Offshore Sequences Research Articles

Bering Strait gateway currents and their implications for sediment distribution, shelf conceptual models and economic potential

This paper addresses a gap in our knowledge about strong geostrophic current control of epicontinental shelf deposits. The northward current that accelerates towards the Bering Strait gateway affects sediment distribution in the northern Bering Sea leading to the Strait, in the Strait and in the Chukchi Sea where currents slack. Leeside sand bodies develop when the northward geostrophic current shears and accelerates against topographic projections into the current. When currents slack downstream, leeside sand bodies deposit. Yearly 1 m and decadal catastrophic up to 5 m storm surges resuspend the Yukon sediment. Most of this sediment is then entrained by strong northward currents and transported up to 1000 km north, bypassing Chirikov Basin and the Bering Strait gateway, to deposit in the Chukchi Sea. Northern Bering Sea depositional patterns reinforce evidence that epicontinental shelf sedimentary sequences are different from more regular proximal to distal offshore sequences on marginal shelves. Compared with marginal shelves, strong currents, Coriolis force and topographic constrictions result in sediment bodies detached from the shoreline and large areas of sediment bypass on epicontinental shelves. The Bering Sea currents prevent Holocene sediment from covering both relict gravel with the world's largest offshore gold deposits and large areas of surficial sediment with heavy-mineral concentrations >16% that contain scarce rare earth metals.

Evolution of an Accretionary Complex (LeMay Group) and Terrane Translation in the Antarctic Peninsula

AbstractThe LeMay Group accretionary complex of Alexander Island (Antarctic Peninsula) comprises a 4 km thick succession of variably deformed turbidites associated with thrust slices of ocean floor basalts. The depositional age and provenance of the succession is uncertain with estimates ranging from Carboniferous to Cretaceous. The accretion history is also poorly established and whether the LeMay Group developed allochthonously and accreted during an episode of Cretaceous terrane translation. We have examined the geochronology and geochemistry of 22 samples from across the entire accretionary complex to determine its depositional, provenance and accretion history. The accretionary complex has been subdivided into four separate groups based on detrital zircon U‐Pb age and Lu‐Hf provenance analysis. Groups 1 and 2 are interpreted to be a continuation of the extensive Permian accretionary complexes of West Gondwana and have a depositional age of c. 255 Ma, with volcaniclastic input from the extensive silicic volcanism of the Choiyoi Province. Accretion of the LeMay Group to the continental margin developed during the mid‐Triassic, potentially related to the Peninsula Orogeny and an episode of flat‐slab subduction of the proto‐Pacific plate. Group 3 is only identified from an island to the west of Alexander Island and has a mid‐Cretaceous depositional age and provenance akin to offshore sequences from Thurston Island. A para‐autochthonous origin is suggested, with mid‐Cretaceous accretion associated with the melange belts of central Alexander Island. Group 4 is also a distinct unit with an Early Jurassic depositional age and a source more closely related to the Antarctic Peninsula.

Reservoir properties and reactivity of the Faroe Islands Basalt Group: Investigating the potential for CO2 storage in the North Atlantic Igneous Province

Offshore injection of CO2 into volcanic sequences of the North Atlantic Igneous Province may present a large-scale, permanent storage option through carbonate mineralization. To investigate this potential, onshore studies of reservoir properties and reactivity of the subaerially erupted Faroe Islands Basalt Group have been conducted. Outcrop and borehole samples reveal that the lava flow crusts commonly contain vesicles that have been filled with secondary minerals due to hydrothermal fluid circulation, however, unmineralized and highly porous layers do occur. Bulk density measurements, micro-computed tomography (µ-CT) image analysis, and microscope studies of samples from onshore boreholes give present-day porosities ranging from 0.5% to 36.2% in the volcanic sequences. The unmineralized brecciated lava flow crusts contain the largest estimated porosity and simulated absolute permeability (reaching up to 10−12 m2). µ-CT studies of the mineralized, brecciated flow crusts indicate initial porosities reaching up to 45%, before clogging. Kinetic experiments of rock dissolution show that the reactivity of the basalt and volcaniclastic sediments depends on the alteration state with more altered basalt being less reactive. However, the presence of reactive, high porosity, and high permeability flow crusts prior to clogging indicate the existence of promising and very large CO2 reservoirs in less altered offshore sequences.

