Deep-water Basin Research Articles

A phase of transient subsidence, sediment bypass and deposition of regressive–transgressive cycles during the breakup of Iberia and Newfoundland

Seismic, outcrop and well data from West Iberia and Newfoundland are used to investigate sediment stacking patterns during continental breakup as a function of tectonic subsidence. In West Iberia, two breakup sequences are revealed on seismic data by marked strata offlap oceanwards from the present-day continental shelf. This character is similar to Newfoundland, where correlative strata comprise Lower Cretaceous–Cenomanian coarse-grained siliciclastics accumulated around local sediment-source areas. The interpreted data reveal that the two breakup sequences: 1) materialise sediment bypass onto continental-slope depocentres that experienced important tectonic subsidence during continental breakup, but without showing typical syn-rift growth packages; 2) generate specific forced-regressive stratigraphic intervals that relate to uplift and exhumation of the proximal margin. Subsidence and sediment stacking patterns in both West Iberia and Newfoundland reflect similar continental breakup processes as they evolved from the upper lithosphere- to their mantle-breakup stages. On both margins, coarse-grained siliciclastic units on the proximal margin give rise to thick shaley successions in deep-water basins. This work also confirms that in a setting dominated by a significant sediment influx, yet lacking the burial rates of continental slope basins in Newfoundland, West Iberia comprised accommodation-driven basins during continental breakup, not necessarily sediment starved. As a corollary of our analysis, we classify breakup sequences around the world based on the characteristic lithologies of their regressive–transgressive depositional cycles.

Hydrochemistry and nutrient distribution in southern deep-water basin of the Caspian Sea

In this work, the results of hydrochemical studies aboard the R/V Iran Behshahr in southern Caspian Sea in late-winter 2014 were presented. Salinity, temperature, dissolved oxygen, pH, total dissolved inorganic carbon, total alkalinity, nitrate, phosphate and silicate concentrations in water column of Neka-Amir Kabir oil platform section in the southern Caspian Sea were measured to study the status of hydrochemistry of this area. Results showed that the hypoxia continues to intensify in the deep-water basin of the South Caspian Sea. Near-zero concentration of dissolved oxygen and accumulation of phosphate, silicate and total dissolved inorganic carbon in near-bottom layers in the study area showed that vertical winter mixing of water column did not reach the near-bottom layers at the time of this survey. Nitrate showed its maximum concentration at the intermediate maximum depth of 300m.

Cyclostratigraphic constraints on the duration of the Datangpo Formation and the onset age of the Nantuo (Marinoan) glaciation in South China

Constructing an accurate timeline is critical for reconstructing the Earth systems through critical transitions in climate, geochemistry, and life. Existing dates constrain synchronous initiation (ca. 717 Ma) and termination (ca. 660 Ma) of the Sturtian glaciation from multiple continents. The termination of the younger Marinoan glaciation is also well dated at ca. 635 Ma, but the onset of this glaciation is only roughly constrained as ≤ ca. 654 Ma (South China) and ≥ ca. 639 Ma (Namibia). To test if the Marinoan glaciation started close to ca. 654 Ma or ca. 639 Ma, we have conducted a cyclostratigraphic study on the Cryogenian non-glacial Datangpo Formation that conformably overlies and underlies Sturtian and Marinoan glacial diamictites, respectively, in a deep-water basin section in South China. A total of 28,765 magnetic susceptibility (MS) measurements from a drillcore of the 292-m-thick, muddy siltstone- and shale-dominated Datangpo Formation are used for cyclostratigraphic analysis. The results reveal significant decameter- to meter-scale sedimentary cycles of 16–12 m, 3.6–3.0 m, 1.0–0.8 m, and 0.6–0.4 m. The ratios of these cycle wavelengths match well with those of the Milankovitch cycles calibrated for the Cryogenian Period. The established astrochronologic time scale suggests that the duration of the Datangpo Formation is about 9.8 million years. Together with the radiometric age of ca. 660 Ma for the termination of the Sturtian glaciation, the cyclostratigraphic data suggest that the Nantuo (Marinoan) glaciation in South China initiated at ca. 650 Ma, which is slightly younger than but consistent with the ca. 654 Ma U–Pb age from the top of the Datangpo Formation in shelf sections. This age, however, is significantly older than the ages obtained from Marinoan-age glacial diamictites in South China (ca. 636 Ma) and Namibia (ca. 639 Ma). Given that most of the shelf sections may have suffered from glacial erosion, obtaining the onset age of the Marinoan glaciation should focus on relatively complete, deep-water successions.

