Syn-rift Setting Research Articles

Tomographic imaging of South Rewa basin, Central India: Implications of Gondwana rifting and Late Cretaceous volcanism

We have imaged thick (3.0–5.0 km) sub-trappean Gondwana sediments deposited in a typical rift-environment of the south Rewa basin in Central India masked by the Deccan basalts. The basalts intrude through the basin forming several dykes, sills, and other intrusions along with large-scale undulations of the basement showing complex geology and tectonic settings due to Late Cretaceous volcanism (∼65 Ma). To delineate complex subsurface geological structures and understand the tectonic settings, we use long-offset seismic reflection data along the N-S trending 127 km Kuvari-Shahdol profile of the basin. The conventional stack and pre-stack time migration techniques fail to image the complex subsurface structures dominated by steeply dipping faults. The steeply dipping faults with small-scale structures are influenced by strong lateral and vertical velocity variations that cause problems in imaging the basin. Hence, a robust tomographic inversion with pre-stack depth migration technique is adopted to image fine-scale subtle subsurface geological structures with smooth velocity variations. The tomographic velocity model and pre-stack depth migration seismic image could decipher basaltic trap thickness, numerous dyke intrusions, alternate horsts and grabens with deposition of thick hydrocarbon-bearing sub-trappean Gondwana rocks along with the basement configuration. The deep basinal faults with highly distorted basement structures represent the intense tectonic activity of this Gondwana rift-basin. The deep-seated hydrocarbon reservoir is discovered with an excellent trapping mechanism forming both pre-rift and syn-rift settings of the rift-basin. The seismic image is further constrained and corroborated by the inversion of residual Bouguer gravity anomaly data to obtain corresponding density model derived from tomographic velocity model. The large basement faults, horsts and grabens, several intrusives like dyke, sill, laccolith, and thick sub-trappean Gondwana formations with Deccan trap cover indicate complex geology and tectonic settings of south Rewa basin forming the Late Cretaceous Volcanic Province of India.

Upper Jurassic – Lower Cretaceous of eastern Wollaston Forland, North-East Greenland: a distal marine record of an evolving rift

Two drill cores covering the Upper Jurassic – Lower Cretaceous succession in Wollaston Forland, NE Greenland, offer an exceptional insight into mud-accumulation in an evolving distal fault block. Previous studies have revealed the presence of long-lasting black mudstone accumulation extending through the oxygen-restricted early rift and rift climax phases (Bernbjerg and Lindemans Bugt Formations). Here, we revisit the depositional evolution recorded in these cores to present a detailed description of the sedimentary succession extending into the late syn-rift settings (Palnatokes Bjerg and Stratumbjerg Formations). The results indicate that the Kimmeridgian – lower Volgian early rift-phase was characterized by suspension settling, laminae-scale event deposition in a tectonically-affected, prodeltaic offshore setting. The event-related depositional processes are expressed by starved wave-ripples, scour-and-fill structures, putative mud floccule ripples, and mud-dominated gravity-flow deposits. During the middle Volgian – Ryazanian rift climax phase, the depositional environment evolved into a narrow half-graben that was detached from the proximal depocentre flanking the coarse sediment fueled deltaic coastline. The correlative sedimentary facies in the detached half-graben are bioclastic and pyrite-rich black mudstones documenting suspension settling and gravity flow/mass wasting deposition in sub-storm wave-base slope and basin-floor environments. Black shale sedimentation ended abruptly in the late Ryazanian when the accumulation of condensed, bioturbated deep sea marls was initiated linked to broader oceanographic reorganization concomitant with waning rift activity in the west. Deposition of red bioclastic mudstones with a common gravity-flow component characterized the Hauterivian, potentially representing final draping of the submerged fault block crest. The top of the cored succession is demarcated by the appearance of dark grey bioturbated mudstones of Barremian age, reflecting the onset of regionally continuous deep-sea mud accumulation in thermally subsidizing basins. Although superficially monotonous, the mudstone-dominated succession reveals a highly dynamic depositional system that reflects shifting marine processes during almost a full rift cycle.

