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Precambrian Carbonates: Evolution of Understanding

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Precambrian Carbonates: Evolution of Understanding PRECAMBRIAN CARBONATES EVOLUTION OF UNDERSTANDING JOHN P GROTZINGER MA 02139 U SA Department ofEarth Atmospheric and Planetary Sciences Massachusetts Institute ofTechnology Cambridge AND NOEL P JAMES K7L 3N6 Canada Department ofGeological Sciences Queen s University Kingston Ontario a of abiotic and ABSTRAcr In the Precambrian world devoid of higher organisms except near its end carbonate sediments formed by variety and climatic These ancient rocks demonstrate that the microbial processes with patterns of deposition determined by tectonic eustatic processes were initiated in earth with the basic attributes of carbonate sedimen fundamental tenets of carbonate production and accumulation early history tation well established by Neoproterozoic time evolution of the earth The broad temporal patterns of Precambrian carbonate facies composition and disposition parallel the long term s oceans sea whereas the record and atmosphere Archean and Paleoproterozoic carbonates commonly contain abundant floor precipitates Neoproterozoic are transitional and is dominated by clastic textured facies and abundant carbonate mudstones Mesoproterozoic carbonates Mesoproterozoic structure Grainstones dominated ooids early Neoproterozoic carbonates also contain abundant quantitites of the enigmatic molar tooth by giant unusual carbonates a of with centimeter scale diameters are characteristic of many Neoproterozoic carbonates Texturally featuring reprise Archean style sea floor precipitates often cap glacial deposits of middle Neoproterozoic age Precambrian reefs Archean reefs are The influence of biology on sediment texture is best expressed in the history of through Mesoproterozoic of and calcite the dominantly stromatolite based Lamination textures reveal the progressive shift from in situ precipitation aragonite encrusting and stromatolites to textures consistent with accretion of loose sediment trapping binding sea floor in Archean through Paleoproterozoic through of abiotic factors and the concomittant increase of in Neoproterozoic stromatolites This trend is interpreted to reflect the progressive decrease reefs witness the of more textures that benthic microbial mats on controlling stromatolite growth Neoproterozoic appearance complex likely in the seafloor its surface and involve the participation of calcified microbes and noncalcified higher algae colonizing increasing complexity Terminal Proterozoic thrombolitic reefs contain the first resulting in highly porous frameworks for the first time in geologic history additionally calcified metazoans most carbonates such as INTRODUCTION simple question asked of Phanerozoic Were the sediments produced biologicallyand answered af Precambrian carbonate rocks have within them a legible rec firmatively in scores of introductory level textbooks and sum ord of earth history that spans more than three billion years articles is not so easily resolved for the Precambrian rec From the period of first continental accretion to the advent of mary ord For rocks of this age the absence of coarse skeletal debris ecologically diverse biomineralizing metazoans these sedimen in all but terminal Proterozoic carbonates does notprovide an tary rocks contain chemical and proxies biological physical that are vast and the easy for the many platforms as for past tectonic regimes environmental change and evo explanation diverse as any of Phanerozoic age The prob lution of life Their physical attributes reflect tectonic subsi compositionally lem of the of Phanerozoic carbonate mud so easily ex dence and sea level fluctuation their chemical variability pro origin through the post mortem disintegration of green algae vides insight into carbon burial rates continental growth and plained does notfind much basis in accounting for the mudstones pres surficial redox their paleontology illustrates how microorgan ent across the 600 000 km2 late Archean Transvaal platform isms have evolved and how the structure of early ecosystems at least 1 5 billion older than the first direct evidence for developed years in the fossil record Precambrian carbonate sedi Studies of Precambrian carbonates like analyses of their green algae have thus had to rely on a number of different younger counterparts generally fall into two categoriesone mentologists to to address some of these essen in which the former sedimentary facies their constituentgrains approaches begin simple yet tial and so the of sedimentological trace and the platforms they form are of primary interest and the questions integration element biomarker and data has be other in which the sediments are viewed principally as carriers isotopic paleobiological more even for studies devoted to the e is