Biostratigraphy and sequence stratigraphy of the deep offshore Niger Delta and implications for future hydrocarbon exploration in the Paleogene sequences

The Niger Delta Deep Offshore Basin has been the latest frontier within the Niger Delta for hydrocarbon exploration and production activities. Until the onset of drilling activities in the early 1990s, little was known about the biostratigraphy of this frontier setting. Thus, uncertainties existed in the ages ascribed to the various stratigraphic sequences. Presently, several wells have been drilled in the deep offshore Niger Delta and three-pronged biostratigraphic data and some sequence stratigraphic interpretations have become available. We have integrated biostratigraphic data comprising palynological, foraminiferal, and calcareous nannoplankton zonation and biofacies, wireline log information, and regional-3D seismic data, with simple basic sequence stratigraphic analysis for a detailed well correlation, regional mapping of hydrocarbon-bearing intervals, and evaluation of the thicknesses of promising stratigraphic intervals. Results indicate a significant effect on not only the earlier conceptualized age of the deepest prospective reservoirs, but also the ages of possible source rocks in the Paleogene of the Niger Delta offshore sequence. Our findings will help in the realistic assumptions of source rock characterization, charge modeling/prediction, and the stratigraphic thicknesses of the offshore sequences and their reservoirs. We examine the specific derivable age interpretations from the integration of at least six wells, biostratigraphic data with seismic, and the implications of these interpretations for hydrocarbon prospectivity of the Paleogene sequences. Furthermore, we document the biostratigraphy work done in the area, in all the structural belts where drilling has occurred including the deepest well drilled in the deep offshore Niger Delta, Bosi-006. The results of this study have a critical impact on the current regional geologic understanding of the deep offshore Niger Delta.

The petrophysical and petrographical properties of hyaloclastite deposits: Implications for petroleum exploration

Offshore sequences of volcaniclastic rocks (such as hyaloclastite deposits) are poorly understood in terms of their rock properties and their response to compaction and burial. As petroleum exploration targets offshore volcanic rifted margins worldwide, understanding of volcanic rock properties becomes important both in terms of drilling and how the rocks may behave as seals, reservoirs, or permeability pathways. The Hawaiian Scientific Drilling Project phase II in 2001 obtained a 3 km-(2-mi)-long core of volcanic and volcaniclastic rocks that records the emergence of the largest of the Hawaiian islands. Core recovery of 2945 m (9662 ft) resulted in an unparalleled data set of volcanic and volcaniclastic rocks. Detailed logging, optical petrology, and major element analysis of two sections at depths 1831–1870 and 2530–2597 m (6007–6135 and 8300–8520 ft) are compared to recovered petrophysical logs (gamma ray, resistivity, and P-wave velocity). This study concludes deviation in petrophysical properties does not seem to correlate to changes in grain size or clast sorting, but instead correlates with alteration type (zeolite component) and bulk mineralogy (total olivine phenocryst percentage component). These data sets are important in helping to calibrate well-log responses through hyaloclastite intervals in areas of active petroleum exploration such as the North Atlantic (e.g., Faroe-Shetland Basin, United Kingdom, and Faroe Islands, the Norwegian margin and South Atlantic margins bordering Brazil and Angola).

