Segui questo link per vedere altri tipi di pubblicazioni sul tema: Stratigraphic Proterozoic.
Articoli di riviste sul tema "Stratigraphic Proterozoic"
Cita una fonte nei formati APA, MLA, Chicago, Harvard e in molti altri stili
Vedi i top-50 articoli di riviste per l'attività di ricerca sul tema "Stratigraphic Proterozoic".
Accanto a ogni fonte nell'elenco di riferimenti c'è un pulsante "Aggiungi alla bibliografia". Premilo e genereremo automaticamente la citazione bibliografica dell'opera scelta nello stile citazionale di cui hai bisogno: APA, MLA, Harvard, Chicago, Vancouver ecc.
Puoi anche scaricare il testo completo della pubblicazione scientifica nel formato .pdf e leggere online l'abstract (il sommario) dell'opera se è presente nei metadati.
Vedi gli articoli di riviste di molte aree scientifiche e compila una bibliografia corretta.
1
Wu, He Yuan, e Bin Hao. "Third-Order Sequence Division of Yunmengshan and Baicaoping Formation of Proterozoic in Yuxi District of China: an Example from Xiatang Profile in Lushan". Advanced Materials Research 998-999 (luglio 2014): 1492–97. http://dx.doi.org/10.4028/www.scientific.net/amr.998-999.1492.
Testo completoAbstract (sommario):
There are controversies on the Proterozoic stratigraphic genesis, division, correlation and palaeogeographical evolution of western Henan in China. Based on the basic description of sedimentary facies, Yunmengshan and Baicaoping formation of Proterozoic typical section in western Henan is divided into 4 third-order sequences. Sequence stratigraphy framework which reflects sedimentary and overlap is established with basis of two kinds of facies-change surface and two kinds of diachrononism in stratigraphical records. Although chronostratigraphic belonging of Precambrian strata is controversial and Precambrian sequential stratigraphic study is tremendously challenging, the establishment of sequence stratigraphy framework of proterozoic Yunmengshan and Baicaoping formation in western Henan provides actual data to reshape palaeogeographic pattern of Palaeoproterozoic North China craton. What is more, it becomes a typical example of characteristics and exploration of stratigraphic accumulation under the background of tidal action.
2
Halverson, Galen P., Susannah M. Porter e Timothy M. Gibson. "Dating the late Proterozoic stratigraphic record". Emerging Topics in Life Sciences 2, n. 2 (13 luglio 2018): 137–47. http://dx.doi.org/10.1042/etls20170167.
Testo completoAbstract (sommario):
The Tonian and Cryogenian periods (ca. 1000–635.5 Ma) witnessed important biological and climatic events, including diversification of eukaryotes, the rise of algae as primary producers, the origin of Metazoa, and a pair of Snowball Earth glaciations. The Tonian and Cryogenian will also be the next periods in the geological time scale to be formally defined. Time-calibrating this interval is essential for properly ordering and interpreting these events and establishing and testing hypotheses for paleoenvironmental change. Here, we briefly review the methods by which the Proterozoic time scale is dated and provide an up-to-date compilation of age constraints on key fossil first and last appearances, geological events, and horizons during the Tonian and Cryogenian periods. We also develop a new age model for a ca. 819–740 Ma composite section in Svalbard, which is unusually complete and contains a rich Tonian fossil archive. This model provides useful preliminary age estimates for the Tonian succession in Svalbard and distinct carbon isotope anomalies that can be globally correlated and used as an indirect dating tool.
3
Sergeeva, Nina Dmitrievna, e Viktor Nikolaevich Puchkov. "REGIONAL STRATIGRAPHIC SCHEME OF THE UPPER AND FINAL RIPHEAN AND VENDIAN DEPOSITS OF THE SOUTHERN URALS (PROJECT 2022)". Geologicheskii vestnik, n. 2 (14 luglio 2022): 3–14. http://dx.doi.org/10.31084/2619-0087/2022-2-1.
Testo completoAbstract (sommario):
The need to correct individual stratigraphic levels of the Regional Stratigraphic Scheme of the Upper Precambrian deposits of the Urals, existing since 1993, is due to the receipt of new data from lithological-stratigraphic, geotectonic and isotope-geochronological studies of the Upper Precambrian of the Southern Urals. Significant changes and clarifications in the stratigraphy of the Upper Precambrian formations of the region occurred in the Upper Riphean and Vendian of the Bashkir meganticlinorium in the Southern Urals, where the sections stratotypical for the Riphean and reference for the Vendian are located.
The results of dating igneous (primarily volcanic) rocks in the Riphean by modern methods made it possible to refine the geochronological basis of the Ural and General Stratigraphic Scale of the Upper Proterozoic of Russia and identify a new event level: the final Riphean (Arshinian), corresponding to the Arshinian series. Changes and clarifications to the correlation of local stratigraphic sections of the Upper and Final Riphean and Vendian of the Southern Urals are reflected in the draft scheme.
4
Roscoe, S. M., e K. D. Card. "The reappearance of the Huronian in Wyoming: rifting and drifting of ancient continents". Canadian Journal of Earth Sciences 30, n. 12 (1 dicembre 1993): 2475–80. http://dx.doi.org/10.1139/e93-214.
Testo completoAbstract (sommario):
Striking stratigraphic and sedimentological similarities between the Early Proterozoic Huronian Supergroup of the Canadian Shield and the Snowy Pass Supergroup of Wyoming suggest that they were deposited in a single, broad, epicratonic basin developed atop a large Archean continent that included the Superior and Wyoming geological provinces. Breakup of the continent after the 2.2 Ga intrusion of widespread gabbro sheets and dykes resulted in the separation of the Archean Superior and Wyoming cratons and their Early Proterozoic covers. These crustal fragments were subsequently reassembled during Early Proterozoic (~1.85 Ga) orogenesis, the end result being the present 2000 km separation of the Huronian and Snowy Pass supergroups and their Archean basements.
5
Martins-Neto, Marcelo A. "Sequence stratigraphic framework of Proterozoic successions in eastern Brazil". Marine and Petroleum Geology 26, n. 2 (febbraio 2009): 163–76. http://dx.doi.org/10.1016/j.marpetgeo.2007.10.001.
Testo completo
6
Phillips, Bruce J., Alan W. James e Graeme M. Philip. "THE GEOLOGY AND HYDROCARBON POTENTIAL OF THE NORTH-WESTERN OFFICER BASIN". APPEA Journal 25, n. 1 (1985): 52. http://dx.doi.org/10.1071/aj84004.
Testo completoAbstract (sommario):
Recent petroleum exploration in EP 186 and EP 187 in the north-western Officer Basin has greatly increased knowledge of the regional stratigraphy, structure and petroleum prospectivity of the region. This exploration programme has involved the drilling of two deep stratigraphic wells (Dragoon 1 and Hussar 1) and the acquisition of 1438 km of seismic data. Integration of regional gravity and aeromagnetic data with regional seismic and well data reveals that the Gibson Sub-basin primarily contains a Proterozoic evaporitic sequence. In contrast, the Herbert Sub-basin contains a Late Proterozoic to Cambrian clastic and carbonate sequence above the evaporites. This sequence, which was intersected in Hussar 1, is identified as the primary exploration target in the Western Officer Basin. The sequence contains excellent reservoir and seal rocks in association with mature source rocks. Major structuring of the basin has also been caused by compressive movements associated with the Alice Springs Orogeny. The northwestern Officer Basin thus has all of the ingredients for the discovery of commercial hydrocarbons.
7
Cook, Frederick A., e Samantha M. Siegel. "From Proterozoic strata to a synthesized seismic reflection trace: implications for regional seismic reflection patterns in northwestern Canada". Canadian Journal of Earth Sciences 43, n. 11 (1 novembre 2006): 1639–51. http://dx.doi.org/10.1139/e06-040.
Testo completoAbstract (sommario):
Calculation of a synthetic seismic reflection trace from detailed descriptions of exposed Proterozoic strata in northwestern Canada permits correlation of reflections on regional seismic profiles to surface outcrop. Approximately 5.4 km (composite thickness) of Paleo- and Mesoproterozoic strata are exposed in the Muskwa anticlinorium that is located within the foreland of the Cordillera in northeastern British Columbia. The Tuchodi anticline is the easternmost structure of the Muskwa anticlinorium and has the deepest levels of Proterozoic strata exposed. At this location, prominent seismic reflection layering rises toward the surface and is easily correlated to the deeper formations of the Muskwa assemblage stratigraphy. These layers are followed westward into the middle crust, where they are overlain by dramatically thickened (by about five times) strata, primarily of the Tuchodi Formation. Along the same line of section, the Muskwa assemblage reflections overlie additional subparallel layered reflections at depth whose lithology and origin are unknown. However, coupled with other observations, including regional refraction results that indicate the crustal layers have both low seismic p-wave velocities and low ratios of p- and s-velocities, regional gravity observations that indicate the layers are low density, and correlation to similar layers on other seismic profiles that exhibit characteristic seismic stratigraphic features, the subparallel layers that are present beneath the known Muskwa assemblage are most easily interpreted as layered Proterozoic (meta-) sedimentary rocks. These results provide the basis for interpreting the Muskwa anticlinorium as a crustal-scale structure that formed when a deep basin of Proterozoic strata was inverted and thrust over an ~20 km high footwall ramp during Cordilleran orogenesis.
8
Dub, S. A. "Upper Precambrian General Stratigraphic Scale of Russia: Main problems and proposals for improvement". LITHOSPHERE (Russia) 21, n. 4 (28 agosto 2021): 449–68. http://dx.doi.org/10.24930/1681-9004-2021-21-4-449-468.
