Restricted Marine Setting Research Articles

Timing and origin of the Lomagundi-Jatuli Event: Insights from U[sbnd]Pb geochronology, C-O-Fe isotopes and REE compositions from the Jingshan Group, North China Craton

The Lomagundi-Jatuli Event (LJE) represents perhaps the largest and longest positive carbon isotope excursion of Earth history. However, the synchroneity, scale, and linkage of the LJE to Earth's early history of atmospheric oxygenation remain controversial. Strata of the Paleoproterozoic Jingshan Group of the North China Craton that preserve the isotopic record of the LJE excursion in marble layers and abundant graphite deposits provide an opportunity to elucidate the significance of the LJE. Carbon isotopic values (δ13CV-PDB) of the LJE based on analysis of 20 samples of the Lugezhuang Formation, lower Jingshan Group, range from −0.8 to +9.6 ‰ and display positive co-variance with stable oxygen isotope values (δ18OV-PDB). The positive carbon isotope excursion is constrained to 2140.6 ± 8.5 Ma (MSWD = 0.82, n = 25) based on magmatic zircon UPb geochronology of biotite granulite. Variation of facies-dependent carbon isotope values, the presence of graphite deposits of the Douya Formation, upper Jingshan Group, and the absence of a Ce anomaly in PAAS normalized REE patterns of marble samples suggest that the positive carbon isotope excursion is not linked to a marked increase of organic carbon burial and associated significant atmosphere oxygenation. Elevated concentrations of iron and PAAS-normalized middle REE enrichment of analyzed Jingshan Group marble samples point to anoxic and ferruginous oceanic conditions during accumulation of Jingshan Group carbonate. A positive Eu anomaly (average = 1.58), low La (average = 0.23), (Nd/Yb)N (average = 1.27), and Y/Ho (average = 36.6) anomalies, and negative iron isotope values (average δ56Fe = −0.12 ‰) are consistent with accumulation of Paleoproterozoic Jingshan Group carbonate in a restricted marine setting that was affected by high temperature hydrothermal fluids. Enrichment of the studied samples in heavy carbon isotope suggests elevated bio-productivity in the restricted, redox stratified marine setting in which Jingshan Group carbonate accumulated. Thus, it is likely that the positive stable carbon isotope excursion associated with Jingshan Group strata as well as other contemporaneous isotope excursions are local signals that are linked to a global perturbation of the carbon cycle.

Effect of Paleoenvironmental Conditions on the Distribution of Lower Carboniferous Shale in Yaziluo Rift Trough, South China: Insights from Major/Trace Elements and Shale Composition

Paleoenvironmental conditions significantly influence the distribution patterns and organic matter enrichment of shale. This study investigated the vertical variations of major elements, trace elements, and total organic carbon (TOC) in the Lower Carboniferous marine shale from the Yaziluo Rift Trough, South China, to understand the paleoenvironmental conditions, including redox conditions, terrigenous detrital input, paleoproductivity, and paleo-seawater depth. The Lower Carboniferous formation consists of three sedimentary facies: basin facies, lower slope facies, and upper slope facies. From the basin to the lower slope and then to the upper slope facies, TOC, quartz, and pyrite contents gradually decrease, whereas the carbonate mineral content shows an increasing trend. A continuous decline in paleo-seawater depth transformed a deep-water anoxic environment with high paleoproductivity and low detrital input in the basin facies into a semi-deep-water environment with dysoxic-oxic conditions and moderate detrital influx in the lower slope facies, evolving further into a suboxic environment with high detrital flux in the upper slope facies. The geochemistry results suggest that anoxic conditions and high paleoproductivity were the primary controls on organic matter enrichment in the siliceous shale of the basin facies. In contrast, redox conditions significantly influenced organic matter accumulation in the mixed shale of the lower slope facies, attributed to relatively low paleoproductivity in a more restricted marine setting. Additionally, the adsorption of carbon components by clay minerals facilitated the preservation of organic matter in the calcareous shale of the upper slope facies.

