Vertical Facies Variations Research Articles

Platform Evolution in an Oligo–Miocene Back‐arc Basin: An Example from the Central Iran Basin

AbstractThe Qom Formation is the most important hydrocarbon reservoir target in Central Iran. The Qom platform developed in a back‐arc basin during the Oligo–Miocene due to the closing of the Tethyan Seaway. This formation consists of a variety of carbonate and non‐carbonate facies deposited on a platform ranging from supratidal to basin. A combination of tectonic and eustatic events led to some lateral and vertical facies variations in the study area. Six third‐order depositional sequences and related surfaces were identified regarding vertical facies changes in the studied sections of this Oligo–Miocene succession. According to all results and data, this succession was initially deposited during the Chattian upon a distally steepened ramp of siliciclastic‐carbonate composition, including the Bouma sequence. Then, from the late Chattian to the Aquitanian, the platform changed into a homoclinal carbonate ramp with a gentle profile. With respect to tectonic activity, this phase was a calm period during the deposition of the Qom Formation. Finally, a drowned carbonate platform and a rimmed shelf emerged during the Burdigalian, terminated by the continental deposits of the Upper Red Formation. Regarding all geological characteristics, three main tectono‐eustatic evolutionary phases have been recognized in the Qom back‐arc basin.

High-resolution lithostratigraphy and reconnaissance sedimentology of Changotaung structure, Chittagong Tripura fold belt, Bengal Basin, Bangladesh

Unlike other structures in the vicinity of the Chittagong Tripura Fold Belt, the Changotaung anticline is one of Bangladesh's least explored structures. An attempt has been made for the first time to understand and document sedimentary deposits, environments, structure, and tectonic activity based on the high-resolution outcrop and reconnaissance study with the knowledge of broad-brush geology. We found that Changotaung is a symmetrical box-folded structure with an extensive western flank where the amount of dip varies between 11° and 45°. The exposed Cenozoic succession was categorized into three separate sedimentary sequences and correlated with the conventional stratigraphic unit. A first-order simple Markovian approach was presented for the exposed litho-section in an effort to illustrate vertical facies variations in the Upper Surma group. We quantified that heterolithic bed mostly overlies both trough cross-bedding ({P}_{ij}U = 0.706) and parallel laminated bed ({P}_{ij}U=0.955) according to the facies transition probability matrix. According to the results of the stationary distribution, there is a 40% chance of coming across heterolithic beds within the Upper Surma group during any given event that is completely random whereas trough cross-bedding, parallel laminated bed, cumulative sandstone facies, and cumulative shale facies shows around 10.8%, 15.2%, 20.6%, 13.4% probability. We hypothesized, based on the interpretive facies analysis, that the Chittagong Tripura fold belt region's Upper Surma Group underwent three interrelated depositional settings (wave-dominated shallow marine, tide-dominated shallow marine, and fluvio-deltaic distributary).

Assessment of cation exchange as conditioning processes of water chemistry in freshwater lenses.

Freshwater lenses are groundwater sources of limited dimensions that can be usually found in a variety of climates worldwide. These aquifers' quality is important for socioeconomic development, being cation exchange one of the most important geochemical processes that can change the water geochemistry. This study aims to assess the cation exchange processes which determine the chemistry of freshwater lenses in a multilayer aquifer type, considering the center-east of the Pampean Region (Argentina) as a case study. Water samples were taken from the freshwater lenses at different depths to analyze major ions in the laboratory. In addition, geological profiles were made along with the extraction of sediment samples for XRD and laboratory tests to analyze the cation exchange capacity. The results show that water stored in the lenses has a vertical facies variation from Ca-HCO3 to Na-HCO3 . According to the laboratory results, the change of water facies mainly occurs in the clayey sediments which divide the carbonate bioclastic material above and the loessic sediment below, being cation exchange the most important process.

