Abstract Following the early successes of subsurface platform and pinnacle reef exploration in central Alberta, the Upper Devonian Leduc Formation of eastern Alberta has over the last two decades seen sporadic exploration and production, as well as limited research. In the heavy oil belt south of Lloydminster, the uppermost several meters of the Leduc are oil bearing within erosional karst remnants formed by sub-Cretaceous erosion. The highs have been rendered producible by horizontal drilling and the presence of reservoir quality dolomite. The Leduc Formation in east-central Alberta is composed of an impressive 200 m platformal accumulation that typifies the rapid carbonate growth during the Frasnian Stage. From bottom to top, the Leduc becomes increasingly more restricted, as indicated partly by the decrease in skeletal carbonate and increase in restricted peritidal facies. The lower Leduc contains a series of prograding stromatoporoid reefal and interreefal facies, which grade upward into back-reef facies, then finally into restricted lagoonal and peritidal facies of the upper Leduc. Leduc members have been thoroughly correlated in the area, and the youngest two from which oil is produced were sampled across the region for petrographic and geochemical analyses. Contrary to most other models for dolomitization of the inner Leduc platform of the Western Canada Sedimentary Basin, we interpret pervasive dolomitization to have replaced the original mineralogy of the upper Leduc early, during shallow burial, as a result of intraformational brine generation and reflux within peritidal facies. The upper Leduc dolomites recrystallized with burial to produce nonferroan dolomites, with an average δ 18 O value of −4.1‰ Vienna Pee Dee Belemnite (VPDB), and an average δ 13 C value of +1.0‰ VPDB. The dolomites are nearly stoichiometric, with cloudy cores and in some samples, clear rims. After initial burial, laterally extensive pre-Cretaceous erosion created an approximate 260 million year gap between Upper Devonian carbonates and upper Lower Cretaceous siliciclastics. The proximity of the sub-Cretaceous Unconformity to the upper Leduc dolomites is interpreted to have led to local dedolomitization in some lithologies, yielding low-magnesium calcites with very negative, meteoric δ 18 O compositions. The erosion of the overlying seal exposed areas of the upper Leduc to replacement calcite associated with dedolomitization, and an influx of Cretaceous clays, both of which are a detriment to reservoir quality. Proximity of the sub-Cretaceous Unconformity to the Leduc reservoir could increase the likelihood that these processes took place. Resolving the position of the unconformity relative to the upper Leduc reservoir is thus a critical tool in predicting reservoir quality.

Abstract Australia's western margin is adjacent to a low–moderate-relief, semi-arid hinterland extending from northern tropical to southern temperate latitudes. Swell waves occur throughout, and cyclonic storms and tidal influences decline from north to south. The margin is influenced by the poleward-flowing, warm, nutrient-poor Leeuwin Current. There is limited upwelling and localized downwelling of saline water on to the shelf. The North West Shelf (NWS) is an ocean-facing ramp with palimpsest sediments – formed during Marine Isoptope Stage (MIS) 3 and 4; stranded ooids and peloids formed early during the post-Last Glacial Maximum (LGM) sea-level rise – and Holocene particles. Changing oceanography during sea-level rise profoundly affected sediment character. The SW Shelf (SWS) comprises the subtropical sediment-starved Carnarvon Ramp in the north and the incipiently rimmed, flat-topped, steep-fronted Rottnest Shelf in the south. The inner Carnarvon Ramp includes the Ningaloo Reef and hypersaline Shark Bay. The mid ramp is relict or stranded foraminifer-dominated sand, and represents attenuated carbonate production due to downwelling incursions of Shark Bay water on to the ramp; the outer ramp is planktic foraminiferal sand or spiculitic mud. Rottnest Shelf has coralline algal-encrusted hardgrounds, larger symbiont-bearing foraminifers with abundant cool-water elements including bryzoans, molluscs and smaller foraminifers. The SWS is transitional between warm- and cool-water carbonate realms.