Biostratigraphic ages and depositional environments of the upper Oligocene to lower Miocene Veldhoven Formation in the central Roer Valley Rift System (SE Netherlands-NE Belgium)

Abstract Discussions on the age and the depositional environments of the Veldhoven Formation and its members are persistent in Belgium and the Netherlands. Uncertainties on stratigraphy and the constructive process of sediment accumulation continue today as a result of lack of data on this succession within the Roer Valley Rift System. The present study provides new information on the bio- and lithostratigraphy and facies from two boreholes based on dinoflagellate cyst taxa. The results were correlated by gamma-ray logs towards other key boreholes in the region and show a good consistency for stratigraphy and depositional environments for the members of the Veldhoven Formation. After marginal to restricted marine conditions in the latest Rupelian (early Oligocene), the start of deposition of the Veldhoven Formation marked the transition towards a higher sea level, expressed by increased glauconite contents and gamma-ray values. The Voort Member in the lower part of the Veldhoven Formation has an early to late Chattian (Late Oligocene) age and comprises predominantly shallow marine (fluctuating restricted to open marine) conditions. The lithology in the lower part of this unit is often very clayey but is coarsening upward into sands. The superjacent Wintelre Member has a latest Chattian to early Aquitanian (early Miocene) age. This member is distinct by its clayey nature which is expressed by relatively high gamma-ray values. Earlier studies suggest a deeper marine facies for the Wintelre Member compared to the Someren and Voort members. However, the dinoflagellate cyst assemblages in this unit are mostly dominated by a single genus indicating a restricted marine setting, including salinities that deviate from normal marine conditions, most probably due to minor ventilation by narrow or lack of connection to the Atlantic Ocean. A glacio-eustatic sea-level fall around the Oligocene/Miocene boundary limited the sea coverage to the strongest subsiding areas, where deposition of the Wintelre Member is recorded, while non-deposition or erosion occurred in the surrounding highs, hence creating an isolated (sub)basin. The superjacent Someren Member was deposited during the late Aquitanian to middle Burdigalian and consists of shallow to open marine clayey fine sands. Increasing clay contents indicate a gradual development towards a higher sea level, which coincide with upward increasing gamma-ray values. The biostratigraphic results of this study suggest that no major hiatuses are present in the differentially subsiding blocks of the Roer Valley Rift System during the late Oligocene to early Miocene.

Multi-episodic recrystallization and isotopic resetting of early-diagenetic dolomites in near-surface settings

ABSTRACTThe lower Eocene Rus Formation in Qatar reflects carbonate deposition in a semirestricted to fully restricted marine setting on a shallow ramp. Petrographic, mineralogical, and geochemical evidence from three research cores show early diagenesis has extensively altered nearly every petrological attribute of these rocks despite not having been deeply buried. In southern Qatar, the lower Rus (Traina Mbr.) consists of fabric-retentive dolomite intervals that preserve mudstone, wackestone, and packstone textures that are interbedded with depositional gypsum beds. In northern Qatar, the same member is dominated by fabric-destructive planar-e dolomite, and evaporites are absent. In both northern and southern Qatar, the upper Rus (Al Khor Mbr.) is composed of fabric-retentive dolomite intervals as well as limestone intervals rich with Microcodium textures that display evidence of dedolomitization. Geochemical analysis reveals that the limestones have an average δ18Ocal of –10.73‰ VPDB and δ13Ccal of –7.84‰ VPDB, whereas average dolomite δ18Odol is significantly higher (–1.06‰ VPDB) but δ13Cdol values (–3.04‰ VPDB; range –10 to 0‰) overlap with δ13Ccal values. Additionally, δ13Cdol trends toward normal marine values with depth away from the calcite–dolomite contact in all three cores. Petrographic observations demonstrate that dolomite crystals are commonly included in calcite and partially to completely replaced by calcite in these intervals and suggests that dolomite formed before calcite in the Microcodium-bearing intervals. Furthermore, the dolomites are commonly cemented by gypsum in the Traina Mbr. in southern Qatar, suggesting that dolomitization may have also occurred before, or concurrent with, bedded gypsum formation and indicates that dolomitization occurred early. Early dolomites were subsequently replaced by Microcodium-bearing limestones at and immediately below paleo-exposure surfaces, and at greater depths recrystallized in mixed marine–meteoric fluids, producing a negative δ13Cdol signature that trends toward more positive values away from the limestone–dolomite contact. Lastly, the dolomites underwent another phase of recrystallization in either marine-dominated fluids or possibly a well-mixed aquifer setting, resulting in a near-0‰ δ18Odol signature but retaining the negative δ13C signature. These findings thus have implications for reconstructing the diagenetic history of carbonate rocks, as they suggest that early diagenesis of carbonates can be extremely complex, resulting in multiple stages of mineral replacement and isotopic exchange in meteoric and shallow marine fluids before significant burial. Furthermore, this study shows that dolomitization of a limestone does not necessarily prevent additional early diagenesis and multiple recrystallization events. Lastly, it emphasizes the importance of incorporating petrographic observations with geochemical data when interpreting the diagenetic history of carbonate rocks.

