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High-resolution bathymetric and shallow seismic data along the northeast Red Sea margin reveal a previously disregarded mechanism for carbonate platform drowning at a steep-flanked rift basin. At the seafloor, salt extrusions highlight the influence of extensional salt tectonics, with a salt flow from the southern flank of the Al Wajh carbonate platform that likely originates from below. Salt-flow direction, morphology, and kilometer-sized slumps and rotated blocks indicate platform-margin disintegration and rafting of platform blocks toward the southwest. The outlines of several smaller detached or semi-detached carbonate platforms to the south of the main platform can be refitted to the larger platform margin by counter-moving the direction of mass wasting. Several platforms, reaching heights above the seafloor of up to 650 m, are partially or fully submerged in the mesophotic zone and appear to be in danger of drowning. We conclude that the southern outer rim of the Al Wajh platform is breaking apart owing to salt withdrawal, which indicates that carbonate platforms on top of salt sequences grow on mobile ground, leading to platform disintegration, basinward rafts, and the demise of broken-off, smaller pieces of platform. Salt displacement also controls the growth geometries of individual platform rafts, with keep-up reef growth (growth rate equal to sea-level rise) and drowning occurring in close spatial proximity. Therefore, the interplay between salt diapirism and platform growth is not limited to platforms growing on the apexes of diapirs and is more complex than previously thought.

A supplemental figure showing rafting dispersed isolated platforms of the southern Al Wajh area based on Figure 2A, and restoration of the present-day situation to a pre-extensional state.

The Red Sea is a unique place to study a young oceanic rift basin and the interplay between magma and tectonics at a young divergent plate boundary. The spreading rate of the Red Sea rift changes from ~17 mm/yr in the south to ~7 mm/yr in the north, and so does the morphology. The southern Red Sea is a continuous and well-developed oceanic rift, whereas the so-called deeps characterize the central portion with oceanic crust separated by shallower inter-trough zones, and the northern part contains more widely spaced deeps with extensive areas covered by sediments in between. While the central Red Sea morphology has been extensively studied, the structure of the northern Red Sea and its link to the central Red Sea are less clear. Indeed, the northern Red Sea rift, marked at its southern end by Mabahiss Deep, is offset by about 60 km to the central Red Sea axis by the still poorly understood Zabargad Fracture Zone.Here we aim to improve the understanding of the volcano-tectonic setting of the Mabahiss Deep area with new high-resolution bathymetric data from multiple multibeam surveys with R/V Thuwal and R/V Pelagia. Our results show that the 15 km long, 9 km wide, and 2250 m deep Mabahiss Deep, and the 800 m high and 5 km wide central volcano, are the most prominent structures of the area. The deep is bordered by a series of Red Sea parallel normal faults on both sides, forming a graben-like structure and thus suggesting a rift-like morphology. The central volcano has a 2 km wide summit caldera containing several volcanic cones. Several normal faults cut its southern flank, and radial fractures are present on its summit. In the multibeam backscatter data, several recent lava flows (Our new high-resolution bathymetric mapping allows a more precise structural and geomorphological analysis of the Mabahiss Deep area that represents a starting point for understanding the overall structure of the poorly studied northern Red Sea.