Deepwater sedimentation and Cenozoic deformation in the Southern New Caledonia Trough (Northern Zealandia, SW Pacific)

Abstract The New Caledonia Trough (NCT) is a 2000–3000 m deep bathymetric feature that extends 2500 km from Taranaki, New Zealand, to the western margin of New Caledonia (Northern Zealandia, SW Pacific). The underlying sedimentary basin originates from Cretaceous extension, but underwent a significant Eocene tectonic event that shaped its present physiography. We present an analysis of the basin based on multibeam data, seismic profiles and rock samples collected on the TAN1312 and TAN1409 Expeditions onboard R/V Tangaroa, combined with legacy data. We focus on the southern part of the basin, where new data reveal a link between compressive deformation of Paleocene strata and their potential reworking into the basin. On the western basin side, Upper Cretaceous to Paleocene strata were deformed by local reverse faults and folds that created a sub-circular bathymetric ridge and seabed exposure. This folded unit (seismic Unit 2) is sharply overlain by a restricted interval imaged as chaotic high-amplitude reflections that are interpreted as syntectonic mass-transport deposits due to slope oversteepening (seismic Unit 1b). This unit is stratigraphically below the main basin onlap surface and seismic mapping revealed that it rapidly thins away from the mouth of a present day submarine canyon imaged on the western slope of the basin, and that is diverted by exposures of Unit 2 deformed strata. Deformed and syntectonic intervals are in turn overlain by a flat-lying unit (seismic Unit 1a) we interpret as reflecting a passive basin fill. Our new data provide insight into an extensive deep-water basin that is remote from terrigenous sediment sources, and new constraints on its Cenozoic tectonic history. Stratigraphic ages are constrained by seismic ties to Taranaki basin petroleum wells and biostratigraphic dating of dredged samples. This specific site has particular significance for understanding tectonic events in the southwest Pacific. Indeed, our observations show that deformation is younger than the Paleocene and is envisaged to be a local expression of a widespread Cenozoic compressive event (called “TECTA”, Tectonic Event of the Cenozoic in the Tasman Area), which affected the region after the Mesozoic rifting. On a regional perspective, this study provides new insights on the evolution of the submerged Zealandia continent and associated geodynamic processes such as Gondwana break-up and initiation of the Tonga-Kermadec subduction.

Structural style, formation of low angle normal fault and its controls on the evolution of Baiyun Rift, northern margin of the South China Sea

Recent studies of high-resolution seismic reflection profiles in the northern rifted margin of the South China Sea revealed that low angle normal faults dominated the structural evolution of deepwater basins rather than high angle normal faults solely as was previously assumed; however, none of the studies proposed kinematic restorations of the fault system and the relationship to continental margin formation. The Baiyun Rift is located in the slope of the northern margin of the South China Sea underlain by extremely thinned continental crust. It is filled with a relatively complete sequence of Cenozoic sediments up to 10 km and has been considered the best-documented deepwater basin. Basin-wide 3D seismic data together with the available deepwater well data have allowed the structural framework to be built and tectono-stratigraphic history of the Baiyun Rift to be traced in detailed from the rift stage; a quantitative study of the primary geometry and kinematics of the major faults can be performed consequently. Results show the inception of the rift is characterized by faults slipped at high angles; the later stage (mid-Eocene) of intensive extension is dominated by a fault system with large horizontal displacements (>10 km) and initial low dips (<30°). The observations can be best explained by a low angle normal faulting (faults started out at shallow angles during mid-Eocene or being rotated from high angles) following a scattered high angle normal faulting. As a consequence, the polyphase faulting created a large accommodation space vertically and horizontally and formed a deep and wide shape of the Baiyun Rift. Besides, the phenomenon that the cessation of the rift in the Baiyun Rift predated the final breakup of the continental margin suggests the timing of the rifting events along the northern margin of the South China Sea is non-instantaneous. This study implies the progressive deformation dominated by low angle normal faulting causes an uneven distribution of the continental crustal thickness and may favor the breakup location in the northern South China Sea.