come widespread origin of a geochemical record g C and Sr isotopes which itself of the sediment itself the object of study In the first case investigations are motivated it was notclear what facies Pre by the desire to interpret sedimentation patterns that result from Thirty years ago comprised in the cambrian carbonates if they formed differentiated platforms physical chemical and biological processes whereas and if were well for detailed study The second studies are oriented more toward understanding biogeo they enough preserved view was and to some extent still is that Precam chemical cycles and the ancient ocean atmosphere system prevailing fall into either brian carbonate rocks were endless hectares of stromat In this volume we have assembled papers that simply dolostone in an is olites and fabric destructive Several key papers category Having done so emerging trend obvious however answers to these several of these studies accomplish both This degree of inte the mid 1970s provided distinct and demonstrated the clear for additional gration is motivated by the realization that the clearest records questions potential more detailed Hoffman 1974 Serebryakov and Semik of biogeochemical events are elucidated through careful study study hatov Beukes 1977 and 1978 These of sedimentary and diagenetic patterns Conversely sedimen 1974 Cecile Campbell combination with the ofextensive fields of tation patterns are being explored in the context of the local studies in disovery in a effort to better un and global microbiological and physicochemical variability that stromatolites Shark Bay ignited major and might influence textures and accumulations rates derstand the paleoenvironmental paleobiological signifi Walter The study of Precambrian carbonate platforms necessitates cance of stromatolites in platform carbonates 1976 effort in turn to a of studies in such integrated approaches to problem solving An apparently This led second generation Carbonate Sedimentationand Diagenesis in the Evolving Precambrian World Copyright @ 2000 SEPM Society for Sedimentary Geology Special Publication 67 ISBN 1 56576 072 7 4 PRECAMBRIAN CARBONATES EVOLUTION OF UNDERSTANDING which platforms were mapped facies were interpreted in the velopment of Precambrian carbonate platforms is identical to context of modern and analogs complementary diagenetic modern ones Important controls on platform geometry include studies aimed at were trying to unravel primary mineralogy and patterns of differential subsidence eustatic fluctuations silici carbonate precipitation mechanisms Kerans 1982 Grotzinger clastic sediment flux and paleoclimate Ramps and rimmed and Read Bertrand 1983 Sarfati and Moussine Pouchkine shelves are both present although Neoproterozoic rocks show 1983 Tucker 1983 Teitz and Mountjoy 1985 Grey and a dominance of ramps over rimmed shelves The reasons for Thorne 1985 1986a 1986b Hofmann and Jack this trend Grotzinger of abundant Neoproterozoic ramps are unclear but son Beukes Fairchild and 1987 1987 Spiro 1987 Zempolich may be related to the abundance of grainstones in some systems et aI 1988 Syntheses of Precambrian carbonates at the close e g Knoll and Swett 1990 Clough and Goldhammer this of the decade Grotzinger 1988 1989b summarized existing volume the general decline of stromatolites that might have data and demonstrated that to a first order approximation the formed effective barriers Grotzinger 1988 1990 and the rise of their a geometries carbonate platforms primary mineralogies of higher algae Butterfield et 1988 that might have com and the distribution of since at least the late general facies Ar peted effectively for substrate space Knoll and Swett 1990 chean similar were to those present in Paleozoic through Recent Nevertheless many platforms beginning with the late Ar carbonates chean Campbellrand Malmani structure Beukes 1980 1987 This of research however also out that show phase pointed large morphologic development from an initial ramp that un the parts of Precambrian record were non actualistic with no dergoes progressive transition to a rimmed shelf Fig 1 analogues in the modern or for that matter in the Phanerozoic Younger examples include the Paleoproterozoic Pethei and The last ten years has been a watershed in our understanding Rocknest platforms Hoffman 1974 Grotzinger 1986b Sami in this regard as Precambrian carbonates have been interpreted and James 1993 the Neoproterozoic Yellowhead platform on own and not their merits viewed simply as variants on Phan Teitz and Mountjoy 1985 1989 and terminal Proterozoic car erozoic models Furthermore new analytical techniques have bonates of the Gourma basin Bertrand Sarfati and Moussine allowed heretofore unimagined correlation and thus revealed Pouchkine 1983 and the central Oman basin Mattes and Con unknown attributes of sediment previously dynamics
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