Subaerial Exposure Surfaces Research Articles

Three‐dimensional stratigraphic architecture, secondary pore system development and the Middle Pleistocene transition, Sandy Point area, San Salvador Island, Bahamas

AbstractThe Bahamian Archipelago has been the subject of extensive studies of stratigraphy, carbonate island morphology and diagenetic overprinting for many decades. A recently‐acquired dataset comprised of high‐resolution light detection and ranging, tightly spaced cored boreholes with image logs, thin sections, porosity and permeability measurements, and isotopic geochemical analyses provides a unique opportunity to perform detailed sequence stratigraphic analysis. This multi‐faceted study first establishes a new high‐resolution sequence stratigraphic framework for the Sandy Point area of San Salvador Island, encompassing six unconformity bound sequences (exposure surfaces) representing varying lengths of missing time. The stratigraphically lowest sequence is Late Pliocene in age and is dominated by reef facies capped by an extensive laminar caliche that represents 1.5 million years of exposure. The overlying Early Pleistocene is split into two sequences, EP1 and EP2, which are both dominated by low‐energy subtidal facies. The upper three sequences are tied to the Marine Isotope Stage 11, 9 and 5e interglacials, and are distinctly different in facies composition and architecture, being dominated by higher energy facies distributed in a more complex mosaic. The new stratigraphic framework highlights significant changes in depositional style and facies architecture that can be linked to increasing sea‐level amplitude oscillations and greater climate gradient at the Mid Pleistocene Transition. The well‐constrained framework is then used to provide key context for observed complex diagenetic evolution through repeated sea‐level inundation and subaerial exposure. These results suggest that well‐developed subaerial exposure surfaces drive increases in non‐matrix features in the subsequent stratigraphic package following exposure as the well‐cemented exposure surface acts to concentrate fluid flow and dissolution. Integration of non‐matrix features within this framework highlights the importance of dissolution/cementation patterns on formation of these modified unconformity surfaces for focusing later meteoric diagenesis and creating enhanced fluid flow pathways for continued multicyclic diagenetic events.

Development and dolomitization of Anisian isolated carbonate platforms in the Transdanubian Range, Hungary

Abstract In the Middle Anisian, extensional tectonic movements led to the development of isolated carbonate platforms in the area of the southwestern part of the Transdanubian Range. The platforms are made up of meter-scale peritidal–lagoonal cycles bounded by subaerial exposure surfaces. One of the platform successions (Tagyon Platform) consists predominantly of limestone that contains partially and completely dolomitized intervals, whereas the other one (Kádárta Platform) is completely dolomitized. Drowning of the platforms took place in the latest Pelsonian to the early Illyrian interval when submarine highs came into existence and then condensed pelagic carbonate successions with volcanic tuff interbeds were deposited on the top of the drowned platforms from the late Illyrian up to the late Ladinian. The comparative study of dolomitization of the coeval platforms, affected by different diagenetic histories, is discussed in the current paper. Traces of probably microbially-mediated early dolomitization were preserved in the slightly dolomitized successions of the Tagyon Platform. This might also have been present in the successions of the Kádárta Platform, but was overprinted by geothermal dolomitization along the basinward platform margin and by pervasive reflux dolomitization in the internal parts of the platform. The Carnian evolution of the two submarine highs was different, and this may have significantly influenced the grade of the shallow to deeper burial dolomitization.

Transgressive-regressive cycles in saline lake margin oolites: paleogeographic implications (Priabonian, Vistrenque basin, SE France)

The integrative analysis of a lacustrine carbonate succession from Butte Iouton hill (Vistrenque basin, SE France) brings new insights into depositional models and stacking patterns of oolitic saline lake margins and provides new details regarding the late Eocene paleogeography of southeast France. Depositional facies analysis and paleoenvironmental reconstructions allow reconstructing an oolitic lacustrine ramp model, displaying from the proximal to the distal areas: (1) shallow marginal saline lake domain with deposition of planar microbial laminites, and molluscan-ostracodal wackestone, (2) a more distal and open lacustrine environments with low to moderate energy characterized by the deposition of peloidal grainstones and (3) a domain of higher energy with accumulation of ooids mixed with peloids in the vicinity of the area of ooid production. Lake margin carbonate sedimentation dominantly occurred during stages of lake transgression while subaerial exposure surfaces developed during periods of negative inflow-evaporation balance (forced regression). Carbon and oxygen isotopes together with vertical trends in salinity inferred from molluscan associations show that lake transgression does not result from increasingly positive freshwater-evaporation balance volume but from the combination of subsidence and outflow from neighbouring saline waterbodies. The Butte Iouton carbonate margin is part of a set of interconnected saline lakes, occupying continental basins from Languedoc and Rhodanian region during the Priabonian, with a siliciclastic-dominated sedimentation in the southern margin, sourced by erosion of Pyrenean reliefs, and a carbonate-dominated northern margin with significant oolitic sedimentation in high-energy nearshore area.