Two new short-ranged Calcidiscus species from the offshore marine Neogene Niger Delta sequences

Two new species, Calcidiscus salamis sp. nov. and C. odebodea sp. nov. are described from themarine offshore Niger Delta Neogene sequence. Calcidiscus salamis is structurally similar to C. fuscus but has a closed central area. Calcidiscus odebodea differs from C. leptoporous in having strongly curved sutures. The new forms consistently occur throughout the LateMiocene to Early Pliocene offshore eastern and western Niger Delta sequences. They have the same stratigraphic range: NN11-NN13 zones (Messinian to Zanclean). The combination of their widespread geographic occurrence and short stratigraphic rangemakes them excellent index species for the high-resolution biostratigraphy of theNiger Delta marine offshore sequences where most Calcidiscus forms have much longer ranges. KEYWORDS: Calcareous nannofossil, Biostratigraphy, Calcidiscus, Niger Delta.

Understanding the offshore flood basalt sequence using onshore volcanic facies analogues: an example from the Faroe–Shetland basin

AbstractFlood basalts in associated volcanic rifted margins, such as the North Atlantic Igneous Province, have a significant component of lavas which are preserved in the present day in an offshore setting. A close inspection of the internal facies architecture of flood basalts onshore provides a framework to interpret the offshore sequences imaged by remote techniques such as reflection seismology. A geological interpretation of the offshore lava sequences in the Faroe–Shetland Basin, using constraints from onshore analogues such as the Faroe Islands, allows for the identification of a series of lava sequences which have characteristic properties so that they can be grouped. These are tabular simple flows, compound-braided flows, and sub-aqueously deposited hyaloclastite facies. The succession of volcanic rocks calculated in this study has a maximum thickness in excess of 6800 m. Down to the top of the sub-volcanic sediments, the offshore volcanic succession has a thickness of about 2700 m where it can be clearly identified across much of the area, with a further 2700 m or more of volcanic rock estimated from the combined gravity and seismic modelling to the north and west of the region. A large palaeo-waterbody is identified on the basis of a hyaloclastite front/apron consisting of a series of clinoforms prograding towards the eastern part of the basin. This body was > 500 m deep, must have been present at the onset of volcanism into this region, and parts of the water body would have been present during the continued stages of volcanism as indicated by the distribution of the hyaloclastite apron.

Seismic stratigraphy of the quaternary Yellow River delta, Bohai Sea, eastern China

The upper 40 m of stratigraphy of the Yellow River (Huang He) subaqueous delta has been well documented, but the nature of the underlying strata is currently unknown at high-resolution. To address this deficiency we used a Geopulse seismic system to image shallow sedimentary deposits up to 120 m deep on the Yellow River delta. High-resolution seismic reflection images were processed with a series of specific techniques (e.g. swelling attenuation, dynamic s/n filter; f-x deconvolution, predictive deconvolution dipscan stack), and used with borehole data to investigate the Quaternary offshore sequences in the Yellow River (Huang He) delta. Repetitive sequences were observed and interpreted as containing layers of transgressive and regressive deposits. Six seismic transgressive and regressive cycles are identified. Unit M6F–C6F correlates with a relative sea-level rise (173–157 ka) and fall (231–173 ka), while Unit M5F–C5F is associated with a relative sea-level rise (124–100 ka) and fall (157–124 ka). Unit M4F–C4F spans a period of sea-level fall at 100–87 ka, followed by a rise at 87–76 ka. Unit M3F–C3F is a transgressive–regressive cycle dated as 76–58 ka. Unit M2F–C2F correlates with relative sea level fall at 58.2–36 ka and subsequent rise at 36–22 ka. Unit M1F–C1F was deposited during relative sea level fall (22–18 ka), followed by a rise, especially since 8.5 ka.