Controlling factors of acoustic properties in continental carbonates: Implications for high-resolution seismic imaging

Continental carbonates are characterized by multi-scale heterogeneous porous networks, making the geological interpretation of seismic imaging difficult. We investigate two sedimentary sections exhibiting a similar facies succession, combining geological characterizations and multi-scale acoustic measurements. Based on outcrop investigation and petrographic description, we define nine sedimentary facies displaying contrasted early diagenetic evolutions. According to the vertical facies variations, we develop a depositional model corresponding to a low gradient valley fed by freshwaters, subdivided into three main domains (alluvial plain, palustrine and paludal). To understand the acoustic properties of the studied sedimentary rocks while remaining representative of their multi-scale heterogeneity, we acquire acoustic measurements at two different scales: i) at log-scale, directly on the outcrop surfaces using a frequency of 250 kHz; and ii) at plug-scale as usually done in laboratory using a frequency of 500 kHz. Based on these multi-scale geophysical acquisitions, we link in-situ P-wave velocities with the different sedimentary facies while characterizing centimeter-scale Representative Elementary Volumes (REVs). Conversely, based on laboratory measurements and thin-section petrography, we define relationships between P-wave velocity, porosity, facies, and diagenesis, corresponding to millimeter-scale REVs. Using both in-situ P-wave velocity measurements and plug densities, we construct 1-D synthetic seismograms showing meter-scale seismic reflectors equivalent to crosswell seismic frequency ranges. This approach shows the following: i) high-amplitude seismic reflectors fit with facies changes associated to diagenetic contrasts (e.g. cemented versus uncemented carbonates); ii) reflection free-zones match with a succession of facies changes affected by diagenetic homogenization (e.g. intensely to pervasively recrystallized and cemented carbonates). Our work highlights the importance of relating an extensive geological description of carbonates (facies, depositional model, diagenesis) together with multi-scale acoustic measurements and synthetic seismic modelling to predict the high-resolution heterogeneities of subsurface reservoirs.

FACIES ANALYSIS AND STRATIGRAPHIC SEQUENCE ARCHITECTURE IN A BACK-ARC BASIN IN CENTRAL IRAN: A CASE STUDY FROM THE EARLY MIOCENE OF THE QOM FORMATION

Marine deposits of the Qom Formation, which is an important gas reservoir in Central Iran with the age of Early Oligocene to Early Miocene, is studied to determine facies, sedimentary paleoenvironment, and depositional sequences. The primary lithology is limestone, which is accompanied by a conglomerate, sandy marl, marl, and sandy limestone. Based on siliciclastic content, textural analysis, and the biotic constituents, ten facies have been identified. These facies belong to five depositional settings, including delta, a tidal inlet, lagoon, shoal, and open marine. According to the absence of continuous and large barrier reefs, gradual vertical variation in facies from the transitional environment (delta) to shallow open marine, the absence of oncoid, pisoid and aggregate grains that are mostly present in rimmed carbonate shelf environments, the absence of calciturbidites and slump and slide structures, the Qom Formation has been deposited in a homoclinal ramp setting (inner, middle and outer ramp). Field studies and vertical facies variation architecture in the framework of depositional system tracts led to the recognition of two 3rd order depositional sequences in the Early Miocene (Aquitanian) time. Sedimentary facies in the Qom Formation that mainly occurred in the middle ramp setting reveal a mostly aggredational stacking pattern in depositional sequences. The Early Miocene sequences stratigraphic architecture of the Qom Formation based on correlation charts are similar to the regional sequences of the Arabian plate and Zagros basin.