Early-Middle Devonian paleosols and palustrine beds of NW Canada in the context of land plant evolution and global spreads of anoxia

Dozens of subaerial exposure surfaces are assessed in cores from cyclic peritidal carbonates of the Emsian–Eifelian (~410–385 Ma) age. These surfaces range from incipient erosional surfaces to paleokarst profiles and thick (>1 m) calcretic-clayey paleosols. Palustrine carbonates intervening at multiple levels in same strata are the earliest known occurrence of a typical palustrine facies in a coastal carbonate plain environment. None of paleokarst and paleosol profiles contain traces of vascular-plant root penetration, and only palustrine facies exhibit swarms of thin rhizoliths. These findings are within the context of Devonian paleosols on marine carbonate substrates where root traces and laminar calcretes are extremely rare (only 2 reports), and no instances of root penetrations are trackable from unconformities in pre-Givetian carbonate successions. The δ13C and δ18O signatures indicate variable diagenetic reset of isotopic composition. Modest δ13Cvpdb offset towards lighter values is preserved in two thicker paleosols (−3.4 to −8.0‰ in calcretic matrix vs. -1.35 to −6.5‰ in parent limestone). Such offsets are very rare in pre-Late Devonian pedogenic carbonates, and their attribution to plant-derived CO2 is dubious. It is inferred that the land surface in calcimagnesian landscapes remained a regolith or primary desert (i.e., was never colonized by tracheophytes) long after the spread of vascular plants in more favorable wetland settings. Furthermore, the advent of seed reproductive strategy in the latest Devonian and plant adaptations to aridic habitats manifesting in fossil floras only since Pennsylvanian indicate that much of land surface remained within the realm of primary desert long after the afforestation of wetlands. The direct (plant roots, shoots and spores) and indirect (sedimentary features) fossil records of land greening during the Paleozoic are not corroborating the hypothesis holding the primordial embryophytic cover on land accountable for the Late Ordovician atmospheric oxygenation-decarbonization event. It is further argued that the evolution of root systems and plant stature could hardly trigger anoxic events and biotic crises in the Devonian marine realm.

Facies analysis of a Bartonian–Aquitanian siliciclastic‑carbonate system, Costa Rica

Costa Rican Eocene–Oligocene carbonate deposits have been known for more than a hundred years, however, there are few scientific contributions detailing the relation between facies analysis and regional basin evolution. Based on a detailed study in southern Costa Rica, a chronologic history of a Bartonian–Aquitanian sedimentary succession is proposed. The carbonate facies analysis allows for eight microfacies (MF1 to MF8) to be distinguish. The siliciclastic facies analysis permits identification of two main periods of sandstone deposition and one of shale formation. The mixed sandstone/carbonate succession exhibits three main periods of deposition: (1) Bartonian–lower Priabonian, represented by distal, relatively deep slope-to-basin environments. Deposits comprise thick accumulation of sandstones, calcareous breccias, and calciturbidites. This period terminates with a stratigraphic erosion surface; (2) Upper Priabonian–Rupelian, represented by more proximal, shallow to mid depth inner to middle ramp environments. Deposits are composed of peloidal/cortoidal grainstone, foraminiferal-algal rudstones, and rhodolith rudstones. This period terminates with a second stratigraphic erosion surface; (3) Chattian–Aquitanian, represented by proximal to distal, shallow to coastal and offshore environments. Deposits are composed of a mixed siliciclastic carbonate thin complex overlain by dark shale.The vertical and lateral facies changes permit a basin evolution model that is closely controlled by subsidence. Local tectonic uplift may have formed the first subaerial exposure surface. The second subaerial exposure surface may have been controlled by eustatic sea-level variations over relatively shallow water deposits. These results, the first-of-their-kind in the southern Costa Rican Fila Costeña, could permit improvements of lateral regional correlations by bringing new lithostratigraphic layers of reference.

