Mixed Siliciclastic-carbonate Ramp Research Articles

Miocene scleractinian corals from a mixed siliciclastic–carbonate system: Bakhtiari succession, Zagros Basin (central-western Iran)

Colonial scleractinian corals were sampled from three levels within a Miocene marine unit of the Bakhtiari succession, Zagros Basin, central-western Iran. The first two coral-bearing intervals, A and B, contain small-scale scattered colonies and show a poor coral diversity, whereas the third, consisting of a strongly lithified limestone package, reflects a well-developed biostromal framework with higher coral skeletal volume within the Bakhtiari succession. The Bakhtiari succession coral assemblages are characterized by Porites sp. cf. P. maigensis, Porites sp. cf. P. mancietensis, Porites sp. cf. P. collegniana, Tarbellastraea reussiana, Favia sp., Montastrea sp. cf. P. tchihatcheffi, Favites sp. cf. P. neugeboreni, Favites sp. cf. P. neuvillei, Agathiphyllia sp. and Acropora sp. Sedimentological and palaeontological data indicate that the depositional environment is consistent with a mixed carbonate–siliciclastic ramp that was gently deepening basinwards from the shoreline. The hemispherical and massive growth forms of colonies and sparse branching forms dominated the well-illuminated euphotic zone. Abundant domestone and dense pillarstone coral growth fabrics interdigitating with coarse-grained terrigenous sediments developed in the shallow inner ramp environment. Branching forms and meandroid branching colonies together with some massive forms mostly inhabited the low-energy conditions of the lower euphotic to oligophotic zones of the middle ramp. In the middle parts of the mixed carbonate–siliciclastic ramp, sparse pillarstone together with domestone comprises a mixstone coral growth fabric. Fluctuations in nutrient and clastic sediment input, salinity and the growth of red algae likely terminated coral growth.

The Madotz Urgonian platform (Aralar, northern Spain): Paleoecological changes in response to Early Aptian global environmental events

Sudden addition of carbon dioxide to the atmosphere can reduce the CaCO3 saturation and weaken the biocalcification potential of marine organisms in shallow water and in open marine settings. In this study, the response of an Aptian neritic carbonate environment to sudden addition of carbon dioxide at the beginning of Oceanic Anoxic Event 1a is investigated. The beginning of the OAE1a was coupled with a major perturbation on the carbon cycle as indicated by a negative carbon isotope excursion in the sedimentary record. This isotope anomaly is regarded as a proxy for massive addition of volcanic or methane-derived CO2 to the atmosphere within only a few 104years. The impact of a rapid change in atmospheric pCO2 on biocalcifiers in low latitude shallow-water settings can be studied in a well preserved Aptian carbonate shelf succession cropping out today in the Aralar mountains (NE Spain). The Madotz section (N Spain) preserves a continuous shallow water record that was deposited on a mid-latitude, Atlantic-oriented mixed siliciclastic–carbonate ramp. Lower Aptian sediments consist of two neritic limestone successions separated by orbitolinid-rich marlstone enriched in organic matter. The lower neritic limestone succession ends with a submarine hardground and the transition from the lower neritic limestone to the orbitolinid marlstone coincides with a negative spike in the organic carbon isotope record. This negative spike can be correlated with the negative carbon isotope anomaly marking the base of OAE1a. The paleoecological change coinciding with the base of OAE1a occurred at a time of sea level rise and it coincided with a demise of heavily calcified nannoconids in the Tethys and Pacific Oceans. The paleoecological change observed in the Madotz section corresponds to a comparable change seen in the more distal and more expanded carbonate ramp section (Igaratza) at the Aralar Mountains. Ocean acidification caused by sudden increase in pCO2 may explain reduced calcification potential of some shallow water calcifiers. Calcification crisis was amplified by rising sea level, increasing temperatures and increased flux of detrital material and nutrients from continents into coastal seas.

Environmental change in the Early Permian of NE Svalbard: from a warm-water carbonate platform (Gipshuken Formation) to a temperate, mixed siliciclastic-carbonate ramp (Kapp Starostin Formation)