Interplay of eustatic, tectonic and autogenic controls on a Late Devonian carbonate platform, northern Canning Basin, Australia

AbstractFrasnian reef complexes along the northern margin of the Canning Basin in northwestern Australia evolved during rifting of the Fitzroy Trough. Geological investigations of the Frasnian Hull platform, which developed on an active tilted fault‐block, reveal significant lateral and vertical facies variations superimposed on prominent metre‐scale cyclicity. This study uses numerical analyses of facies and magnetic susceptibility data from three measured sections along the Hull platform to test whether a tectonic signal can be distinguished from eustatic and other signals.Geostatistical analysis of facies variations reveals an exponential distribution of thin (<3 m) facies, characteristic of stochastic depositional processes. Thick subtidal facies predominate in the Guppy Hills (GH) and southeastern Hull Range (SHR) sections near the hangingwall margin, and thick shallow‐subtidal to intertidal facies dominate the Horse Springs drillcore (HD 14) section near the footwall margin. Power and wavelet spectral analyses indicate a strong periodic component; Average Spectral Misfit and spectral optimisation methods confirm the presence of Milankovitch eccentricity signals and suggest the presence of obliquity and precession signals. However, the results also expose strong temporal and spatial variation providing evidence for tectonic control. Spectral analyses show strongest periodicity is recorded in short intervals that are not correlated across the platform and provide evidence of variations in sedimentation rate and hiatuses. Time series for the neighbouring GH and SHR sections show no overall statistical correlation, and Markov analysis indicates weakly ordered vertical facies transitions that do not correlate across the platform. Subtidal to intertidal facies data from HD 14 core suggest that at least 35% of the section is absent, almost obscuring the Milankovitch signal. The results indicate a complex set of controls on deposition on the Hull platform with local tectonic effects having produced spatio‐temporal moderation of the underlying eustatic signals and autogenic processes adding a localised stochastic response.

Hiperpicnitos (Inunditos) como análogos de reservatórios: um exemplo nos depósitos Gonduânicos Triássicos do Gráben Arroio Moirão, Rio Grande do Sul

Os arenitos associados a modelos episódicos de inundação, como os Inunditos provenientes de fluxos hiperpicnais em corpos de água rasa, podem ser reservatórios de hidrocarbonetos. No entanto, há poucos estudos para entender a arquitetura e heterogeneidade desses modelos, o que resulta em dificuldades para exploração e explotação desses reservatórios. O presente trabalho visa à análise de pacotes triássicos da Bacia do Paraná pertencentes à Formação Santa Maria, que ocorrem como fragmentos isolados sobre o Escudo Sul-Rio-Grandense na região do Gráben Arroio Moirão (Rio Grande do Sul). Para tanto, fez-se um estudo comparativo de modelos de inundação e como metodologia de obtenção de dados utilizou-se mapeamento sistemático, análise de heterogeneidade de fácies e estratigrafia de sequências, que incluem levantamento de perfis colunares, identificação de associações e sucessões de fácies e superfícies-chave. Secundariamente, algumas análises petrográficas foram realizadas. Foi possível delimitar os principais depósitos de arenitos, dos quais predominam geometria tabular, grãos mal selecionados e alguma matriz argilosa. Distinguiram-se cinco ciclos deposicionais granodecrescentes ascendentes, limitados na base e no topo por superfícies erosivas, que marcam heterogeneidades recorrentes. As fácies-reservatório são subarcósios, de matriz argilosa com agregados de caulinita, porosidade intergranular e shrinkage. Identificaram-se potenciais fácies-reservatório constituídas de arenitos grossos a conglomeráticos, com estratificações cruzada tangencial e plano-paralela. A integração dos dados resultou na elaboração de um modelo de variação lateral e vertical de fácies de hiperpicnitos lacustres, como um potencial modelo análogo de reservatório do tipo compartimentado. Esses resultados possibilitam prospectar outros depósitos arenosos dessa unidade estratigráfica da Bacia do Paraná, para fins de dimensionar regionalmente o análogo de reservatório.