Rhizoliths in Lower Pliocene alluvial fan deposits of the Sorbas Basin (Almería, SE Spain)

Root traces are a valuable tool for paleoenvironmental interpretation and a significant indicator of the potential preservation of paleosol horizons. As such, the analysis of rhizolith-bearing intervals can help resolve high-frequency stratigraphic variability generated at the marine to continental sedimentary interface. This type of stratigraphic complexity is exhibited by the Lower Pliocene sedimentary infill of the intermontane Sorbas Basin (Almería, SE Spain), where we document a distinctive, ~90-m long and ~ 1–2 m thick continuous horizon of well-preserved, densely spaced rhizoliths, occurring within conglomerate to coarse sandstone deposits along the northern margin of the basin (Moras Member of the Cariatiz Formation). The root traces are locally very abundant, mostly vertical, with very low sinuosity, averaging 10–19 cm long and 0.8–1.9 cm in diameter. The rhizoliths can be classified as root casts, with concentric laminar detrital infill. They are abruptly truncated at the top by a pebble conglomerate bed and overlain by a tabular unit of structureless sand, containing fossils of marine affinity.The rhizoliths occur at the contact between alluvial and marine facies. Facies analysis of the studied interval indicates a retrogradational trend, with upward transition from proximal to intermediate/distal alluvial fan deposits to shallow marine fossiliferous sand. The verticality of the root traces extending through several underlying coarse beds suggests the episode of plant colonization occurred in the presence of a relatively low water table. A sudden pulse of coarse clastic input killed the plants, and was followed by the onset of marine transgression.This case study illustrates an example of rhizolith development and preservation in coarse-grained alluvial fan deposits, and demonstrates the stratigraphic value of root traces as markers of composite discontinuity surfaces, where a surface of subaerial exposure is superimposed by a marine flooding surface.

Calibration of bulk carbonate strontium isotopes to ammonite zones: Implication for global stratigraphic correlation of Callovian–Kimmeridgian strata in Central Saudi Arabia

This study evaluates a strontium isotopic ratio (87Sr/86Sr) dataset from bulk rock carbonate samples of Middle–Upper Jurassic strata in central Saudi Arabia (upper part of the Tuwaiq Mountain Formation and entire Hanifa Formation). The objective is to use the 87Sr/86Sr dataset to determine ages to improve correlation to equivalent European ammonite zones to better link paleogeographic, paleoceanographic, and ecological events across the Tethys Ocean. The results show a scattered vertical pattern of 87Sr/86Sr, most likely caused by the introduction of radiogenic 87Sr during postdepositional (post-Jurassic) diagenetic modification. Data collected from mud-dominated facies not associated with subaerial exposure surfaces with pedogenesis or subaqeous marine firmground surfaces with Glossifungites Ichnofacies show a smoother sample-to-sample variation, is interpreted to be less altered by meteoric diagenesis. These data show distinct 87Sr/86Sr ranges that fall within the stratigraphic ranges and are in stratigraphic order. The absolute ages estimated from these data, however, show discrepancies with the ages previously interpreted by biostratigraphic data. The two age estimations (from 87Sr/86Sr and biostratigraphy) for the studied succession agree when the 87Sr/86Sr values are corrected for radiogenic (rubidium) contribution to mass 87. The corrected 87Sr/86Sr values are used to estimate the numerical ages and assign the ammonite zones of the studied succession. The proposed ammonite zones provide a temporal framework to link events that occurred in the Middle–Late Jurassic at different geographic locations in the Tethys Ocean.

Diagenesis and reservoir characteristics of the Lithocodium–Bacinella facies in a Lower Cretaceous reservoir, eastern Persian Gulf Basin

In this paper, the importance of Lithocodium–Bacinella facies (LB facies) as reservoir rock in the Lower Cretaceous carbonates of the Persian Gulf will be addressed. The studied interval belongs to the reservoir zone of upper Fahliyan Formation, which occurs in one of the giant offshore oilfields. A detailed examination of a continuous core (58 m) from this formation allows clarifying the relationship between facies distribution, diagenetic evolution, and porosity development in function of relative sea level changes. Seven facies types are identified based on core descriptions and microfacies analysis. These facies are interpreted to have been deposited in lagoonal, shoal, and open marine settings during Valanginian–Hauterivian times. Petrographic studies and stable carbon and oxygen isotope analyses indicate that these facies have been affected by different diagenetic processes, starting with early marine diagenesis and followed by meteoric and/or burial diagenesis. Based on facies stacking patterns and evidence of subaerial exposures, four high-frequency sequences are interpreted in terms of variations in accommodation space. Combination of facies analysis, diagenetic studies and poroperm data reveals (with the exception of sequence I) that the lower transgressive and the upper highstand intervals of these sequences show different facies types, diagenetic modifications and reservoir properties. The transgressive system tracts (TST’s) correspond to tight zones (Z-1, 3, 5), with poor reservoir quality (Φ < 15% and K < 1 md). These intervals are composed of an alternation of open to restricted marine facies. The facies are characterized by extensive bioturbation, high clay content (argillaceous limestones), displaying marine (micritization) and shallow to deep burial diagenetic processes (pyrite mineralization and solution seams). The reservoir intervals preferentially occur within the highstand system tracts (HST’s), which are commonly composed of vertically stacked Lithocodium-Bacinella facies. This facies with aggradational patterns is capped by subaerial exposure surfaces at the top of the sequences and is associated with leaching and cementation. Meteoric dissolution during subaerial exposure resulted in the formation of vuggy and moldic pores as the main pore types in these system tracts, which survived in significant amounts until present-day. Therefore, the LB facies usually have greater than 15% porosity and 1 md permeability. Diagenetic studies in these units show that the marine diagenesis was interrupted by subaerial exposure and meteoric diagenesis before burial and development of stylolites. Our findings show that despite the uncertain taxonomy and paleoenvironmental setting of Lithocodium aggregatum and Bacinella irregularis, the LB facies has great reservoir importance.