Supracrustal faults of the St. Lawrence rift system, Québec: kinematics and geometry as revealed by field mapping and marine seismic reflection data

The St. Lawrence rift system from the Laurentian craton core to the offshore St. Lawrence River system is a seismically active zone in which fault reactivation is believed to occur along late Proterozoic to early Paleozoic normal faults related to the opening of the Iapetus ocean. The rift-related faults fringe the contact between the Grenvillian basement to the NW and Cambrian–Ordovician rocks of the St. Lawrence Lowlands to the SE and occur also within the Grenvillian basement. The St. Lawrence rift system trends NE–SW and represents a SE-dipping half-graben that links the NW–SE-trending Ottawa–Bonnechère and Saguenay River grabens, both interpreted as Iapetan failed arms. Coastal sections of the St. Lawrence River that expose fault rocks related to the St. Lawrence rift system have been studied between Québec city and the Saguenay River. Brittle faults marking the St. Lawrence rift system consist of NE- and NW-trending structures that show mutual crosscutting relationships. Fault rocks consist of fault breccias, cataclasites and pseudotachylytes. Field relationships suggest that the various types of fault rocks are associated with the same tectonic event. High-resolution marine seismic reflection data acquired in the St. Lawrence River estuary, between Rimouski, the Saguenay River and Forestville, identify submarine topographic relief attributed to the St. Lawrence rift system. Northeast-trending seismic reflection profiles show a basement geometry that agrees with onshore structural features. Northwest-trending seismic profiles suggest that normal faults fringing the St. Lawrence River are associated with a major topographic depression in the estuary, the Laurentian Channel trough, with up to 700 m of basement relief. A two-way travel-time to bedrock map, based on seismic data from the St. Lawrence estuary, and comparison with the onshore rift segment suggest that the Laurentian Channel trough varies from a half-graben to a graben structure from SW to NE. It is speculated that natural gas occurrences within both the onshore and offshore sequences of unconsolidated Quaternary deposits are possibly related to degassing processes of basement rocks, and that hydrocarbons were drained upward by the rift faults.

Siliceous sedimentary record of the last 280 kyr in the Canary basin (NW Africa)

Siliceous microfossils were quantified in four sedimentary sequences off Northwest Africa in order to disclose the productivity story of the last 280 kyr. Three sediment cores retrieved along an E–W profile, at about 29°N (GeoB4240, GeoB4241 and GeoB4242), and one core (GeoB4216) from the highly productive area off Cape Ghir in the Canary Island region. At 29°N, along all but the most offshore sequences (GeoB4242), coincidence of high diatom accumulation rates (DAR) and high abundances of Chaetoceros spores, the upwelling-related species, indicate glacial stages (isotope stages (IS) 4 and 2) as more productive than the interglacial intervals (IS5 and Holocene). The DAR in the coastal area of the Canary basin during IS2 is of the same order of magnitude (10 8 valves cm −2 kyr −1) reported for Cape Blanc and one order of magnitude higher than reported for the Iberian margin. The deviation observed between the offshore (higher DAR at IS3 and 6) and the coastal diatom records (higher DAR at IS2 and 4) may be a reflection of local productivity generated by wind input of nutrients and advection from coastal sources. The occurrence of DAR spikes at the timing of some Heinrich events seems to support the hypothesis of productivity maxima associated with the presence of meltwater off NW Africa, as previously proposed by other authors.

Biodiversity during the Deccan volcanic eruptive episode

This paper gives a detailed overview of biotic assemblages recovered from the Deccan trap intercalated sedimentary sequences (infra- and intertrappean beds) of peninsular India as a result of extensive research done during the last 20 years. The infra- and intertrappean beds contain remnants of Gondwanan forms such as myobatrachinae frogs, pelomedusid turtles, dinosaurs (i.e. titanosaurids and abelisaurids), and mammals. Apart from these Gondwanan elements, the infra- and intertrappean beds also contain forms of Laurasian affinity though recently doubt has been cast on such relationships. Based on previous fossil records, Laurasiatic forms were considered to be represented by a great variety of micro- and megavertebrate assemblages such as discoglossid and pelobatid frogs, anguid lizards, alligatorid crocodiles, palaeoryctid mammals, charophytes and ostracodes. The biotic assemblages show a remarkable similarity between the infra- and intertrappean beds indicating a short time period for the deposition of these Deccan volcano-sedimentary beds. The recovered biotic assemblages strongly indicate a Maastrichtian age for the initiation of Deccan volcanic activity and the sedimentary beds associated with it. The Cretaceous/Tertiary boundary as such remains to be defined in any known sections in sedimentary sequences in so far investigated localities of peninsular India. What have been identified are Maastrichtian age beds in the east-central and western Narmada river region on the basis of pollens, vertebrate assemblage and planktonic foraminiferas in infratrappean offshore sequences. A Palaeocene intertrappean bed at Lalitpur (Uttar Pradesh) that is among those lacking dinosaurian remains but having palynological assemblages similar to those from well established Palaeocene sequences, suggests the presence of Palaeocene intertrappeans, but the K/T boundary is yet to be properly defined.