Testo completoAbstract (sommario):
Research subject. Main problems of the General Stratigraphic Scale (GSS) of the Upper Precambrian including uncertainties in the hierarchy of subdivisions are analyzed.Results. Prospects for detailing the Upper Precambrian GSS are discussed, along with questions of its correlation with International Chronostratigraphic Chart (ICSC) and establishing the lower boundaries of chronostratigraphic subdivisions. The importance of unifying the existing views is emphasized.Conclusions. It is proposed to carry out the following reforms of GSS: to abolish Acrothemes / Acrons; to approve the Proterozoic (as well as the Archean) as an Eonotheme / Eon; to minimize the use of terms “Upper Proterozoic” and “Lower Proterozoic”; to assign the Riphean and Vendian to the rank of Erathem / Era (while preserving the status of the Vendian as a System / Period); to consider Burzyanian, Yurmatinian, Karatavian and Arshinian as Systems / Periods of the Riphean. Attention is focused on the Upper Riphean-Vendian interval. The lower boundary of the Upper Riphean (Karatavian) was proposed to establish according to the first appearance of the Trachyhystrichosphaera sp. microfossils. Then, the Terminal Riphean (Arshinian) lower boundary should be traced to the base of the tillites formed during the global Sturtian glaciation (which approximately corresponds to the base of the Cryogenian in ICSC). Apparently, the Vendian lower boundary may be raised to the level of the top of the Gaskiers tillites, as the deposits of the last major glaciation in the Precambrian. The indicated proposals are substantiated. It is necessary to form work groups to develop solutions.
9
Hiatt, Eric E., T. Kurtis Kyser, Paul A. Polito, Jim Marlatt e Peir Pufahl. "The Paleoproterozoic Kombolgie Subgroup (1.8 Ga), McArthur Basin, Australia: Sequence stratigraphy, basin evolution, and unconformity-related uranium deposits following the Great Oxidation Event". Canadian Mineralogist 59, n. 5 (1 settembre 2021): 1049–83. http://dx.doi.org/10.3749/canmin.2000102.
Testo completoAbstract (sommario):
ABSTRACT Proterozoic continental sedimentary basins contain a unique record of the evolving Earth in their sedimentology and stratigraphy and in the large-scale, redox-sensitive mineral deposits they host. The Paleoproterozoic (Stratherian) Kombolgie Basin, located on the Arnhem Land Plateau, Northern Territory, is an exceptionally well preserved, early part of the larger McArthur Basin in northern Australia. This intracratonic basin is filled with 1 to 2 km-thick, relatively undeformed, nearly flat-lying, siliciclastic rocks of the Kombolgie Subgroup. Numerous drill cores and outcrop exposures from across the basin allow sedimentary fabrics, structures, and stratigraphic relationships to be studied in great detail, providing an extensive stratigraphic framework and record of basin development and evolution. Tectonic events controlled the internal stratigraphic architecture of the basin and led to the formation of three unconformity-bounded sequences that are punctuated by volcanic events. The first sequence records the onset of basin formation and is comprised of coarse-grained sandstone and polymict lithic conglomerate deposited in proximal braided rivers that transported sediment away from basin margins and intra-basin paleohighs associated with major uranium mineralization. Paleo-currents in the upper half of this lower sequence, as well as those of overlying sequences, are directed southward and indicate that the major intra-basin topographic highs no longer existed. The middle sequence has a similar pattern of coarse-grained fluvial facies, followed by distal fluvial facies, and finally interbedded marine and eolian facies. An interval marked by mud-rich, fine-grained sandstones and mud-cracked siltstones representing tidal deposition tops this sequence. The uppermost sequence is dominated by distal fluvial and marine facies that contain halite casts, gypsum nodules, stromatolites, phosphate, and “glauconite” (a blue-green mica group mineral), indicating a marine transgression. The repeating pattern of stratigraphic sequences initiated by regional tectonic events produced well-defined coarse-grained diagenetic aquifers capped by intensely cemented distal fluvial, shoreface, eolian, and even volcanic units, and led to a well-defined heterogenous hydrostratigraphy. Basinal brines migrated within this hydrostratigraphy and, combined with paleotopography, dolerite intrusion, faulting, and intense burial diagenesis, led to the economically important uranium deposits the Kombolgie Basin hosts. Proterozoic sedimentary basins host many of Earth's largest high-grade iron and uranium deposits that formed in response to the initial oxygenation of the hydrosphere and atmosphere following the Great Oxygenation Event. Unconformity-related uranium mineralization like that found in the Kombolgie Basin highlights the interconnected role that oxygenation of the Earth, sedimentology, stratigraphy, and diagenesis played in creating these deposits.
10
Ansdell, Kevin M., T. Kurtis Kyser, Mel R. Stauffer e Garth Edwards. "Age and source of detrital zircons from the Missi Formation: a Proterozoic molasse deposit, Trans-Hudson Orogen, Canada". Canadian Journal of Earth Sciences 29, n. 12 (1 dicembre 1992): 2583–94. http://dx.doi.org/10.1139/e92-205.
Testo completoAbstract (sommario):
The Missi Formation in the Flin Flon Basin forms part of a discontinuous series of molasse-type sediments found throughout the Early Proterozoic Trans-Hudson Orogen in northern Saskatchewan and Manitoba. The Flin Flon Basin contains a sequence of proximal-fan to braided-stream fluvial conglomerates and sandstones, which unconformably overlie subaerially weathered Amisk Group volcanic rocks. Stratigraphic way-up indicators have been preserved, even though these rocks have undergone greenschist-facies metamorphism and polyphase deformation. The sedimentary rocks are crosscut by intrusive rocks, which provide a minimum age of sedimentation of 1840 ± 7 Ma.Detrital zircons from each of the six stratigraphic subdivisions of the Flin Flon Basin were analyzed using the single-zircon Pb-evaporation technique. Euhedral to slightly rounded zircons dominate each sample, and these zircons give ages of between about 1854 and 1950 Ma. The Missi sediments were thus deposited between 1840 and 1854 Ma. Possible sources for the detrital zircons are Amisk Group felsic volcanic rocks and post-Amisk granitoid rocks and orthogneisses in adjacent domains within the Trans-Hudson Orogen. However, the immature character of the sedimentary rocks, the composition of clasts, the euhedral character of many of the zircons, and the range in ages suggest that most were likely derived from Amisk Group and granitoid rocks in the western Flin Flon Domain. Rounded zircons are uncommon but provide evidence for the reworking of older Proterozoic sedimentary rocks, or a distant Archean or Early Proterozoic granitoid terrane.
11
Cosgrove, G. I. E., L. Colombera e N. P. Mountney. "Eolian stratigraphic record of environmental change through geological time". Geology 50, n. 3 (22 novembre 2021): 289–94. http://dx.doi.org/10.1130/g49474.1.
Testo completoAbstract (sommario):
Abstract The terrestrial sedimentary record provides a valuable archive of how ancient depositional systems responded to and recorded changes in Earth's atmosphere, biosphere, and geosphere. However, the record of these environmental changes in eolian sedimentary successions is poorly constrained and largely unquantified. Our study presents the first global-scale, quantitative investigation of the architecture of eolian systems through geological time via analysis of 55 case studies of eolian successions. Eolian deposits accumulating (1) under greenhouse conditions, (2) in the presence of vascular plants and grasses, and (3) in rapidly subsiding basins associated with the rifting of supercontinents are represented by significantly thicker eolian dune-set, sand-sheet, and interdune architectural elements. Pre-vegetation eolian systems are also associated with more frequent interactions with non-eolian environments. The interplay of these forcings has resulted in dune-set thicknesses that tend to be smallest and largest in Proterozoic and Mesozoic successions, respectively. In the Proterozoic, the absence of sediment-binding plant roots rendered eolian deposits susceptible to post-depositional wind deflation and reworking by fluvial systems, whereby highly mobile channels reworked contiguous eolian deposits. During the Mesozoic, humid greenhouse conditions (associated with relatively elevated water tables) and high rates of basin subsidence (associated with the breakup of Pangea) favored the rapid transfer of eolian sediment beneath the erosional baseline. The common presence of vegetation promoted accumulation of stabilizing eolian systems. These factors acted to limit post-depositional reworking. Eolian sedimentary deposits record a fingerprint of major environmental changes in Earth history: climate, continental configuration, tectonics, and land-plant evolution.
12
Mitchelmore, Marlene Dredge, e Frederick A. Cook. "Inversion of the Proterozoic Wernecke basin during tectonic development of the Racklan Orogen, northwest Canada". Canadian Journal of Earth Sciences 31, n. 3 (1 marzo 1994): 447–57. http://dx.doi.org/10.1139/e94-041.
Testo completoAbstract (sommario):
New deep seismic reflection data coupled with regional stratigraphic correlations, drill-hole information, and potential field data are interpreted to provide images of Middle Proterozoic Wernecke Supergroup (meta-)sedimentary layers that were uplifted during tectonic development of the ca. 0.9–1.3 Ga Racklan Orogen in Canada's western Northwest Territories. The reflection data are located at the eastern front of the Mackenzie Mountains portion of the Canadian Cordillera and on the western flank of the Fort Simpson structural trend that is a prominent Proterozoic structure in the subsurface throughout the region. Along three parallel profiles, layers that are correlated with thick Wernecke Supergroup sedimentary rocks produce prominent reflections between about 3.0 and 9.0 s (about 7.5 and 23 km) that were arched prior to deposition of younger Proterozoic (probably Mackenzie Mountains Supergroup) and Phanerozoic sedimentary rocks. The strata are considered to be Wernecke basin sedimentary rocks that were uplifted during deformation associated with the development of the Racklan Orogen.