PALEOGENE PALYNOLOGY OF THE CENTRAL SUMATERA BASIN

Unlike rich pollen assemblage of other Paleogene sediments in western Indonesia (as seen in NanggulanFormation of Central Java, Tanjung Formation of South Kalimantan and Malawa Formation of SouthSulawesi), pollen assemblage of Paleogene sediments in the Central Sumatera Basin is considerablylow. Referring to the occurrence of Oligocene spore Cicatricosisporites dorogensis supported by pollenPalmaepollenites kutchensis and Meyeripollis naharkotensis, it is inferred that the studied sediment isassigned to Oligocene age. This is strengthened by the disappearance of many key Indian affi nities whicharrived in the Sundaland during Eocene. Palynologically, this study separates the Brown Shale from theUpper Red Bed. The Brown Shale is dominated by fresh water pollen without brackish element suggestingnon-marine environment. This sequence might have been formed in syn-rift setting. Surprisingly, lacustrineindicators of fresh water algae Botriococcus and Pediastrum (as found in the lacustrine sediment of TalangAkar Formation of Sunda-Asri Basin) are absent. Mean while, the Upper Red Bed is marked by signifi cantoccurrence of brackish palynomorphs suggesting the infl uence of marine environment during post-rift period.This condition proves the existence of the transgressive phase where sedimentation started in freshwaterenvironment during the Brown Shale deposition which gradually shifted into transition (shallow marine)environment during the Upper Red Bed sedimentation.

Complex stratigraphic fill of a small, confined syn-rift basins: an Upper Jurassic example from offshore mid-Norway

Abstract Extensional rift basin systems have been a focus of study for hydrocarbon exploration and have provided significant economic reserves in recent decades, which has led to advances in our understanding of their tectono-sedimentary evolution. However, with the increasing maturity of such settings, focus is shifting from pre-rift structures to the underexplored syn- and late-rift plays. This change in focus brings with it a significant increase in complexity when trying to develop an integrated understanding of the sedimentological, sequence stratigraphic and structural conditions that control the distribution of syn-rift reservoirs. Here we present a subsurface example from offshore Norway of a small, confined, syn-tectonic basin sourced from a local basement high. 3D seismic data, sedimentary facies analysis and stratigraphic correlations based on well and core data from six key wells constrain the local and regional tectonic controls. The PL586 licence partners drilled several wells between 2014 and 2015 targeting syn-rift Upper Jurassic stratigraphy on the southern Halten Terrace. The wells penetrated a complete syn-rift stratigraphic sequence, which provided an excellent subsurface record of the inherent complexities encountered in small, confined (3 by 5 km) syn-rift basins. Seismic data reveals that the Late Jurassic hanging-wall basin architecture and the footwall hinterland of the Frøya High were controlled by activity on the basin-bounding Vingleia Fault Complex. Facies analysis from well and core data reflects a complete syn-rift succession. The early-rift succession comprises mainly mud and siltstone with minor fine-grained sandstones interpreted to be deposited by sediment gravity flows in isolated mini-basins. The peak-rift deposits comprise coarse-grained sediment gravity flows that were sourced locally from the uplifting footwall and deposited as coarse-grained submarine fan systems in the immediate hanging wall. Significant footwall uplift during the peak-rift phase resulted in fault scarp instability and the emplacement of several large landslide complexes into the basin. The late-rift phase is dominated by a mud-rich sequence with minor coarse-grained apron fans shedding sediment off the basin bounding fault and shoreface deposits flanking the hanging-wall dip-slope and subtle intra-basinal highs. This subsurface study from the southern Halten Terrace is significant in the number of wells that have specifically targeted syn-rift stratigraphy and the level of detail at which it captures the stratigraphic variability of these deposits within a small, confined basin system. Integration of seismic geomorphology and well data with robust provenance, dating and stratigraphic correlation has enabled the development of a tectono-stratigraphic model specific to the scale and local tectonic setting. The model reveals that structural evolution ultimately controlled the development of sediment supply routes, their age and distribution, and allows several conclusions to be drawn: The stratigraphy of small, confined syn-rift basins is broadly comparable with more typically documented larger syn-rift settings, although lateral facies changes occur more rapidly. The cannibalistic nature of syn-rift systems, where a large volume of pre- and syn-rift sediments are uplifted and eroded then rapidly re-deposited makes determining the relative age of laterally variable depositional settings difficult. In order to adequately constrain the timing and depositional extent of syn-rift stratigraphy, it is essential to understand the structural evolution of the system and integrate this information with other disciplines and datasets. This integrated understanding will ultimately improve predictions of reservoir presence and quality.

