TY - JOUR A1 - Huck, Stefan A1 - Wohlwend, Stephan A1 - Coimbra, Rute A1 - Christ, Nicolas A1 - Weissert, Helmut T1 - Disentangling shallow-water bulk carbonate carbon isotope archives with evidence for multi-stage diagenesis BT - an in-depth component-specific petrographic and geochemical study from Oman (mid-Cretaceous) JF - The depositional record N2 - Disentangling shallow‐water bulk carbonate carbon isotope archives into primary and diagenetic components is a notoriously difficult task and even diagenetically screened records often provide chemostratigraphic patterns that significantly differ from global signals. This is mainly caused by the polygenetic nature of shallow‐water carbonate substrates, local carbon cycle processes causing considerable neritic–pelagic isotope gradients and the presence of hiatal surfaces resulting in extremely low carbonate preservation rates. Provided here is an in‐depth petrographic and geochemical evaluation of different carbonate phases of a mid‐Cretaceous (Barremian–Aptian) shallow‐water limestone succession (Jabal Madar section) deposited on the tropical Arabian carbonate platform in Oman. The superposition of stable isotope signatures of identified carbonate phases causes a complex and often noisy bulk carbon isotope pattern. Blocky sparite cements filling intergranular pores and bioclastic voids evidence intermediate to (arguably) deep burial diagenetic conditions during their formation, owing to different timing or differential faulting promoting the circulation of fluids from variable sources. In contrast, sparite cements filling sub‐vertical veins reveal a rock‐buffered diagenetic fluid composition with an intriguing moderate enrichment in 13C, probably due to fractionation during pressure release in the context of the Miocene exhumation of the carbonate platform under study. The presence of abundant, replacive dedolomite in mud‐supported limestone samples forced negative carbon and oxygen isotope changes that are either associated with the thermal breakdown of organic matter in the deep burial realm or the expulsion of buried meteoric water in the intermediate burial realm. Notwithstanding the documented stratigraphically variable and often facies‐related impact of different diagenetic fluids on the bulk‐rock stable isotope signature, the identification of diagenetic end‐members defined δ13C and δ18O threshold values that allowed the most reliable ‘primary’ bulk carbon isotope signatures to be extracted. Most importantly, this approach exemplifies how to place regional shallow‐water stable isotope patterns with evidence for a complex multi‐stage diagenetic history into a supraregional or even global context. KW - Arabian carbonate platform KW - Barremian-Aptian KW - multi-stage diagenesis KW - shallow-water chemostratigraphy Y1 - 2017 U6 - https://doi.org/10.1002/dep2.35 SN - 2055-4877 VL - 3 SP - 233 EP - 257 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Huck, Stefan A1 - Stein, Melody A1 - Immenhauser, Adrian A1 - Skelton, Peter W. A1 - Christ, Nicolas A1 - Foellmi, Karl B. A1 - Heimhofer, Ulrich T1 - Response of proto-North Atlantic carbonate-platform ecosystems to OAE1a-related stressors JF - Sedimentary geology : international journal of applied and regional sedimentology N2 - Integrated biostratigraphic-chemostratigraphic studies provide evidence that the proto-North Atlantic realm witnessed major changes in carbonate platform production in the run-up of the Early Aptian oceanic anoxic event (OAE) 1a. Whereas pervasive growth of Lithocodium microencrusters represents an early harbinger of OAE1a-related environmental perturbation, the subsequent replacement of oligotrophic rudist-coral-nerineid by mesotrophic orbitolinid-oyster communities was clearly associated with the event itself. In order to test the supra-regional relevance of this major community replacement, two shallow-water sections in the southern Lusitanian Basin (Portugal) are investigated by means of geochemistry (carbon and oxygen isotopes), cement petrography and detailed sedimentological analysis. The focus is on a regional, prominent discontinuity surface (S4) at the transition between oligotrophic and mesotrophic carbonate platform production, which might indicate that the major biotic change could have been associated with a phase of non-sedimentation and possibly erosion. The studied sections (Sao Julia, Crismina) provide evidence that the major Early Aptian biotic turnover was