Did Supercontinental Amalgamation
Trigger the “Cambrian Explosion”?
Martin D. Brasier and John F. Lindsay
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 NeoproterozoicCambrian interval (ca. 600 to 500 Ma). Peritidal carbonate platforms were drowned, to be followed in places by phosphorites and black shales, while thick evaporites 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; Dalziel 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. 750– 700 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