Quaternary carbonate islands have contributed significantly to the fundamental understanding of the interplay between climate and early diagenetic processes in carbonates. However, most of the studied islands, such as the carbonate islands on the Great Bahamas Bank, are situated in humid climate zones. Contrary to this, the Al-Wajh carbonate platform, situated within the arid African-Arabian desert belt on the NE Red Sea shelf (Saudi-Arabia), hosts a plethora of poorly studied carbonate islands. These islands were likely formed during the Last Interglacial (LIG) sea level highstand, commonly defined by Marine Isotope Stage 5e (MIS 5e: 124 – 119 ka). As such, these islands provide an excellent opportunity to give new insights into the paragenesis of carbonate islands within an arid climatic setting and an overall regressive/transgressive sequence.This study investigates Shurayrah Island, located in the southern part of the Al-Wajh platform lagoon. Shaped by the prevailing NW wind direction, Shurayrah Island has an elongated shape, while a reef belt is established on the upwind NW side and carbonate sand spits accumulate on the leeward SE side. The main data base consisted of five drill cores with a total recovered length of 61 m and 150 thin sections. Eight lithofacies (LFT) and 17 microfacies types (MFT) were differentiated, including, amongst others, coral framestones, coral float- & rudstones and ooid-bioclast grainstones. Diagenetic analysis was based on a detailed petrographic investigation, while porosities (Φ) were measured from thin sections with digital image analysis (n = 150) and core plugs using a helium porosimeter (n = 102). Results reveal generally high porosities (mean Φ from thin sections = 29 %; mean Φ from core plugs = 45 %). Pore types are dominated by primary pores in the growth framework of coral framestones and secondary moldic & vuggy pores. Dissolution features are most pronounced in coral framestones, which show almost complete dissolution of original aragonite microstructures. Cement types include dog tooth, pore-filling and bladed cements, with a dominance of dog tooth cements in terms of frequency. Aragonite fibrous cements only occur scarcely and can be overgrown by dog tooth cements. Additionally, dog tooth and bladed cements are frequently observed to grow inside moldic pores.The diagenetic analysis clearly reveals a dominance of porosity creating processes (dissolution) vs. porosity reducing processes (cementation) during paragenesis. In addition, results emphasize the importance of facies-controlled diagenesis: high primary porosities combined with metastable mineral composition of aragonite in coral framestones, result in a high meteoric diagenetic potential. Cement stratigraphy indicates a shift in the diagenetic realm, transitioning from marine (MIS 5e) to meteoric (MIS 5d – MIS 2) conditions, followed by a return to a marine setting with the Holocene Transgression (MIS 1). The overall strong meteoric diagenetic overprint suggests the influence of temporary humid phases (MIS 5c & a), during the overall >100 ka long subaerial exposure period. The observations highlight the significance of short-term climate fluctuations introducing meteoric waters for the diagenesis of carbonate islands in arid climate belts.

Carbonate tepee structures are believed to initiate through cement growth in shallow marine hardgrounds causing lateral expansion and leading to upward buckling of cemented layers commonly along polygonal boundaries. They reportedly form in subtidal to supratidal marine settings and are stratigraphically important markers for exposure and cycle boundaries in ancient rock sequences. Yet in modern carbonate settings only minor occurrences have been reported from the Arabian Gulf in Abu Dhabi and Qatar as well as in Australia. We have discovered two spectacular fields of giant polygonal tepee structures on Sheybara Island, a part of the Al Wajh carbonate platform in the NE Red Sea, KSA. Satellite and drone data were used to measure the dimensions of polygons. Samples have been collected from three transects and two boreholes for age dating, petrographic and geochemical analysis. The tepee fields cover an area of 420,000 m2 and 130,000 m2, respectively, in the supratidal to intertidal environment on the ocean facing side of the island. Individual tepees are composed of chaotically superimposed rugged slabs reaching 3-10 cm in thickness. Tepee ridges range in height from 10-50 cm. Tepees are aligned along larger structures of well-defined polygonal shapes. Their diameters range from 5m to 55m (n =100) with the majority having a diameter of 10-25 m (n=69). Peculiar to many polygons is a central domal buckle with extensional fracture patterns. The tepees have formed in a well-cemented layer of shallow marine bioclastic sand to gravel-sized sediments composed predominantly of coral, red algae, benthic foraminfera, bivalve and gastropod debris that overlie a paleo-reef flat. Grains are heavily micritized, cemented by clotted micrite and fibrous to acicular rim cements and occasionally covered by lace-like meshes of organic matter, likely indicating microbial activity. SEM images from tepee samples show evidence for the presence of microbial activity - biofilms, morphologies that strongly resembles filamentous and coccoidal cyanobacteria, and mineralized cyanobacterial mats. Environmentally corrected C14 age data indicate that polygons formed between 3000 to 1000 years before present (b.p.) correlating with a sealevel regression from a mid-Holocene sealevel highstand some 4000 to 5000 years b.p. Dead and blackened finger corals commonly encrust tepees indicating that the elevated tepee crusts provide preferential seeding for coral colonialization upon re-submergence.

