Paleomagnetism of the Western Cape Fold Belt, South Africa, and Its Bearing on the Paleozoic Apparent Polar Wander Path for Gondwana

Earth and Planetary Science Letters, 84 (1987) 487-499 487 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [51 Paleomagnetism of the western Cape Fold belt, South Africa, and its bearing on the Paleozoic apparent polar wander path for Gondwana Valerian Bachtadse a,., Rob Van der Voo a and Ingo W. H~ilbich 2 1 Department of Geological Sciences, University of Michigan, Ann Arbor, 341 48109-1063 (U.S.A.) : Department of Geology, University of Stellenbosch, Stellenbosch, 7600 (South Africa) Received January 30, 1987; revised version received May 7, 1987 In order to test two different proposals for the poorly defined African Paleozoic apparent polar wander path (APWP), a paleomagnetic study was carried out on Ordovician through Carboniferous clastic sediments from the Cape Fold belt, west of the 22nd meridian. One proposal involves a relatively simple APWP connecting the Ordovician Gondwana poles in North Africa with the Late Paleozoic poles to the east of South Africa in a more or less straight line crossing the present equator in the Devonian. The other proposal adds a loop to this path, connecting Ordovician poles in North Africa with poles to the southwest of South Africa and then returning to central Africa. This loop would occur mainly in Silurian time. New results reported herein yield paleopoles in northern and central Africa for Ordovician to lowermost Silurian and Lower to Middle Devonian formations. The best determined paleopole of our study is for the Early Ordovician Graafwater Formation and falls at 28 ° N, 14°E (k = 25, 0t95 = 8.8 °, N = 28 samples). The other paleopoles are not based on sufficient numbers of samples, but'can help to constrain the apparent polar wander path for Gondwana. Our results give 0nly paleopoles well to the north of South Africa and we observe no directions within the proposed loop. Hence, if the loop is real, it must have been of relatively short duration (60-70 Ma) and be essentially of Silurian/Early Devonian age, implying very high drift velocities for Gondwana (with respect to the pole) during that interval. 1. Introduction Excluding earlier poles based on incomplete demagnetization analysis as well as results from Because the paleomagnetic data base for the possibly displaced terranes [1], there are essen- Paleozoic of Gondwana is relatively poor, many tially no reliable published poles for the 100 Ma uncertainties exist about the plate motions and interval between the Late Ordovician and the Late paleogeographic positions of this supercontinent. Devonian: the only paleomagnetically acceptable As a consequence, models for Paleozoic continent- pole is for the Mereenie Sandstone [2] but it is continent collisions involving the present Circum- very poorly dated. Even the Middle to Late De- Atlantic continents are very poorly constrained. vonian apparent polar wander path (APWP) seg- The reasons for the paucity of data are manifold, ment for Gondwana is ambiguous, with a previ- and include a scarcity in general of Paleozoic ously acceptable result for the Msissi Norite in rocks in the cratonic parts of the Gondwana conti- Morocco [3] now discarded [4], and a result from nents, more particularly a lack of formations sui- Devonian rocks of the Gneiguira Supergroup table for paleomagnetic analysis, a lack of precise arguably based on late Paleozoic remagnetizations ages for the rocks that do exist, the possibility of [5-7]. Upper Devonian carbonates from the in- widespread remagnetization, and uncertainties tracratonic Canning Basin in northwestern about the positions of non-cratonic areas, such as Australia have yielded a pole in African coordi- the eastern Australian fold belts, which have nates (on the basis of the Smith and Hallam [8] yielded several paleomagnetic results previously. reconstruction) near to the equator in central Africa, which appears to give the best estimate for * Now at: Department of Earth Sciences, University of Ox- the Late Devonian paleofield of Gondwana thus ford, Parks Road, Oxford, OX1 3PR, United Kingdom. far [7]. 488 13 ~~ PATH X the African APWP, and hence the paleogeo- -- graphic positions of Gondwana during this time. 2. Geologic setting and sampling Clastic sedimentary rocks of the Cape Super- group (7-9 km in thickness) unconformably over- lie Precambrian/Cambrian metasediments and granitic rocks with ages as young as 530 Ma [15], PATH _ and are overlain by the Carboniferous Dwyka ~/)5475 Ma __ y Formation of the Karroo sequence. The Cape Supergroup consists of the Table Mountain, the 0 '~ Bokkeveld and the Witteberg Groups. A schematic stratigraphic column is presented in Fig. 2, and a geological map with sampling sites is shown in Fig. 3. Folding is thought to be mostly of Triassic age [17,18] and fold axes are mainly paral- Fig. 1. Alternative Paleozoic apparent polar wander paths for lel to the present-day coastline of South