Occurrence and Turnover of Biogenic Sulfur in the Bering Sea During Summer

AbstractThe horizontal/geographical variations in dissolved dimethylsulfide (DMS), its precursor dimethylsulfoniopropionate (DMSPd and DMSPp), and chlorophyll a (Chl a), as well as the oceanographic parameters influencing the concentrations of dimethylated sulfur compounds, were investigated in the Bering Sea from July to August 2012. Similar to Chl a, the surface DMS and DMSPp levels, as well as DMS(P) production and consumption rates, exhibited a declining gradient from the central basin to the continental shelf, with high‐value areas appearing in the central basin, the slope regions, and Anadyr Strait but a low‐value area occurring on the outer‐middle continental shelf. Considerably high values of DMS and DMSP were measured in the saline Bering Sea Basin Deep Water (&gt;2,000 m) located at the southwest of the Bering Basin because of the release of resuspension in 2,000 m depth and the DMSP production from endogenous benthic bacteria and cyanobacteria population. Chl a was positively correlated with DMSPp and DMS in the surface waters and the upper water of the basin, whereas significant negative correlations were found between DMS and nutrients (dissolved inorganic nitrogen [DIN], phosphorus, and silicate) in the inner shelf of the Bering Sea. DMS microbial consumption was approximately 6.26 times faster than the DMS sea‐air exchange, demonstrating that the major loss of DMS in the surface water occurred through biological consumption relative to evasion into the atmosphere. Average sea‐to‐air DMS fluxes were estimated to be 4.66 μmol/(m2·d), and consequently oceanic biogenic DMS emission had a dominant contribution to the sulfur budget over the observational area.

Geodynamics of the origin of granulites in the Sangilen block of the Tuva–Mongolian terrane, Central Asian Orogenic Belt

The Tuva–Mongolian terrane of the Central Asian Orogenic Belt is a composite structure with a Vendian–Cambrian terrigenous–carbonate cover. The Sangilen block in the southern part of the belt is a smaller composite structure, in which tectono–stratigraphic complexes of different age that were produced under various conditions were amalgamated in the course of Early Paleozoic tectonic cycle. The P–T parameters of the Early Paleozoic metamorphism in the western part of the Sangilen block corresponded to the amphibolite facies. The gneisses of the Erzin Complex contain relict granulite-facies mineral assemblages. The granulites are dominated by metasediments typical of deep-water basins on passive continental margins. The only exception is granulites of the Lower Erzin tectonic nappe of the Chinchlig thrust system: these rocks are metatholeiites, tonalites, and trondhjemites, whose REE patterns are similar to those of MORB. The composition of these granulites and their high Sm/Nd ratios indicate that the rocks were derived from juvenile crust that had been formed in an environment of a mature island arc or backarc basin. It is reasonable to believe that these rocks are fragments of the Late Riphean basement of the Sangilen block. The average 206Pb/238U zircon age of the garnet–hypersthene granulites is 494 ± 11 Ma. With regard for the zircon age of the postmetamorphic granitoids, the granulite-facies metamorphism occurred within the age range of 505–495 Ma. The peak metamorphic temperature reached 910–950°C, and the pressure was 3–4 kbar, which corresponds to ultrahigh-temperature/low-pressure (UHT–LP) metamorphism. The garnet–hypersthene orthogranulites were formed at a temperature that decreased to ~850°C and pressure that increased to ~5.5‒7 kbar. It can be hypothesized that the earlier UHT–LP granulites were produced at an elevated heat flux and were later (in the course of continuing collision) overlain by a relatively cold tectonic slab, and this leads to a certain temperature decrease and pressure increase. This relatively cold slab could consist of fragments of the Vendian elevated-pressure metamorphic belt whose development terminated at the Vendian–Cambrian boundary before the onset of the Early Paleozoic regional metamorphism.

Paleoclimatic and paleolatitude settings of accumulation of radiolarian siliceous–volcanogenic sequences in the middle Mesozoic Pacific: Evidence from allochthons of East Asia

