Ooid Grainstone Research Articles

Pennsylvanian carbonate platforms adjacent to deltaic systems in an active marine foreland basin (EscaladaFm.,CantabrianZone,NW Spain)

AbstractThePennsylvanian marine foreland basin of the Cantabrian Zone (NWSpain) is characterized by the unique development of kilometre‐size and hundred‐metre‐thick carbonate platforms adjacent to deltaic systems. DuringMoscovian time, progradational clastic wedges fed by the orogen comprised proximal alluvial conglomerates and coal‐bearing deltaic sequences to distal shelfal marine deposits associated with carbonate platforms (EscaladaFm.) and distal clay‐rich submarine slopes. A first phase of carbonate platform development (EscaladaI, upperKashirian‐lowerPodolskian) reached a thickness of 400 m, nearly 50 km in width and developed a distal high‐relief margin facing a starved basin, nearly 1000‐m deep. Carbonate slope clinoforms dipped up to 30° and consisted ofin situmicrobial boundstone, pinching out downslope into calciturbidites, argillaceous spiculites and breccias. The second carbonate platform (EscaladaII, upperPodolskian‐lowerMyachkovian) developed beyond the previous platform margin, following the basinward progradation of siliciclastic deposits. Both carbonate platforms include: (1) a lower part composed of siliciclastic‐carbonate cyclothems characterized by coated‐grain and ooid grainstones; and (2) a carbonate‐dominated upper part, composed of tabular and mound‐shaped wackestone and algal‐microbial boundstone strata alternating at the decametre scale with skeletal and coated‐grain grainstone beds. Carbonate platforms initiated in distal sectors of the foreland marine shelf during transgressions, when terrigenous sediments were stored in the proximal part, and developed further during highstands of 3rd‐order sequences in a high‐subsidence context. During the falling stage and lowstand systems tracts, deltaic systems prograded across the shelf burying the carbonate platforms. Key factors involved in the development of these unique carbonate platforms in an active foreland basin are: (1) the large size of the marine shelf (approaching 200 km in width); (2) the subsidence distribution pattern across the marine shelf, decreasing from proximal shoreline to distal sectors; (3) Pennsylvanian glacio‐eustacy affecting carbonate lithofacies architecture; and (4) the environmental conditions optimal for fostering microbial and algal carbonate factories.

Topographic and sea level controls on oolite-microbialite-coralgal reef sequences: The terminal carbonate complex of southeast Spain

The terminal carbonate complex of southeast Spain is a Miocene (Messinian) unit of oolite, microbialite, and coralgal reefs deposited in association with glacioeustasy and evaporitic drawdown. The relationship between paleotopography and sea level history is useful for prediction of microbialite and oolite reservoir facies in the subsurface. Four sequences record sea level change with minimum amplitudes of 32–77 m (105–253 ft). Sequences commonly have local basal stromatolites overlain by local thrombolites, ooid grainstone, volcaniclastic-rich planar-bedded ooid grainstone, and fenestral ooid grainstone. At low substrate positions, thrombolite boundstones are thicker and laterally more continuous than at higher positions. At intermediate substrate positions (relative to sea level history), sequences have a build-and-fill architecture, characterized by a relief-building phase and a relief-filling phase, with thin sequences draping paleotopography. Microbialites dominate during the relative sea level rises and build topographic relief. Oolites dominate during the relative sea level falls and fill topographic relief. At higher substrate positions, close to highstand, sequences thicken and yield stratigraphic character that is inconsistent with a build-and-fill model. Apparently, the build-and-fill model requires an intermediate-elevation substrate position and nonoptimal carbonate productivity during rapid sea level change. Sequences progressively show increasing diversity and more normal marine organisms, possibly caused by decreasing aridity. Lithofacies of the La Molata area show evidence of more restricted conditions compared to the La Rellana-Ricardillo area lithofacies, likely because La Molata was in an embayment. These results show that distribution of oolite, microbialite, and reef facies are predictable given known interaction among sea level, paleotopography of the depositional surface (substrate), and coastline configuration.

Reservoir analog model for oolite-microbialite sequences, Miocene terminal carbonate complex, Spain

