Since Eldridge Moores (1991) proposed the "SWEAT" hypothesis--that Antarctica was the Neoproterozoic conjugate rift margin to western North America--many continental reconstructions for Late Proterozoic time have been proposed (Hoffman, 1991; Kirschvink, 1992a; McKerrow et al., 1992; Dalziel, 1992; Dalziel et al., 1994). Some models (e.g., those by Hoffman and Dalziel) rely heavily on pre-existing tectonic units, such as the ~1-Ga "Grenvillian" orogenic belts, as "piercing points" between reconstructed continental margins. In addition, two unpaired passive margins of the same age (in this case Vendian-Cambrian) can be rejoined to a supposed pre-rift state. Unfortunately, the solutions to this problem are non-unique; if a Late Proterozoic supercontinent (Rodinia) existed as many now suspect, we should find many penecontemporaneous orogenic belts contributing to its formation and many similarly-aged passive margins resulting from its demise.

Paleomagnetism and geochronometry of Neoproterozoic rocks can position the continents to within about +/-5° latitude and +/-10 Ma. Many rocks of this age seem well enough preserved to retain primary magnetization. At present, however, most Precambrian paleomagnetic studies have not proven that magnetizations are primary, i.e., the same age as the rock; and some Late Proterozoic continental blocks (e.g., Rio de la Plata craton) have not been sampled at all (Van der Voo, 1993). Broadening the paleomagnetic database with "anchor" paleopoles from various cratons is now the most efficient step toward reconstructing the Neoproterozoic world.

Last year Joe Kirschvink and I submitted a successful three-year proposal to determine the positions of four Neoproterozoic continents (Laurentia, Baltica, East Gondwanaland, and Kalahari) to test the validity of the various reconstructions. We began work in early 1995, beginning with sample acquisition. Emphasis is placed on proving primary magnetizations, through fold, conglomerate, baked-contact, and reversals tests. In some cases this may discover that established paleomagnetic poles from previous studies are only remagnetizations. The reversals test, as applied to stratigraphic sequences, can also lead to magnetic stratigraphy which has been used to correlate Cambrian sequences among different basins (Kirschvink et al., 1991). Magnetostratigraphy may help correlation among Vendian assemblages as well.

Preliminary results from the Lower Cambrian Puerto Blanco Formation near Caborca, Mexico, are published in abstract only (Barr and Kirschvink, 1983). The work's value centers upon a conglomerate test of volcanic clasts, considered positive by the authors. Nonetheless, the test only involved seven samples, whose mean, albeit somewhat dispersed, is similar to the mean of the stratigraphically adjacent basalt flow-units. In March I revisited the Eocambrian section in the Sierra El Rajon and sampled 25 more clasts of the basalt in the conglomeratic facies of the Puerto Blanco Formation. When these samples are analyzed, they should readily determine a positive or negative conglomerate test. If positive, then the Puerto Blanco paleopole (at moderate paleolatitude) will provide constraint on the timing of Laurentia's equatorial return from its Vendian jaunt to the South Pole, for which there is growing evidence (Park, 1994).

Joe and I returned from a sampling expedition to South Africa and Namibia, where we sampled copiously from the Eocambrian Nama Group, comprising carbonate and siliciclastic deposits on the western margin of the Kalahari craton. Previous paleomagnetic study of these rocks (Kröner et al., 1980) failed to identify magnetization ages. I am presently analyzing samples from red mudstones of the Cambrian Fish River Subgroup, which, by comparison with contemporaneous paleopoles from Morocco and Australia (Kirschvink, 1978; Kirschvink, 1980) can test the coherence of Gondwanaland during earliest Paleozoic time. These results will be presented in November, at both the GSA annual meeting and a symposium of Proterozoic tectonics in southern Brazil.

During the summer of 1995, we continued our sampling spree in Russia, at beautifully preserved exposures of fossiliferous, Vendian siltstones along the White Sea coast. Our goal is to test the recent paleomagnetic results of Torsvik and colleagues (Geology, 1995) that situated Baltica at high latitudes, and also to erect a coordinated magneto- and bio-stratigraphy for the Baltic Vendian. This may aid in intercontinental correlation of Vendian deposits.

An exciting corollary of the paleomagnetic project involves Neoproterozoic tillites, which may be useful as global chronostratigraphic markers, providing "snapshots" of Earth during Rapitan-Chuos-Nantuo-Sturtian (~750 Ma) and Ice Brook-Numees-Varanger-Marinoan (~620 Ma) times. Unfortunately, global synchroneity of Neoproterozoic glaciogenic deposits, however permissable given existing isotopic data, has yet to be demonstrated conclusively. Furthermore, tillites may have inherent problems of broad scatter of paleomagnetic directions from samples at a single locality, as indicated by a preliminary study of the Chuos diamictite in northern Namibia (Evans et al., 1994). Nonetheless, direct paleomagnetic study of Neoproterozoic tillites can determine the severity of these glacial episodes, an issue that has been spurred by a recent debate of the Snowball Earth model (Kirschvink, 1992b; Meert and Van der Voo, 1994; Williams et al., 1995; Schmidt and Williams, 1995).