Petrofacies and taphofacies analyses of coquinas as a tool for the establishment of a stratigraphic evolution model of the Morro do Chaves Formation (Sergipe-Alagoas Basin, NE Brazil)

Petrofacies and taphofacies defined for the Morro do Chaves Formation (Late Barremian/Early Aptian) outcropping in the Sergipe-Alagoas Basin (NE Brazil) enable the interpretation of the biostratinomic and fossil-diagenetic processes that acted on the abundant bivalve shells and enclosing sediments. Such processes played an important role in the circulation of fluids within the coquina deposits and therefore on the resulting porosity–permeability of the rocks. Ten taphofacies and four lithofacies are recognized, allowing 36 petrofacies to be defined, based on the nature of the sedimentary and taphonomic processes. Five of these petrofacies are distinguished for those showing almost total replacement of the original constituents. The relationship between petrofacies with the geometry of sedimentary bodies and their lateral and vertical variations, forms the basis of a stratigraphic model for the depositional system of the coquinas of the Morro do Chaves Formation that reflects the spatial and temporal distribution of the identified petrofacies. The integration of the evolutionary taphonomic analysis of the unit with the analysis of multiscale properties through the CAMURES Methodology allowed us to develop a theoretical model for the alteration and modification of taphonomic elements and coquinas, which reflect the rise and fall of the relative base level in a lacustrine to the restricted marine setting. This model leads to more accurate identification and classification of coquina reservoir facies, as well as the delimitation of reservoir zones for 3D geocellular modeling and flow simulation of hydrocarbon-producing reservoirs, thus improving predictive analyses.

Climate-ocean control on the depositional watermass conditions and organic matter enrichment in lower cambrian black shale in the upper Yangtze Platform

The lower Cambrian sediments including the Qiongzhusi and Maidiping formations in the Sichuan Basin are generally considered as the main source rocks of Precambrian-Cambrian petroleum system. A comprehensive study integrated with multiple geochemical methods was conducted on 119 lower Cambrian drillcore samples collected from the upper Yangtze Platform. The results show that the fluctuation of sea level and climate during the deposition of the Qiongzhusi and Maidiping formations act as the critical factors controlling the watermass conditions and thus OM accumulation. During the deposition of upper Maidiping Formation (Є1m), the paleoclimate was arid and sea level was relatively low in the study area. The depositional environment indexes (UEF-MoEF, gammacerane index, β-carotane and VEF) suggested that the upper Є1m was deposited in a strong restricted marine setting with high water salinity and anoxic water conditions in Ziyang-Weiyuan area. However, during the early Qiongzhusi Formation period, the paleoclimate became humid and sea level rose in the study area. High paleoproductiviy caused by high concentrations of nutrients brought by seawater and freshwater plays an important role in OM accumulation in the basal part of the Qiongzhusi Formation (Є1q). Furthermore, the model of paleoproductivity promotion caused by the coordinated control of climate and extensive transgression in the lower part of Є1q may also be applicable to interpret the origin of high organic carbon abundance in the strata deposited in the same period with the Є1q (such as Shiyantou, Shuijingtuo and Niutitang formations) in other parts of the upper Yangtze Platform.