preceded by numerous subordinate but significant changes in platform ecology, which mirrored a series of progressive short-term environmental changes in the course of OAE1. Several transient mass occurrences of orbitolinids indicate repeated phases of ecological stress arguably due to enhanced nutrient input and deepening. Small-scale sea-level changes at parasequence level below the major discontinuity surface are revealed by alternations of rudist assemblages dominated by clinger or recumbent forms as well as intercalated hardground and subaerial exposure stages. Expanded phases of subaerial exposure, however, can be largely ruled out following the geochemical and cement-petrographic data presented here. Enhanced continent-derived siliciclastic input characterising the lower orbitolinid-oyster dominated limestones is in support of a shift to more humid conditions during the middle Early Aptian. This is in line with palaeoclimatic data, which propose a southward movement of the mid-latitude arid climate belt during this time. The documented rapid replacement of oligotrophic assemblages by various environmental-stress adapted carbonate platform communities might be seen as explanation for ongoing Early Aptian proto-North Atlantic carbonate production during a time of widespread platform demise and drowning in the northern Tethyan realm. (C) 2014 Elsevier B.V. All rights reserved. KW - Aptian KW - Oceanic anoxic event 1a KW - Proto-North Atlantic KW - Discontinuity surfaces KW - Carbonate platform response KW - Carbon and oxygen isotopes Y1 - 2014 U6 - https://doi.org/10.1016/j.sedgeo.2014.08.003 SN - 0037-0738 SN - 1879-0968 VL - 313 SP - 15 EP - 31 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Amour, Frederic A1 - Mutti, Maria A1 - Christ, Nicolas A1 - Immenhauser, Adrian A1 - Agar, Susan M. A1 - Benson, Gregory S. A1 - Tomas, Sara A1 - Alway, Robert A1 - Kabiri, Lachen T1 - Capturing and modelling metre-scale spatial facies heterogeneity in a Jurassic ramp setting (Central High Atlas, Morocco) JF - Sedimentology : the journal of the International Association of Sedimentologists N2 - Each simulation algorithm, including Truncated Gaussian Simulation, Sequential Indicator Simulation and Indicator Kriging is characterized by different operating modes, which variably influence the facies proportion, distribution and association of digital outcrop models, as shown in clastic sediments. A detailed study of carbonate heterogeneity is then crucial to understanding these differences and providing rules for carbonate modelling. Through a continuous exposure of Bajocian carbonate strata, a study window (320 m long, 190 m wide and 30 m thick) was investigated and metre-scale lithofacies heterogeneity was captured and modelled using closely-spaced sections. Ten lithofacies, deposited in a shallow-water carbonate-dominated ramp, were recognized and their dimensions and associations were documented. Field data, including height sections, were georeferenced and input into the model. Four models were built in the present study. Model A used all sections and Truncated Gaussian Simulation during the stochastic simulation. For the three other models, Model B was generated using Truncated Gaussian Simulation as for Model A, Model C was generated using Sequential Indicator Simulation and Model D was generated using Indicator Kriging. These three additional models were built by removing two out of eight sections from data input. The removal of sections allows direct insights on geological uncertainties at inter-well spacings by comparing modelled and described sections. Other quantitative and qualitative comparisons were carried out between models to understand the advantages/disadvantages of each algorithm. Model A is used as the base case. Indicator Kriging (Model D) simplifies the facies distribution by assigning continuous geological bodies of the most abundant lithofacies to each zone. Sequential Indicator Simulation (Model C) is confident to conserve facies proportion when geological heterogeneity is complex. The use of trend with Truncated Gaussian Simulation is a powerful tool for modelling well-defined spatial facies relationships. However, in shallow-water carbonate, facies can coexist and their association can change through time and space. The present study shows that the scale of modelling (depositional environment or lithofacies) involves specific simulation constraints on shallow-water carbonate modelling