Jurassic–Cretaceous siliceous–volcanogenic rocks from nappes of tectonostratigraphic sequences of the East Asia Middle Cretaceous Okhotsk–Koryak orogenic belt are represented by a wide range of geodynamic sedimentation settings: oceanic (near-spreading zones, seamounts, and deep-water basins), marginal seas, and island arcs. The taxonomic compositions of radiolarian communities are used as paleolatitude indicators in the Northern Pacific. In addition, a tendency toward climate change in the Mesozoic is revealed based on these communities: from the warm Triassic to the cold Jurassic with intense warming from the Late Jurassic to the Early Cretaceous. Cretaceous warming led to heating of ocean waters even at moderately high latitudes and to the development of Tethyan radiolarians there. These data are confirmed by a global Cretaceous temperature peak coinciding with a high-activity pulse of the planetary mantle superplume system, which created thermal anomalies and the greenhouse effect. In addition, the Pacific superplume attributed to this system caused accelerated movement of oceanic plates, which resulted in a compression setting on the periphery of the Pacific and the formation of the Okhotsk–Koryak orogenic belt on its northwestern framing in the Middle Cretaceous, where Mesozoic rocks of different geodynamic and latitudinal–climate settings were juxtaposed into allochthonous units.

Building a model with realistic pore pressures: Modifying the SEAM Phase I subsalt model

Predrill pore-pressure prediction, particularly in overpressured reservoirs, is of paramount importance for hazard avoidance in deepwater basins like those in the Gulf of Mexico. The SEG Advanced Modeling (SEAM) Pore Pressure Prediction project has been developing a model for a region of the Gulf of Mexico that can be used for seismic simulation to generate a seismic data set for use in testing pore-prediction methodologies. The approach was to modify the deepwater model developed for the SEAM Phase I project to include a realistic pore-pressure distribution. The methodology for determining the pore pressures was to use basin simulation with the Phase I model as a basis. We experimented with a range of parameters and pore-pressure-generation mechanisms to better understand their impact on the final distribution of predicted pore pressures. The experiments and results obtained from them will be described. Following completion of the basin simulation, a rock-physics model was developed to transform the geology and derived pore pressures into anisotropic elastic values that were then used for seismic simulation.

PETROLOGICAL INVESTIGATION OF CARBONATE ROCKS FROM THE IONIAN ZONE (ETOLOAKARNANIA, WESTERN GREECE)

The petrographic features of the carbonate rocks from the Ionian Zone in the Etoloakarnania Prefecture are presented. They are represented by limestones with minor dolomite. The limestones include: (i) wackestones (or sparse micrites), with poor presence of allochems within a mud matrix; (ii) packstones (or packed micrites) with increasing levels of carbonate grains that are still surrounded by micrite matrix, and (iii) grainstones (or sparites) containing allochems that are cemented with sparry calcite crystals, while the mud matrix is absent. Bioclasts are the dominant carbonate components in most of the samples accompanied by infrequent pelloids, intraclasts, lithoclasts and ooids. Crystalline limestones were not identified. Quartz, apatite, barite, anhydrite, halite, clay minerals, magnetite and ilmenite have been determined as accessory phases. The results show that mineralogical and petrographic features of the analyzed carbonate rocks are related to their evolution during the development of the Ionian Zone from a shallow-marine platform to a deep water basin.

The position and age of flysch deposits in the Crimean Mountains (Southern Ukraine).

The Crimean Mountains (CM) are regarded as part of the Alpine-Himalaya orogenic belt related to the collision of the Eurasian and African plates. Our research in the CM has allowed confirming the existence of at least two flysch formations of different ages: the Taurida Flysch Formation (Upper Triassic/Lower–? Middle Jurassic) and the Sudak Formation (uppermost Jurassic/Lower Cretaceous) in the western and eastern sectors of the CM, respectively. After the Middle Jurassic volcanism, the freshwater claystones with coal-bearing intercalations, as well as local alluvial fan conglomerates were deposited. Then, following the Oxfordian/Kimmeridgian marine transgression, three separated Tithonian/Berriasian carbonate platforms developed: Baydarska, Chatyr-Dag and Demerji/Karabi. At the turn of the Late Jurassic, the deep-water Sudak Basin (eastern sector of the CM) began to develop in the eastern periphery of the Demerji/Karabi carbonate platform. This basin, dominated by turbiditic deposition, was probably connected with the Great Caucasus sedimentary system. Finally, distal flysch sedimentation in the Sudak Basin was followed by debris-flow deposits, with huge blocks of Upper Jurassic limestones. These blocks were derived from destruction of the Karabi carbonate platform. During the Berriasian/Valanginian, the eastern sector of the CM began to subside. As a result, both carbonate platforms (western CM) and the debris flow fans of the Sudak Basin (eastern CM) were overlain by marly shales and/or distal turbidites. This type of deposition lasted until the Aptian/Albian. In the post-Albian period, the Alushta-Salgir tectonic zone was transformed into a SE-dipping thrust fault with at least 10 km amplitude of overthrusting.