Static three-dimensional (3-D) reservoir analog models were constructed for the Miocene terminal carbonate complex (TCC) in southeastern Spain. The models used field data collected from two areas containing exceptional 3-D exposures (La Molata; La Rellana-Ricardillo). Four TCC sequences in each area are composed of oolite, microbialite (thrombolites, stromatolites), bioclastic sands, and coralgal reefs deposited over paleotopographic relief of 33–76 m (108–249 ft). The models integrate field, laboratory, and petrophysical data with results providing a workflow and reservoir analogs useful in evaluating oolilte and microbialite reservoir characteristics in relation to paleotopography and sea level change. Results from this study reveal favorable reservoir-quality values with the potential for substantial hydrocarbon storage for many lithofacies. Flow and baffle facies were distinguished for the models based on thickness, lateral distribution, porosity, and permeability values. Trough cross-bedded ooid grainstone is volumetrically the most abundant lithofacies within both models, is laterally extensive across the entirety of sequences, has large storage capacities with good permeability, and has good connectivity with other flow facies. This facies represents the best reservoir-quality facies and would be the primary target for hydrocarbon exploitation. Microbialites act both as reservoir and baffle facies. Thrombolites, in particular, are concentrated downslope and in a more restricted embayment but vary between porous and nonporous facies. Stromatolites and fenestral ooid grainstones are concentrated at sequence boundaries and would create laterally extensive baffles with significant thicknesses at the sequence boundaries. Sea level interacting with both paleotopography and paleogeography were identified as the main controls on sequence development and reservoir heterogeneity. An understanding of these controls can aid in exploitation and identification of oolite-microbialite sequences.

Calcareous crusts on exposed Pleistocene limestones: A case study from Grand Cayman, British West Indies

Calcareous crusts, which include laminar (up to 6cm thick) and non-laminar calcretes, have developed on ooid grainstones and skeletal wacke/pack/grain/floatstone in the Pleistocene Ironshore Formation exposed at Morgan's Harbour, Tarpon Springs, and Rogers Wreck Point on Grand Cayman, British West Indies. Non-laminar calcretes, which consist largely of micrite with variably altered skeletal fragments and lithoclasts derived from the host limestone, contain peloids in their basal parts. In contrast, the laminar calcretes are formed of porous laminae and low-porosity laminae. Calcified roots are common in the porous laminae but rare in the low-porosity laminae. The δ18O values of calcretes, from ‐5.22‰ to +0.16‰, have a positive correlation with the δ13C values that vary from ‐10.03‰ to ‐6.41‰ at Tarpon Springs, from ‐11.57‰ to ‐5.43‰ at Morgan's Harbour, and from ‐9.89‰ to ‐8.68‰ at Rogers Wreck Point.The non-laminar calcretes reflect progressive micritization of the bedrock with a basal peloidal zone that passes upward into the micrite zone. In contrast, laminar calcretes are attributed to accretionary build-up dictated by microscale processes that were controlled largely by the interplay of plant roots and their associated microorganisms. The positive correlation between the δ18O and δ13C values from the calcareous crusts indicates that evaporation influenced calcite precipitation. The δ13C values of the laminar calcretes record a change from vegetation dominated by C3 plants that grew in a cool, wet climate, to a C3/C4 vegetation that grew in a hotter, drier climate. The Cayman calcretes resemble those found on the Pleistocene Miami Limestone in Florida, and highlight the possibility that these calcareous crusts may reflect regional climates and may therefore enable regional stratigraphic correlation.

Arabian carbonate reservoirs: A depositional model of the Arab-D reservoir in Khurais field, Saudi Arabia

The Upper Jurassic Arab Formation in the Arabian Peninsula, the most prolific oil-bearing interval of the world, is a succession of interbedded thick carbonates and evaporites that are defined stratigraphically upsection as the Arab-D, Arab-C, Arab-B, and Arab-A. The Arab-D reservoir is the main reservoir in Khurais field, one of the largest onshore oil fields of the Kingdom of Saudi Arabia. In Khurais field, the Arab-D reservoir is composed of the overlying evaporitic Arab-D Member of the Arab Formation and the underlying upper part of the Jubaila Formation. It contains 11 lithofacies, listed from deepest to shallowest: (1) hardground-capped skeletal wackestone and lime mudstone; (2) intraclast floatstone and rudstone; (3) pelletal wackestone and packstone; (4) stromatoporoid wackestone, packstone, and floatstone; (5) Cladocoropsis wackestone, packstone, and floatstone; (6) Clypeina and Thaumatoporella wackestone and packstone; (7) peloidal packstone and grainstone; (8) ooid grainstone; (9) crypt-microbial laminites; (10) evaporites; and (11) stratigraphically reoccurring dolomite. The Arab-D reservoir lithofacies succession represents shallowing-upward deposition, which, from deepest to shallowest, reflects the following depositional environments: offshore submarine turbidity fans (lithofacies 1 and 2); lower shoreface settings (lithofacies 3); stromatoporoid reef (lithofacies 4); lagoon (lithofacies 5 and 6); shallow subtidal settings (lithofacies 7 and 8); peritidal settings (lithofacies 9); and sabkhas and salinas (lithofacies 10). The depositional succession of the reservoir represents a prograding, shallow-marine, reef-rimmed carbonate shelf that was subjected to common storm abrasion, which triggered turbidites.