A detailed facies study of Early Permian strata within NE Svalbard reveals a fundamental change of the depositional setting, from a restricted-marine, warm-water carbonate platform to an open-marine, temperate-water, mixed siliciclastic-carbonate ramp. The uppermost strata of the Gipshuken Formation (Templet and Sorfonna members; Sakmarian–early Artinskian?) consist of microbialites (algal mats), mudstones, bioclastic/peloidal limestones, carbonate breccias and Microcodium facies reflecting peritidal platform areas and supratidal sabkhas. A mixed heterozoan/reduced photozoan assemblage indicates temperate-water conditions within neighboring deeper, open-marine mid-platform areas, while warm-water conditions still prevailed within inner platform zones. In contrast, the lowermost strata of the overlying Kapp Starostin Formation (Voringen Member; late Artinskian?–Kungurian) show a fully heterozoan biotic assemblage reflecting temperate water conditions within open-marine, storm-dominated, nearshore to transitional offshore areas of a mixed carbonate-siliciclastic ramp. The Voringen Member comprises three facies associations, which form a shallowing-upward sequence subsequent to an initial transgression. The sediments reflect bryozoan bioherms in most distal areas, followed by stacked tempestites of sandy brachiopodal shell banks and Skolithos piperocks, grading into broad sand flats in most proximal areas of the inner ramp. The above environmental change is regarded as a regional event taken place across the entire shelf along the northern margin of Pangea and is attributed to paleoclimatic, paleoceanographic, as well as paleogeographic changes, possibly related to the overall northwards drift of the supercontinent. An abrupt increase in terrigenous input coinciding with this change is ascribed to the uplift of a new local source area, probably to the north or east of the investigation area.

Changes in size and growth rate of ‘Lilliput’ animals in the earliest Triassic

Marine invertebrate taxa that survived the late Permian (latest Changhsingian) mass extinction event are all much smaller than they were prior to the event, an example of the Lilliput effect. New taxa that first appeared in the immediate extinction aftermath are also small compared to their size in younger strata. The Lilliput effect is a temporary phenomenon, with most surviving taxa returning to pre-extinction size in the first two conodont zones of the Triassic. In this study, the growth rates, of some of the Early Triassic Lilliput animals, estimated by growth-line analysis, were compared from different stratigraphic levels to determine how this variable fluctuated as body size returned to pre-extinction values after the event. Specimens were collected from the Mazzin and Siusi members of the Werfen Formation in northern Italy, from facies deposited in a tropical, mixed siliciclastic-carbonate ramp setting. Measurements of the bivalves Unionites and Claraia, the gastropod Bellerophon and the brachiopod ‘ Lingula’ were made in the field, with subsequent growth-line analysis conducted on well-preserved specimens of ‘Lingula’. Specimens collected from the basal Triassic Mazzin Member were smaller than those from the younger Siusi Member. Growth-line analysis indicates that the smaller Mazzin Member specimens had a greater number of more closely spaced growth lines than the larger Siusi Member specimens. The small, ‘Lilliput’ animals living in the immediate aftermath of the Late Permian event therefore suffered more interruptions to growth and had overall slower growth rates than larger individuals living in later recovery times. Frequent interruptions to growth and an overall slower growth rate in specimens from the Hindeodus parvus and Isarcicella isarcica zones of the Mazzin Member (lower Induan) reflects suboptimal environmental conditions, such as the frequency or severity of low-oxygen episodes, temperature extremes, and/or disruptions to primary productivity.

High-resolution sequence stratigraphy of a Late Jurassic mixed carbonate-siliciclastic ramp, Lower Saxony Basin, Northwestern Germany

High-resolution sequence stratigraphy of a Late Jurassic mixed carbonate-siliciclastic ramp, Lower Saxony Basin, Northwestern Germany

The western edge of the Mediterranean Pelagian Platform: A Messinian mixed siliciclastic–carbonate ramp in northern Tunisia

The marine Messinian deposits of Tunisia cover a narrow littoral strip some 300 km long between the northern Bizerte and Cap Bon areas and the central–eastern Sahel region. Litho- and biofacies analysis of six stratigraphic sections reveals the distinctive features of these deposits. The lower Messinian deposits are characterized by ubiquitous siliciclastics and abundant oolitic/bioclastic limestones organized in an eastward facing ramp. Westward (landward), this ramp changes into coastal lagoons, sometimes containing evaporites. Eastward, the ramp passes to the reefal Pelagian Platform extending as far as Lampedusa. Two main sedimentary cycles are distinguished: 1) an early Messinian siliciclastic retrogradational then oolitic/bioclastic progradational cycle (Beni Khiar Formation and lower Oued bel Khedim Formation); 2) a late Messinian brackish to continental cycle that probably accumulated in rapidly subsiding lagoons (Oued el Bir Formation and upper Oued bel Khedim Formation). The Tunisian early Messinian cycle is partly eustatically controlled, but the late Messinian cycle cannot be confidently correlated to other well-known Messinian series because of tectonic movements. The lower Messinian deposits of Tunisia are also characterized by abundant suspension-feeding organisms (molluscs and bryozoans) and rare corals, calcareous algae, echinoids, and larger benthic foraminifers. The proposed palaeoenvironmental model shows that the lower Messinian ramp of Tunisia was located on a current-protected margin and subjected to continent-derived sediment and nutrient supply. Eastward, nutrient influx diminished and a shallow-water isolated carbonate platform with coralgal facies developed between the western and the eastern Mediterranean basins. The main hydrological connection between these two basins occurred through a narrow seaway situated to the northeast of the Pelagian Platform, south of Sicily and Malta.

