Formation sampled from the OR1 research core. The age is interpreted here as a minimum constraint due to a proposed Pb-loss event that has likely influenced a number of published results. A comparison with the Geomagnetic Polarity Time Scale as well as published U-Pb TIMS ages from the overlying Beaufort Group supports a ca. 290-265 Ma age for deposition of the Ecca Group.

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The timing of the Cape Orogeny: New 40 Ar/ 39 Ar age constraints on deformation and cooling of the Cape Fold Belt, South Africa

Cited by 60 publications

References 60 publications

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

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

Deflating the shale gas potential of South Africa’s Main Karoo basin

Refining the chronostratigraphy of the Karoo Basin, South Africa: magnetostratigraphic constraints support an early Permian age for the Ecca Group

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