Back-Reef Depositional Environments in a Lower Cretaceous (Sligo) Shelf-Margin Complex: Insights into Ultradeep Reservoir Preservation and Controls on Stacking Patterns in an Outer Platform Setting

The Early Cretaceous Sligo and Stuart City shelf-margin trends in Texas have been a subject of interest for over 60 years, attracting attention both because of their striking complex reef lithologies and their continued potential for oil and gas exploration. Numerous gas reservoirs have been discovered since the late 1950s, with a major push for revitalization and expansion occurring in the 2000s. The shelf-margin complexes originated as steep-walled reefs rimming the Comanche Platform. They evolved into a narrow (<6 mi) highly heterogeneous belt of banks, islands, shoals, and tidal flats with an associated highly productive shallow-water carbonate factory. Most characterization efforts for these systems have focused on known reservoirs in South Texas; very little work has investigated the margins outside of this region, particularly with respect to the Sligo shelf margin. This investigation presents the first well-documented long stratigraphic section for the Sligo margin in East Texas, which serves as an initial step in understanding this major shelf-margin complex. Facies are mainly associated with back-reef to back-reef lagoon environments and exhibit significant heterogeneity on the basis of variations in water depth and wave energy, with facies including argillaceous carbonate mudstones, oncolitic packstones, rudist biostromes, reef-derived grainstones, ooid grainstones, and algal laminate bindstones. Four depositional sequences are delineated based on broad changes in facies and stacking patterns related to water depth and energy regime, yielding new insight into controls on facies partitioning in a stable long-term shelf-margin complex. The pore network, which is limited to micropores, is largely facies-controlled and is related to the presence of originally Mg–calcite allochems such as miliolids and Lithocodium/Bacinella. As East Texas has significant structural complexity, the ultradeep buried Sligo Formation is an underexplored area with the potential for undiscovered reservoirs related to structural features. Geologic characterization of this core is an important step in understanding some of the porous lithofacies present within the Sligo shelf margin in East Texas.

Interplay between depositional facies, diagenesis and early fractures in the Early Cretaceous Habshan Formation, Jebel Madar, Oman

Diagenesis and fracturing can significantly alter petrophysical properties of subsurface carbonate reservoirs, but the impacts of these processes at the inter-well scale are hard to predict. However, the initial distribution of sedimentary facies is easier to predict, and could template future diagenetic transformation and potentially influence fracturing as well. This paper provides a case study investigating the mutual influence of depositional facies on diagenesis and interpreted early fractures in the Berriasian–Valanginian Habshan Formation in Jebel Madar (Oman). The Habshan Formation mostly comprises ooidal limestones but dolomitized beds a few metres in thickness and laterally correlatable over at least 5 km are present. Discordant (vertical) dolomite streaks are present in the limestone but are rare. The various sedimentary and diagenetic processes were investigated using fieldwork, XRD analysis, petrography and geochemistry (major and trace elements, stable carbon and oxygen isotopes and strontium isotopes). The lithologies consist of mud-lean ooidal grainstones, mud-rich ooidal packstones and bioturbated limestones. The changes in the amount of fine-grained sediment suggest changing energy conditions at a backshoal position of a carbonate platform. The mud-lean versus mud-rich depositional textures impacted on early diagenesis by promoting abundant calcite cementation in the mud-lean ooidal grainstones and dolomitization in the mud-rich packstones. The discordant bodies, interpreted as sediment-filled early fractures, are dolomitized in the same way as the mud-rich ooidal packstones. Dolomitization occurred probably during the Valanginian–Hauterivian in a shallow burial setting, and was most likely caused by seepage reflux. Salt-doming post-dated dolomitization of the Habshan Formation. The results of this study document the inter-relationship between sedimentary facies, early diagenesis and early fractures.

Microfacies and Sedimentary Environments of Dalan Formation at Surmeh Mountain, Folded Zagros Zone, Southwestern Iran

The Permian carbonates- evaporates Dalan Formation is one of the most important gas reservoir in the Zagros Basin. The Zagros are located on the boundary between the Arabian and Eurasian lithosphere plates and are the Orogenic response to a collision between Eurasia and advancing Arabia during the Cenozoic. The study area is located at the eastern part of Surmeh Mountain, ~120 km south of Shiraz in the Folded Zagros Zone. Dalan Formation at Surmeh Mountain, with a thickness of 650 meters, is formed of three lithostratigraphic units. These units, from bottom to top, consist of Lower Dalan, Nar Member and Upper Dalan. The careful examination of thin sections led to the recognition of six microfacies which was performed on the basis of skeletal and non- skeletal grains and matrix (fenesteal dolomitic mudstone, layered anhydrite, lagoonal skeletal wackstone/ packstone, fine- grained ooid, peloid grainstone with lagoonal skeletal debris, medium- grained ooid grainstone with oomoldic porosity, open marine fossiliferous mudstone). Microscopic and macroscopic evidences show that Dalan Formation facies can be divided into four facies belts, on the basis of depositional features, specific of carbonate platforms of homoclinal ramp type. These belts include tidal flats, lagoons, shoal and basin environments.

