Albian Sequence Research Articles

Aptian–Albian sequence stratigraphy, biostratigraphy, chemostratigraphy, and chronostratigraphy: Sonoran shelf and Tamaulipas Basin, Mexico

The Late Jurassic extensional Bisbee Basin in northwestern Mexico and southwestern United States lay between the Pacific magmatic arc and subduction zone and the cratonic Comanche shelf. Aptian to late Albian fluvial, deltaic, and shallow marine sediments and patch reefs of the Bisbee Group filled the basin. The upper Barremian(?)–lower Aptian Cerro de Oro Formation and its equivalent Rancho La Colgada Formation are overlain by the Morita, Mural, Cintura, and La Juana formations. The paralic Morita Formation and the marine Mural Limestone grade southeast into the marine Agua Salada and Lampazos formations, which correlate with deep-water facies in the Chihuahua trough and Tamaulipas Basin. Three late Aptian to middle Albian long-term depositional cycles were approximately 2.5 myr in duration.Foraminifers, calcareous algae, bivalves, rudists, corals, dinoflagellates and scarce ammonites compose the Mural Limestone biostratigraphy. First-occurrence datums of Aptian–Albian benthic Foraminifera are integrated with planktonic Foraminifera and rudists and define biostratigraphic zones that correlate with the Comanche shelf. In the Chihuahua trough and Tamaulipas Basin planktonic Foraminifera and nannofossils identify global zones. Stage boundaries on the Sonora shelf are correlated by a comprehensive biostratigraphic database.Carbon isotope chemozones correlate with Tethyan chemostratigraphy. The lower Aptian OAE 1a and uppermost Aptian–lowest Albian OAE1b chemozone are in the Lampazos section. The negative δ13C excursions and fossil evidence correlate with global Jacob, Kilian, Paquier, and Leenhardt levels. Low-oxygen water masses flooded the shallow shelf and even affected coral-rudist buildups.

Formation evaluation and reservoir quality assessment of the Albian clastic sequence in the SWM Oilfield, Southwest the Mesopotamian Basin, Iraq

This work intends to delineate the petrophysical properties and the reservoir quality of the Albian sequence in the Mesopotamian Basin which has been deposited as a siliciclastic sequence. The well log data (including the caliper, gamma-ray, density, photoelectric factor 'PEF', neutron, sonic, and resistivity logs), derived from eight wells in the SWM (Southwest Mesopotamian) Oilfield, was studied and interpreted using the appropriate software (IP3.6). These borehole data indicate the necessity to divide the sequence into five reservoir rock units (U1, U2, M, L1, and L2 units). This enabled depicting the porosity, shale volume, water saturation, and net-pay thickness in the vertical and horizontal directions. It is indicated that the M and L1 units are the most promising units with good to very good porosity (∅ ≥ 17.2%), low water saturation (Sw ≤ 28.0%), and acceptable shale volume (Vsh ≤ 8.4%). The net-pay thickness varies between 32.8 and 77.4 m in the different wells of the SWM Oilfield. The well log data were supported by more detailed conventional core data including porosity (∅He) and permeability (k), from which the reservoir quality parameters were estimated precisely. In the present study, reservoir quality assessment based on conventional core data is considered for the first time for evaluating and discriminating the Albian–Aptian sequence in the Southwest Mesopotamian field into reservoir zones and hydraulic flow units. The estimated reservoir parameters include the Flow Zone Indicator (FZI), the Reservoir Quality Index (RQI), and the effective pore radius (R35). Based on the available core data and the estimated reservoir quality parameters, the studied sequence was divided into three reservoir rock types (RRT1–3). The best petrophysical properties have been assigned for the medium-grained sandstone of the RRT1 that dominates in the L1 and the M units (av. ∅He = 23.0%, av. k = 1581 mD, av. R35 = 23.7 μm, av. FZI = 7.368 μm, and av. RQI = 2.304 μm). On the other side, the fine-grained sandstone of the RRT3 is characterized by the lowest petrophysical properties and reservoir quality parameters (av. ∅He = 17.1%, av. k = 5.49 mD, av. R35 = 0.92 μm, av. FZI = 0.603 μm, and av. RQI = 0.136 μm). The integration between the well log and core data of the studied Albian sequence stated that the M and L1 units are the most promising reservoir units. They are predominated by medium-grained and fine to medium-grained sandstones of the RRT1 and the RRT2, respectively. This well log-core data integration is applicable to the different oilfields in the different fields in the Mesopotamian Basin and other basins in south Iraq and elsewhere.

