Ramp Facies Research Articles

La Zona de Yangtzeplacognathus crassus (Conodonta), Darriwiliano de la Precordillera Central, San Juan, Argentina

THE YANGTZEPLACOGNATHUS CRASSUS (CONODONTA) ZONE, DARRIWILIAN OF THE CENTRAL PRECORDILLERA, SAN JUAN, ARGENTINA. Elements of Yangtzeplacognathus crassus (Chen and Zhang) were recovered from samples collected in the San Juan Formation at three section in the Central Precordillera, San Juan Province, Argentina. The last meters of the San Juan Formation are exposed at the Quebrada del río de Las Chacritas, Cerro La Chilca and Quebrada Del Aluvión where these gray limestone strata are represented by grainstone, packstone-wackestone y dolopackstone. These limestone outcrops correspond to different carbonate ramp facies, from distal ramp, middle ramp, and lagoon. The key conodont Yangtzeplacognathus crassus documents the Yangtzeplacognathus crassus Zone in these sections, pointing towards a middle Darriwilian age for the conodont-bearing strata in the San Juan Formation and allowing correlation among these sections. The middle Darriwilian biostratigraphic frame of Baltica and central-south China is verified for the argentine Precordillera.

Stratigraphy of the Middle Eocene – Lower Oligocene successions in northwestern and eastern Jordan

Rocks of Eocene and Oligocene age, consisting mainly of carbonate ramp facies, are widely distributed in Jordan, but no previous stratigraphic works have dealt with the Eocene – Oligocene contact. Stratigraphic analysis, based on field exploration, and detailed lithostratigrapic and biostratigraphic preliminary data detected a major hiatus that separates the Middle Eocene (Lutetian) from the Early Oligocene (Rupelian) in the studied exposures in the Wadi El Ghadaf and Ash Shunah areas of Jordan. Our results reveal that the planktonic foraminifera Turborotalia cerroazulensis/Pseudohastigerina spp. (P18) Zone and the calcareous nannoplankton Ericsnia subdisticha (NP21) Zone of early Oligocene age were recorded immediately above the Morozovelloides lehneri (E11) Zone and the Discoaster tanii nodifer (NP16) Zone indicating, a period of non-deposition as a result of tectonic activity and/or sea level changes. A correlative hiatus and time gap were also reported in different parts of the Arabian and African plates, which representing the contemporaneous paleo-high areas. The presence of the early Oligocene shallow marine calcareous planktonic assemblages indicates that shallow shelves of the Tethys Ocean occupied the paleotopographic lows of Jordan, with deepening of the depositional basin toward the north.

A Triassic homoclinal ramp from the Western Tethyan realm, Western Balkanides, Bulgaria: Integrated insight with special emphasis on the Anisian outer to inner ramp facies transition

The Triassic carbonate succession exposed in the Western Balkanides is analyzed in terms of depositional evolution, sequence stratigraphy, sedimentation rate, environmental constraints, and biota distribution. Deposition took place on a storm-dominated ramp which existed at the NW end of the Tethys Ocean from late Olenekian to early Carnian time. Subtidal, intertidal and supratidal sediments were accumulated in three zones of the ramp profile: inner ramp, mid-ramp and outer ramp. The calculated average sedimentation rate is typical for other ancient marine shallow water carbonates including those deposited on ramps. Newly accomplished sedimentological study on part of the Anisian interval reveals a homoclinal ramp model with episodic initial distal steepening. Two main phases of the ramp evolution are distinguished on the basis of changing relative paleobathymetry — transgressive, e.g. retrogradational depositional trend, and regressive, e.g. progradational depositional trend. The maximum flooding zone is marked by the Terebratula Beds having late Pelsonian age. The inferred environmental constraints of the ramp system comprise: normal marine to hypersalinity; warm, well-illuminated water; aerobic and rarely dysaerobic bottom conditions; dominantly soft lime mud substrate; intensive storm activity; temporal seismic influence presumably related to Early Triassic rifting. The vertical distribution of marine fauna indicates shifting from Germanic (Peri-Tethyan) affinity to Alpine (Tethyan) affinity in Anisian time. The sedimentation was not characterized by formation of large buildups because of the paucity or lack of metazoan and other reef builders. The studied rocks are well correlated with the Triassic ramp succession outcropping in the Tizsa Meagunit of Southern Hungary. Similarities to the German, Polish and Spanish Muschelkalk are also clearly outlined and some parallel can be drawn with other platform carbonates from the NW Tethys shelf area and Peri-Tethys basins.