Abstract The Port Willunga Formation is a cool–water, marine, quartzose, clay–rich, biosiliceous, and calcareous sedimentary succession of Early Oligocene age that accumulated in a series of proximal estuarine paleoenvironments along the eastern side of the St. Vincent Basin, South Australia. Coeval strata in two of the paleo–embayments are interpreted to record deposition during one ~ 3.5 My–long eustatic sea–level fluctuation. Transgressive facies above a ravinement surface comprise quartzose sands (subaqueous marine tidal dunes) that grade upward into fossiliferous floatstones and mudstones (shoreface to shallow basin–floor environments) that accumulated in a protected embayment. Highstand sediments are distinctly cyclic at the meter scale and consist of epifaunal bryozoan–pecten–echinoid clay–rich floatstones that become less fossiliferous but more spiculitic and chert–rich upward in each cycle. Whereas cyclic sediments in one embayment (Willunga) are interpreted to have accumulated on a current–swept, illuminated seafloor, those in the other (Noarlunga) are thought to have been deposited in a lower–energy, sub–photic setting. Cyclicity is interpreted to record the increasing influence of fluvial fresh water in the system during each sea–level fluctuation. Comparison with underlying strata reveals a striking similarity in depositional style and stratigraphic packaging between Late Eocene and Early Oligocene deposits; both are interpreted as paleoestuarine. Differences between the dark, organic–rich, biosiliceous, and low–diversity Eocene highstand deposits and the light, more calcareous, and more diverse Oligocene highstand deposits are interpreted to be due to local depositional controls. An important implication of local controls is that several postulated unconformities in the succession are not due to global eustatic changes but are ravinement surfaces related to estuarine sedimentation dynamics. Such controls, specifically terrestrial climate, hydrodynamic energy, and trophic resource levels were more important in determining sediment composition than eustasy and Southern Ocean cooling. Similar biosiliceous–carbonate sedimentary facies are a recurring feature of cool–water deposition throughout the Phanerozoic.

Abstract Lower to Middle Miocene warm temperate-water carbonates of the Murray Supergroup, comprising both fossiliferous heterozoan and foraminifer photozoan facies, accumulated on a low-energy, marine, mesotrophic epeiric ramp in a broad, shallow, intracratonic basin. Depositional conditions ranged from temperate-water with abundant nutrients supplied from the surrounding hinterland under a cool, moist climate regime in the early Miocene, to warm-temperate and subtropical seas with reduced levels of trophic resources associated with an increasingly arid regional climate in the middle Miocene. The succession is separated into three, metre-scale, 1.25–2.75 Ma, transgressive–regressive sequences, or stratigraphic packages. Transgressive carbonates range from Lower Miocene, heterozoan, open-marine echinoidal and bryozoan facies to Middle Miocene, bryozoan-rich, open-marine foraminifer-photozoan facies. Regressive deposits are mainly heterozoan, varying from restricted nearshore marine clays to molluscan seagrass meadow facies, all of which are interpreted to represent highly mesotrophic water conditions. The turn-around from transgression to regression, the transition zone, is variably preserved depending upon the ability of the depositional system to track changes in falling sea-level and is critical to the completeness of the succession. Stratigraphic completeness dramatically increases from the Lower to Middle Miocene stratigraphic packages as rates of sediment production and accumulation increased in conjunction with higher water temperatures and climatic aridity set against a background of increasing eustatic amplitudes. The result is a greenhouse-style succession generated in an evolving icehouse world. This highlights the principle that while allogenic repetition is produced by eustatic forcing, the nature and completeness of the 'cool-water' carbonate stratigraphic record can vary significantly due to subtle changes in the nature of the carbonate factory, which are, in turn, determined by sea-water temperature and trophic resources related to climate.

Abstract The Waimai, Orahiri and Otorohanga limestones of the upper Eocene to lowermost Miocene Te Kuiti Group, North Island, New Zealand, provide examples of cool-water carbonate sedimentation within seaways. The limestones are dominated by grainstones and packstones, and contain abundant bryozoans, benthic foraminifers and echinoderm fragments, with only small amounts of siliciclastic material. Horizontally bedded and cross-bedded lithofacies represent sedimentation in wave-dominated and current-dominated settings, respectively; biostrome lithofacies are less abundant and tend to be associated with the current-generated deposits. The migration of dune fields and the associated biostromes generate laterally restricted cyclic alternations of lithofacies. Stratigraphic partitioning of cross-bedded and wave-dominated lithofacies demonstrate that the Te Kuiti seaway alternated between current-dominated, wave-dominated and mixed-energy conditions. Eustatically and/or tectonically produced changes in water depth had the most important influence, with current-dominated conditions prevailing when water depth was optimal to allow flow acceleration through the seaway. Wave-dominated or mixed-energy conditions prevailed when water depth was less or more than this optimum depth. Mixed-energy conditions were most prevalent in widest seaways. The controls on seaway sedimentation inferred have application to other carbonate and siliciclastic seaway successions.

