Proterozoic-Phanerozoic Boundary Research Articles

The subduction-related Great Unconformity in the Tarim intracraton, NW China

The Great Unconformity across the Proterozoic-Phanerozoic boundary records a significant change of the Earth's continents, atmosphere, environment and life. Its origin has been assumed to be glacioeustatic mechanism in the cratonic interior. On the contrary, we provide evidences for the sub-Cambrian Great Unconformity that can be traced across the Tarim Craton (NW China) from recent geochronological and seismic data. In the interior basement uplift, a distinct unconformity over an area of 300, 000 km2 presents a stratigraphic gap from the Paleoproterozoic (ca. 1.9 Ga) to the end Ediacaran. There is significant denudation beneath the Cambrian with compressional uplift. Given that the ca. 590–580 Ma Hankalchough diamictite (correlated with the Gaskiers glaciation) is absent in most areas, as much as 40 m.y. of stratigraphic gap constrained maximum span of the latest sub-Cambrian unconformity. The Precambrian uplift and denudation, and the age patterns of detrital zircon grains, overlap ca. 560–540 Ma advancing subduction process along the convergent southern margin of the Tarim Craton. This Great Unconformity may be linked to subduction-related uplift along the convergent southern Tarim margin, as occurred across peri-Gondwana. These relationships suggest tectonic driver rather than glaciation could lead to Great Unconformity in the cratonic interior as well as the continental margin

Enhanced deep carbon cycle marked by the upsurge of silica-undersaturated nephelinitic magmatism at the Proterozoic-Phanerozoic boundary

Abstract The temperature of the upper mantle was a principal factor controlling the style of plate tectonics and influencing magmatism and metamorphism on Earth over geological history. Recent studies emphasized that Earth’s tectonic style has transited into the modern plate tectonics since the late Neoproterozoic, which is characterized by a global network of plate boundaries with deep and cold oceanic plate subduction. However, the consequence of the establishment of modern plate tectonics to Earth’s mantle temperature and deep carbon cycle has not been fully understood. Here we apply statistical analysis on the geochemical data of continental igneous rocks and identify an increased magnitude of nephelinitic volcanism at the end of the Ediacaran. Nephelinitic rocks, a silica-undersaturated high-alkaline rock group, are mostly formed by low-degree melting of carbonated mantle sources. We link their widespread emergence with an enhanced mantle cooling event and a dramatically increased flux of crustal carbonates transporting to the mantle. The rapid cooling of the mantle was ascribed to the onset of modern-style plate tectonics with global-scale cold oceanic and continental subduction since the late Neoproterozoic. The declined upper-mantle temperature could not only favor the low-degree melting but also allow the subduction of carbonates into the deep mantle without decarbonation at shallow depth. Considering the high oxygen fugacity feature of the nephelinitic rocks and some other high-alkaline volcanism, the establishment of modern plate tectonics and thereafter enhanced mantle cooling and deep carbon cycle might contribute to the high-level atmospheric oxygen content during the Phanerozoic.

Geomorphic expression of shear zones in Southern Brazilian and Uruguayan Shields

Earth's surface is a result of complex tectonic and erosional processes shaping landscapes and producing transient signs of different evolutionary stages. These transient signs are formed by a gradual adjustment of rivers to an equilibrium stage through channel incision and uplift. Processes have different magnitudes according to lithological contrasts and base-level changes that result in distinct disequilibrium phases of bedrock rivers. This integrated study of geomorphic indices in bedrock rivers of the southernmost Brazilian and Uruguayan Shields is developed to identify key signs of transience associated with surface processes and compare with contrasting drainage basin results. The study area is composed of igneous-metamorphic rocks of Precambrian age of the Dom Feliciano Belt amalgamated during the Proterozoic-Phanerozoic boundary in the Brasiliano Orogeny. Digital elevation models (DEM) are used to extract hypsometric indices, χ-plots for each basin, and an interpolated Ksn map through interactive MATLAB tools emphasizing erosional stages and uplifted regions. Hypsometric integral values varying from 0.20 to 0.45 and a mean concavity of 0.5 show an overall eroded landscape, however, knickzones, positive χ-curves and Ksn anomalies of up until 45 reveal transient landscape signs in the area. Combined hypsometric, χ-plots and Ksn anomalies reveal distinctive patterns of erosion and uplift components with knickpoints aligned to structural signatures of lineaments. In the Sul-rio-grandense Shield (SRGS), these patterns point to a rugged and reworked relief controlled by shear zones, whereas the Uruguayan Shield (UYS) shows a denudated and more stable topography. Major tectonic events are well constrained in the Dom Feliciano Belt, but the timing and driving forces of the geomorphological features remains difficult to estimate because of the long geological history involved and intraplate context.