13
Ming, Yan, Liu Yingjun e Ma Dongsheng. "Stratigraphic geochemistry of Upper-Middle Proterozoic Suberathem in northern Guangxi, China". Chinese Journal of Geochemistry 14, n. 3 (luglio 1995): 231–42. http://dx.doi.org/10.1007/bf02842046.
Testo completo
14
Bailey, A. H. E., E. Grosjean, L. Wang, C. J. Boreham, G. A. Butcher, C. J. Carson, A. J. M. Jarrett et al. "Resource potential of the Carrara Sub-basin from the deep stratigraphic well NDI Carrara 1". APPEA Journal 62, n. 2 (13 maggio 2022): S378—S384. http://dx.doi.org/10.1071/aj21075.
Testo completoAbstract (sommario):
NDI Carrara 1 is a deep stratigraphic well completed in 2020 as part of the MinEx CRC National Drilling Initiative (NDI), in collaboration with Geoscience Australia and the Northern Territory Geological Survey. It is the first stratigraphic test of the Carrara Sub-basin, a newly discovered depocentre in the South Nicholson Region. The well intersected Proterozoic sediments with numerous hydrocarbon shows, likely to be of particular interest due to affinities with the known Proterozoic plays of the Beetaloo Sub-basin and the Lawn Hill Platform, including two organic-rich black shales and a thick sequence of interbedded black shales and silty-sandstones. Alongside an extensive suite of wireline logs, continuous core was recovered from 283.9 m to total depth at 1750.8 m, providing high-quality data to support comprehensive analysis. Presently, this includes geochronology, geochemistry, geomechanics and petrophysics. Rock-Eval pyrolysis data demonstrate the potential for several thick black shales to be a source of hydrocarbons for conventional and unconventional plays. Integration of these data with geomechanical properties highlights potential brittle zones within the fine-grained intervals where hydraulic stimulation is likely to enhance permeability, identifying prospective Carrara Sub-basin shale gas intervals. Detailed wireline log analysis further supports a high potential for unconventional shale resources. Interpretation of the L210 and L212 seismic surveys suggests that the intersected sequences are laterally extensive and continuous throughout the Carrara Sub-basin, potentially forming a significant new hydrocarbon province and continuing the Proterozoic shale play fairway across the Northern Territory and northwest Queensland.
15
Alsop, G. I. "The geometry and structural evolution of a crustal-scale Caledonian fold complex: the Ballybofey Nappe, northwest Ireland". Geological Magazine 131, n. 4 (luglio 1994): 519–37. http://dx.doi.org/10.1017/s0016756800012139.
Testo completoAbstract (sommario):
AbstractThe gross geometries exhibited by crustal-scale fold nappes are considered a consequence of both original stratigraphic relationships associated with sub-basin configuration, coupled with the nature of the structural regime and tectonic processes involved in the generation of the nappe pile. The Neo-Proterozoic Dalradian metasediments of northwestern Ireland provide a well-constrained and correlatable stratigraphy which defines a sequence of sub-reclined, tight-isoclinal Caledonian (c. 460 Ma) fold nappes. Within this fold complex, the dominant structure is the crustal-scale Ballybofey Nappe, which may be traced for 40 km along strike and is responsible for a regional (500 km2) stratigraphie inversion. The gentle, NE-plunging attitude of this fold results in a complete spectrum of tectonic levels and deformation gradients being exposed. Relatively low strains in the upper fold limb gradually increase down through the nappe, resulting in the generation of composite foliations and lineations and the development of a 10 km thick shear zone which culminates in a high strain basal detachment with underlying pre-Caledonian basement. The Ballybofey Nappe nucleated and propagated along a major zone of lateral sedimentary facies variation, coincident with the margin of a major Dalradian sub-basin. The large amplitude of the nappe is strongly influenced by the lateral heterogeneity within the metasedimentary sequence, and is associated with a minimum of 25–30 km ESE-directed translation concentrated within the overturned limb. Additional significant displacement is also focused along the basal décollement. Generation of the nappe complex resulted in significant crustal thickening and amphibolite facies metamorphism consistent with 15–18 km of burial, induced by a sequence of nappes propagating in the direction of overshear. The ESE-directed translation of the major fold nappes is away from the Caledonian foreland and a gravity-driven mechanism of nappe emplacement is suggested. Rigorous structural analysis within the cohesive stratigraphie framework enables relationships between the tectonic evolution and stratigraphic patterns to be distinguished, thus allowing models of fold nappe generation and mid-crustal deformation to be evaluated.
16
Kulikov, V. S., V. V. Kulikova e A. K. Polin. "NEW CHRONOSTRATIC SCHEME OF SOUTH-EASTERN FENNOSCANDIA AND ITS USE IN THE PREPARATION OF SMALL-SCALE GEOLOGICAL MAPS OF THE PRECAMBRIAN REGIONS". Proceedings of higher educational establishments. Geology and Exploration, n. 5 (28 ottobre 2017): 5–12. http://dx.doi.org/10.32454/0016-7762-2017-5-5-12.
Testo completoAbstract (sommario):
A new chronostratic scheme of South-East (SE) Fennoscandia has been developed, based on the International Stratigraphie Scale, taking into account some elements of the Common Stratigraphic Scale of Russia and the regional stratigraphic scheme of the North-West (NW) of Russian Federation. A rank of Archean and Proterozoic stratons has been determined (including supersystems for Riphean and Archean geonotems), compatable in dutation to the Phanerozoic systems (Mesozoic and Paleozoic ones). An original coloring for the geological maps of the newly allocated systems and their analogues in the Precambrian, as well as the digital indexing of all stratons of the rank of systems instead of the traditional alphabetic one, have been proposed. Based on the extensive geological materials of the Institute of Geology of Karelian Research Centre of the Russian Academy of Sciences, in view of the new approaches and reliable geochronological data, an areal geological map of SE Fennoscandia in scale 1: 750 000 has been created, which includes the territory of Karelia and adjacent areas of the Russian Federation and eastern Finland. The proposed chronostratic scheme can serve as a basis for developing legends of small-scale state geological maps of the new generation, especially in the regions of the Precambrian development.
17
Marian, Melinda L., e Robert H. Osborne. "Petrology, petrochemistry, and stromatolites of the Middle to Late Proterozoic Beck Spring Dolomite, eastern Mojave Desert, California". Canadian Journal of Earth Sciences 29, n. 12 (1 dicembre 1992): 2595–609. http://dx.doi.org/10.1139/e92-206.
Testo completoAbstract (sommario):
The Beck Spring Dolomite is the medial unit of the Middle to Late Proterozoic Pahrump Group, the oldest sequence of sedimentary rocks in eastern California. Stratigraphic sections of the Beck Spring Dolomite examined in the eastern Mojave Desert and Death Valley regions consist of four members. These are, in ascending order, a lower cherty member, a lower laminated member, an oolitic–pisolitic member, and an upper cherty member. More than 80% of the Beck Spring Dolomite is algal-laminated dolomite with a possible Middle to Late Riphean stromatolite assemblage characterized by cf. Conophyton, eroded, irregular columnar forms similar to Kussiella or Baicalia, and several types of stratiform Stratifera. Petrographic, X-ray diffraction, and atomic absorption spectroscopic analyses indicate that the formation is composed of well-ordered replacement dolomite with less than 25% acid-insoluble residue. Concentrations of Fe and Mn are two to six times higher in the algal-laminated members than in the oolitic–pisolitic member, whereas the concentrations of Ca, Mg, Ba, Sr, Na, and K show no systematic variations. Stratigraphic relationships, primary and secondary sedimentary structures, petrology, and stromatolite assemblages suggest deposition during Middle to Late Proterozoic time on a platform that most likely included offshore shoals, restricted lagoons, and broad tidal flats with ponds, channels, and levees.
18
GEHLING, JAMES G., SÖREN JENSEN, MARY L. DROSER, PAUL M. MYROW e GUY M. NARBONNE. "Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland". Geological Magazine 138, n. 2 (marzo 2001): 213–18. http://dx.doi.org/10.1017/s001675680100509x.
Testo completoAbstract (sommario):
The range of Treptichnus pedum, the index trace fossil for the Treptichnus pedum Zone, extends some 4 m below the Global Standard Stratotype-section and Point for the base of the Cambrian Period at Fortune Head on the Burin Peninsula in southeastern Newfoundland. The identification of zigzag traces of Treptichnus isp., even further below the GSSP than T. pedum in the Fortune Head section, and in other terminal Proterozoic successions around the globe, supports the concept of a gradational onset of three-dimensional burrowing across the Proterozoic–Cambrian boundary. Although T. pedum remains a reasonable indicator for the base of the Cambrian Period, greater precision in the stratotype section can be achieved by a detailed re-evaluation of the stratigraphic ranges and the morphological variation of ichnotaxa included in the T. pedum Zone.
19
Sevigny, James H., Frederick A. Cook e Elizabeth A. Clark. "Geochemical signature and seismic stratigraphic setting of Coppermine basalts drilled beneath the Anderson Plains in northwest Canada". Canadian Journal of Earth Sciences 28, n. 2 (1 febbraio 1991): 184–94. http://dx.doi.org/10.1139/e91-018.