Africa Gondwana from [12]. (a) Path X connecting the Early Paleo- (Fig. 3). Deformation was most intense in the zoic poles in northern Africa in a more or less straight line with eastern Cape Province leading to penetrative the Carboniferous and younger poles. (b) Path Y, showing a loop in the Ordovician through Late Devonian interval, super- cleavage [19], whereas the rocks in the western imposed on path X. Ages are indicated in Ma. syntax area (Fig. 3), where we sampled, have been folded to a somewhat lesser degree. It is not surprising, therefore, that mutually and conflicting APWP's have been proposed for the fences southern continents on the basis of paleomagnetic ~.brotes, fish, )morphs, data [1,9-12], or on the basis of glacial relicts [13]. s (42) Two models constitute the extremes among the different possibilities. One path connects the Ordovician paleopoles in northern Africa in a ~sc/d hsh ?D) direct line southward through the continent with m i ospor'es 31) the Late Paleozoic poles just to the east of south- oleptus (28) ern Africa (Fig. la); the other, based on glacial ophyl tu m (26) occurrences and paleomagnetic results from the tebrotes (27) East Australian fold belt, adds a south-southwest ~ody foss)ts looping hairpin as an additional complexity of the 'ebrates * path (Fig. lb). These two models have been labeled spores(2124) path X and path Y, respectively, by Morel and Irving [12]. Ring complexes in Niger, which give )ody fossils an Early Silurian age, have provided results in support of path Y, with pole positions to the west fossils (16) and southwest of southern Africa [14]. ss/ls We report here paleomagnetic results from a ssits very thick series of Paleozoic clastics in South ) isochron Africa, comprising the Table Mountain, Bokke- t5Mo (15) i veld and Witteberg Groups. Because ages span the Fig. 2. StratigraphJc column of the Paleozoic formations in interval from Ordovician to Carboniferous, it was South Africa, with sampling marked for those formations hoped that paleopoles derived from the forma- discussed in this paper. The right-hand column gives diagnostic tions in these groups might shed further light on fossil occurrences and the references for these. 489 CAPE SUPERGROUP II WITTEBERG GROUP • 30°S BOKKEVELD GROUP TABLE MTN. GROUP 30°S . 0 KILOMETRES 300 Pakhuis Pass Gydo Pass hk\~ hoed' P.as.s Wotceste Hex River Pass,~'~'~ ~Rooihogte Pas~ .34°S (~ ELIZABETH Chapman's Peak 20°E I 251°E Fig. 3. Schematic geological map with sampling sites (dots) indicated. Tankard and Hobday [20] interpreted the de- the Bokkeveld Group; see Fig. 2). Below, geologi- positional environment of the Table Mountain, cal descriptions are given for these four forma- Bokkeveld and Witteberg Groups as typical for a tions only. basin that evolved as the failed arm of a major rift The Graafwater Formation is the third oldest in system, with more or less continuous Paleozoic the Table Mountain Group. It was sampled along subsidence leading to the immense thickness of at Chapman's Peak Drive, south of Cape Town (Fig. least 7000 m in a miogeosynclinal setting. Rock 3). Trace fossils indicate an Early Ordovician age types include prominent quartz-arenites, con- [16], but the age is really only firmly bracketed by glomerates, diamictites and mudstones, as well as the basement (530 Ma or older) and the overlying shales. While they are locally hematite bearing, well-dated Upper Ordovician formations. The such generally coarse rock types do not usually rocks are red sediments of varying grain size, provide good paleomagnetic data and the finer- which were sampled at 12 stratigraphically distrib- grained strata have been preferred for sampling. uted sites, with minor tilt of the strata only. Even so, we have encountered highly unstable The Pakhuis Formation consists of gray massive magnetic behaviour in many of the formations diamictite, varying in thickness from 2 to 150 m sampled, and only four formations gave magneti- [21], which is related to the Winterhoek glaciation zations of presumably Paleozoic age (the Graaf- [15,22] that also affected the overlying Cedarberg water, Pakhuis, Cedarberg Shale Formations and Shale Formation [13]. These and coeval glacial 490 deposits elsewhere in Africa and South America gasoline-powered portable drill and oriented with indicate that the center of the ice sheets was a standard Brunton compass, with a correction for located in northwestern Africa and adjacent Brazil the local geomagnetic declination. [13,23]. The Pakhuis Formation is well dated on the basis of brachiopods, trilobites and Chitinozo- 3. Laboratory techniques ans as Upper Ordovician [24]. Fresh outcrops are rare, but 2 suitable sites were located at the type The oriented cores of 2.5 cm diameter were cut locality at Pakhuis Pass along the road between to 2.14 cm length and the uppermost more Clanwilliam and Wuppertal (Fig. 3); there the weathered portions of the cores were discarded.

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