Diagenetic processes and reservoir properties in the ooid grainstones of the Asmari Formation, Cheshmeh Khush Oil Field, SW Iran

Original mineralogy of ooids affects both their diagenetic history and, therefore, the reservoir quality of ooid grainstones in the Oligo-Miocene Asmari Formation in the Cheshmeh Khush Oil Field. In the Asmari ooid grainstones, the primary fabric is dominantly affected by dolomitization and selective dissolution. Therefore, determination of primary fabric in this type of ooid grainstones is impossible. Nevertheless the concentric fabric of some ooids can be distinguished by petrographical studies. The aragonitic composition of the studied ooids seems to be responsible for selective dissolution, intra-cortical and moldic porosity development. On the other hand, presence of some un-dissolvable coats on the ooids may be the cause of primary fabric preservation during the aragonite replacement by calcite.Integration of petrographic and petrophysical data shows that these ooid grainstones with different mineralogical composition (dolostone, dolomitic limestone and limestone) have different reservoir properties. The ooid grainstones of the studied well were grouped into 6 reservoir rock types.The ineffective moldic porosity, although considerable, is associated with low permeability in both dolomitic limestone and dolomitic types. Anhydrite cementation has played a significant role in the reducing of porosity and the size of pore spaces, accordingly the permeability. Mechanical compaction has also reduced porosity and permeability of some limited samples. Abundant inter-granular porosity (up to 200μ in size) and lack of anhydrite cement in one of the oolitic intervals have resulted in average permeability up to 523mD. Such characteristics make this interval as the best reservoir rock type in the studied well. The calcite-cemented lime ooid grainstones have minor effective porosity and low permeability, so dolomitization has played a significant role in controlling the reservoir characteristics. Most rock types in which common pore spaces are around or less than 7μ in size show insignificant permeability, but rock type with pore spaces greater than 7μ show significant permeability.

Glacioeustatic cyclicity of a Pennsylvanian carbonate platform in a foreland basin setting: An example from the Bachende Formation of the Cantabrian Zone (NW Spain)

By analysis of outcrop data, sequence stratigraphy was used to interpret the stratal architecture and evolution of a Moscovian (Pennsylvanian) carbonate platform (Bachende Formation) located in the Variscan foreland basin of the Cantabrian Zone (NW Spain). The Bachende Formation represents a delta-top carbonate platform developed in highly subsiding (average subsidence rate of 254m/My) distal shelf areas of the basin. It reached a thickness of 650m and covered an area of ~170km2 (after conservative palinspastic restorations). The platform interior succession is mostly composed of low-relief mound-shaped micritic boundstones, alternating with bioclastic mudstones/wackestones, peloidal–foraminiferal packstones/grainstones, calcareous algal bafflestones and skeletal and ooidal grainstones. Landwards (to the W), carbonate platform beds alternate and interfinger with deltaic sandstones and shallow marine shales.The platform interior consists of three sequences (S1, S2 and S3), ranging from 180 to 230m in thickness and from 1.3 to 0.5My in age. These sequences are in turn subdivided into several meter-scale (10.6m average thickness), high-frequency (110 to 15ky), subtidal transgressive–regressive cycles. These cycles are bounded by subaerial exposure or marine flooding surfaces and are inferred to be laterally continuous across the platform-top. They vary from mixed carbonate-siliciclastic to carbonate-dominated cycles.Cyclicity on the Bachende platform was strongly influenced by glacioeustatic sea level variations and changes in subsidence and sedimentation rates. Subaerial exposure surfaces capping high-frequency cycles are better developed in the sequence 1 (late Kashirian/early Podolskian), when subsidence was relatively low and glacioeustatic sea level fluctuations could have exposed subtidal deposits during base-level falls. On the basis of the conservative estimated duration of each cycle and spectral analysis of cycle thickness, the precessional (~20ky) and obliquity (~40ky) cycles were detected in all the sequences, the short eccentricity cycle (~100ky) in the sequence 1, and the long eccentricity cycle (~400ky) in the sequences 2 and 3 (Podolskian). Nevertheless, precessional and obliquity frequencies seem to dominate during sequences 2 and 3, which were deposited under significant higher subsidence and sedimentation rates. Additionally, spectral peaks at ~60–80ky are randomly present across the whole succession, and they cannot be explained as a linear response of the climate system to the Milankovitch forcing.

Carbonate platform dominated by peloidal sands: Lower Ordovician La Silla Formation of the eastern Precordillera, San Juan, Argentina