Testo completoAbstract (sommario):
Basalts drilled in the Petro-Canada – Canterra Tweed Lake M-47 well in the Anderson Plains of northwestern Canada have geochemical characteristics, including of major, trace, and rare earth elements, that are similar to those of the most enriched Coppermine lavas but significantly different from those of the younger Proterozoic volcanics, such as the Natkusiak basalts. This result provides a geological tie and timing constraint for structures observed on seismic reflection data in this area. Correlation of stratigraphic data from the well to seismic data shows that the lavas are within a sequence of layered reflections that onlap, and are thus younger than, easterly verging structures, and that were themselves probably uplifted prior to deposition of the Mackenzie Mountains Supergroup. These relationships thus show that at least two stages of Proterozoic compressional deformation, one that predated and one that postdated the basalts, produced structures beneath the Anderson Plains.
20
Carevic, Ivana, Miroljub Milincic e Velimir Jovanovic. "Contribution to the study of geological setting of lower Aleksandrovac Zupa". Glasnik Srpskog geografskog drustva 89, n. 3 (2009): 135–46. http://dx.doi.org/10.2298/gsgd0903135c.
Testo completoAbstract (sommario):
Aleksandrovac Zupa represents the example of territorial area with composite geological structure and furthermore compound geotectonic framework. Due to the character of some formations, framework and time relation of deformation, these areas belong among the most compound on the Balkan Peninsula. On its territory, the big and composite structural-facies zones are being confronted and became imbued. In a stratigraphic point of view, the largest extend have the Miocene deposits that overlain the crystalline schists of Proterozoic. .
21
Konstantinova, Larisa N., Igor A. Gubin, Sergey A. Moiseev, Andrey M. Fomin e Elena N. Kuznetsova. "CORRELATION RESULTS OF THE PROTEROZOIC-PHANEROZOIC SECTIONS OF THE ALDAN-MAYA OIL AND GAS BEARING REGION ON DEEP DRILLING DATA". Interexpo GEO-Siberia 2, n. 1 (21 maggio 2021): 102–10. http://dx.doi.org/10.33764/2618-981x-2021-2-1-102-110.
Testo completoAbstract (sommario):
The article discusses the controversial issues of well log correlation of the Riphean, Vendian, Cambrian and Mesozoic sequences, which is prospects for oil and gas within the Aldan-Maya petroleum region. The author's version of the stratigraphic correlation is based on the interpretation of logging data, seismic surveys, core description, archival and published materials. The results obtained can be used for stratigraphic picks correction and structural maps plotting.
22
Skuf’in, P. K. "SOME PROBLEMS GEOCHEMISTRY OF VOLCANITES OF THE PECHENGA STRUCTURE (KOLA PENINSULA)". Globus 7, n. 2(59) (4 aprile 2021): 11–14. http://dx.doi.org/10.52013/10.52013/2658-5197-59-2-2.
Testo completoAbstract (sommario):
Some problems of dating the most important stratigraphic units of the Early Proterozoic Pechenga Structure are considered. The structure is composed of rocks of the Pechenga Complex, which in turn is divided into well-studied formations of the North-Pechenga subcomplex and poorly studied formations – the South-Pechenga subcomplex. This structure is one of the most studied Early Precambrian structures in the world. Volcanites of the Pechenga Structure have been studied for nearly half a century. However, many determinations of the isotopic ages of reference levels of geological crossection were carried out by insufficiently accurate Rb-Sr, Sm-Nd, and other methods. Nevertheless, in recent years, using the modern U-Pb method, age characteristics of the most important stratigraphic units of the North-Pechenga subcomplex have been obtained.
23
Birkenmajer, Krzysztof. "Polish Geological Research in Svalbard". Earth Sciences History 11, n. 2 (1 gennaio 1992): 81–87. http://dx.doi.org/10.17704/eshi.11.2.n2747185001nv261.
Testo completoAbstract (sommario):
The Polish geological investigations started in the Svalbard archipelago in 1934. The most diversified stratigraphic, tectonic, sedimentological and palacontological work, along with detailed geological mapping, was carried out in the Hornsund area, south Spitsbergen, between 1957 and 1960, in connection with the Illrd International Geophysical Year and its prolongation (International Geophysical Co-operation). Between 1962 and 1970, some Polish geologists worked in Spitsbergen for the Norwegian Polar Institute. From 1974 onwards, the Polish geological investigations extended from southern to central and eastern Spitsbergen. In south Spitsbergen, at Hornsund, they are based at the Polish Scientific Station built already in 1957, and renovated in 1978. The paper presents a review of mainly pre-Quaternary (Proterozoic through Tertiary) geological, structuraltectonic, petrological, stratigraphic, palaeontologiesl and sedimentologtcal problems of Svalbard elaborated by the Polish geologists.
24
Kyrylyuk, V. P., e O. V. Gaiovskyi. "REGIONAL METAMORPHISM AND STRATIGRAPHY OF THE BASEMENT OF UKRAINIAN SHIELD". Geological Journal, n. 4 (28 dicembre 2022): 3–30. http://dx.doi.org/10.30836/igs.1025-6814.2022.4.261979.
Testo completoAbstract (sommario):
Stratigraphic complexes of the Lower Precambrian are everywhere metamorphosed. That is why the stratigraphic dismemberment of the Lower Precambrian has always been inextricably linked with the study of metamorphism. For some time, metamorphism was even used as a sign of the relative age of stratigenic metamorphic complexes according to the principle – “the higher the degree of metamorphism, the older the complex”. The high-temperature complexes of granulite and amphibolite facies belonged to the Archean, while the less metamorphosed complexes belonged to the Proterozoic. But already at the beginning of the widespread use of methods of isotope dating of metamorphic shield complexes, the most ancient age determinations were obtained from weakly metamorphosed complexes. After that, the degree of metamorphism was no longer taken into account in the stratigraphic dismemberment of the shield basement. The degree of metamorphism of the complexes has not been taken into account for a long ago when compiling official stratigraphic schemes of the Precambrian of the Ukrainian Shield, in which isotope dating plays a leading role. This led, according to many geologists, to distortions in the schemes of the real stratigraphy of the basement of the region. The authors of the article believe that the degree of metamorphism can still be used in the development of the stratigraphy of the Ukrainian Shield and, above all, in the stratigraphic study of individual megablocks. The possibility of such use of metamorphism is considered in the cycle of publications in which this is the first article. The article presents the characteristics of stratigraphy and metamorphism of all megablocks of the Ukrainian Shield. In the structure of the megablocks of the Ukrainian Shield, as well as all the shields of ancient platforms, the following stratigenic metamorphic complexes of the Lower Precambrian have been established, differing in the type and degree of metamorphism: a) granulite-gneiss, b) amphibolite-gneiss, c) greenstone (metavolcanogenic), d) ferruginoussiliceous- shale (metavolcanogenic-chemogenic-terrigenous), e) gneiss-shale (metacarbonate-terrigenous). It has been established that in all megablocks granulite-gneiss complexes are represented and are the most ancient, and all younger complexes are always less metamorphosed than the previous ones. Thus, the relationship between the degree of metamorphism and relative age in individual megablocks is preserved, but this feature cannot be extended to the entire shield. Coeval complexes of neighboring megablocks may have different degrees of metamorphism, and younger complexes of one of the megablocks are sometimes more intensely metamorphosed than older ones in the neighboring megablock.
25
Gibson, Gail G. "Trace fossils from Late Precambrian Carolina slate belt, south-central North Carolina". Journal of Paleontology 63, n. 1 (gennaio 1989): 1–10. http://dx.doi.org/10.1017/s0022336000040889.
Testo completoAbstract (sommario):
The volcanosedimentary sequence of the Carolina slate belt in south-central North Carolina was long thought to be unfossiliferous; however, the 5,484–7,315 meters of dominantly evenly bedded siltstones and mudstones have recently yielded body fossils of the late Precambrian Ediacarian fauna and a Middle Cambrian trilobite assemblage. Ongoing stratigraphic studies in the Carolina slate belt of southern North Carolina have now revealed trace fossils representing the ichnotaxa Gordia arcuata?, ?Helminthopsis sp., Monocraterion sp., Neonerites biserialis, N. uniserialis, ?Neonerites sp., Planolites beverlyensis, P. montanus, ?Planolites sp., Syringomorpha nilssoni?,? Tomaculum sp., Torrowangea sp., and three additional indeterminate ichnogenera. These trace fossils, lacking ornamentation and complex patterns, compare favorably with ichnofossil assemblages from Late Proterozoic stratigraphic sequences (Ichnofossil Zone I) elsewhere and support the late Precambrian age interpretation for the Carolina slate belt in south-central North Carolina.
26
McDonough, Michael R., e Randall R. Parrish. "Proterozoic gneisses of the Malton Complex, near Valemount, British Columbia: U–Pb ages and Nd isotopic signatures". Canadian Journal of Earth Sciences 28, n. 8 (1 agosto 1991): 1202–16. http://dx.doi.org/10.1139/e91-108.