Detrital zircon U-Pb ages of the Cretaceous strata in the southern Songliao Basin, NE China: Constraints on basin-and-range evolution

The basin-and-range evolution of Northeast China could provide a comprehensive understanding of the subduction of the Paleo-Pacific Plate. The Songliao Basin preserved the essential and complete records of the Cretaceous tectonic evolution. The systematic detrital zircon UPb geochronological study was conducted on the Cretaceous sequences in the southern Songliao Basin, including Yingcheng Formation, Denglouku Formation, Quantou Formation, Qingshankou Formation, Yaojia Formation, Nenjiang Formation from bottom to up, providing new insights of provenances and tectonic evolution of the basin-range system. The Aptian and Albian sequences with the dominant age populations at ca. 133–125 Ma were derived from the Great Xing'an Range (GXAR). The Cenomanian and Early Turonian sequences with the distinct age populations at ca. 243–227 Ma received the source from the southernmost part of the Zhangguangcai Range (ZGCR). The Late Turonian to Campanian sequences experienced the gradual provenance transition to the GXAR. The main interim period with the dominant peaks at ca.1.8 and 2.5 Ga, and subdominant peaks at ca. 277 Ma, reveals the provenances from the southern margin, including the Inner Mongolia Paleo-Uplift (IMPU) and northern North China craton (NNCC) during the Late Turonian to Early Santonian. Since then, the provenance was skewed towards the GXAR manifesting the distinct age populations at ca. 139 Ma. The multi-provenance transitions reveal two orogeneses of the GXAR and ZGCR with the difference of 9 Ma. The gradual uplift from north to south in the Jiamusi Massif and the ZGCR at ca. 120–90 Ma was related to magmatic activity under the transition from the low angle subduction to slab rollback of the Pacific-Plate. The integral uplift of the GXAR at ca. 90–83.8 Ma was related to the transient compression due to the changes in direction and convergence rate of the Paleo-Pacific Plate. The compression-induced uplift was more intense than magmatism-induced uplift. Our findings can contribute to a better understanding of the basin-and-range evolution of Northeast China and the response to the subduction of the Paleo-Pacific Plate.

Application of seismic stratigraphy in reservoir characterisation: a case study of the passive margin deposits of the northern Orange Basin, South Africa

The Barremian-Cenozoic depositional sequences in the northern Orange Basin, SW, South Africa, were investigated using the principles of seismic stratigraphy to understand the interplay of tectonics and sedimentary processes in the distribution of potential hydrocarbon reservoirs. A seismic stratigraphic workflow (seismic sequence, seismic facies and lithofacies analysis) was completed by utilising three seismic lines (L1, L2 and L3) tied to Wireline data (gamma, checkshots and sonic) in two exploration wells (A1 and A2). Seven depositional sequences were mapped followed by the creation of lithofacies log interpreted from the gamma-ray log (GR) by setting maximum GR value at 60 API for Sandstone, 60–100 API for Siltstone and above 100 API for Shale. Six seismic facies units are recognised based on internal geometry and configurations of the seismic reflectors; Tangential-Oblique (SF1), Hummocky (SF2), Wavy-Parallel (SF3), Chaotic (SF4), Sub-parallel/parallel (SF5) and Divergent (SF6). SF4 is dominant within the Barremian-Aptian sequence and expressed in an incised valley fill, suggesting mass transport deposition accompanied by strong hydrodynamic conditions. Evidence of sedimentary basins progradation is seen within the Late-Albian-Turonian sequences, because of the occurrences of SF2, SF6 and SF 4 facies. SF5 facies is prominent in the Maastrichtian/Campanian sequence, indicating that the deposition of sediments may have been accompanied by uniform margin subsidence after the Late-Cretaceous uplift of the Africa margin. The occurrence of SF1 and SF4 facies within the Cenozoic sequence indicates terrigenous pro-deltaic deposits and mass transport deposits, respectively. Further results from seismic-lithofacies modelling reveal that sand deposits of Barremian-Aptian (SF4 facies unit) and Albian sequences (SF2 and SF6 facies units) are potential stratigraphic reservoirs in this part of the basin.