Abstract In the Precambrian world, devoid of higher organisms except near its end, carbonate sediments formed by a variety of abiotic and microbial processes, with patterns of deposition determined by tectonic, eustatic, and climatic processes. These ancient rocks demonstrate that the fundamental tenets of carbonate production and accumulation were initiated early in earth history, with the basic attributes of carbonate sedimentation well established by Neoproterozoic time. The broad temporal patterns of Precambrian carbonate facies composition and disposition parallel the long-term evolution of the earth’s oceans and atmosphere. Archean and Paleoproterozoic carbonates commonly contain abundant sea-floor precipitates, whereas the Neoproterozoic record is dominated by clastic-textured facies and abundant carbonate mudstones; Mesoproterozoic carbonates are transitional. Mesoproterozoic and early Neoproterozoic carbonates also contain abundant quantitites of the enigmatic molar-tooth structure. Grainstones, dominated by giant ooids with centimeter-scale diameters, are characteristic of many Neoproterozoic carbonates. Texturally unusual carbonates, featuring a reprise of Archean-style sea-floor precipitates, often cap glacial deposits of middle Neoproterozoic age. The influence of biology on sediment texture is best expressed in the history of Precambrian reefs. Archean through Mesoproterozoic reefs are dominantly stromatolite-based. Lamination textures reveal the progressive shift from in situ precipitation of aragonite and calcite encrusting the sea floor in Archean through Paleoproterozoic stromatolites to textures consistent with accretion of loose sediment through trapping and binding in Neoproterozoic stromatolites. This trend is interpreted to reflect the progressive decrease of abiotic factors and the concomittant increase of benthic microbial mats on controlling stromatolite growth. Neoproterozoic reefs witness the appearance of more complex textures that likely involve the participation of calcified microbes and noncalcified higher algae in colonizing the seafloor, increasing its surface complexity and resulting in highly porous frameworks for the first time in geologic history. Terminal Proterozoic thrombolitic reefs additionally contain the first calcified metazoans.

Abstract In the Precambrian world, devoid of higher organisms except near its end, carbonate sediments formed by a variety of abiotic and microbial processes, with patterns of deposition determined by tectonic, eustatic, and climatic processes. These ancient rocks demonstrate that the fundamental tenets of carbonate production and accumulation were initiated early in earth history, with the basic attributes of carbonate sedimentation well established by Neoproterozoic time. The broad temporal patterns of Precambrian carbonate facies composition and disposition parallel the long-term evolution of the earth’s oceans and atmosphere. Archean and Paleoproterozoic carbonates commonly contain abundant sea-floor precipitates, whereas the Neoproterozoic record is dominated by clastic-textured facies and abundant carbonate mudstones; Mesoproterozoic carbonates are transitional. Mesoproterozoic and early Neoproterozoic carbonates also contain abundant quantitites of the enigmatic molar-tooth structure. Grainstones, dominated by giant ooids with centimeter-scale diameters, are characteristic of many Neoproterozoic carbonates. Texturally unusual carbonates, featuring a reprise of Archean-style sea-floor precipitates, often cap glacial deposits of middle Neoproterozoic age. The influence of biology on sediment texture is best expressed in the history of Precambrian reefs. Archean through Mesoproterozoic reefs are dominantly stromatolite-based. Lamination textures reveal the progressive shift from in situ precipitation of aragonite and calcite encrusting the sea floor in Archean through Paleoproterozoic stromatolites to textures consistent with accretion of loose sediment through trapping and binding in Neoproterozoic stromatolites. This trend is interpreted to reflect the progressive decrease of abiotic factors and the concomittant increase of benthic microbial mats on controlling stromatolite growth. Neoproterozoic reefs witness the appearance of more complex textures that likely involve the participation of calcified microbes and noncalcified higher algae in colonizing the seafloor, increasing its surface complexity and resulting in highly porous frameworks for the first time in geologic history. Terminal Proterozoic thrombolitic reefs additionally contain the first calcified metazoans.

