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Rameil N.,Ruhr University Bochum | Rameil N.,Neftex Petroleum Consultants Ltd | Immenhauser A.,Ruhr University Bochum | Csoma A.E.,ConocoPhillips | Warrlich G.,Petroleum Development Oman
Sedimentology | Year: 2012

Discontinuity surfaces in shallow-marine carbonate successions may represent significant time gaps in the geological record of ancient epeiric-neritic seas. Understanding the hidden geological information contained in major discontinuities is thus of key significance in palaeo-environmental analysis, sequence stratigraphy, reconstructions of sea-level change and basin evolution. In the present paper, the Aptian top Lower Shu'aiba Formation discontinuity in the Sultanate of Oman is taken as a prominent example of a regionally extensive (>100000km 2) surface with a long (up to 10Myr) and complex geological history. The top Shu'aiba discontinuity formed on the topographically elevated domain of the Oman platform and represents in essence the Late Aptian time interval. Coeval carbonates in the intrashelf Bab Basin and oceanic rim indicate forced regression and sequence-wise, gradual down-stepping. Available regional, sedimentological, sequence-stratigraphic, petrographic, palaeontological and geochemical evidence from outcrops and cored wells in Oman is summarized, in part complemented by new data, and reviewed in a process-oriented context. In the field, the discontinuity is expressed as a low relief, stained surface with evidence for a marine hardground stage being dominant. Indistinct features that indicate a transient meteoric precursor stage (isotope shifts, meteoric cements, circumgranular cracks, etc.) are present but their interpretation requires careful and detailed work. This feature is remarkable, as a series of relative sea-level falls with amplitudes of up to several tens of metres from the Early to Late Aptian boundary to the end of the Aptian are reported from the Middle East and elsewhere. Despite the palaeogeographic position of the study area in the tropical climate zone, evidence of deep-cutting karst features, characteristic for many long-term exposure surfaces worldwide is scarce. Acknowledging the fact that the modern world offers no genuine analogues for the Lower Aptian carbonate system in Oman, morphological similarities between actualistic, wave-eroded coastal terraces and the top Shu'aiba discontinuity are discussed critically. This discussion may imply that, during an exposure time of several million years, the top Shu'aiba discontinuity experienced repeated stages of shallow flooding and emergence, with each transgression removing portions of the underlying rock record. The data shown here exemplify the complexity of hiatal surfaces in epeiric-neritic carbonates and may serve as a case example for other major discontinuities. © 2011 The Authors. Journal compilation © 2011 International Association of Sedimentologists.

Marshall C.,University of Birmingham | Marshall C.,VNG Norge AS. Postboks 720 Sentrum | Thomas A.T.,University of Birmingham | Boomer I.,University of Birmingham | Ray D.C.,Neftex Petroleum Consultants Ltd
Bulletin of Geosciences | Year: 2012

High resolution δ 13C calb data are presented for two composite sections in England covering much of the Homerian Stage. Micrite samples collected at ~0.5 m intervals from outcrop and core in the Dudley area, West Midlands, span the upper-most Coalbrookdale, Much Wenlock Limestone and basal Lower Elton formations. Deposition there occurred in a mid-shelf setting. A similar suite of samples from the Wenlock type area, Wenlock Edge, Shropshire, represents a coeval sequence deposited closer to the shelf-basin margin. The successions concerned extend from the upper lundgreni to nilssoni graptolite biozones and provide a detailed record of variation in stable carbon isotope ratios across the well-known double-peaked Homerian positive excursion (Mulde Excursion), a time of significant global biological and chemical perturbation. In the West Midlands, this excursion occurs in the Much Wenlock Limestone Formation. The lower peak (Lower Quarried Limestone Member) has δ 13C calb values rising to +5.5%o VPDB. Values fall to +0.8%c VPDB higher in the section before rising again to +4.1%o VPDB (Nodular Beds Member). Analysis of lithofacies varia-tion in this interval indicates two transgressive-regressive cycles, the two positive peaks of the excursion correlating with relative sea-level lows and the intervening dip with a relative sea-level high, the local expression of Johnson's (2006) Highstand 5A. The double-peaked nature of the excursion at Dudley resembles that previously recorded for the area; however, our δ 13C carb values are consistently 2%c higher, and accord more closely with values published for sec-tions elsewhere. The lower of the two peaks found in the West Midlands cannot be identified on Wenlock Edge, where 813Ccarb values fluctuate somewhat around +2%c VPDB. The upper peak, though less distinct, can be identified on Wenlock Edge with values rising to +3.8%c VPDB. Correlations based on biostratigraphy, sequence stratigraphy and bentonite geochemistry suggest that not all changes in δ 13C calb occurred synchronously in the two areas studied, de-spite their close proximity.