A combination of genetic inversion and seismic frequency attributes to delineate reservoir targets in offshore northern Orange Basin, South Africa

Abstract The reported occurrence of Albian- and Cenomanian-aged braided fluvio-deltaic channels in the Orange Basin, South Africa, opens a window of exploration activities to characterize these channels as they are renowned to form some of the world’s giant oil field. In this study, a seismic acoustic impedance inversion and seismic attributes (instantaneous frequency and iso-frequency) analysis is used to investigate potential Albian and Cenomanian fluvio-deltaic channels in offshore, northern Orange Basin. Reservoirs were mapped using a well and 3D seismic volume (8-bit) after initial dip-steering coherency filtering had been performed on the seismic volume to remove incoherent noise and improve data resolution. Model-based acoustic impedance inversion was applied on the seismic volume to delineate fluvio-deltaic channels in addition to using the RMS (root mean square) amplitude attribute. Iso-frequency using the cosine correlative transform (CCT) method was equally applied to delineate these channels. Instantaneous frequency attribute was analyzed for potential hydrocarbon-charged sediments. This was achieved by utilizing thirty-three seismic traces as an input in the Hilbert transform window, after which trace envelope and instantaneous phase were transformed into instantaneous frequency. Acoustic impedance inversion results reveal the presence of two channels within the Cenomanian sequence, which shows high porosity (∼40%) along its geometry. The CCT method shows that the 8 Hz frequency window resolved the presence of a channel within the Albian sequence. A meandering channel within the Albian sequence was equally delineated by the RMS, while the application of instantaneous frequency (IF) attribute indicates the presence of hydrocarbon-charged sediments of Cenomanian age in proximity to a listric normal fault because of the attenuation of frequency observed close to the fault. This study demonstrates a case study of the application of seismic impedance inversion and seismic attributes for the delineation of potential reservoirs and hydrocarbon-charged sediments in a basin.

Combining geology, hydrogeology and groundwater flow for the assessment of groundwater in the Zahrez Basin, Algeria

The Zahrez basin is one of the endorheic basins of the vast steppes region, which is located in central northern Algeria, about 300 km from Algiers, covering a surface area of approximately 9000 km2. Geological, hydrogeological, hydrochemical, and groundwater flow data are integrated and used to evaluate and characterize groundwater resources and to identify the main factors controlling groundwater flow in the region. The hydrogeological characteristics of the aquifer system show that Turonian sediments form the principal aquifer in the north-western part of the study area. This aquifer is continuous with an aquifer of Miocene-Pliocene-Quaternary age on the northern flank of the Djelfa Syncline. The mean hydraulic conductivity of the Turonian aquifer is about 10-4 m/s. Albian sandstone sequences constitute the most extensive aquifer in the Saharan Atlas mountain range, with typical hydraulic conductivities of between 10−6 and 10−2 m/s. A Barremian aquifer within the basin is most productive in the downstream parts, where the hydraulic conductivity is about 1.3 A— 10-5 m/s. Piezometric maps from 1994 to 2011 show groundwater flow within this aquifer converging from all directions towards the endorheic depression of the Zahrez Chergui and Zahrez Gharbi chotts, which form the main natural discharge areas.

Statistical analysis of forward stratigraphic models in complex salt provinces: The central Scotian Basin case study

Predicting vertical and lateral facies variations in various depositional environments remains a major challenge in the oil and gas industry because it impacts petroleum system assessments and the associated exploration-risking phase. The use of multidisciplinary constraints (geomorphology, geology, geophysics) in forward stratigraphic models sheds light on the complex interaction of local, regional, and global driving mechanisms that influence sediment transport and deposition along continuously evolving landscapes. In this paper, we develop an integrated statistical approach to examine the sensitivity of forward stratigraphic models in complex salt provinces to several parameters, including water discharge, sedimentary load, grain size and associated diffusion coefficients, and slope. This statistical analysis was applied to the Barremian–Albian sequence of the central Scotian Basin (Canada) and highlights the influence of complex salt kinematics on sediment pathway diversion and accumulation around salt domes and canopies. Forward stratigraphic modeling results point to regions of higher probability of Lower Cretaceous sandy reservoirs. Automating simulation runs significantly reduced the time required to achieve a statistically valid number of simulations and allowed the sensitivity of the model to be evaluated.

Carbonate production, accumulation and sea-level change in fault-block platform setting: a case study on the Albian sequences in Central Tunisia