methods. KW - 3D facies modelling KW - carbonate ramp KW - facies heterogeneity KW - Jurassic KW - modelling algorithms KW - scale Y1 - 2012 U6 - https://doi.org/10.1111/j.1365-3091.2011.01299.x SN - 0037-0746 VL - 59 IS - 4 SP - 1158 EP - 1189 PB - Wiley-Blackwell CY - Malden ER - TY - JOUR A1 - Christ, Nicolas A1 - Immenhauser, Adrian A1 - Amour, Frederic A1 - Mutti, Maria A1 - Preston, Rosalind A1 - Whitaker, Fiona F. A1 - Peterhänsel, Arndt A1 - Egenhoff, Sven O. A1 - Dunn, Paul A. A1 - Agar, Susan M. T1 - Triassic Latemar cycle tops - Subaerial exposure of platform carbonates under tropical arid climate JF - Sedimentary geology : international journal of applied and regional sedimentology N2 - The Triassic Latemar platform in the Dolomites, Italy, is the site of several ongoing controversies. Perhaps the most interesting debate focuses on apparent cyclic deposition within the Latemar platform, whose nature and duration are still open to debate. Further disagreement concerns the lack of meteoric diagenesis-related isotope shifts at cycle tops that bear circumstantial petrographic evidence for subaerial emergence. Here, an evaluation of the nature of Latemar cycle tops is presented combining evidence from previous work and new field, petrographic and geochemical data. Cycle tops are ranked according to increasing exposure duration and spatial extent: type I surfaces lacking unequivocal evidence of prolonged supratidal conditions; type II dolomite caps formed in warm, evaporitic, intertidal lagoonal waters followed by exposure of perhaps intermediate duration; type III clastic-rich, red calcareous horizons with some showing platform-wide extent, representing prolonged supratidal conditions, and type IV discontinuities in tepee belts, genetically related to type II and III surfaces, but likely representing shorter-lived exposure stages. Petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking. This is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that, where present, are here interpreted as marine-vadose in origin. The scarcity of carbon and oxygen isotope signatures commonly assigned to subaerial exposure stages is best explained in the context of mid-Triassic climate. The low latitude, tropical but arid setting of the Latemar, situated in the western extension of the Tethys ocean, its isolation from nearby continental areas and overall short-term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops. High amounts of aeolian clastic material beneath some cycle tops, along with high Fe and Mn elemental abundances argue for intermittent subaerial conditions. This study proposes an enhancement of the classical Allan and Matthews (1982) isotope model for subaerial exposure under strongly arid climates. As the subaerial exposure nature of Latemar cycle tops, and therefore eustasy as the cause for cyclicity, have been previously challenged due to the lack of meteoric-induced isotopic signatures, the outcome of this study is of significance for the ongoing Latemar stratigraphic controversy. KW - Triassic KW - Latemar KW - Subaerial exposure KW - Carbonate platforms KW - Carbon and oxygen stable isotopes KW - Diagenesis Y1 - 2012 U6 - https://doi.org/10.1016/j.sedgeo.2012.02.008 SN - 0037-0738 VL - 265 IS - 28 SP - 1 EP - 29 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Amour, Frederic A1 - Mutti, Maria A1 - Christ, Nicolas A1 - Immenhauser, Adrian A1 - Benson, Gregory S. A1 - Agar, Susan M. A1 - Tomas, Sara A1 - Kabiri, Lahcen T1 - Outcrop analog for an oolitic carbonate ramp reservoir a scale-dependent geologic modeling approach based on stratigraphic hierarchy JF - AAPG bulletin N2 - Considerable effort has been devoted to the development of simulation algorithms for facies modeling, whereas a discussion of how to combine those techniques has not existed. The integration of multiple geologic data into a three-dimensional model, which requires the combination of simulation techniques, is yet a current challenge for reservoir modeling. This article presents a thought process that guides the acquisition and modeling of geologic data at various scales. Our work is based on outcrop data collected from a Jurassic carbonate ramp located in the High Atlas mountain range of Morocco. The study window is 1 km (0.6 mi) wide and 100 m (328.1 ft) thick. We describe and model the spatial and hierarchical arrangement of carbonate bodies spanning from largest to smallest: (1) stacking pattern of high-frequency