Rameil N.,Ruhr University Bochum | Rameil N.,Neftex Petroleum Consultants Ltd | Immenhauser A.,Ruhr University Bochum | Warrlich G.,Petroleum Development Oman | And 3 more authors.
Sedimentology | Year: 2010

Lithocodium aggregatum and Bacinella irregularis are now extinct, shallow marine life forms of unknown taxonomic origin. Forming part of the tropical platform biota during much of the Mesozoic, these organisms experienced bloom periods and temporarily replaced rudist-coral assemblages during parts of the Early Aptian. Within the limitations of time resolution, this 'out-of-balance' facies is coeval with the Oceanic Anoxic Event 1a-related black shale deposition in oceanic basins but the triggering factors remain poorly understood. Here, a platform-wide comparison of Lithocodium-Bacinella geobodies and morphotypes from the Sultanate of Oman is presented and placed in its environmental, bathymetric and physiographic context. Lithocodium-Bacinella geobodies reach from kilometre-scale 'superstructures' to delicate centimetre-sized growth forms. Clearly, scale matters and care must be taken when drawing conclusions based on spatially limited observational data. Whilst the factors that cause Lithocodium-Bacinella expansion should probably be considered in a global context, regional to local factors affected growth patterns in a more predictable manner. Here, the unresolved taxonomic relationship remains the main obstacle in any attempt to unravel the response of Lithocodium-Bacinella to specific or interlinked environmental parameters as different organisms respond differently to changing environment. Acknowledging these limitations, the following tentative patterns are observed: (i) Lithocodium-Bacinella tolerated a wide range of hydrodynamic levels and responded to differences in energy level or physiographic settings (margin, intrashelf basin, inner platform) by obtaining characteristic growth forms. (ii) Lithocodium-Bacinella favoured low-sediment input but had the ability to react to higher sedimentation rates by enhanced upward growth; a feature perhaps pointing to a phototrophic metabolism. Circumstantial evidence for continuous growth within the upper-sediment column is debated. (iii) The availability of accommodation space had a direct influence on the maximum size of geobodies formed. (iv) Fluctuating nutrient levels and sea water alkalinity may have affected the growth potential of Lithocodium-Bacinella. Understanding the relationship between Lithocodium-Bacinella morphogenesis on a wide range of scales and local environmental parameters allows for better prediction of the spatial distribution of reservoir properties and also results in an improved interpretation of palaeoenvironments. This study might represent a useful first step in this direction. © 2010 The Authors. Journal compilation © 2010 International Association of Sedimentologists.

Hampson G.J.,Imperial College London | Jewell T.O.,Imperial College London | Jewell T.O.,Neftex Petroleum Consultants Ltd | Irfan N.,Imperial College London | And 3 more authors.
Journal of Sedimentary Research | Year: 2013