The Albian series have been deposited on both sides of a major E–W paleogeographic fault, which was inherited in the early stages of the Tethyan-rifting phase. To the south of this fault, a shallow platform domain (footwall block or horst) occurred and is relayed northward by a slope-deep shelf setting (hanging wall block or graben). The platform to basin transition was analyzed northward of this fault nearby M’rhila Mountains. A detailed sedimentological study, coupled with a tectonic analysis, indicates that the Albian sedimentary deposits consist of four third-order depositional sequences. The three lower ones are only present in the deep-shelf setting and include substantial carbonate accumulations. The latter deposits are generally arranged into alternating mud-rich-bedded limestone and conglomeratic levels. Most of this material was exported from the adjacent shallow platform by mass flow and suspension transport processes. In the studied series, the maximum carbonate accumulation is recorded in the deep shelf during the moderate transgressive increasing and the late highstand decreasing rates of sea-level rise. However, the minimum carbonate sedimentation is thought to be linked to both the minimum lowstand and the maximum transgressive sea-level rise. The uppermost depositional sequence—which occurs on both sides of the major fault, is different from the three lower ones as it includes only scarce thin re-sedimented limestone beds. The youngest sequence is related to an important interplay between a pulse of tectonic subsidence and the major see level rise that happened during the upper Albian (100–98.9 Ma), resulting in a regional collapse of the substratum and the drowning of the carbonate factory situated southward. Therefore, the uppermost sequence is considered as a tectonically originated depositional sequence rather than a “pure” eustatic one. The organic-rich deposits associated to this sequence south and north of the major fault correlate well with the global anoxic events. The present study case represents a good example illustrating the role of local tectonics and global eustatic fluctuations in controlling not only the facies and thickness distribution but also the carbonate production and accumulation.

Seismic interpretation, attribute analysis, and illumination study for targets below a volcanic-sedimentary succession, Santos Basin, offshore Brazil

During the past decade, a significant exploration effort into deeper water and deeper targets in offshore areas brought more knowledge about the distribution of igneous rocks in the sedimentary basins along many continental margins. Nonhomogeneous illumination effects may occur below shallow, high-impedance igneous rock bodies. The seismic processing, depth imaging, interpretation, and attribute analysis require a special attention when these magmatic bodies affect the illumination of deeper targets. Usually, those structures are not considered in illumination studies, and the salt diapirs govern the analysis. In this work, seismic attributes are quite relevant to constrain the geologic model used to simulate the amplitude maps of the deeper target and to quantify the shadow effects observed on it. These amplitude shadows may either create (false) or hide (true) attribute anomalies. We have modeled a Tertiary volcano-sedimentary succession (VSS) mapped in the northeastern Santos Basin, offshore Brazil, to simulate the effect in the amplitude response of deeper targets. There were multiple magmatic events through the Santos Basin Cretaceous-Tertiary sedimentary succession, intrusive and extrusive. The igneous extrusive rocks intercalated with sediments forming VSS marked by low- and high-amplitude responses with tough lateral discontinuity. The amplitude found well-preserved architectural elements interpreted as lava flows and volcanoes, contrasting with layered sediments. We have defined geobodies constrained by relative impedance and 3D edge detection to build the 3D geologic model of the igneous successions used for seismic simulation. From the geobodies, we modeled two VSS to run the P-waves ray-tracing propagation to simulate the migrated amplitude map of the deeper Lower Albian sequence top. We computed the P-velocity and the density from well logs, and the seismic acquisition geometry was similar to the original. Comparison between real and simulated amplitudes showed the impact of shadow zones caused by shallower igneous bodies.

Transpressional structures and hydrocarbon potential along the Romanche Fracture Zone: a review

Abstract The Romanche Fracture Zone was originally a corridor of Aptian-age dextral transtensional rifting along the Equatorial Atlantic margins. Late Albian plate tectonic compression occurred due to a change in plate vectors, when the African and South American continents were still in contact across a 500 km-long section of the Romanche Fracture Zone. This dextral compression produced reactivation of the rift faults to produce asymmetric landward-vergent anticlines and thrusts that trend ENE to NE. Fold-axial planes dip seaward, parallel to the rift faults. Minor asymmetric anticlines were developed on the long seaward-dipping fold limbs and these have subvertical axial planes. The asymmetry of the minor folds is due to the southward stratal dip having been oblique to the horizontal maximum principal stress during the Albian inversion. The folds on the African margin were subsequently tightened by compression in Santonian and Oligo-Miocene times. Aptian-age ENE strike-slip faults were reactivated during the compression phases to produce broad positive flower structures up to 30 km wide that formed topographical ridges along the original strike-slip faults. The intervening and broader flat-bottomed synclines do not appear to be associated with rift faults. The folding and thrust faulting created seabed relief of 1–2 km at the end of the Albian; evidenced by the amount of subsequent erosion that removed the better-quality reservoirs in the upper Albian sequence from the major fold crests. Consequently, there has been a significant number of failed oil exploration wells drilled along the fold crests. The fold ridges would have diverted turbidite channels in the onlapping Cenomanian–Campanian sequence and these will be preferentially located on the landward side of the anticlinal crests. Late Cretaceous stratigraphic and structural traps located between the major anticlines have not yet been explored for hydrocarbons along the Romanche Fracture Zone margins.