Active synsedimentary tectonism on a mixed carbonate-siliciclastic continental margin: third-order sequence stratigraphy of a ramp to basin transition, lower Sekwi Formation, Selwyn Basin, Northwest Territories, Canada

The lower part of the Early Cambrian Sekwi Formation in the Selwyn Basin of the Northwest Territories, Canada, is composed of two regional, unconformity-bounded sequences, S0 and S1, which record the first widespread carbonate deposition during the initial Palaeozoic transgression onto the western margin of Laurentia. These Early Cambrian sequences are unique to the western North American Cordillera, representing the only record of primarily deep-water deposition on a tectonically active, mixed carbonate–siliciclastic ramp during this period. More specifically, the geometry of the Sekwi ramp changed during deposition of S0 and S1, from a shallowly dipping homoclinal ramp during the S0 transgressive systems tract to a steeply dipping tectonically modified ramp during the early highstand systems tract of S0. The steeply dipping ramp profile of S0 was preserved into the early transgressive systems tract of S1. The Sekwi ramp returned to a gently sloping ramp during the late highstand systems tract of S1 and remained so throughout the remainder of Sekwi deposition. The evolving shape of the Sekwi ramp is attributed to syndepositional ‘down to the basin’ faulting during deposition of both S0 and S1 and is recorded by: (i) the westward thickening, irregular geometries of S0 and S1; (ii) geographical restriction of deep-water facies (including sediment gravity flow deposits); (iii) the presence of large allochthonous blocks; and (iv) the clast composition of sediment gravity flow deposits. Sediment gravity flow deposits play an unusually important role in the sequence stratigraphic interpretation of the lower Sekwi Formation, as they delineate depositional packages, including the maximum flooding zone, the transitions between portions of systems tracts, and the inferred locations of syntectonic extensional faults. Syntectonic faults increased accommodation basinward of an extensive ooid-shoal complex that developed along the Sekwi ramp crest, greatly influencing sequence geometry and initiating the downslope motion of sediment gravity flows. The syndepositional faulting probably was a continuation of extension that began during the latest Neoproterozoic rifting of western Laurentia. The composition of sediment gravity flow deposits track changing accommodation space on the lower Sekwi ramp and can be used to differentiate systems tracts that probably were related more to tectonism than eustasy.

Stratigraphic organisation, spatial distribution, palaeoenvironmental reconstruction, and demise of Lower Cretaceous (Barremian-lower Aptian) carbonate platforms of the Western Pontides (Black Sea region, Turkey)

Barremian-Lower Aptian platform carbonates (“Urgonian limestones”) of the northern margin of the Istanbul zone extend from Zonguldak to the Kurucasile area along the Black Sea coast. New stratigraphic data on the “Inpiri” Formation of the Inpiri-Kurucasile area are based on the identification of calcareous algae, foraminifera, and rudists. They show that this lithostratigraphic unit is stratigraphically and lithologically equivalent to the Öküşmedere Formation from Zonguldak. Some of the biostratigraphic markers are reported for the first time in Anatolia. Foraminifera are represented by several forms with a significant biostratigraphic potential used to distinguish the Barremian from the lower Aptian. Lower Aptian beds also yield relatively advanced caprinid rudists. The Öküşmedere Formation is relatively thin, terrigeneous-rich, and rudist-free or rudist-poor in the Kurucasile sector, and thick, terrigeneous-poor, and rudist-rich from Amasra to Zonguldak, with a set of marker beds including either charophytes or Palorbitolina and capped by a coral unit underlying ammonite bearing marls. Terrigeneous-rich carbonates from the eastern sector are interpreted as marginal marine coastal, infralittoral environments and grade distally, northward, to marly basinal sediments. By contrast “Urgonian type” limestones from the Zonguldak-Amasra region possess a wide extent and no transition to coastal or basinal sediments has been observed. A transition from a typical platform westward to a mixed siliciclastic-carbonate ramp eastward was controlled by both the nature of the adjacent exposed area and tectonic factors affecting the overall continental margin that is a northward downwarping. The exposed area was flanked southward by a belt of coastal siliciclastics grading southward and eastward to deep water sediments of the Ulus basin. In mid-Bedoulian time, carbonate platform demise from the western region was drowned below deeper marly sediments whereas the eastern siliscilastic-carbonate ramp was buried below coastal clastics.