Calcareous nannofossil biostratigraphy of the External Dinarides flysch (Vrčić‐Staravasa Pag Island, Croatia): A key to an Eocene tectono‐stratigraphic and paleoenvironmental interpretation

It is largely accepted that in the Dinaric shallow foredeep basin, the onset of Cenozoic synorogenic sedimentation is diachronous along strike and shows orogen‐parallel southward younging, creating a large uncertainty in the age of the Dalmatian flysch. Calcareous nannofossil biostratigraphy was studied to investigate a turbiditic succession exposed in Pag Island (Croatia) with the main purpose of constraining its age, which is still a matter of debate. The age assignment for the turbiditic deposition bears important implications for better understanding of the geodynamic and paleoenvironmental evolution of the External Dinarides. We logged a well‐exposed sedimentary section in the south‐western limb of the Pag anticline and performed calcareous nannofossil quantitative analyses revealing a high species diversity and abundant assemblage. The age diagnostic species indicate CNE14‐CNE15 biozones, suggesting that flysch deposition occurred in the Lutetian–Bartonian. This age can be further restricted to the CNE14 (42.37–40.51 Ma) by means of the lowest occurrence of Reticulofenestra reticulata and the highest occurrence of Sphenolithus furcatolithoides. Vertical facies variation and paleoecological indications suggest an increase in paleobathymetric depth during deposition. Our data support a post‐Lutetian age of folding and thrusting for the Pag Island region and suggest a relatively short time span during the middle‐late Eocene period for the main contractional stage in this sector of the External Dinarides. This result has important implications for the evolution of the Dinaride foreland basin system in Cenozoic times.

Grain size fractionation by process‐driven sorting in sandy to muddy deltas

AbstractModern and ancient analogues are often consulted by geologists to help understand subsurface systems. While modern analogues provide information on the areal relationship between facies, ancient systems provide detailed data on the vertical facies variations, typically along a two‐dimensional outcrop. Combining data from modern and ancient systems effectively requires translating areal morphology, which is often still evolving, to the related sediments preserved in three dimensions. Process‐based models simulate both depositional processes while preserving stratigraphy. These models can be employed to unravel the relationship between sediment supply and preserved deposits in natural systems and to help integrate field data. Four synthetic deltas were modelled using different sediment supply compositions, from coarse to very fine sand systems. The resultant sedimentary deposits are classified into architectural elements, and the grain size composition of each architectural element is studied over time. Facies that are extensive in their horizontal dimensions are often less abundant in three‐dimensional preserved deposits. Between deltas, grain size compositions of a specific architectural element type (e.g. mouth bars) are more similar than their corresponding sediment supply compositions. This is due to selective deposition of grain size classes across each architectural element type. This selective deposition causes overrepresentation of the same range of grain sizes, even for systems with different sediment supply compositions. When a particular supply composition does not contain enough of the overrepresented grain size class for a particular architectural element, that element will be under‐supplied and constitute a smaller proportion of the overall delta deposits. It is imperative to account for over‐representation of grain size classes in particular architectural elements when estimating palaeo‐sediment supply, delta architecture and morphology from field data. Even when data availability/accessibility does not allow the inclusion of distal deposits in field studies, process‐based simulations can contribute valuable information on sediment sorting patterns in three dimensions.

Recognizing metasedimentary sequences potentially hosting concealed massive sulfide accumulations in the Iberian Pyrite Belt using geochemical fingerprints

