Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion

Journal name:
Nature
Volume:
484,
Pages:
363–366
Date published:
DOI:
doi:10.1038/nature10969
Received
Accepted
Published online

The transition between the Proterozoic and Phanerozoic eons, beginning 542million years (Myr) ago, is distinguished by the diversification of multicellular animals and by their acquisition of mineralized skeletons during the Cambrian period1. Considerable progress has been made in documenting and more precisely correlating biotic patterns in the Neoproterozoic–Cambrian fossil record with geochemical and physical environmental perturbations2, 3, 4, 5, but the mechanisms responsible for those perturbations remain uncertain1, 2. Here we use new stratigraphic and geochemical data to show that early Palaeozoic marine sediments deposited approximately 540–480Myr ago record both an expansion in the area of shallow epicontinental seas and anomalous patterns of chemical sedimentation that are indicative of increased oceanic alkalinity and enhanced chemical weathering of continental crust. These geochemical conditions were caused by a protracted period of widespread continental denudation during the Neoproterozoic followed by extensive physical reworking of soil, regolith and basement rock during the first continental-scale marine transgression of the Phanerozoic. The resultant globally occurring stratigraphic surface, which in most regions separates continental crystalline basement rock from much younger Cambrian shallow marine sedimentary deposits, is known as the Great Unconformity6. Although Darwin and others have interpreted this widespread hiatus in sedimentation on the continents as a failure of the geologic record, this palaeogeomorphic surface represents a unique physical environmental boundary condition that affected seawater chemistry during a time of profound expansion of shallow marine habitats. Thus, the formation of the Great Unconformity may have been an environmental trigger for the evolution of biomineralization and the ‘Cambrian explosion’ of ecologic and taxonomic diversity following the Neoproterozoic emergence of animals.

At a glance

Figures

  1. Sauk Sequence in North America.
    Figure 1: Sauk Sequence in North America.

    Distribution and age of the oldest Phanerozoic sedimentary rocks in North America.

  2. Phanerozoic sedimentation patterns in North America.
    Figure 2: Phanerozoic sedimentation patterns in North America.

    a, Minimum area of crystalline basement rock exposed at surface before burial by sediments in each epoch. b, Estimated minimum total carbonate burial flux in each epoch, based on lithostratigraphic units that are composed primarily of carbonate (Fig. 3c). c, Proportion of glauconite-rich siliciclastic sedimentary rock units (Fig. 3d). Grey bars span the Cambrian–Early Ordovician Sauk Sequence. Error bars, ±1 standard error. O, Ordovician; S, Silurian; D, Devonian; C, Carboniferous; P, Permian; Tr, Triassic; J, Jurassic; K, Cretaceous; Pg, Palaeogene; Ng, Neogene.

  3. Middle to Late Cambrian palaeoenvironments and sedimentology.
    Figure 3: Middle to Late Cambrian palaeoenvironments and sedimentology.

    c, Generalized marine shelf profile16. b, Cathodoluminescence photomicrograph of the Kaili Formation, an outer detrital belt mudstone from south China. Lamination tops are defined by authigenic carbonate cements that glow orange under cathodoluminescence; clay-rich lamina bases are dark. c, Exposure of Notch Peak Formation in Utah. d, Plane-polarized light photomicrograph of a carbonate-cemented, glauconite-bearing sandstone from the Au Train Formation, inner detrital belt, northern Michigan. e, Great Unconformity in Wind River Canyon, Wyoming, with ~510- Myr-old marine Flathead Sandstone on 2,900-Myr-old granitic basement. Coin is US dime (17.9mm diameter). gl, glauconite; cal, calcite; qtz, quartz.

  4. Summary of major tectonic, geochemical and sedimentary patterns over the past 900[thinsp]Myr.
    Figure 4: Summary of major tectonic, geochemical and sedimentary patterns over the past 900Myr.

    The shift from widespread continental denudation to widespread sedimentation on the continents defines the Great Unconformity. 87Sr/86Sr estimates (dark grey envelope) from ref. 2 and references therein; εNd estimates (light grey envelope) from ref. 26. Vertical blue bars indicate times of major global glaciations; vertical green bars identify continent-scale sedimentary sequences12, including the Cambrian–Early Ordovician Sauk Sequence (dark green bar). Approximate times of widespread continental denudation are identified by horizontal red bars at top; time of widespread sediment accumulation on continents is identified by partially overlapping light yellow bar. Periods of major supercontinents (Rodinia, Pangea) and their break-up are identified, including recent mountain building (mtns.).

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Author information

Affiliations

  1. Department of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA

    • Shanan E. Peters
  2. Geology Department, Pomona College, Claremont, California 91711, USA

    • Robert R. Gaines

Contributions

S.E.P. contributed Macrostrat-derived data, R.R.G. contributed sample-derived data. Both authors contributed to the development of ideas and writing.

Competing financial interests

The authors declare no competing financial interests.

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Correspondence to:

Data for aspects of this analysis derive from Macrostrat (http://macrostrat.org).

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    This file contains Supplementary Figures 1-8, Supplementary Tables 1-3 and additional references.

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