Comparison of differential diagenesis of two oolites on the Lower Triassic platform margin, NE Sichuan Basin: Implications for the co-evolution of rock structure and porosity

Two oolites of the Lower Triassic Feixianguan Formation on the margins of the Kaijiang-Liangping (K-L) Trough in the NE Sichuan Basin were deposited in the upper parts of similar shallowing-upward sequences, and are capped by a subaerial exposure surface. However, they have contrasting diagenetic histories and reservoir qualities: the sour oolitic dolostone reservoirs on the eastern side of the K-L Trough have significantly higher porosities than the sweet oolitic limestone reservoirs on the western side. A petrologic and geochemical comparison of the two oolites was carried out so as to investigate the role of diagenetic processes on the co-evolution of oolite structure and pore network. On both sides of the K-L Trough, eogenetic meteoric digenesis led to an inversion of texture with ooid dissolution and interparticle cementation in the oolite layers close to the exposure surface. The layers relatively far from the exposure surface mostly underwent recrystallization in association with connate seawater, resulting in neomorphic fabrics in the ooid cortices and less interparticle calcite cement. During the subsequent dolomitization, the early-formed oolite structure were mostly inherited by the dolostones on the eastern side, with fabric-retentive textures in the layers close to the exposure surface and fabric-destructive textures in the layers far from the exposure surface. During deeper burial the oolitic limestones were subjected to extensive calcite cementation and porosity loss, whereas the dolostones generally retained more porosity and have better connectivity of the micropore network. The higher porosity of the dolostones is ascribed to lesser chemical compaction and associated cement precipitation, rather than the creation of new pores. Porosity enhancement is closely dependent on the absence of the calcite cement related to pressure solution. The heterogeneity of the dolostone fabrics and the porosity was further enhanced by the subsequent recrystallization and the dissolution-reprecipitation process induced by thermochemical sulfate reduction, during which small amounts of net porosity are calculated to have been created. The oolitic limestones are the best reservoirs close to the exposure surface, whereas the most porous dolostones with the highest degree of fabric destruction are commonly some distance from the exposure surface. These results provide some insights into the diagenetic controls on oolite structure and porosity, and have implications for predicting the porosity of deeply-buried oolitic reservoirs, so have global theoretical and practical significance.

Facies and architecture of the SCIc formation (Schisto-Calcaire Group), Republic of the Congo, in the Niari-Nyanga and Comba subbasins of the neoproterozoic west Congo basin after the marinoan glaciation event

The Neoproterozoic Schisto-Calcaire Group (630 to ca. 580 Ma) was deposited on an extensive carbonate shelf in the margin of the Congo Craton in the Niari-Nyanga and Comba subbasins (Gabon and Republic of the Congo). It consists of three carbonate-dominated subgroups (SCI to SCIII, up to 1300 m-thick) recording relative sea-level changes. The SCIc Formation, at the upper part of the SCI Subgroup, is a carbonate succession of meter-scale shallowing-upward cycles, composed of a standard sequence of 7 facies grouped in 5 facies associations recording the evolution of a marine ramp from distal carbonate muds and giant stromatolitic bioherms (F1–F2) and extensive ooid shoals (F3), to proximal settings submitted to evaporation near a sabkha (F7). Fifth-order ‘meter-scale’ (or elementary parasequences) packages are grouped into fourth-order sequences (parasequence sets), which are not correlative in the whole basin. Two categories of fifth-order elementary parasequences are recognized, on the basis of physical bounding surfaces: (i) subtidal cycles bounded by marine flooding surfaces across which subfacies deepen; and (ii) peritidal cycles bounded by subaerial exposure surfaces. These cycles are the result of the interplay of relative sea-level changes due to eustatic variations related to periodic extensional tectonic events affecting the whole basin. The Niari-Nyanga and Comba subbasins experienced basin tectonics in the general context of the rifting of Rodinia creating changes of relative sea-level in the different parts of the shelf. The SCIc cycles are enclosed into a third-order sequence with two major transgressive-regressive phases, related to the deposition of the SCI Subgroup. The most typical sedimentologic feature of the SCIc Formation is the deposition of giant stromatolitic bioherms (stacked up to 20 m) topped by ooid shoals (up to 75 m thick) deposited during high systems tract prograding and forced regressive systems tract phases that ended with a lowstand systems tract phase with evaporitic and karstic conditions at the top of the SCIc Formation. The elementary parasequences and parasequence sets are probably the result of the migration of lateral environments related to the variation of the energy in relation to tectonic setting. As a result, a regional sea-level increase is for the first time highlighted in transgressive systems tract phase (composed of microbial induced sedimentary structures Facies) in the lower part of the SCIc Formation. The third-order succession can be followed more than 100 km in the Republic of the Congo and several hundred meters from South of Gabon to the Lower Congo in the Democratic Republic of the Congo. Tentative detailed sequence stratigraphy correlations between both Congo's highlight the role of tectonics affecting both areas.

