Interbasinal Correlation Research Articles

Palaeomagnetic results from the Lancer 1 stratigraphic drillhole, Officer Basin, Western Australia, and implications for Rodinia reconstructions

A partial record of the positions of Australia during Middle to Late Neoproterozoic time is provided by palaeomagnetic results for samples from the Lancer 1 stratigraphic drillhole in Western Australia. Lancer 1 was drilled vertically to 1501 m, through essentially horizontal Neoproterozoic strata of the western Officer Basin. We studied 123 samples from 28 intervals of drillcore which were oriented by matching features (fractures, cross-beds, etc.) in the core with oriented acoustic scanner images of the drillhole walls. Three new palaeopoles are reported for red mudstones and sandstones (redbeds) of the Browne (44.5°N, 141.7°E, dp = 5.1°, dm = 9.0°), Hussar (62.2°N, 85.8°E, dp = 7.3°, dm = 14.6°), and Kanpa (74.0°N, 128.8°E, dp = 7.7°, dm = 14.8°) Formations of the ca 830 – 720 Ma Buldya Group (Supersequence 1), which exhibit stable, two-polarity magnetisations carried by fine-grained hematite and magnetite. The overlying ca 610 – 590 Ma Wahlgu Formation glaciogenic diamictite (Supersequence 3) yielded dispersed directions and an imprecise palaeopole that overlaps results from the glaciogenic Elatina Formation and other Late Neoproterozoic rock units. The results help to elaborate the Middle to Late Neoproterozoic apparent polar wander path for Australia and indicate, in agreement with palaeoclimatic data and previous palaeomagnetic studies, that the continent was slow-moving and occupied low latitudes at this time. Assuming that Australia and Laurentia were still joined at ca 780 Ma, comparison of the new Hussar Formation palaeopole with coeval Laurentian data favours AUSMEX, rather than SWEAT or AUSWUS, as the most likely configuration of these two continents in Rodinia. This preliminary study of Lancer 1 demonstrates the utility of acoustic scanner logs for orienting drillcores, as well as the scope for additional sampling and palaeomagnetic studies of Lancer 1, and other oriented drillcores, to yield a more continuous record of Australia's past motions and to provide magnetostratigraphic data for enhancing inter-basin correlations.

Sea level in the Ordovician: Sharp fluctuations in subsidence rates of the Siberian Craton crust

Most researchers relate rapid fluctuations in water depths of epicontinental sedimentary basins to eustatic sea level fluctuations [1 and others]. Of particular interest for oil-and-gas prospecting are third-order cycles or eustatic events 1–3 Ma long with amplitudes of 20 to 200 m. They are used as reference levels for interbasin correlation [2, 3, and others]. Using the data on low sedimentation rates in extremely shallow settings ( ≤ 10 m) of the eastern Baltic and East Siberia regions, it has been shown that third-order sea level fluctuations in the terminal Cambrian–Silurian did not exceed 10–20 m [4, 5]. At the same time, fluctuations in water depths up to 100–200 m over periods of 1–3 Ma long were characteristic of many epiplatformal sedimentary basins on many continents. Since the sea level was almost stable, such fluctuations indicate frequent crustal uplifts and subsidences in platforms. This is a quite unexpected result, since such areas are usually considered relatively calm in terms of tectonics.

Dictyotidium senticogremium sp. nov., a new prasinophyte (Chlorophyta) phycoma from the Upper Devonian of northern Iran

A new, morphologically distinctive, murornate prasinophyte phycoma, Dictyotidium senticogremium sp. nov., is described from Upper Devonian strata of the Geirud Formation, Alborz Mountains, northern Iran. It is characterized by a small, near-circular vesicle, the surface of which is regularly divided by low membranous muri into few relatively large polygonal lacunae; central parts of the latter bear prominent granulate sculpture. Dictyotidium senticogremium sp. nov. occurs in a relatively diverse acritarch, prasinophyte, and miospore assemblage collectively indicating an early Late Devonian (Frasnian) age. The species is persistent, relatively common and has a presumably short stratigraphic range; therefore it is a potentially useful biostratigraphic index fossil for inter-basinal correlation across Iran.