Geophysical and geological signatures of relative sea level change in the upper Wheeler Formation, Drum Mountains, West-Central Utah: A perspective into exceptional preservation of fossils

The Middle Cambrian Wheeler Formation in Utah is renowned for its exceptionally preserved fossils. Herein, geophysical techniques (gamma ray spectrometry and magnetic susceptibility), carbonate analyses, lithofacies, and fossils are used to characterize limestones and calcareous mudrocks of the upper portion of the middle Wheeler Formation and the upper Wheeler Formation in the Drum Mountains, West-Central Utah. This succession was deposited on a mixed carbonate-siliciclastic ramp within the House Range Embayment, an eastward incursion of deeper water, created and bounded on the southeast by a normal fault. Geophysical data clearly reveal variations in carbonate versus clay content. Observed patterns among outcrop geophysical data, lithofacies, and fossils are interpreted within the contexts of relative sea level changes and a sequence stratigraphic model. Above a basal interval of rhythmically-bedded limestones interpreted to have been deposited on the middle part of the ramp during a period of relatively low sea level, lithofacies, rising gamma ray and rising magnetic susceptibility track a transgression, which culminates in a maximum flooding surface (MFS). This MFS is near the base of a “hot zone”: an ~ 6-m-thick interval of exceptional preservation ( Konservat-Lagerstätte), highest magnetic susceptibility, highest gamma ray values, highest clay content, and with a bulk carbonate content of ~ 4–20%. The basal layer of this zone contains abundant, fully articulated specimens of agnostids and tiny polymerid trilobites. Exceptionally preserved fossils, such as non-trilobite arthropods, priapulid and annelid worms, hyolithids, phyllocarid arthropods with soft parts, algae, and fragile sponges, are much more abundant within this hot zone than in other strata of the succession. These specimens are preserved in very thin-bedded to laminated mudrock, only rarely interrupted by macroscopic signs of bioturbation. Stratigraphically above the hot zone is a thick unit of mudrock in which both gamma ray and magnetic susceptibility decrease, gradually at first and then precipitously to very low levels; in concert, few soft-bodied specimens have been recovered from this upper interval. The gradual decrease in geophysical responses is interpreted to be the result of gradual shallowing during highstand, caused mainly by progradation of the carbonate factory; it was followed by an abrupt regression. Mudrocks are capped by burrow-mottled grainstone and stromatolitic boundstone, representing a lowstand or the earliest transgressive phase of the next cycle. These observations lead to the following conclusions: (1) the upper Wheeler Formation represents most of a third or fourth-order cycle of relative sea level change on a mixed carbonate-siliciclastic ramp, rather than a period of shallow-water lagoonal sedimentation as previously proposed; (2) superimposed on this overall cycle are several (perhaps many) higher-order fluctuations in relative sea level; (3) when the present results are coupled with results of earlier workers, who interpreted the lower and middle Wheeler Formation as a major transgression and regression, the Wheeler Formation of the Drum Mountains involves two major sea level cycles, rather than one; (4) the Konservat-Lagerstätte was preserved in the deepest-water, early-highstand portion of the upper Wheeler sequence; and (5) these results support earlier hypotheses that Konservat-Lagerstätten in mixed carbonate-siliciclastic successions are most likely to occur during late transgression to early highstand, given that low energy, anoxic conditions prevailed.

Constraints on 87 Sr/ 86 Sr of Late Ediacaran seawater: insight from Siberian high-Sr limestones

Abstract: In SE Siberia, carbonate formations with δ 13 C carb values ranging between −12‰ and −7‰ (V-PDB) and Sr concentrations of up to 2.5% occupy an area of 40 000 km 2 . Several successions exceed 1000 m in thickness and represent the world's largest known exposures of sedimentary carbonates exhibiting extreme depletion in 13 C. The carbonates were deposited on a carbonate platform evolving from a mixed carbonate–siliciclastic ramp to a carbonate ramp, and then from a peritidal rimmed shelf to a deep-water open shelf. All sequences reveal a facies-independent, upward rise in marine δ 13 C carb from −12‰ to −7‰. The trend and magnitude of the values mimic those that are characteristic of the 600–550 Ma Shuram–Wonoka isotope event. A coincident stratigraphic rise in 87 Sr/ 86 Sr from 0.70802 to 0.70862 in several sections of limestones, containing 4400 μg g −1 Sr on average, is considered to be by far the best available constraint on a temporal variation of seawater isotopic composition through the Late Ediacaran. If the greatest temporal rate of change in seawater 87 Sr/ 86 Sr observed in the Cenozoic is applied to the Siberian sections, the calculated minimum duration for the Suram–Wonoka event is 10 Ma. Supplementary material: XRF and ICP-AES analysis, C, O and Sr isotopic data are available at http://www.geolsoc.org.uk/SUP18324 .