A window into the Great Unconformity: Insights from geochemistry and geochronology of Ediacaran glaciogenic rocks in the North China Craton

The “Great Unconformity” across the Proterozoic-Phanerozoic boundary has long been recognised as critical for understandings of the Cambrian Explosion and changes in sediment preservation. However in many parts of the Earth no rock strata are preserved for hundreds of millions of years leading up to the Cambrian Period. This study reports detrital zircon age and whole rock geochemical data from 43 samples of glaciogenic rocks in the Ediacaran Luoquan and Dongpo Formations. Detrital zircon age populations show a distinct peak at ca. 2.5 Ga and minor peaks at 2.2–1.6 Ga, typical of Mesoproterozoic provenance. It has been established that glaciogenic strata are uniquely suited as a proxy for the average upper continental crust (UCC). Applying this principle, the samples presented herein are used to provide insights into the UCC present in the North China Craton (NCC) during the Great Unconformity. This reveals UCC older than the Neoproterozoic but younger than the Great Oxygenation Event. Based upon these findings a tectonic and climatic model is proposed, using a modern analogue from Antarctica, wherein accommodation space was created by glacial abrasion whilst uplift continued throughout the NCC. This overdeepening process provides an explanation for the preservation of these glaciogenic strata, despite the Great Unconformity that surrounds them on all sides and this study contribute towards understanding of processes in the NCC during a critical time when little has survived into the rock record.

Systematics of Early Cambrian Paleomagnetic Directions from the Northern and Eastern Regions of the Siberian Platform and the Problem of an Anomalous Geomagnetic Field in the Time Vicinity of the Proterozoic–Phanerozoic Boundary

Representative paleomagnetic collections of Lower Cambrian rocks from the northern and eastern regions of the Siberian platform are studied. New evidence demonstrating the anomalous character of the paleomagnetic record in these rocks is obtained. These data confidently support the hypothesis (Pavlov et al., 2004) that in the substantial part of the Lower Cambrian section of the Siberian platform there are two stable high-temperature magnetization components having significantly different directions, each of which is eligible for being a primary component that was formed, at the latest, in the Early Cambrian. The analysis of the world’s paleomagnetic data for this interval of the geological history shows that the peculiarities observed in Siberia in the paleomagnetic record for the Precambrian–Phanerozoic boundary are global, inconsistent with the traditional notion of a paleomagnetic record as reflecting the predominant axial dipole component of the geomagnetic field, and necessitates the assumption that the geomagnetic field at the Proterozoic–Phanerozoic boundary (Ediacaran–Lower Cambrian) substantially differed from the field of most of the other geological epochs. In order to explain the observed paleomagnetic record, we propose a hypothesis suggesting that the geomagnetic field at the Precambrian–Cambrian boundary had an anomalous character. This field was characterized by the presence of two alternating quasi-stable generation regimes. According to our hypothesis, the magnetic field at the Precambrian–Cambrian boundary can be described by the alternation of long periods dominated by an axial, mainly monopolar dipole field and relatively short epochs, lasting a few hundred kA, with the prevalence of the near-equatorial or midlatitude dipole. The proposed hypothesis agrees with the data obtained from studies of the transitional fields of Paleozoic reversals (Khramov and Iosifidi, 2012) and with the results of geodynamo numerical simulations (Aubert and Wicht, 2004; Glatzmayer and Olson, 2005; Gissinger et al., 2012).