Problems with Paleocene Palynozones in the Rockies: Hell's Half Acre Revisited

The scheme of palynostratigraphic zonation most widely applied to interbasinal correlation of Paleocene rocks in the Rocky Mountain region of North America is fundamentally flawed. In its initial publication (based on juglandaceous fossil pollen), basic principles of biostratigraphy were violated, the taxonomy was typological, and the phylogenetic analysis was methodologically unsound. Youngest parts of Paleocene time were not sampled during development of the zonation, and the basal two zones involved strata of Cretaceous age. Implications of paleoenvironmental influences on sequences of palynostratigraphic occurrences were not considered. Subjective assertions at many levels remain neither verifiable nor falsifiable, because data from specimens utilized in the published formulation of Paleocene palynozones P1–P6 cannot be reliably tied in their defining area to identifiable stratigraphic sequences at known positions in actual rocks. A rigorous and geologically useful palynological zonation for Paleocene strata in the Rockies is possible, and its development should be pursued. Procedurally, however, any such scheme must (1) be constructed according to sound principles of biostratigraphy; (2) contain accurate information about locations and stratigraphic positions of all samples; and (3) provide publicly accessible and verifiable taxonomic data on compositions of all samples.

Tree-ring footprint of joint hydrologic drought in Sacramento and Upper Colorado river basins, western USA

Growing and changing demands on water supply, along with natural climate variability and possible anthropogenically induced climate change, make water resource management and planning increasingly challenging, particularly in arid regions. Instrumental climate and gaged streamflow records provide just a snapshop of recent natural hydrologic variability. In this paper, we use tree-ring-based annual streamflow reconstructions for the Sacramento River in California and the Blue River in western Colorado to analyze the temporal and spatial variability of widespread drought simultaneously affecting both basins over the past five centuries. Stability of joint-drought episodes and the covariation of reconstructed flows in the two basins are analyzed with sliding correlations, spectral analysis and a hypergeometric test. Year-to-year spatial patterns of moisture anomalies in a singular joint-drought episode in the late-1500s are mapped with a network of tree-ring data. Climatological aspects of joint droughts of the 20th century are investigated with 500-mb geopotential height data and climatic indices. Although flow in the two rivers is only very weakly correlated over the full 538-yr reconstruction period, more years of joint drought occur than would be expected by chance alone. Covariation in reconstructed flows is stronger in the late 1500s and mid-1700s than at any time since 1800. The late 1500s period of drought is not characterized as a decades-long unbroken drought, but as a series of drought impulses broken by wet years, with widespread moisture deficits in joint dry years. Periods of high inter-basin correlation in reconstructed flow are characterized by coherency at frequencies within the ENSO band. However, joint droughts in instrumental gage records do not display any consistent relationship with ENSO or the Pacific Decadal Oscillation (PDO), and so it is difficult to infer either as a causal mechanism for joint droughts in the past.

Global correlation for strontium isotope curve in the Late Cretaceous of Tibet and dating marine sediments

87Sr/86Sr ratios of marine carbonate samples collected from a sedimentary section of the Late Cretaceous in the south of Tibet were measured. Based on the absence of cathodo-luminescence and a very low Mn/Sr ratio (average 0.06) of the samples, it is thought that they contain information on the original seawater strontium isotope composition. The strontium isotope evolution curve of the Late Cretaceous in Tibet we established here, is consistent with other coeval curves from Europe, North America and Antarctica, supports the notion that the strontium isotope composition of seawater is governed by global events, which provides a new approach for the inter-continental and inter-basinal correlations of Late Cretaceous in the area and is a complementarity for biostratigraphy. In addition, we attempt to determine the age of the boundaries for Campanian/Santonian and Maastrichtian/Campanian by87Sr/86Sr ratios for Gamba section in southern Tibet. The two boundaries are located in the thickness of 217 m (83.5 Ma) and 291 m (71.3 Ma), respectively.

Sequence Stratigraphy and Depositional Facies of Lower Ordovician Cyclic Carbonate Rocks, Southern Missouri, U.S.A.