Testo completoAbstract (sommario):
Proterozoic gneisses of the Malton Complex in the vicinity of Valemount, British Columbia, occur in a series of lithologically and structurally complex, fault-bounded slices of crystalline basement and interleaved cover. Gneisses of the Malton Complex span the Southern Rocky Mountain Trench and underlie the western part of the Rocky Mountain fold and thrust belt and the eastern part of the Omineca Belt of the Canadian Cordillera. Structural and stratigraphic relationships indicate that they formed the basement upon which an enigmatic quartzite unit and the Late Proterozoic Windermere Supergroup were deposited.The Yellowjacket and Bulldog gneisses, on the east side of the Rocky Mountain Trench, have yielded four U–Pb zircon crystallization ages of ca. 1870 Ma, with εNd(T) values of −2.6 to −3.4. The Hugh Allan gneiss, also on the east side of the trench but separated from the Yellowjacket gneiss by a major thrust fault, includes leucocratic granite gneiss having a zircon U–Pb age of [Formula: see text], which has intruded an older (undated) lithologically heterogeneous assemblage of gneiss. The basal Windermere succession of the Valemount region is inferred to be younger than ca. 740 Ma, since these intrusions are not found within the Late Proterozoic stratified rocks.Augen granitoid orthogneiss of the Malton Range on the west side of the Rocky Mountain Trench has been dated as [Formula: see text] using zircons. A second sample yields data suggesting an age between 2050 and 2100 Ma, but its interpretation is uncertain because of scatter in analyses and possible zircon inheritance. The latter sample has an εNd(T) at 1990 Ma of −2.6. Nd model ages for the Malton, Yellowjacket, and Bulldog samples range from 2.45 to 2.56 Ga, indicating that the igneous protoliths were derived from a source that probably had some component of Archean crustal material involved.The U–Pb ages and Nd model ages are quite similar to those of rocks underlying portions of Alberta and the western Canadian Shield, specifically the Fort Simpson terrane, the Great Bear magmatic zone, and parts of the Thelon–Taltson arc. This evidence, as well as structural and stratigraphic arguments, links the Malton Complex gneisses with those of the Canadian Shield, precluding their derivation by large-magnitude displacements from the southwestern United States. Structural analysis indicates that they restore to locations 100–200 km southwest of their present exposure.Structural, stratigraphic, and isotopic data indicate that the Southern Rocky Mountain Trench is not a suture.
27
GEYER, G. "The Fish River Subgroup in Namibia: stratigraphy, depositional environments and the Proterozoic–Cambrian boundary problem revisited". Geological Magazine 142, n. 5 (settembre 2005): 465–98. http://dx.doi.org/10.1017/s0016756805000956.
Testo completoAbstract (sommario):
The Fish River Subgroup of the Nama Group, southern Namibia, is restudied in terms of lithostratigraphy and depositional environment. The study is based on partly fine-scaled sections, particularly of the Nababis and Gross Aub Formation. The results are generally in accordance with earlier studies. However, braided river deposits appear to be less widely distributed in the studied area, and a considerable part of the formations of the middle and upper subgroup apparently were deposited under shallowest marine conditions including upper shore-face. Evidence comes partly from sedimentary features and facies distribution, and partly from trace fossils, particularly Skolithos and the characteristic Trichophycus pedum. Environmental conditions represented by layers with T. pedum suggest that the producer favoured shallow marine habitats and transgressive regimes. The successions represent two deepening-upward sequences, both starting as fluvial (braided river) systems and ending as shallow marine tidally dominated environments. The first sequence includes the traditional Stockdale, Breckhorn and lower Nababis formations (Zamnarib Member). The second sequence includes the upper Nababis (Haribes Member) and Gross Aub formations. As a result, the Nababis and Gross Aub formations require emendation: a new formation including the Haribes and Rosenhof and possibly also the Deurstamp members. In addition, four distinct sequence stratigraphic units are deter-minable for the Fish River Subgroup in the southern part of the basin. The Proterozoic–Cambrian transition in southern Namibia is most probably located as low as the middle Schwarzrand Subgroup. The environmentally controlled occurrence of Trichophycus pedum undermines the local stratigraphic significance of this trace fossil which is eponymous with the lowest Cambrian and Phanerozoic trace fossil assemblage on a global scale. However, occurrences of such trace fossils have to be regarded as positive evidence for Phanerozoic age regardless of co-occurring body fossils. Other suggestions strongly dispute the concept of the formal Proterozoic–Cambrian and Precambrian–Phanerozoic boundary. Carbon isotope excursions and radiometric datings for the Nama Group do not help to calibrate precisely the temporal extent of the Fish River Subgroup. Fossil content, sequence stratigraphy and inferred depositional developments suggest that this subgroup represents only a short period of late orogenic molasse sedimentation during the early sub-trilobitic Early Cambrian.
28
Jansa, L. F., G. Pe-Piper e B. D. Loncarevic. "Appalachian basement and its intrusion by Cretaceous dykes, offshore southeast Nova Scotia, Canada". Canadian Journal of Earth Sciences 30, n. 12 (1 dicembre 1993): 2495–509. http://dx.doi.org/10.1139/e93-216.
Testo completoAbstract (sommario):
Aeromagnetic data collected between eastern Nova Scotia and southern Newfoundland provide new information about the offshore extension of the Avalon and Meguma terranes. A zone of short-wavelength anomalies that delineates Scatarie Ridge extends westward to the Late Proterozoic Fourchu Group in southeastern Cape Breton Island and eastward towards the Burin Peninsula of Newfoundland, suggesting that both regions belong to the same tectono-stratigraphic province of the Avalon composite terrane. A different zone of short-wavelength, discontinuously lineated anomalies at the northern edge of the Canso Ridge correlates with amphibolite-facies metamorphic rocks of the Meguma terrane on the Canso Peninsula, interpreted as an exhumed deeper metamorphic level of the Meguma terrane at its boundary with the Avalon terrane. The S-shaped pattern of long linear magnetic trends, characteristic of lower grade Meguma rocks on the southern flank of the Canso Ridge, indicates plastic deformation of the Meguma terrane during the Acadian orogeny when emplaced against the rigid Cape Breton Island block indentor. Analogous patterns occur off western Nova Scotia, suggesting little strike-slip motion occurred between the Meguma and Avalon terranes since the Acadian orogeny.Late Proterozoic rocks on Scatarie Ridge are intruded by Cretaceous diabase dykes. The diabase is alkaline with a within-plate geochemical signature, similar in composition to basalt flows in the Orpheus half-graben. A depleted-mantle model age TDM (Nd) of 731 Ma, εNd = +6.5, suggests that the magma was sourced from a lithospheric mantle reservoir involved in Late Proterozoic magmatic activity. Aeromagnetic data interpretation confirms the distribution of Cretaceous basalt flows and sills within Mesozoic sedimentary strata of the Orpheus half-graben previously outlined by seismic methods but was unable to differentiate between Proterozoic and Mesozoic intrusive rocks where the Proterozoic rocks lay near to the ocean floor.
29
Myrow, P. M., N. C. Hughes e N. R. McKenzie. "Reconstructing the Himalayan margin prior to collision with Asia: Proterozoic and lower Paleozoic geology and its implications for Cenozoic tectonics". Geological Society, London, Special Publications 483, n. 1 (21 novembre 2018): 39–64. http://dx.doi.org/10.1144/sp483.10.
Testo completoAbstract (sommario):
AbstractReconstructing the stratigraphic architecture of deposits prior to Cenozoic Himalayan uplift is critical for unravelling the structural, metamorphic, depositional and erosional history of the orogen. The nature and distribution of Proterozoic and lower Paleozoic strata have helped elucidate the relationship between lithotectonic zones, as well as the geometries of major bounding faults. Stratigraphic and geochronological work has revealed a uniform and widespread pattern of Paleoproterozoic strata >1.6 Ga that are unconformably overlain by <1.1 Ga rocks. The overlying Neoproterozoic strata record marine sedimentation, including a Cryogenian diamictite, a well-developed carbonate platform succession and condensed fossiliferous Precambrian–Cambrian boundary strata. Palaeontological study of Cambrian units permits correlation from the Indian craton through three Himalayan lithotectonic zones to a precision of within a few million years. Detailed sedimentological and stratigraphic analysis shows the differentiation of a proximal realm of relatively condensed, nearshore, evaporite-rich units to the south and a distal realm of thick, deltaic deposits to the north. Thus, Neoproterozoic and Cambrian strata blanketed the northern Indian craton with an extensive, northward-deepening, succession. Today, these rocks are absent from parts of the inner Lesser Himalaya, and the uplift and erosion of these proximal facies explains a marked change in global seawater isotopic chemistry at 16 Ma.
30
Alvan, Aldo A., Yacory F. Bustamante, Elvis A. Sánchez e Mirian I. Mamani. "Arquitectura estratigráfica, paleogeografía y proveniencia sedimentaria de las rocas cenozoicas del sur de Perú (Tacna, 18° S)". Andean Geology 47, n. 2 (29 maggio 2020): 351. http://dx.doi.org/10.5027/andgeov47n2-3168.
