Low-angle Ramp Research Articles

Siliciclastic input controlling carbonate deposition on a low-angle ramp system: New insight from the Upper Triassic Luning Formation (Western-Central Nevada)

Although more than one century has passed since the first studies, knowledge about the Upper Triassic shallow-water carbonates from the Panthalassa Ocean remains lower than their Tethyan counterparts. Characterising these sedimentary rocks represents a unique way to deepen our understanding of the biosphere's evolution and recovery after the major Permian–Triassic biological crisis. The Triassic Luning Formation in Western–Central Nevada is a crucial sedimentary succession due to its considerable thickness and areal extent. Among the three members forming the Luning Formation, the present study has only described the upper one in detail due to the minor areal distribution of the Lower Member and the pure siliciclastic nature of the middle Member. All accessible localities in this region, 21 in total, have been extensively sampled for facies and micropalaeontological analyses, including conodonts and poorly preserved foraminifera, leading to new insights about the depositional conditions and exact age of the Luning Formation's Upper Member. A depositional model is proposed for the first time, corresponding to a low-angle ramp with homogeneous bathymetric and hydrodynamic conditions subjected to frequent siliciclastic input from the continent. The regular terrigenous influx causes a significant cyclicity in this portion of the formation, comprising heterogeneously thick limestone beds. The environment-diagnostic fauna is characterised by pioneer organisms that regenerate carbonate production after the main break related to the pure siliciclastic Middle Member's deposition. Numerous conodont specimens indicate that the entire Upper Member is middle Norian in age. Identifying the observed organisms, tied to a reliable stratigraphic interval, allows comparing the Luning Formation with other Upper Triassic carbonates from the Panthalassa Ocean, now cropping out at different latitudes on the eastern and western coasts of the Pacific Ocean. Despite some faunal affinities, especially with carbonate build-ups developed at middle-palaeolatitudes, the Luning Formation differs from most other East Panthalassa carbonates because of its short palaeo-distance from the continent (i.e. the American craton), being deposited near the mainland coast instead of on an isolated volcanic arc.

Quantitative analysis of geological attributes for reservoir heterogeneity assessment in carbonate sequences; a case from Permian–Triassic reservoirs of the Persian Gulf

This study integrates quantitative analysis of reservoir properties with geological parameters to discriminate the reservoir heterogeneities in the Permian–Triassic sequences of the Persian Gulf. To achieve this goal, geological analysis of core samples and thin sections were integrated with petrophysical logs and porosity–permeability data. For quantitative assessment of reservoir heterogeneity, Coefficient of Variation (CV), Dykstra-Parsons Coefficient (VDP) and Lorenz Coefficient (VLC) were used along with the concept of Hydraulic Flow Units (HFU). Depositional facies, diagenetic alterations and sequence stratigraphic framework of the reservoir were used as a basis for the interpretation of geological heterogeneities. Accordingly, 14 microfacies types and 5 facies belts have been defined and indicate a low-angle (homoclinal) ramp model. Complex diagenesis history of these sequences includes marine, hypersaline and shallow to deep burial diagenesis that considerably altered the primary distribution of reservoir properties. A comparison of tested methods revealed that the CV is the best approach for quantification of heterogeneity in targeted reservoir. Consequently, the K4 and K2 are proposed as the most homogeneous units with a dominance of grain-supported facies of shoal setting, intense effects of hypersaline dissolution and lesser effect of compaction and cementation. The K3 and K1 units show high levels of heterogeneity resulting from the combined facies changes and diagenetic alterations (dolomitization, cementation and compaction). Finally, sequence stratigraphic framework of reservoir heterogeneities is discussed for all reservoir units and can be used as a basis for ongoing reservoir studies of the Permian–Triassic sequences in the Persian Gulf and the Middle East.

Complex co- and postseismic faulting of the 2017–2018 seismic sequence in western Iran revealed by InSAR and seismic data