Classification of the Paleoproterozoic Gulcheru Formation: Implications on Early Paleogeographic and Tectonic Evolution of the Cuddapah Basin, India

Abstract The Papaghni sub-basin of the Cuddapah basin is an arcuate NW-SE trending intracratonic basin, stretching from Kurnool in the NW to Guvvalacheruvu in the SE, in southern peninsular India. Despite of large number of researches, an understanding of the evolution of the basin is still elusive. The sub-basin preserves records of repeated opening and closing of Paleoproterozoic rifts along the zone of NW-SE trending axis in the East Dharwar craton. Present study was carried out along the southwestern margin of the Papaghni sub-basin, in and around Tadpatri-Gooty-Dhone area, where the successive litho-units are represented by two major unconformity-bound sequences. Each of these sequences are characterized by a distinctive set of litho-assemblage, deposited under diverse tectonic environments with highly variable modes and tempos of sedimentation in an extensional basin. The basin initiated at about 2 Ga and shortly evolved into a shallow sea, with development of wide siliciclastic shelf in an open marine condition, the Gulcheru Formation. Gradually, with time the siliciclastic shelf was replaced by an extensive stromatolitic carbonate platform, the Vempalle Formation. Dedicated fieldwork exemplifies that the initial basin-fill is characterized largely by several cycles of simple, but amalgamated and stacked fanglomerates (alluvial fan to fan-delta) systems developed from several single stream breaches onto the proximal setting of the basin, in a syn-rift setting. These fanglomerates display a fining upward sequence, which progressively passes up to high-energy coastal, tide-storm dominated shelf environments of post-rift setting. This study justifies the subdivision of the Gulcheru Formation into two formal lithostratigraphic members, the Daditota Member and the Karvipalle Member. The Daditota Member directly overlies the granitoid basement and grades upward to Karvipalle Member. Karvipalle Member, comprising predominantly of medium to fine-grained, reddish brown and white quartz arenites of stable shelf deposits, distinctly differs from the conglomerate and very coarse-grained mass-flow dominated deposits of the Daditota Member. The Vempalle Formation overlies the Gulcheru Formation with a gradational contact and consists of stromatolitic dolomite, oolite and shale, developed in a ramp type carbonate platform setting. Gradual transition from siliciclastic to carbonate sequence points to passive margin setting developed during the breakup of the supercontinent Kenorland. Disposition of proximal to distal sequences and northeastern, eastern and southeastern paleocurrent of the sequences advocate a NW-SE shoreline and eastern paleo-slope of the basin.

Geological Challenges of Archaeological Prospecting: The Northern Peloponnese as a Type Location of Populated Syn-Rift Settings

The Northern Peloponnese is not only home of a series of ancient poleis that are being studied by archaeologists, but it is also located on the southern shoulder of the most active extensional crustal structure in the world; the Corinthian rift. This rift has shaped the Northern Peloponnese as we now see it today since the Pliocene. Normal faulting, the tectonic uplift of syn-rift sediments and sea level changes, has shaped a landscape of steps rising from the coast to the ridges in the hinterland that provides challenging conditions to a geophysical survey. Where we can find coarse grained slope and delta deposits of conglomerate on top of banks of marl on ridges and slopes, the lower marine terraces and the coastal plain as well as valleys show the protective caprock eroded and the marl covered by young alluvial deposits. These materials show only a small contrast in their magnetic properties, which reduces the importance of magnetic mapping for the archaeological prospection in this region. The human utilization of the coastal plain and the urban areas pose additional challenges. These challenges have been overcome through various approaches that are shown in exemplary case studies from Aigeira and Sikyon. Whereas a combination of magnetic mapping and ground-penetrating radar (GPR) works very well on the ridges and along the slopes where we find coarser sediments in addition to the magnetic mapping, it is not suitable in the coastal plain due to the attenuating properties of the alluvial sediment. Here, electrical resistivity tomography (ERT) proved to be very successful in mapping entire parts of a settlement in great detail. Seismic soundings were also sucessfully applied in determining the bedrock depth, the detection of walls and in the question of locating the harbor basin. In the presented six exemplary case studies, the following findings were made: (1) A fortification wall and building foundations at a depth of 0.4–1.2 m on a plateau northwest of the acropolis of Aigeira was found by 400 MHz GPR. (2) A honeycomb-shaped pattern of magnetic anomalies that suggested cavities could be identified as a weathering pattern of conglomerate rocks. (3) A rock basement 2.3 m deep and remains of an enclosing wall of the Aigeira theater area were found by shear wave refraction measurements. (4) Extensive ERT surveys detected several building remains in Sikyon like a potential building and grave monuments as well as several small houses. (5) A silted-up depression in the sediments of the coastal plane located through Love wave measurements, could be taken as evidence for either a silted harbor or a navigable riverbed.