Early Callovian ingression in southwestern Gondwana. Palaeoenvironmental evolution of the carbonate ramp (Calabozo Formation) in southwestern Mendoza, Neuquen basin, Argentina

The carbonatic sequence of the Calabozo Formation (Lower Callovian) developed in southwestern Gondwana, within the northern area of the Neuquén basin, and is widespread in thin isolated outcrops in southwestern Mendoza province, Argentina. This paper describes the facies, microfacies and geochemical-isotopic analysis carried out in five studied localities, which allowed to define the paleoenvironmental conditions of a homoclinal shallow ramp model, highly influenced by sea level fluctuations, where outer, mid and inner ramp subenvironments were identified. The outer ramp subenvironment was only recognized in the south of the depocenter and is characterized by proximal outer ramp facies with shale levels and interbedded mudstone and packstone layers. The mid ramp subenvironment is formed by low energy facies (wackestone) affected by storms (packstones, grainstones and floatstones). The inner ramp subenvironment is the most predominant and is characterized by tidal flat facies (wackestones, packstones and grainstones) over which a complex of shoals (grainstones and packstones) dissected by tidal channels (packstone, grainstones and floatstones) developed. In the north area, protected environment facies were recorded (bioturbated wackestones and packstones). The vertical distribution of facies indicates that the paleoenvironmental evolution of the Calabozo Formation results from a highstand stage in the depocenter, culminating in a supratidal environment, with stromatolitic levels interbedded with anhydrite originated under restricted water circulation conditions due to a progressive isolation of the basin. δ13C and δ18O values of the carbonates of the Calabozo Formation suggest an isotopic signature influenced by local palaeoenvironmental parameters and diagenetic overprints. The δ13C and δ18O oscillations between the carbonates of the different studied sections are related with lateral facies variations within the carbonate ramp accompanied with dissimilar reactivities in relation to diagenetic fluids. The δ18O values of all sections exhibit a rather broad scatter which may be attributed to diagenesis and recrystallisation while the carbon isotopic composition has been less affected by those processes. Carbon isotope system has best retained the primary isotopic signal and δ13C values (0–3.9‰) are within the Callovian isotope range. The 87Sr/86Sr ratios of the bulk carbonates of El Plomo creek, La Vaina creek and Potimalal River sections are in agreement with the Callovian seawater Sr-isotope curve.

Facies heterogeneity at interwell-scale in a carbonate ramp, Upper Jurassic, NE Spain

The well-exposed upper Kimmeridgian carbonate ramp near Arroyo Cerezo, Iberian Basin, Eastern Spain, provides an excellent analog to stratigraphically equivalent subsurface reservoirs, such as the carbonate ramps of the Arab-D of the Middle East and the Smackover of the Gulf of Mexico. Critical questions regarding interwell-scale heterogeneity and correlation motifs for low-angle ramp systems can be addressed using continuous exposures that encompass a full range of inner to outer ramp facies. Outcrops in this area provide a complete exposure of the ramp succession in depositional dip direction.At Arroyo Cerezo, a 40-m thick and 1.3-km long, dip-oriented, continuous outcrop has been studied. Lithofacies and bounding surfaces have been mapped on a continuous photomosaic to build a detailed 2D cross-section, complemented by five stratigraphic sections (∼300 m apart) and petrographic analysis. The reconstructed section shows the facies transition from relative proximal- to distal carbonate ramp settings. Carbonate facies associations are grouped according to the dominant carbonate types: buildup-dominated systems and coated-grain dominated systems. Detailed mapping of facies and bounding surfaces documents distinct ramp depositional units with downdip facies changes occurring within the 1.3 km length of the outcrop. The Arroyo Cerezo outcrop indicates that the ramp-facies continuity, when based on a 5–10 km distance well-log correlation, is mostly apparent. Use of detailed spatial and temporal analysis of high-quality outcrop analogs promotes more realistic models for understanding the interwell, meter-scale heterogeneity. And the scale of these depositional heterogeneities, although being below resolution of subsurface tools, governs in fact (along with diagenetic modifications) the fluid flows within a reservoir. The scale of these stratigraphic heterogeneities needs to be considered in order to optimize and enhance hydrocarbon production and last recovery.