ABSTRACT Lower Ordovician cyclic carbonate strata of southern Missouri were deposited in a warm, shallow, epeiric sea on a fully aggraded carbonate platform. Sedimentological characteristics distinguish the Jefferson City and Cotter dolomites from the underlying Gasconade and Roubidoux formations. Mixed carbonate-siliciclastic sedimentation characterizes the Roubidoux Formation, with sandstones accounting for up to 60% of sedimentation. The Gasconade, Jefferson City, and Cotter dolomites exhibit an increased occurrence of chalcedonic chert nodules in very similar shape and texture to the gypsum and anhydrite nodules common on modern sabkha supratidal flats. Casts of halite and ghosts of gypsum laths also exist in the Jefferson City and Cotter strata but are rarely found in the underlying units. Facies analysis from drill cores and outcrop sections provides the basis for identifying two major meter-scale cycle types. Type I cycles consist of algal stromatolites, tidal-flat laminites (mechanical and algal), ooid grainstones, wavy peloidal wackestones, and quartz sandstones interpreted as peritidal facies. They are the dominant components of the Roubidoux Formation, Jefferson City Dolomite, and Cotter Dolomite. Type II cycles consist mostly of subtidal facies such as strongly burrowed mudstone, thrombolite boundstone, and stromatolites. Type I cycles are thinner and represent highstand systems tracts, whereas the thicker type II cycles represent transgressive systems tracts and are dominant in the Gasconade Dolomite. The cycle stacking patterns, facies changes, and the intrabasinal correlatability of Fischer plots made from the widely spaced sections argue for a eustatic control on sea-level fluctuation on the platform. Interbasinal correlation with other North American basins is possible using biostratigraphic information and comparison of Fischer plots. Five Missouri sequences correlate with those described for other regions. The continent-wide uniformity in cycle stacking patterns indicates a primarily eustatic control on Lower Ordovician meter-scale cycle development. Regional tectonic and autocyclic controls probably account for general differences in sedimentation pattern among the correlated basins.

Permian brachiopods from the Tupe Formation, San Juan Province, Precordillera, Argentina

The brachiopod fauna from the Tupe Formation at La Herradura Creek, located on the west flank of Perico Hill, San Juan Province, Argentina, palaeogeographically belongs to the western sector of the Paganzo basin (‘Guandacol embayment’). The stratigraphical section of the Tupe Formation at La Herradura Creek is the stratotype of the Tivertonia jachalensis-Streptorhynchus inaequiornatus biozone, was previously regarded as being of Late Carboniferous age but here is assigned to the earliest Permian (Asselian). We describe and review the biozone assemblage, which consists of Streptorhynchus inaequiornatus, Tivertonia jachalensis, Kochiproductus sp., Costatumulus sp., Coronalosia argentinensis, Tupelosia paganzoensis, Trigonotreta pericoensis, Septosyringothyris sp. aff. Septosyringothyris jaguelensis and Crurithyris? sp. This brachiopod assemblage is related to Indian and Australian Early Permian faunas and its presence in the La Herradura Creek section provides new evidence in support of an Asselian (Early Permian) age for the Tivertonia jachalensis-Streptorhynchus inaequiornatus biozone. This assemblage is also important for intra- and inter-basinal correlation because several of its characteristic species have been identified from other sections of the Paganzo basin and the Río Blanco basin. The proposed age for this biozone is consistent with the age of palynological data from slightly above the marine faunas from the stratotype locality.

Palynostratigraphy of the tertiary sediments of Meghalaya, Northeastern India - Present status and gaps