Origin of seismic reflections in a carbonate gas field, Lower Miocene, offshore Myanmar

The geological origin of seismic reflections, within a carbonate gas reservoir, has been determined on the basis of an integrative analysis of cores, well-logs, pre-production and 4D time-lapse-seismic data. The gas reservoir is subdivided into meter to decameter-thick diagenetic units which are characterized by distinct acoustic properties. Low impedance units are dominant and consist of highly microporous and vuggy foraminiferal/coralline algal floatstones intervals whose porosity has been significantly enhanced in burial diagenetic environments. High to moderate acoustic impedance units (thickness: 1–20 m) are related to various depositional and diagenetic fabrics: 1) early-lithified coral floatstones, 2) foraminiferal rudstone whose intergranular space is occluded by shallow burial sparry calcite and 3) carbonate breccia with neomorphosed corals occurring below an intra-formational subaerial exposure surface. Within the gas zone, seismic reflection horizons may form at the boundary between two diagenetic units with contrasted acoustic impedance or may result from the interference between reflection at base and top of such intervals. A stratigraphy-cross-cutting reflection horizon has been shown to form at a major and laterally continuous shift in porosity which is related to a differential diagenetic evolution between the gas and the water zone, during or after hydrocarbon emplacement. Finally, the analysis of the geological origin of seismic reflections has been proved to help predicting and mapping the major heterogeneities with significant impact on flow during production.

Rolling window regression of δ13C and δ18O values in carbonate sediments: Implications for source and diagenesis

AbstractDiagenetic alteration can produce, modify or erase significant biogeochemical information recorded in carbon and oxygen (δ13C and δ18O) values of marine carbonates throughout geological time. Understanding the type and extent of alteration experienced by a carbonate deposit can improve sedimentological and geochemical interpretations of events in Earth history. In this study, we present a new application of a statistical approach to aid in the identification and interpretation of sedimentological surfaces in a shallow marine carbonate sequence using a rolling window regression (RWR) analysis. RWR analysis evaluates the degree of covariation between two records and how it changes through time. Geological application of this statistical technique permits a new perspective on the fine scale variability in carbon and oxygen isotope records and the processes that generate them and provides a complementary tool for sedimentological interpretations. In this study, we apply RWR to δ13C and δ18O values from the Clino drilled into the western margin of Great Bahama Bank, a core that has been extensively altered by diagenetic processes, within the vadose, phreatic and marine burial zones. This core penetrates ~676 m of shallow marine to deeper slope deposits and contains a variety of both sedimentological and diagenetic events, including facies transitions, subaerial exposure surfaces, marine hardgrounds, firmgrounds and periods of reduced sedimentation rate. Using more than 1,200 analyses of paired δ13C and δ18O values, we have applied a RWR analysis to evaluate how the correlation between these two proxies changes at scales of 10, 30 and 100 m. The results of this study highlight the dynamic evolution of correlations between δ13C and δ18O values within diagenetic zones and in association with diagenetic surfaces and provide context for interpreting covariance in δ13C and δ18O values from the geological record.

Sedimentology and sequence stratigraphy of the middle and upper Eocene succession from Jebel Kabbara (central Tunisia)