Evaluating the Origin of Seepage Water in the Golgohar Iron Mine, Iran

The Golgohar mine produces iron ore from an open pit in south-central Iran that is now more than 100 m below the water table, and dewatering has not solved the mine’s water seepage problem. Previous studies had reported that Kheirabad (Sirjan) Playa, 13 km north of the mine, was the most probable groundwater source. A combination of geological, hydrogeological, hydrochemical, and isotopic techniques were used to evaluate the hydraulic connectivity between the mine and the lake, and to characterize the probable water sources. Thirty-two surface and groundwater samples were collected and analyzed for major and minor (Li, B, and Br) constituents. Ten water samples were analyzed for oxygen-18 and deuterium and three samples were analyzed for tritium. The results indicated that impermeable formations and very low-permeability alluvial deposits lie between lake and the pit. In addition, the chemical and isotopic signatures of the two water sources indicated different origins. Inverse geochemical modeling and the mine water’s isotopic signatures suggest that the water is coming from the surrounding alluvial aquifer, or is deep basin water from within the fractured bedrock, or a mixture of the two.

Deep-water clastic systems in the Upper Carboniferous (Upper Mississippian–Lower Pennsylvanian) Shannon Basin, western Ireland

The Upper Carboniferous Shannon Basin of western Ireland contains a more-than-2300-m-thick (7540 ft) basin-fill succession, shallowing upward from deep-water to deltaic and incised fluvial deposits. The deep-water basin floor and slope succession is world renowned as an analog for hydrocarbon-bearing deep-water sandstones on several continental margins such as the basins in East and West Africa, South America, the Gulf of Mexico, and not least offshore northwest Europe. The Shannon Basin is frequently visited by both academia and industry for research and training purposes. A series of behind-outcrop research boreholes reveals the subsurface expression of the deep-water rocks and is complemented by seismic-scale cliff exposures. The succession is interpreted as a first-order basin-scale linked sedimentary system. This system can be analyzed using the principles of source-to-sink analysis and leaves the visitor with a complete picture of the basin fill, enhanced by spectacular sedimentological and stratigraphic detail.

Experimental study of vortex-induced vibration for drilling risers under uniform flow current

An experiment was conducted in a deep water basin to investigate the vortex-induced vibration mechanism of a drilling riser. Various measurements were obtained by the fiber Bragg grating strain sensors, and data was analyzed by modal analysis method. Results show that the vibration mode of the drilling riser increases with the increasing flow velocity, and the vibration amplitude in the CF direction is larger than that in the IL direction. The vibrations in the CF and IL directions interact and mutually affect each other. The vibration mode in the IL direction is usually larger than that in the CF direction as the dominant vibration frequency is twice of that in the CF direction. Higher stresses may occur rather in the IL direction than in the CF direction. Hence, fatigue induced by the IL direction should also be taken into consideration when analyzing the fatigue life of a drilling riser. The three-time harmonic appears, and the phenomenon becomes more obvious as the flow velocity increased for the effect of the “2T” wake mode under the experiment condition. Displacement trajectories are significantly influenced by dominant vibration frequency and phase angle between the CF and IL directions. Crescent shapes, figure-eight, and inclined figure-eight are appeared in the experiment with the increasing flow velocity.

Zambezi continental margin: compartmentalized sediment transfer routes to the abyssal Mozambique Channel

Sediment delivery to the abyssal regions of the oceans is an integral process in the source to sink cycle of material derived from adjacent continents and islands. The Zambezi River, the largest in southern Africa, delivers vast amounts of material to the inner continental shelf of central Mozambique. The aim of this contribution is to better constrain sediment transport pathways to the abyssal plains using the latest, regional, high-resolution multibeam bathymetry data available, taking into account the effects of bottom water circulation, antecedent basin morphology and sea-level change. Results show that sediment transport and delivery to the abyssal plains is partitioned into three distinct domains; southern, central and northern. Sediment partitioning is primarily controlled by changes in continental shelf and shelf-break morphology under the influence of a clockwise rotating shelf circulation system. However, changes in sea-level have an overarching control on sediment delivery to particular domains. During highstand conditions, such as today, limited sediment delivery to the submarine Zambezi Valley and Channel is proposed, with increased sediment delivery to the deepwater basin being envisaged during regression and lowstand conditions. However, there is a pronounced along-strike variation in sediment transport during the sea-level cycle due to changes in the width, depth and orientation of the shelf. This combination of features outlines a sequence stratigraphic concept not generally considered in the strike-aligned shelf-slope-abyssal continuum.

