Intracratonic Basin Research Articles

Tectonic significance of abrupt immature sedimentation in a shallow cratonic margin basin: The Arkose Level, Mesoproterozoic Paranoá Group

Facies analysis, aided by X-ray diffraction, QemScan analysis, U-Pb detrital ages and field geological data are used in a detailed sedimentary characterization of the Arkose Level, an immature sedimentary unit that occurs abruptly and discontinuously in the upper Paranoá Group, a sequence of mature marine rhythmites deposited in a shallow cratonic margin basin. This basin was developed during the Mesoproterozoic in the western margin of the São Francisco Craton, central Brazil. The study reveals three facies associations in the Arkose Level: (1) Facies Association I (C), (2) Facies Association II (MC) and (3) Facies Association III (FM). The lithofacies are characterized by plane-parallel lamination and planar cross-bedding, trough cross-stratification, channel-fills, overturned cross-stratification, and convoluted structures. The sedimentological and paleocurrent data acquired in this study leads to the interpretation that this unit represents a record of braided-river deposit, which was influenced by marine reworking in the uppermost portion. The depositional model comprises a transitional continental-marine environment, with channels in the continental domain interpreted as a high-energy braided river system around the margins of a shallow intracratonic basin during the Mesoproterozoic. Compiled and newly acquired LA-ICP-MS detrital zircon U-Pb ages reveal a new source area contribution of 1.1 Ga in the Upper portion of the Paranoá Group. The abrupt deposition of braided-river deposits amid the mature Paranoá basin is most probably associated with local faulting, responsible for exposing the crystalline basement and the sedimentary rocks of previous basins, providing new and near-source areas of crystalline basement rocks. The inferred faulting can be associated to a Stenian unsuccessful rifting event, or to the early stages of opening the Goiás-Pharusian ocean, in the Tonian.

Onset of the Late Paleozoic Glacioeustatic Signal: A Stratigraphic Record from the Paleotropical, Oil-Shale-Bearing Big Snowy Trough of Central Montana, U.S.A.

AbstractIn the Big Snowy Mountains of central Montana, USA, late Visean to Bashkirian strata preserve a nearly complete, but poorly documented, paleotropical stratigraphic succession that straddles the range of current estimates of the onset of the Late Paleozoic Ice Age (LPIA). Sedimentologic and stratigraphic investigation of the Otter (late Visean to Serpukhovian) and Heath (Serpukhovian) formations, with secondary focus on the overlying Tyler (late Serpukhovian to Bashkirian) and Alaska Bench (Bashkirian) formations, facilitated an appraisal of paleotropical environmental change preserved in this succession. Three facies associations reminiscent of environments currently forming in Shark Bay, Australia, were identified in the Otter Formation: shallow semi-restricted littoral platform, intertidal platform, and supratidal plain. Five facies associations broadly comparable to modern environments present in the Sunda Shelf and southern coast of the Persian Gulf were identified in the Heath Formation: offshore outer ramp, mid- to outer ramp, inner ramp, coastal plain, and sabkha. Facies associations preserved in the Heath Formation are here explained in the context of a protected, homoclinal carbonate ramp situated in a partially silled epicontinental embayment. A shift from low-magnitude relative sea-level oscillations preserved in the Otter Formation to a cyclothemic stratigraphic pattern entailing ≥ 6 fourth-order, high-frequency and high-magnitude relative sea-level fluctuations in the Heath Formation is here interpreted to record the main eustatic signal of the LPIA in central Montana. Current published biostratigraphic constraints for the observed stratigraphy estimate the main eustatic signal of the LPIA to have occurred approximately between 331 (base Serpukhovian) and 327 Ma in central Montana. A distinct upward transition from coal and paleosol-bearing depositional sequences in the lower Heath to evaporite and limestone-bearing depositional sequences in the upper Heath preserves a broad humid to arid paleoclimate shift during deposition of this unit, which influenced hydrographic circulation patterns and the resultant distribution of anoxic environments in the Big Snowy Trough during this time interval. Improved depositional and sequence stratigraphic models of the Heath Formation proposed in this study permit new insight into the theoretical distribution of, and water depth necessary to preserve, black, organic-rich claystone and shale in partially silled intracratonic basins, in addition to new temporal constraints on LPIA onset in paleotropical western Laurentia.