Thick and extensive Tertiary sediments. belonging to both shelf and geosynclinal facies, occupy the southern part of the Meghalaya Plateau. The basal part of these sediments is represented by the Langpar Formation which is followed by Jaintia (Therria. Sylhet Limestone and Kopili formations), Barail (Laisong. Jenam and Renji formations), Surma (Bhuban and Bokabil formations), Tipam (Tipam Sandstone and Girujan Clay formations), Dupitila and Dihing groups. In Garo Hills, these sediments are classified into various other differently named formations. A considerable amount of palynological information has so far been published from these sediments, which provides useful data for biostratigraphic zonation, correlation and age determination and for interpreting palaeoclimate, palaeogeography and environment of deposition. An effort has been made, in the present paper, to synthesize the published palynological work on these sediments in order to understand the present status of the Tertiary palynostratigraphy of Meghalaya. It has been noticed that the various palynozones established in the Tertiary sequence of Meghalaya can be recognized by their peculiar palynofossil assemblages. This is particularly useful in the correlation of homotaxial stratigraphic units in Garo, Khasi and Jaintia Hills and in the interbasinal correlation. In spite of a good deal of palynodata available, there are plenty of gaps in our knowledge regarding Tertiary palynostratigraphy of Meghalaya that deserve attention of palynologists. These gaps have been identified for the future studies.

Application of the parasequence concept to the Paleoproterozoic record of the northern Fennoscandian Shield

Paleoproterozoic (ca 2450 to 2000 Ma) sedimentary sequences covering Archean blocks of the Fennoscandian Shield outcrop as discontinuous belts in northwest Russia, eastern and northern Finland, northern Sweden and northern Norway. This fragmentary distribution hampers conventional correlations. More robust inter-basin correlations can be established by applying principles of sequence stratigraphy to particularly well-preserved sections such as the Kovasinvaara Formation (Kainuu belt) and the Rukatunturi Quartzite Formation (Kuusamo belt) exposed in northern Finland. In the East Puolanka Group (Kainuu Belt) depositional systems (fluvial, high wave-energy shoreline, tidally influenced shoreline, inner shelf) define four transgressive systems tracts followed by highstand periods. Rocks in the Kuusamo Belt, ca 130 km to the north, display a similar stratigraphic evolution. Extensive tidal deposits in these areas record periods of high sea level. The sets of identified parasequences in both areas are retrogradational and indicates migration of shoreline landwards enable the deeper water tidal channel-fill deposits to occur at the top of an overall transgressive system tracts. Parasequences identified in this study are related to high-frequency cycles of relative sea-level change. Major sequences reflect lower order fluctuations related to heterogeneous lithospheric subsidence. As the eustatic origins for individual boundaries become more firmly established, it may be possible to proceed to more detailed correlations between coeval sequences deposited on other early Precambrian cratons.

Ediacarian sequence and isotope stratigraphy of the Officer Basin, South Australia

The Ediacarian was a time of active tectonism over large areas of the Australian craton and sedimentary rocks of this age are relatively widespread. In spite of their wide distribution and the perception that they offer some of the most likely Neoproterozoic petroleum source rocks, their stratigraphy is not well understood and interbasinal correlation has proved difficult. Using available seismic and well‐log data, 19 depositional sequences contained within two tectonically defined megasequences are recognised in the Ediacarian succession in the eastern Officer Basin. Systematic carbon and strontium isotope chemostratigraphy, combined with the sequence stratigraphy, has enabled correlation of the main sequence boundaries between the Officer Basin and the Adelaide Geosyncline and Amadeus Basin. A significant number of sequence boundaries recognised in the exceptionally complete Officer Basin succession are either missing or not expressed in the Amadeus Basin and Adelaide Geosyncline successions. This suggests that only the major tectonically enhanced sequence boundaries (megasequence boundaries) have been recognised in the adjacent basins. The megasequences bounded by these surfaces define major basin phases.

The application of sequence stratigraphy to Upper Carboniferous fluvio-deltaic strata of the onshore UK and Ireland: implications for the southern North Sea