The largest earthquake in the Zagros Mountains struck the city of Azgeleh on the Iran–Iraq border on 12 November 2017. This Mw 7.3 earthquake was followed by an intense seismic sequence. Implementing the double-difference earthquake location technique, we relocate 1069 events recorded by our local seismic network, deployed after the mainshock. The spatial distribution of the epicenters indicates linear alignments of the events nucleated along at least four notable clusters. The clusters are characterized by at least one significant earthquake, such as the Tazehabad earthquake of 25 August 2018 (Mw 5.9) along a dense, east–west trending cluster and the Sarpol-e Zahab earthquake of 25 November 2018 (Mw 6.3) along the cluster with a northeast–southwest trend. We use two-pass differential SAR interferometry (DInSAR) and Small BAseline Subset (SBAS) methods to study the coseismic permanent displacements of the Azgeleh, Tazehabad and Sarpol-e Zahab events as well as the one-year postseismic deformation field of the 2017–2018 seismic sequence, respectively. We use non-linear and linear optimization algorithms to derive the source geometry and the slip distribution along the fault planes. The inversion is conducted by introducing also seismological constraints, leading to the definition of a listric geometry for the Azgeleh mainshock rupture that accommodates the slip area at depth of 10–16 km along a sub-horizontal plane (dipping ~3°) and a low-angle (~16°) ramp. The thrust and dextral movements along this NNW-striking (~345°) fault have triggered a tear fault responsible for the Tazehabad event ruptured an east–west trending (~267°), north-dipping (~78°) sinistral shear fault. We present the dextral slip distribution of the Sarpol-e Zahab event along a NE-striking (~34°) fault, as a synthetic Riedel structure for the southern segment of the Khanaqin fault, dipping 63° to the southeast. We find the postseismic deformation field associated with the seismic sequence is not confined only to the mainshock source (the Azgeleh fault), but also develops along the Tazehabad and Sarpol-e Zahab faults. We additionally propose afterslip along a duplex, flat-ramp-flat structure down-dip and up-dip of the Azgeleh coseismic slip area. The up-dip afterslip develops onto the shallow detachment (~3°) at depth of ~8 km and the down-dip afterslip propagate onto the mid-crustal décollement level within the Pan-African basement. The Azgeleh, Tazehabad, Sarpol-e Zahab and Khanaqin faults mark the Lurestan Arc–Kirkuk Embayment sharp margin in the Northwest Zagros and play a key role in the lateral escape of the Lurestan Salient and vertical strain partitioning in the Zagros front.

Implications of sea-level rise in a modern carbonate ramp setting

This study addresses a gap in our understanding of the effects of sea-level rise on the sedimentary systems and morphological development of recent and ancient carbonate ramp settings. Many ancient carbonate sequences are interpreted as having been deposited in carbonate ramp settings. These settings are poorly-represented in the Recent. The study documents the present-day transgressive flooding of the Abu Dhabi coastline at the southern shoreline of the Arabian/Persian Gulf, a carbonate ramp depositional system that is widely employed as a Recent analogue for numerous ancient carbonate systems. Fourteen years of field-based observations are integrated with historical and recent high-resolution satellite imagery in order to document and assess the onset of flooding. Predicted rates of transgression (i.e. landward movement of the shoreline) of 2.5myr−1 (±0.2myr−1) based on global sea-level rise alone were far exceeded by the flooding rate calculated from the back-stepping of coastal features (10–29myr−1). This discrepancy results from the dynamic nature of the flooding with increased water depth exposing the coastline to increased erosion and, thereby, enhancing back-stepping. A non-accretionary transgressive shoreline trajectory results from relatively rapid sea-level rise coupled with a low-angle ramp geometry and a paucity of sediments. The flooding is represented by the landward migration of facies belts, a range of erosive features and the onset of bioturbation. Employing Intergovernmental Panel on Climate Change (Church et al., 2013) predictions for 21st century sea-level rise, and allowing for the post-flooding lag time that is typical for the start-up of carbonate factories, it is calculated that the coastline will continue to retrograde for the foreseeable future. Total passive flooding (without considering feedback in the modification of the shoreline) by the year 2100 is calculated to likely be between 340 and 571m with a flooding rate of 3.40–8.64myr−1. However, adopting the observation that global sea-level rise only accounts for 15% of the recorded shoreline retreat, this figure rises dramatically to a total likely dynamic flooding (considering modifications to the shoreline) of between 2.3 and 3.8km. Loss of microbial and mangal habitats will subject the exposed shoreline to increasing erosion. Shoreline retreat will threaten existing coastal infrastructure.