Lower Oligocene non-geniculate coralline red algal (Corallinales, Rhodophyta) assemblage from Poljšica pri Podnartu (Upper Carniola, Slovenia)

The Lower Oligocene Gornji Grad beds from Poljšica pri Podnartu consist of marly limestone, mudstone, several layers of limestones and two layers of sandstones, and were deposited on a mixed carbonate-siliciclastic ramp. Especially the limestones contain rich fossil fauna and non-geniculate coralline red algae. These were systematically collected from four horizons and researched in thin sections under an optical microscope. Genera Lithoporella, Neogoniolithon, Spongites, Lithothamnion, Mesophyllum and Spongites were recognized. Surface area for each genus was calculated and the differences in the coralline assemblages in the four horizons were analysed. The corallines originate from two source areas: sandy-muddy bottom of a shallow marine environment, and small coral bioherms with its encrusters.

Facies architecture of a Late Jurassic carbonate ramp: the Korallenoolith of the Lower Saxony Basin

The sedimentary succession of a Late Jurassic (Oxfordian to basal Kimmeridgian) carbonate ramp is described and interpreted. The study area is located in the central part of the Lower Saxony Basin in NW Germany, which forms part of the Central European Basin. Eight well-exposed and undeformed sections of the study area (Suntel area, Wesergebirge and eastern part of the Wiehengebirge) provide detailed information about lithofacies and lateral thickness variations. Biostratigraphically, the age of these sediments is poorly constrained. Twenty microfacies types are recognized that can be grouped into seven facies associations: (a) strongly bioturbated marlstones deposited near storm wave base (SWB), (b) foraminifera-rich wackestones, (c) wackestones and floatstones with biostromes and (d) bioclastic limestones deposited between SWB and fair-weather wave base (FWWB), (e) oolitic and iron-oolitic limestones and (f) siliciclastic sediments deposited above FWWB, and (g) lagoonal deposits. These facies associations characterize a storm dominated shallow mixed carbonate-siliciclastic ramp. Based on facies changes, quartz content, and gamma ray logs, the Korallenoolith Formation can be subdivided into a lower carbonate-dominated and an upper siliciclastic-dominated part, build up by different scales of small- to large-scale deepening- and shallowing-upward cycles. A preliminary correlation of measured outcrops of this formation is presented.

Facies development of a Late Ordovician mixed carbonate-siliciclastic ramp proximal to the developing Taconic orogen: Lourdes Formation, Newfoundland, Canada

The Upper Ordovician (late Whiterockian to Mohawkian) Lourdes Formation represents a narrow (tens of kilometers), short-lived [∼5–7 million years (my)], open-ocean (high-energy) mixed siliciclastic-carbonate ramp that onlapped allochthonous strata along the orogen side of the local Taconic foreland basin. Platform development followed a 6–8 my hiatus during which weathering had concentrated chemically mature siliciclastics that were admixed with initial carbonate sediments. A cross-platform facies gradient contains paleokarst and peritidal carbonates and sandstones, shallow-ramp carbonate bioherms and skeletal shoals, and deeper ramp calcareous shales. Transgressive systems tracts are marked by ramp-wide sheets and shoals of skeletal grainstone and low accumulation rates, and highstand systems tracts are marked by significant admixture and interbedding of siliciclastics with cross-ramp carbonate facies. Platform demise coincides with increased siliciclastic input, which is likely tectonically influenced. The Lourdes platform is equivalent to epicontinental foreland ramps along eastern Laurentia, but its narrower width precluded formation of oceanographically restricted platform-interior facies.