Modeling Stratigraphic Architecture Using Small Unmanned Aerial Vehicles and Photogrammetry: Examples From the Miocene East Coast Basin, New Zealand

Abstract: The ability to view and characterize outcrops that are difficult to study from the ground is greatly improved by aerial investigation. We describe the application of flying a small, unmanned aerial vehicle (UAV) to collect photographic data for modeling rock outcrops and creating detailed digital terrain models (DTMs), which can be used for multi-scale digital feature mapping in true three-dimensional (3-D) space. Georeferenced digital outcrops can complement field measurements, as well as enhance classroom to field learning experiences for undergraduate education. High-resolution photographs, acquired at various elevations and azimuths by a small UAV, are used to convert stratigraphic features to 3-D digital representations with spatial accuracy approaching 1 cm. Complex topography, recorded in 2-D image sequences, is rendered digitally in 3-D surface models using Structure from Motion (SfM) photogrammetry. The resulting DTMs are populated with the high-resolution photographs for stratigraphic interpretation at the centimeter scale. The method of imaging and modeling outcrops is demonstrated in the Miocene East Coast Basin, New Zealand, where steep, coastal cliff exposures of continental-slope deposits and extensive, wave-cut platform exposures of steeply dipping, deep-water basin deposits offer exceptional opportunities to investigate depositional processes and the resulting stratigraphic architecture. Our approach makes it possible to characterize deposits that are exposed in inaccessible, 150-m-high, vertical sea cliffs and along 2000 m of wave-cut platforms, and to quantify their geometric variability. Results yield a spatial and temporal understanding of two depositional systems at a scale that was previously unattainable by conventional field techniques.

Technology Update: Retrofitting MPD Systems to Deepwater Rigs Aids Drilling, Efficiency, and Process Safety

Technology Update The unique properties of deepwater formations pose significant challenges to the capabilities of conventional drilling rigs. By retrofitting rigs with automated drilling systems, including managed-pressure drilling (MPD) equipment, operators and contractors in global deepwater basins can optimize efficiency and economics while maintaining the level of safety that the industry and regulatory community have mandated for this high-stakes arena. MPD, an adaptive drilling process that actively controls annular pressure throughout the wellbore, is increasingly being integrated into the risers of existing deepwater drilling vessels (Fig. 1). It is the cornerstone of an automated well-control strategy that augments and enhances standard well-control protocols. The technique provides distinct drillability, efficiency, and process safety benefits in deepwater environments. It addresses the higher level of priority that the industry has placed on process safety, which largely involves the ability to accurately and actively monitor and control operations while drilling. Long used in land and shallow-water drilling, MPD-ready rigs are proving their value in deepwater operations by delivering a reliable level of automation and process control when the risk of a well-control incident is highest. Fluctuations in wellbore pressure often create drilling hazards that result in lost circulation, kicks, and other problems that can significantly increase rig time and cost, or lead to catastrophic well-control incidents. An automated MPD system installed onto a deepwater semisubmersible rig or drillship can effectively monitor and maintain the annular hydraulic pressure profile by applying common MPD variants, such as constant bottomhole pressure (CBHP) and pressurized mud cap drilling (PMCD). The ability to continuously monitor various wellbore parameters—particularly those related to downhole pressure—while drilling and making connections also enhances personnel preparedness by serving as an extra set of eyes to supplement those of less-experienced drillers. While expanding the drilling envelope and monitoring wellbore pressure profiles, an MPD system that provides automated advanced flow detection, including early kick detection, also reduces the risks of human intervention by mitigating the possibility of gas in the riser.