Lifestyles of the palaeoproterozoic stromatolite builders in the Vempalle Sea, Cuddapah Basin, India

The distribution and changing pattern of stromatolites in the Vempalle Formation of the Cuddapah Basin, India, have been investigated with emphasis on external morphologies, internal fabrics, growth patterns and sediment associations. The stromatolitic limestone occurs in a low angle ramp type carbonate platform, with complete exposures from shallow shelf to basinal facies, provides record of changes in reef-building capacity of stromatolites with change in the depositional milieu. Changing pattern and style of the stromatolites clearly reflect depth partitioned growth of the microbial community. Small and scattered stromatolites, commonly with wavy parallel laminations or stratifications occurred in inner-ramp settings, where they are build up to the sea level. Tepee structure and desiccation cracks in associated sediments and salt pseudomorphs point to shallow water to occasional exposure condition. Large bioherms with columnar and conical stromatolites developed in the headland (mid ramp), which were in continuous interaction with strong waves of the open sea. Growth of the bioherms changed the landform with time and generated a high sloping edge with a sheer drop that extended out into the sea, forming distally steepened ramp. Outer ramp lithofacies characterized by thick layers of shale succession with thin beds of laterally persistent stromatolitic beds with low synoptic relief build-ups. These biotic components, along with the absence of wave-related structures, place the outer ramp below the base of wave action zone. Ooid banks developed in the mid ramp area are in the high surfing zone. High flux of nutrients and high-light show positive correlation; thus the high growth pattern is best observed within the photic zone where wave action is maximum. The ability to change from low synoptic relief algal laminites to high synoptic relief columnar stromatolites point to their adaptive power. The key factor that controlled the shape of these Palaeoproterozoic stromatolites is the dynamic interactions between mat growth, currents and sediment supply. Presence of concentric and radial fibrous ooids indicates change in climate and salinity of the sea.

Depositional paleoenvironments of the Lower Permian (upper Cisuralian) carbonate succession of Paso Hondo Formation in Chiapas State, southeastern Mexico

The Paso Hondo Formation outcrops around of the Chicomuselo region, Chiapas State, Mexico. It is a Permian lithostratigraphic unit mainly composed of massive limestone which has been dated for the Artinskian-Kungurian (late Cisuralian). A microfacies analysis carried out on the carbonate rocks of a stratigraphic section, allowed for the first time the recognition of the depositional conditions that prevailed in the Chicomuselo region at the end of the Cisuralian. The facies associations studied allowed identifying different marine paleoenvironments related with a homoclinal carbonate ramp. The presence of anhydrite nodules, a mud-dominated carbonate production in euphotic zone (precursor of mudstones and packstones) and the presence of a diverse fauna mainly composed of photic-independent biota (mollusks, bryozoans, brachiopods and crinoids), indicate that the studied section was deposited in a relatively uniform low angle ramp. Thus, facies of different environments of inner ramp were detected, including those of a lagoon close to a peritidal area, with periodical restricted or open circulation, and open waters deposits influenced by the storm zone. In addition, mid-ramp facies were also observed.Facies associations of the basal levels on the studied section were mainly correlated with lagoonal shallow marine environments, being ostracods, calcispheres and peloids the dominant allochems. In contrast, there is a shift upwards to facies of open waters and mid-ramp environments, characterized by abundant skeletal grains of bryozoans, brachiopods and crinoids. The paleoenvironments recorded through the stratigraphic section were related with specific bathymetries, having a general tend towards the sea level rise. This record coincides with the global transgression event occurred during the Early Permian which have also been described for coeval localities of Texas and New Mexico in the United States and western Venezuela.

Facies Analysis and Depositional Environment of Upper Permian Rocks in Darin Section, East-Central Iran

Deposits of Permian rocks in Kalmard Block are recognized with Khan Group, showing various characteristics in different outcrops. This group is made up of three informal formations, namely Chili, Sartakht and Hermez. Upper Permian deposits (Hermez Formation) are composed chiefly of lateritic and carbonate rocks. This formation is composed of 65.5 m lateritic soils, lateritic sandstone, limestone, dolomite and dolomitic limestone in the Darin section. This formation unconformably overlies middle Permian siliciclastic and evaporite deposits (Sartakht Formation), where as it is depicted underlying an erosional unconformity above lateritic deposits of lower Triassic (Sorkh Shale Formation). According to lithologic and microscopic investigations, the deposits of Hermez Formation can be divided into 1 siliciclastic petrofacies and 14 carbonate microfacies. Field observations, along with microscopic examinations, have resulted in identifying tidal flat, lagoon, shoal and open marine environments in the rocks of the studied formation. Vertical changes of microfacies and depth variation curve point to the high thickness of the microfacies of lagoon, tidal flat and shoal environments and low thickness of open marine microfacies. Hermez Formation rocks in Darin section are deposited in a low-angle homoclinal ramp, mostly in the inner ramp, located in the southern Paleo-Tethys Ocean.