Interplay of Autogenic and Allogenic Processes On the Formation of Shallow Carbonate Cycles in a Synrift Setting (Lower Pliensbachian, Traras Mountains, NW Algeria)

AbstractMeter-scale shallowing-upward cycles are recorded in many carbonate successions around the world. It is often difficult to recognize whether they represent autocycles, formed through intrinsic controls, or allocycles, resulting from orbital forcing or tectonic movements, or both. Here, we review the criteria used in the identification of the two types of cyclicity and apply them to two newly described lower Pliensbachian outcrops in the Traras Mountains, northwestern Algeria. Throughout the investigation of six sections, the deposits are suggested to have formed in intertidal–supratidal to shallow subtidal environments on a tropical ramp in the Western Tethys. In this area, shallowing-upward small-scale peritidal and subtidal cycles have been shown to be, and are assumed to be, ordered. The carbon isotope data mirror the recorded cycles and indicate different lengths of subaerial exposures. These cycles, in a developed within synrift setting, have been interpreted as produced mainly by autocyclic processes, but interacting with allocyclic factors. Peritidal cycles are thought to be generated by progradation of intertidal and supratidal flats into lagoonal sediments, while subtidal cycles are interpreted to have been controlled by lateral migration of shoals. The impact of the minor fluctuations of eustatic sea level is weakly marked, and only long subaerial exposure can reveal the contribution of these fluctuations to the formation of the recorded cycles. Tectonic movements resulting from spreading of the Tethys are interpreted to have controlled cycle distribution and thickness at a regional scale. However, synsedimentary tectonic features are rare in the studied area; this suggests that sediment transport would control the thickness and duration of cycles instead of the rate at which accommodation was created.

Geochronological, geochemical and Sr–Nd isotopic fingerprinting of Neoproterozoic mafic dykes in the western margin of the Yangtze Block, SW China: Implications for Rodinia supercontinent breakup

The Neoproterozoic magmatic suites in South China have been central to the debate over the assembly and disruption of the Rodinia supercontinent. In this study, we report LA-ICP-MS zircon U-Pb ages, as well as mineralogical, geochemical and Sr-Nd isotopic data on the Mid-Neoproterozoic mafic dykes in the Xichang area along the western margin of the Yangtze Block. The zircon U-Pb data yield ages in the range of 795–743 Ma. Based on available age data, the magmatism along the western margin of Yangtze Block during this period can be divided into three episodes: Episode-I (800–790 Ma), Episode-II (790–780 Ma) and Episode-III (780–730 Ma). Episode-I was widespread and dominated by High Ti-Alkaline (AL-HT) type of mafic–ultramafic rocks, which possibly formed during “active” rifting or the pre-rift stage. This type is characterized by OIB-like geochemical features, higher clinopyroxene crystallization temperature and some pyroxenite (eclogite) component in source, implying picritic source triggered by a mantle plume with involvement of minor pyroxenite (eclogite). Episode-II marks magmatic quiescence with only minor mafic rocks. The mafic dykes in Episode-III display a rang from OIB (AL-HT type) to E-MORB/N-MORB (Enriched/Normal Mid-Ocean Ridge Basalts; Transitional type and TH-LT, Low Ti-Tholeiite), suggesting possible syn-rift setting, and involved mixing between enriched (OIB-like) and depleted sources (asthenospheric mantle), followed by variable degrees of partial melting. Based on the temporal evolution and geochemical features of the mafic–ultramafic rocks, we propose that the 800–730 Ma magmatic events in the western margin of Yangtze Block involved the entrainment of a dense eclogite-derived material generated through subduction processes during the Sibao orogenesis at the base of mantle, together with instabilities caused by thermochemical mantle plume. This process also marks the transformation from intercontinental rift to the opening of oceanic basin, ultimately leading to the breakup of the Rodinia supercontinent.