Constraining the climate and ocean pH of the early Earth with a geological carbon cycle model

The early Earth's environment is controversial. Climatic estimates range from hot to glacial, and inferred marine pH spans strongly alkaline to acidic. Better understanding of early climate and ocean chemistry would improve our knowledge of the origin of life and its coevolution with the environment. Here, we use a geological carbon cycle model with ocean chemistry to calculate self-consistent histories of climate and ocean pH. Our carbon cycle model includes an empirically justified temperature and pH dependence of seafloor weathering, allowing the relative importance of continental and seafloor weathering to be evaluated. We find that the Archean climate was likely temperate (0-50 °C) due to the combined negative feedbacks of continental and seafloor weathering. Ocean pH evolves monotonically from [Formula: see text] (2σ) at 4.0 Ga to [Formula: see text] (2σ) at the Archean-Proterozoic boundary, and to [Formula: see text] (2σ) at the Proterozoic-Phanerozoic boundary. This evolution is driven by the secular decline of pCO2, which in turn is a consequence of increasing solar luminosity, but is moderated by carbonate alkalinity delivered from continental and seafloor weathering. Archean seafloor weathering may have been a comparable carbon sink to continental weathering, but is less dominant than previously assumed, and would not have induced global glaciation. We show how these conclusions are robust to a wide range of scenarios for continental growth, internal heat flow evolution and outgassing history, greenhouse gas abundances, and changes in the biotic enhancement of weathering.

Palaeobiology of latest Ediacaran phosphorites from the upper Khesen Formation, Khuvsgul Group, northern Mongolia

Microfossil assemblages that include large acritarchs with complex processes, known as Doushantuo-Pertatataka-type acritarchs, are recovered from early Ediacaran successions globally. They are commonly found in shale and chert lithologies, but their diversity and palaeobiological significance is greatest when they are phosphatized. The best-known examples are from the Doushantuo Formation, South China, which preserves over 60 taxa including possible embryonic forms which may represent the oldest fossil animals. Fossils have only been recorded in four Ediacaran phosphorite deposits. Here we report the fifth such occurrence, from phosphorites of the upper Khesen Formation, Khuvsgul Group, northern Mongolia, where preservation rivals that in the Doushantuo Formation. The assemblage includes the likely cyanobacteria Obruchevella delicata, O. magna, O. parvissima and O. valdaica, as well as various Siphonophycus filaments, the possible alga Archaeophycus yunnanensis, and the Doushantuo-Pertatataka-type acritarchs Appendisphaera grandis, A. fragilis, A. tenuis, Cavaspina basiconica, Variomargosphaeridium gracile and V. aculeiparvum, sp. nov. The phosphorites also preserve the multicellular embryo-like taxon Megasphaera, which is represented by M. minuscula sp. nov. and potentially by M. puncticulosa. Geological and chemostratigraphical data suggest a latest Ediacaran age for the Khesen assemblage, immediately prior to the Proterozoic–Phanerozoic boundary. Thus, this is the youngest Doushantuo-Pertatataka-type microfossil assemblage yet described. It extends the range of Appendisphaera, Cavaspina, Megasphaera and Variomargosphaeridium upward by tens of millions of years. The assemblage adds to a growing database of Ediacaran fossils and emphasizes the importance of Mongolian strata to understanding the transition from a broadly microbial Proterozoic Eon to a Phanerozoic Eon where macroscopic animals acted as geobiological agents.

Ocean and Atmosphere Geochemical Proxies Derived from Trace Elements in Marine Pyrite: Implications for Ore Genesis in Sedimentary Basins