Promising advances in the development of new greenfield and brownfield exploration methods and methodologies for massive sulfide ore-systems in the Iberian Pyrite Belt (IPB) have been made over the last two decades. However, the effects of lateral and vertical facies variations in metasedimentary piles forming the two main lithostratigraphic units that potentially host massive sulfide deposits [the Phyllite-Quartzite Group (PQG) and the Volcanic-Sedimentary Complex (VSC)] on mineral exploration remain unknown. Herein we report on results of a multi-element geochemical study of 133 samples representing PQG and VSC metasedimentary rocks from the Albernoa area (≈713 km2); this dataset is complemented by whole-rock Sr, Nd and Pb isotopic data for a subset of 12 VSC metasedimentary rock samples.The prevalent siliciclastic component in the analyzed rocks derives from an evolved “felsic source” (as suggested by the Zr/Sc, Th/Sc, Eu/Eu* relationships) that, according to CIA values, should have been affected by significant weathering before they were deposited in the basin from Middle Devonian to Visean time. The siliciclastic component comprises clay and (fine-grained) sand whose relative abundance can be distinguished on the basis of Al2O3/TiO2, Sc/TiO2, Ga/TiO2, Zr/TiO2 and SiO2/Al2O3 ratios. These terrigenous components are, in places, variably mixed with volcanic-derived fractions, leading to the formation of tuffaceous metapelites typical of the VSC sequences but also present in the upper stratigraphic sections of the PQG. In general, VSC samples display the following parameters 2.5 ≤ SiO2/Al2O3 ≤ 6.5, 0.02 ≤ TiO2/Al2O3 ≤ 0.06 and 0.2 ≤ (CaO + Na2O + K2O)/Al2O3 ≤ 0.25, whereas PQG samples display 2.5 ≤ SiO2/Al2O3 ≤ 12, 0.08 ≤ TiO2/Al2O3 ≤ 0.58 and (CaO + Na2O + K2O)/Al2O3 ≤ 0.3. Fe2O3/TiO2 ratios ≥ 10 and Al2O3/(Al2O3 + Fe2O3 + MnO) ratios ≤ ≈0.6 indicate the strong influence of an early-developed (prior to Variscan metamorphism and deformation) hydrothermal component on the prevalent siliciclastic component sometimes mixed with a variable volcanic-derived fraction. Moreover, values of (As + Sb)/Sc, (Cu + Zn + Pb)/Sc and 5 × [(Fe2O3 + MgO + MnO)/Al2O3] ratios between 1 and 10 indicate the influence of seafloor metasomatism processes, and ratios ≥ 10 indicate close proximity to hydrothermal discharge potentially associated with massive sulfide mineralization.The Sr, Nd, Pb isotopic data show that VSC metasedimentary rocks are mostly derived from old, reworked, upper continental crust. Isotope ratios characterizing the main siliciclastic component are somewhat disturbed by the significant presence of volcanic-derived components and/or mineral changes due to early hydrothermal alteration/mineralization processes, which should have involved isotopically distinct fluids.

Ground penetrating radar in the medieval oyster shell middens of Saint-Michel-en-l'Herm (Vendée, France)

This study presents the integrated results of GPR (Ground Penetrating Radar) and sedimentological analysis, performed on the giant shell middens of the medieval site of Saint-Michel-en-l'Herm (Vendée, France). Avoiding the use of destructive methods like large-scale digging, the purpose of this study was to determine the internal structure of the northern midden and the links with its substrate, and to obtain insight on the manner of its construction. Four GPR profiles were obtained along the main axis of the midden, on the shell mound, and at its foot, in an area leveled by the former extraction of shells. Sedimentological analysis is also presented from study of the extant parts of the midden and with hand auger cores taken along one of the GPR profiles. The integrated results show that each sedimentological facies is related to a specific radar facies, allowing the mapping of lateral and vertical facies variations along the GPR sections. Within these deposits, the major tilted reflectors correspond to thin soils and trampled surfaces. Their geometry and the stacking pattern highlight multiphase deposition triggered by successive inputs of large volumes of shells (in the range of 100m3), from west to east. Below the exploited parts of the midden, the data are interpreted to show the presence of undisturbed shells, although, an ancient dug area was also identified by GPR. Finally, sedimentological and GPR data both show evidence of a midden established on the west bank of a channel, progressively infilling the channel eastward. These results illustrate the relevance of GPR in answering major questions common to all large shell middens.