Abstract Outcrop-derived gamma-ray curves, lithofacies, and U-Pb SHRIMP zircon ages are integrated to provide a better understanding of accommodation history in the Paleo-Mesoproterozoic Nathan Group of northern Australia. This chronostratigraphic analysis significantly revises earlier lithostratigraphic interpretations of a 1200-m-thick succession of sandy carbonates. Rather than a continuous succession deposited in a complex series of lacustrine environments, it consists of three completely separate second-order supersequences, each a few hundred meters thick and deposited over a few million years. These supersequences are separated by major stratigraphic breaks (tectonically enhanced sequence boundaries) each approaching a duration of probably 10 million years. Each supersequence comprises several third-order sequences, which themselves contain many higher-order cycles, deposited in a series of continental, shoreline, and inner-rarnp to outer-ramp environments. Transgressive, high-energy, continental to shallow marine, mixed clastic-carbonate facies dominate most of the sequences. The middle supersequence, however, preserves deeper-water (mostly sub-storm) stromatolitic facies in one sequence, and storm-reworked clastics in another. These are interpreted as condensed intervals deposited around their respective maximum flooding surfaces and are succeeded by regressive facies that probably represent highstand systems tracts. New correlations between the Paleoproterozoic carbonate successions of the McArthur Basin and approximately time equivalent clastic successions in the Mt. Isa area, some 400 km to the southeast, are proposed.

Abstract Stratigraphic forward models are a significant asset in reconstructing the important processes controlling sedimentary basin development, stratigraphic architecture, and the distribution of facies. Generally, the models are used to make predictions concerning the rates and magnitudes of geologic processes operating at various temporal and spatial scales. Here, we use a recently developed stratigraphic forward modeling package (STRATA) to evaluate the impact of varying accommodation space and sediment fluxes, as well as sampling intervals, on the structure of chemostratigraphic curves preserved in the rock record. The method assumes that a primary signal, say secular variations in the chemical composition of seawater, is embedded within the stratigraphic record of platform carbonate deposits. In principle, the method and underlying assumptions are generally independent of the particular primary signal that is being recorded (e.g., Sr isotopes), but for the sake of illustration we apply the approach to the terminal Proterozoic δ 13 C record. If enough is known about the processes that regulate the primary signal, then a map of the primary signal can be used to infer how the controlling processes have varied through time and space—the stratigraphic record itself may be of little interest, depending on the question that is being asked. However, stratigraphic processes can exert a fundamental control on the structure of the primary signal, particularly if they are unsteady in time. Therefore, a forward stratigraphic model can be an essential tool in illuminating which processes may have influenced the final form of the primary signal as it is preserved in the rock record. Geologic processes such as variations in sea level, subsidence, and sediment supply can clearly influence the form of δ 13 C curves as a result of variations in accommodation space and sediment preservation. Another important influence on the construction of chemostratigraphic curves is the bias that is introduced through variations in sampling intervals. Samples are often collected in the field with spacings of 10–20 meters or more. The numerical experiments presented here show, particularly for epicratonic cover sequences (such as the Siberian platform), that sample spacings of >10 m can result in potentially severe distortion of the terminal Proterozoic and Early Cambrian δ 13 C primary signal. On the other hand, sample spacings of 1–2 m or less result in recovery of even short duration events—provided that the data gaps associated with unconformities and siliciclastic intervals can be accounted for, as well as the overprinting effects of diagenesis.

ABSTRACT Early diagenetic chert in the upper Kotuikan and Yusmastakh formations, northeastern Siberia, preserves an exceptional record of carbonate textures and microfossils in an early Mesoproterozoic peritidal carbonate platform. Silicified lithologies include carbonate precipitates that formed at or near the sediment-water interface, as well as micritic event laminae that appear to have lithified more slowly. Precipitated textures include (1) radial-fibrous laminae nucleated on organic horizons and locally forming botryoids that stack vertically to produce microdigitate structures; and (2) micron-scale carbonate laminae. Both radial-fibrous carbonates and microlaminites contain abundant microfossils, some of which are preserved as uncompressed casts and molds that retain cellular detail. These indicate that lithification preceded microbial decay; actualistic taphonomy experiments on filamentous cyanobacteria suggest that lithification occurred on a timescale of days to weeks. In other silicified textures, microfossils show evidence of extensive post-mortem decay and compression, suggesting less rapid lithification. Papier-mâché carbonate sedimentation, characterized by essentially instantaneous lithification, appears to have been locally common in restricted tidal-flat environments during the Mesoproterozoic and earlier eras but uncommon in Neoproterozoic and later times.