Depositional environments and sequence architecture of the Raha and Abu Qada formations (Cenomanian–Turonian), west central Sinai, Egypt

Cenomanian–Turonian deposits are important reservoirs for many oil fields in the Western Desert and the Gulf of Suez region of Egypt. Study of the Raha and Abu Qada formations (Cenomanian–Turonian), from five dip-oriented outcrop locations in west central Sinai; indicates deposition of a mixed siliciclastic-carbonate system on a ramp setting. The inner ramp facies (bivalve and benthonic foraminiferal wackestone) grades northward to the mid ramp facies (echinoderm calcisphere packstone, and oyster floatstone), and outer ramp facies (planktonic foraminiferal wackestone and calcisphere wackestone).The two studied formations comprise one second-order depositional sequence (duration of approximately 10 Million years). This large scale sequence includes four third-order depositional sequences, three of which are observed in the Raha Formation, with the other one recorded in the Abu Qada Formation. Because west central Sinai was tectonically stable during the Cenomanian and Turonian, the main factor controlling the lateral and vertical distribution of facies tracts is likely due to changes in the relative sea level. The Cenomanian–Turonian boundary event is known as the largest oceanic anoxic event during the Cretaceous. This global event has been documented in three of the studied sections. The recorded δ13C excursions range from+3.04‰ to+5.24‰. These high positive excursions in δ13C are associated with highly negative values of δ18O (values range from −6.01‰ to −1.38‰).

Facies related phylostratigraphy of the benthic neoselachian Ptychodus from the Late Cretaceous (Cenomanian/Turonian) of the Pre-North Sea Basin of Europe

The phylostratigraphy, taphonomy and palaeoecology of the Late Cretaceous neoselachian Ptychodus of northern Germany appears to be facies related. Ptychodus is not present in lower Cenomanian shark-tooth-rich rocks. First P. oweni records seem to relate to middle Cenomanian strata. P. decurrens appears in the middle to upper Cenomanian mainly in non-coastal environments of the shallow marine carbonate ramp and swell facies which isolated teeth were found partly in giant ammonite scour troughs on the Northwestphalian-Lippe High submarine swell in the southern Pre-North Sea Basin. They are recorded rare in deeper basin black shales facies (upwelling influenced, OAE Event II). P. polygyrus seems to be restricted to upwelling influenced basin and deeper ramp facies mainly of the uppermost Cenomanian and basal lower Turonian (OAE II Event). P. mammillaris is mostly represented during the lower to middle Turonian in the inoceramid-rich ramp and the near shore greensand facies along the Münsterland Cretaceous Basin coast north of the Rhenish Massif mainland. Finally, P. latissimus is recorded by two new tooth sets and appears in the upper Turonian basin swell facies and the coastal greensands. Autochthonous post-Turonian Ptychodus remains are unrecorded in the Santonian–Campanian of Germany yet. Reworked material from Cenomanian/Turonian strata was found in early Santonian and middle Eocene shark-tooth-rich condensation beds. With the regression starting in the Coniacian, Ptychodus disappeared in at least the Münster Cretaceous Basin (NW-Germany), but remained present at least in North America in the Western Interior Seaway. The Cenomanian/Turonian Ptychodus species indicate a rapid neoselachian evolution within the marine transgression and global high stand. A correlation between inoceramid shell sizes, thicknesses and their increasing size during the Cenomanian and Turonian might explain the more robust and coarser ridged enamel surfaces in Ptychodus teeth, if Ptychodus is believed to have preyed on epifaunistic inoceramid bivalves.