Ediacaran (Sinian) palaeogeographic reconstruction of the Upper Yangtze area, China, and its tectonic implications

ABSTRACT The Ediacaran (ca. 635–541 Ma) was a critical period in Earth history, which saw a dramatic transformation in global biological, chemical and climatic conditions, making it one of the most intensively investigated time intervals worldwide. As the first mixed clastic–carbonate succession deposited on the Upper Yangtze Craton, the Ediacaran succession overlies Neoproterozoic continental rifts, and records a complex basin infill history. In this study, we focus on the tectonostratigraphic environment and basin infill process, integrating multiple data sets, including boreholes, outcrops, well correlations, seismic data, isopach maps and the spatial distribution of sedimentary facies. The Ediacaran System includes the Doushantuo Formation and overlying Dengying Formation which can be subdivided into four lithostratigraphic members (D1–D4). The Doushantuo Formation and D3 Member consist of mixed clastic–carbonate sediments, while carbonates predominate in the D1, D2 and D4 members. Intracratonic depressions and continental margin rifts dominated the Upper Yangtze area during the Ediacaran. During deposition of the Doushantuo Formation, the Upper Yangtze area contained a variety of environments, including terrigenous shorefaces, mixed tidal flats, lagoons, shelves, slopes and bathyal seas. A rimmed carbonate platform developed during deposition of the D1 and D2 members. The Deyang–Anyue Trough, which is related to post-rift subsidence in the intracratonic basin, began to develop in this interval. A relatively intense extension during deposition of the D3 Member resulted in intensification of intracratonic and continental margin rifting, creating distinct tidal flat, deep-water shelf, slope and bathyal environments. With the expansion of the Deyang–Anyue Trough during deposition of the D4 Member, the rimmed carbonate platform was divided into two segments. We suggest that the Deyang–Anyue Trough was a structurally controlled intracratonic rift-sag and likely arose from multiple episodes of rifting in the cratonic basement, as well as differential uplifting and exposure caused by the ongoing Tongwan Movement. Our model highlights the role of basement heritage on the structure and evolution of intracratonic rift-sag.

Mixed carbonate-siliciclastic microfacies from Permian deposits of Western Gondwana: Evidence of gradual marine to continental transition or episodes of marine transgression?

At the end of the Paleozoic, several intracratonic basins developed in Gondwana, including the Paraná Basin in Western Gondwana. Permian sedimentary units include the Passa Dois Group, which encompasses four formations: the Irati, Serra Alta, Teresina, and Rio do Rasto, and represents a transition from marine to continental depositional environments. Its strata correlate extensively, from Argentina to South Africa, and the Irati and Teresina formations in particular present mixed carbonate-siliciclastic successions. This interval has been speculated to have formed under several different paleoenvironmental scenarios; however, Permian Gondwana basin microfacies, with no open sea influence, have rarely been studied. In addition, in these basins there is a scarcity of micropaleontological studies and geochronological calibration, besides an absence of marine index fossils. In view of this, a comprehensive microfacies study for the unit is developed, with a large and representative surface and subsurface sample analysis of the Passa Dois Group, divided into three large geographic domains: north, central, and south of the Paraná Basin, Brazil. Thin sections distributed among the four units were analyzed with an emphasis on carbonate layers where the microfossils have a better chance to be recognized. The data were interpreted within a revised biochronostratigraphic framework and integrated with bivalves, macroflora, palynology, conchostracans, and vertebrates biozones. Microfacies analysis resulted in the recognition of 13 microfacies: four siliciclastic, eight carbonate, and one siliceous (chert). We observed a predominance of fine siliciclastic microfacies, rich in organic matter, in the Southern Domain; rudaceous carbonate microfacies in the Central Domain, with diverse allochemicals and the greater diversity of bioclasts; and an increase on siliciclastic sediments influx in the Northern Domain of the basin, where only the Irati and Teresina formations occur. A shortage of microfossils was observed in the South Domain, where the carbonate microfacies are less varied than those to the north. In the Central Domain, coarse-grained carbonates are present. Bivalves, ostracods, and sponge spicules are the main bioclasts on rudstones and grainstones from the Irati and Teresina formations, and have been interpreted as allochthonous to parautochthonous. The association of detrital and authigenic glauconite to bioaccumulations of sponge spicules is interpreted as episodes of marine transgression.

