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Difficulties in correlating the Cambrian magmatic, depositional, structural and metamorphic events along the proto-Pacific margin of East Gondwana have led to subdivision of the region into the Delamerian, Lachlan and Tyennan Orogens in Australia, the Ross Orogen in Antarctica and the Takaka Terrane in New Zealand. As a result, the Cambrian tectonic evolution of the region is poorly understood. We present here a revised lithostratigraphic section from the late Early to Mid-Cambrian rocks exposed near Stawell in western Victoria, which is used as the basis for correlating geological events in East Gondwana. These data show that the Cambrian tectonic evolution of East Gondwana's >4000km long proto-Pacific margin involved predominantly compressional orogenesis separated by major short-lived extensional events at c. 516-514 and 504-500 Ma. The most significant extensional event, at c. 516-514 Ma, involved extensive slab rollback along the proto-Pacific margin in response to major changes in global plate motions and plate-boundary stresses following the termination of East-West Gondwana collision. Partial subduction of a ribbon of buoyant continental crust led to localized subduction-zone failure and obduction of the young hot forearc lithosphere in Tasmania at c. 510 Ma. Collision of the continental ribbon also significantly modified the architecture of the proto-Pacific margin and ultimately controlled the extent of the second major extensional event associated with slab rollback at c. 504-500 Ma. Tectonic evolution of the proto-Pacific margin of East Gondwana thus involved the complex interaction between convergent-margin processes and collisional orogenesis. [ABSTRACT FROM AUTHOR]

U-Th-Pb Secondary Ion Mass Spectrometry (SIMS) data from detrital zircons extracted from the South Stack Group, Anglesey, UK, indicate that: (1) the maximum depositional age of the Holyhead Formation (South Stack Group, Monian Supergroup) is 501 ± 10 Ma; (2) the Monian Supergroup was deposited between c. 500 and 475 Ma and is part of the Cambrian-Lower Ordovician succession found in southern Britain and Ireland; (3) Avalonia was a major sediment source (age maxima at 543-552 and 604-627 Ma); (4) Amazonia probably also provided zircons (common Neoarchaean-Mesoproterozoic grains) weakening suggestions that Avalonia had rifted off Gondwana by Cambrian times. [ABSTRACT FROM AUTHOR]

The Brunovistulian was one of the first tectonic units of Central Europe to be defined as a 'terrane'. In spite of extensive studies, the Early Palaeozoic palaeogeographical position and provenance of this unit remains unclear. A palaeomagnetic study of the Lower Cambrian red beds and a study of the trilobite fauna were performed to provide constraints on the palaeogeographical position of the terrane in Early Cambrian time. Good quality palaeomagnetic data obtained from the Lower Cambrian red beds suggest the mixed nature of the Early Cambrian geomagnetic field and a nearly equatorial position (a palaeolatitude of c. 7°) of the Brunovistulian terrane in the Early Cambrian. Comparison of this palaeolatitude with existing palaeogeographical models leads to the conclusion that at this time the Brunovistulian terrane was separated by a great distance from the Avalonian margin of Gondwana. The terrane was, located within the Cadomian belt, occupying a position north of the present-day northern margin of Africa. It was coupled to the present-day southern margin of Baltica during the Cambrian, when Baltica moved along the Cadomian margin of Gondwana. Another possibility is that the Brunovistulian terrane could have remained near this margin of Baltica since Grenvillian time and was incorporated into the Cadomian orogen. [ABSTRACT FROM AUTHOR]