Stratigraphic and geochemical framework of the Agouron drill cores, Transvaal Supergroup (Neoarchean-Paleoproterozoic, South Africa)

Two long drill cores through the Transvaal Supergroup (Neoarchean-Paleoproterozoic) record the vertical transition from a mixed siliciclastic-carbonate ramp (Schmidtsdrif Subgroup) through the Campbellrand-Malmani carbonate platform (Campbellrand Subgroup) to deeper-water banded iron formations (Asbestos Hills Subgroup). The cores were drilled in proximal (core GKF01) and distal (core GKP01) positions relative to the margin of the Campbellrand-Malmani carbonate platform. Correlation between cores is based on three types of marker horizons: sequence stratigraphic, volcanic/impact and lithologic markers. Several surfaces can be correlated to the platform succession. Gamma-ray data and bulk-rock XRF geochemistry of siliciclastic mudstones and tuff beds support the correlation. Following flooding of a shallow-water carbonate ramp (Boomplaas Formation of Schmidtsdrif Subgroup), deposition of slope carbonates with minor siliciclastic mudstones, chert and banded iron formation (BIF) occurred below storm wave base. Cycles of in situ microbialites and reworked slope carbonates are prominent in both cores. A final transgressive pulse near the top of the succession led to deposition of thick cherts and iron formations across the former platform, slope and basinal areas. Deposition on the slope was controlled by cementation potential in microbial facies and by sediment input, which in turn was linked to variations in platform margin geometry and changes in accommodation space. Results indicate preferred sediment accumulation in the proximal slope position, while chemical sediments were deposited preferentially beyond the reach of carbonate and siliciclastic detrital sediment. Similarly, sediment input and possibly early marine cementation influenced cycles of microbialites and reworked carbonates. Sequence stratigraphic analysis and siliciclastic mudstone geochemistry suggest a possible sea level fall and a change in provenance of the siliciclastic detritus at the transition from the Monteville Formation to the Nauga Formation (basal Campbellrand Subgroup). These changes were probably related to a major depositional hiatus on the platform.

Sequence Stratigraphic Development of the Neoarchean Transvaal carbonate platform, Kaapvaal Craton, South Africa

The similar to 2.67 to similar to 2.46 Ga lower Transvaal Supergroup, South Africa, consists of a mixed siliciclastic-carbonate ramp that grades upward into an extensive carbonate platform, overlain by deep subtidal banded iron-formation. It is composed of 14 third-order sequences that develop from a mixed siliciclastic-carbonate ramp to a steepened margin followed by a rimmed margin that separated lagoonal environments from the open ocean. Drowning of the platform coincided with deposition of banded iron-formation across the Kaapvaal Craton. The geometry and stacking of these sequences are consistent with more recent patterns of carbonate accumulation, demonstrating that Neoarchean carbonate accumulation responded to subsidence, sea level change, and carbonate production similarly to Proterozoic and Phanerozoic platforms. The similarity of carbonate platform geometry through time, even with significant changes in dominant biota, demonstrates that rimmed margins are localized primarily by physiochemical conditions rather than growth dynamics of specific organisms. Stratigraphic patterns during deposition of the Schmidtsdrift and Campbellrand-Malmani subgroups are most consistent with variable thinning of the Kaapvaal Craton during extrusion of the similar to 2.7 Ga Ventersdorp lavas. Although depositional patterns are consistent with rifting of the western margin of the Kaapvaal Craton during this time, a rift-to-drift transition is not required to explain subsidence. Heating and thinning during Ventersdorp time can produce the observed thermal subsidence from similar to 2.7 to similar to 2.45 Ga if the thermal diffusivity of the craton was moderately low.

Facies distribution patterns and environment reconstruction of the upper member of the Láncara Formation in the Somiedo-Correcilla unit (Lower-Middle Cambrian, Cantabrian zone, NW Spain) with special respect to biofacial investigations

The microfauna of the upper member of the Lancara Formation in the Somiedo-Correcilla unit (Lower-Middle Cambrian, Cantabrian zone, northwestern Spain) is described in detail and discussed in respect to biofacies and palaeoecology. The study of about 140 thin sections allows to reconstruct a sedimentological model based on bio- and lithofacial trends. A vertical faunal differentiation is detected in the three studied outcrops. The base of the upper member of the Lancara Formation (Beleno facies) is characterised by a high amount of sessile suspension feeders (echinoderms) which indicate a low or absent siliciclastic influx. The amount of trilobites (mobile deposit feeders) and sessile filter feeders (brachiopods) is clearly lower. This echinoderm-rich facies characterises the lower few meters and is called Faunal Assemblage A. The transition to Faunal Assemblage B is characterised by a decrease of sessile suspension feeders, whereas the mobile organisms increase in percentage. This evolution continues in “Faunal Assemblage C”, which is dominated by mobile suspension- and sessile filter-feeders. The continuous lithological and faunal succession, but overall low faunal and facial gradients of the upper member of the Lancara Formation are representative for a ramp environment. The deposition is interpreted as a turnover from an eastward-tilted homoclinal carbonate ramp (lower member of the Lancara Formation) to a mixed carbonate-siliciclastic ramp (Beleno and Barrios facies, respectively Faunal Assemblages A, B, C, D). The abrupt appearance of the Faunal Assemblage A, points to a sudden subsidence of the environment. A continuous increase in the siliciclastic content is accompanied with a successive change in the faunal composition from a fauna dominated by sessil suspension feeders to a mobile deposit feeder-dominated one. The increase of siliciclastic material indicates a more or less constant rise of sea-level that caused on onshore shift of the facies belts, and mirrors the general drowning of the ramp.