Neoarchaean banded iron formation within a ∼2620 Ma turbidite-dominated deep-water basin, Slave craton, NW Canada

Banded iron formations (BIFs) are iron- and silica-rich chemical metasedimentary rocks that span much of the Archaean and early Palaeoproterozoic rock record. Given that BIFs precipitated from ambient seawater, they provide an important archive of Earth’s ancient marine environments. Within the Slave craton occurs one of the largest and well-preserved Achaean turbidite basins in the world. Interstratified within the greywacke-mudstone turbidites is BIF that is commonly associated with ferruginous sediments and locally with a felsic tuff bed. Field investigations were conducted at four BIF locations from west to east across the Slave craton. These BIFs are composed of micro- and mesobands of quartz and magnetite, with various amounts of metamorphic grunerite, ferro-actinolite, ferro-hornblende as well as greenalite and stilpnomelane, the latter two are interpreted as retrograde metamorphic phases. As the BIF was deposited in a deep-water turbidite-dominated basin(s), the chemical component was occasionally diluted by clastic detritus, as illustrated by highly variable Al2O3 content, and Al2O3/TiO2 and Cr/Ni that are similar to the clastic sediments. On a layer-by-layer basis, the iron content in the BIF ranges from as low as 4wt.% and up to ∼75wt.% Fe2O3, whereas the SiO2 content ranges from 20wt.% to as high as ∼94wt.%. This, together with a hyperbolic germanium/silicon distribution, suggests that continentally-sourced silica comprised the background precipitation, which was periodically interrupted by pulses of hydrothermally-sourced iron. High-temperature hydrothermal fluids contributed, with an average shale-normalized positive Eu anomaly of 1.92, which indicates input of reduced Eu2+ to the ambient seawater. The BIF shows characteristic aqueous fluid-derived rare earths+yttrium fractionation (Pr/Yb<1) and positive La and Y anomalies. A negative Ce anomaly in parts of the BIF indicates the presence of an oxidizing agent able to fractionate Ce3+ to Ce4+. The age and trace element signature of the felsic tuff is similar to arc-type volcanic rocks, potentially linking the BIF and turbidite basin(s) to the Defeat Suite plutons (ca. 2630–2620Ma). The slow chemical rainout behind BIF deposition presumably took place on the basin plain adjacent to a steep submarine ramp where turbidite currents and slump-generated debris flows, sourced and triggered by the tectonically active arc-terrain, episodically delivered clastic sediment.

Challenges and lessons learned from environmental approvals for drilling in frontier offshore basins, using the Great Australian Bight as an example

The recent withdrawal of BP Developments Australia Pty Ltd from its exploration drilling efforts in the Great Australian Bight (GAB) has brought focus to the challenges of obtaining environmental approvals for offshore petroleum exploration projects, particularly in frontier, deep-water basins such as the GAB. In preparing the first two environment plan (EP) submissions for the BP GAB drilling exploration project, Aventus Consulting Pty Ltd has come across numerous such challenges, which are summarised in the present paper.

Rift systems of the Russian Eastern Arctic shelf and Arctic deep water basins: link between geological history and geodynamics

In our study, we have developed a new tectonic scheme of the Arctic Ocean, which is based mainly on seismic profiles obtained in the Arctic-2011, Arctic-2012 and Arctic-2014 Projects implemented in Russia. Having interpreted many seismic profiles, we propose a new seismic stratigraphy of the Arctic Ocean. Our main conclusions are drawn from the interpretation of the seismic profiles and the analysis of the regional geological data. The results of our study show that rift systems within the Laptev, the East Siberian and the Chukchi Seas were formed not earlier than Aptian. The geological structure of the Eurasian, Podvodnikov, Toll and Makarov Basins is described in this paper. Having synthesized all the available data on the study area, we propose the following model of the geological history of the Arctic Ocean: 1. The Canada Basin formed till the Aptian (probably, during Hauterivian-Barremian time). 2. During the Aptian-Albian, large-scale tectonic and magmatic events took place, including plume magmatism in the area of the De Long Islands, Mendeleev Ridge and other regions. Continental rifting started after the completion of the Verkhoyansk-Chukotka orogenу, and rifting occurred on the shelf of the Laptev, East Siberian, North Chukchi and South Chukchi basins, and the Chukchi Plateau; simultaneously, continental rifting started in the Podvodnikov and Toll basins. 3. Perhaps the Late Cretaceous rifting continued in the Podvodnikov and Toll basins. 4. At the end of the Late Cretaceous and Paleocene, the Makarov basin was formed by rifting, although local spreading of oceanic crust during its formation cannot be excluded. 5. The Eurasian Basin started to open in the Early Eocene. We, of course, accept that our model of the geological history of the Arctic Ocean, being preliminary and debatable, may need further refining. In this paper, we have shown a link between the continental rift systems on the shelf and the formation history of the Arctic Ocean.