Stromatolites are the vestige of first life on earth and were the dominating carbonate forming marine biota in the Archean and Proterozoic. During the course of the Phanerozoic their importance in producing carbonates has been reduced to niche occurrences usually found in challenging environments, such as hypersaline marine settings and alkaline lakes. Most recently, the discovery in 2010 of a new chlorophyll type - chlorophyll f - from stromatolites in Hamelin Pool in Shark Bay, Western Australia has sparked much additional interest in the genesis and composition of modern stromatolites. We report the discovery of stromatolites in the NE Red Sea on Sheybara Island, Al Wajh carbonate platform, KSA. Based on satellite and drone surveys calibrated by site surveys, the Red Sea stromatolites are distributed over an area of about 50,000 m2 in an intertidal to very shallow subtidal setting on a paleo-reef flat facing the open sea. Two principal growth shapes are recognized: (i) elongated rhomboidal structures 10-100 cm in length, up to 5-50 cm in width and up to 10 cm in height and (ii) low relief (height During a sampling campaign in March 2021 salinity, pH and dissolved oxygen have been measured with average values at 42ppt, 7.8±0.1 and 5.9±0.5mg/L, respectively, typical for coastal Red Sea surface marine waters. Water temperatures range from 18°C in the winter to 29°C in the summer. During exposure at low tides surface temperatures over the tidal flats may fall as low as 12°C in the winter exceeding 43°C in the summer. Large numbers of cerithid gastropods were found grazing on the stromatolite surfaces apparently not affecting their growth.Hence, the setting and conditions are overall similar to some of the stromatolites found on the Exuma Islands in the Bahamas, the only other known occurrence of stromatolites in normal marine waters. Research is continuing on the environmental conditions, the aerial distribution, the microbial diversity and chemical composition of these modern stromatolites to determine why they form in this particular location and if they are similar or not to other reported occurrences of stromatolites.

The NW Iberian continental margin is characterised by a complex morphology and by a sedimentary system which was highly dynamic over glacial to interglacial times. The sedimentary history of the continental slope was strongly influenced by the interaction of bottom currents with topographic highs of structural origin leading to the accumulation of several sediment drifts. A combined analysis of gravity cores from different water depth with hydroacoustic data reveals the vertical behaviour of the upper Mediterranean Outflow Water (MOW) core after the Last Glacial Maximum (LGM). A coarser grained interval during Deglacial and early Holocene times (17.2 to 9.9 cal ka BP) points to an increase in bottom current strength. This increase in velocity was probably related to oceanic density fronts, which migrated through the 300 m thick transition zone between the underlying Labrador Sea Water and the overlying MOW. Radiocarbon dates timed the current strengthening to 17.2 cal ka BP, and a following weakening of the bottom current to 13.3 cal ka BP at 1965 m water depth and to 9.9 cal ka BP at 1885 m water depth. The depth-dependent current weakening suggests an upward shifting of the transition zone by 80 m that was related either to an overall shallowing of MOW or a vertical contraction of this water mass. The upward movement happened over a time interval of approximately 3.4 thousand years. In addition sediment core analysis reveals significant lateral heterogeneities within cm to dm thick sediment layers in the contourite drift. These heterogeneities suggest a need of a detailed core coverage across current-influenced deposits for palaeoceanographic studies to minimize misinterpretations.