ABSTRACT The precipitation of carbonate into alkalinc silicate solutions results in the formation of self-assembled crystal aggregates with noncrystallographic morphologies. These precipitates emulate biologically induced mineral textures as well as display forms typical of primitive microfossils. The precipitation behavior varies with pH, i.e., as a function of the species created by dissociation of the silicic acid under alkaline conditions. Calcite single crystals and crystal aggregates precipitated in these media display complex forms derived From the specific inhibition of some crystal faces, and eventually, noncrystallographic shapes such as sheaf-of-wheat with self-organized banding develop. When strontianite and witherite precipitate in these environments at pH higher than 10, their crystal aggregates display in addition very specific morphologies, such as target patterns, scrolls, twisted ribbons, spirals, fingers, etc., with typical sizes ranging from microns to millimeters. The crystallites of the metal carbonate are embedded in a silicate matrix and are co-oriented and parallel to each other, suggesting that both the loci for nucleation and the orientation of the carbonate groups are controlled by the silica phase. The silica concentration (>250 ppm SiO 2 ), ionic force, and pH values (>8.5) required for the phenomenon to be observed are well within the range of values measured in contemporary alkaline lakes. A number of geological scenarios where the phenomenon could occur have been identified, among which are: a) contemporary lakes and thermal springs associated with alkaline magmatism such as those in the African rift valley; b) Precambrian (particularly Archean) terranes where cherts formed as a result of direct precipitation of silica; and c) a scenario on Earthlike planets where the existence of a silica-rich environment derived From hydrolysis of alkaline rocks is predicted.

ABSTRACT Silicified Mesoproterozoic stratifom stromatolites of the ca. 1400 Ma Gaoyuzhuang Formation in northern China contain microbial fossils preserved in a synsedimentary context rich in carbonate precipitates. Benthic microbial fossils were preserved by early silicification in growth position. Carbonate precipitation took place concurrently with accumulation of fine-grained sediment, and within the time frame of microbial growth and movements. The kinetics of the sedimentary process is thus calibrated by the rates commensurate with the behavioral responses of ancient microorganisms. Since both mineral and organic components of these ancient stromatolites remained preserved, their mutual relationship could be assessed. Extensive microbial growth, mat formation, and accumulation of organic matter required time and indicated the extent of sedimentary pauses. Carbonate precipitation took place in the absence of microorganisms, inhibiting their successful colonization and growth. The interplay between biological and abiotic forces in the formation of Gaoyuzhuang stromatolite permits an approximation of actual rate of carbonate precipitation, which often exceeds that of microbial settlement and growth. The relationship between microbial growth and precipitates in stromatolites under study is generally antagonistic, indicating limited involvement of microbial activities in the precipitation process.

Abstract A unique lufa and stromatolite succession, represented by the uppermost 10 m of the 1.8 Ga Hearne Formation (Pethei Group), northern Canada, developed across a large carbonate platform during a transition from normal marine to evaporitic conditions. In ascending order, the facies that document this transition consist of den-dritically branching tufa, irregularly laminated flat to domal stromatolites, and even, isopachously laminated domal stromatolites. The morphologies and textures of these tufas and stromatolites are similar to structures produced in heavily mineralized depositional environments (e.g., hot-spring and hypersaline depositional systems). Comparison with structures produced in the mineralizing systems, as well as with laboratory experiments of biological growth and abiotic mineral precipitation, provide insight into the mechanistic processes that contributed to development of the unusual facies of the uppermost Hearne Formation. This comparison suggests that the Hearne tufa and stromatolites were formed by biotic and abiotic processes whose influence on morphology fluctuated during the deposition of these facies. The key to understanding the dominant role of abiotic processes in development of these unusual carbonate fabrics lies in recognizing that these features formed during a transition from normal marine to evaporite conditions when seawater became warmer, increasingly saline, and more conducive to in situ mineralization. The tufa facies and domal, isopachously laminated stromatolite facies are both considered to have resulted from abiotic precipitation of carbonate mud induced by progressive oversaturation of seawater associated with increasing temperature and salinity during restriction of the Pethei basin. These facies are not observed in normal marine carbonates of this age and younger, and so the presence of such extreme environmental conditions are considered essential for the development of this facies. The generic growth mechanism of diffusion-limited aggregation (or similar depositional process) is invoked here to account for growth of micritic, dendritically branching tufa as a dominantly abiotic process. Similarly, domal stromatolites with even, isopachous laminae and evidence for surface-normal growth may have been produced mainly by abiotic mineral precipitation of micrite cement at the sediment-water interface. Whether or not micrite precipitation was kinetically aided by the presence of microbes remains uncertain, because there is no preserved evidence of such structures. However, the characteristically irregular lamination of the flat to domal stromatolites is most consistent with the former presence of discontinuous microbial mats, which would have trapped and bound loose sediment. Abundant precipitation is not indicated in this facies, because no calcified sheaths are preserved.