Trace element concentrations in marine pyrite, measured by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), have the potential to open a new window into deep-time ocean chemistry, atmosphere oxygenation, and genesis of basin-hosted ore deposits. Only early-formed syngenetic and early diagenetic marine pyrite preserves the trace element chemistry of the past oceans, as late diagenetic and metamorphic recrystallization of pyrite changes the trace element budget. A database of over 5,000 marine pyrite trace element analyses by LA-ICP-MS has enabled the development of deep-time proxies for nutrient supply, productivity, ocean pH, and atmosphere oxygenation. These proxies suggest that the Archean ocean was enriched in Fe, Ni, Co, As, Au, and Hg compared with modern oceans, probably related to composition of erosive flux from the continents and active seafloor hydrothermal activity. This was also a time for major iron, gold, and nickel ore formation in sedimentary and greenstone settings. In the Paleoproterozoic, there was a decrease in Ni, Co, As, and Au, replaced by increasing Cu, Zn, and ![Formula][1] in the oceans and O2 in the atmosphere. The first appearance of red beds and evaporites is a response to the rise in O2 and ![Formula][2] , and provided the conditions necessary for sediment-hosted Cu and stratiform Pb-Zn-Ag sedimentary exhalative (SEDEX) deposits. Through 1700 to 1500 Ma, phosphorous, gold, and most other nutrient trace elements dropped to a minimum in the ocean, possibly related to tectonic stasis and changes in atmosphere O2 and/or ocean pH. Sediment-hosted Au, orogenic Au, and volcanic-hosted massive sulfide (VHMS) deposits are virtually absent from this period, whereas mineral systems that required relatively oxidized ore fluids, such as SEDEX Zn-Pb, iron oxide copper-gold (IOCG), and unconformity uranium became more abundant, due to these changed conditions. All redox-sensitive and nutrient trace elements rose dramatically in concentration at the Proterozoic-Phanerozoic boundary and peaked in the mid- to Late Cambrian oceans, accompanied by black shale deposition enriched in Mo, Se, Ni, Ag ± Au, and platinum group elements. Cyclic variation in nutrient trace elements increased in frequency through the Phanerozoic on a wavelength of 50 to 100 m.y., compared with 500 to 1,000 m.y. in the Proterozoic. The more frequent Phanerozoic cycles relate to repeated episodes of continent collision, mountain building, and increased erosive flux of trace elements into the oceans. Ore deposit cycles in the Phanerozoic of SEDEX Zn-Pb, orogenic sediment-hosted Au, and VHMS have a time frame similar to the tectonic and seawater chemistry cycles. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif

The first magnetostratigraphic data on the stratotype of the Lopata Formation, Northeastern Yenisei Ridge: Problems of its age and paleogeography of the Siberian Platform at the Proterozoic–Phanerozoic boundary

New paleomagnetic and magnetostratigraphic data are presented for the stratotype of the Upper Riphean Lopata Formation (Teya River, Yenisei Ridge). The paleomagnetic pole calculated is significantly distinct from the Phanerozoic and Riphean poles of the Siberian Platform and is similar to the Late Vendian–Early Cambrian poles of the Madagascar Group. The stratigraphic range studied is characterized by an anomalously high frequency of geomagnetic inversions (15 zones of magnetic polarity), which is comparable with the inversion frequency of the Late Vendian sections of Baltica. These data, along with previous paleontological findings, indicate an age of the Lopata Formation of 555–540 Ma.

A molecular genetic timescale for the diversification of autotrophic stramenopiles (Ochrophyta): substantive underestimation of putative fossil ages.

BackgroundStramenopiles constitute a large and diverse eukaryotic clade that is currently poorly characterized from both phylogenetic and temporal perspectives at deeper taxonomic levels. To better understand this group, and in particular the photosynthetic stramenopiles (Ochrophyta), we analyzed sequence data from 135 taxa representing most major lineages. Our analytical approach utilized several recently developed methods that more realistically model the temporal evolutionary process.Methodology/Principal FindingsPhylogenetic reconstruction employed a Bayesian joint rate- and pattern-heterogeneity model to reconstruct the evolutionary history of these taxa. Inferred phylogenetic resolution was generally high at all taxonomic levels, sister-class relationships in particular receiving good statistical support. A signal for heterotachy was detected in clustered portions of the tree, although this does not seem to have had a major influence on topological inference. Divergence time estimates, assuming a lognormally-distributed relaxed molecular clock while accommodating topological uncertainty, were broadly congruent over alternative temporal prior distributions. These data suggest that Ochrophyta originated near the Proterozoic-Phanerozoic boundary, diverging from their sister-taxon Oomycota. The evolution of the major ochrophyte lineages appears to have proceeded gradually thereafter, with most lineages coming into existence by ∼200 million years ago.Conclusions/SignificanceThe evolutionary timescale of the autotrophic stramenopiles reconstructed here is generally older than previously inferred from molecular clocks. However, this more ancient timescale nevertheless casts serious doubt on the taxonomic validity of putative xanthophyte/phaeophyte fossils from the Proterozoic, which predate by as much as a half billion years or more the age suggested by our molecular genetic data. If these fossils truly represent crown stramenopile lineages, then this would imply that molecular rate evolution in this group proceeds in a fashion that is fundamentally incompatible with the relaxed molecular clock model employed here. A more likely scenario is that there is considerable convergent morphological evolution within Heterokonta, and that these fossils have been taxonomically misdiagnosed.