Carbonate REE + Y signatures from the restricted early marine phase of South Atlantic Ocean (late Aptian – Albian): The influence of early anoxic diagenesis on shale-normalized REE + Y patterns of ancient carbonate rocks

Rare earth elements plus yttrium have been extensively applied in paleoenvironmental studies of ancient carbonate successions due to their fractionation in the marine environment. However, in modern marine anoxic environments, seawater REE + Y signatures can be suppressed in reducing conditions (e.g. stagnant basins and/or early diagenesis) by REE remobilization from several sedimentary components (e.g. detrital siliciclastics, oxides, organic compounds). We present the shale-normalized REE + Y signatures for a transgressive marine carbonate succession of the primitive South Atlantic Ocean (latest Aptian-Albian), in a restricted marine setting with anoxic bottom conditions, to provide and evaluate the REE + Y record deposited in such conditions. Based on well-constrained paleoenvironmental reconstruction through microfacies analysis, three shale normalized REE + Y patterns were identified, varying according to the microfacies associations (MA) and related diagenetic environments: a) light REE-enriched patterns in the more proximal MA associated with burial diagenesis at the base of section; b) flat patterns towards distal MA related to anoxic and sulfidic early diagenesis; and c) middle REE-enriched patterns in MA related to dissolution of FeMn oxy-hydroxide grains under reducing early diagenesis. Cerium anomalies are absent due to stagnant water mass conditions, in accordance with previous studies for the early marine South Atlantic. Y/Ho ratios are lower than modern seawater values, with an increasing trend towards more distal facies. The “non-seawater” shale-normalized REE + Y patterns of the marine carbonates from the primitive South Atlantic Ocean corroborate studies in modern marine reducing environments, in which REE can be remobilized from detrital phases, oxides and organic compounds, suppressing the primary seawater REE + Y signature of authigenic minerals. Caution is suggested for the sole use of carbonate REE + Y signatures as paleoenvironmental proxies, especially in ancient successions lacking proper microfaciological components.

Extent and persistence of photic zone euxinia in Middle–Late Devonian seas – Insights from the Canning Basin and implications for petroleum source rock formation

Abstract Late Devonian mass extinctions attributed to extensive anoxia and/or euxinia of the oceans are associated with widespread deposition of organic-rich shales. Also in the epeiric waters of the Canning Basin (Western Australia), photic zone euxinia (PZE) prevailed during the Givetian–Frasnian, with geochemical evidence for PZE on the northern (Lennard Shelf)–, and southern (Barbwire Terrace) margins of the Fitzroy Trough. On the Lennard Shelf, shales record episodic pulses of PZE associated with high algal activity due to enhanced nutrient supply, whereas a restricted marine setting on the Barbwire Terrace is thought to be the main driver for the development of persistent PZE and associated bacterial predominance. Structural evidence indicates that the Fitzroy Trough was a confined basin during the Late Devonian with the possibility of limited ocean circulation. Widespread PZE is expected to have developed in the poorly mixed water column, if the basin received sufficient nutrient supply for enhanced primary production. Notwithstanding the presence of anoxia during deposition of potential source rocks, only two small Devonian-sourced oil fields and numerous oil shows have been found in the Canning Basin. Biomarker assemblages show that the oils produced from the Lennard Shelf fields (i.e. Blina-1, Blina-4 and Janpam North-1) have substantially different molecular compositions to the minor oil recovered from Mirbelia-1 on the Barbwire Terrace. A correlation was established between the Lennard Shelf oils and rock extracts from the Gogo Formation at Blina-1 and McWhae Ridge-1 based on their hopane, sterane and carotenoids abundances. A definitive source correlation was not obtained for the Mirbelia-1 oil, but it did show some genetic affinity to the Givetian–Frasnian extracts from the Barbwire Terrace, suggesting that local source rocks are developed in the region.