Submarine allochthonous salt sheets: Gravity-driven deformation of North African Cretaceous passive margin in Tunisia – Bled Dogra case study and nearby salt structures

We used structural, stratigraphic and sedimentologic data, together with a comparison of nearby structures and a Bouguer gravity map, to evaluate the evolution of the Bled Dogra salt structure (northern Tunisia) during the Cretaceous. Triassic salt sheets are recognized in the northwestern region of the Tunisian Atlas. These salt sheets are the result of Cretaceous thick and/or thin-skinned extension along the south Tethyan margin. The Bled Dogra salt structure is one of these submarine allochthonous salt sheets, which was emplaced during the Early Cretaceous. The geologic framework, during this period, produces conditions for a predominantly gravity-driven deformation: extension has produced space for the salt to rise; vigorous differential sedimentation created differential loading that resulted in the emplacement and extrusion of a large volume of Triassic salt and formation of large submarine salt sheets. Geologic field data suggest an interlayered Triassic salt sheet within Albian sequences. Salt was extruded at the sea floor during the Early–Middle Albian and was initially buried by Middle–Late Albian strata. The Coniacian corresponds to a second transgressive cover onto the salt sheet after the gliding of the first salt cover (Late Albian–Turonian). In addition, this northwest Tunisian area exposes evidences for salt flow and abundant slump features at the base of a northward facing submarine slope, which was probably dominant from the Early Cretaceous to Santonian. Two gravity deformation processes are recognized: gravity gliding and gravity spreading. Acting concurrently, these two processes appear indistinguishable in this geologic context. Like the present-day salt-involved passive margins – such as the northern Gulf of Mexico, the Atlantic margin of Morocco, the Brazilian Santos basin, the Angola margin, Cadiz in western Iberia, and the Red Sea – the North African Cretaceous passive margin in Tunisia provides evidences that deformation in a passive-margin salt basin is predominantly gravity-driven deformation.

Seismic architecture of a Lower Cretaceous platform-to-slope system, Santa Agueda and Poza Rica fields, Mexico

Two three-dimensional seismic data sets over the Albian western Golden Lane margin and time-equivalent basinal deposits of Poza Rica field allowed us to investigate the linked architecture of a steep-sided carbonate platform (El Abra Formation) and a thick accumulation of redeposited carbonate sediment at the toe of the slope and in the basinal area (Tamabra Formation). Regional seismic cross sections show that the most aggrading Albian platform has an eroded platform top, a scalloped margin, and a channelized slope that are equivalent to a 20-km (12.4-mi)-wide, westward-thinning, thick toe-of-slope apron made of chaotic, contorted, mounded, moderate- to high-amplitude reflections. Detailed reflection geometries in the Albian toe-of-slope and basinal deposits consist of chaotic to short, discontinuous, low-amplitude reflection at the toe of the slope of the Golden Lane platform, laterally changing to a discontinuous mounded, shingling reflection, which ultimately turns into high-amplitude parallel reflections. We interpret this lateral change to reflect the seismic signature of the change from the block- and debris-flow–dominated toe-of-slope area, to debris-flow and concentrated density flow deposits in the basin that ultimately grade laterally into pelagic deposits. On a flattened seismic slice, mounded reflections correspond to lobate to fan-shaped seismic events several kilometers wide that are interpreted as a carbonate basin-floor fan. Comparison between core and seismic data shows a dominance of debris flows in the lower two Albian sequences (Albian 1 and Albian 2) that grade vertically into more lobate concentrated density flows and turbidites in the upper two Albian sequences (Albian 3 and Albian 4). Seismic data used in this study, combined with core observations, do not support the interpretation of the Albian Tamabra Formation being of shallow-water origin. Seismic features identified as basin-floor fan, channel, and debris-flow deposits have a shape and size that are similar to those of other redeposited basinal carbonate deposits elsewhere. The seismic architecture shows that the Poza Rica field is a typical example of thick accumulation of grainy porous carbonate deposits in a basinal setting. This example shows the potential of a large hydrocarbon accumulation in a tectonically modified stratigraphic trap around shallow-water carbonate platforms.

The uppermost Middle and Upper Albian succession at the Col de Palluel, Hautes-Alpes, France: An integrated study (ammonites, inoceramid bivalves, planktonic foraminifera, nannofossils, geochemistry, stable oxygen and carbon isotopes, cyclostratigraphy)