During the Maastrichtian-Middle Eocene interval, a resistant area (Kasserine Island) occurred in central Tunisia. The emerged area narrowed especially on its northern edge because of the eustatic rise and associated compressive tectonics having marked central Tunisia during the Paleocene-Eocene transition. Located at the NE boundary of the Kasserine Island, the study area saved the shoreline variations and therefore the sedimentary environment evolution. In this area, sedimentological and sequence stratigraphy analysis of middle and upper Eocene deposit are scarce in previous works. In this context, this paper aims to study the depositional facies, early diagenesis and sequence stratigraphy of the Cherahil Formation (Middle and Late Eocene) in Jebel Kabbara (central Tunisia) in order to refine the deposit facies and environment and paleographic variations in the NE boundary of the Kasserine Island during Middle and Late Eocene.The studied succession (Cherahil Formation) outcrops at Jebel Kabbara, SW of Jebel Cherahil and is located 42 km from Kairouan city. This Formation is subdivided into lower and upper lithostratigraphic members separated by Siouf member. The field examination and microscope observation of thin sections and XRD clays mineralogy set apart five carbonate facies. The depositional facies occurred broadly within a shallow-water carbonate platform with alternatively episodes of open marine conditions and episodes of restricted lagoonal conditions.The analysis of the depositional facies stacking and the field examination of the unconformity surfaces allow subdividing the Cherahil Formation into eight third-order depositional sequences, including only transgressive and high-stand systems tracts. The sequence boundaries generally consist of erosive surfaces corresponding to superimposed subaerial exposure surfaces and transgressive ones. According to the available biostratigraphic data, the depositional sequences of Cherahil Formation may be broadly correlated with the Haq's et al. TA3 2nd order global eustatic cycles.

Anatomy of the Book Canyon conglomerate: a sequence boundary at the top of the Bear Gulch Limestone in the Big Snowy Trough

The Serpukhovian Book Canyon Conglomerate is an unpublished limestone conglomerate contained within the Tyler Formation in central Montana. It overlies and contains clasts of the Bear Gulch Limestone, a plattenkalk deposit yielding amazing paleontological detail. The Book Canyon Conglomerate is up to 2 m thick, markedly lensoid, and laterally discontinuous in its outcrop for a distance of 2 km but likely extends beyond the study area. Well logs and cores indicate its presence in the subsurface between the underlying Bear Gulch Limestone (Serpukhovian) and overlying Tyler Formation (Serpukhovian–Morrowan). This conglomerate provides new information regarding the transition of the marine-dominated Bear Gulch Limestone to the overlying Tyler Formation. The Book Canyon Conglomerate is dominantly clast-supported and includes sedimentary structures including pebble and cobble imbrication, tangential cross-stratification, and an overall upward-fining character. The basal surface of the conglomerate is sharp and irregular where the top of the underlying Bear Gulch Limestone has been erosionally removed. The Book Canyon Conglomerate is therefore interpreted as a fluvial channel deposit formed on a surface of subaerial exposure at the top of the Bear Gulch Limestone. In areas where the conglomerate is thin or absent, the top of the Bear Gulch Limestone is interpreted as a paleosol developed in a semi-arid climate where it is preserved between channel incisions of the Book Canyon Conglomerate. The paleosol contains microcrystalline silica cement and limestone clasts that have weathered to a black color. Freshwater exposure and channelized fluvial erosion early in the post-depositional history of the Bear Gulch Limestone explain variations in the contacts between units. These variations contribute to the debate concerning stratigraphic relationships. Further analysis of unconformities and fauna at the base and top of the unit clarifies the Bear Gulch Limestone’s position in time, and its relationship to the Heath and Tyler Formations.

The impact of Aptian glacio‐eustasy on the stratigraphic architecture of the Athabasca Oil Sands, Alberta, Canada

AbstractHigh‐frequency incised valley systems and subaerial exposure surfaces are prevalent in Aptian‐aged units, having been documented in numerous basins around the world. In light of a growing body of palynological, geochemical and sedimentological evidence, a glacio‐eustatic mechanism for their generation has been suggested, driven by the waxing and waning of polar ice caps. The densely‐penetrated McMurray Formation of north‐eastern Alberta, Canada, provides a unique opportunity to test for an Aptian glacio‐eustatic signal within the Western Canadian Foreland Basin. This study combines detailed drill core description with wireline log‐based stratigraphic analysis to map and evaluate a series of drainage networks located along the cratonic edge of the Western Canadian Foreland Basin. Four stratigraphically distinct composite channel‐form bodies are documented across an area &gt;14 000 km2. Composite channel‐form bodies are up to 50 km wide and locally exceed 70 m thick, subtending from upward‐coarsening parasequence sets 2 to 12 m thick; bounding marine flooding surfaces are mappable over distances exceeding 280 km along the north–south trending basin. Detailed internal fill characterization of the four composite channel‐form bodies suggests that three represent tidal–fluvial incised valley networks, and one represents a deltaic distributary system. At least six cycles of transgression and regression are apparent within the McMurray Formation stratigraphy. Recent biostratigraphic assessment has constrained deposition of the McMurray Formation between ca 125 Ma and 118 Ma, and periods on the order of 0·7 to 1·7 Ma are estimated for the mapped transgressive–regressive cycles. The high frequency nature of these Aptian sequences suggests that the sea‐level oscillations recorded in McMurray Formation strata are consistent with a glacio‐eustatic driver. Linking the internal stratigraphic architecture of the McMurray Formation with interpreted global cooling events and sea‐level drops provides the first evidence of glacio‐eustasy within Early Cretaceous strata of western Canada.