Carbonate ramp reservoirs may show a much higher degree of complexity than the schematic and often layer-cake type depositional models which are widely used in exploration and reservoir characterisation, especially in three dimensions. This complexity was investigated in an outcrop study of the Ladinian (Middle Triassic) Quaderkalk Formation (Upper Muschelkalk) from SW Germany. The formation was deposited on an epicontinental, gently inclined carbonate ramp in the regressive part of a third-order depositional sequence and represents a sub-seismic-scale coquina-dominated shoal reservoir analogue. In the study area, coquina carbonates make up a complex system of four stratigraphically separated shoal complexes. The present paper investigates the 1-D to 3-D facies distribution, reservoir architecture and sequence stratigraphic evolution of the largest shoal complex of the Quaderkalk Formation, the Oberer Hauptquader. The object of this outcrop analogue study was to improve the understanding of internal reservoir heterogeneities (such as connectivity and continuity) in similar carbonate shoal complexes in the subsurface. To that end, detailed analyses of 71 quarry outcrop and core samples were carried out, together with the study of more than 400 thin sections, the 2-D analysis of three quarry wall panels and regional-scale 2-D correlations, and facies mapping. This investigation documents significant sedimentological heterogeneities at different scales within the studied shoal reservoir analogue. Instead of a continuous facies belt, a number of separate potential reservoirs form a shoal complex “mosaic”. Shoal development is controlled by sequence stratigraphic architecture, mainly by cycles which were responsible for shoal expansion during times of regression (forming potential reservoir-volume) and shoal drowning during times of transgression (forming seals and potential stratigraphic traps). Within the individual shoal complexes, thickness and lithofacies types show gradational lateral changes. Correspondingly, lithofacies associations within the studied shoal analogue are not defined by sharp boundaries. Thus the present “shoals” do not in fact represent discrete “bodies”, as often depicted in reservoir models and as commonly used in reservoir simulation. In addition, shoal-internal sedimentological heterogeneities (potential flow baffles in the subsurface) are strongly influenced by a combination of small-scale cyclicity and event-type deposition. This study therefore demonstrates the need for sedimentological studies of reservoir heterogeneities at various scales in order to decrease uncertainties and risks in exploration and production.

The characterization and modelling of carbonate reservoirs can still be significantly improved to account for complex property and fracture network heterogeneities at scales difficult to resolve in the subsurface. The objective of this research is to develop and establish workflows for high fidelity geological modelling and characterization using modern and ancient carbonate outcrop analogues.As a first step, we carefully selected high quality modern and ancient analogues to create comprehensive data sets on depositional heterogeneities. Advanced instrumentation and techniques were used such as 3D drone surveys, high-resolution surface geophysical surveys (50 MHz-100 MHz, and seismic), chirp sub-bottom profiler and high-resolution bathymetry mapping. These high-end techniques are paired with tried and tested standard geological techniques of measuring stratigraphic sections anchored by outcrop spectral gamma ray logs, analysis of sediment samples (texture, grain size, mineralogy, geochemistry) and fracture/fault surveys all integrated with full cores drilled in the outcrops. Using these, data models can be created for depositional and fracture heterogeneities at different scales and populated with ranges of property data like those found in actual reservoirs. The outcome will be a series of models for various carbonate reservoir settings and well location patterns with the goal of supporting drilling/exploration operations and reducing future development costs.The project is based on two large-scale research projects of Jurassic carbonates outcropping in central KSA and a large modern carbonate platform in the Red Sea. Jurassic outcrops were analyzed using a unique dataset of measured sections including spectral gamma ray logs (300 vertical m), drone photogrammetry data (4×4 km2 overflight and several km's of vertical cliffs), seismic data (2 km), and GPR data (8 km). Data expose lateral heterogeneities, facies dimensions, and fracture networks at different scales. The modern carbonate outcrops are an ideal laboratory to investigate lateral facies heterogeneities and their relation to environmental factors influencing sediment distribution (prevailing winds versus storms, climate and nutrients). Around 800 km of hydroacoustic data, 50 sediment cores and 200 sea-floor samples were collected exposing significant and complex heterogeneities.The outcome of these research projects significantly increases our understanding of property heterogeneity, facies distribution, fracture networks, and architecture of complex carbonate reservoirs. Resulting multi-scale modelling approaches and associated facies templates will improve the prediction of spatial heterogeneities of facies in subsurface reservoirs of similar settings. In addition, these datasets can be used as input for static analogue models and dynamic simulations to test sensitivities and determine optimum development scenarios for improving ultimate recovery.