Rare earth element geochemistry of Cambrian phosphorites from the Yangtze Region

Analysis on P2O5 and REE (rare earth elements) in basal Cambrian phosphorites from the Yangtze Region, showed that the phosphorites from Bailongtan in Yunnan Province, Zhijin, Jinsha, Xishui, Zunyi, Tianzhu and Tongren in Guizhou Province, Shangrao in Jiangxi Province, Jiangshan in Zhejiang Province and Nanjing in Jiangsu Province contained high REE contents in phosphorites, especially those from Tianzhu and Tongren in Guizhou Province, Shangrao in Jiangxi Province, Jiangshan in Zhejiang Province and Nanjing in Jiangsu Province. Among them, the highest REE contents reached up to 1361.59×10−6 and P2O5 contents up to 29.45%. In contrast, phosphorites from Kaiyang, Qingzhen and Zhenyuan in Guizhou Province had relatively low total REE contents and low P2O5 contents, with the lowest total REE content of 53.19×10−6, and the lowest P2O5 content of 0.07%. REE contents exhibited a positive correlation with phosphorous contents, indicating a close genetic relationship between REE and phosphorous element. The main reason was the difference of the sedimentary environments and the contents of phosphorous and REE in deep water mass formed in upwelling currents, at an upper slope facies (Slu) in reducing condition due to relatively closed sedimentary environment. It was very difficult for the upswelling phosphorous and REE to deposit, massive phosphorous deposits were hard to be formed. At a deep water ramp facies (DRa) in the ascending process of currents, phosphorus and REE underwent differentiation rather than sedimentation. Consequently, the contents of P2O5 sediments and REE were low. At a shallow water ramp facies (including shoals and tidal flats) (SRa), with active seawater circulation and phosphatic supplement, sunny weather, high nutrition and rapid growth of algae, were benefitial for the physical enrichment of carbonate sediments. Therefore, massive phosphorite deposits were easy to be formed with abundant REE minerals, and finally turned into high REE bearing phosphorous deposits.

Characteristics of Milankovitch Cycles in the Mid‐Permian Liangshan and Qixia Formations of the Sichuan Basin—Examples from Well‐Long17 and Well‐Wujia1

AbstractThe Liangshan and Qixia formations in the Sichuan Basin of central China were formed in the earlier middle Permian. Based on outcrop observation of the Changjianggou section at Shangsi, Guangyuan region and 3rd‐order sequence division in typical drillings, one‐dimensional spectrum analysis has been used to choose the better curve between the natural gamma ray spectrometry log(ln (Th/K)) in Well‐Long17 and the gamma ray log(GR) in Well‐Wujia1, respectively, for identifying Milankovitch cycles in Sequence PSQ1 which comprises the Liangshan and Qixia formations, and then to identify the variation in the Milankovitch cycle sequences. On this basis, the system tract and 4th‐order sequence interfaces in Sequence PSQ1 were found via two‐dimensional spectral analysis and digital filtering. Finally, a high‐frequency sequence division program was established. Among these cycles, long eccentricity (413.0 ka) and short eccentricity (123.0 ka) are the most unambiguous, and they are separately the major control factors in forming 4th‐order (parasequence sets) and 5th‐order (parasequences) sequences, with the average thicknesses corresponding to the main cycles being 11.47 m and 3.32 m in Well‐Long17, and 14.21 m and 3.79 m in Well‐Wujia1, respectively. In other words, the deposition rate in the beach subfacies is faster than that of the inner ramp facies. The ln(Th/K) curve is more sensitive than the GR as the index of relatively ancient water depth in carbonate deposition. One‐dimensional spectrum analysis of ln(Th/K) curve could distinguish the Milankovitch cycle sequences that arose from the Precession cycle (20.90 ka), with a much higher credibility. Sequence PSQ1 in Well‐Long17 contains 10 4th‐order sequences, and the growth span of Sequence PSQ1 consisting of the Liangshan and Qixia formations is about 4.13 Ma. The single deposition thickness of the long eccentricity cycle sequence has the characteristics of thinning and then thickening in the two‐dimensional spectrum, which could be used to identify the system tract interface of the 3rd‐order sequence. The precession sequence thickness remains stationary. As a result, the early deposition rate in the mid‐Permian of the Sichuan basin was very slow, remaining nearly stationary, and this reflects a sustained depositional environment. Whole‐rock carbon and oxygen isotope curves could also prove this point Milankovitch cycle sequence studies provide a basis for paleoenvironmental analysis and, as such, can be used to analyze ancient climate change, calculate deposition rate and deposition time, and carry out fine isochronous stratigraphic correlation.