depositional sequences, (2) facies association, and (3) lithofacies. Five sequence boundaries were modeled using differential global position system mapping and light detection and ranging data. The surface-based model shows a low-angle profile with modest paleotopographic relief at the inner-to-middle ramp transition. Facies associations were populated using truncated Gaussian simulation to preserve ordered trends between the inner, middle, and outer ramps. At the lithofacies scale, field observations and statistical analysis show a mosaiclike distribution that was simulated using a fully stochastic approach with sequential indicator simulation. This study observes that the use of one single simulation technique is unlikely to correctly model the natural patterns and variability of carbonate rocks. The selection and implementation of different techniques customized for each level of the stratigraphic hierarchy will provide the essential computing flexibility to model carbonate settings. This study demonstrates that a scale-dependent modeling approach should be a common procedure when building subsurface and outcrop models. Y1 - 2013 U6 - https://doi.org/10.1306/10231212039 SN - 0149-1423 VL - 97 IS - 5 SP - 845 EP - 871 PB - American Association of Petroleum Geologists CY - Tulsa ER - TY - JOUR A1 - Tomas, Sara A1 - Homann, Martin A1 - Mutti, Maria A1 - Amour, Frederic A1 - Christ, Nicolas A1 - Immenhauser, Adrian A1 - Agar, Susan M. A1 - Kabiri, Lahcen T1 - Alternation of microbial mounds and ooid shoals (Middle Jurasssic, Morocco) - response to paleoenvironmental changes JF - Sedimentary geology : international journal of applied and regional sedimentology N2 - The occurrence of neritic microbial carbonates is often related to ecological refuges, where grazers and other competitors are reduced by environmental conditions, or to post-extinction events (e.g. in the Late Devonian, Early Triassic). Here, we present evidence for Middle Jurassic (Bajocian) microbial mounds formed in the normal marine, shallow neritic setting of an inner, ramp system from the High Atlas of Morocco. The microbial mounds are embedded in cross-bedded oolitic facies. Individual mounds show low relief domal geometries (up to 3 m high and 4.5 m across), but occasionally a second generation of mounds exhibits tabular geometries (<1 m high). The domes are circular in plan view and have intact tops, lacking evidence of current influence on mound preferred growth direction or distribution patterns, or truncation. The mound fades consists almost entirely of non-laminated, micritic thrombolites with branching morphologies and fine-grained, clotted and peloidal fabrics. Normal marine biota are present but infrequent. Several lines of evidence document that microbial mound growth alternates with time intervals of active ooid shoal deposition. This notion is of general significance when compared with modern Bahamian microbialites that co-exist with active sub-aquatic dunes. Furthermore, the lack of detailed studies of Middle Jurassic, normal marine shallow neritic microbial mounds adds a strong motivation for the present study. Specifically, Bajocian mounds formed on a firmground substratum during transgressive phases under condensed sedimentation. Furthermore, a transient increase in nutrient supply in the prevailing mesotrophic setting, as suggested by the heterotrophic-dominated biota, may have controlled microbial mound stages. KW - Microbial mounds KW - Thrombolites KW - Ooid shoals KW - Paleoenvironment KW - Jurassic Y1 - 2013 U6 - https://doi.org/10.1016/j.sedgeo.2013.05.008 SN - 0037-0738 VL - 294 SP - 68 EP - 82 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Christ, Nicolas A1 - Immenhauser, Adrian A1 - Amour, Frederic A1 - Mutti, Maria A1 - Tomas, Sara A1 - Agar, Susan M. A1 - Alway, Robert A1 - Kabiri, Lahcen T1 - Characterization and interpretation of discontinuity surfaces in a Jurassic ramp setting (High Atlas, Morocco) JF - Sedimentology : the journal of the International Association of Sedimentologists N2 - Discontinuity surfaces are widely recognized but often poorly understood features of epeiric carbonate settings. In sedimentary systems, these features often represent hiatus surfaces below biostratigraphic resolution and may represent a considerable portion of the time contained in the sediment record. From an applied perspective, discontinuities may represent horizontal flow barriers and result in reservoir compartmentalization. Here, a