Sedimentology and mechanism of a lacustrine syn-rift fan delta system: A case study of the Paleogene Gaobei Slope Belt, Bohai Bay Basin, China

Abstract Fan delta systems have long been studied as a kind of good hydrocarbon reservoirs. Although many models have been raised according to their local conditions, there still exists some controversies regarding the formations of the ones developed in continental syn-rift settings. Taking Paleogene Es32+3 of Gaobei Slope Belt as an example, this paper conducted a comprehensive geological study and a forward depositional modeling. The results show that the sediment within the study area came mainly from the northeastern Baigezhuang uplift and the northwestern Xinanzhuang uplift over a short distance. The sedimentary system is characterized by subaerial debris flow, subaerial braided channels, subaqueous distributary channels, front sand bars and front sand sheets. Furthermore, the study area is a balance-filled basin that experienced a 3rd order normal regression followed by a transgression during the development of the study interval. The results also show that, a stable subsidence rate with small-magnitude climate changes would be a plausible explain for the entire sequence of the fan delta system in the study area. The tectonic subsidence was indeed the dominant driving force to a continental syn-rift fan delta system, although the climate, sediment supply or lake level might bring some lower order (e.g., 4th and 5th order) superimpositions. This paper proposed a likely model for fan deltas developed in continental syn-rift settings, which is expected to gain some insights into the forming mechanism and provide some supports for the reservoir characterization of similar and correlative sedimentary systems.

Petrogenesis of Mesoproterozoic mafic rocks in Hainan (South China) and its implication on the southwest Hainan-Laurentia-Australia connection

New U-Pb geochronological, elemental and Nd-Hf isotopic data for the metamorphic mafic rocks from the Baoban Complex in southwest Hainan provide new constraints on their petrogenesis and geodynamic setting. Magmatic zircons from the representative samples yielded weighted mean 207Pb/206Pb ages of 1441–1424 Ma, representing their crystallization ages. These samples show subalkaline to alkaline characteristics and can be geochemically subdivided into three groups in spite of similar formation time. Group 1 has TiO2 of 2.85–3.18 wt.%, Nb of 20.5–21.6 ppm, Nb/U of 45–49, Nb/La of 1.22–1.37, εNd (t) of +6.8–+6.9 and zircon εHf (t) of +9.3–+12.7, suggestive of the derivation from OIB-like source. Group 2 shows Nb contents ranging from 1.0 ppm to 12.6 ppm, εNd (t) values from +5.5 to +5.9 and zircon εHf (t) values from +10.3–+12.4 with right-sloping REE-normalized pattern, indicative of an E-MORB source. Group 3 is characterized by negative Nb-Ta anomalies with Nb/La ratios of 0.51–0.88 and εNd (t) value of ∼+5.1, indicative of the derivation from a previously-metasomatized mantle source. The coeval mafic and felsic rocks in the Baoban Complex are geochronologically and geochemically comparable to the Mesoproterozoic intra-plate magma in southwest Laurentia. The Mesoproterozoic Baoban igneous rocks are generated in a syn-rift setting in response to the Columbia breakup. Southwest Hainan might connected with west Laurentia and Australia at ∼1.43 Ga.

Novel 3D sequence stratigraphic numerical model for syn-rift basins: Analysing architectural responses to eustasy, sedimentation and tectonics