ABSTRACT Large crystal pseudomorphs, composed of limestone and dolomite, that radiate upward to form centimeter- to meter-tall fans are known from every well-preserved Late Archean carbonate platform on earth. In many cases these crystal fans are an important facies, constituting as much as 50% of the observed volume of carbonate rock. Texturally, the fans are composed of elongate blades consisting of a mosaic of crystals with randomly oriented optic axes. In some pseudomorphs, trains of inclusions define the fibrous character of the precursor mineral, and the blades exhibit blunt terminations when draped by micrilic sediment. Some of the pseudomorphs contain strontium concentrations of up to 3700 ppm. Associated facies include strongly elongate giant stromatolites, hummocky cross-stratified sandstones, ooid-intraclast packstone to grainstone, small domal stromatolites, and several thinly laminated micritic facies that may display desiccation cracks. Previously, some of these crystal fans have been interpreted as calcite-and dolomite-replaced pseudomorphs after gypsum, formed under restricted conditions resulting from evaporative concentration of seawater. However, replacement textures and elevated strontium concentrations suggest that the crystal fans are more likely the result of neomorphism of large botryoids of aragonite that formed thick crusts directly on the sea floor. Furthermore, occurrence of the crystal fans in direct association with strongly elongate giant stromatolites and hummocky cross-stratified sediments suggests precipitation of the fans in open marine, wave- and current-swept environments. Although evaporation of seawater may have contributed to the growth of fans in some peritidal environments, most occurrences are not associated with any other indicators of evaporitic conditions such as halite or gypsum pseudomorphs. The reinterpretation of most reported occurrences of Late Archean gypsum pseudomorphs as aragonite pseudomorphs indicates that calcium sulfate precipitation from Late Archean seawater was rare, and that precipitation of aragonite as thick crusts on the sea floor was significantly more abundant than during any subsequent time in earth history. Rapid aragonite precipitation rates and the paucity of calcium sulfate precipitation can be accounted for in a model for Late Archean seawater featuring, relative to present-day seawater, higher supersaturation with respect to calcium carbonate and high HC0 3 concentrations.

Abstract Laterally extensive beds of acicular, radiating carbonate fans, locally known as “Coxco needles”, are particularly common within a distinct stratigraphic interval (—1640 Ma) in the Proterozoic of northern Australia. In the southern McArthur Basin, they are the distinctive feature of the Coxco Dolomite Member and occur throughout a number of lithofacies across the platform. Mounding and onlapping of sediment laminae, their upwardly divergent aspect, brecciated Coxco needle clasts infilling synsedimentary fractures, and their intimate association with stromatolites supports the precipitation of Coxco fans from ambient seawater directly onto the seafloor. Individually, they consist of acicular crystal casts up to 10 cm long, which form radiating, bottom-nucleated fans. Needle terminations are commonly blocky or square and in cross section appear pseudohexagonal with crystal casts generally having six-sided forms. Needles consist internally of an irregular mosaic of dolospar cement easily distinguished from the more finely crystalline dolomicrite matrix. These features are entirely consistent with criteria for the recognition of aragonite in ancient carbonate sequences and imply an original aragonitic mineralogy for the Coxco fans. The sequence through the middle McArthur Group (Emmerugga Dolomite, Teena Dolomite, Coxco Dolomite Member, and Barney Creek Formation) is broadly transgressive, and precipitation of Coxco needles occurred during the onset of a period of tectonically induced subsidence. The mechanism for the widespread chronostratigraphic precipitation of CaCO 3 is thought to be upwelling of highly alkaline, HCO 3 − -rich anoxic bottom water onto the carbonate platform coeval with changes in the bathymetry of the basin. Mixing with relatively Ca 2+ -rich surface waters resulted in widespread precipitation of carbonate seafloor cement (i.e., Coxco fans) across the platform in several distinct lithofacies. The reason that macroscopic carbonate cement formed in preference to widespread precipitation of finely crystalline micrite remains unclear, although it is suggested that elevated concentrations of Fe 2+ and Mn 2+ in the basin waters may have inhibited micrite precipitation and thus favored development of macroscopic seafloor Coxco fans.