AbstractThe Lower Ordovician La Silla Formation of the Precordillera of west‐central Argentina is part of the west‐facing early Palaeozoic, tropical carbonate platform succession that comprises the core of the Cuyania terrane. Up to 360 m thick, it is exposed in several thrust sheets over a distance of some 250 km along and across depositional strike over a palinspastically unrestored distance of about 35 km. La Silla Formation is a strikingly pure limestone with subordinate finely crystalline dolomite and rare chert. It accumulated on a more or less uniformly subsiding passive margin. Copious precipitation of microcrystalline calcite, probably influenced by microbial activity to varying degrees, led to the generation of peloids, ooids and aggregates of these grains, as well as small amounts of lime mud, intraclasts, stromatolites and thrombolites. Rare bioclasts are limited mostly to scattered gastropods and trilobite sclerites; bioturbation is present locally. The array of carbonate rock types is grouped into eight recurring lithofacies, in order of decreasing abundance: (i) peloidal grainstone; (ii) laminated dolostone; (iii) intraclastic rudstone; (iv) microbial laminite; (v) peloidal packstone; (vi) ooidal grainstone; (vii) thrombolite boundstone; and (viii) mudstone. These facies represent sediments that formed solely in a shallow subtidal marine environment, with no evidence of restricted conditions, hypersalinity or subaerial exposure. No events of eustatic sea‐level change are recorded. By far the dominant facies is grainstone composed of well‐sorted, fine sand‐sized peloids and peloidal aggregates in homogeneous, tabular to gently undulating, medium to thick beds; cross‐lamination is scarce. Clusters of sub‐metre‐sized microbial patch reefs developed sporadically. The shallow platform is envisaged to have been covered by extensive peloidal sand flats and low‐relief banks, and little lime mud was generated. The setting was probably microtidal and may not have been affected by strong trade winds. It was washed by frequent, relatively gentle wave action but without experiencing powerful storms. In the middle member, anomalous lenses of intraclastic rudstone and laminated dolostone occur as graded beds overlying sharply downcut scoured surfaces up to 20 cm deep; these are interpreted to indicate a phase when accretion was punctuated occasionally by tsunamis generated from rift‐faulting seaward of the platform margin. The remarkably uniform peloidal grainstone composition over a broad area shows that, given the appropriate combination of climate, environmental and ecological factors, large portions of some early Palaeozoic platforms were dominated by grainy sediment and remained under well‐agitated conditions within fair‐weather wave‐base, without distinct lateral facies differentiation or tidal‐flat aggradation.

Polygonal sandcracks: Unique sedimentary desiccation structures in Bahamian ooid grainstone

Abstract Sandcracks, which are ubiquitous in Holocene eolian and beach backshore carbonate grainstone on Alligator Point, Cat Island, Bahamas, resemble polygonal mudcracks, but formed in ooid sand without muddy matrix. In experiments on Cat Island beach sand, sediment surfaces cracked polygonally in the absence of mud or biofilms while drying at room temperature due to contraction generated by capillary effects related to surface tension attraction of interstitial water. Gravitational collapse of irregular open pores and repacking of sand grains due to loss of cohesion between particles caused by evaporation of water enhance the cracking process and appearance of polygons by providing space for cracks to expand. The polygons are held together by any remaining capillary moisture and associated meniscus cement, which precipitates as the sand dries. Polygonal sandcracks can be preserved by rapid lithification of carbonate sand, but have been documented only rarely from other localities because their formation requires well-sorted, well-rounded spherical grains rather than those making up the more common, heterogeneous skeletal and peloidal sediment in carbonate settings. Interpretation of this primary sedimentary desiccation structure provides new insights into sedimentation and diagenesis of ooid-rich deposits and can aid in recognizing ancient subaerial exposure horizons.

Structural analysis of the Itapucumí Group in the Vallemí region, northern Paraguay: Evidence of a new Brasiliano/Pan-African mobile belt