An integrated study of the ammonites, inoceramid bivalves, planktonic foraminifera, calcareous nannofossils, geochemistry, stable carbon isotopes, and cyclostratigraphy is provided for the upper Middle to upper Upper Albian sucession exposed in the Col de Palluel section east of Rosans in Hautes-Alpes, France. The Albian-Cenomanian boundary interval described by Gale et al. at Mont Risou is re-examined, a total thickness of 370 m of the Marnes Bleues Formation. Zonal schemes based on ammonites, inoceramid bivalves, planktonic foraminifera, and calcareous nannofossils are integrated with the stable carbon isotope curve and key lithostratigraphic markers to provide a sequence of more than 70 events in the uppermost Middle Albian to basal Cenomanian interval. Time series analysis of the Al2O3 content of the 500 m Albian sequence present in the Col de Palluel and Risou sections reveals the presence of the 20 kyr precession, 40 kyr tilt, 100 kyr short eccentricity, and 406 kyr long eccentricity cycles. Correlation using planktonic foraminiferan and nannofossil data provide a link between the Col de Palluel and Risou sections and the Italian sequence at Gubbio, and in the Piobbico core. This provides a basis for the extension of the orbital time scale of Grippo et al. to the sequence. It reveals a major break in the Col de Palluel succession at the top of the distinctive marker bed known as the Petite Vérole that may represent as much as 2 Ma. It also provides a basis for the estimation of the length of the Albian Stage at 4.12Ma, 0.8 Ma for the early Albian, 2.84 Ma for the Middle Albian, and 3.68 Ma for the late Albian substages.

Regional subsurface correlations of Albian sequences north of the Peace River in NE British Columbia: northward extent of sandstones of the Falher and Notikewin members along the eastern flank of the foredeep

Abstract The highly explored Peace River Region and Elmworth Deep Gas Basin have yielded a large database of geophysical wireline well logs and cores. Recent exploration and research within the Trutch, Fort Nelson and the Liard Basin regions are providing a growing database of geological information, allowing for reliable correlations between these two regions, filling the gap between 55.5° N and 58° N. In northeastern British Columbia and northwestern Alberta, four major phases of marine inundation can be traced regionally outlining the geometries resulting from the presence of the foredeep to the west. These phases are referred to paleogeographically in ascending order, as the Moosebar/Clearwater Sea, Hulcross/ Harmon Sea, Joli Fou Seaway, and Mowry Sea. Within the western foredeep, Albian sedimentation to the northwest is preserved as distinct marine shales and mudstones of the Buckinghorse Formation. To the south, this shale package is interrupted by distinct stacked, coarsening-upward sandstone packages. Along the eastern margin of the foredeep, regional wireline cross-sections are interpreted to show northward extensions of the Falher and Notikewin members, and the preservation of a Notikewin lowstand facies attached to the highstand. The highstand shoreline is interpreted to have been oriented in a southwest to northeast trend, with sandstone progradation occurring in a northwesterly direction. A core located within the lowstand deposit also provides evidence of tidally influenced incised valley fill deposits. The northward extent of the sandstone may be a result of proximity to the eastern flank of the foredeep, transport of sand north during storms over an overall shallow shelf setting and strong basin-wide currents allowing for the northward transport of these coarser deposits. In the southeast, as a result of erosion between the Paddy and Cadotte members, deposits of the Joli Fou Formation are not well preserved. In the north and west, however, an equivalent conformable suite of mudstones is thinning in an easterly direction. The equivalent Viking Formation marker bed in the foredeep can only be detected by a subtle resistivity change.

Lithology and geochemistry of Albian-Cenomanian sandy sequences of the Voronezh Anteclise and their paleogeographic implication

The paper proposes subdivision of Albian and Cenomanian monotonous sandy sequences of the Voronezh anteclise on the basis of textural-structural features and distribution of accessory minerals. Distribution of rare earth, trace, and major elements was studied in sandy rocks and early diagenetic formations (glauconites and phosphorites). Dependence of the distribution on tectonic factor is noted.

Aggradation of gravels in tidally influenced fluvial systems: upper Albian (Lower Cretaceous) on the cratonic margin of the North American Western Interior foreland basin