Testo completoAbstract (sommario):
The Cenozoic rocks lying in the Province of Tacna (18° S), southern Perú, represent approximately 600 m of stratigraphic thickness. This stacking groups the Sotillo (Paleocene), Moquegua Inferior (Eocene), Moquegua Superior (Oligocene), Huaylillas (Miocene) and Millo formations (Pliocene), and these are the sedimentary fill of the Moquegua Basin. The sediments of the three latter formations are organized into nine sedimentary facies and five architectural elements. Their facies associations suggest the existence of an ancient highly channelized multi-lateral fluvial braided system, with upward increase of pyroclastic and conglomeratic depositions. The heavy mineral spectra make each lithostratigraphic unit unique and distinguishable, being the sediments of the Moquegua Superior Formation rich in garnets, titanites and zircons; while the sediments of the Huaylillas and Millo formations in clinopyroxenes. This mineral arrangement becomes an excellent tool for stratigraphic correlations between outcrops and subsurface stratigraphy (by means of well cores studies) and allow to sketch out a new stratigraphic framework and a complex of rocky blocks bounded by normal faults, often tilted. The sediment mineralogy also suggests that the rocks conforming the Western Cordillera were the main source of sediments for the Moquegua Basin in Tacna. In this context, the detritus of the Moquegua Superior Formation derives mainly from the erosion of the rocks forming the Coastal Basal Complex (Proterozoic), the Ambo Group (Carboniferous) and the Junerata/Chocolate Formation (Early Jurassic). The Huaylillas Formation is a pyroclastic and sedimentary unit which components derived mainly from the Huaylillas volcanism (Miocene) and partly from the denudation of the Toquepala Group (Late Cretaceous). The Huaylillas Formation widely contrasts to the underlying Moquegua Superior Formation due its mineralogy and facies. Finally, the detritus of the Millo Formation derived mostly from the rocks forming the Barroso Formation (Pliocene), and their facies represent a higher contrast in relation to the underlying units due its notorious conglomerate facies.
31
Kemnitz, Helga, Bodo-Carlo Ehling, Olaf Elicki, Hans-Joachim Franzke, Gerd Geyer, Ulf Linnemann, Dietmar Leonhardt et al. "Proterozoikum–Silur in der Stratigraphischen Tabelle von Deutschland 2016 The Stratigraphic Table of Germany 2016: Proterozoic to Silurian". Zeitschrift der Deutschen Gesellschaft für Geowissenschaften 168, n. 4 (1 marzo 2018): 423–46. http://dx.doi.org/10.1127/zdgg/2017/0134.
Testo completo
32
Knoll, Andrew H. "The advent of the Phanerozoic world: Vendian stratigraphy, environmental change, and evolution". Paleontological Society Special Publications 6 (1992): 169. http://dx.doi.org/10.1017/s2475262200007292.
Testo completoAbstract (sommario):
The Vendian interval (ca. 610–540 Ma) links Proterozoic and Phanerozoic worlds of sharply contrasting character. Despite decades of study, the nature of this transition remains unclear, in part because of our limited ability to correlate Vendian successions or evaluate shifts in global environments. New data on secular variations in the C and Sr isotopic compositions of Vendian carbonates (and organic matter) provide an improved stratigraphic and biogeochemical framework for understanding latest Proterozoic biological and environmental evolution.Biologically, the Vendian interval is best known for the Ediacaran radiation of macroscopic animals, but this event is set within a broader Neoproterozoic diversification of higher eukaryotes. All three principal groups of multicellular algae radiated well before the beginning of the Vendian, as did a host of unicellular protists. In particular, successions deposited immediately after the Varanger Ice Age (ca. 610–590 Ma) are characterized by a high diversity of large and morphologically complex acritarchs; most of these forms disappeared after the first appearance of Ediacara-grade metazoans but before the eponymous fauna preserved in South Australia.Stratigraphic ordering of the earliest faunas is made possible by chemostratigraphy. Contrary to some published expectations, the morphologically complex petalonemids and skeletalized cloudinids of the lower Nama Group, Namibia, appear to predate, perhaps significantly, the classic faunas of South Australia, eastern Siberia, and elsewhere. Zircon ages for tuffs promise an absolute chronology for biological and biogeochemical events. The presence in pre-Cambrian rocks of Cloudina, calcareous algae and (?)siliceous discs comparable to chrysophyte scales demonstrates that eukaryotic calcite, aragonite, and silica biomineralization all predate the beginning of the Cambrian; however, sedimentological and petrographic features of carbonates and cherts suggest that skeletons first emerged as globally significant components of the carbon and silica cycles with the basal Cambrian radiation.Vendian evolution must also be evaluated within a broader context of environmental change. The Sr and C isotopic data that enhance stratigraphic correlation also record patterns of hydrothermal emission and organic carbon burial that must have affected pO2. Independent models by Derry and others and Knoll and Walker suggest that latest Proterozoic reductions in the hydrothermal flux of reduced materials into the oceans coupled with high burial rates of organic carbon resulted in a significant increase in global oxygen levels immediately prior to the great Ediacaran radiation.Many questions about Vendian evolution remain unresolved. Some will surely require fresh insights into the development and functional morphology of early metazoans, but it is becoming increasingly clear that a satisfactory accounting of Ediacaran animal diversification will not be achieved without a better understanding of the stratigraphic, environmental, and biological context in which it occurred.
33
HARLAND, W. BRIAN. "Origins and assessment of snowball Earth hypotheses". Geological Magazine 144, n. 4 (6 giugno 2007): 633–42. http://dx.doi.org/10.1017/s0016756807003391.
Testo completoAbstract (sommario):
Brian Harland was for many years an editor of this journal. He was also a seminal figure in the origins of the current ‘snowball Earth’ debate, having recognized in 1964 the significance of coupling emerging palaeomagnetic data on palaeolatitude with his interpretations of diamictites. Harland worked extensively in the Arctic and knew well many of the workers involved in the arguments surrounding the origin of diamictites. He thus had a unique perspective on the evidence and the disputes surrounding it. This was his last paper but he was not able to complete it before he died. However, with the help of Professor Ian Fairchild to whom we are indebted, the editors have lightly revised this work which is presented as the personal view of one of the key figures with a very broad stratigraphic appreciation of the problems of ‘snowball Earth’.Records of Precambrian glaciation onwards from the late nineteenth century led to the concept of one or more major ice ages. This concept was becoming well advanced by the mid 1930s, particularly through the compilation of Kulling in 1934. Even so tillite stratigraphy shows that glaciation was exceptional rather than typical of Earth history. Some Proterozoic tillites, sandwiched between warm marine facies, indicate low, even equatorial palaeolatitudes as determined magnetically, and more recently led to ideas of a snow- and ice-covered ‘snowball Earth’. However, interbedded non-glacial facies as well as thick tillite successions requiring abundant snowfall both militate against the hypothesis of extreme prolonged freezing temperatures referred to here as an ‘iceball Earth’ in which all oceans and seas were sealed in continuous ice cover. On the other hand tropical environments were interrupted by glaciation several times in the Proterozoic, something that did not recur in the Phanerozoic. The term ‘snowball Earth’ is consistent with the established view of extremely widespread Proterozoic glaciation, but the ‘iceball Earth’ version of this is not compatible with the geological record.
34
Dawes, P. R., e D. C. Rex. "Proterozoic basaltic magmatic periods in North-West Greenland: evidence from K/Ar ages". Rapport Grønlands Geologiske Undersøgelse 130 (31 dicembre 1986): 24–31. http://dx.doi.org/10.34194/rapggu.v130.7937.
Testo completoAbstract (sommario):
This paper is the third of a series reporting on K/Ar whole rock ages of Proterozoic basaltic rocks of the Thule district, North-West Greenland. The dating programme is a co-operative venture between the Geological Survey of Greenland and The University, Leeds, V.K., and was set up in connection with a 1:500 000 mapping project of the region between 75°N and 78° 45'N (Melville Bugt - Inglefield Land, fig. 1). Field work by one of us (PRD) has shown that several episodes of unmetamorphosed Proterozoic basic igneous rocks can be distinguished stratigraphically in the region (see below). The routine K/Ar isotopic work, carried out concurrently with the field work, has aimed at dating these episodes, thereby providing minimum ages for deposition or consolidation of the host rocks. The two earlier reports (Dawes et al., 1973, 1982a) dealt with sills and dykes from the northern part of the region (Inglefield Land - Prudhoe Land); intrusions that have known or inferred stratigraphic relationships with the Proterozoic Thule Group. The present report gathers together all hitherto unpublished K/Ar dates - 18 in total on both extrusive and intrusive rocks - mainly from the central and southern parts of the region (Inglefield Bredning and Melville Bugt). Six of the samples represent cross-cutting intrusions in the Precambrian Shield that have uncertain age relationships with the overlying Thule Group.
35
Green, J. W., A. H. Knoll e K. Swett. "Microfossils from silicified stromatolitic carbonates of the Upper Proterozoic Limestone-Dolomite 'Series', central East Greenland". Geological Magazine 126, n. 5 (1 settembre 1989): 567–85. http://dx.doi.org/10.1017/s0016756800022858.
Testo completoAbstract (sommario):
AbstractSilicified flake conglomerates andin situstratiform stromatolites of the Upper Proterozoic (c.700–800 Ma) Limestone-Dolomite ‘Series’, central East Greenland, contain well preserved microfossils. Five stratigraphic horizons within the 1200 m succession contain microbial mat assemblages, providing a broad palaeontological representation of late Proterozoic peritidal mat communities. Comparison of assemblages demonstrates that the taxonomy and diversity of mat builder, dweller, and allochthonous populations all vary considerably within and among horizons. The primary mat builder in most assemblages isSiphonophycus inornatum, a sheath-forming prokaryote of probable but not unequivocally established cyanobacterial affinities. An unusual low diversity unit in Bed 17 is dominated by a different builder,Tenuofilum septatum, while a thin cryptalgal horizon in Bed 18 is built almost exclusively bySiphonophycus kestron.Although variable taphonomic histories contribute to observed assemblage variation, most differences within and among horizons appear to reflect the differential success or failure of individual microbial populations in colonizing different tidal flat microenvironments. Twenty-two taxa are recognized, of which two are described as new:Myxococcoides stragulescensn.sp. andScissilisphaera gradatan. sp.
