Did Supercontinental Amalgamation Trigger the Cambrian Explosion

A global overview of sediment patterns and accumulation rates, and carbon, strontium, and neodymium isotopes confirms that increasing rates of subsidence and uplift accompanied the dramatic radiation of animal life through the Neoproterozoic-Cambrian interval (ca. 600 to 500 Ma). Peritidal carbonate platforms were drowned, to be followed in places by phosphorites and black shales, while thick evap-orites accumulated in interior basins. This drowning of cratons during the latest Neoproterozoic-Cambrian could have brought about major taphonomic changes. The shoreward spread of oxygen-depleted and nutrient-enriched waters favored the preservation of thin skeletons by secondary phosphate and chert in peritidal carbonates and, later, the occurrence of Burgess Shale— type preservation in deeper-water shales. The burial of event sands in rapidly subsiding basins also allowed the paradoxical preservation of deep-water Nereites ichnofacies in shallow-water sediments.

THIS CHAPTER ATTEMPTS to put the "Cambrian explosion" into the wider context of events in the lithosphere. The formation and later rapid extensional subsidence of supercontinents in the Neoproterozoic have recently become apparent from a wide range of disciplines, including paleomagnetism, facies and fossil distributions, subsidence curves, and isotopic studies (e.g., Bond et al. 1984; Lindsay et al. 1987; Dal-ziel 1991; McKerrow et al. 1992; Derry et al. 1992, 1994). At some time before ca. 900 Ma b.p., Antarctica, Australia, Laurentia, Baltica, and Siberia appear to have been united in a Neoproterozoic supercontinent called Rodinia or Kanatia (Torsvik et al. 1996). It is possible that this may have begun to rift apart as early as 800 Ma (e.g., Lindsay and Korsch 1991; Lindsay and Leven 1996); certainly early rift successions can preserve deposits of the older, Rapitan-Sturtian glaciations (ca. 750700 Ma; Young 1995). At some point after 725 Ma, the western margins of Laurentia and Antarctica-Australia were certainly separated and moving apart (Dalziel 1992a,b; Powell et al. 1993). By ca. 600-550 Ma, Laurentia, Baltica, and Siberia were also in

the process of rifting apart (McKerrow et al. 1992; Torsvik et al. 1996), and here the rift sequences may preserve deposits of the younger, Varangerian (or Marinoan) glaciations (ca. 620-590 Ma; e.g., Young 1995).

The assembly of another supercontinent, Gondwana, also took place during the Ediacarian to Early Cambrian interval. (Ediacarian is here used to indicate that period of the Late Neoproterozoic between the Marinoan glaciation at ca. 600 Ma and the base of the Cambrian at ca. 543 Ma). This involved the amalgamation of the separate

Figure 4.1 Isotopes, sea level, fossil taphonomy, and global tectonic changes during the Ven-dian-Cambrian interval. Basic dykes in Baltica and Laurentia indicate a final phase of rifting: Tr = Troms, Norway (582 ± 30 Ma; Torsvik et al. 1996); TH = Tibbit Hill, Quebec (554 Ma; Kumara-peli et al. 1989). Latest Pan-African plutonic events may indicate the final phases of amalgamation in West Gondwana: EG = Ercall Granophyre, England (560 ± 1 Ma, U/Pb zircon; Tucker and Pharaoh 1991); Ah = Ahaggar plutons, West Africa (556 ± 12 Ma, U/Pb zircon; Betrand-Sarfati et al. 1995); Hq = granite and ignimbrite below Huqf Group, Oman (556 ± 10 Ma, Rb/Sr; Burns et al. 1994); ME = granites from the Mount Everest region, Nepal, Himalaya (550 ± 16 Ma, Rb/Sr; Ferrara et al. 1983); MG = Marystown Group volcanics, southeastern Newfoundland (552 ± 3 Ma, U/Pb zircon; Myrow and Hiscott 1993); Oz = Ourzazate volcanics, Morocco (563 ± 2.5 Ma, U/Pb zircon; Odin et al. 1983); SG = postorogenic quartz syenite, Skelton Group, Antarctica (551 ± 4 Ma, U/Pb zircon; Rowell et al. 1993); VC = Vires-Carolles granite, Brioverian France (540 ± 10 Ma, U/Pb monazite; Dupret et al. 1990). Thick rock salt accumulated during rapid subsidence of extensional basins: A = Ara Salt Formation, Oman (Burns and Matter 1993; Loosveld et al. 1996); H = Hormuz Salt Formation, Iran (Brasier et al. 1990; Husseini and Hus-seini 1990). Burgess Shale-type faunas are confined to the medial Lower to Middle Cambrian (Butterfield 1996). Phosphatic sediments with early skeletal fossils first appear in the transition to more rapid subsidence and/or flooding of the platforms (sources cited in figures 4.2 and 4.3). £Nd(t) data recalculated from Thorogood 1990, using revised ages. The carbon isotope curve is composite, compiled from the Vendian of southwestern Mongolia (Brasier et al. 1996), Early to Middle Cambrian of Siberian Platform (Brasier et al. 1994), and Middle to Upper Cambrian of the Great Basin, USA (Brasier 1992b). The strontium isotope curve is based on least-altered samples (compiled from Burke et al. 1982; Keto and Jacobsen 1987; Donnelly et al. 1988, 1990; Derry et al. 1989, 1992, 1994; Narbonne et al. 1994; Nicholas 1994, 1996; Smith et al. 1994; Brasier et al. 1996). The sea level curve is based on data in Brasier 1980, 1982, and 1995; Notholt and Brasier 1986; Palmer 1981; and Bond et al. 1988.

crustal blocks of Avalonia, Europa, Arabia, Africa, Madagascar, South America, and Antarctica (together forming West Gondwana) and resulted in the compressional Pan-African orogeny, which culminated between ca. 560 and 530 Ma. Orogenic closure of the Pan-African compressional basins was accompanied in many places by igneous intrusions. In figure 4.1, we have plotted some of the youngest dated phases of igneous activity, as well as the riftogenic dyke swarms of Laurentia. Although geologic evidence indicates that East Gondwana (India, South China, North China, Australia) collided with West Gondwana along the Mozambique suture between ca. 600 and 550 Ma, recent paleomagnetic evidence has also suggested that final amalgamation did not take place until the Early Cambrian (Kirschvink 1992; Powell et al. 1993).

Pan-African amalgamation of Gondwana appears to have been accompanied by the widespread development of subsiding foreland basins, as documented in figures 4.14.3. Sediments of "rift cycle 1" (sensu Loosveld et al. 1996) begin with the Sturtian Ghadir Mangil glaciation in Arabia, dated to ca. 723 Ma (Brasier et al. 2000). The development of thick salts in the Ara Formation, once thought to be rift deposits of Tommotian age (Loosveld et al. 1996; Brasier et al. 1997), now appear to be foreland basin deposits of late Ediacarian age (Millson et al. 1996; Brasier et al. 2000).

Subductive margins were also developed along the borders of eastern Australia and Antarctica (e.g., Millar and Storey 1995; Chen and Liu 1996) and Mongolia (e.g., Sen-gor et al. 1993; but see also Ruzhentsev and Mossakovsky 1995) in the Early to

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