The Neoproterozoic (Ediacaran) Itapucumí Group in northern Paraguay is composed of carbonate and siliciclastic rocks, including ooid grainstones, marls, shales and sandstones, containing Cloudina fossils in the eastern region. It is almost undeformed over the Rio Apa Cratonic Block but shows a strong deformational pattern at its western edge. A detailed structural analysis of the Itapucumí Group was conducted in the Vallemí Mine, along with a regional survey in other outcrops downstream in the Paraguay River and in the San Alfredo, Cerro Paiva and Sargento José E. López regions. In the main Vallemí quarry, the structural style is characterized by an axial-plane slaty cleavage in open to isoclinal folds, sometimes overturned, associated with N–S trending thrust faults and shear zones of E-vergence and with a low-grade chlorite zone metamorphism. The structural data presented here are compatible with the hypothesis of a newly recognized mobile belt on the western side of the Rio Apa Cratonic Block, with opposite vergence to that of the Paraguay Mobile Belt in Brazil. Both belts are related to the Late Brasiliano/Pan-African tectonic cycle with a Lower Cambrian deformation and metamorphism age. The deformation could be due to the late collision of the Amazonian Craton with the remainder of Western Gondwana or to the western active plate boundary related to the Pampean Belt. The structural and lithologic differences between the western Itapucumí Group in the Vallemí and Paraguay River region and the eastern region, near San Alfredo and Cerro Paiva, suggest that this group could be divided into two lithostratigraphic units, but more stratigraphic and geochronological analyses are required to confirm this possibility. O Grupo Itapucumí é constituído por rochas carbonáticas e siliciclásticas, caracterizadas pela presença de grainstones com oóides, margas, folhelhos e arenitos de idade neoproterozóica (ediacarana), devido à ocorrência do fóssil Cloudina na sua porção leste. Apresenta-se quase indeformado quando recobre o bloco cratônico do Rio Apa, porém mostra um intenso padrão deformacional na sua porção ocidental. Análise estrutural detalhada foi realizada na mina Vallemí, norte do Paraguai, bem como reconhecimento de outros afloramentos ao longo do rio Paraguai e na região entre San Alfredo, Cerro Paiva e Sargento José E. López. Na mina principal em Vallemí, o estilo estrutural caracteriza-se pelo desenvolvimento de clivagem ardosiana plano-axial de direção aproximada N–S em dobras abertas a isoclinais, por vezes com flanco inverso e associadas a empurrões com vergência para E com desenvolvimento de metamorfismo de baixo grau, na zona da clorita. Os dados estruturais levantados reforçam a hipótese da existência de uma nova faixa móvel na margem oeste do Bloco Rio Apa, com vergência oposta à da Faixa Paraguai Meridional, relacionada ao final do evento Brasiliano/Pan-Africano, com deformação e metamorfismo no Cambriano Inferior. A deformação pode ser atribuída à colisão tardia entre o craton Amazônico e o restante do Gondwana Ocidental, ou à borda de placa ativa associada à Faixa Pampeana. As diferenças estruturais e em parte litológicas, entre as exposições da parte ocidental do Grupo Itapucumí, na região de Vallemí, das da parte oriental, nos arredores de San Alfredo e Cerro Paiva, conduzem à possibilidade do Grupo Itapucumí vir a ser dividido no futuro em duas unidades estratigráficas distintas, o que requer estudos estratigráficos e geocronológicos mais pormenorizados para uma conclusão definitiva. Un análisis estructural detallado fue realizado en la Mina Vallemí en el norte de Paraguay. Afloran allí litotipos del Grupo Itapucumí, de edad neoproterozoica, constituído por calcarenitas oolíticas, margas, lutitas y areniscas. En la mina principal, la sucesión presenta deformación relativamente intensa, con desarrollo de clivaje de plano axial y rumbo N–S. Se observan pliegues abiertos a isoclinales, a veces con flanco invertido y asociados a cabalgamientos con vergencia al NE y E. Se registra asimismo metamorfismo de grado bajo (zona de clorita). Los datos estructurales refuerzan la hipótesis de una nueva faja móvil en el margen oeste del Bloque Río Apa, con vergencia opuesta a la del Cinturón Paraguay meridional, este último relacionado al evento Brasiliano/Pan-Africano. Las diferencias estructurales y en parte litológicas entre las exposiciones de la parte occidental del Grupo Vallemí y las de la parte oriental, en los alrededores de Colonia San Alfredo, sugieren la posibilidad de que el Grupo Itapucumí pueda ser subdividido en dos unidades litoestratigráficas distintas. Para llegar a una conclusión definitiva son necesarios estudios estratigráficos y geocronológicos más pormenorizados.

The Middle Jurassic basinal deposits of the Surmeh Formation in the Central Zagros Mountains, southwest Iran: facies, sequence stratigraphy, and controls

The lower part of the Lower to Upper Jurassic Surmeh Formation consists of a succession of shallow marine carbonates (Toarcian–Aalenian) overlain by a deep marine basinal succession (Aalenian–Bajocian) that grades upward to Middle to Upper Jurassic platform carbonates. The termination of shallow marine carbonate deposition of the lower part of the Surmeh Formation and the establishment of deep marine sedimentation indicate a change in the style of sedimentation in the Neotethys passive margin of southwest Iran during the Middle Jurassic. To evaluate the reasons for this change and to assess the basin configuration during the Middle Jurassic, this study focuses on facies analysis and sequence stratigraphy of the basinal deposits (pelagic and calciturbidite facies) of the Surmeh Formation, referred here as ‘lower shaley unit’ in the Central Zagros region. The upper Aalenian–Bajocian ‘lower shaley unit’ overlies, with an abrupt contact, the Toarcian–lower Aalenian platform carbonates. It consists of pelagic (calcareous shale and limestone) and calciturbidite facies grading to upper Bajocian–Bathonian platform carbonates. Calciturbidite deposits in the ‘lower shaley unit’ consist of various graded grainstone to lime mudstone facies containing mixed deep marine fauna and platform-derived material. These facies include quartz-bearing lithoclast/intraclast grainstone to lime mudstone, bioclast/ooid/peloid intraclast grainstone, ooid grainstone to packstone, and lime wackestone to mudstone. The calciturbidite layers are erosive-based and commonly exhibit graded bedding, incomplete Bouma turbidite sequence, flute casts, and load casts. They consist chiefly of platform-derived materials including ooids, intraclasts/lithoclasts, peloids, echinoderms, brachiopods, bivalves, and open-ocean biota, such as planktonic bivalves, crinoids, coccoliths, foraminifers, and sponge spicules. The ‘lower shaley unit’ constitutes the late transgressive and the main part of the highstand systems tract of a depositional sequence and grades upward to platform margin and platform interior facies as a result of late highstand basinward progradation. The sedimentary record of the ‘lower shaley unit’ in the Central Zagros region reveals the existence of a northwest–southeast trending platform margin during the Middle Jurassic that faced a deep basin, the ‘Pars intrashelf basin’ in the northeast. The thinning of calciturbidite layers towards the northeast and the widespread Middle Jurassic platform carbonates in the southern Persian Gulf states and in the Persian Gulf area support the existence of a southwest platform margin and platform interior source area. The platform margin was formed as a result of tectonic activity along the preexisting Mountain Front fault associated with Cimmerian continental rifting in northeast Gondwana. Flooding of the southwest platform margin during early to middle Bajocian resulted in the reestablishment of the carbonate sediment factory and overproduction of shallow marine carbonates associated with sea-level highstand, which led to vertical and lateral expansion of the platform and gradual infilling of the Pars intrashelf basin by late Bajocian time.