Fracture distribution along an Upper Jurassic carbonate ramp, NE Spain

Understanding the distribution of natural fractures in sedimentary systems is of high relevance for the exploration and production of fluids in the subsurface. This study focuses on a Kimmeridgian mixed siliciclastic-carbonate ramp system, which is part of the Jurassic limestones outcropping in the northeastern Iberian Chain. The study area is located north of the Ricla village, fifty kilometers southwest of Zaragoza. The outcrop stretches over six kilometers in length, it allows for recording detailed fracture patterns and facies variations. A GIS-based software-package 'DigiFract' is used to measure and digitize fractures in vertical outcrops. Fracture orientation measurements obtained from vertical and horizontal outcrops are used to create a conceptual three-dimensional image of the fracture distribution of the mixed ramp system. The fracture data are processed by integrating outcrop logs and sample-data obtained from thin-sections and rock property analysis. The continuous facies belts of the shallow low-angle ramp system show limited lateral variations. The studied sediments are subdivided in four main facies tracts (FT's): (FT-1) Bioclastic siltstones, (FT-2) alternating sandy limestones and marls, (FT-3) cross-bedded and channelled oolitic-bioclastic sand- and grainstones and (FT-4) coralgal float-to mudstones with eventites. Measured fracture orientations of the vertical and horizontal outcrops are identical throughout the entire exposure. Two main fracture sets are identified, the first set (Set 1) has a N–S direction and the second set (Set 2) has a NE–SW direction. The lateral homogeneity of the sedimentary system, thus facies, layer thickness and slope angle, can be translated to the observed fracture patterns. Within one single facies belt, fractures tend to behave the same in the proximal, middle and distal part of the ramp system. However, vertical facies variations are an important factor for the measured vertical fracture-heterogeneity. Fine-grained mud-supported facies correspond to periods of a sea-level highstand; coarse-grained cemented facies on the other hand are related to a sea-level lowstand. The physical contrast of the sediments caused by sea-level fluctuations forces fractures to solely concentrate in the brittle layers. Fracture density and termination patterns observed on this ramp stand in contrast to fracture geometries observed in flat-topped carbonate platforms. Lateral facies heterogeneity and platform anatomy of flat-topped carbonate platforms are key parameters for the eventual fracture distribution. For the studied mixed ramp-type system the vertical facies variations are key parameter.

Seismic chronostratigraphy and basin development at a Mid-Cretaceous intrashelf basin margin

The Mid-Cretaceous Wasia Formation represents one of the most productive hydrocarbon carbonate sequences in the Middle East. In Saudi Arabia, limited integrated studies have assessed the complexity and spatial distribution of its reservoir depositional systems near to an intrashelf basin margin. This study was focused on an assessment of its 3D seismic chronostratigraphy by integrating key well and seismic data to evaluate the gross depositional history of the mixed carbonate-clastic system. A seismic chronostratigraphy approach was introduced to assess the geometric relationships of depositional cycles and lithologic associations in response to relative sea-level changes. The main step entailed the correlation of well log chronostratigraphy from core and biostratigraphy interpretations with closely spaced, semiautomatically generated seismic horizons from seismic reflection data. A 3D seismic chronostratigraphy cube was built to interactively assess the basin history through the Mid-Cretaceous stages. Seismic sequences were selected to assess isopachs and gross depositional trends for demarcation of shelf, shelf margin, and slope from horizon attributes including root-mean-square amplitude and frequency decomposition. The results found the oldest Albian Safaniya member to be a generalized low-angle ramp shelf slope with thinning of sequences toward an intrashelf basin. Later, in the Albian, Cenomanian, and Turonian, carbonate factories developed distinct seismic sequences with steepened prograding systems in the Mauddud, Ahmadi/Rumaila, and Mishrif members at the intrashelf basin margin. Generally, areas immediately landward of the shelf margin possessed the most favorable reservoir lithologies of rudist-bearing platform carbonates, with source-rock distribution confined to basinal equivalents of sequences. The seismic chronostratigraphy approach proved to be fundamental to understanding the Wasia carbonate depositional system because it provided a technique to assess the varied stratal architectures of the main productive sequences. The integrated technique represents a unique methodology for exploration targeting of conventional reservoirs and unconventional source rocks.