36
Terleev, A. A., A. A. Postnikov, A. S. Gibsher, O. V. Sosnovskaia, B. G. Kraevskii, G. N. Bagmet e D. A. Tokarev. "РЕГИОНАЛЬНАЯ СТРАТИГРАФИЧЕСКАЯ СХЕМА ВЕРХНЕПРОТЕРОЗОЙСКИХ ОТЛОЖЕНИЙ АЛТАЕ-САЯНСКОЙ СКЛАДЧАТОЙ ОБЛАСТИ". Geology and mineral resources of Siberia, n. 8s (giugno 2019): 4–32. http://dx.doi.org/10.20403/2078-0575-2019-8s-4-32.
Testo completo
37
Syme, E. C., e A. H. Bailes. "Stratigraphic and tectonic setting of early Proterozoic volcanogenic massive sulfide deposits, Flin Flon, Manitoba". Economic Geology 88, n. 3 (1 maggio 1993): 566–89. http://dx.doi.org/10.2113/gsecongeo.88.3.566.
Testo completo
38
Sims, P. K., W. R. Van Schmus, K. J. Schulz e Z. E. Peterman. "Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen". Canadian Journal of Earth Sciences 26, n. 10 (1 ottobre 1989): 2145–58. http://dx.doi.org/10.1139/e89-180.
Testo completoAbstract (sommario):
The Early Proterozoic Penokean Orogen developed along the southern margin of the Archean Superior craton. The orogen consists of a northern deformed continental margin prism overlying an Archean basement and a southern assemblage of oceanic arcs, the Wisconsin magmatic terranes. The south-dipping Niagara fault (suture) zone separates the south-facing continental margin from the accreted arc terranes. The suture zone contains a dismembered ophiolite.The Wisconsin magmatic terranes consist of two terranes that are distinguished on the basis of lithology and structure. The northern Pembine–Wausau terrane contains a major succession of tholeiitic and calc-alkaline volcanic rocks deposited in the interval 1860–1889 Ma and a more restricted succession of calc-alkaline volcanic rocks deposited about 1835 – 1845 Ma. Granitoid rocks ranging in age from about 1870 to 1760 Ma intrude the volcanic rocks. The older succession was generated as island arcs and (or) closed back-arc basins above the south-dipping subduction zone (Niagara fault zone), whereas the younger one developed as island arcs above a north-dipping subduction zone, the Eau Pleine shear zone. The northward subduction followed deformation related to arc–continent collision at the Niagara suture at about 1860 Ma. The southern Marshfield terrane contains remnants of mafic to felsic volcanic rocks about 1860 Ma that were deposited on Archean gneiss basement, foliated tonalite to granite bodies ranging in age from about 1890 to 1870 Ma, and younger undated granite plutons. Following amalgamation of the two arc terranes along the Eau Pleine suture at about 1840 Ma, intraplate magmatism (1835 Ma) produced rhyolite and anorogenic alkali-feldspar granite that straddled the internal suture.
39
Nagovitsin, K. E., A. M. Stanevich e T. A. Kornilova. "Stratigraphic setting and age of the complex Tappania-bearing Proterozoic fossil biota of Siberia". Russian Geology and Geophysics 51, n. 11 (novembre 2010): 1192–98. http://dx.doi.org/10.1016/j.rgg.2010.10.004.
Testo completo
40
Patranabis Deb, Sarbani, e Asru K. Chaudhuri. "Stratigraphic architecture of the Proterozoic succession in the eastern Chattisgarth Basin, India: tectonic implications". Sedimentary Geology 147, n. 1-2 (marzo 2002): 105–25. http://dx.doi.org/10.1016/s0037-0738(01)00192-0.
Testo completo
41
Jinbiao, Chen. "An explanatory note on proterozoic stratigraphic nomenclature used in the People's Republic of China". Precambrian Research 29, n. 1-3 (giugno 1985): 3–4. http://dx.doi.org/10.1016/0301-9268(85)90054-3.
Testo completo
42
Devlin, William J. "Stratigraphy and sedimentology of the Hamill Group in the northern Selkirk Mountains, British Columbia: evidence for latest Proterozoic – Early Cambrian extensional tectonism". Canadian Journal of Earth Sciences 26, n. 3 (1 marzo 1989): 515–33. http://dx.doi.org/10.1139/e89-044.
Testo completoAbstract (sommario):
Three informal stratigraphic divisions are recognized in the uppermost Proterozoic – Lower Cambrian Hamill Group in the northern Selkirk Mountains of British Columbia. These informal divisions include a lower sandstone unit, a greenstone–graded-sandstone unit, and an upper sandstone unit. Both the lower and upper sandstone units display sedimentary characteristics that are uniform along strike and indicate a shallow-marine environment of deposition. As is typical of other exposures of the Hamil Group in southeastern British Columbia, the lower sandstone unit is coarser grained and more poorly sorted than the mature quartz arenites of the upper sandstone unit.The greenstone–graded-sandstone unit is a complex assemblage of mafic metavolcanic rocks and associated sandstone facies. This unit is highly variable along strike but essentially consists of a thick succession of subaqueous extrusive rocks overlain by a variety of sediment gravity-flow deposits. These latter deposits include resedimented conglomerates, debris-flow deposits, and trubidites (deposited from both high- and low-density turbidity currents). Stratigraphic sections of this unit are described in detail from three different localities and are examined in terms of their transport and depositional mechanisms.The stratigraphic succession of the Hamill Group indicates that deposition of the shallow-marine sands of the lower sandstone unit was abruptly interrupted by a period of volcanism, the creation of a paleoslope, and the deposition of a large volume of sediment gravity-flow deposits of the greenstone–graded-sandstone unit. These relations are attributed to an episode of syndepositional normal faulting. The inferred fault(s) could have served as the conduit for the extrusion of the volcanics. Offset along the fault(s), the tilting of fault blocks, and the consequent formation of an unstable slope adjacent to a fault scarp created an environment favorable for deposition of the sediment gravity flows. In general, deposition of proximal, base-of-slope deposits was followed by an aggradational basin-fill phase of sedimentation. With the waning of tectonic activity and the filling of the fault-bounded basin, depositon of shallow-marine sands resumed (the upper sandstone unit). The stratigraphic relations of the Hamill Group in the northern Selkirk Mountains are considered direct evidence for an episode of latest Proterozoic – Early Cambrian extensional tectonism. The evidence for an episode of rift-related tectonism in the northern Selkirk Mountains supports inferences concerning the timing of this event as derived from tectonic subsidence analyses of post-rift strata of the Cordilleran miogeocline.
43
Jackson, J., I. P. Sweet e T. G. Powell. "STUDIES ON PETROLEUM GEOLOGY AND GEOCHEMISTRY, MIDDLE PROTEROZOIC, McARTHUR BASIN NORTHERN AUSTRALIA I: PETROLEUM POTENTIAL". APPEA Journal 28, n. 1 (1988): 283. http://dx.doi.org/10.1071/aj87022.
Testo completoAbstract (sommario):
Mature, rich, potential source beds and adjacent potential reservoir beds exist in the Middle Proterozoic sequence (1400-1800 Ma) of the McArthur Basin. The McArthur and Nathan Groups consist mainly of evaporitic and stromatolitic cherty dolostones interbedded with dolomitic siltstone and shale. They were deposited in interfingering marginal marine, lacustrine and fluvial environments. Lacustrine dolomitic siltstones form potential source beds, while potential reservoirs include vuggy brecciated carbonates associated with penecontemporaneous faulting and rare coarse-grained clastics. In contrast, the younger Roper Group consists of quartz arenite, siltstone and shale that occur in more uniform facies deposited in a stable marine setting. Both source and reservoir units are laterally extensive (over 200 km).Five potential source rocks at various stages of maturity have been discovered. Two of these source rocks, the lacustrine Barney Creek Formation in the McArthur Group and the marine Velkerri Formation in the Roper Group, compare favourably in thickness and potential with rich demonstrated source rocks in major oil-producing provinces. There is abundant evidence of migration of hydrocarbons at many stratigraphic levels. The geology and reservoir characteristics of the sediments in combination with the distribution of potential source beds, timing of hydrocarbon generation, evidence for migration and chances of preservation have been used to rank the prospectivity of the various stratigraphic units in different parts of the basin.
44
Evans, James E., Joshua T. Maurer e Christopher S. Holm-Denoma. "Recognition and significance of Upper Devonian fluvial, estuarine, and mixed siliciclastic-carbonate nearshore marine facies in the San Juan Mountains (southwestern Colorado, USA): Multiple incised valleys backfilled by lowstand and transgressive systems tracts". Geosphere 15, n. 5 (9 agosto 2019): 1479–507. http://dx.doi.org/10.1130/ges02085.1.
Testo completoAbstract (sommario):
Abstract The Upper Devonian Ignacio Formation (as stratigraphically revised) comprises a transgressive, tide-dominated estuarine depositional system in the San Juan Mountains (Colorado, USA). The unit backfills at least three bedrock paleovalleys (10–30 km wide and ≥42 m deep) with a consistent stratigraphy of tidally influenced fluvial, bayhead-delta, central estuarine-basin, mixed tidal-flat, and estuarine-mouth tidal sandbar deposits. Paleovalleys were oriented northwest while longshore transport was to the north. The deposits represent Upper Devonian lowstand and transgressive systems tracts. The overlying Upper Devonian Elbert Formation (upper member) consists of geographically extensive tidal-flat deposits and is interpreted as mixed siliciclastic-carbonate bay-fill facies that represents an early highstand systems tract. Stratigraphic revision of the Ignacio Formation includes reassigning the basal conglomerate to the East Lime Creek Conglomerate, recognizing an unconformity separating these two units, and incorporating strata previously mapped as the McCracken Sandstone Member (Elbert Formation) into the Ignacio Formation. The Ignacio Formation was previously interpreted as Cambrian, but evidence that it is Devonian includes reexamined fossil data and detrital zircon U-Pb geochronology. The Ignacio Formation has a stratigraphic trend of detrital zircon ages shifting from a single ca. 1.7 Ga age peak to bimodal ca. 1.4 Ga and ca. 1.7 Ga age peaks, which represents local source-area unroofing history. Specifically, the upper plate of a Proterozoic thrust system (ca. 1.7 Ga Twilight Gneiss) was eroded prior to exposure of the lower plate (ca. 1.4 Ga Uncompahgre Formation). These results are a significant alternative interpretation of the geologic history of the southern Rocky Mountains.