Subsurface Seawater Intrusion, an Additional Factor Influencing the Limnological History of the Dead Sea Basin

<p class="emsd-body"><span lang="EN-GB">A deep core hole, drilled in the middle of the Dead Sea penetrated the Pleistocene- Holocene section, revealed an alternating sequence of fresh water and evaporitic (gypsum, halite) deposits. The vertical facies variations were interpreted as related mainly to lake level changes during this period. The present study, however, proposes an additional factor that influenced these changes, namely subsurface seawater intrusion from the Mediterranean Sea to the endorheic Dead Sea Basin. This proposed process is controlled by the elevation and head difference between both base levels at a given time, because the Mediterranean Sea level also fluctuated during the discussed period. We find that in times of smaller head differences, and assumed lower seawater intrusion, a gypsum facies prevailed in the Dead Sea Basin. In times of greater head differences and assumed more abundant seawater intrusion a halite facies prevailed because of greater sodium chloride input into the Dead Sea.</span></p>

A retreating fan-delta system in the Northwestern Junggar Basin, northwestern China—Characteristics, evolution and controlling factors

In this study, we use seismic reflection, logging and core data to investigate the depositional systems and their controlling factors in the lower Triassic in the northwestern margin of Junggar basin. The succession shows strong lateral and vertical facies variation. The facies associations indicate the development of conglomeratic fan-delta system, with characteristics of fining-upward and landward migration of facies belts. A critical balance between progradation, aggradation, and retrogradation controlled the evolution of delta successions within an overall transgressive regime. Migration of the locus of active faulting exerted the primary control on fan-delta thickness. Variations on the sandy component and gravel type within the fan-delta system among different units indicate the control of provenance. The climate significantly influenced the mode of sediment transfer from the feeder system to the basin. The significant climate change from arid to humid played a primary role in controlling the fan-delta architecture. Our study suggests that in tectonic-active basin, the controls of tectonics on the sequence stratigraphy are still fundamental. However, during the tectonic quiescence or rift-depression transition stage, significant changes of climate would play a primary role in controlling the sediment dispersal. The current study reveals that the hydrocarbon reservoir shows a landward migration trend within a retreating fan-delta system.

Fracture distribution along an Upper Jurassic carbonate ramp, NE Spain

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

The syntectonic evolution of foredeep turbidites related to basin segmentation: Facies response to the increase in tectonic confinement (Marnoso-arenacea Formation, Miocene, Northern Apennines, Italy)

The Marnoso-arenacea Formation (MAF) records the progressive closure of the foredeep due to the north-eastward propagation of thrust fronts. The latter produced a complex foredeep characterized by the uplift of the basin inner portions and by the north-eastward shifting of the main depocentres. This work discusses the evolution of the MAF inner basin and the first deposits of the outer basin in a transect perpendicular to the paleocurrents and the main tectonic structures that are NW–SE-oriented. Specifically, this study focuses on the upper Serravallian transitional stage, which records the closure of the inner basin and the deposition shifting to an outer basin due to the growth of an important regional structure represented by the NW–SE-oriented M. Castellaccio thrust-related fold (CTRF). The growth of this structure produces progressive segmentation of the foredeep through the progressive isolation of the inner basin that becomes, in the upper Serravallian phase of basin narrowing, a ponded piggyback basin. During this evolution, the structural control and associated morphologic confinement increased over time, controlling the lateral and vertical distribution of turbidite facies that is very similar to that of intra-slope minibasins. In such a situation, vertical and lateral facies changes, associated with evident stratigraphic pinchings and onlap relationship, have played a key role in showing this phase of strong foredeep segmentation and basin confinement. This phase is marked by a well-determined vertical facies variation: contained-reflected beds pass upward into huge accumulations of sandstone lobes featuring the appearance of Ophiomorpha trace fossils and coarse-grained massive sandstone beds often overlain by mud-draped scours and tractive structures (megaripples and traction carpets). These beds are interpreted as related to flow decelerations induced by structurally-controlled basin confinement that favour the deposition of massive sandstones through hydraulic jumps and flow decoupling with bypass of upper turbulent flows that can produce well-developed tractive structures.