This paper presents a comprehensive analysis of multiscale geological, petrophysical and rock mechanical data acquired from 35 m of outcrop well core from the Late Jurassic (Kimmeridgian) Upper Jubaila Formation, Saudi Arabia. In this location the sequence is a typical shallow marine heterogeneous carbonate sequence and a direct analogue to a part of the prolific Arab D reservoir sequence in the subsurface towards the east. Four main lithofacies types were identified with a general shoaling upwards trend, varying from nodular bioturbated peloidal mudstone/wackestone facies to sharp-based intraclast-peloidal, skeletal rudstone/floatstone facies. The mineralogy in the upper section of the well cores is predominantly calcite, while the lower section is also comprised of some dolomite and quartz. Stratiform dedolomitization as a telogenetic overprint is only evident in the upper 18 m of the core and controlled by vertical flow of meteoric fluids. The core plug porosity distribution is lower than that of the subsurface equivalent reservoir zones, and is attributed to meteoric cementation. Thalassinoides burrow features in mudstone and wackestone facies in the upper sections of the core control horizontal permeability distribution at the centimeter to meter-scale. Micrite morphology and dedolomitization appear to be key controls on plug permeability, rock strength and sonic velocity. Based on the trends observed in compressive rock strength, p-wave velocity and plug porosity distributions, the cored section was divided into five main rock mechanical layers. Despite reduced porosity, the average compressive rock strength in the outcrop core is lower than that of the Arab-D subsurface equivalent rocks, indicating overall ‘weakening’ of the rock due to telogenetic processes and surface weathering.

The Arabian Peninsula has a unique setting to resolve how proximity to tectonically diverse plate margins, eustacy, and glacial isostatic adjustment (GIA) influences coastal sea level fluctuations. New 14C AMS and U series dating of coastal sediments from the western, southern, and eastern plate margins (Al-Wajh and Thuwal, Saudi Arabia; Bar Al-Hikman, Oman; Abu Dhabi, U.A.E) have been integrated with an archive of dated coastal sediments (n = 145, 31 locations). Each dated sample is recalibrated for its elevation based on a high-resolution coastal DEM dataset, tidal and tectonic correction, and reservoir correction, to develop a new-suite of relative sea level (RSL) plots, separated into six tectonically distinct zones. A mid-Holocene highstand has been identified in most of the zones (1, 2, 4, 6), however, with complex spatial and temporal variability. No data is available for zone 3, whereas no Holocene highstand was observed in zone 5. The results suggest that eustacy, varying rates of vertical tectonic change (max 1 mm/year) and glacial isostatic adjustment (max 0.5 mm/year) were the major drivers of sea level fluctuations since Mid-Holocene albeit in different proportions along the different sections of Arabian shorelines. Furthermore, the topography of a shoreline, tidal range, and presence/absence of a coastal barrier also significantly influences the coastal inundation. Based on our findings, eastern Arabia will in the near future experience the largest coastal flooding (largest 0–10 m elevation area, > 2 m tidal amplitude, no barriers), followed by western (1–2 m tidal amplitude, discontinuous coral reef barriers) and southern Arabia shorelines (>3 m tidal amplitude, no barriers) shorelines. The presented insights are critical in supporting communities along the coasts of the Arabian Peninsula, who will have to cope with flooding from rising oceans over the next few decades due to climate change.