Syn-rift clastic sedimentary systems preserve a complicated stratigraphic architecture that records the interplay of tectonics, eustatic sea level and storage and routing of sediments. Previous conceptual models describe and explain changes in depositional stacking patterns along a fault segment. However, stacking patterns, and the nature of key stratigraphic surfaces, is challenging to predict accurately with conventional sequence stratigraphic models that do not consider the three-dimensional interplay of subsidence, sedimentation, and eustasy. We present a novel, geometric, 3D sequence stratigraphic model (‘Syn-Strat’), which applies temporally- and spatially-variable, fault-scale tectonic constraints to stratigraphic forward modelling, as well as allowing flexibility in the other controls in time and space. Syn-Strat generates a 3D graphical surface that represents accommodation. Although the model has the capacity to model footwall variation, here we present model results from the hangingwall of a normal fault, with temporal and spatial (dip and strike) predictions made of stacking patterns and systems tracts for a given set of controls. Sensitivity tests are tied to the depositional architecture of field-based examples from the Loreto Basin, Gulf of California and Alkyonides Basin, Gulf of Corinth. Here, the relative influence of major sedimentary controls, different subsidence histories, varying sedimentation distribution, including along-strike variation in stacking patterns, are assessed and demonstrate the potential of Syn-Strat for reducing subsurface uncertainties by resolving multiple scenarios. In addition, the model demonstrates the nature of diachroneity of key stratigraphic surfaces that can arise in syn-rift settings, which could be represented by a bypass surface (sequence boundary) or reservoir seal (maximum flooding surface) in the rock record. Enabling a quantitative assessment of these surfaces is critical for prospect analysis in hangingwall half-graben-fills, where these surfaces are heavily relied upon for well correlations that are used for hydrocarbon volume and production rate predictions.

Stratigraphic architecture of an Early–Middle Jurassic tidally influenced deltaic system (Plover Formation), Browse Basin, Australian North West Shelf

Understanding the internal stratigraphic architecture of sand-dominated deltas is critical to assessing the extent and distribution of petroleum reservoirs. The stratigraphic architecture and evolution of a major Early–Middle Jurassic fluvio-deltaic system (Plover Formation) on the Australian North West Shelf has been established through integrated analysis of core, borehole image logs and wireline logs for the Calliance field in the Browse Basin. Six facies associations identified in cored intervals are interpreted as tidally influenced channel- and tidal channel-fill complexes (FA1–FA2), crevasse-splay deposits and interchannel marshes (FA3), heterolithic mouthbars and sandflats (FA4), sandy mouthbars (FA5) and offshore transition to offshore (FA6). Therefore, the overall depositional system in the study area is proposed to be a tidally influenced deltaic system, in which FA6 represents prodelta deposits, FA5 and FA4 constitute distributary mouthbar deposits of the delta front and tidally influenced channel and interchannel deposits represent the lower delta plain (FA2, FA1, FA3). Analysis of image lithology and fabric are used to extend interpretation to uncored intervals and to identify intrusive and extrusive igneous units and associated volcaniclastic facies within the formation. Five third-order stratigraphic sequences (S1–S5) record progradational (S1, S2 and S4) and retrogradational (S3 and S5) phases of delta evolution. Paleocurrent indicators derived from borehole image logs indicate common southerly directed sediment dispersal in S2 and S3 and increasingly complex with westerly directions in S4 and S5. Two rift-related depositional phases are recognised separated by a phase of uplift between S3 and S4. The stratigraphic succession of S2 is consistent with the depositional pattern expected in a synrift setting and deposition of the over-thickened sandy succession (FA5 in S4), which is also the major reservoir interval, was most likely controlled by syn-tectonic faulting.

Mid-Permian Khuff Sequence KS6: Paleorelief-influenced facies and sequence patterns in the Lower Khuff time-equivalent strata, Oman Mountains, Sultanate of Oman