Stratigraphic Record of River-Dominated Crevasse Subdeltas With Tidal Influence (Lajas Formation, Argentina)

Abstract Crevasse subdeltas develop on modern river-dominated delta plains, and may be affected by the interaction of river currents and marine processes. However, their sedimentology and stratigraphic architecture is poorly constrained, leading to simplistic depositional models of delta-plain systems in the ancient record. Extensive exposures of the Middle Jurassic Lajas Formation permit the architecture, main stratigraphic surfaces, and lateral and vertical facies variations of crevasse subdelta deposits to be constrained. Lower-delta-plain successions studied in the Lajas Formation consist of up to 5-m-thick distributary channels and interdistributary-bay deposits, interpreted as crevasse subdeltas. Crevasse subdelta deposits consist of small-scale lenticular units (∼ 1–2 m thick) interpreted as crevasse channels and upward-coarsening and upward-thickening packages (∼ 2 m thick) with clinothems interpreted as crevasse mouth bars. These deposits preserve interbedding of coarser and finer sediments that are interpreted as river flood and interflood couplets associated with variations in river discharge. River flood beds are commonly structureless and erosionally based, and show little evidence of tidal action and brackish-water conditions. Interflood deposits show rhythmically distributed mudstone drapes, bimodality, and brackish trace fossils. This study highlights an important but largely undocumented component of interdistributary deposits consisting of tide-influenced, but strongly river-dominated, prograding depositional bodies. An implication is that some coarsening-upward, forward-accreting units previously interpreted from the rock record as interchannel “tidal bars” may instead represent minor mouth bars of tide-influenced crevasse subdeltas. Furthermore, present-day crevasse subdeltas are restricted to river-dominated delta systems that flow into semi-enclosed or enclosed seas and lakes with microtidal conditions and limited wave action, which is comparable to paleogeographic reconstructions for the Neuquen Basin during the Middle Jurassic.

Nature of the Campano-Maastrichtian Sub-Basins in the Gongola Basin, Nigeria

Geological mapping was carried out and outcrop sections were examined alongside palaeocurrent directions from the greater part of the Campano-Maastrichtian outcrops in the Gongola Basin in Nigeria. Palaeocurrent measurements were obtained from asymmetrical ripples, cross bedding and dune bedforms within the channel-filling sub-facies of the Arowa Formation, and the foresets of the tabular cross bedding displayed in the Duguri formation. A north-westerly to a northerly paleocurrent direction was determined for the Arowa formation, indicating that the open sea was located to north-north-west, while a unimodal palaeocurrent directions characterize the Duguri Formation. These currents transported sediment into the Dukku, Akko and Bashar sub-basins with an overall centripetal pattern. The main structural features of the western part of Gongola Basin from E-W, are the N-S, NE-SW and NW-SE trending faults. These series of faults controll the basin subsidence and deposition of the Campano-Maastrichtian succession in the Dukku, Akko and Bashar sub-basins.The lateral and vertical facies variation within the Campano-Maastrichtian succession are in consistence with the northerly retreat of the sea.