Microfacies Analysis, Environmental Reconstruction and Determination of Sea-Level Fluctuations in Permian Deposits of the Kalmard Area, Khan Gorup (Kuh-e-Ahaki Section)

Lower Permian deposits in the Kalmard block, which are known as Khan Formation, enjoys different characteristics in various outcrops. This formation, from bottom to top, is interpreted to be a group of three formations, namely Chili, Sartakht and Hormoz. It is mainly composed of carbonate rocks, dolomite, sandstones, shale and marl. The Kuh-e-Ahaki section in Chili formation is 87 meters thick. According to lithological and microscopic studies, deposits of Khan Gorup in Kuh-e-Ahaki section are deposited in beach sub-environments, open to semi-restricted lagoon sub-environments, shoal sub-environment and open submarine sub-environment in the rocks of this part of formation, triggering fluctuations in sea level. Vertical changes of microfacies and the curve of its depth change point to the high thickness of the facies of shoal and open marine sub-environments. The rocks of Khan Gorup are deposited in a low-angle homoclinal ramp located in southern Paleotethys Ocean.

Unconventional-Reservoir Development in Mexico

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 164545, ’Unconventional-Reservoir Development in Mexico: Lessons Learned From the First Exploratory Wells,’ by P.A. Parra, N. Rubio, C. Ramirez, B.D. Guerra, V.A. Exler, I.R. Campos, M.D. Trejo, J. Olguin, C.H. Vargas, R. Valbuena, D.F. Soler, M.I. Weimann, V. Lujan, and P. Bonningue, Schlumberger, and P.G. Reyes, R. Martinez, R. Munoz, E. Rodriguez, and M. Garcia, Pemex, prepared for the 2013 SPE Unconventional Resources Conference—USA, The Woodlands, Texas, USA, 10-12 April. The paper has not been peer reviewed. In 2010, exploration of gas-rich and possibly liquid-rich shale reservoirs began in northern Mexico. A two-stage integrated workflow was developed to achieve set objectives. The drilling stage used a petrophysical and geomechanical static model to identify the most prospective interval in the reservoir, define the best drilling azimuth and landing point, and reduce drilling risk. Real-time geosteering was implemented to achieve the targeted navigation window. In the completion stage, reservoir-centric completion-and-stimulation software was used to optimize completion and stimulation design. Introduction The initial target formation was the Upper Cretaceous Eagle Ford; the second was a Jurassic formation locally called Pimienta. The Eagle Ford shale is a Late Cretaceous hydrocarbon-producing formation composed of organic-rich calcareous mudstones with mineralogical composition with ranges of 5–20% quartz, 15–25% clay, and 65–80% carbonate, and with a total-organic-carbon (TOC) range of 1–6%. The Eagle Ford shale is overlain by the Austin chalk and overlies the Buda limestone. It has been divided into two units, a lower unit deposited during transgressive sequences and an upper unit deposited in a regressive sequence. Both units were deposited in a low-angle sloping ramp. As of July 2012, five wells had been drilled in the Eagle Ford formation: AEF, BEF, CEF, DEF, and EEF. The origins of the Pimienta formation date to the Late Jurassic. It is an organic-rich source rock with potential for hydrocarbon production as an unconventional reservoir. Its mineralogical composition exhibits ranges of 20–40% quartz, 25–45% clay, and 20–40% carbonate, with an average TOC range of 3–4%. As of July 2012, one well (AP) had been drilled in the Pimienta formation.

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.

Upper‐crustal extension during oblique collision: the Temsamane extensional detachment (eastern Rif, Morocco)

Terra Nova, 24, 505–512, 2012AbstractThe subducted North Maghrebian passive margin was exhumed in the Tortonian (11–7 Ma) by an upper‐crustal brittle‐ductile extensional detachment and brittle low‐angle normal faults in a continental subduction transform setting. The Temsamane detachment in the eastern Rif is defined by a ductile shear zone approximately 100‐m thick with a low‐angle ramp geometry that cuts down into the Temsamane fold‐nappe stack. The shear zone shows south‐westward kinematics and separates lower‐greenschist (≈400 °C) metapelites of the Temsamane units below from the anchizone and diagenetic rocks (<300–200 °C) of the Ketama‐Aknoul units above. To the east, the detachment becomes brittle, branching into a listric‐fan that cuts through 10–6 Ma sediments and volcanoclastics in the Tres Forcas cape. Both the North Maghrebian and the South Iberian subducted passive margins were exhumed in the Betic and Rif branches of the Gibraltar arc by SW‐directed brittle‐ductile detachments during the late Miocene in an oblique collisional setting.