ABSTRACTKhuff Sequence KS6 was studied in the Al Jabal al-Akhdar, Oman Mountains, in an area of 30 x 50 square kilometers by means of detailed sedimentological logging of five time-equivalent outcrop sections of the Saiq Formation. KS6 represents one transgressive-regressive, third-order sequence, and is composed of four facies associations each representing particular environments of deposition (backshoal, shoal, foreshoal and offshoal) with distinct sedimentological characteristics. Facies stack to form cycles and cycle sets that were used for correlation at a subregional scale and to reveal the KS6 stratigraphic architecture. During the initial phase of basin-fill, clastic sediments (“Basal Saiq Clastics”) were deposited in paleolows above the “Sub-Saiq Unconformity”. In contrast to younger Upper Khuff sequences KS4 to KS1, the underlying paleorelief strongly affects the thickness and facies composition of KS6. The correlation strategy to follow paleolandscape surfaces using all available sedimentological, biostratigraphic and lithostratigraphic data resulted in a stratigraphic architecture with subtle shingle geometries.Sequence KS6 shows a strong facies partitioning resulting in the necessity of two separate facies models for the transgressive (crinoidal ramp) versus regressive hemisequence (oolitic/peloidal carbonate ramp). This study revealed potential reservoir units in KS6, commonly regarded as non-reservoir in the subsurface of Oman and other parts of the Gulf region. The abundance, nature and lateral extent of reservoir facies strongly varies with stratigraphic position. In the transgressive part of KS6, crinoidal grainstones are concentrated around the margin of a gentle paleohigh. They might have the best reservoir potential, although early diagenetic cementation is common in most settings. Oolitic/peloidal grainstones in the upper regressive part have a much higher diagenetic reservoir potential and are laterally much more widespread. Thus, Khuff Sequence KS6 differs profoundly in its stratigraphic architecture from the more “layer-cake”-like KS4 to KS1 sequences. Facies and thickness patterns are controlled by a marked paleohigh to paleolow configuration, resulting from the antecedent uneven topography during the Neo-Tethyan syn-rift setting, in contrast to the post-rift setting with low tectonic activity during KS4 to KS1.

The petroleum potential of Tertiary coals from Western Indonesia: Relationship to mire type and sequence stratigraphic setting

More than 500 deltaic sediments from 14 basins in Western Indonesia were subjected to detailed geochemical, petrographic and palynological analysis. The resultant data represent each of the main peat-forming tectonostratigraphic episodes (Paleogene syn-rift transgressive, Paleogene–Neogene post-rift transgressive and Neogene syn-tectonic regressive) of the Indonesian Tertiary. The coals are hydrogen-rich, with average hydrogen index (HI) of 250–300 and atomic H/C ratios of 1.0–1.3 where immature. Paleogene coals from syn-rift transgressive systems tracts are more hydrogen-rich than Neogene coals from syn-tectonic regressive systems tracts, and upon pyrolysis, yield lower gas/oil ratios, implying they are more oil-prone. Pliocene coals from a strongly regressive sequence in the Sumatran fore-arc are very hydrogen-poor with average HI of 138. Indonesian coals display a systematic increase in HI with increasing rank, peaking at a Tmax of c. 430 °C, suggesting that Rock-Eval pyrolysis underestimates petroleum potential in low rank coals. The increase in HI is accompanied by a decrease in S1/TOC ratio and the disappearance of suberinite and resinite, reflecting the assimilation of these macerals into the coal matrix with increasing diagenesis. Petrographic analyses reveal a very poor correlation between liptinite content and HI. The term Detrovitrinite Index (DI) is coined to describe the ratio of detrovitrinite/total vitrinite. A better relationship between HI and DI suggests that vitrinite type is a more important control on petroleum potential than liptinite content. The coals are classified into four sub-types (I–IV) on the basis of their pyrolysis potential and petrographic composition. Sub-types I, II and III are low ash coals and are likely to have been deposited in raised mires. Sub-type I is hydrogen-rich and primarily represents Middle to Late Miocene coals, deposited in a regressive megasequence. Sub-type II coals are hydrogen-poor and occur in multiple tectonostratigraphic settings. Sub-type III consists of very hydrogen-poor Pliocene coals from the rapidly subsiding West Sumatran fore-arc. Sub-type IV is hydrogen-rich, and consists of high ash Eocene coals, deposited in submerged mires in a transgressive, syn-rift setting. A close relationship between HI and tissue preservation index (TPI) suggests that the degree of degradation of the peat is an important factor in determining petroleum potential. Highly degraded peats result in hydrogen-rich coals with a higher proportion of vitrinite of detrital or allochthonous origin. The degree of degradation is most likely related to the amount of time the peat spends within the zone of influence of the water table, and is unlikely to be directly related to tectonostratigraphic setting.