Stratigraphical correlations and fades interpretations of Upper Carboniferous fluvio-deltaic strata have been based traditionally on cyclothems bound by marine flooding surfaces (marine bands). The recent recognition of major, regionally extensive erosional unconformities (Exxon-style sequence boundaries) within selected cyclothems questions their validity as units of genetically related strata. U sing examples from the Carboniferous of the onshore UK and Ireland, we present sedimentological criteria for the recognition of sequence boundaries, placing particular emphasis on the regional context of these surfaces. Sequence boundaries comprise widespread, deeply eroded surfaces at the base of major fluvial sandstone complexes, and laterally equivalent palaeosols developed on interfluves at the margins of the fluvial complexes. These sequence boundaries define units of genetically related strata (sequences) which contain other key surfaces of time-stratigraphic significance, including marine bands and regionally extensive coals. The recognition of key surfaces enables the construction of a high resolution stratigraphic framework within which coeval facies relationships can be interpreted. Sequence boundaries can be correlated between individual basins in the onshore UK, by reference to their position in relation to a particular marine band. For example, the sequence boundary at the base of the Farewell Rock in the South Wales Basin can be correlated with that at the base of the Rough Rock in the Pennine Basin, northern England, since both these sandstone bodies are directly overlain by the Subcrenatum Marine Band. Interbasinal correlations of this nature imply that potential fluvial sandstone reservoirs within major incised valley fills in the Upper Carboniferous strata of the southern North Sea can be predicted by correlation with the onshore UK. The stratigraphical framework can be extended and tested using core and well-log data, particularly spectral gamma-ray data, which are able to identify key sequence stratigraphic surfaces.

Slope-apron deposition in an Ordovician arc-related setting: the Vuelta de Las Tolas Member (Suri Formation), Famatina Basin, northwest Argentina

The Ordovician Suri Formation is part of the infill of the Famatina Basin of northwest Argentina, which formed in an active setting along the western margin of early Paleozoic Gondwana. The lower part of this formation, the Vuelta de Las Tolas Member, records sedimentation on a slope apron formed in an intra-arc basin situated on a flooded continental arc platform. The coincidence of a thick Arenig-Llanvirn sedimentary succession and volcanic-plutonic arc rocks suggests an extensional or transtensional arc setting, and is consistent with evidence of an extensional regime within the volcanic are in the northern Puna region.The studied stratigraphic sections consist of volcanic rocks and six sedimentary facies. The facies can be clustered into four facies associations. Association 1, composed of facies A (laminated siltstones and mudstones) and B (massive mudstones and siltstones), is interpreted to have accumulated from silty-muddy high- and low-density turbidity currents and highly fluid, silty debris flows, with subsequent reworking by bottom currents, and to a lesser extent, hemipelagic suspension in an open-slope setting. Facies association 2 is dominated by facies C (current-rippled siltstones) strata. These deposits are interpreted to record overbank sedimentation from fine-grained turbidity currents. Facies E (matrix-supported volcanic breccias) interbedded with andesitic lava units comprises facies association 3. Deposition was contemporaneous with subaqueous volcanic activity, and accumulated from cohesive debris flows in a coarse-grained wedge at the base of slope. Facies association 4 is typified by facies D (vitric fine-grained sandstones and siltstones) and F (channelized and graded volcanic conglomerates and breccias) deposits. These strata commonly display thinning- and fining-upward trends, indicating sedimentation from highly-concentrated volcaniclastic turbidity currents in a channelized system. The general characteristics of these deposits of fresh pyroclastic detritus suggest that their accumulation was contemporaneous with, or post-dated shallow-water or subaereal explosive volcanism.The Vuelta de Las Tolas Member tends to show an overall random facies patterns reflecting the strong influence of non-cyclical episodic processes related to arc volcanism and slope sedimentation. The scarcity of resident ichnofaunas and the presence of thick packages of uniform mudstones suggest deposition under oxygen-depleted conditions in a topographically confined, ponded sub-basin. Interbasinal correlations favor comparison with Middle Arenig slope-apron successions formed in the northern Puna Basin and suggest a southward prolongation of the Arenig volcanic arc.