Abiod Formation in Boukhil section (Jebel M'rhila, Central Tunisia): Facies, geochemistry, mineralogy and deposition sequences

The field study and the petrographic observations of the limestone samples particularly from the Boukehil section allowthe identificationof four carbonate facies: (1) calcisphere-rich mudstone-wackestone, (2) Conglomeratic limestone, (3) bioclastic wackestone-packstone, and (4) siliceous dolostones. According to the vertical change of the deposition facies and the occurrence of unconformities, it was possible to divide the AbiodFormation in this area into three third-order depositional sequences bounded either by subaerial exposure surfaces or by submarine erosional surfaces. These sequences include only transgressive and highstand systems tracts. The SEM observation of the limestone shows three types of micrite: bio-detrital, biological and chemical. Despite poor occurrence in our samples, the existence of some biological structures similar to bacterial fabrics highlights the role of bacteria in the carbonatogenesis. The XRD mineralogy and the calcimetry analyses show that the limestone of the Abiod Formation is very rich in calcium carbonate. The CaCO3 concentrations vary with the systems tracts and the depositional sequences controlled by the sea-level fluctuation. For instance, higher concentrations of CaCO3are recorded in the transgressive intervals of the eustatic cycles (with some samples yielding 100 wt% of CaCO3), whereas through highstand intervals, the CaCO3 concentration falls to 83 wt %. The atomic absorption geochemical analysis of limestones shows that the major elements concentrations change with systems tracts and thus with sea level fluctuation. For example the highest concentrations of Fe in the carbonates are observed in the marine highstand and regressive intervals, and those of Mg and Ca observed in the transgressive intervals.

Transgressive deposits and morphological patterns in the equatorial Atlantic shallow shelf (Northeast Brazil)

The aim of this work is to identify and characterize the Late Quaternary morphological patterns of the west coast of the Ceará shelf, Northeast Brazil, based on Landsat 8, sedimentology, and seismic data. The shallow shelf has three regions: the Acaraú high (AH), unconsolidated floor (UF), and Itapagé bank (IB). The AH occurs in the inner shelf (0–5 m water depth) and shows various bedforms and in part a rocky bottom. The UF (>5 m) has many subaqueous dunes with bioclastics, structures for catching lobsters, and an escarpment. The IB is an extensive feature (delimited by the 20-m isobath) with large dunes and a shipwreck. Three seismic boundaries were identified as follows: transgressive surface (TS), subaerial exposure (SE), and miscellaneous surface (MS = TS + SE). Four seismic units were defined as follows: U1 — transparent facies with low amplitude, underlying the SE; U2 — chaotic to parallel facies with moderate to high amplitude, overlying the SE and underlying the TS; U3 — chaotic facies above the SE or TS and below the TS or sea floor, with the largest spatial extension; U4 — periodically occurring chaotic facies above the TS and below the seafloor. U1 is probably related to the falling stage systems tract or previous highstand systems tract. U2 and U3 belong to the transgressive systems tracts and U4 is associated with bedforms of the highstand systems tract. Ancient topography favored the development of transgressive deposits and modern bedforms. The AH, escarpment, and IB are morphostructures influenced by Acaraú sub-basin faults. The inner shelf is characterized by shallow geomorphology strongly influenced by structural inheritance (Precambrian and post-breakup of Pangea), bedrock control, and the Holocene transgressive systems tract (the largest thickness occurs with subaqueous dunes).

Environmental change during the Triassic–Jurassic boundary interval of an equatorial carbonate platform: Sedimentology and chemostratigraphy of the Ghalilah Formation, United Arab Emirates

The sedimentology and chemostratigraphy (δ13Ccarb, δ13Corg, Sr concentrations) of the Triassic-Jurassic (Tr-J) boundary interval has been investigated in detail in sections of the Ghalilah Formation exposed in Wadi Ghalilah, Wadi Milaha and Wadi Naqab, northeastern United Arab Emirates. The sequence of events recorded across the Tr-J boundary interval in Ras Al Khaimah reflects significant environmental change that can ultimately be related to CAMP volcanism. Uppermost Triassic limestones of the Sumra Member rich in coral, calcareous sponges and large-size bivalves are interbedded with siliciclastic mudstones and deposited in an open lagoonal environment. These are abruptly followed by oolites with intercalated calcareous mudstones of the Sakhra Member that are devoid of macrofossil. Thrombolitic microbialites that occur in the lowermost Sakhra Member are interpreted to reflect the demise of hypercalcifying marine biota during the Tr-J boundary interval. The oolites of the Sakhra Member are capped by a subaerial exposure surface, followed by peritidal deposits of the Shuba Member. The available, sparse biostratigraphic data of the Ghalilah Formation are critically discussed and support a position of the Tr-J boundary at the base of the Sakhra Member. A negative excursion in δ13Corg occurs in the upper Sumra Member and possibly correlates with the Late Triassic Spelae carbon-isotope event. The δ13Ccarb and δ13Corg values increase across the boundary interval, and an extended positive excursion in δ13Ccarb occurs in the oolites of the Sakhra Member. Sr concentrations increase in the bioclastic limestones of the uppermost Sumra Member and peak in the oolites of the upper Sakhra Member. High Sr concentrations also occur at correlative stratigraphical levels in the Lombardy Basin (Italy), and are interpreted as an Early Hettangian seawater signal, reflecting the global-scale reduction of biological aragonite precipitation and an increased continental weathering flux.