Paleoclimatic and paleoenvironmental records of the Oligocene–Miocene transition, central Jylland, Denmark

A multidisciplinary study of the Oligocene/Miocene (O/M) transition was carried in two boreholes (Harre-1 and Horn-1) from the Danish land area in order to improve the understanding of the paleoclimatological and environmental changes across the Mi-1, the earliest Miocene cooling event. Dinoflagellate cyst (dinocyst) biostratigraphy, supported by re-evaluation of the foraminiferid biostratigraphy, a new set of magnetic data and stable carbon isotope (δ13C) records were applied to improve the age model for the studied succession; as well as the positioning of the O/M boundary. These data further enabled a correlation of the two investigated profiles with the well-established stratigraphical framework for the Danish area. The BIT index (an organic proxy indicating the relative input of soil-derived organic matter), dinocyst assemblages and palynofacies have been applied for establishing the paleoenvironmental changes across the Oligocene/Miocene (O/M) boundary. Our data indicate a shallowing upward trend in the latest Oligocene, resulting in the establishment of a very restricted marine setting in the earliest Miocene. Our study confirms that the O/M boundary is located at a sequence boundary (the local sequence boundary B — SB B) and corresponds to an increase in terrestrial organic matter input. A mean air temperature record based on the MBT′/CBT proxy reveals an ~ 2 °C drop in temperature at the O/M and the sequence boundary. Our findings confirm previous suggestions that this sequence boundary is a result of the glacio-eustatic sea level fall related with the Mi-1 cooling event. The unconformity at the sequence boundary probably correlates with the coldest part of Mi-1, and thus no deposits from this period have been preserved. This suggests that the temperature drop might have been greater than 2 °C.

Barremian bivalves from the Huitrín Formation, west-central Argentina: taxonomy and paleoecology of a restricted marine association

The Cretaceous Huitrín Formation in west-central Argentina records the final connection of the Neuquén Basin to the Pacific Ocean. This formation is comprised of a variety of continental to marginal-marine sediments deposited behind an Andean volcanic arc under warm, arid paleoclimatic conditions. Here we focus on a bivalve fauna from carbonate ramp deposits within the Barremian La Tosca Member of the Huitrín Formation. This fauna is very abundant and widely distributed within the basin but, surprisingly, it has not yet been studied in detail. In addition, paleoenvironmental affinities remain unresolved, with the fauna variously interpreted as having freshwater, brackish, and marine affinities. We studied the fauna's taxonomy and paleoecology based on more than 500 specimens collected at ten fossil localities in combination with new field observations. The bivalve assemblage was recorded from middle to outer carbonate ramp deposits and is composed of five taxa of marine affinity:Phelopteria huitrinianan. sp.,Isognomoncf.I. nanus(Behrendsen),Placunopsis?pichin. sp.,Anthonya jarain. sp., andArgenticyprina mulensisn. gen. n. sp.; the first three may be regarded as eurytopic and/or opportunistic. Reduced diversity, low evenness, overall small size (length <4 cm), thin shells, eurytopic or opportunistic life strategies, and high endemism point to a restricted marine setting for the La Tosca Member. The most important limiting factors likely were low primary productivity and fluctuating salinity and temperature, as conditions inferred for the unit include high evaporation rates combined with low continental runoff and reduced rainfall. Thick evaporite deposits below and above La Tosca Member and thin intercalated gypsum beds support a restricted, hypersaline setting.