Alluvial conglomerates were widely distributed around the margin of the Early Cretaceous North American Cretaceous Western Interior Seaway (KWIS). Conglomerates, sandstones, and lesser amounts of mudstones of the upper Albian Nishnabotna Member of the Dakota Formation were deposited as fill-in valleys that were incised up to 80 m into upper Paleozoic strata. These paleovalleys extended southwestward across present-day northwestern Iowa into eastern Nebraska. Conglomerate samples from four localities in western Iowa and eastern Nebraska consist mostly of polycrystalline quartz with lesser amounts of microcrystalline (mostly chert), and monocrystalline quartz. Previous studies discovered that some chert pebbles contain Ordovician–Pennsylvanian invertebrate fossils. The chert clasts analyzed in this study were consistent with these findings. In addition, we found that non-chert clasts consist of metaquartzite, strained monocrystalline quartz and ‘vein’ quartz from probable Proterozic sources, indicating that parts of the fluvial system’s sediment load must have travelled distances of 400–1200 km. The relative tectonic stability of this subcontinent dictated that stream gradients were relatively low with estimates ranging from 0.3 to 0.6 m/km. Considering the complex sedimentologic relationships that must have been involved, the ability of low-gradient easterly-sourced rivers to entrain gravel clasts was primarily a function of paleodischarge rather than a function of steep gradients. Oxygen isotopic evidence from Albian sphaerosiderite-bearing paleosols in the Dakota Formation and correlative units from Kansas to Alaska suggest that mid-latitude continental rainfall in the Albian was perhaps twice that of the modern climate system. Hydrologic fluxes may have been related to wet-dry climatic cycles on decade or longer scales that could account for the required water supply flux. Regardless of temporal scale, gravels were transported during ‘high-energy’ pulses, under humid climatic conditions in large catchment areas. An overall rising sea level during the late Albian created accommodation space for the gravelly lithofacies equivalent to the Kiowa-Skull Creek rocks. As Western Interior sea level rose, regional stream gradients were reduced, resulting in regional fluvial aggradation. The conglomeratic lower parts of the Nishnabotna Member of the Dakota Formation formed the transgressive systems tract within an upper Albian sequence that is defined by two unconformities that can be traced from marine Kiowa strata in western Kansas northeastward into western Iowa (Brenner et al., 2000). Mud-draped cross-bedded sandstone bodies, laminated mudstone intervals, and vertical burrows in the lower strata of the Nishnabotna Member indicate that estuarine conditions existed at the mouths of the river system, and tidal effects were transmitted at least 200 km inland from the interpreted late Albian coast. These observations suggest that estuarine conditions stepped up the incised valleys as fluvial sediments aggraded in response to regional transgression that continued through the Late Albian.

Geology and Geochemistry of the Reocin Zinc-Lead Deposit, Basque-Cantabrian Basin, Northern Spain

The Reocin Zn-Pb deposit, 30 km southwest of Santander, Spain, occurs within Lower Cretaceous dolomitized Urgonian limestones on the southern flank of the Santillana syncline. The Reocin deposit is one of the largest known strata-bound, carbonate-hosted, zinc-lead deposits in Europe. The total metal endowment of the deposit, including past production and remaining reserves, is 62 Mt of ore grading 8.7 percent Zn and 1.0 percent Pb. The epigenetic mineralization consists of sphalerite and galena, with lesser marcasite and trace pyrite with dolomite as gangue. Microprobe analyses of different generations of dolomite revealed nonstoichiometric compositions with various amounts of iron (up to 14 mol % of FeCO3). Replacement of host dolomite, open-space filling of fractures, and cementation of breccias derived from dissolution collapse are the principal types of ore occurrence. Detailed cross-section mapping indicates a stratigraphic and structural control on the deposit. A stratiform morphology is present in the western part of the orebody (Capa Sur), whereas mineralization in the eastern part is highly discordant but strata bound (Barrendera). Stratigraphic studies demonstrate that synsedimentary tectonic activity, related to the rifting of the North Atlantic (Bay of Biscay), was responsible for variation in sedimentation, presence of unconformities (including paleokarsts), local platform emergence and dolomitization along the N60 fault trend. In the Reocin area, two stages of dolomitization are recognized. The first stage is a pervasive dolomitization of the limestone country rocks that was controlled by faulting and locally affected the upper part of the Aptian and the complete Albian sequence. The second dolomitization event occurred after erosion and was controlled by karstic cavities. This later dolomitization was accompanied by ore deposition and, locally, filling of dolomite sands and clastic sediments in karstic cavities. The circulation of hydrothermal fluids responsible for sulfide deposition and the infilling of karst cavities were broadly contemporaneous, indicating a post-Albian age. Vitrinite reflectance data are consistent with previously measured fluid inclusion temperatures and indicate temperatures of ore deposition that were less than 100°C. Carbon and oxygen isotopic data from samples of regional limestone, host-rock dolostone and ore-stage dolomite suggest an early hydrothermal alteration of limestone to dolostone. This initial dolomitization was followed by a second period of dolomite formation produced by the mixing of basinal metal-rich fluids with local modified seawater. Both dolomitization events occurred under similar conditions from fluids exhibiting characteristics of basinal brines. The δ 34S values of sulfides are between –1.8 and +8.5 per mil, which is consistent with thermochemical sulfate reduction involving organic matter as the main source of reduced sulfur. Galena lead isotope compositions are among the most radiogenic values reported for Zn-Pb occurrences in Europe, and they are distinct from values reported for galena from other Basque-Cantabrian deposits. This suggests that a significant part of the lead was scavenged from the local underlying Asturian sediments. The stratigraphic and structural setting, timing of epigenetic mineralization, mineralogy, and isotopic geochemistry of sulfide and gangue minerals of the Reocin deposit are consistent with the features of most of Mississippi Valley-type ore deposits.