Sequence Development of A Mixed Carbonate-siliciclastic High-Relief Ramp, Mississippian, Kentucky, U.S.A.

ABSTRACT The discovery of the supergiant oil fields of the Caspian region has focused attention on eustatic, tectonic, and climatic controls on Lower Carboniferous facies. This paper documents the development of a highly complex, mixed carbonate-siliciclastic ramp succession of the Lower Mississippian (Upper Osagean-Lower Meramecian) Fort Payne-Salem interval, Kentucky, that formed peripheral to an abandoned delta system (Borden Formation) following drowning of the paleoshelf, during a time of arid climate and tectonic quiescence. The antecedent Borden deltaic-marine paleoshelf stood 100 m above the starved basin plain to the southwest and had marginal slopes of less than 0.5°. The succession is a supersequence (0 to 150 m thick, approximately 5 to 10 My duration) composed of two Upper Osagean (Fort Payne) and two Lower Meramecian (Warsaw-Salem) depositional sequences which correlate with the global sea-level cycles of Ross and Ross (1988). The supersequence lowstand systems tract on the basin floor consists of the lower part of a unit of green shale-and-mounds. Mound growth appears to have continued during supersequence transgression, culminating in some mounds up to 60 m thick. On the Borden paleoshelf, the supersequence transgressive systems tract is a condensed glauconite horizon(s). The supersequence highstand tract consists of marine quartz sandstone and shale, peritidal carbonates, high-energy crinoidal-bryozoan grainstone banks, and deep ramp and slope siliceous calcisiltite. Third-order sequences have local lowstand-transgressive sand bodies; those in sequence 2 are clinoformed strike sand lobes up to 50 m thick, while those in the younger sequences are discontinuous sheets-dip oriented lobes. The siliciclastic units appear to be associated with a paleodrainage system off the Borden paleoshelf. In Upper Osagean sequences 1 and 2, third-order highstand systems tracts are aggradational on the paleoshelf and prograded to the southwest as a series of clinoforms into the starved basin. Lower Meramecian sequences 3 and 4 highstand systems tracts are relatively layer-cake, reflecting loss of accommodation during Late Osagean basin filling, and they prograded more to the west into the subsiding Reelfoot Graben-Illinois Basin. The extremely complex facies relations reflect the interaction of the antecedent shelf-slope-basin topography, the mixed carbonate and siliciclastic systems, mounded basin-floor topography, the highly channeled margin morphology, eustasy, and relatively stable tectonics.

Paleoproterozoic intracratonic basin processes, from breakup of Kenorland to assembly of Laurentia: Hurwitz Basin, Nunavut, Canada

Abstract The evolution of Hurwitz Basin records discontinuous tectonic processes in the Hearne domain (northern Canada) resulting from changing distant boundary conditions. Magmatic, eustatic and paleoclimatic influences, although significant, are considered to have been of secondary importance. In agreement with recent models, lower Hurwitz Group units record intracratonic basin formation during initial breakup stages of a speculative Neoarchean supercontinent (Kenorland). In contrast to recent models, new Sm–Nd and Pb–Pb data and recent detrital zircon geochronology document a hitherto unsuspected time break (at least 200 My) within the Hurwitz Group (ca. 2.11–1.91 Ga). These data indicate that the deposition of upper Hurwitz Group units overlapped with ca. 2.0–1.7 Ga amalgamation of the Slave, Superior, Sask, Wyoming and Nain cratons during assembly of Laurentia. Initiation of Hurwitz Basin and deposition of predominantly continental siliciclastic rocks in sequence 1 (Noomut, Padlei and Kinga Formations) were due to regional sagging in response to early lithospheric stretching. Immature deep-water pelites of sequence 2 (Ameto Formation) reflect basin-margin arching and basin-centered deepening related to a second episode of stretching. Gabbro sills and dykes (2.11 Ga), emplaced after lithification of the Ameto Formation represent continued stretching that ultimately led to dispersion of Kenorland's daughter fragments. The gap in the sedimentary record signifies a time when processes required to generate permanent subsidence were lacking. Mixed siliciclastic–carbonate ramp and fluvial deposits of sequences 3 and 4 (Watterson, Ducker and Tavani Formations) were deposited after ca. 1.91 Ga, during shortening from the west (related to Taltson–Thelon and Wopmay orogens), but boundary conditions to the south are less certain. These may have involved either extension related to late fragmentation and dispersion of the southern Hearne margin, or shortening due to early (ca. 1.87 Ga) collisional processes in western Trans-Hudson orogen. Similar to the classic Phanerozoic intracratonic basins of North America, we infer that tectonic subsidence in Hurwitz Basin is a reflection of plate tectonics, reinforcing similarities in basin-forming processes between the early Paleoproterozoic and the Phanerozoic.