Oligocene dinoflagellate cyst biostratigraphy of the southern North Sea Basin

The Rupelian (Lower Oligocene) and Chattian (Upper Oligocene) stratotype sections are both defined on the basis of the southern North Sea Basin sedimentary successions. The characterisation of biotic events occurring within the stratotypes (and equivalents) is vital for the recognition of these stages outside the North Sea Basin. Although the restricted marine setting of the North Sea Basin during most of the Paleogene clearly hampers ‘traditional’ calcareous microfossil calibration, organic-walled dinoflagellate cysts (dinocysts) are increasingly successful in the stratigraphic analysis and calibration of the marginal-marine North Sea Basin successions. Here we present a high-resolution Oligocene dinocyst biostratigraphic zonation scheme for the southern North Sea Basin based on previously published and new dinocyst studies from Belgium, northern Germany and The Netherlands. Eight (southern) North Sea Oligocene (NSO) dinocyst zones (biozones) and four subzones are here defined. Their application on a regional and inter-regional scale is discussed. The stratigraphic important Late Oligocene dinocyst taxon Triphragmadinium demaniae gen. and sp. nov. is formally described.

Pre-Rodinian (Mesoproterozoic) supercontinental rifting along the western margin of Laurentia: geochemical evidence from the Belt-Purcell Supergroup

Recent efforts at supercontinental reconstruction for the late Precambrian (e.g., the SWEAT hypothesis Moores, 1991) have commonly invoked an uncertain lacustrine interpretation for deposition of the Mesoproterozoic Belt-Purcell Supergroup, with the Belt basin placed centrally within the assembled landmass. The utility of the Belt-Purcell system in addressing Proterozoic continental configurations has improved with refined constraints on the timing and setting of deposition. Contrary to a lacustrine interpretation, elemental carbon–sulfur (C–S) relationships and stratigraphic S isotope trends for black shales of the Helena Embayment of western Montana, the easternmost extension of Belt deposition, reveal that the early Belt basin received at least episodic fluxes of open marine water into a variably restricted setting. In the present study, this model of restricted marine deposition was tested and corroborated by generation of an analogous S isotope stratigraphy for the time equivalent Prichard Formation, located to the west of the embayment within the main Belt basin. Data for both pyrite and coexisting pyrrhotite show comparatively high δ 34S values and systematic S isotope shifts in excess of 20‰ over stratigraphic intervals spanning several hundred meters, suggesting limited and temporally varying sulfate availability in a bacterially driven marine system. Additional geochemical and sedimentological proxies (e.g. δ 13C of carbonate lithologies, diagnostic abiotic carbonate precipitates, and the presence of glauconite) further support a repeated or continued marine linkage throughout Belt deposition. A model for a restricted marine setting, when viewed in light of independent tectonic evidence (i.e. syndepositional mafic magmatism with peaks in activity at approximately 1470 and 1370 Ma; hydrothermal activity/mineralization along basin-bounding, high-angle normal faults; and high rates of sedimentation that resulted in sediment thicknesses of up to ∼16 km) defines a history of active and likely episodic rifting for the Belt basin. Collectively, these observations are temporally consistent with new models for Precambrian supercontinent cyclicity and suggest a complex history for western Laurentia that includes both episodes of convergence and at least incipient rifting prior to the final assembly of Rodinia. This relationship reflects a stepwise continental assembly perhaps analogous to the formation of Gondwana and the subsequent assembly of Pangea.