Magnetic susceptibility and its relationship with paleoenvironments, diagenesis and remagnetization: examples from the Devonian carbonates of Belgium

To better understand the origin of the initial magnetic susceptibility (χin) signal in carbonate sequences, a rock magnetic investigation that includes analysis of acquisition curves of the isothermal remanent magnetization (IRM) and hysteresis parameters, was undertaken on Devonian carbonates from the Villers and Tailfer sections, Belgium. Both sections are divided into a lower unit, dominated by biostromal and external ramp facies (biostromal unit) and an upper unit, only consisting of lagoonal facies (lagoonal unit). The variations in χin signal are mainly driven by magnetite variation, mostly pseudo-single-domain (PSD) magnetite. Clay minerals, pyrite, hematite and obviously calcite and dolomite are also present but their contribution to the χin pattern is not significant. There is a correlation between detrital proxies (Zr, Rb, Al2O3, TiO2) and χin for the Tailfer biostromal unit and the entire Villers section. The pervasive presence of fine-grained magnetite is interpreted as related to remagnetization. In absence of external fluids, the iron released during the smectite to illite transition remains in situ. In those situations χin may reflect an inherited primary synsedimentary signal. In the lagoonal unit of the Tailfer section, remagnetization appears to have obscured the original detrital information prompting the need for an evaluation of the composition of the susceptibility signal for individual case studies.

Stratigraphic architecture of an outer ramp/basinal turbidite fan carbonate system in a foreland basin (“Scaglia Toscana” Fm., Chianti Mts, Northern Apennines, Italy)

The Chianti Mts, part of the Northern Apennines orogen, represent an ideal case study to depict the relationships between outer carbonate ramps and basin plain carbonate turbidite systemsin foreland basins. The main object of this research is the Scaglia Toscana Fm., a ca. 200 metres thick Eocene-Oligocene succession of polychromous shales, marls, pelagic limestones and turbidite calcarenites, with marked lateral/vertical complexity, having its type-section in the Chianti Mts. During fi eldwork, the fm. has been subdivided in four partially interfingered members (from the bottom and northernmost): Sugame Marls; Cintoia Shales; Montegrossi Calcarenites; Dudda Shales and Limestones. The formation relates to three depositional systems: a middle to outer carbonate ramp dominated by hemipelagic deposition; a basinal plain whose sediments distribution was mainly forced by redox/oxic conditions and calcite-lysocline fluctuations; a thinning- and fining-upward, coarse- to fine-grained carbonate turbidite system. The Scaglia Toscana basin was located on the Adria continental micro-plate, foreland of the developing Northern Apennines orogen, whose facies distribution was mainly controlled by block faulting and differential subsidence. The Chianti Mts. represent then a crucial sector, since they expose the junction between a northern SSE-dipping ramp and a southern basinal plain with turbidite fl oor fans. A general southwardand upward-shift from ramp, basinal plain and turbidite fan facies has been recognized and remarked since the pioneering studies of the 1960s. Nonetheless, an exhaustive facies and architectural analysis is still lacking in literature. To fill this gap, the partial and preliminary results of a field logging, integrated with sedimentary facies analysis are reported here.

Ramp facies in an intracratonic basin: A case study from the Upper Devonian and Lower Carboniferous in central Hunan, southern China

Detailed studies on Late Devonian to Early Carboniferous carbonate rocks in central Hunan, southern China have led to the recognition of 25 lithofacies which can be grouped into: (1) inner ramp peritidal platform, (2) inner ramp organic bank and mound, (3) mid ramp, (4) outer ramp, and (5) shelf basin facies associations. The peritidal platform facies association dominates the Zimenqiao Formation (Namurian A or late Datangian) and is characterized by gypsum and dolostone-containing sequences, indicating a peritidal platform environment. The other four facies associations dominate the Menggongao Formation (late Famennian), Liujiatang Formation (Tournaisian or Yangruanian), Shidengzi Formations (early Visean or early Datangian). Five upward-shallowing cycles were distinguished in these three Formations. The predominant facies associations developed in each Formation demonstrate an overall transgression–regression cycle in the Late Devonian to Early Carboniferous in central Hunan. The overall transgressive sequence was preserved in the Shaodong, Menggongao, and Liujiatang Formations, and the overall regressive sequence was preserved in the Liujiatang, Shidengzi, Ceshui and Zimenqiao Formations.