Seismic Attributes Mapping of Late Palaeozoic Glacial Deposits on the Australian Northwest Shelf

During the Permo-Carboniferous the Gondwana supercontinent experienced the most extensive Phanerozoic glacial event, as recorded by widespread glaciogenic sediments from South America to Australia. Simultaneously the northern margin of the supercontinent was affected by the onset of the Neotethys rifting. During that period, several elements such as glacial surfaces or glacially eroded valleys suggest the presence of a terrestrial ice sheet in Western Australia, and the glacial Lyons Group has been reported from various locations in the Carnarvon Basin. Such deposits are proposed to form the lower part of the sedimentary cover of the Mermaid Nose area (Dampier Sub-basin, Northern Carnarvon Basin, NW Shelf).Therefore, a classical seismic stratigraphic approach combined with recently developed seismic attributes and innovative workflows is used to define the morphology of the Late Palaeozoic sequences, despite severe limitations in the resolution of the seismic data.Six stacked units are defined within a half-graben in a syn-rift setting. They are interpreted as a series of basal moraines followed by deposits associated with deglaciation periods.Six stacked units are defined within a half-graben in a syn-rift setting. They are interpreted as a series of basal moraines followed by deposits associated with deglaciation periods.In addition, the glacially related hydrocarbon-rich deposits of the Al Khlata Formation in east-central Oman provide a most interesting parallel with the Permo-Carboniferous syn-rift deposits observed on the Australian Northwest Shelf.

Growth of wedge-shaped plutons at the base of active half-grabens

ABSTRACTCombined field and geophysical data show that plutons from the Bega Batholith are elongate, meridional, wedge-shaped bodies which intruded during a period of regional east–west extension in the Palaeozoic eastern Lachlan orogen, eastern Australia. Plutons within the core of the batholith have intruded coeval, syn-rift sediments and co-magmatic volcanics. The batholith is bound by high-temperature, dip-slip faults, and contains several major NE-trending transtensional faults which were active during batholith construction. In the central part of the batholith, the Kameruka pluton is an asymmetric, eastward-thickening, wedge-shaped body with the base exposed as the western contact, which is characterised by abundant, shallow-dipping schlieren migmatites which contain recumbent folds and extensional shear bands. A shallow (<30°), east-dipping, primary magmatic layering in the Kameruka pluton steepens progressively westward, where it becomes conformable to the east-dipping basal migmatites. The systematic steepening of the layering is comparable to sedimentary units formed during floor depression in syn-rift settings. The present authors suggest that the wedge-shaped plutons of the Bega Batholith are the deeper, plutonic expression of a hot, active rift. The batholith was fed and sustained by injection of magma through sub-vertical dykes. Displacement along syn-magmatic, NE-trending faults suggests up to 25 km of arc-perpendicular extension during batholith construction. The inferred tectonic setting for batholith emplacement is a continental back-arc, where modern half-extension rates of 20–40 mm yr−1 are not unusual, and are sufficient to emplace the entire batholith in ∼1 Ma. This structural model provides a mechanism for the emplacement of some wedge-shaped plutons and is one solution to the ‘room problem’ of batholith emplace

Continental margin off Western India and Deccan Large Igneous Province

Massive, transient late syn-rift-to-breakup volcanism during separation between the Seychelles microcontinent and India formed the Deccan continental flood basalts and their equivalents on the Seychelles-Mascarene Plateau and on the conjugate continental margins, i.e. the Deccan Large Igneous Province. We estimate an original extrusive area of at least 1.8×106 km2, and a volume >1.8×106 km3, and suggest a plate tectonic model comprising: (1) development of the Seychelles microplate by fan-shaped spreading in the Mascarene Basin, and continental extension followed by fan-shaped spreading between India and the Seychelles during A29-27 time. (2) Cessation of fan-shaped spreading just after A27 time, followed by spreading along the India-Seychelles plate boundary. (3) Margin subsidence, modified south of Goa by the persistent, time-transgressive effects along the plume trail. The margin is divided into three regional provinces by the prolongation of regional transforms which formed the east and west boundaries of the Seychelles microplate during breakup and early sea floor spreading. In some aspects, the conjugate margins are different from other volcanic margins; e.g. regional wedges of seaward dipping reflectors along the continent-ocean transition have not yet been reported. We ascribe this to the eruption of the most voluminous lavas during chron 29r, i.e on continental lithosphere in a late syn-rift setting. The enigmatic Laxmi Ridge is a complex marginal high comprised of both continental and oceanic crust. It was probably created during breakup, but may have experienced later magmatic and/or tectonic deformation.