Dinoflagellates from the Miocene Rudeis and Kareem formations borehole GS-78-1, Gulf of Suez, Egypt

The Miocene Rudeis and the Kareem formations encountered in the Neogene part of the GS-78-1 borehole, Gulf of Suez produced diverse assemblages of dinoflagellate cysts, spores and pollen. The Early Miocene (Burdigalian) age assigned to the Rudeis Formation and the Early-?Middle Miocene (Langhian–Serravallian) age postulated for the Kareem formation is based on the presence of dinoflagellate cysts. These offer a good basis for biostratigraphic correlation of the Miocene deposits in the Gulf of Suez with those in the Nile Delta and Sinai in Egypt, and also with those present in key sections from the Mediterranean, the Canadian offshore sequences, Northwest Europe and from the North Atlantic. The terrestrial palynoflora (spores and pollen) affords no really precise, independent testimony as to the age of the samples, apart from being generally indicative of a Neogene age, in accord with the established gross age of the sediments, derived mainly from planktonic forams, calcareous nannoplankton and dinoflagellate cysts. The Rudeis Formation was deposited in a relatively deep water environment, based on the abundance fluctuations in miospores and dinoflagellates. However, the miospores recovered from the Rudeis Formation give an equivocal signal with respect to depositional environment. Such observed incursions of terrestrial elements in the Rudeis Formation could indicate that they might have been carried about within the basin of deposition by the waters of the Mediterranean Sea, or that they were displaced into a deep water setting. The overlying Kareem Formation was identified as an outer continental shelf deposit to upper bathyal (distal) environment because it contains a higher percentage of marine dinoflagellate cysts in most investigated samples, except in its uppermost part which shows the lowest percentage of marine forms. These include Spiniferites ramosus, S. pseudofurcatus, Operculodinium centrocarpum, Polysphaeridium zoharyi, Systematophora placacantha and Lingulodinium machaerophorum. The consistent presence of P. zoharyi in the Kareem Formation indicates that the Gulf of Suez was at times in the tropical to subtropical belt during the Early-?Middle Miocene age.

Sedimentology, C-S-Fe relationships and stable isotopic compositions in Devonian black mudrocks, Mackenzie Mountains, Northwest Territories, Canada