Concept-driven sequence stratigraphic models of alluvial-to-coastal-plain successions suggest that fluvial architecture and style should transition from isolated, single-story, channelized sandbodies deposited by single-thread (typically meandering) rivers in mudstone-prone, high-accommodation intervals into densely stacked, amalgamated sandbodies deposited by multiple-thread (typically braided) rivers in low-accommodation intervals. Model predictions of changing fluvial style are tested by comparing the facies character, internal architecture, dimensions, and formative paleohydraulic conditions of representative, major fluvial sandbodies developed at different stratigraphic levels of an alluvial-to-coastal-plain succession developed under a progressively decreasing rate of accommodation creation (Late Cretaceous Blackhawk Formation, Wasatch Plateau, central Utah, USA). The major fluvial sandbodies have a similar facies composition, and consist mainly of cross-bedded, medium-grained sandstone with subordinate mudclast conglomerate, folded sandstone, and inclined sandstones and intercalated siltstones. Facies are arranged within a hierarchy of architectural components. Channel stories (mean width and depth of 90 m and 7.2 m, respectively) represent the migration of a paleochannel segment and adjacent bar, and they are amalgamated laterally into channel belts (mean width and depth of 400 m and 9.2 m, respectively) that in turn are stacked vertically into channel-belt complexes (mean width and depth of 400 m and 20 m, respectively). Channel stories and belts were deposited by a combination of three paleochannel types that occur together in most major sandbodies: (1) single-thread "cut-and-fill" channels; (2) singlethread, laterally accreting channels of low-to-moderate sinuosity; and (3) multiple-thread, wandering-to-braided channels. Estimated paleochannel slopes are uniformly low (< 0.04°), most sandbodies contain sparse, potential marine indicators (e.g., Teredolites-bored logs and tidally modulated? carbonaceous drapes along cross-bed foresets), and paleodischarge estimates imply that multiple-thread channels may have narrowed and branched downstream to form distributary networks. All of these features are consistent with a delta-plain setting. Channel story and belt stacking patterns within each major sandbody (channel-belt complex) are highly non-uniform, such that (1) there are no systematic trends shared by the sandbodies, (2) sandbodies do not result from systematic, short-term changes in accommodation, such as those associated with the incision and fill of coastal incised valleys, and (3) variability within each sandbody is more pronounced than variability between sandbodies. These results suggest that local variations in sediment flux and transport capacity, combined with local avulsion history, were the principal controls on the architecture and dimensions of the major sandbodies. The similarities in architecture between major fluvial sandbodies imply that these controls were not predominantly governed by proximity to the coeval shoreline (c. 40-100 km) or by long-term tectonic subsidence rate (c. 80-700 m/Myr), which controlled creation of accommodation. Copyright © 2013, SEPM (Society for Sedimentary Geology).

Ray D.C.,University of Portsmouth | Ray D.C.,Neftex Petroleum Consultants Ltd. | Collings A.V.J.,Arup | Worton G.J.,Dudley Museum and Art Gallery | Jones G.,248 The Broadway
Geological Magazine | Year: 2011

The upper Wenlock Series (Homerian Stage) of the northern Midland Platform, England, contains numerous volcanic bentonite clay layers.AtWren'sNestHill, Dudley, 15 bentonites have been investigated and comparisons with the type-Wenlock have been made by means of two key sections alongWenlock Edge, Shropshire. In total 22 bentonites have been investigated and their clay and sandgrade mineralogies determined. Rare earth element (REE) and yttrium concentrations of apatite grains contained within ten of the bentonites have been established allowing geochemical fingerprinting as an indication of provenance of source magmas and identification of geochemical marker beds. Based on the analysis of REE and yttrium concentrations it seems likely that the majority of these bentonites originated from a granodiorite magmatic source. Comparisons with published Llandovery and lower Wenlock age bentonites indicate generally more enrichment in light REEs relative to heavy REEs. In addition, close geochemical similarities between bentonites along Wenlock Edge and at Wren's Nest Hill strongly argue for their presence as precise stratigraphic equivalents within the upper Much Wenlock Limestone Formation. These correlations are further supported by geophysical data from borehole wire-line logs across the West Midlands. Finally, a chemically distinct mid-Homerian episode of volcanism is identified and represents a potentially important marker interval between the study area and other similarly aged bentonites reported from the Island of Gotland, Sweden. © 2010 Cambridge University Press.