Summary The work presented here is part of a multidisciplinary research project that aims to develop high fidelity geomodels, of Jurassic hydrocarbon reservoirs in Saudi Arabia, suitable for IOR/EOR reservoir simulation studies through careful integration of outcrop and subsurface data. An integrated petrophysical and rock mechanics characterization of 35 meters outcrop well cores from the Upper Jubaila section was performed, which provides an analogue to the lower Arab-D reservoir unit. Preliminary petrographic analysis indicated a strong diagenetic overprint of the core by telogenetic products. The diagenetic factors as well as the rock mechanical and petrophysical properties are the main controls on the fine-scale mechanical stratigraphy. The findings from this study will contribute to the development of high fidelity geomodels of Jurassic hydrocarbon reservoirs in Saudi Arabia, suitable for IOR/EOR reservoir simulation studies.

The distribution of pores in carbonate rocks is controlled by the synergy of different factors such as the initial sediment, environment processes and diagenesis. Prediction of the texture-related pore distribution to evaluate reservoir quality is still a challenging task in geothermal and hydrocarbon exploration. In order to degrease these uncertainties, 3D facies models are produced on the basis of facies types and their relationship to petrophysical properties. However, several studies have shown that these relationships are often rather controlled by diagenesis that is often described as a sequence for an entire reservoir. Two outcrops as part of the same bioclastic shoal reservoir analogue present two different architectural styles: (I) layer-cake architecture and (II) high-angle foreset architecture. In order to identify petrophysical trends and related controlling factors, both outcrops were mapped and studied in detail. Based on the investigation of 41 thin sections, microfacies types were defined and diagenesis was analysed. In addition permeability-porosity was measured and pores were digitally characterized. Although both outcrop successions studied are associated with the same reservoir interval, varied architecture leads to fundamental differences in the porosity-permeability distribution. Petrophysical trends in the layer-cake architecture are mainly controlled by cyclicity and channel features. Compared to layer-cake architecture, trends in the high-angle foreset architecture are dominantly controlled by the depositional environment. Both outcrops show differences in their diagenesis. While layer-cake architecture suffered the entire sequence, large parts of the high-angle foreset architecture were not influenced by late burial and telogenetic processes. As a consequence, primary porosity is persevered within the high-angle foreset architecture. In addition, results of this investigation indicate that primary porosity has a bigger influence on good permeability values compared to telogenetic associated vug-enhancement. This investigation therefore demonstrates that individual carbonate reservoirs can suffer different diagenetic pathways. Moreover, the study highlights the potential of outcrop studies to improve the principal understanding of fundamental trends within potential subsurface reservoirs.

Summary In order to improve the understanding of sedimentological heterogeneities on varies scales, from micro-scale thin section to macro-scale correlations, a joint project was initiated between the University of Tubingen (Germany) and ENI S.p.A. This outcrop study focused on the characterization and modelling of the largest and thickest wedge (wedge 3) of the Quaderkalk Formation. The Middle Triassic Quaderkalk Formation in SW Germany composed of four lithostratigraphic separated bioclast-rich wedges and represents an outcrop analogue for sub-seismic scale carbonate ramp reservoirs. Facies range from mud-rich, low-energy offshoal (non-reservoir facies) to bioclast-rich, high-energy shoal (potential reservoir facies), deposited on an epeiric carbonate ramp. Instead of a continuous facies belt, tens of kilometres-scale correlations indicate several separated high-energy shoal areas forming a shoal complex “mosaic”, which is controlled by a cycle architecture. Integrating all data of an intensive field study, two 3D facies models were produced: (1) a 30 × 30 km model to simulate the large-scale facies pattern and (2) a more detailed 10 × 10 km small-scale model to simulate internal shoal reservoir heterogeneities. The results highlight the importance of realistic geological, especially sequence-stratigraphic input in order to optimize the simulations of carbonate shoals.

Mitgliederkarte für den Kongress in St. Petersburg, ausgestellt vom Organisationskomitee des Congrès Géologique International Russie 1897 (20.8.-4.9.1900 in St. Petersburg), mit den Unterschriften von u.a. Aleksandr Karpinskij, Präsident des Organisationskomitees, M. Tschernyschew, Generalsekretär des Organisationskomitees. Weiterer Ausweis in russischer Sprache, datiert, ETH-Bibliothek