Structural Diagenesis in Carbonate Rocks as Identified in Fault Damage Zones in the Northern Tarim Basin, NW China

The identification of structural diagenesis and the reconstruction of diagenetic paragenesis in fault damage zones is important for understanding fault mechanisms and fluid flow in the subsurface. Based on the examination of core and sample thin section data, we deciphered the diagenetic parasequence and their fault controls for Ordovician carbonates in the northern Tarim intracratonic basin in NW China (Halahatang area). In contrast to the uniform nature of diagenesis observed in country rocks, there is a relatively complicated style of compaction and pressure solution, multiple fracturing, and cementation and dissolution history along the carbonate fault damage zones. The relative paragenetic sequence of the structure related diagenesis suggests three cycles of fracture activities, following varied fracture enlargement and dissolution, and progressively weaker calcite cementation. These processes of structure related diagenesis are constrained to the fault damage zones, and their variation is affected by the fault activities. The results of this study suggest that the carbonate reservoir and productivity could be impacted by the structure related diagenesis locally along the fault damage zones.

Tectonic evolution of the Paleozoic Alto Tapajós intracratonic basin - A case study of a fossil rift in the Amazon Craton

The Alto Tapajós Basin is an intracratonic basin located in the Amazon Craton, consisting predominantly of Paleozoic siliciclastic sedimentary rocks cutted by Mesozoic dyke swarms. With an area of about 97,000 km2, the size of Portugal, some parts density cover by the Amazon forest, it has few detailed studies regarding geological structures and potential for hydrocarbons, with scarce surface mapping, absence of seismic surveys and only one well drilled to the SE. It is a NW-SE oriented branch of the ENE-WNW Amazonas Basin. Geological maps, topographic, gravimetric and magnetometric data are here interpreted and integrated to analyse its tectonic evolution. The sedimentary strata are deposited on reactivation zones related to NW-SE normal faults and NNE-SSW oblique faults, that control the spatial arrangement of the basement highs. The former is parallel, and some places coincident, with a major tectonic Paleoproterozoic crustal boundary that separates the Tapajós-Parima and Rondônia-Juruena provinces within the Amazon Craton. The NNE-SSW oblique faults are interpreted as representing a transfer zone that accommodates the Paleozoic oblique rifting mode. However, the rifting age is still not well stabilished. Cambrian mafic dikes could link the initiation of this basin to the breakup of a supercontinent in the early Cambrian (Pannotia), in the dawn of the Gondwana. To account for the Devonian to Permian basin subsidence, crustal stretching due to intraplate passive rifting might have played the role, in response to farfield stresses regarding a convergent tectonic setting on the western margin of Gondwana (Terra Australis and Alleghanian orogenies). This is corroborated by the shift on the basin depocenter axis, from NW-SE (Devonian-Carboniferous) to NNE-SSW (Permian), that might be related to the change on orientation of stresses due to these two orogenies. In addition, the Paleozoic sedimentary units occur basically within the same area of the Paleoproterozoic cratonic sedimentary units from a ca. 1.7 Ga rift. This suggests that the Alto Tapajós Basin Paleozoic rifting reactivated an ancient fossil rift, also described in other cratons throughout the world. In the Jurassic, Alto Tapajós Basin was submitted to a post-sedimentation tectono-thermal event, related to the disintegration of Pangea and expansion of the Central Atlantic Magmatic Province (CAMP). During this episode, 180–220 Ma mafic dike swarms intruded its central-southwest compartment, following the NNE-SSW structures. The actual positive topographic feature, Cachimbo Range and plateau, and the absence of thick Cenozoic deposits, unlike the Amazonas Basin, indicate that the recent evolution of the Alto Tapajós region is related to uplift within an intraplate setting.

Radiogenic isotope chemostratigraphy reveals marine and nonmarine depositional environments in the late Mesoproterozoic Borden Basin, Arctic Canada