total of 80 condensed surfaces (S1), firmgrounds (S2) and hardgrounds (S3) from a Jurassic (Middle and Upper Bajocian Assoul Formation) ramp setting of the High Atlas in Morocco are carefully documented with respect to their morphology, their secondary impregnation by Fe and Mn oxides and phosphates and their palaeoecological record. A statistical frequency distribution of two surfaces of the S1 type, 1.1 surfaces of the S2 type and 0.4 surfaces of the S3 type per 10 section metres is observed along a 220 m long carbonate succession. Based on two stratigraphically and spatially separated study windows and correlative sections, the stratigraphic frequency distribution, the lateral extent and the nature of facies change across discontinuities are documented in a quantitative manner. Specific features of the study site include the considerable stratigraphic thickness of the Assoul Formation and the conspicuous absence of subaerial-exposure-related features. Based on the data presented here, firmground and hardground surfaces are best interpreted as maximum-regression-related features. Relative sea-level lowstand results in a lowered wave base, and wave orbitals and currents result in sea floor omission and lithification. Care must be taken to avoid overly simplistic interpretations, as differences in bathymetry and carbonate facies result in marked changes in discontinuity characteristics in proximal-distal transects. The data shown here are of significance for those concerned with the interpretation of shoal water carbonate environments and are instrumental in the building of more realistic carbonate reservoir flow models. KW - Atlas Mountains KW - carbonate ramp KW - discontinuity surfaces KW - hardgrounds KW - hydrodynamic level KW - Jurassic KW - palaeoecology KW - relative sea-level Y1 - 2012 U6 - https://doi.org/10.1111/j.1365-3091.2011.01251.x SN - 0037-0746 VL - 59 IS - 1 SP - 249 EP - 290 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Agada, S. A1 - Chen, F. A1 - Geiger, S. A1 - Toigulova, G. A1 - Agar, Susan M. A1 - Shekhar, R. A1 - Benson, Gregory S. A1 - Hehmeyer, O. A1 - Amour, Frédéric A1 - Mutti, Maria A1 - Christ, Nicolas A1 - Immenhauser, A. T1 - Numerical simulation of fluid-flow processes in a 3D high-resolution carbonate reservoir analogue JF - Petroleum geoscience N2 - A high-resolution three-dimensional (3D) outcrop model of a Jurassic carbonate ramp was used in order to perform a series of detailed and systematic flow simulations. The aim of this study was to test the impact of small- and large-scale geological features on reservoir performance and oil recovery. The digital outcrop model contains a wide range of sedimentological, diagenetic and structural features, including discontinuity surfaces, shoal bodies, mud mounds, oyster bioherms and fractures. Flow simulations are performed for numerical well testing and secondary oil recovery. Numerical well testing enables synthetic but systematic pressure responses to be generated for different geological features observed in the outcrops. This allows us to assess and rank the relative impact of specific geological features on reservoir performance. The outcome documents that, owing to the realistic representation of matrix heterogeneity, most diagenetic and structural features cannot be linked to a unique pressure signature. Instead, reservoir performance is controlled by subseismic faults and oyster bioherms acting as thief zones. Numerical simulations of secondary recovery processes reveal strong channelling of fluid flow into high-permeability layers as the primary control for oil recovery. However, appropriate reservoir-engineering solutions, such as optimizing well placement and injection fluid, can reduce channelling and increase oil recovery. Y1 - 2014 U6 - https://doi.org/10.1144/petgeo2012-096 SN - 1354-0793 VL - 20 IS - 1 SP - 125 EP - 142 PB - Geological Soc. Publ. House CY - Bath ER - TY - JOUR A1 - Shekhar, R. A1 - Sahni, I. A1 - Benson, Gregory S. A1 - Agar, Susan M. A1 - Amour, Frédéric A1 - Tomas, Sara A1 - Christ, Nicolas A1 - Alway, Robert A1 - Mutti, Maria A1 - Immenhauser, A. A1 - Karcz, Z. A1 - Kabiri, L. T1 - Modelling and simulation of a Jurassic carbonate ramp outcrop, Amellago, High Atlas Mountains, Morocco JF - Petroleum geoscience N2 - Carbonate reservoirs pose significant challenges for reservoir modelling and flow prediction due to heterogeneities in rock properties, limits to seismic resolution