Subtidal stromatolites from the Sinemurian of the Lusitanian Basin (Portugal)

Well-preserved dome-shaped carbonate stromatolites occur in the lowermost part of the Sinemurian of the Lusitanian Basin (Portugal), at S. Pedro de Moel region (W of the basin). Deposition in the region took place on a westward-dipping carbonate ramp. The stromatolitic mounds are not found anywhere else in the Sinemurian of the basin and therefore are regarded as specific bioevents. In contrast to marginal-marine stromatolitic crusts, subtidal carbonate mounds other than sponge-mounds have been seldom reported in the Lower Jurassic, in particular in the Sinemurian, either from Europe or North-Africa. Therefore, the case documented here contributes to enhance the knowledge on stromatolites of this age in the Peri-Tethyan and Proto-Atlantic regions. The depositional setting of the studied succession is interpreted as a mainly low-energy, restricted marine one, punctuated by higher-energy episodes and, locally, subjected to more open marine influence. The existence of a topographic high and detached shoals at a more distal location of the ramp is likely, considering regional seismic evidence, the record in offshore (to the W) wells of peloidal/ooid wacke-packstones with detrital quartz and occurrence of a few ooid grainstones in the studied section. The inferred positive relief would act as a physical constraint that, coupled with the low-gradient of the ramp, defined an embayment-like environment in which the prevailing ecological conditions must have been, for the part of the succession bearing the stromatolites, unfavorable for many benthic organisms, favoring the microbial community. The upper part of the succession suggests stepwise environmental openness to more marine influence alternating with frequent environmental restriction.

Microfacies and sequence stratigraphy of the Amapá Formation, Late Paleocene to Early Eocene, Foz do Amazonas Basin, Brazil

On the basis of thin-section studies of cuttings and a core from two wells in the Amapá Formation of the Foz do Amazonas Basin, five main microfacies have been recognized within three stratigraphic sequences deposited during the Late Paleocene to Early Eocene. The facies are: 1) Ranikothalia grainstone to packstone facies; 2) ooidal grainstone to packstone facies; 3) larger foraminiferal and red algal grainstone to packstone facies; 4) Amphistegina and Helicostegina packstone facies; and 5) green algal and small benthic foraminiferal grainstone to packstone facies, divisible locally into a green algal and the miliolid foraminiferal subfacies and a green algal and small rotaliine foraminiferal subfacies. The lowermost sequence (S1) was deposited in the Late Paleocene–Early Eocene (biozone LF1, equivalent to P3–P6?) and includes rudaceous grainstones and packstones with large specimens of Ranikothalia bermudezi representative of the mid- and inner ramp. The intermediate and uppermost sequences (S2 and S3) display well-developed lowstand deposits formed at the end of the Late Paleocene (upper biozone LF1) and beginning of the Early Eocene (biozone LF2) on the inner ramp (larger foraminiferal and red algal grainstone to packstone facies), in lagoons (green algal and small benthic foraminiferal facies) and as shoals (ooidal facies) or banks ( Amphistegina and Helicostegina facies). Depth and oceanic influence were the main controls on the distribution of these microfacies. Stratal stacking patterns evident within these sequences may well have been related to sea level changes postulated for the Late Paleocene and Early Eocene. During this time, the Amapá Formation was dominated by cyclic sedimentation on a gently sloping ramp. Environmental and ecological stress brought about by sea level change at the end of the biozone LF1 led to the extinction of the larger foraminifera ( Ranikothalia bermudezi).