Abstract The ca. 1050 Ma Bylot Supergroup in Arctic Canada is one of the best-preserved archives of late Mesoproterozoic geochemistry and biology and offers evidence that this period of Earth history may have been more biogeochemically dynamic than previously appreciated. The Bylot Supergroup was deposited in the Borden Basin and is the most thoroughly studied stratigraphic succession from a series of broadly contemporaneous late Mesoproterozoic intracratonic basins known as the Bylot basins. This ∼6-km-thick mixed carbonate-siliciclastic succession has undergone minimal postdepositional deformation and is now exposed on Baffin and Bylot Islands, Nunavut, Canada. Deep-water and tidally influenced carbonate facies, traditionally interpreted as marine, have yielded important insights into the evolution of Proterozoic seawater chemistry; however, more recent studies indicate that the Borden Basin was restricted marine or lacustrine for portions of its depositional history. Here, we present new multiproxy radiogenic isotope chemostratigraphic data spanning the Bylot Supergroup. A comparison of stratigraphic trends in radiogenic isotope data from hydrogenous (black shale 187Os/188Os and limestone 87Sr/86Sr) and detrital (whole-rock mudstone 143Nd/144Nd) sedimentary phases elucidates the complex hydrologic history of the Borden Basin and reconciles these disparate interpretations. Episodic coupling and decoupling between the composition of basin waters (from Os and Sr isotopes) and the local weathering input to the basin (from Nd isotopes) indicate that depositional environments within the Borden Basin fluctuated between marine and nonmarine (i.e., lacustrine). Variations in basin hydrology controlled secular sedimentation patterns through changes in basin water chemistry. These interpretations help to characterize the environment in which the early red algal fossil Bangiomorpha pubescens evolved. Episodically restricted epeiric seaways, such as within the Borden Basin, were relatively widespread within Rodinia and may have exerted unique selective pressures on eukaryotic evolution in the Mesoproterozoic Era. Hydrogenous and detrital radiogenic isotope chemostratigraphy, as implemented in this study, may provide a useful paleoenvironmental framework for future paleontological studies aimed at testing the role of freshwater environments in eukaryotic evolution. In addition, 87Sr/86Sr compositions from 81 new middle Bylot Supergroup marine limestone samples, calibrated by recent Re-Os geochronology, contribute to the terminal Mesoproterozoic marine 87Sr/86Sr curve. These data display a rise from ∼0.705 to 0.706 that reflects weathering of the active Grenville orogenic belt and demonstrates a global increase in chemical weathering during the amalgamation of Rodinia.

Post‐Great Oxidation Event Orosirian–Statherian iron formations on the São Francisco craton: Geotectonic implications

AbstractThe protocratonic core of the São Francisco craton assembled during the 2.1–2.0 Ga Transamazonian orogeny. Orosirian Fe‐rich sequences that extend from the northwestern border of the São Francisco protocraton (Colomi Group) to the southeast under the Espinhaço Belt (the < 1.99 Ga Serra da Serpentina Group) record the opening of an intracratonic basin with the episodically developed ferruginous waters prior to the initiation of the Espinhaço rift at 1.8 Ga. Ferruginous conditions developed again during deposition of the Canjica Iron Formation of the < 1.7 Ga Serra de São José Group in the Espinhaço rift (contemporaneously with felsic magmatism; Conceição do Mato Dentro Rhyolite and Borrachudos Granitic Suite) and extensive sandstones of the < (1666 ±32) Ma Itapanhoacanga and < (1683 ±11) Ma São João da Chapada Formations. In the upper São João da Chapada Formation, banded hematitic phyllite also records input of Fe‐rich fluids. The young age of these iron formations with respect to the conventionally accepted 1.88 Ga age for the youngest shallow‐marine Paleoproterozoic iron formations, the apparent absence of granular facies (granular iron formations), and yet shallow‐water (above fair‐weather base) depositional environment indicate that an unusual setting developed in a large basin after the Great Oxidation Event, in the aftermath of the Transamazonian orogeny. We propose that mantle plumes led to the opening of a previously unrecognized rift system, that could have caused the magmatism, supplied hydrothermal Fe and led to the opening of the Espinhaço, Pirapora, and Paramirim rifts, later obliterated by the Araçuaí orogenic belt during the Neoproterozoic to Early Paleozoic Brasiliano orogeny. The rift system did not develop into an open continental margin but probably evolved into a broad sag basin, stretching across the São Francisco and Congo cratons.

Facies analysis and paleosol characterization of Urucuia group in the Sao Domingos - GO region

The Sanfranciscana basin is a wide intracratonic basin sub-divided in Abaeté sub-basin and Urucuia sub-basin which includes four Groups of Neo Paleozoic to Neo Cretaceous ages, among them there is the target of this study, the Urucuia Group (Upper Cretaceous). The Urucuia Group has the largest rock volume and areal distribution of the basin. Its rocks constitute two Formations, the basal named Posse overlaid by Serra das Araras Formation. In a continental environment where there is a few fossil contents, the paleosol study can bring important information about climatic and geomorphologic conditions, and biological activity being possible to do an environmental reconstruction. Therefore, this project aims to elaborate a paleoenvironmental model to the Upper Cretaceous of the Sanfraciscana basin through facies analysis and paleosol characterization. The fieldwork was accomplished in the outcrops of the Urucuia Group situated near to São Domingo (GO) town. Outcrop descriptions, photographic registration, and sample collection to laboratory analysis were the methodology applied. The outcrop description was made by the identification of lithologies, textures, sedimentary structures, paleocurrents, fossil content, bed geometry, as well as paleosol macromorphologic description.