45
Croon, Marcel, Joshua Bluett, Luke Titus e Raymond Johnson. "Formation evaluation case study: Glyde unconventional Middle Proterozoic play in the McArthur Basin, northern Australia". APPEA Journal 55, n. 2 (2015): 429. http://dx.doi.org/10.1071/aj14064.
Testo completoAbstract (sommario):
The Glyde–1 and Glyde Sidetrack–1 wells were drilled by Armour Energy in the Glyde Sub-basin of the McArthur Basin, NT, Australia in August 2012. This program was to evaluate the unconventional hydrocarbon potential of the Barney Creek Shale source rock and the conventional potential of the Coxco Dolomite of the McArthur Group. The Glyde wells discovered gas in both formations. Transtensional faults in this region allowed to form a series of fault-bounded depocentres. The target gas source of the Glyde discovery is located in 1640 Ma organic-rich black shales of the Barney Creek Formation. Weatherford was contracted to acquire both vertical and lateral advanced log suites and perform subsequent log interpretation to constrain the in situ minimum and maximum horizontal stress regimes to assist with maximising gas production from future lateral placement pilot programs in the Coxco Hydrothermal Dolomite (HTD) Play. Two stratigraphic and structural domains were defined by the observed features in the image log data; a dolostone dominated, fractured strata below an erosional surface. Above this stratigraphic timeline is a monotonous package of laminated, lower-energy Barney Creek Formation sediments. Observed changes in azimuths and dips of the measured beddings suggest a phase of compression after deposition of the Barney Creek Formation, resulting in gentle folding of the formations. The porous gas-charged HTD play is drilled in top of the anticline, which is further characterised by a significant number of conductive fractures, likely indicative of open fractures.
46
Gubin, Igor A., e Vladimir A. Kontorovich. "Seismogeological structure model of the Anabar-Olenek region". Georesursy 23, n. 1 (30 marzo 2021): 70–77. http://dx.doi.org/10.18599/grs.2021.1.7.
Testo completoAbstract (sommario):
The velocity characteristics of the Upper Proterozoic-Phanerozoic sedimentary cover of the Anabar-Olenek region were studied, in particular, the bimodal character of the distribution interval P-wave velocities was established. Taking into account modern ideas about the chronostratigraphy of sediments encountered by the Charchykskaya-1, Burskaya-3410 and Khastakhskaya-930 deep boreholes, stratification of reflecting horizons was carried out and time sections from previous years were reinterpreted. From the perspective of seismic stratigraphic and seismic facies analysis, the Cambrian, Vendian, and Riphean intervals of the section were examined in detail. In the course of the analysis, adjustments to the stratigraphic breakdown of the Burskaya-3410 and Charchykskaya-1 boreholes are proposed. The study shows that the Lapar Formation, which underwent Prepermian erosion, increase in the thickness multiple in an eastward direction. The distribution areas of the Tuessal Formation, the Lower and Middle Cambrian clinoform complex, as well as the areas of the Upper Riphean Formations reaching the Prevendian erosion surface are contoured. An Intrariphean tectonic disagreement between the Kulady Formation and older deposits was established.
47
SAYLOR, BEVERLY Z., JANICE M. POLING e WARREN D. HUFF. "Stratigraphic and chemical correlation of volcanic ash beds in the terminal Proterozoic Nama Group, Namibia". Geological Magazine 142, n. 5 (settembre 2005): 519–38. http://dx.doi.org/10.1017/s0016756805000932.
Testo completoAbstract (sommario):
At least twenty silicified volcanic ash beds have been identified in the Kuibis and Schwarzrand subgroups of the terminal Proterozoic Nama Group of Namibia. Nineteen of the Nama ash beds are in the Schwarzrand Subgroup in the Witputs subbasin. Two of these are in the siliciclastic-dominated lower part of the subgroup, which consists of the Nudaus Formation and Nasep Member of the Urusis Formation and comprises two depositional sequences. Identification and correlation of these ash beds are very well known based on stratigraphic position. Sixteen ash beds are contained within the carbonate-dominated strata of the Huns, Feldschuhhorn and Spitskop members of the Urusis Formation. These strata comprise four large-scale sequences and eighteen medium-scale sequences. Ash beds have been found in three of the large-scale sequences and seven of the medium-scale sequences. Correlations are proposed for these ash beds that extend over large changes in facies and stratal thickness and across transitions between the seaward margin, depocentre and landward margin of the Huns-Spitskop carbonate shelf. A study of whole rock and in situ phenocryst compositions was conducted to evaluate the feasibility of independently testing sequence stratigraphic correlations by geochemically identifying individual ash beds. Whole rock abundances of Al, Fe, Mg, K and Ti vary inversely with Si, reflecting variations in phenocryst concentration due to air fall and hydrodynamic sorting. These sorting processes did not substantially fractionate whole rock rare earth element abundances (REE), which vary more widely with Si. REE abundances are higher in samples of the Nudaus ash bed than in samples of the Nasep ash bed, independent of position in bed, phenocryst abundance, or grainsize, providing a geochemical means for discriminating between the two beds. Variations in the position of chondrite-normalized whole rock REE plots similarly support suspected correlations of ash beds between widely separated sections of the Spitskop Member. Abundances of Fe, Mg and Mn in apatite plot in distinct clusters for Spitskop ash beds that are known to be different and in clusters that overlap for ash beds suspected of correlating between sections. Abundances of REE in monazites differ for the Nudaus, Nasep and Spitskop ash beds in which these phenocrysts were identified. Multivariate statistical analysis provided a quantitative analysis of the discriminating power of different elements and found that whole rock abundances of Ge, Nb, Cs, Ba and La discriminate among the whole rock compositions of the Nudaus and Nasep ash beds and the Spitskop ash beds that are thought to correlate between sections. Each of the above geochemical signatures, by itself, is not definitive because the differences between beds are comparable to the variability within beds and because some signatures are shared by beds known to be different. Taken together, however, weight-of-evidence arguments based on multiple components and phases can successfully discriminate among Nama ash beds. Results from this study support sequence stratigraphic correlations of Spitskop ash beds that document stratal truncations and gaps in the record related to onlap and erosion.
48
Clarke, G. L., J. P. Burg e C. J. L. Wilson. "Stratigraphic and structural constraints on the proterozoic tectonic history of the Olary Block, South Australia". Precambrian Research 34, n. 2 (dicembre 1986): 107–37. http://dx.doi.org/10.1016/0301-9268(86)90053-7.
Testo completo
49
Bhattacharya, Purbasha, e Sarbani Patranabis-Deb. "Stratigraphic evolution of the Proterozoic succession in the western part of the Chattisgarh basin, India". Journal of the Geological Society of India 87, n. 3 (marzo 2016): 287–307. http://dx.doi.org/10.1007/s12594-016-0396-7.
Testo completo
50
Abell, P. I., J. McClory, H. E. Hendry e K. L. Wheatley. "Stratigraphic variations in carbon and oxygen isotopes in the dolostone of the Carswell Formation (Proterozoic) of northern Saskatchewan". Canadian Journal of Earth Sciences 26, n. 11 (1 novembre 1989): 2318–26. http://dx.doi.org/10.1139/e89-197.
Testo completoAbstract (sommario):
Petrographic and stable isotopic analyses of stromatolitic sediments deposited in nearshore environments provides us with some of the best information available on ancient environments. Diamond drill hole CAR 58 penetrated 110 m of sediments in the lowermost part of the Proterozoic (probably Helikian age) Carswell Formation of northern Saskatchewan and gave us such an opportunity. The rocks are mainly dolostone and include, in descending order of abundance, cyanobacterial laminites, stromatolites, dolomicrites, dolorudites, breccias, and oolites. Stromatolites and Cyanobacterial laminites increase in abundance up-section, and deposition is interpreted as having taken place in conditions of increasingly restricted water circulation through time. The carbon isotope ratios vary from about −0.5 to −1.5‰ (Pee Dee Belemnite (PDB)) in the section except near the base where they assume values near −2.5‰. The oxygen isotope ratios (vs. PDB) increase from about −9.3‰ at the base to −7‰ at the top, with anomolously high values, more positive than −7‰, at two positions in the sequence. Original depositional structures and textures are still visible in most of the rocks, but gypsum has been replaced by dolomite, there has been some silicification, and original features have been obliterated by dolomite rhombs in a few samples. The upward trend to less-negative values of the oxygen isotope ratios is interpreted in terms of changing depositional environment involving a deepening but more protected basin, with increased evaporational concentration of the heavier isotope. Scatter diagrams of carbon and oxygen isotope ratios place the Carswell Formation dolomites close to the mainstream of other Proterozoic stromatolites but indicating some evaporative alterations during deposition.