The oxygen isotope evolution of seawater: A critical review of a long-standing controversy and an improved geological water cycle model for the past 3.4 billion years

Controversy over the oxygen isotope composition of seawater began in the 1950's, since which time there has been no agreement over whether the oxygen isotope composition of the oceans has changed over time. Resolving this uncertainty would allow the δ 18O values of demonstrably well preserved marine authigenic precipitates to be used to reconstruct surface climate trends back to early Archean times and would help towards a more quantitative description of Earth's global water cycle on geological time scales. Isotopic studies of marine carbonate and silica reveal a trend of increasing δ 18O values with decreasing age since the Archean. This trend has been interpreted by some to reflect a progressive increase in seawater δ 18O through time; however, it is generally accepted on the basis of ophiolite studies and theoretical considerations that seawater δ 18O cannot change significantly because of the buffering effects of ocean crust alteration at mid-ocean ridges. As a result many alternative interpretations have been proposed, including meteoric alteration; warmer paleoclimates; higher seawater pH; salinity stratification and biased sampling. Here we review these interpretations in the light of an updated compilation of marine carbonate δ 18O from around the world, covering the Phanerozoic and Precambrian rock records. Recent models of the geological water cycle demonstrate how long-term trends in chemical weathering and hydrothermal circulation can indeed influence the O-isotope composition of the global ocean to the extent necessary to explain the carbonate δ 18O trend, with residual variation attributed to climatic fluctuations and post-depositional alteration. We present the further development of an existing model of the geological water cycle. In the model, seawater δ 18O increased from about − 13.3‰ to − 0.3‰ over a period of 3.4 Ga, with average surface temperatures fluctuating between 10 °C to 33 °C, which is consistent with known biological constraints. Similar temperature variations are also obtained, although with higher starting seawater δ 18O composition, when more conservative approaches are used that take into account the systematic effects of diagenetic alteration on mean calcite δ 18O values. In contrast to much published opinion, the average δ 18O value of ocean crust in the model remained relatively unchanged throughout all model runs. Invariable ophiolite δ 18O values can, therefore, not be used as a definitive argument against changing seawater δ 18O. The most likely explanation for the long-term trend in seawater δ 18O invokes two stepwise increases in the ratio of high- to low-temperature fluid/rock interactions. An initial increase may have occurred close to the Archean–Proterozoic boundary, but a possibly more significant increase took place near the Proterozoic–Phanerozoic boundary. Possible explanations for extremely low seawater δ 18O during the Archean include higher continental weathering rates caused by a combination of higher atmospheric pCO 2 (necessarily combined with high CO 2 outgassing rates), a greater abundance of relatively easily weathered volcanic rocks in greenstone belts and partial emergence of spreading ridges. The second increase may have been caused by the suppression of low-temperature overprinting of ocean crust alteration by the formation of a thick sediment cover on ridge flanks due to the emergence of shelly plankton at the beginning of the Phanerozoic. Postulated increases in spreading ridge depths since the Archean would also have enhanced the efficiency of vertical heat flux and changed the depth at which hydrothermal fluids boil, both of which would favour high- over low-temperature interactions with time.

Changes in sub‐water table fluid flow at the end of the Proterozoic and its implications for gas pulsars and MVT lead–zinc deposits

AbstractA fundamental change in the nature of sub‐water table fluid flow occurred at roughly the Proterozoic–Phanerozoic boundary when organic matter began to be buried in sufficient quantities that nonaqueous fluids could occupy a significant fraction of the pore space. This allowed the formation of remarkably durable capillary seals that could trap gas in large portions (hundreds of kilometers) of a basin for hundreds of millions of years. Gas loss from these gas zones can be highly dynamic, especially during gas generation. Under the right circumstances, hundreds of cubic kilometers of gas can be rapidly discharged into adjacent permeable aquifers. In Pennsylvanian/Permian time, the Arkoma Basin may have repeatedly discharged such volumes of gas into the very permeable Cambrian sandstone and karstic carbonate aquifers of the mid‐continent of the United States. This could have displaced brines rapidly enough to form the Mississippi Valley‐type (MVT) lead–zinc deposits of this age that are associated with the Arkoma Basin, heating them only briefly as required by maturity indicators. Sea level rise accompanying the melting of Permian continental glaciers may have triggered the gas expulsion events. This radically new mechanism for the formation of MVT deposits is just one example of the nonlinear dynamics of gas accumulations that are possible since Late Proterozoic time.