Geothermal play typing in Germany, case study Molasse Basin: a modern concept to categorise geothermal resources related to crustal permeability

AbstractThe majority of running geothermal plants worldwide are located in geological settings with convection- or advection-dominant heat transport. In Germany as in most regions in Europe, conduction is the dominating heat transport mechanism, with a resulting average geothermal gradient. The geothermal play type concept is a modern methodology to group geothermal resources according to their geological setting, and characteristic heat transport mechanisms. In particular, the quantity of heat transport is related to fluid flow in natural or engineered geothermal reservoirs. Hence, the permeability structure is a key element for geothermal play typing. Following the existing geothermal play type catalogue, four major geothermal play types can be identified for Germany: intracratonic basins, foreland basins and basement/crystalline rock provinces as conduction-dominated play types, and extensional terrains as the convection-dominated play type. The installed capacity of geothermal facilities sums up to 397.1 MWthby the end of 2018. District heating plants accounted for the largest portion, with about 337.0 MWth. The majority of these installations are located in the play type ‘foreland basin’, namely the Molasse Basin in southern Germany. The stratigraphic unit for geothermal use is the Upper Jurassic, also known as ‘Malm’ formation, a carbonate reservoir with high variability in porosity and permeability. Recently drilled wells in the southernmost Molasse Basin indicate the Upper Jurassic as a tight, fracture-controlled reservoir, not usable for conventional hydrothermal well doublets. Our new data compilation including the recently drilled deep geothermal well Geretsried reveals the relation of porosity and permeability to depth. The results suggest that obviously diagenetic processes control permeability with depth in carbonate rock, diminishing the predictability of reservoir porosity and permeability. The play type concept helps to delineate these property variations in play type levels because it is based on geological constraints, common for exploration geology. Following the general idea of play typing, the results from this play analysis can be transferred to geological analogues as carbonate rock play levels in varying depth.

VIRGIANID BRACHIOPODS OF THE MICHIGAN BASIN, AND ITS IMPLICATIONS FOR POST-EXTINCTION DIVERSIFICATION OF THE SILURIAN PENTAMERIDE FAUNA IN LAURENTIA

Three virgianid genera are present in the Michigan Basin. The oldest, Virgiana mayvillensis Savage from the Mayville Dolomite, is upper Rhuddanian in age and coeval with the same species on Anticosti Island in eastern Canada. “Virgiana” major Savage, 1916, from the uppermost Lime Island (uppermost Rhuddanian), has an incipient cruralium supported anteriorly by a low median ridge, for which Virgianoides gen. nov. is proposed in this study. Platymerella in the Elwood Formation (uppermost Rhuddanian) was the most southerly virgianid occurrence in the American mid-continent. The early evolution of the Brevilamnulella-Viridita-Virgiana lineage was represented by the early-middle Rhuddanian fossil record of Anticosti Island. Available fossil data indicate that the Virgiana invasion into intracratonic basins did not begin until late Rhuddanian time, represented by the excellent record of V. mayvillensis in the Michigan Basin, and V. decussata in the Hudson Bay and Williston basins. Despite its late arrival, virgianids thrived for a somewhat longer geological time in the Michigan Basin, represented by Virgianoides and Platymerella in the latest Rhuddanian, when virgianids largely became extinct in other basins of Laurentia.

Two billion years of evolution enclosed in hydrothermal rutile: Recycling of the São Francisco Craton Crust and constraints on gold remobilisation processes

The Quadrilátero Ferrífero in the São Francisco Craton, Brazil is known for its world-class gold deposits hosted by the Rio das Velhas greenstone belt (RdV). As in other greenstone belts, gold is also concentrated in late Archean/early Paleoproterozoic supracrustal units; in this case within the Moeda Formation. This unit has been compared to the Witwatersrand basin where there has been a long-term debate over the nature of the gold and whether it is detrital (placer), hydrothermal or hybrid (modified placer). Presently, links between gold in the Moeda Formation and RdV are tenuous. To enlighten this area of ambiguity, a new approach using chemical, isotopic and mineral inclusion data from detrital rutile found in the gold-bearing horizons of the Moeda Formation is presented. Grain textures and mineral inclusions have contributed to establishing a primary hydrothermal origin for the rutile, related to As-Fe-Cu-Sb-Pb-W rich mineralising fluids. Fe, Nb and V indicate that the likely source of the gold-related rutiles is the Archean banded iron formations within the RdV, where most of the lode-gold occurs. Average Nb/Ta values of these rutiles are lower than average continental crust ratios which suggests fractionation driven by fluid precipitation and/or competing mineral phases. U-Pb data yield primary crystallization ages older than deposition ages (>2.58 Ga), supporting the detrital nature of these rutiles. Rutiles record either resetting or new growth at 2.1 Ga and at ca. 500 Ma, during which gold was remobilised. Hence, this study supports a modified placer model for the gold-bearing horizons of the Moeda Formation. These fluid-assisted gold remobilisation events can be found in other correlated Paleoproterozoic gold-bearing horizon units that represent intracratonic basins related to the rifting of the Kenorland Supercontinent. This study reinforces the suitability of rutile and mineral inclusions as a powerful tool to elucidate processes and provenance.