Highstands during Marine Isotope Stage 5: evidence from the Ironshore Formation of Grand Cayman, British West Indies

The Ironshore Formation on Grand Cayman is formed of six unconformity-bounded packages (units A–F). Units A, B, C, and D, known from the subsurface in the northeastern part of Grand Cayman, formed during Marine Isotope Stages (MIS) 11(?), 9, 7, and 5e, respectively. Unconformities at the tops of units A, B, and C are highlighted by terra rossa and/or calcrete layers. Strata in core obtained from wells drilled in George Town Harbour and exposed on the west part of Grand Cayman belong to unit D, and the newly defined units E and F. Corals from unit E yielded Th/U ages of ∼104ka whereas conch shells from unit F gave ages of ∼84ka. Unit E equates to MIS 5c whereas unit F developed during MIS 5a.Th/U dating of corals and conchs from the Ironshore Formation on the western part of Grand Cayman shows that unit D formed during the MIS 5e highstand whereas units E and F developed in association with highstands at 95–110ka (MIS 5c) and 73–87ka (MIS 5a). Unit E, ∼5m thick in the offshore cores, is poorly represented in onshore exposures. Unit F, which unconformably overlies unit D at most localities, is formed largely of fossil-poor, cross-bedded ooid grainstones. The unconformity at the top of unit D, a marine erosional surface with up to 2.5m relief, is not characterized by terra rossa or calcrete in the offshore cores or onshore exposures. Unit D formed with a highstand of +6m asl, whereas units E and F developed when sea level was +2 to +5 asl and +3 to +6m asl, respectively. Thus, the highstands associated with MIS 5e, 5c, and 5a were at similar elevations.

Fracture intensity vs. mechanical stratigraphy in platform top carbonates: the Aquitanian of the Asmari Formation, Khaviz Anticline, Zagros, SW Iran

Outcrop analogue studies can significantly improve the understanding of fracture distribution and their impact on fluid flow in hydrocarbon reservoirs. In particular, the outcrops may reveal details on the relationships between mechanical stratigraphy and fracture characteristics. This has been investigated in an integrated sedimentological-structural geological study in the Aquitanian sequence of the Asmari Formation on the NE limb of the Khaviz Anticline in the Zagros foothills in SW Iran. The Aquitanian sequence was deposited in a platform top setting and is characterized by well-defined bedding planes and relatively thin layers (<4 m) with rapid changes in textures from laminated peritidal mudstones to bioclast and ooid grainstones. Fractures in the studied area dominantly strike parallel to the fold axis, have a high angle to bedding and are stratabound. In the literature it is often reported that fracture spacing or the inverse fracture intensity (FI) is controlled by the mechanical layer thickness (MLT). However, in the present study area a rather poor correlation between FI and MLT was observed. Instead, the Dunham texture appears to be more important for the FI. Mud-supported textures (mudstone and wackestone) have higher FI than grain-supported (packstone and grainstone) ones. The degree of dolomitization does not appear to have any significant effect on FI within each texture class. A strong relationship between FI and MLT is observed generally in cases where there has been one single phase of extension and when interbed contacts are weak, e.g. interbedded competent limestones and incompetent shales. However, in the present study area a rather complex deformation history exists and well-developed shales between fractured carbonate layeres are lacking. It is suggested that in such cases the MLT is of minor importance for the FI, which is controlled by the texture.

Transgressive oversized radial ooid facies in the Late Jurassic Adriatic Platform interior: Low-energy precipitates from highly supersaturated hypersaline waters

Dark-gray oolitic units characterized by oversized ooids with primary radial calcite fabrics occur in the interior of the Late Jurassic Late Tithonian, Adriatic Platform, a large Mesozoic, Tethyan isolated platform in Croatia. They differ from open-marine, platform-margin ooid grainstones in their dark color, cerebroid outlines, broken and recoated grains, abundant inclusions, highly restricted biota, and lack of cross-stratifiThey have been interpreted as being of vadose origin (“vadoids”) at tops of upward-shallowing parasequences. However, detailed sections show that most oolitic units occur at bases of precessional parasequences, overlying erosional surfaces on fenestral carbonates. The oolitic units are similar to quiet-water ooids that form today in low-energy settings. They developed in an arid climate during initial transgression of supratidal fl ats, along lowenergy shores seaward of tidal fl ats, and along the margins of restricted lagoons and intertidal ponds. Superimposed fenestral fabrics, meniscus micrite cements, and grain breakage occurred as they aggraded to high-tide level and were subjected to wetting and drying, thermal expansion and contraction, and wind transport. They migrated landward with transgression, forming extensive sheets, and were overlain by subtidal lagoonal facies that shallow up into fenestral carbonates. These distinctive facies may have been overlooked in the geological record, or their geological distribution requires juxtaposition of calcite seas, high-calcite supersaturation states, arid climate, and presence of fl at-topped carbonate platforms in a greenhouse world.

Upper Dalan Member and Kangan Formation between the Zagros Mountains and offshore Fars, Iran: depositional system, biostratigraphy and stratigraphic architecture

Upper Dalan Member and Kangan Formation between the Zagros Mountains and offshore Fars, Iran: depositional system, biostratigraphy and stratigraphic architecture