Questions of extreme climatic fluctuations in Earth history extend to the Early Proterozoic, whence glaciogenic deposits have been found in North America, South Africa, and Europe. In conjunction with Prof. Nic Beukes at Rand Afrikaans University in Johannesburg, we have conducted paleomagnetic studies of the Transvaal Supergroup in the Northern Cape Province of South Africa. A paleolatitude of ~15° for the basaltic-andesitic Ongeluk Formation, we believe, applies to the underlying Makganyene diamictite (ms in prep.). The diamictite contains faceted and striated clasts, indicative of ice contact. Widespread extent of these deposits throughout the Kaapvaal craton (Visser, 1971) demonstrate that these deposits formed near the terminus of a large ice sheet. Indeed, existing age constraints for the Huronian (Ontario) and Makganyene (South Africa) deposits permit synchroneity of a global icehouse at ~2.3-2.4 Ga. Whereas such severity has been proposed in the past (Ojakangas, 1988), this is the first paleomagnetic evidence for the Early Proterozoic Snowball Earth.

References:

Barr, T.D., and J.L. Kirschvink, 1983. The paleoposition of North America in the early Palaeozoic: New data from the Caborca sequence in Sonora, Mexico [abst]. EOS, Trans. of the A.G.U., v.64, p.689-690.

Dalziel, I.W.D., 1992. On the organization of American plates in the Neoproterozoic and the breakout of Laurentia. GSA Today, v.2, no.11, p.237, 240-241.

Dalziel, I.W.D., L.H. Dalla Salda, and L.M. Gahagan, 1994. Paleozoic Laurentia-Gondwana interaction and the origin of the Appalachian-Andean mountain system. Geological Society of America Bulletin, v.106, no.2, p.243-252.

Embleton, B.J.J., and G.E. Williams, 1986. Low paleolatitude of deposition for the late Precambrian periglacial varvites in South Australia: Implications for palaeoclimatology. Earth and Planetary Science Letters, v.79, p.419-430.

Evans, D.A., J.L. Kirschvink, and J.W. Holt, 1994. Paleomagnetism of the Chuos Formation, Otavi Mountains, Namibia [abst]. Windhoek conference on Proterozoic tectonics and metallogeny, Aug 29-Sept 1.

Evans, D. A., A. Yu. Zhuravlev, C.J. Budney, and J.L. Kirschvink, 1996. Paleomagnetism of the Bayan Gol Formation, western Mongolia. Geological Magazine, in review.

Hambrey, M.J., and W.B. Harland, 1981. Earth's Pre-Pleistocene Glacial Record. Cambridge Univ. Press, U.K., 1004pp.

Hoffman, P.F., 1991. Did the breakout of Laurentia turn Gondwanaland inside out? Science, v.252, p.1409-1412.

Kirschvink, J.L., 1978. The Precambrian-Cambrian boundary problem: Paleomagnetic directions from the Amadeus Basin, central Australia. Earth and Planetary Science Letters, v.40, p.91-100.

Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of paleomagnetic data. Geophysical Journal of the Royal Astronomical Society, v.62, p.699-718.

Kirschvink, J.L., 1992a. A paleogeographic model for Vendian and Cambrian time. In: J.W. Schopf and C. Klein, eds., The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge Univ. Press, p.569-581.

Kirschvink, J.L., 1992b Late Proterozoic low-latitude global glaciation: The Snowball Earth. Ibid., p.51-52.

Kirschvink, J.L., M. Magaritz, R.L. Ripperdan, A.Yu. Zhuravlev, and A.Yu. Rozanov, 1991. The Precambrian-Cambrian boundary: Magnetostratigraphy and carbon isotopes resolve correlation problems between Siberia, Morocco, and South China. GSA Today, v.1, no.4, p.69-91.

Kröner, A., M.O. McWilliams, G.J.B. Germs, A.B. Reid, and K.E.L. Schalk, 1980. Paleomagnetism of Late Precambrian to Early Paleozoic mixtite-bearing formations in Namibia (South West Africa): The Nama Group and Blaubeker Formation. American Journal of Science, v.280, p.942-968.

McKerrow, W.S., C.R. Scotese, and M.D. Brasier, 1992. Early Cambrian continental reconstructions. Journal of the Geological Society, London, v.149, p.599-606.

Meert, J.G., and R. Van der Voo, 1994. The Neoproterozoic (1000-540 Ma) glacial intervals: No more snowball earth? Earth and Planetary Science Letters, v.123, p.1-13.

Moores, E.M., 1991. Southwest U.S.-East Antarctic (SWEAT) connection: A hypothesis. Geology, v.19, no.5, p.425-428.

Ojakangas, R.W., 1988. Glaciation: An uncommon "mega-event" as a key to intracontinental and intercontinental correlation of Early Proterozoic basin fill, North American and Baltic cratons. In: K.L. Kleinspehn and C. Paola, eds., New Perspectives in Basin Analysis. Springer-Verlag, New York, p.431-444.

Park, J.K., 1994. Palaeomagnetic constraints on the position of Laurentia from middle Neoproterozoic to Early Cambrian times. Precambrian Research, v.69, p.95-112.

Schmidt, P.W., and G.E. Williams, 1995. The Neoproterozoic climatic paradox: Equatorial palaeolatitude for Marinoan glaciation near sea level in South Australia. Earth and Planetary Science Letters, v.134, p.107-124.

Van der Voo, R., 1993. Paleomagnetism of the Atlantic, Tethys and Iapetus Oceans. Cambridge Univ. Press, U.K., 411pp.

Visser, J.N.J., 1971. The deposition of the Griquatown Glacial Member in the Transvaal Supergroup. Transactions of the Geological Society of South Africa, v.74, p.187-199.

Williams, G.E., P.W. Schmidt, and B.J.J. Embleton, 1995. Comment on 'The Neoproterozoic (1000-540 Ma) glacial intervals: No more snowball earth?' by Joseph G. Meert and Rob van der Voo. Earth and Plan. Sci. Letters, v.131, p.115-122.