Depositional history and provenance of cratonic “Purana” basins in southern India: A multipronged geochronology approach to the Proterozoic Kaladgi and Bhima basins

Peninsular India is a collage of Archaean cratonic domains separated by Proterozoic mobile belts. A number of cratonic basins, known as “Purana basins” in the Indian literature, formed in different parts of the Indian Peninsula during extensional tectonic events, from Paleoproterozoic through Neoproterozoic times. In this contribution, we present a diversity of new geochronological data for different units within the Kaladgi and the Bhima basins, which overlie the western and eastern Dharwar cratons, respectively. The new geochronology data are discussed in terms of depositional history and provenance of these poorly understood Proterozoic intracratonic basins. For the Kaladgi Group, a U–Pb baddeleyite age of 1,861 ± 4 Ma obtained for a dolerite dyke intruding the Yendigere Formation is used to constrain the minimum age of deposition of the lower Kaladgi Group. This result demonstrates that this part of the succession is comparable in age to the Papaghni Group of the Cuddapah Basin, heralding onset of Purana sedimentation at ~1,900 Ma. The detrital zircon populations from the clastic rocks of the Kaladgi and Bhima basins show unique and distinct age patterns indicating different source of sediments for these two basins. Palaeocurrent analysis indicates a change in provenance from south or southeast to west or northwest between the Kaladgi and Bhima clastic sedimentation. New U–Th–Pb and Rb–Sr radiometric dates of limestones and glauconite‐bearing sandstones of the Bhima Group (Bhima Basin) and the Badami Group (Kaladgi Basin) indicate deposition at around 800–900 Ma, suggesting contemporaneity for the two successions. Thus, the unconformity between the Kaladgi Group and the overlying Badami Group represents a time gap of up to 1,000 Myr. These new results demonstrate the complex multistage burial and unroofing history of the Archaean Dharwar Craton throughout the Proterozoic, with important implications for exploration of metal deposits and diamonds in Peninsular India.

Stratigraphic modeling of a transgressive barrier-lagoon in the Permian of Paraná Basin, southern Brazil

Barrier-lagoon systems occur in most coastal regions worldwide. The sedimentary record of these environments is of scientific and economic relevance because important coal deposits are associated with them. We show the evolution of an ancient coal-bearing barrier-lagoon system in the subsurface on the southern Paraná Basin, Brazil, from a stratigraphic modeling approach. The main objective is to simulate and understand the origin and evolution of this system controlled by relative sea level changes and sediment supply. Database comprises 75 logged boreholes that provided data about the main coal bed deposited in the back-barrier, as well as 19 cored and logged boreholes distributed along two stratigraphic sections, one parallel (NE-SW) and another perpendicular (NW-SE) to the paleocoastline. The dataset was uploaded in the Diffusive Oriented Normal and Inverse Simulation of Sedimentation (DIONISOS) software to establish basic parameters and simulate different scenarios. Described facies are interpreted as swamp, lagoon, sandy barrier and tide-influenced deposits, developed during a relative sea level rise of 16 m. The coal bearing succession was deposited during the transgressive system tract of fourth-order depositional sequence and included peat deposition in a swamp in the back-barrier, barrier breakup and landward migration. Results are of high relevance for understanding this paleoenvironment evolution and similar systems in intracratonic basins. This environment occurs both in ancient and recent coasts, and the present model supports the understanding of several barrier-lagoon systems with coal formation in the world.

Late-Neoproterozoic ferroan granitoids of the Transversal subprovince, Borborema Province, NE Brazil: petrogenesis and geodynamic implications

ABSTRACT Ferroan granites (585–530 Ma) have been described in the Transversal subprovince of the Borborema Province (BP) and in Pan-African counterparts. They comprise two groups: Group 1 – slightly peraluminous to metaluminous, alkali-calcic rocks, with low Fe# mica and crystallized under intermediate fO2 (Aroeiras Complex and Serra Branca – Coxixola dike swarms); Group 2 metaluminous to slightly peraluminous, alkalic to alkali-calcic rocks, with high Fe# mica and crystallized under low fO2 (Queimadas and Prata intrusions). Group 1 marks transition from collision to transcurrence (ca. 585 Ma), or from transcurrence to uplift and transtension (ca. 545 Ma). Group 2 – represents granitoids intruded during extensional tectonics in transcurrent setting (ca. 550 Ma), or coeval with deposition of transtensional intracratonic basins (ca. 530 Ma). Hf and Nd model ages are older than 2.0 Ga, suggesting that the ferroan granitoids involved partial melting of Paleoproterozoic rocks. The data presented in this paper show that the ferroan magmatism was widespread in the BP and its counterparts in Africa in pre-drift reconstructions.