and limited constraints on subsurface data. Hence, a systematic and streamlined approach is needed to construct geological models and to quickly evaluate key sensitivities in the flow models. This paper discusses results from a reservoir analogue study of a Middle Jurassic carbonate ramp in the High Atlas Mountains of Morocco that has stratigraphic and structural similarities to selected Middle East reservoirs. For this purpose, high-resolution geological models were constructed from the integration of sedimentological, diagenetic and structural studies in the area. The models are approximately 1200 x 1250 m in size, and only faults (no fractures) with offsets greater than 1 m are included. Novel methods have been applied to test the response of flow simulations to the presence or absence of specific geological features, including proxies for hardgrounds, stylolites, patch reefs, and mollusc banks, as a way to guide the level of detail that is suitable for modelling objectives. Our general conclusion from the study is that the continuity of any geological feature with extreme permeability (high or low) has the most significant impact on flow. Y1 - 2014 U6 - https://doi.org/10.1144/petgeo2013-010 SN - 1354-0793 VL - 20 IS - 1 SP - 109 EP - 123 PB - Geological Soc. Publ. House CY - Bath ER - TY - JOUR A1 - Christ, Nicolas A1 - Maerz, Sven A1 - Kutschera, Edgar A1 - Kwiecien, Ola A1 - Mutti, Maria T1 - Palaeoenvironmental and diagenetic reconstruction of a closed-lacustrine carbonate system BT - the challenging marginal setting of the Miocene Ries Crater Lake (Germany) JF - Sedimentology : the journal of the International Association of Sedimentologists N2 - Chemostratigraphic studies on lacustrine sedimentary sequences provide essential insights on past cyclic climatic events, on their repetition and prediction through time. Diagenetic overprint of primary features often hinders the use of such studies for palaeoenvironmental reconstruction. Here the potential of integrated geochemical and petrographic methods is evaluated to record freshwater to saline oscillations within the ancient marginal lacustrine carbonates of the Miocene Ries Crater Lake (Germany). This area is critical because it represents the transition from shoreline to proximal domains of a hydrologically closed system, affected by recurrent emergent events, representing the boundaries of successive sedimentary cycles. Chemostratigraphy targets shifts related to subaerial exposure and/or climatic fluctuations. Methods combine facies changes with C-13-O-18 chemostratigraphy from matrix carbonates across five closely spaced, temporally equivalent stratigraphic sections. Isotope composition of ostracod shells, gastropods and cements is provided for comparison. Cathodoluminescence and back-scatter electron microscopy were performed to discriminate primary (syn-)depositional, from secondary diagenetic features. Meteoric diagenesis is expressed by substantial early dissolution and dark blue luminescent sparry cements carrying negative C-13 and O-18. Sedimentary cycles are not correlated by isotope chemostratigraphy. Both matrix C-13 and O-18 range from ca -75 to +40 parts per thousand and show clear positive covariance (R=097) whose nature differs from that of previous basin-oriented studies on the lake: negative values are here unconnected to original freshwater lacustrine conditions but reflect extensive meteoric diagenesis, while positive values probably represent primary saline lake water chemistry. Noisy geochemical curves relate to heterogeneities in (primary) porosity, resulting in selective carbonate diagenesis. This study exemplifies that ancient lacustrine carbonates, despite extensive meteoric weathering, are able to retain key information for both palaeoenvironmental reconstruction and the understanding of diagenetic processes in relation to those primary conditions. Also, it emphasizes the limitation of chemostratigraphy in fossil carbonates, and specifically in settings that are sensitive for the preservation of primary environmental signals, such as lake margins prone to meteoric diagenesis. KW - Lacustrine carbonates KW - meteoric diagenesis KW - Miocene KW - ostracod shells KW - palaeoenvironmental reconstruction KW - sedimentary cycles KW - subaerial exposure KW - C-13 and O-18 chemostratigraphy Y1 - 2017 U6 - https://doi.org/10.1111/sed.12401 SN - 0037-0746 SN - 1365-3091 VL - 65 IS - 1 SP - 235 EP - 262 PB - Wiley CY - Hoboken ER -