Abstract The Boot Inlet Formation (Reynolds Point Group, Shaler Supergroup) is an early Neoproterozoic (< 1077 MA, >723 Ma) succession that crops out within the Minto Inlier on northern Victoria Island in the Canadian Arctic archipelago, and consists of strata that accumulated on a carbonate ramp. Inner-ramp facies comprise molar-tooth lime mudstone and current-bedded ooid grainstone (locally herringbone cross-laminated) with scalloped erosional surfaces. Ooid shoals ( 3–4 m thick) and sheets (0.5–1.0 m thick) are interbedded with 10–15 m thick stromatolite bioherms and biostromes forming complexes 0.5 to 5.0 km wide. The most common mid-ramp facies is parted to ribbon-bedded limestone with conspicuous ripples, gutter casts, hummocky cross-stratification, and intraformational breccias readily interprétable as storm deposits; these finegrained rocks form shallowing-upward, meter-scale cycles capped by oolitic limestone and small reefs. Outer-ramp facies comprise shale with large carbonate concretions. Reefs are most common in the lower half of the succession, where overall sea-level rise combined with higher-order transgressions to produce maximum accommodation space. A pronounced zonation of reef types occurs across the ramp. A current-oriented biostrome of Baicalia? is the only reef type on the inner ramp. Patch reefs and table reefs characterize the inner- to mid-ramp transition, and consist of stacked meter-scale bushes of Tungussia that pass upward into broad domal sheets of parallel, columnar stromatolites (Baicalia) oriented at a high angle to the sheets. Overall upward decrease in diversity of growth form is accompanied by evidence for increasing wave and current energy. Concentric-sheet bioherms up to 60 m in diameter and 15 m high, composed of sheets of closely spaced “pencil stromatolites” (Jurusania), grew in outer-ramp facies during rapid transgression. The Boot Inlet reefs are similar to other Prolerozoic reefs in being composed entirely of stromatolites, including some of the same forms as characterize other early Neoproterozoic patch reefs. Calcimicrobes are conspicuously absent, despite their abundance in coeval deeper-water reefs in the Mackenzie Mountains. The presence of kalyptra-like stromatolitic structures in the Boot Inlet reefs is similar to that of Early Cambrian calcimicrobe-archaeocyathan reefs, and lends support for the view that the Phanerozoic reef archetype originated during the Neoproterozoic.

Abstract Giant reefs of the early Neoproterozoic Little Dal Group, Mackenzie Mountains, N.W.T., Canada, differ from most previously described Proterozoic buildups in containing a calcimicrobial and thrombolitic framework. Systematic vertical changes in composition permit the identification of five framework stages. Each stage contains a persistent community of calcimicrobes, yet the expression of element morphologies throughout the reefs is exceedingly varied, indicating that environment exerted the predominant control over framework attributes. Framework development is correlated with extrinsic paleoenvironmental controls, namely change in relative sea level. Deepest-water intervals are characterized by accretion of dense, layered crusts (Stage IV), intermediate water depths are reflected by intricately anastomosing, morphologically diverse framework elements (Stages I and 111), and shallowing on reef tops is expressed as thin successions of erect, well-ordered, columnar microbialites (Stages II and V). Reef growth occurred in low- to moderate-energy regimes, within the photic zone, on hard substrates, and in the absence of significant settling of carbonate or terrigenous mud. The growth window is interpreted to have been delimited by the base of the photic zone at depth, and by excessive fragmentation near the water surface. Optimal growth occurred in moderate water depths, between fair-weather wave base and a limit determined by light attenuation at depth. The Little Dal reefs record a major inflection point in the development of reefa) ecosystems: although they display a combination of attributes from both Proterozoic and Paleozoic reef ecosystems, there is a preponderance of Phanerozoic-style features, including mineralized reef-building organisms, complex framework complete with growth cavities containing internal sediment and synsedimentary cement, vertical and lateral framework zonation, and large-scale accretion style that varies with relative-sea-level change. They are therefore the earliest known representatives of “modern”-style reef growth.