A terrestrial auxiliary stratotype point and section for the Plio-Pleistocene boundary in the Turkana Basin, East Africa

The Turkana Basin of northern Kenya and southern Ethiopia preserves an extensive record of terrestrial sediments and fossils dating from the Oligocene through the Holocene. Of particular significance is the Plio-Pleistocene Omo Group, which documents changes in environments and faunas through the Pli-Pleistocene. Omo Group strata have been intensively studied for nearly three decades, and a tightly constrained stratigraphic framework has been developed. The lithostratigraphic sequence shows that a fluvial system dominated the basin for most of the past five million years, with six relatively short but significant intervals of lacustrine deposition. The sequence is punctuated by some 130 tephra, most of which have been geochemically characterized, and can be used as a basis for local and inter-basinal correlation. These tephra reach as far as the Gulf of Aden and the Indian Ocean, providing a means of tightly correlating the marine and terrestrial records. Isotopic dating and magnetic polarity stratigraphy allow good temporal control on the sequence, with over 30 control points in the interval from 4.35 to 0.6 Ma. Extensive paleontological collections from this context provide critical calibration of East African biostratigraphic markers. The Plio-Pleistocene boundary can be closely constrained in the Omo Group deposits based on magnetic polarity stratigraphy. This allows the boundary to be related to components of the stratigraphic framework recognized in the terrestrial sequence there, and thence related to other East African terrestrial localities by a variety of correlation tools. Establishment of the Omo Group sequence as an Auxiliary Stratotype Point for the Plio-Pleistocene boundary will thus allow close relation of these terrestrial localities on the basis of tephro- and biostratigraphy, the most useful correlation tools in the East African terrestrial record.

Integrated biostratigraphic and sequence stratigraphic framework for Upper Cretaceous strata of the eastern Gulf Coastal Plain, USA

Upper Cretaceous (Santonian-Maastrichtian stages) strata of the eastern US Gulf Coastal Plain represent a relatively complete section of marine to nonmarine mixed siliciclastic and carbonate sediments. This section includes three depositional sequences which display characteristic systems tracts and distinct physical defining surfaces. The marine lithofacies are rich in calcareous nannoplankton and planktonic foraminifera which can be used for biostratigraphic zonation. Integration of this zonation with the lithostratigraphy and sequence stratigraphy of these strata results in a framework that can be used for local and regional intrabasin correlation and potentially for global interbasin correlation. Only the synchronous maximum flooding surfaces of these depositional sequences, however, have chronostratigraphic significance. The sequence boundaries and initial flooding surfaces are diachronous, and their use for correlation can produce conflicting results. The availability of high resolution biostratigraphy is critical for global correlation of depositional sequences.

Cross‐platform architecture of a sequence boundary in mixed siliciclastic‐carbonate lithofacies, Middle Cambrian, southern Great Basin, USA

ABSTRACTStratigraphic analysis of mixed siliciclastic‐carbonate lithofacies within the Middle Cambrian Bonanza King Formation of the southern Great Basin reveals three distinct facies associations that record a range of depositional environments from semi‐arid tidal flats to deeper subtidal, restricted lagoons. Stratigraphic trends, cross‐platform facies variations and correlation of individual surfaces across 250 km of the study area suggest that these mixed lithofacies were deposited in three temporally distinct phases. (1) Extensive progradation of mixed peritidal environments culminated in a prolonged episode of subaerial exposure marked by an areally extensive intraclast breccia (0·5–1·2 m thick) that we interpret to be a major Type 1 sequence‐bounding disconformity. (2) Abrupt flooding of the exposed platform resulted in the deposition of mixed deeper subtidal lithofacies, including a condensed interval of fissile, fossiliferous shale. (3) Progressive shallowing and aggradational accumulation was accompanied by a decrease in siliciclastics and a shift to pure carbonate deposition. Deep‐water siliciclastics and megabreccias record deposition along the base‐of‐slope off the Middle Cambrian shelf‐edge, and are interpreted to represent lowstand deposits emplaced during the prolonged episode of subaerial exposure of the shallow shelf.The presence of fine siliciclastics in both peritidal facies and sharply overlying deeper subtidal facies of the study interval within the Bonanza King suggests a variable, but relatively continuous, influx of terrigenous material throughout an extended period of accommodation change, apparently asynchronous with respect to the predictive model of reciprocal sedimentation. We suggest that the primary siliciclastic source changed with relative sea‐level position. During lowered sea level, aeolian processes acting upon the unvegetated Cambrian craton transported fine siliciclastics onto peritidal and shallow‐subtidal environments. During higher sea level, coastal siliciclastic reservoirs supplied sediment that was transported for long distances by geostrophic currents flowing along the submerged platform.As opposed to many Cambro‐Ordovician grand cycles that are commonly interpreted to consist of a transgressive shaly half‐cycle grading upward into a regressive carbonate half‐cycle, the sequence boundary within this Middle Cambrian succession occurs within siliciclastic‐rich, mixed lithofacies rather than in adjoining purer carbonates, implying that some ‘grand cycles’ should not be considered synonymous with ‘sequences’. Interbasinal correlations of the Type 1 sequence boundary within the mixed unit are speculative, primarily because of the inherent imprecision of available trilobite biostratigraphy. However, there is evidence that an extended episode of subaerial exposure may have been continent‐wide during the Ehmaniella trilobite biochron.