Terminal Neoproterozoic and Cambrian evaporite rocks worldwide record a marked sulphur isotope excursion to very heavy δ34S isotopes that reflect a period of significant burial of reduced sulphur. New δ34S data from anhydrite and pyrite of the Ara Group in Oman extend this record to Arabia. Depositional history of the Ara Group spans c. 8 Ma across the terminal Neoproterozoic Cambrian boundary. Therefore, these rocks provide a detailed record of sulphur isotopic evolution in this important time interval. Ara Group sulphur isotope values include a mean δ34Sanhydrite of 39.4‰ ± 2.6‰ (1σ range 32.4-46.4‰; n = 61), and a mean δ34Spyrite of 1.4‰ ± 7.3‰ (1σ; range -10.8‰ to 11.4‰; n = 12). The isotope signatures were not altered significantly during diagenesis and are representative of the isotopic composition of parent waters in the basin. Marine bromine concentrations in halite and marine fossils in associated carbonate rocks indicate that these waters had a marine origin. Bacterial sulphate reduction was the dominant process leading to the segregation between isotopically heavy sulphur incorporated in evaporite rocks and isotopically light sulphur buried in pyrite and organic matter. Strong sulphate reduction operated in a hypersaline and partly anoxic basin: these environmental conditions were also conducive to the preservation of organic material and make the Ara Group a prolific hydrocarbon province. Significant variation of the observed values is attributed to the inherent variability of bacterial sulphate reduction, including partially closed-system behaviour, disproportionation processes, limited diagenesis, and possibly secular variation of sulphur isotopes within the basin. Ara Group δ34Sanhydrite values overlap with data from other terminal Neoproterozoic to Cambrian... [ABSTRACT FROM AUTHOR]

During the latest Neoproterozoic to Mid-Cambrian time (580-505 Ma ago), the Earth underwent significant changes in palaeogeography that included rifting of a possible supercontinent and the near simultaneous formation of a second, slightly smaller supercontinent. It is against this tectonic backdrop that the Cambrian radiation occurred. Although the general tectonic setting during this interval is fairly well constrained, models of the exact palaeogeography are controversial because of the lack of reliable palaeomagnetic data from some of the continental blocks. Palaeogeographical models based on palaeomagnetic data range from a high-latitude configuration for most continents, to a low-latitude configuration for most continents, or to rapid oscillations in continental configurations triggered by inertial changes within the planet. Palaeobiogeographical data can also be used to help constrain palaeogeographical models. To this end we use vicariance patterns in olenellid trilobites to determine their compatibility with three end-member palaeogeographical models derived from palaeomagnetic data for the Neoproterozoic and early Cambrian. The most congruent palaeogeographical model with respect to the palaeobiogeographical data described herein is the high-latitude configuration for most continents. Those palaeomagnetic models that predict inertial interchange true polar wander or multiple episodes of true polar wander differ significantly from the results from palaeobiogeography. The low-latitude palaeogeographical models also differ from the results from palaeobiogeography, but this may partly arise because of a tack of palaeomagnetic and palaeobiogeographical data from many parts of present-day South America and Africa. [ABSTRACT FROM AUTHOR]

The stratigraphic value of trilobite burrows has recently been questioned because of a seemingly Ordovician form in the Lower Cambrian of Canada. Closer examination shows, however, that the Cambrian homeomorph ( Cruziana pectinata n. sp.) has a significantly different scratch pattern. This raises the question whether the multiple scratches of the Ordovician Cruziana rugosa were also made by exopodites or by similarly shaped endopodites. [ABSTRACT FROM AUTHOR]

As extant bilaterian phyla emerged during the Lower Cambrian, these clades acquired morphological features that separated them from their stem groups. At the same time, morphological variants on the body plan within a phylum emerged that we recognize as classes and subphyla. In many cases, the emergence of body plan variants within a phylum is associated with major changes in patterns of early regional specification. Subsequently these different patterns of regional specification were stabilized, probably because later developmental events depended on them. As a consequence, the frequency of new body plan variants involving early development declined in these lineages at later periods during their history. This hypothesis is explicated here by examining the process of early regional specification in the different subphyla of brachiopods and in pairs of species from the same subphylum belonging to different clades that originated during the Jurassic, Ordovician, and Lower Cambrian. [ABSTRACT FROM AUTHOR]