A mid-Cretaceous age for the Palmer Land event, Antarctic Peninsula: implications for terrane accretion timing and Gondwana palaeolatitudes

New structural and age data suggest that West Gondwana may have been at lower palaeolatitudes than previously interpreted from Albian sequences in Gondwana marginal suspect terranes. The Palmer Land event, which juxtaposed Mesozoic terranes on the Gondwana margin, deformed granitoids in the southern Antarctic Peninsula. U–Pb SHRIMP dating of zircons from a microgranite dyke yields a crystallization age of 106.9± 1.1 Ma. This result and re-interpretation of the structural position of another granite pluton date the Palmer Land event, and probable terrane collision, as late Early Cretaceous, and not latest Jurassic as formerly interpreted.

Gamma-ray measurements in the Kirchrode I and II boreholes

The downhole γ-ray logs in the Kirchrode I and II boreholes through the Albian southeast of Hannover in Northwest Germany were used (a) to correlate the two boreholes, (b) to calculate the clay content of the sediments, and (c) to detect cyclicity in changes in sediment composition. The cross-correlation of the downhole γ-ray logs of the Kirchrode I and II showed that a shift of the Kirchrode II borehole downwards by 89 m provides the best correlation with the Kirchrode I borehole. A comparison of the γ-ray log of Kirchrode II with that of the nearby Prakla 2 borehole, for which biostratigraphic data is available, also yields a close correspondence. Using this correlation of the logs and adopting the stratigraphic subdivision of the Prakla 2 borehole by Kemper (1989), the Upper/Middle Albian and Middle/Lower Albian boundaries in Kirchrode II are at depths of 152 and 252 m, respectively. The γ-ray logs were used to calculate the clay content. It varies between 30 and 60%, indicating only small lithologic changes. This result is in agreement with values derived from analysis of the quartz and CaCO 3 contents of core samples. The montmorillonite, illite, and kaolinite contents of the clay fraction determined by Kühn et al. (2001), are the basis for an independent calculation of the clay contents of the core using mean literature values for the thorium and potassium contents of these three clay minerals. The periodicities identified by spectral analysis of the γ-ray measurements made every 5 cm throughout the recovered Albian sequence, can be interpreted in terms of Milankovitch cycles. Sliding window spectral analysis shows that the sedimentation rates were relatively constant during the Late Albian (8–12 cm/ka). Sedimentation rates of 4 cm/ka were determined for the Middle Albian.

Organic geochemistry and depositional history of Upper Albian sediments from the Kirchrode I borehole, northern Germany

In the framework of a multidisciplinary research program, an organic geochemical study was carried out on a drill core which comprises a 245 m thick sequence of light-colored, Upper Albian marlstones that were deposited in the central part of the Lower Saxony basin (northern Germany). For part of the Upper Albian sequence, high-resolution measurements of carbonate contents reveal cycles which can be related to earth orbital forcing. Based on these data, sediment accumulation rates were calculated to be in the order of 15 g/m 2yr 1. These high accumulation rates contrast with very low organic carbon contents and an extremely poor preservation of the autochthonous organic matter. Most of the sedimentary organic matter is of terrigenous origin and mainly derived from the erosion of older sedimentary rocks. Organic petrography reveals only a very small fraction of marine organic particles. Carbon/sulphur ratios, pristane/phytane ratios as well as the predominance of resedimented organic particles over autochthonous organic particles suggest that aerobic degradation processes rather than anaerobic processes (sulphate reduction) were responsible for the degradation of the organic matter. Furthermore, the scarcity of terrigenous organic particles (vitrinite) indicates that there was little vegetation on nearby land areas. To explain these analytical results, a depositional model was developed which could explain the scarcity of organic matter in the Upper Albian sediments. This model is based on downwelling of oxygen-rich, saline waters of Tethyan origin, which reduces the nutrient content of surface waters and thus primary bioproductivity while degradation of primary organic matter in the water column is enhanced at the same time. These conditions contrast to those which existed in Barremian and early Aptian times in this basin, when limited water exchange with adjacent oceans caused oxygen deficiency and the deposition of numerous organic carbon-rich black shales. The thick, organic matter-poor Upper Albian sequence of northern Germany also contrasts with comparatively thin, time-equivalent, deep-sea black shales from Italy. This discrepancy indicates that local and regional oceanographic factors (at least in this case) have a greater influence on organic matter deposition than global events.