Rodinia and the Mesoproterozoic Earth–Ocean System

Precambrian Research 111 (2001) 1–3 www.elsevier.com/locate/precamres Preface Rodinia and the Mesoproterozoic earth–ocean system We could hardly introduce this volume without For those working in Precambrian geology, mentioning the now infamous words of our re- such an approach is familiar. After all, the con- spected colleague, Roger Buick, who, in a state- cept of a Rodinian supercontinent emerged from ment somewhere between a tongue-in-cheek fragmentary evidence, reconstructed in a new caricature and an honest representation of avail- way. Recognized first by the Neoproterozoic able data, claimed that the Mesoproterozoic Era rifted margins it left behind, the Mesoproterozoic was ‘the dullest time in Earth history’ (Buick et assembly of Rodinia began to take shape when al., 1995). If anything is certain, though, it is that ancient mobile belts were rejoined in geologic models. The result was the SWEAT hypothesis the Mesoproterozoic was not dull. Rather, the (Moores, 1991), in which Moores proposed spe- Mesoproterozoic is emerging as a time of signifi- cific tests of his hypothesis, challenged future cant change in terms of global carbon cycling, workers to continue the search for a superconti- biogeochemical cycling of redox sensitive ele- nent, and set the SWEAT hypothesis up as the ments, and evolutionary diversification in both ‘straw man’ for subsequent hypotheses. Such a prokaryotic and eukaryotic communities, includ- simple, elegant model is a tough act to follow. ing the appearance of the first unambiguous mul- The contributions to this volume reflect the ticellular algae. Furthermore, many of these complexity of the questions regarding the Meso- major biogeochemical transitions may have been proterozoic era, and perhaps the complexity of concurrent with, and perhaps causally linked to, the answers. Large-scale tectonic interpretations, the global tectonic events that resulted in the grounded in fieldwork and geochronology, assembly of the supercontinent Rodinia. provide not only the framework for addressing The contributions to this special issue are, to these questions, but also a wealth of testable say the least, an eclectic mix of Mesoproterozoic hypotheses. Because each tectonic scenario carries geology. Rather than finding answers in this vol- specific implications for nearby basins, single- ume, we hope that the reader will find questions. basin studies allow questions of plausibility to be We have, therefore, asked the authors, while re- addressed. In this volume, smaller scale studies maining acutely aware of the constraints of their each focus on aspects of Rodinia, and each draws data, to broaden the expression of their ideas in local or regional conclusions that must be incor- order to foster new lines of inquiry. Toward this porated into the emerging global framework. end, we have not discouraged the presentation of This special issue contains 10 papers, with 10 potentially conflicting hypotheses — it is at the different perspectives on Rodinia. The astute points of conflict that breakthrough understand- reader will note that there is no consensus on ing will occur. what Rodinia looked like, precisely how it assem- 0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0301-9268(01)00153-X 2 Preface bled, or even when these events occurred. Each linkages between deposition in western Laurentia paper presents a piece of the puzzle, and it falls to and geochronologically constrained tectonic those of us working on these problems to find a events, thereby providing a number of chrono- solution appropriate to all the data. We have, logic ‘piercing points’ with which to test possible therefore, simply organized the volume to begin reconstructions. with regional overviews and end with more local- In the fifth paper, Geraldes et al. turns to the ized investigations. poorly known eastern portions of Rodinia with Karlstrom and coworkers present a somewhat one of the first comprehensive tectonic and controversial reconstruction of Rodinia, based on geochronometric studies of the Amazonian craton a wide variety of field-based evidence. By with respect to its position early in the assembly combining paleomagnetic data, detrital zircon of Rodinia. These authors highlight some of the geochronology, and outcrop patterns of sedimen- difficulties with reconstructing the position of tary basins and deformation fronts within Lau- Amazonia during the Mesoproterozoic. rentia, they attempt to ‘fingerprint’ piercing points In the second half of the volume, we return to surrounding Laurentia. They conclude that the Laurentia, with five papers focused on individual southern margin of Laurentia represents an active basins in northern Laurentia and Siberia. Al- margin through much of the pre-Rodinian though the