Abstract The Katakturuk Dolomite records an unsurpassed history of Neoproterozoic passive-margin cyclic sedimentation in Arctic Alaska and offers new insights into the evolution of Precambrian carbonate platforms in response to interpreted eustatic sea-level changes. The Katakturuk depicts a south-dipping, low-angle, distally steepened carbonate ramp complex with a complete spectrum of facies types, from proximal, updip tidal-flat complexes to distal, downdip, sub-wave-base allodapic turbidites, debates, and rhythmites. The ramp margin is marked by thick stacks of amalgamated grainstone shoal complexes separating distally steepened downdip facies from ramp-interior facies. Using analysis of cycle stacking patterns, the 2500-m-thick Katakturuk can be subdivided into four second-order supersequences (of roughly equal thickness), each of which is made up of two to four third-order sequences (average a few hundred meters thick). The high-frequency cyclic architecture of a single third-order depositional sequence (lower gray craggy dolomite member) provides an example of systems-tract development in the Katakturuk Dolomite. On the basis of physical bounding surfaces, two types of cycles are recognized: cycles bounded by marine flooding surfaces across which subfacies deepen, termed “subtidal cycles”, and “peritidal cycles”, that are bounded by subaerial exposure surfaces (e.g., peritidal lamjnites). The systematic vertical variation in cycle type (peritidal vs. subtidal) and cycle thickness, combined with vertical subfacies trends and the recognition of significant subaerial exposure surfaces (karsts, stacked tepees or peritidal breccias) define the transgressive and highstand systems tracts of thirteen third-order depositional sequences. The third-order sequences in tum stack to build larger second-order accommodation cycles. Coinciding second-order and third-order rises in relative sea level resulted in two major backstepping events, which were recorded in the deposition of outer-ramp slope facies directly on peritidal facies. The top of the Katakturuk is marked by a complete spectrum of karst facies, representing a supersequence lowstand superimposed on a third-order late highstand.

Abstract Rocks of the lower McNamara Group (including the Torpedo Creek, Gunpowder Creek, Paradise Creek, and Esperanza formations) form part of the extensive late Paleoproterozoic sedimentary cover in northwestern Queensland. The succession varies in thickness from 450 to 1250 m and consists largely of dolomite, chert, and siliciclastic rocks deposited in nonmarine, marginal marine, and marine environments. The package is divisible into two distinct basin-filling supersequences (second-order sequences): Prize, a lower suite of fault-bounded, storm-dominated siliciclastic ramps; and the overlying Gun, a regionally extensive, southeast-facing, storm-dominated mixed carbonate/siliciclastic ramp. A regionally correlative unconformity in the lower to middle Gunpowder Formation, representing a depositional hiatus of over 28 My, records the boundary between these two supersequences. Combining facies architecture with gamma-ray logs derived from hand-held spectrometers enables further subdivision into three third-order depositional sequences, (1) Prize 2, Torpedo Creek-lower Gunpowder Creek, (2) Gunl, upper Gunpowder Creek-Paradise Creek, and (3) Gun 2, upper Paradise Creek-Esperanza, each of which is bounded by regionally correlative disconformilies. The Prize 2 depositional sequence consists largely of siliciclastic storm-dominated ramp sediments deposited in locally fault-controlled depocenters. The overlying Gun 1 sequence marks the transition from a siliciclastic to a carbonate ramp as storm-dominated shallow-water carbonate facies prograded easl/southeastward over the underlying siliciclaslics. Storm processes continued to dominate sedimentation. Formation of stromatolitic rim complexes had a significant hydrodynamic effect on this second ramp and promoted flattening of the inner ramp and steepening of the outer ramp to form a distally steepened ramp to rimmed platform. Unlike the underlying succession, the Gun 1 depositional sequence represents a regionally continuous depositional system with only minor thickness variations. The Gun 2 depositional sequence similarly represents a storm-dominated siliciclastic/carbonate ramp. The most striking difference between the Gun 1 and 2 sequences is the relatively abrupt shift from micrite-rich domal-columnar stromatolites to spar-rich microdigitate and digitate columnar stromatolites. Unlike most other Paleoproterozoic carbonate successions documented to date, the lower McNamara platform was dominated by storm-deposited and reworked sediments, predominantly intraclasts, quartz and peloid silt, and muds, rather than precipitated carbonate cements. The preponderance of silt-size grains, rather than mud-size, differentiates this succession from most other detrital Proterozoic platforms. The internal architecture and evolution of the succession is instead similar to sediment-rich Phanerozoic carbonate ramps.