Tectonic triggering of slump sheets in the Upper Cretaceous carbonate succession of the Porto Selvaggio area (Salento peninsula, southern Italy): Synsedimentary tectonics in the Apulian Carbonate Platform

Soft-sediment deformation structures crop out in the Upper Cretaceous carbonate succession in Porto Selvaggio cove in the western Salento peninsula, Apulian foreland, southern Italy. The deformed interval is about 13m thick and occurs between shallow-water limestones and dolostones formed in peritidal and shallow subtidal environments. It comprises well-bedded grey mudstones interlayered with dark grey laminated microbioclastic wackestones characterized by couplets of closely spaced dark and bright laminae marked by the parallel orientation of calcareous microbioclasts and thin-shelled bivalves. The low biological diversity, scarcity of burrowing biota, and presence of a well preserved fish fauna provide evidence of anoxic conditions occurring in morphological depressions within the platform, and a stagnant, stratified water body affected by weak bottom currents, indicating the sudden development of a localised and short-lived intraplatform basin. Two soft-sediment deformation horizons (slump sheets) separated by undeformed limestones with similar facies occur in this part of the succession. The lower, thicker slump sheet (1.0–1.3m thick) contains asymmetric and box folds. Well-developed décollement surfaces (locally containing thick brecciated zones) cut the folds, forming small-scale thrust-sheets and indicating mixed plastic to brittle behaviour. The upper, thinner slump sheet (0.25–0.35m thick) contains only asymmetric folds, indicating plastic behaviour only. The differences in deformation style are attributed to differences in facies. Measurements of fold-axis orientations in the slump sheets show that they moved in similar directions, recording the development of a local, gently dipping palaeoslope. Autogenic (internal) trigger mechanisms are ruled out by a detailed consideration of facies. The slump sheets were triggered by allogenic, tectonic effects, either the weakening of sediment by seismic activity or the tectonically induced steepening of slopes, or a combination of both. Tectonically induced steepening is consistent with localised and sudden vertical facies changes related to the creation of an intraplatform basin. The occurrence of slump sheets in carbonate platform successions is unusual since carbonate platforms are normally associated with shelves or low-angle ramps.

The changing climate framework and depositional dynamics of Triassic carbonate platforms from the Dolomites

The Triassic of the Dolomites offers a valuable opportunity to investigate the relationships between climate fluctuations and the changing depositional dynamics of carbonate platforms. The reconstruction of the large palaeoclimatic modification is based on the synthesis of multiple sedimentological, palaeobotanical, pedological, and geochemical proxies. The Triassic climate fluctuations were generally fast in nature and were probably associated with a latitudinal shift of the monsoon belts. During the Triassic, the Dolomites were placed at an intertropical northern latitude of about 16–18° and were generally dominated by dry climate and elevated temperature. At least five pulses toward moister climate are however documented, dated to Early Olenekian, Middle Anisian, Late Ladinian, “Middle” Carnian, and Late Norian times. Within this variable climatic scenario, the carbonate systems record a global evolutionary trend from Late Permian–Early Triassic regional shelves, rich in loose micrites and bio-calcarenites, to Anisian–Ladinian, synsedimentary cemented, steep sided (35–40°), high-relief platforms, and back to Carnian low angle ramps. Moist transgressive episodes were surprisingly prone to the spreading of corals and other colonial reef organisms, probably because of the reduced space competition by syndepositional cementation structures. The Permo-Triassic boundary extinction dramatically impacted on the calcified organism communities, but the carbonate depositional architecture nevertheless stayed almost unaffected through the boundary. The Early Triassic and earliest Anisian were commonly associated with hyperhaline, sometimes evaporitic conditions. Intervals of moister climate and efficient river discharge are however documented by earliest Induan and early Olenekian sediments. The latter terrigenous deposits matched a crisis in the carbonate production. After a return to arid conditions, during the Anisian, a fluctuation toward moister climate developed, associated to a massive continental vegetation cover. The Anisian tectonic activation of the region fragmented the previous carbonate shelf, triggering the development of three generations of fast prograding, but globally backstepping, isolated platforms, recording an increasing role of the synsedimentary cementation. A dry climate dominated the Late Anisian and Early Ladinian interval. At the time, a fast subsidence pulse was associated with platform drowning and basinal anoxia. Only a few aggrading pinnacles were able to survive, providing the nuclei for larger prograding platforms. During a dry climate interval, massive syndepositional cementation generated a major source of carbonate. Carbonate production stayed active through the Ladinian magmatic phase, but the onset of volcanism was associated with a large modification in the platform carbonate facies, which became dominated by automicrites. Moist climate phases are well documented again during the Late Ladinian, followed by a return to dry conditions. The Late Ladinian–Early Carnian platforms prograded toward the eastern Dolomites depocentre, eventually infilling almost the whole of the accommodation space. The middle portion of the Carnian recorded renewed, sharply moist phases, associated with the demise of the rimmed platforms and with the return to loose sediment ramps. At least four separated humid fluctuations can be identified in this Carnian interval alone. Climate then returned to persistent aridity, until the Late Norian moist phase.