northern margin of Laurentia has Proterozoic and suggest that placement of Aus- received much attention, the position of Siberia tralia along the southwestern Laurentian margin with respect to Laurentia is still poorly con- best explains the distribution of geologic data. In strained, as are the relationships among the sev- contrast, Thorkelson et al. reevaluate age con- eral Siberian and northern Laurentian straints and emplacement mechanisms of the Wer- sedimentary basins. In the absence of a well- necke breccias in northwestern Canada and defined ancient suture, consensus on the relative suggest that mineralogical and textural similarities position of Siberia with respect to Laurentia has of these breccias to the Olympic Dam breccias in not emerged through classical methods. Rainbird Australia support a more northerly placement of et al. (1998) took a different approach, suggesting Australia with respect to Laurentia. Obviously, that the position of Siberia could be constrained Australia cannot be in both places in Rodinia. by the presence and paleocurrent direction of However, the juxtaposition of competing hy- Neoproterozoic detritus derived from the potheses may suggest tests to distinguish between Grenville orogen in eastern Laurentia. In this them. volume, Khudoley et al. extend this approach to In the third paper, Ross et al. examines the Mesoproterozoic rocks in eastern Siberia. detrital grains within sedimentary sections and Within an emerging geochronologic framework, tries to account for their derivation. Such an we can begin to examine the geochemical frame- approach provides a potentially powerful means work of sedimentary successions in some detail. of testing regional scale tectonic hypotheses. Ross Bartley et al. compile d13Cand87Sr/86Sr data et al. suggest an intermediate position for Aus- from coeval successions of eastern and western tralia, as detritus from westerly sources is de- Siberia, within the context of global tectonic posited in the Muskwa and related basins. In the events. Similarly, Kah et al. examine the sulfur fourth paper, Luepke and Lyons take a pale- isotopic record of sulfates and sulfides in an ex- oceanographic approach, using sulfur isotopic ceptionally well-preserved succession from north- composition to constrain the extent of restriction ern Laurentia. Within the emerging framework of in the basin. They conclude that the lower Belt geochronology and tectonic change, such geo- Supergroup was deposited in a marine setting chemical studies provide evidence for and tests of with episodes of restriction driven by regional linkages among tectonic evolution, geochemical tectonic events along western Laurentia. By ob- cycles, and biospheric evolution. serving paleoceanographic change through stratig- In the penultimate paper of this volume, But- raphy, Luepke and Lyons propose specific terfield explores the potential paleobiological con- Preface 3 sequences of inferred biogeochemical changes. In find the time and resources to organize this effort. the evolving ocean-atmosphere system of the Finally, we would like to extend personal thanks Mesoproterozoic, the first demonstrably multicel- to our graduate and postgraduate mentors — lular algae appear in the fossil record. These algae John Grotzinger, Andrew Knoll, Paul Hoffman, compete with the ancient microbial systems for Timothy Lyons, and Bill Schopf — who taught space, thereby altering the fundamental nature of us that grand scientific ideas ultimately derive the sediments they inhabit. from detailed examination of the data. Although the current evidence for changes in This volume is dedicated to the memory of the Mesoproterozoic derives largely from geo- Christopher McArthur Powell, whose tireless ef- chemical, paleontological, and tectonic datasets, it forts to unite scientists to the cause of Rodinia has not gone unrecognized that the fundamental will be reflected in the scientific literature for nature of carbonate rocks may also have under- years to come. gone significant transformation during this inter- val. In the Proterozoic, as in the Phanerozoic, carbonate buildups provide a sensitive indicator References of ocean geochemistry, sea level change, and con- tinental margin geometry. In the final paper of Buick, R., Des Marais, D.J., Knoll, A.H., 1995. Stable isotopic compositions of carbonates from the Mesoprotero- this issue, Petrov and Semikhatov explore the zoic Bangemall Group, northwestern Australia. Chem. sequence stratigraphic architecture of a Meso- Geol. 123, 153–171. proterozoic pinnacle reef from Siberia. This pa- Moores, E.M., 1991. Southwest U.S.–East Antarctic per, though seemingly remote from the larger (SWEAT) connection; a hypothesis. Geology 19, 425–428. subject of Rodinia, reminds us of the

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