Initiation of ultrahigh‐pressure metamorphism and its significance on the Proterozoic–Phanerozoic boundary

Ultrahigh‐pressure (UHP) metamorphism began at 540–530 Ma near the Precambrian–Cambrian boundary, and repeated more than 10 times through the Phanerozoic, indicating one of the most prominent tectonic aspects of the Phanerozoic Earth. The preceding subduction zone metamorphism back to the Archean had been of the intermediate‐ to low‐P type. An apparent change in the P/T conditions of subduction zone metamorphism occurred at the Precambrian–Cambrian boundary at ca 544 Ma. At ∼ 200 million years earlier than the initiation of UHP metamorphism, blueschist facies metamorphism occurred along the northern margin of the Tarim craton. A transitional period between the Proterozoic and Phanerozoic at 750–540 Ma corresponds to the initiation of blueschist facies metamorphism at the subduction zone; the cooling Earth dropped its geothermal gradient which intersected the blueschist facies P–T field. An abrupt change in the geothermal gradient from 22–15 °C/km (Archean–Proterozoic) to 6–7 °C/km (Phanerozoic) to initiate UHP metamorphism may correspond to a rather abrupt increase of slab‐pull driving force at the beginning of the Phanerozoic. Large amounts of heat loss from the Earth's interior may have occurred just before 540 Ma; presumably it may correspond to the duration from the breakup of supercontinent Rodinia at 750 Ma, to the formation of the next supercontinent Gondwana at 540 Ma.Through the transient period 750–540 Ma the most extensive glaciation in Earth's history ended at ca 600 Ma, when the world's largest‐scale unconformity covered nearly all the continents on the Earth. Huge landmasses from ∼ 5 to 30% of the Earth's surface first raised above sea‐level during 600–540 Ma; such change may have initiated since ca 750 Ma, and repeated four times until 540 Ma. Emergence of large continental masses above sea‐level resulted in extensive erosion and diversity of surficial environment, and significant modification in the composition of seawater and world climates. Coincidentally or accidentally with the breakup of the latest Proterozoic supercontinent, the Cambrian life explosion began after the transient period in which the Ediacara fauna evolved along rifted seaways with hot springs in a partly rifted supercontinent.The complex scenario of these phenomena may be triggered and linked by a sudden increase in the plate‐driving force of slab‐pull during 750–540 Ma, which allowed buoyant continental crust to be subducted to a 150 km depth and subjected to UHP metamorphism. The increased slab‐pull force reflects an increase in plate thickness, and is the surface manifestation of the loss of a large amount of internal heat from the Earth's interior before the initiation of UHP metamorphism. It occurred in a short time span (750–540 Ma) from rifting of Rodinia to amalgamation of the next supercontinent Gondwana. Since then, old slabs with deep oceans survived, large landmasses raised above sea‐level, the surface environment diversified, and the explosion of life accelerated before and after the mass extinction that occurred at 550–540 Ma.

Changes in the trace fossil biota across the Proterozoic-Phanerozoic boundary

Trace fossils became relatively diverse in shallow-water clastic seas in the late Proterozoic (Vendian), with a further significant increase in abundance, diversity and complexity in the Tommotian and Atdabanian. Little change followed in the remainder of the Lower Palaeozoic. Traces typical of deeper-water facies evolved in shallow water during the Vendian and early Cambrian, and may have slowly migrated into the deep ocean during the remainder of the Lower Palaeozoic. There are a limited number of short-ranging ichnogenera which may be useful for correlation but the first appearance of ichnogenera may be a more satisfactory method. Three trace fossil zones covering the ranges Redkino and Kotlin, Rovno and Tommotian-lower Atdabanian are recognized and may be useful for world-wide correlation.