Biostratigraphy, sequence stratigraphy and gamma-ray spectrometry of the Tournaisian-Viséan boundary interval in the Dublin Basin

A study of three carbonate and carbonate-siliciclastic sections at the Tournaisian-Visean (Tn-V) boundary in the Dublin Basin (Ireland), has been carried out using high resolution foraminiferal biostratigraphy and gamma-ray spectrometry. The aim was to identify the Tn-V boundary and trace correlatable log patterns in different environmental settings of the Dublin Basin. The foraminiferal fauna in the heterozoan and mixed heterozoan-photozoan (foramderm and bryonoderm extended) late Tournaisian shallow ramp facies and its calciturbidite products is taxonomically impoverished and resembles the similar impoverished foraminiferal associations of western Canada interpreted as reflecting upwelling zones. The increase of photozoan bioclasts which occurs in the latest Tournaisian is accompanied by the entry of important foraminiferal guides of the Tn-V boundary interval. The combination of biostratigraphic and spectral gamma-ray data proved to be a useful tool for the identification of the sequence boundary just below the Tn-V boundary across the different environmental settings of the Dublin Basin. The recognized sea-level fall is correlatable across the London Brabant Massif from western Ireland through to England, South Wales and Belgium.

Dolomitization on an evaporitic Paleoproterozoic ramp: Widespread synsedimentary dolomite in the Denault Formation, Labrador Trough, Canada

The Denault Formation (2.1–1.9 Ga) crops out in the Labrador Trough, northeastern Québec and western Labrador. Rocks surrounding the town of Schefferville, Quebec contain textural characteristics consistent with deposition on the middle and outer portions of a storm-influenced shallow ramp. Mid-ramp facies consist of intraclastic grainstones with hummocky cross-stratification (HCS), swaley cross-stratification (SCS), current ripples, and graded event beds. Further outboard, grainstones grade into deeper-water laminites that are composed of even, mm-scale couplets of flat-lying organic and dolomudstone laminae. Scours within the laminites suggest periodic storm activity. Laminites gradually grade into outer ramp deep-water shales. An isolated eastern stromatolitic buildup is separated from these ramp facies by 50 km (present day). This succession can be interpreted as the remnant of a near-continuous margin or may simply represent an isolated accumulation that developed on a pre-existing topographic high. The presence of gypsum pseudomorphs in all lithofacies indicates that the Denault margin was restricted and evaporitic. Four paragenetic stages are recognized in the diagenetic evolution of the Denault Formation: (1) carbonate deposition, contemporaneous marine cementation, authigenic gypsum growth, and precipitation of authigenic chert; (2) synsedimentary mimetic dolomite precipitation; (3) pore-rimming and pore-occluding shallow burial dolomite cement; and (4) fabric destructive, sutured, anhedral burial dolomite. Gypsum crystals occur in all lithofacies, form the nuclei of interstitial dolomite rhombs, average 10 μm in length, and often display swallowtail twinning. Paleoproterozoic ocean water had very low concentrations of dissolved sulfate and evaporation in restricted settings would have been required to form gypsum. Formation of microcrystalline gypsum across this restricted ramp facilitated dolomite precipitation by increasing pore water Mg/Ca ratios and lowering its dissolved sulfate concentrations. Such an interpretation may explain why there is an abundance of synsedimentary dolostone in the Precambrian and the relative paucity of Phanerozoic analogs.

Evaluating the Pharmacist-Physician MTM Collaborative Relationship Using Patient Outcomes

The Vaca Muerta Formation (early Tithonian–early Valanginian) is a rhythmic succession of marls and limestones, cropping out in the Neuquén Basin, west-central Argentina. This lithostratigraphic unit was traditionally interpreted as basinal to slope deposits. Detailed facies analysis allows to differentiate seven facies associations, representing basinal to middle ramp facies of a homoclinal ramp system prograding westward from the eastern margin, and slope facies attributed to a distally steepened ramp system that progrades eastward from the Andean volcanic arc in the west. Two sequence hierarchies are recognized: five third order depositional sequences, and fifteen fourth order high-frequency sequences. Fluctuations in organic matter content within the Vaca Muerta Formation suggest relationship with depositional sequences, finding the highest values associated with transgressive system tracts. This work represents an important advance in the understanding of the sedimentary and stratigraphic evolution of this exceptional unconventional reservoir. Our sequence stratigraphic approach contributes to the understanding of the relationship between organic matter, facies, and sea-level changes.