Sulfur geochemical constraints on mesoproterozoic restricted marine deposition: lower Belt Supergroup, northwestern United States

Despite nearly a century of study by a diverse group of Precambrian workers, competing interpretations have persisted that favor both marine and nonmarine depositional settings for the Mesoproterozoic Belt Supergroup of the northwestern United States. Our data for sedimentary pyrite formation in unmetamorphosed organic-rich shales of the Newland Formation of the lower Belt Supergroup, western Montana, argue that at least a portion of the Belt basin, during its early history, was characterized by restricted marine conditions with only episodic incursions of seawater. This model is supported by ratios of organic carbon (C org) to pyrite sulfur that, once corrected for selective diagenetic and thermal loss of C org and the inferred high reactivity of the C org reservoir in the absence of vascular land-plant remains, suggest degrees of sulfate limitation consistent with a restricted marine setting. The isotopically heavy disseminated pyrite that dominates the black shales shows a broad range of δ 34S values of bacterial origin (δ 34S ranges from −8.7 to +36.3‰; mean = +7.6‰, n = 41). The pyrite data overlap with the δ 34S range of coeval (barite) sulfate within the basin and are similar to independent estimates for the sulfate of Mesoproterozoic seawater. These relationships have allowed us to delineate a record of bacteriogenic pyrite that formed in a modified marine reservoir where the supply of sulfate was limited. A bacterial mechanism is further suggested by the morphological character of this early-formed pyrite, including the ubiquitous framboids. After comparing our results with those from present-day localities, such as the Black Sea and sites of lacustrine deposition, we conclude that the abundant 34S-enriched bacteriogenic disseminated pyrite observed in black shales of the Newland Formation must record a style of sulfate limitation possible only within an isolated marine basin receiving an intermittent flux of seawater. Although the entire Belt basin likely remained a restricted marine setting throughout deposition of the lower Belt Supergroup, sulfur isotope data from the Newland Formation at two localities separated by more than 150 km in the eastern Belt basin reveal stratigraphic trends in the strength of the marine connection. Specifically, the connection evolved such that inputs of seawater sulfate increased progressively in frequency and possibly strength during deposition of the Newland Formation. Overall, the hypothesized restricted marine setting, when viewed in light of independent tectonic, sedimentologic and geochronologic evidence, suggests syndepositional and likely episodic continental rifting, which helps constrain the Proterozoic paleotectonic history of western Laurentia.

Geometric and Sedimentologic Characteristics of Mid-Miocene Lowstand Reservoir Sandstones, Offshore Northwest Java, Indonesia: ABSTRACT

Numerous reservoirs in the Upper Cibulakan Formation (Mid-Miocene) of the Offshore Northwest Java shelf occur in sharp-based sandbodies that range from less than 1 m up to 10 m in thickness. Well-log derived net-sand isopach and seismic amplitude maps of these sandbodies depict elongate features, that are 1-2 km wide and 5-8 km long. The orientation of the longest axis of these sandbodies is predominantly north-south. Conventional cores reveal that these sandbodies are burrowed to completely bioturbated sandstones. Common trace fossils associated with these sandbodies include Ophiomorpha, Teichichnus and Thalassinoides. The lower contact of these sands is typically sharp and is commonly associated with a Glossifungites surface and siderite mud clasts. Overlying and underlying mudstones are relatively devoid of burrowing. Benthonic foraminifera assemblages within these mudstones indicate inner to outer neritic conditions in a relatively restricted marine setting. The upper contact of these sandstones is gradational over a 0.5 to 1m interval. Sandbodies of the same age and similar facies were observed in outcrops in onshore west Java. Here, they can be observed to pinch out over a distance of 500 m. The lower bounding contact appears discordant with underlying interbedded sandstones and mudstones. Several of the sandstones contain abundantmore » accumulations of the large, open marine, benthonic foraminifera Cycloclypeus and Lepidocyclina. Occasionally the concentration of these large foraminifera form limestones within the sharp-based sandbodies. These bioclastic deposits commonly exhibit planar-tabular and trough cross-stratification. The sandbodies are interpreted as having been emplaced during relative falls in sea-level within a large Mid-Miocene embayment. Our understanding of their geometry and sedimentologic characteristics is leading to a more effective exploitation strategy for these sandbodies in the Offshore Northwest Java area.« less