Stratigraphic architecture and fracture-controlled dolomitization of the Cretaceous Khami and Bangestan groups: an outcrop case study, Zagros Mountains, Iran

Abstract The Barremian–Aptian upper Khami Group and Albian–Campanian Bangestan Group have been studied at outcrop in Lurestan, SW Iran. The upper Khami Group comprises a thin deltaic wedge (Gadvan Fm) transgressively overlain by shelfal carbonates (Dariyan Fm). The Dariyan Fm can be divided into lower and upper units separated by a major intra-Aptian fracture-controlled karst. The top of the Daryian Fm is capped by the Arabian plate-wide Aptian–Albian unconformity. The overlying Bangestan Group includes the Kazhdumi, Sarvak, Surgah and Ilam formations. The Kazhdumi Fm represents a mixed carbonate-clastic intrashelf basin succession, and passes laterally (towards the NE) into a low-angle Orbitolina- dominated muddy carbonate ramp/shoal (Mauddud Mbr). The Mauddud Mbr is capped by an angular unconformity and karst of latest Albian–earliest Cenomanian age. The overlying Sarvak Fm comprises both low-angle ramp and steeper dipping (5–10°) carbonate shelf/platform systems. Three regionally extensive karst surfaces are developed in the latest Cenomanian–Turonian interval of the Sarvak Fm, and are interpreted to be related to flexure of the Arabian plate margin due to the initiation of intra-oceanic deformation. The Surgah and Ilam Fm represent clastic and muddy carbonate ramp depositional systems respectively. Both The Khami and Bangestan groups have been affected by spectacularly exposed fracture-controlled dolomitization. Dolomite bodies are 100 m to several km in width, have plume-like geometry, with both fracture (fault/joint) and gradational diagenetic contacts with undolomitized country rock. Sheets of dolomite extend away from dolomite bodies along steeply dipping fault/joint zones, and as strata-bound bodies preferentially following specific depositional/diagenetic facies or stratal surfaces. There is a close link between primary depositional architecture/facies and secondary dolomitization. Vertical barriers to dolomitization are low permeability mudstones, below which dolomitizing fluids moved laterally. Where these barriers are cut by faults and fracture corridors, dolomitization can be observed to have advanced upwards, indicating that faults and joints were fluid migration conduits. Comparisons to Jurassic–Cenozoic dolomites elsewhere in Iran, Palaeozoic dolomites of North America and Neogene dolomites of the Gulf of Suez indicate striking textural, paragenetic and outcrop-scale similarities. These data imply a common fracture-controlled dolomitization process is applicable regardless of tectonic setting (compressional, transtensional and extensional).

High-resolution seismic stratigraphy of the Shu'aiba and Natih formations in the Sultanate of Oman: implications for Cretaceous epeiric carbonate platform systems

Abstract The Shu'aiba and Natih Formation carbonates are important hydrocarbon reservoirs in the Sultanate of Oman. They consist of stacked deepening and shallowing-upward depositional cycles within an extensive middle Cretaceous epeiric carbonate platform. Geological models for these units traditionally assume a layer-cake stratigraphy and a high lateral continuity of facies. This is based on the assumption that epeiric platforms consist of ramps with low depositional gradients, broad facies belts and gradual facies transitions. However, high-resolution 3D seismic data covering large areas of the platform have revealed a more complicated internal stratigraphic architecture and have led to a new geometrical model for these epeiric platform systems. The transgressive part of the cycles is dominated by a low angle ramp depositional profile with localized development of algal-dominated mounds. Differential carbonate growth led to a topography of shallow carbonate shoals and intra-platform ‘basins’ with water depths reaching several tens to 100 m. During the regressive part of the cycles these basins were progressively filled-in by prograding carbonate rudist shoal complexes with depositional slopes of 0.5° to more than 30°. Clinoform belts on seismic show a wide range of progradational geometries ranging from closely spaced, laterally continuous ‘tramlines’ to irregular wedges and noses. The cycle tops are characterized by bedrock incisions and the influx of fine clastic sediments that fill in remnants of the intra-platform basins. The seismic images show that previous stratigraphic models for these carbonates oversimplified and flattened the stratigraphy. As a result the stratigraphic trapping potential and the internal reservoir heterogeneity for these systems has been underestimated.