Atmospheric dust from the Pennsylvanian Copacabana Formation (Bolivia): A high-resolution record of paleoclimate and volcanism from northwestern Gondwana

This study documents the occurrence of atmospheric dust from Pennsylvanian carbonates of the Copacabana Formation, recovered in core (Mobil-Oxy Manuripi X-1) from the Madre de Dios basin (Bolivia), within southern mid-latitudes (~35°S) of western Gondwana. The Copacabana Formation spans Pennsylvanian-Early Permian time, and thus formed coeval with and in relative proximity to ice centers and associated glacial deposits located at southern paleolatitudes in adjoining regions of Gondwana (e.g. the Paraná, Tarija, and Paganzo basins in Brazil, southeastern Bolivia, and Argentina, respectively). In Pennsylvanian time carbonate deposition of the Copacabana Formation occurred on a ramp isolated from fluvial-deltaic influx, and thus siliciclastic material in this system reflects atmospheric input. The study interval comprises a series of upwardly shallowing successions 1–3m thick ranging from open marine ramp facies to more restricted inner-ramp facies, commonly capped by horizons of microkarsted and/or red mudstone reflecting subaerial exposure of the carbonate ramp. These horizons mark abnormal exposure and are interpreted to record glacial lowstands.Dust recovered from throughout the study section varies from ~1 to 43wt% in carbonate facies and is quartzo-feldspathic. Grain size modes range from <1 to 97μm, with coarser intervals generally corresponding to peak dust content (wt%), and high-frequency sequence (glacial-stage) boundaries. Provenance indicates two discrete sources of atmospheric input–a western volcanic arc source and eastern continental source, recording both westerly (zonal) and easterly (katabatic) wind directions. The western (volcanic) source records zonal westerlies expected at this mid-latitude (~35°S) locality. In contrast, easterly winds suggest the influence of katabatic winds associated with Gondwanan ice centers. The most likely dust-sourcing regions are the periglacial to proglacial regions of the Gondwanan ice sheets. Non-volcanic peaks in dust occur most commonly associated with subaerial exposure surfaces; this, together with mass accumulation rate estimations suggest that atmospheric dust loading peaked during glacial stages.

Age constraints on intra-formational unconformities in Upper Jurassic-Lower Cretaceous carbonates in northeast Turkey; geodynamic and hydrocarbon implications

Upper Jurassic-lowermost Cretaceous carbonate build-ups are imaged on seismic data in the Black Sea. They form important, untested, hydrocarbon reservoirs that are the focus of active exploration. Outcrop analogues to these build-ups around the Black Sea contain a series of subaerial exposure surfaces. The hiatuses associated with a number of these subaerial exposure surfaces have been dated in a well exposed Callovian or Upper Oxfordian to Barremian shallow-water inner platform carbonate succession (the Berdiga Formation) in the Eastern Pontides using strontium isotope stratigraphy and foraminiferal biostratigraphy. They span the latest Kimmeridgian to Tithonian or Berriasian, and the Hauterivian to Barremian. Less well constrained, but broadly contemporaneous stratigraphic gaps in multiple successions around the Black Sea provide additional insights and point to a regional driving mechanism. The timing of hiatus formation does not correspond to periods of eustatic lowstand. It does coincide, however, with Late Tithonian to Berriasian and Hauterivian to Early Aptian episodes of rifting in the Greater Caucasus Basin, located farther to the north. Thus, it is possible that subaerial exposure was caused by rift flank uplift during periods of regional extension. Uplift due to slab break off is discounted as a control because it post-dates (rather than pre-dates) locally developed Kimmeridgian magmatism. Rift-flank uplift is likely to have also affected carbonate build-ups on the intervening rift shoulders of the eastern Black Sea, the Shatskiy Ridge and the Mid Black Sea High. At outcrop, subaerial exposure is often associated with karstification and secondary porosity development. Similar processes may have occurred in the offshore helping to enhance the reservoir quality of these exploration targets.