Evolution of the Major Gercino Shear Zone in the Dom Feliciano Belt, South Brazil, and implications for the assembly of southwestern Gondwana

The Dom Feliciano Belt developed during the Brasiliano–Pan-African orogenic cycle due to the tectonic interaction between the Rio de la Plata, Congo and Kalahari cratons, along with the amalgamation of smaller continental fragments. Together with its prolongations to the south, the Major Gercino Shear Zone constitutes one of the main lineaments of the orogenic system, establishing a more than 1000 km long NE-trending tectonic boundary between a granitic batholith and a metavolcano-sedimentary association. Based on combined field, structural, microstructural and textural data, together with new and published geochronological data, a refined model for the geological evolution of the Major Gercino Shear Zone is presented. Regional NW-verging, low-dipping structures were generated between 650 and 615 Ma, preceding the formation of the shear zone. This was followed by the main deformation phase, corresponding to pure-shear dominated dextral strike-slip, interpreted do be controlled by regional transpression during oblique convergence of the continental terranes. This stage lasted until ca. 585 Ma and was coeval with the continuous emplacement of granitic magmatism along the structure. Strain partitioning and localization led to the development of mylonitic belts along the intrusion borders mostly under greenschist facies metamorphic conditions. Late-stage ductile deformation along the structure was active during the cooling of the intrusions until ca. 550 Ma, while the deformation front of the orogenic system migrated to counterpart orogenic belts in the African continent. After tectonic stabilization, the Major Gercino Shear Zone recorded episodic brittle reactivation, possibly related to the Phanerozoic evolution of the active margins and intracratonic basins in Gondwana. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.

Influence of basement heterogeneity on the architecture of low subsidence rate Paleozoic intracratonic basins (Reggane, Ahnet, Mouydir and Illizi basins, Hoggar Massif)

Abstract. The Paleozoic intracratonic North African Platform is characterized by an association of arches (ridges, domes, swells, or paleo-highs) and low subsidence rate syncline basins of different wavelengths (75–620 km). The Reggane, Ahnet, Mouydir and Illizi basins are successively delimited from east to west by the Amguid El Biod, Arak-Foum Belrem, and Azzel Matti arches. Through the analysis of new unpublished geological data (i.e., satellite images, well logs, seismic lines), the deposits associated with these arches and syncline basins exhibit thickness variations and facies changes ranging from continental to marine environments. The arches are characterized by thin amalgamated deposits with condensed and erosional surfaces, whereas the syncline basins exhibit thicker and well-preserved successions. In addition, the vertical facies succession evolves from thin Silurian to Givetian deposits into thick Upper Devonian sediments. Synsedimentary structures and major unconformities are related to several tectonic events such as the Cambrian–Ordovician extension, the Ordovician–Silurian glacial rebound, the Silurian–Devonian Caledonian extension/compression, the late Devonian extension/compression, and the Hercynian compression. Locally, deformation is characterized by near-vertical planar normal faults responsible for horst and graben structuring associated with folding during the Cambrian–Ordovician–Silurian period. These structures may have been inverted or reactivated during the Devonian (i.e., Caledonian, Mid–Late Devonian) compression and the Carboniferous (i.e., pre-Hercynian to Hercynian). Additionally, basement characterization from geological and geophysics data (aeromagnetic and gravity maps), shows an interesting age-dependent zonation of the terranes which are bounded by mega-shear zones within the arches–basins framework. The old terranes are situated under arches while the young terranes are located under the basins depocenter. This structural framework results from the accretion of Archean and Proterozoic terranes inherited from former orogeny (e.g., Pan-African orogeny 900–520 Ma). Therefore, the sedimentary infilling pattern and the nature of deformation result from the repeated slow Paleozoic reactivation of Precambrian terranes bounded by subvertical lithospheric fault systems. Alternating periods of tectonic quiescence and low-rate subsidence acceleration associated with extension and local inversion tectonics correspond to a succession of Paleozoic geodynamic events (i.e., far-field orogenic belt, glaciation).