An integrated approach combining C-S-Fe relationships, stable isotopic compositions, and lithofacies characterization was utilized to constrain the palaeoenvironmental and early diagenetic conditions of Middle-Upper Devonian (Eifelian-Frasnian) mudrocks from the Mackenzie Mountains, Northwest Territories, Canada. These rocks include the Hare Indian Formation (informally subdivided into the lower Bluefish Member and the Hare Indian Upper Member), Carcajou Marker and Canol Formation. The Bluefish Member is dominated by black, laminated, organic-rich shales (TOC = 0.35–10.34 wt.%; av. 5.83 wt.) with moderate degrees of pyritization (DOP) of 0.34–0.67 (av. 0.55). These mudrocks were deposited in dysoxic marine bottom-waters that became progressively more oxygenated with time. Variations in TOC, DOP and organic matter δ 13CPDB values (−29.7% to −19.9%; av. −27.2%) are attributed to repeated clastic dilution and increased input of terrestrial organic matter in association with shallowing-upward ramp-clinothem cycles. Pyrite δ 34SCDT values (−32.7% to −18.8%; av. −24.9%) indicate an open system, bacteriogenic seawater-sulphate reduction. Conversely, the overlying Hare Indian Upper Member, characterized by clinothem facies, is composed of grey to green mudstone with minor argillaceous limestones and considerably less organic matter contents (TOC = 0.28–2.99 wt.%; δ 13C = −29.5% to −22.5%). Deposition occurred in oxic to slightly dysoxic waters (DOP = 0.20–0.54; δ 34S = −23.0% to −20.9%), depending on the palaeotopographic location along the depositional slope.A rapid rise in sea level drowned the carbonate 'ramp’ member of the Ramparts Formation and produced the thin, organic-rich Carcajou Marker. Bottom-water stagnation that resulted from subdued ramp palaeotopography produced anoxic sea bottom. Black, laminated, organic-rich shales from the Canol Formation (TOC = 1.37–6.68 wt.%) are very similar to those of the Bluefish Member, and are likewise basinal sediments. However, TOC, DOP and organic-matter δ 13CPDB values (−29.1% to −20.8%; av. −26.2%) do not show pronounced variations and indicate that low-energy, quietwater conditions persisted over relatively long, uninterrupted periods of time. High DOP values (0.72–0.93) throughout the Canol Formation suggest that deposition occurred in anoxic bottom-waters, but as basin-fill conditions continued there was a shift to a dysoxic environment (DOP = 0.55–0.65), which grades into nearshore and offshore sequences of the overlying Imperial Formation. In contrast with the Hare Indian Formation, much heavier δ 34SCDT values of pyrite in Canol mudrocks (−11.1% to +5.3%; av. −3.1%) point to bacterial sulphate reduction in a closed to semi-closed system with respect to seawater sulphate.

Correlation of Miocene North Sea sequences with the Danish land area based on foraminifera

In the present study comparisons are made between the Miocene North Sea sequences and the Danish onshore formations based on foraminiferal biostratigraphy. Foraminiferal faunas of four onshore borings previously analyzed by Kristoffersen have been reviewed. Biozones NSB 9 - NSB 13 of the offshore sequences were all found in the Danish Miocene formations, and the obtained results promise well for a future more detailed correlation between the North Sea and the Danish land area.

Origin and Diagenesis of Clay Minerals in the Monterey Formation, Santa Maria Basin Area, California

AbstractThe clay mineralogy of the Miocene Monterey Formation was determined for onshore and offshore sequences in the Santa Maria basin area, California. The <0.1-μm fraction of clayey, opal-CT porcelanites, siliceous mudstones, and dolostones from the Pt. Pedernales area consists of mixed-layered illite/smectite (I/S) that contains < 10% illite layers. The underlying Tranquillon Volcanics and Obispo Tuff, the presence of bentonite beds, zeolite minerals, and unaltered volcanic ash beds in the Monterey Formation, and the highly smectitic composition of the I/S suggest that a significant amount of the I/S formed from the alteration of vitreous volcanic ash. The alteration of volcanic glass to smectite coincides with the transformation of biogenic opal-A (mostly diatoms) to opal-CT. Alteration of volcanic glass to smectite may inhibit the opal-A to opal-CT transformation by removing pore-water Mg, promote dolomite precipitation by raising the pH, alter the Sr isotopic composition of the pore waters, and provide an additional source of silica to these predominantly biogenic siliceous rocks. The 0.1- to 2-μm fraction contains mica minerals (discrete illite, biotite, and muscovite) as well as I/S. Clayey, opal-A diatomites from the overlying Sisquoc Formation contain detrital kaolinite and random (R = 0) I/S, but the percent illite layers in the I/S is uncertain.The <0.1-μn fraction of quartz rocks from the Lions Head area and the offshore B-2 well is far more variable, and contains random or ordered (R = 1) I/S, kaolinite, and chlorite. Corrensite was observed at the base of the Lions Head area. The percent illite layers in the I/S increases from 10% to 80% over a stratigraphic depth of 0.8 km that corresponds to a present-day temperature range of 80 to 115°C. Initial illitization of smectite coincides with the opal-CT to quartz transformation. Ordered (R = 1) I/S is associated with abundant diagenetic kaolinite and dolomite in several metabentonite beds. The dominance of sodic plagioclase over K-feldspar, and the absence of kaolinite and chlorite in rocks of equivalent age that have not undergone illitization suggest that limited availability of K results in the alteration of smectite to kaolinite, chlorite, and possibly late dolomite. Minor amounts of clinoptilolite were found in opal-A and opal-CT rocks. Minor to trace amounts of analcime and mordenite were tentatively identified, primarily in quartz rocks.