Interbasinal correlation of the Cenomanian Stage; testing the lateral continuity of sequence boundaries

Abstract Sequence stratigraphic analysis of the Aquitaine, Münster and Paris Basins has shown that many of the major discontinuities found can be traced with good biostratigraphic confidence between the basins, and through a variety of sedimentary, tectonic and palaeobiogeographic regimes. Up to five sequence boundaries appear to be synchronous between the basins when account is taken of the biostratigraphic shortcomings. An extra sequence boundary near the Lower-Middle Cenomanian boundary in the Münster Basin may be a tectonic disruption as all evidence for this relative sea-level fall is from the northern, active margin of the basin, where it is associated with mass flow deposits. Notable features are the absence of lowstand deposits in these epieric, hemipelagic settings and the expression of sequence boundaries as hardgrounds. Multiple transgressive surfaces are common in deeper water settings, and surfaces of maximum flooding are very poorly developed. However, deep water deposits do preserve more accurate evidence of sea-level change, due to increased lithological and biostratigraphical completeness and fewer controls on sedimentation.

The carbon-isotopic composition of Proterozoic carbonates: Riphean successions from northwestern Siberia (Anabar Massif, Turukhansk Uplift).

Thick carbonate-dominated successions in northwestern Siberia document secular variations in the C-isotopic composition of seawater through Mesoproterozoic and early Neoproterozoic (Early to early Late Riphean) time. Mesoproterozoic dolomites of the Billyakh Group, Anabar Massif, have delta 13C values that fall between 0 and -1.9 permil versus PDB, with values in the upper part of the succession (Yusmastakh Formation) consistently higher than those of the lower (Ust'-Il'ya and Kotuikan formations). Consistent with available biostratigraphic and radiometric data, delta 13C values for Billyakh carbonates compare closely with those characterizing early Mesoproterozoic carbonates (about 1600-1200 Ma) worldwide. In contrast, late Mesoproterozoic to early Neoproterozoic limestones and dolomites in the Turukhansk Uplift exhibit moderate levels of secular variation. Only the lowermost carbonates in the Turukhansk succession (Linok Formation) have delta 13C values that approximate Billyakh values. Higher in the Turukhansk succession, delta 13C values vary from -2.7 to +4.6 permil (with outliers as low as -5.0 permil interpreted as diagentically altered). Again, consistent with paleontological and radiometric data, these values compare well with isotopic values from 1200 to 850 Ma successions elsewhere. Five sections measured in different parts of the Turukhansk basin show nearly identical patterns of variation, confirming that carbonate delta 13C correlates primarily with time and not facies. The Siberian sections illustrate the potential of integrated biostratigraphic and chemostratigraphic data in the intra- and interbasinal correlation of Mesoproterozoic and early Neoproterozoic rocks.