SEQUENCE STRATIGRAPHY AND RESERVOIR CHARACTERISTICS OF THE TURONIAN‐CONIACIAN KHASIB FORMATION IN CENTRAL IRAQ

Bioturbated chalky limestones of the Khasib Formation (Upper Turonian – Lower Coniacian) form potential reservoir rocks at oilfields and structures in central Iraq. Core and cuttings samples and wire‐line logs from wells in the East Baghdad, Balad, Samarra and Tikrit fields (wells EB‐77, EB‐57, Ba‐1, Ba‐2, Sr‐1, Sr‐2, and Ti‐1) were used to investigate microfacies types and porosity evolution. Facies modelling was applied to predict the relationship between facies distribution and reservoir characteristics to construct a predictive geologic model which will assist future exploration in central Iraq.Microfacies analysis and electrofacies identification and correlations indicate that the limestones of the Khasib Formation were deposited in a ramp setting. The ramp developed over the distal margin of the Upper Cretaceous proforeland basin, adjacent to the evolved forebulge. Inner ramp facies are characterized by carbonate bank bioclastic packstones intercalated with lagoonal green shales. Middle ramp facies dominate the Khasib Formation and consist of bioturbated, chalky, dolomitic and bioclastic limestones. Bioclasts include benthic and planktonic foraminifera. Intense Thalassinoides and less common Palaeophycus bioturbation has enhanced the porosity of this facies. Outer ramp deposits consist of alternating mid‐ramp bioturbated bioclastic chalky limestones and argillaceous and marly limestones. The latter contain basinal bioclasts including planktonic foraminifera, oligosteginids, dwarf rotaliids, and sponge spicules.Sequence stratigraphic analyses of the Khasib Formation indicate that it represents a third‐order depositional cycle. At the base is a type 1 sequence boundary which separates it from the underlying LST deposits of the Kifl Formation. Shaly and argillaceous limestones in the lower part of the Khasib Formation represent the early TST. These are overlain by a thick, deepening‐upwards succession of outer ramp facies. The maximum flooding surface is represented by a thin and extensive horizon of Oligosteginal limestones with Palaeophycus bioturbation. HST deposits are represented by aggradational build‐ups of bioturbated chalky dolomitic limestones, followed by progradational late HST shales and limestones. The boundary with the overlying Tanuma Formation is a type 2 sequence boundary. Bioclastic packstones and intensively bioturbated (Thalassinoides) bioclastic limestones of the mid‐outer ramp are the primary source of fabric‐selective porosity which is greatly enhanced by diagenetic overprints. These two units constitute the prime target for future exploration in central Iraq.

Carbonate ramp facies at the Calabozo Formation

Las calizas de la Formacion Calabozo (Caloviano inferior) afloran en el sudoeste de la Provincia de Mendoza, Argentina, dentro del ambito de la Cuenca Neuquina. En el presente trabajo, se realiza el analisis facial/microfacial de esta unidad en la localidad del arroyo El Plomo, el que permitio inferir las condiciones de sedimentacion y el modelo de rampa carbonatica local. Las calizas de la Formacion Calabozo representan un episodio transgresivo sobre las facies deltaico-marginales de la Formacion Lajas (Bajociano tardio-Caloviano), como asi tambien un cambio del regimen sedimentario silicoclastico a carbonatico. El inicio de esta etapa se manifiesta por un nivel de 'floatstone' de amplia extension areal. En el ambiente de rampa se distinguieron el subambiente de rampa media que es el mejor representado y esta compuesto por un arreglo de facies submareales ('wackestones', 'packstones', 'packstone' con canales y 'floatstones') y el subambiente de rampa interna, caracterizado por un sistema de barras ooidales ('grainstones'), con canales mareales ('floatstones') y con predominio de facies intermareales que pasan a perimareales hasta supramareal ('boundstones'). En la evolucion de la rampa se diferenciaron tres ciclos con tendencia somerizante. Los ciclos inferior y medio son caracteristicos de la rampa media. El ciclo superior es el mejor desarrollado y, en el, se distinguen un hemiciclo inferior, de rampa media y un hemiciclo superior, de rampa interna. La Formacion Calabozo culmina con depositos de ambiente supramareal de circulacion restringida, sobre los cuales se desarrolla la sucesion evaporitica de la Formacion Tabanos del Caloviano inferior.