Huck S.,Ruhr University Bochum | Heimhofer U.,Ruhr University Bochum | Rameil N.,Neftex Petroleum Consultants Ltd | Bodin S.,Ruhr University Bochum | Immenhauser A.,Ruhr University Bochum
Earth and Planetary Science Letters | Year: 2011

A widely accepted hypothesis proposes that the Early Aptian demise of carbonate platforms in the northern Tethyan realm reflects the impact of environmental changes that eventually led to the deposition of organic-rich basinal sediments during oceanic anoxic event (OAE) 1a. In fact, the temporal correlation of oceanic anoxia and platform drowning has not been rigorously tested and evidence indicating causality is debated. The present paper provides a high-resolution carbon- and strontium-isotope chronostratigraphy combined with detailed sedimentological analysis applied to the Barremian-Aptian Urgonian carbonate platform development at the northern rim of the Tethys (Subalpine Chains, Haute-Savoie, ESE France). A characteristic Barremian-Aptian carbon- and strontium-isotope pattern below the onset of OAE 1a interval permits precise platform-to-basin correlation with the Barremian stratotype locality of Angles (SE France). This chemostratigraphic pattern equally allows for a correlation with well-studied shoal-water and pelagic records of the northern and central Tethys. Strontium-isotope stratigraphy provides a high-resolution numerical age-data set, indicating an earliest Aptian age (<124.5 ± 0.4. Ma) of platform demise in the Subalpine Chains. The detailed platform-to-basin correlation obtained in France clearly illustrates that shoal-water carbonate production in the Urgonian platform ceased about 300. kyr before the most negative values of the characteristic negative carbon-isotope anomaly, which marks the beginning of OAE 1a black-shale deposition. The stratigraphic results confirm (i) the theory of an Early Aptian northern Tethyan platform drowning episode predating OAE 1a and (ii) document the temporal response of shoal-water carbonate platforms to the environmental perturbations prior to the OAE acme. © 2011 Elsevier B.V.

van Buchem F.S.P.,Maersk Oil | Simmons M.D.,Neftex Petroleum Consultants Ltd | Droste H.J.,Royal Dutch Shell | Davies R.B.,Neftex Petroleum Consultants Ltd
Petroleum Geoscience | Year: 2011

This stratigraphic article summarizes a number of observations on the mid-Cretaceous stratigraphy of Arabia made during the 2nd Arabian Plate Geology Workshop held in Abu Dhabi in January 2010. These observations pertain to the lithostratigraphic nomenclature applied on the eastern Arabian Plate and its relationship to mid-Cretaceous (Late Aptian- Turonian) depositional systems and sequences recognized plate-wide. It appears that several of the commonly applied lithostratigraphic terms are diachronous as a result of (i) plate-wide migration of carbonate and siliciclastic facies belts; (ii) the occurrence of intra-shelf basins at different times and locations; (iii) differential preservation beneath erosional unconformities; and (iv) simple differences in usage across political boundaries. A plate-wide sequence stratigraphic scheme is thus a powerful tool for regional correlation and mapping, and for reconciling lithostratigraphic differences. The studied interval can be subdivided into two primary sedimentary systems that differ in lithology, depositional geometries and dominant faunal assemblages. First, latest Aptian- and Albian-aged systems, which are characterized by volumetrically significant Arabian Shield-derived siliciclastics (both sandstones and claystones) that alternate with carbonate beds. The carbonate beds thicken up-section and have generally very low-angle, muddy ramp depositional geometries. Organic-rich basinal facies are found only in the Kazhdumi Basin in SW Iran at this time. The faunal composition of the carbonates is dominated by benthic foraminifera (notably orbitolinids). Within this succession three third-order depositional sequences are distinguished (MFS K90, K100 and K110). Secondly, Cenomanian-early Turonian-aged sedimentary systems are characterized by a marked reduction in siliciclastic influx, the development of carbonate platform to intra-shelf basin topography and deposition of basinal source rocks. Rudists are the dominant component of grainy, high-energy platform margin/barrier facies, and are also present in the platform top sediments. Within this succession three Cenomanian sequences can be correlated clearly at the scale of the plate (MFS K120, K130 and K140), whereas the number of sequences in the Turonian interval is less well known owing to significant local erosion and/or non-deposition during a period of tectonic instability. Applying this robust sequence stratigraphic framework in combination with a synthesis of biostratigraphic age calibration demonstrates the diachronous character of the Mauddud, Safaniya, Ahmadi and Mishrif formations and equivalents from the southern to the northern part of the Arabian Plate. © 2011 EAGE/Geological Society of London.