Radiochronological age and correlation of proterozoic sediments in Brazil

A review of available Rb—Sr and K—Ar datings obtained on sedimentary sequences, metamorphosed or not, interbedded volcanics and cross-cutting intrusives of the Precambrian of Brazil yields the following conclusions: 1.(1) The Roraima and Rio Fresco Formations, resting upon the Amazonian craton, have been affected by the Trans-Amazonian orogeny and are of Lower Proterozoic age.2.(2) The Beneficiente Group, in the same region, seems to be of Middle Proterozoic, Lower Riphean age.3.(3) Upon the Sao Francisco craton, and upon the Lower Proterozoic Rio dos Remedios complex, the Paraguaçu Group might be of middle Riphean age and the glacial sequences, Macaubas Group and Bebedouro Formation, date back to between 950 and 1000 Ma.4.(4) The age of the Bambui and Una Groups, in the same region, remains undetermined. It might be either Upper Riphean, or Upper Riphean and Vendian.5.(5) The molassic series associated with the Brazilian orogeny are dated back to between the Proterozoic—Phanerozoic boundary and the Ordovician. (1) The Roraima and Rio Fresco Formations, resting upon the Amazonian craton, have been affected by the Trans-Amazonian orogeny and are of Lower Proterozoic age. (2) The Beneficiente Group, in the same region, seems to be of Middle Proterozoic, Lower Riphean age. (3) Upon the Sao Francisco craton, and upon the Lower Proterozoic Rio dos Remedios complex, the Paraguaçu Group might be of middle Riphean age and the glacial sequences, Macaubas Group and Bebedouro Formation, date back to between 950 and 1000 Ma. (4) The age of the Bambui and Una Groups, in the same region, remains undetermined. It might be either Upper Riphean, or Upper Riphean and Vendian. (5) The molassic series associated with the Brazilian orogeny are dated back to between the Proterozoic—Phanerozoic boundary and the Ordovician.

Earliest Phanerozoic or latest Proterozoic fossils from the Arabian Shield

Cloud, P., Awramik, S.M., Morrison, K. and Hadley, D.G., 1979. Earliest Phanerozoic or latest Proterozoic fossils from the Arabian Shield. Precambrian Res., 10: 73--93. We report here the first biologically definable fossils from pre-Saq (pre-Middle Cambrian) rocks of the Arabian Shield. They include the distinctive helically coiled tubular filaments of the oscillatorialean blue-green alga Obruchevella parva as well as two size classes of spheroidal unicells of uncertain affinity. Also present is the conical stromatolite Conophyton and unidentified stromatolites. All occur in cherty limestones of the Jubaylah Group, northern Saudi Arabia, a nonmarine to locally marine taphrogeosynclinal sequence that fills depressions along the northwest-trending Najd faults. Conophyton has heretofore been found only in strata older than about 680 Ma (except for puzzling records in modern hot springs) while Obruchevella is so far known only from rocks between about 680 and 470 Ma old. Thus it appears that the Jubaylah Group is close to the Proterozoic-Phanerozoic transition. The simple spheroidal nannofossils are not diagnostic as to age. Their relationships within what appears to be early diagenetic chert suggest a classical algal-mat association. The brecciated and microchanneled appearance of much of the fossiliferous rock, its locally dolomitic nature, and the prevalence of cryptalgalaminate favors a very shallow, locally turbulent, and perhaps episodically exposed marine or marginal marine setting. The Jubaylah Group lies unconformably beneath the Siq Sandstone (basal member of the Saq Sandstone) of medial Cambrian age, rests nonconformably on crystalline basement, *The on Precambrian Stratigraphy of the International Union of Geological Sciences in July 1977 recommended the use of the time-honored term Proterozoic for time and events between Archean and Phanerozoic (James, 1978) and this usage has been adopted by the U.S. Geological Survey (Sohl and Wright, 1978). To date the Subcommission has not reached a decision on placement of the ProterozoicPhanerozoic boundary. In this paper, however, we follow the practice of including in the Phanerozoic, and thus regarding as post-Proterozoic, those sub-Cambrian rocks and events that are referable to the paleontologically defined Ediacarian System of Termier and Termier (1960) as discussed by Cloud (1976). For clarity in discussion, the term Precambrian with a capital P is here avoided and rocks older than Ediacarian (or Vendian are designated as pre-Phanerozoic.