Proximal–distal facies relationships and sedimentary processes in a storm dominated carbonate ramp (Kimmeridgian, northwest of the Iberian Ranges, Spain)

Abstract Facies analysis of the upper Kimmeridgian rocks in the outcrops located near Ricla (Zaragoza province, northeast Spain) and the integration of the resultant data in a broader context (the northern part of the Iberian Basin), has produced two general models showing the facies distribution and the processes that controlled the sedimentation in the Kimmeridgian carbonate ramp. Using these two models the transition from shallow to relatively deep environments of the carbonate ramp is examined in detail. Model 1 corresponds to the development of a mixed carbonate-siliciclastic ramp during a slow rise and stillstand of sea level (Sequence 1-HST), whereas Model 2 represents the growth of a pure carbonate ramp during a rapid rise of sea level (Sequence 2-TST). Carbonate production was higher in the shallow ramp domains (coral reefs and oolitic shoals in Model 1 and reefs in Model 2) than in deeper domains, where there is no indication of significant pelagic or benthic production. The activity of unidirectional return flows induced by winter storms and hurricanes, played an important role in the redistribution of the sediment across the ramp, generating different coarse-grained deposits. In the inner and mid-ramp settings dunes, lower scale bedforms and tempestites occur in Model 1, and storm lobes, bars and tempestites in Model 2. Moreover, a significant bulk of the carbonate mud produced in shallow areas would eventually be resedimented in the outer ramp as suspended load in the density currents. Stillstand of sea level in Model 1 involved a rapid progradation of the inner and proximal mid-ramp carbonate and siliciclastic facies. The rapid relative sea level rise of Model 2 is determined by the dominance of the carbonate facies and by the presence of aggradational geometries in the transitional area between shallow and deep-ramp domains. The presence of relatively thick sections in the outer-ramp settings (instead of condensed sections, as observed in Model 1) during times of sea level rise (Model 2) can mainly be explained by the increase of the shallow production in the reef dominated areas.

Sequence stratigraphy of a tidally dominated carbonate–siliciclastic ramp; the Tithonian–Early Berriasian of the Southern Neuquén Basin, Argentina

The Tithonian–Berriasian Vaca Muerta, Carrín Curá and Picún Leufú formations in the southern Neuquén Basin were deposited on a tidally dominated, mixed carbonate–siliciclastic ramp. Basinal, outer, middle, shallow and back ramp facies associations are recognized and a sequence stratigraphic analysis reveals that the ramp record consists of three shallowing‐upwards sequences (Ti1, Ti2 and Ti3) set within a lower‐order progradational cycle. A higher order of cyclicity is superimposed on to the middle (Ti2) sequence. The majority of the ramp facies belong to the transgressive and highstand systems tracts; however, at the base of Ti2, a lowstand systems tract is identified, characterized by a basal unconformity and an abrupt basinward shift of the shallow marine lithofacies. Transgressive systems tracts were characterized by slow sedimentation rates and rapid sea‐level rises that affected carbonate productivity. Highstand systems tracts show the greatest carbonate productivity and an increased progradation rate on account of a reduction in accommodation space generation. Palaeogeography played a major role in the development of the depositional systems. Partial isolation from the Pacific Ocean reflecting the growth of the Andean magmatic arc and geographic restriction due to tectonic inversion in the central part of the basin resulted in a meso‐macrotidal regime that produced a tidally dominated sedimentary record in the shallow and back ramp environments. Coeval anoxic conditions in the central part of the Neuquén Basin favoured distal ramp and basinal black shale deposition during episodes of relative sea‐level rise.