Relationships between foraminiferal assemblages and depositional sequences in Jahrum Formation, Ardal area (Zagros Basin, SW Iran)

The Jahrum Formation was deposited in the foreland basin in southwest Iran (Zagros Basin). The Zagros mountain belt of Iran, a part of the Alpine–Himalayan system, extends from the NW Iranian border through to SW Iran, up to the strait of Hormuz. The various facies of the Jahrum Formation were deposited in four main genetically related depositional environments, including: tidal flat, lagoon, shoal and open marine. These are represented by 14 microfacies. The Jahrum Formation represents sedimentation on a carbonate ramp. Tidal flat facies are represented by fenestral fabric, stromatolitic boundstone and thin-bedded planes. Carbonate deposition in a shallow marine lagoon was characterised by wacke–packstone, dominated by various taxa of imperforate foraminifer. The shoals are made up of medium- to coarse-grained skeletal and peloidal grainstone. This facies was deposited predominantly in an active high energy wave and current regime, and grades basinward into middle ramps facies are represented by wackestones–packstones with a diverse assemblage of echinoderm and large benthic foraminifers with perforate wall. Outer ramp facies consist of alternating marl and limestones rich in pelagic foraminifera. There is no evidence for resedimentation processes in this facies belt. The sequence stratigraphy study has led to recognition of three third-order depositional sequences.

Three-dimensional sequence analysis of a subsurface carbonate ramp, Mississippian Appalachian foreland basin, West Virginia, USA

Well-cuttings, wireline logs and limited core and outcrop data were used to generate a regional, three-dimensional sequence framework for Upper Mississippian (Chesterian), Greenbrier Group carbonates in the Appalachian foreland basin, West Virginia, USA. The resulting maps were used to document the stratigraphic response of the basin to tectonics and to glacio-eustasy during the transition into ice-house conditions. The ramp facies include inner ramp red beds and aeolianites, lagoonal muddy carbonates, mid-ramp ooid and skeletal grainstone shoal complexes, and outer ramp wackestone–mudstone, that grades downslope into laminated silty lime mudstone. The facies make up fourth-order sequences, a few metres to over 90 m (300 ft) thick. The sequences are bounded along the ramp margin by lowstand sandstones and calcareous siltstones. On the ramp, sequence boundaries are overlain by thin transgressive siliciclastics and aeolianites, and only a few are calichified. Maximum flooding surfaces on the outer ramp lie beneath deeper water facies that overlie lowstand to transgressive siliciclastic or carbonate units. On the shallow ramp, maximum flooding surfaces overlie siliciclastic-prone transgressive systems tracts, that are overlain by highstand carbonates with significant grainstone units interlayered with lagoonal lime mudstones. The fourth-order sequences are the major mappable subsurface units; they are bundled into weak composite sequences which are bounded by red beds. In spite of differential subsidence rates across the foreland basin (1 to 3 cm/k.y. up to 25 cm/k.y.), eustatic sea-level changes controlled regional sequence development. Thrust-load induced differential subsidence of fault-blocks, coupled with in-plane stress, controlled the rapid basinward thickening of the depositional wedge, whose thickness and facies were influenced by subtle structures such as arches trending at high angles as well as parallel to the margin.

Acervulinid macroid and rhodolith facies in the Eocene Nummulitic Limestone of the Dauphinois Domain (Maritime Alps, Liguria, Italy)

The Eocene Nummulitic Limestone of the Dauphinois domain in the Argentina Valley (Maritime Alps, Liguria, Italy) is characterized by the local presence of carbonate ramp facies rich in acervulinid macroids, rhodoliths and larger foraminifera. The development of these particular facies is mainly controlled by palaeomorphology of the substratum, tectonics, type and amount of terrigenous supply and global sea level changes.