The Neoproterozoic of southwestern Africa, with emphasis on platform stratigraphy and paleontology

Neoproterozoic sediments in southwestern Africa occur in the Pan-African Kaoko, Khomas, Gariep, Vanrhynsdorp and Saldania Belts. They were deposited in response to rifting, break-up of a supercontinent (formed at ∼ 1000 Ma) and subduction and collisions of the Congo, Kalahari, Rio de la Plata and Malvinas plates. These collisions led to the formation of a Neoproterozoic/Cambrian supercontinent. This paper deals with the geology of the platform successions of the Kaoko, Khomas, Gariep and Vanrhynsdorp Belts. Deposition of the sediments occurring in these belts commenced ∼ 900 to 1000 Ma ago and probably ended between 500 and 520 Ma. The sediments record Wilson cycles that ended with the deposition of red beds. Palaeontological and carbon isotope studies on the Damara Sequence, Gariep Complex, Nama Group and Vanrhynsdorp Group make intra- and inter-basinal correlations possible. Prior to deposition of the Nama Group two, or perhaps three, glaciations took place over an extensive area in southwestern Africa, these being the Kaigas (Chuos) (∼ 725-700 Ma), Numees (∼ 650-600 Ma or ∼ 610-590 Ma) and possibly the Varianto (∼ 745-725 Ma) glaciations. The Numees tillite is overlain by a stratigraphic unit containing pinkish dolomite. During deposition of the Nama Group, the minor Vingerbreek (∼ 600-545 Ma) “glaciation” (or cold-water period) occurred. The major Kaigas (Chuos) and Numees and minor Vingerbreek glaciations correspond with low δ 13C isotope ratios and appear to have taken place at relatively low palaeolatitudes. The Riphean sediments were deposited predominantly during rifting and break-up of a supercontinent. During the late Vendian to early Cambrian, deposition of the Nama Group in southern Namibia and the penecontemporaneous Vanrhynsdorp Group in the Republic of South Africa took place in the Nama foreland basin. Detrital micas indicate that deposition of the Nama Group took place between 635 and 530 Ma. The Nama foreland basin, which was connected to a major ocean to the south, formed as a result of collisions between the Congo, Kalahari and Rio de la Plata plates. With the exception of the Nama Group, little is known about the palaeontology of the Neoproterozoic in southwestern Africa. The lower Nama Group is latest Proterozoic in age. It contains an Ediacaran fauna, a depauperate assemblage of organic-walled microfossils and a relatively well diversified ichnofauna with Brooksella, Diplocraterion and Nereites which are atypical of Vendian strata. Many trace fossils are exceptionally small. The occurrence of the shelly fossil Cloudina and probable calcified metaphytes in the lower Nama Group suggest that skeletalization in animals and metaphytes occurred almost simultaneously towards the end of the Proterozoic. The upper Nama Group is early Cambrian in age as indicated by the occurrence of the trace fossil Phycodes pedum. The upper Nama Group, dominated by red beds, contains a low-diversity ichnofauna with trace fossils of normal size. Bio- and chemostratigraphy combined with sequence stratigraphy will probably establish how the prominent unconformities, which occur in the Neoproterozoic of southwestern Africa, are related to the periods of formation of the Wonoka Canyons in south Australia.

Neoproterozoic variations in the C-isotopic composition of seawater: stratigraphic and biogeochemical implications

The recent proliferation of stratigraphic studies of δ 13C variation in carbonates and organic C in later Neoproterozoic and basal Cambrian successions (∼ 850-530 Ma) indicates a strong oscillating trend in the C-isotopic composition of surface seawater. Alone, this trend does not adequately characterize discrete intervals in Neoproterozoic time. However, integrated with the vectorial signals provided by fossils and Sr-isotopic variations, C isotope chemostratigraphy facilitates the interbasinal correlation of later Neoproterozoic successions. Results of these studies are evaluated in terms of four stratigraphic intervals: (1) the Precambrian/Cambrian boundary, (2) the post-Varanger terminal Proterozoic, (3) the late Cryogenian, and (4) the early Cryogenian. Where biostratigraphic or radiometric data constrain the age of Neoproterozoic sedimentary sequences, secular variations in C and Sr isotopes can provide a level of stratigraphic resolution exceeding that that provided by fossils alone. Isotopic data place strong constraints on the chemical evolution of seawater, linking it to major tectonic and paleoclimatic events. They also provide a biogeochemical framework for the understanding of the initial radiation of macroscopic metazoans, which is associated stratigraphically, and perhaps causally, with a global increase in the burial of organic C and a concomitant rise of atmospheric O 2.