Marine carbonate systems in the Sarmatian (Middle Miocene) of the Central Paratethys: the Zsámbék Basin of Hungary

AbstractThe study of eight stratigraphic sections at the margin of the semi‐enclosed Zsámbék Basin (Hungary) allows the sedimentary anatomy of oolitic–bioclastic systems in the Sarmatian of the Central Paratethys to be reconstructed. The mollusc, foraminiferal and ostracod associations indicate that the carbonate systems are Latest Badenian to Late Sarmatian in age. The Lower–Upper Sarmatian deposits are organized in superimposed subaqueous dunes prograding towards the basin on a low‐angle ramp. During the Late Sarmatian, the ramp underwent subaerial erosion linked to a moderate relative fall in sea‐level. Lagoonal deposits were later formed and microbial–nubeculariid–bryozoan–serpulid buildups were emplaced. The ‘abnormal’ marine conditions of the Sarmatian, conducive to the development of a poorly diversified flora and fauna and dominant non‐skeletal grains, are linked to fluctuating salinities, mesotrophic to eutrophic conditions and perhaps high alkalinity.

The origin and evolution of footslope ramps in the sandstone desert environment of south-west Jordan

In the Cambrian–Ordovician sandstones of south-west Jordan, the spectacular scenery that has developed under arid conditions includes the presence of low-angle (<10°) ramp slopes lying beneath vertical cliffs and running down to adjacent wadi beds or playas. The ramps are depositional features built of fine debris with occasional larger clasts and may be mantled by weathered boulders in their upper parts. These ramps are especially well displayed in the Ishrin Sandstone, but seem to be the landforms rarely, if ever reported from desert environments. The analysis of ramp geomorphology and sedimentology, and of rock slopes above the ramps, strongly suggests that most of the material that makes up the ramps is provided by catastrophic rockfalls. Rockfall debris is subjected to efficient salt weathering and quickly disintegrates into easily transported fine-grained material which, redistributed by surface wash, builds the ramps. The uniqueness of the ramps arises from a combination of lithological and environmental factors, the former accounting for the high frequency of rockfalls in the Ishrin sandstone, the latter being responsible for the efficacy of salt weathering and surface wash.

Aphotic zone carbonate production on a Miocene ramp, Central Apennines, Italy

The lower Miocene Latium–Abruzzi platform was a low-angle ramp that developed under tropical-to-subtropical conditions, but was dominated by bryomol and rhodalgal sediment associations. The Aquitanian to Serravallian sequence described here paraconformably overlies the Cretaceous limestones. It consists of a lowstand systems tract, a transgressive systems tract and a highstand systems tract. Based on facies analysis and on the light dependence of biotic associations, the ramp is divided into three parts: an inner ramp, a middle ramp and an outer ramp. The inner ramp facies are represented by a few metres of coral framestone, rhodolith floatstone–rudstone and balanid macroids floatstone without wave-related structures. The middle ramp consists of structureless bioclastic grainstone to packstone, floatstone and rudstone with rhodoliths and larger foraminifera. The outer ramp facies—proximal sector—are composed of crudely stratified bryozoan-dominated packstone to floatstone which extend over the whole platform. The outer ramp facies—intermediate sector—are represented by wackestone, packstone and rarely grainstone with foraminifera and echinoid fragments. The final depositional profile of the ramp was strongly influenced by the main organisms producing sediment. During the lowstand, the resulting profile is a ramp type. During the transgressive phase, the rapid spreading of the outer ramp facies belt, as a consequence of the enhanced productivity of the light-independent biota, is believed to be promoted by a change from oligotrophic to eutrophic conditions. Climate and/or tectonics are presumed to have played an important role in continental runoff and then in the nutrients delivery. During the highstand phase, the system returns to rates of production uniform throughout the platform. The high rates of carbonate production occurring in the aphotic zone are quite unusual in tropical settings and represent a provocative trend in apparent contrast with the common idea about the locus of the most significant carbonate production, as derived from the “tropical model”. The example described here shows that carbonate production and accumulation below the photic zone might be higher than in the shallow euphotic zone even in tropical conditions.