Temporal and Spatial Patterns of Cenozoic and Late Mesozoic Erosion and Deposition along the Western Margin of Southern Africa: ABSTRACT

Compared with subsidence history and eustatic sea level change, sediment supply has been a neglected component of studies of passive margin stratigraphy. The spatial and temporal pattern of sediment supply to continental margin, however, is a critical factor in determining the architecture of offshore sedimentary sequences. Sediment routing across passive margins is controlled primarily by their tectonic development and the consequent morphological evolution of the subaerial part of the margin. By combining offshore sediment volume and sedimentation rate data based on isopach maps and borehole records with apatite fission-track analysis and denudational modeling onland, the depositional history of the western margin of southern Africa has been related to its geomorphic response to continental rifting. The sediment volume data indicate a declining rate of sedimentation after rifting in the Early Cretaceous despite a probable enlargement of the sediment source area through time. Similarly, apatite fission-track ages and confined track length distributions indicate an Early Cretaceous episode of relatively high erosion rates which affected areas both inland and oceanward of the major topographic discontinuity along the margin represented by the Great Escarpment. Late Cenozoic rates of erosion and sediment supply have been low, although much of the sediment source area is stillmore » at a significant elevation. Although aridity may have contributed to this reduction in sediment supply, the morphological response to the tectonic evolution of the margin has also been crucial.« less

Palynology, palaeoecology, and dinocyst stratigraphy of the Oligocene throuh Pliocene succession in the Qantara-1 well, Eastern Nile Delta, Egypt

Distinctive dinoflagellate cyst assemblages here assigned to the middle-late Oligocene, early, middle and late Miocene and probable early Pliocene are described from sixty-nine cuttings samples recovered from the Tertiary succession penetrated by the Qantara-1 well, drilled at the eastern border of the Nile Delta, Egypt. The identified dinoflagellate cysts offer good basis for biostratigraphic correlation of the Tertiary sediments in Egypt, with those in the Mediterranean, North Pacific and the Canadian offshore sequences. The quantitative distribution of dinocysts, spores and pollen was used to decipher the palaeoenvironmental conditions. Two regressive phases were recorded during the “middle”-late Oligocene, early, late Miocene, and early Pliocene. These were rollowed by twansgressions, a major one occurred in the early-middle Miocene and a brief one was recorded during the middle Miocene.

Late Quaternary stratigraphy of the northern Rockall trough and Faeroe‐Shetland channel, northeast Atlantic Ocean