Fracture attributes in reservoir-scale carbonate fault damage zones and implications for damage zone width and growth in the deep subsurface

The width of reservoir-scale fault damage zone governs fluid flow of the tight reservoirs in subsurface petroliferous basins. Together with cores, thin sections, geophysical well logs, and production data from more than 60 wells, we describe fracture attributes and fault damage zones associated with a conjugate strike-slip fault system in tight Ordovician carbonate rock at depths of between 6500 m and 7500 m in the Tarim intracratonic basin in NW China. Fracture frequency, aperture, and oil production in wells reveals a power-law relationship with the distance from fault cores, with a variation in two orders of magnitude. The cumulative data of fracture frequency, aperture and oil production show multiple gradients in slope with the distance. Except for sampling bias, the varied gradients are possibly related to the overlaying multiple fault zones or multiple sections along a large fault zone, which are useful to constrain damage zone width. Together with fracture frequency, cumulative fracture frequency, aperture and oil production, four major damage zone boundaries are identified at ∼200, ∼420 m, ∼780 m and ∼1100 m. Therefore, a wide fault damage zone with a width of more than 2 km in the carbonate rocks could be inferred, which mostly depends on the interaction and superposition of secondary faults. In addition, there is a relatively high fracture frequency decay exponent (>1) with distance, which is possibly related to fault size, fault splays, background fracture frequency and the lithology. In turn, the growth of wide damage zone also resulted in a large scattered distribution of the fracture attributes with the distance from fault cores. Attributes of fractures of various scales from the deep subsurface provide powerful tools for assessing damage zone width and growth.

3D structural modeling of the Upper Bahariya Member in Abu Gharadig oil and gas field, North Western Desert, Egypt

Abu Gharadig Field lies in the central portion of Abu Gharadig Basin of the northern part of the Western Desert of Egypt, about 256 km west of Cairo. Abu Gharadig Basin is considered as an E-W trending intracratonic basin. The Upper Bahariya Member is a clayey reservoir and represents one of the Late Cretaceous units. There is a need for identifying whether the petrophysical parameters of this member are related to the facies changes or the present structures. Therefore, detailed structural and petrophysical studies are required. The structural configuration of the Upper Bahariya Member is studied in the present case and the results of this study will form the basis for future petrophysical studies. The present study includes the delineation of the main structural elements of the studied member within the study area. The aim of the present study was achieved through building a 3D structural model using Petrel™ Schlumberger Modeling software by integrating twenty 2D seismic sections with four digital well logs and a check-shot survey. The interpretation of the resulted model shows that the Upper Bahariya Member was folded into doubly plunging anticline with a gentle plunge to SW and a relatively steep plunge to NE. The fold was dissected by a series of normal faults trending E-W to NW-SE. The anticline is asymmetric and exhibits sigmoidal shape. The axis has mainly a NE-SW orientation and exhibits changing of this orientation in the NE part to be ENE-WSW and in the SW part to be also ENE-WSW, relating to the effect of a rotational stress. Growth faulting and thinning of the studied member at the crest of the anticline suggest the syn-sedimentary tectonics. Thus, these structures are related to the Late Cretaceous right-lateral shear which led to the opening of the North Atlantic.

Ascocerid cephalopods from the Hirnantian?–Llandovery stages of the southern Paraná Basin (Paraguay, South America): first record from high paleolatitudes

AbstractAscocerid cephalopods are described for the first time from high paleolatitudes of Gondwana. Studied material was collected from the Hirnantian?–Llandovery strata of the Eusebio Ayala and Vargas Peña formations, Paraná Basin, southeastern Paraguay. The specimens are poorly preserved and were questionably assigned to the subfamily Probillingsitinae Flower, 1941, being undetermined at genus and species rank because diagnostic characters are not visible. A particular feature seen in our material is the presence of both parts of the ascocerid conch (the juvenile or cyrtocone and the mature or brevicone) joined together, which is a very rare condition in the known paleontological record. The specimens are interpreted as at a subadult stage of development because fully grown ascocerids would have lost the juvenile shell. A planktonic vertical migrant mode of life with a subvertical attitude is proposed for the juvenile, and a horizontal demersal nektonic mode for the adult form, as has been previously suggested. A subvertical orientation near the bottom is proposed for the subadult stage. We suggest that the immigration of ascocerids to southwestern Gondwana was possible through ocean currents that would carry the planktonic juveniles from low to high latitudes during the end-Ordovician postglacial transgression that flooded the intracratonic basins of the region.