SEQUENCE STRATIGRAPHIC ANALYSIS OF THE MID‐CRETACEOUS MISHRIF FORMATION, SOUTHERN MESOPOTAMIAN BASIN, IRAQ

The middle Cenomanian – early Turonian Mishrif Formation, a major carbonate reservoir unit in southern Iraq, was studied using cuttings and core samples and wireline logs (gamma‐ray, density and sonic) from 66 wells at 15 oilfields. Depositional facies ranging from deep marine to tidal flat were recorded. Microfacies interpretations together with wireline log interpretations show that the formation is composed of transgressive and regressive hemicycles. The regressive hemicycles are interpreted to indicate the progradation of rudist lithosomes (highstand systems tract deposits) towards distal basinal locations such as the Kumait, Luhais and Abu Amood oilfield areas. Transgressive hemicycles (transgressive systems tract deposits) represent flooding of the shallow carbonate platform and are recorded in oilfields such as Amara, Halfaya and Zubair.A sequence stratigraphic framework has been constructed for the Mishrif Formation based on correlation of the transgressive and regressive hemicycles which are separated by maximum flooding surfaces. Three third‐order sequences are identified which show lateral and vertical facies variations depending on relative sea‐level changes. Sequence boundaries are characterized by karstic, exposure and drowning features. Middle Cenomanian – Turonian eustatic sea‐level changes together with regional‐scale tectonic deformation of the Arabian Plate controlled the availability of accommodation space and therefore the depositional profile during development of each sequence. Both of these factors controlled the maximum flooding surfaces and sequence boundaries which have been identified.The sequence stratigraphic key surfaces presented in this study represent typical candidate horizons or datum surfaces for future seismic or resevoir modelling studies. Also, lateral facies variations in each transgressive‐regressive sequence and associated carbonate bodies (i.e. prograding shelf margin, forced regressive wedge) may form important stratigraphic traps in the Mesopotamian Basin.

Geology and geophysics at the archeological park of Vulci (central Italy)

<p>The Vulci archeological site was object of interest by the Soprintendenza ai beni culturali dell’Etruria meridionale (Italian government department responsible for southern Etruria’s cultural heritage) since the beginning of the 20th century. In 2001, the Ministero dei Beni Culturali (Italian ministry of cultural heritage) along with the local authorities, opened a natural-archeological park. In this area, it lies most of the ancient Etruscan city of Velch (today known by its Latin name, Vulci) including the Osteria Necropolis that is the object of this study. Recently, new archaeological excavations were made and the local authorities needed major geological information about the volcanic lithotypes where the Etruscans used to build their necropolis. The aim of this study is to define the geological and geophysical characteristics of the rock lithotypes present in the Vulci park. For this purpose, a geological map of the area (1:10000) has been realized. Moreover, two different geophysical methods were applied: measurements of magnetic susceptibility and electrical resistivity tomography. Magnetic susceptibility analyses clearly identify magnetic contrasts between different lithotypes; the characteristics of the pyroclastic flow that originated the Sorano unit 2 and its vertical facies variations are well recorded by this parameter that along with lithostratigraphic observations provides information about the depositional conditions. Two electrical resistivity tomographies were performed, which show the Sorano unit 2 thickness to be of c. 7 m with resistivity values ranging from 200 to 400 Ω·m. This kind of multidisciplinary approach resulted to be suitable to study this type of archaeological sites, revealing that areas characterized by a relevant thickness and wide areal extension of volcanic lithotypes can be a potential site where Etruscans might have excavated their necropolis.</p>

Sequencing the Sturtian icehouse: dynamic ice behaviour in South Australia

The Cryogenian record of South Australia includes the type region of the Sturtian glaciation, the oldest of three pan-global icehouse intervals during the Neoproterozoic. Data are presented from previously little described sections at Holowilena Creek, Oladdie Creek and Hillpara Creek in the central and southern Flinders Ranges, where five facies associations are recognized. These are (1) diamictite and conglomerate, (2) interbedded heterolithic deposits, (3) hummocky cross-stratified sandstone, (4) lonestone-bearing siltstone, and (5) ferruginous siltstone and sandstone. The succession reveals significant lateral and vertical facies variation, which is linked to a complex inherited palaeotopography and distance from the sediment source. Repeated stratigraphic occurrences of striated clasts and abundant ice-rafted debris strongly support recurrent glacial influence on sedimentation. The intercalation of gravitationally reworked diamictites, dropstone-bearing siltstone and dropstone-free siltstone testifies to dynamic sedimentation within a periodically glacially influenced subaqueous environment. Sequence stratigraphic analysis identifies four glacial advance systems tracts, separated by three glacial retreat systems tracts, wherein hummocky cross-stratified sandstones attest to open water conditions. These findings support dynamic ice sheet behaviour in South Australia, and provide clear evidence for repeated intra-Sturtian ice sheet recession.