AbstractA late Quaternary deep‐water stratigraphic framework has been established for the deep‐water areas (>450m) of the northern Rockall Trough and Faeroe‐Shetland Channel. Four stratigraphic units (1–4) are identified; these are primarily biostratigraphic units based on dinoflagellate cyst evidence. Unit 1 represents the late Weichselian glacial (pre‐13 000 yr BP); unit 2 the Late Glacial Interstadial (11 000‐13 000 yr BP); unit 3 is of Younger Dryas age (10 000‐11 000 yr BP); and unit 4 represents the Holocene interglacial (post‐10 000 yr BP). This stratigraphy is supported by the discovery of the mixed Vedde Ash (10 600 yr BP) and North Atlantic Ash zone 1, and the Saksunarvatn Ash (9000–9100 yr BP), concentrated in units 3 and 4 respectively. The sedimentology indicates that the oceanographic regime underwent a major change between the glacial and interglacial stages. This is marked by the onset of strong bottom current activity, allied to the restoration of overflow of the Norwegian Sea Deep Water into the North Atlantic, towards the end of the Younger Dryas Stadial. Despite intense bioturbation and bottom‐current reworking the basic stratigraphic framework is maintained. Recognition of two volcanic ash markers enables correlation with established onshore and offshore sequences of marine and non‐marine environments.

Geohistory Analysis of Neogene Point Arena Basin, California: Implications for Its Tectonic Evolution: ABSTRACT

The Neogene Point Arena basin of northwestern California is located west of the San Andreas fault system and south of the Mendocino triple junction. Key units of the Point Arena sequence are exposed from Iversen Landing north to Point Arena, California, including the Oligocene-Miocene Iversen Basalt (23.8 Ma), the lower Miocene Skooner Gulch and Gallaway Formations, and the lower to mid-Miocene Point Arena Formation. Lithologic and thickness data, together with evidence of age and paleobathymetry from both onshore and offshore sequences in the Point Arena basin, were used in a geohistory analysis of basin development. The resulting geohistory diagram tracks depths of specific stratigraphic zones, variations in paleobathymetry, and patterns of subsidence and uplift during late Paleogene through Neogene time. Geohistory analysis indicates that the late Paleogene margin was uplifted during approach of the Pacific-Farallon spreading ridge. Subsequently, a pulse of volcanism during latest Oligocene-Miocene signaled initial Neogene subsidence of the margin as marked by the Iversen Basalt. Subsidence likely involved both initial thermal subsidence as well as later transtensional deformation during the passage of the Mendocino triple junction and initiation of the San Andreas fault system. Rapid initial subsidence was accompanied by deposition of turbidites (Skooner Gulch and Gallawaymore » Formation). The highly organic shales and petroliferous sands of the overlying Point Arena Formation indicate an abrupt cessation of turbidite deposition and a slower rate of basin subsidence during the middle Miocene. Episodes of warping from mid-Miocene through Holocene can be attributed to crustal flexing associated with wrench tectonism, with a major event bringing the Point Arena sequence above sea level during the late Pliocene-Pleistocene.« less

Characteristic trace fossils in nearshore to offshore sequences, Upper Cretaceous of east-central Utah

More than 20 trace fossil species occur in marine facies of the Cretaceous Star Point and Blackhawk formations in the Book Cliffs and Wasatch Plateau provinces of Utah. Major genera include Ancorichnus, Arenicolites, Aulichnites, Chondrites, Conichnus, Cylindrichnus, Medousichnus, Ophiomorpha, Palaeophycus, Planolites, Rosselia, Schaubcylindrichnus, Scolicia, Skolithos, Teichichnus, Teredolites, and Thalassinoides. Newly named taxa include Ancorichnus capronus, Medousichnus loculatus, and Rosselia chonoides.Most trace fossils occur in characteristic, albeit intergradational ichnofacies correlative with major lithofacies of regressive nearshore to offshore sequences. The latter include foreshore, foreshore–shoreface transition, shoreface, and offshore facies. Landward facies are typified by clean, well sorted, well stratified, sparsely burrowed sandstones. Seaward facies, except where interrupted by hummocky bedded sandstones, exhibit successively less pure, less well sorted and stratified, more intensely bioturbated, finer grained sandstones, siltstones, and mudstones.Characteristic ichnofacies and lithofacies in the Cretaceous of east-central Utah should provide potentially useful models for reconstruction of nearshore to offshore sequences elsewhere, especially in the Western Interior Region of North America.