Ray D.C.,Neftex Petroleum Consultants Ltd. | Butcher A.,University of Portsmouth
Bollettino della Societa Paleontologica Italiana | Year: 2010

The type Wenlock Series is established around the village of Much Wenlock, Shropshire, England, and consists of the Buildwas, Coalbrookdale and Much Wenlock Limestone formations. Based upon new data and a re-evaluation of lithological and palaeontological data from the Lower Hill Farm Borehole and outcrops, an assessment of relative sea-level change has been made. Broadly, the type Wenlock consists of two shallow water carbonates separated by terrigenous sediments. Within this framework two major depositional sequences have been recognised, along with six higher-order sequences. These transgressive- regressive sequences have been calibrated against graptolite zonation schemes and are in general agreement with previously published Wenlock eustatic sea-level curves.

Davies A.,UK National Oceanography Center | Davies A.,Neftex Petroleum Consultants Ltd. | Kemp A.E.S.,UK National Oceanography Center | Palike H.,UK National Oceanography Center
Geophysical Research Letters | Year: 2011

The first annually resolved sedimentary record from the Cretaceous is used to develop time series of inter-annual and decadal scale climate variability from the Arctic Ocean. Analysis of records spanning 1000 years reveals strong periodicities in the quasi-biennial oscillation and El Niño - Southern Oscillation (ENSO) band as well as a 14 year period, which all closely match periodicities typical of modern high latitude climate variability. This supports the view that an Arctic Ocean free of permanent sea ice would be driven by similar forcing to the present state, implicating tropical ocean atmosphere interaction and demonstrating that stratosphere-troposphere coupling likely played a prominent role in the transmission of Cretaceous equatorial climate forcing to polar latitudes as has recently been established for the modern earth system. On the other hand, the prominent ENSO periodicities in our records argue against the hypothesized link between past warm climates and "permanent El Niño" states. Copyright © 2011 by the American Geophysical Union.

Arnaud E.,University of Guelph | Etienne J.L.,Neftex Petroleum Consultants Ltd
Geological Society Memoir | Year: 2011

This chapter provides an overview and key references of glacial processes and resulting sedimentary products in subglacial, terrestrial proglacial and glaciomarine or glaciolacustrine settings. These settings are characterized by a wide variety of processes ranging from subglacial lodgement and deformation, ice-push and sediment remobilization, which in turn result in a wide range of products such as diamictite, conglomerate, sandstone, siltstone and mudstone. The sedimentary record of proglacial settings exhibits the most lateral and vertical variability due to the dynamic nature of ice margins and the most direct record of climatic fluctuations. Many Neoproterozoic successions, however, preserve glaciomarine deposits that can provide a more continuous and high-resolution (though indirect) record of change. This chapter will enable the reader to identify features that may be used to infer a glacial influence on the formation of ancient deposits. The chapter also outlines some of the important issues that require consideration when evaluating palaeoclimatic models for Neoproterozoic sedimentary successions. These include the equivocal significance of most commonly used proxies such as occurrence of diamictite, outsized clasts in laminated sediments, clast characteristics, lithostratigraphic trends and sequence boundaries. Careful analysis of multiple lines of sedimentary evidence, together with other proxies of climatic changes, can yield meaningful reconstructions and provide a basis for testing palaeoclimate models for this time period. A summary table outlining the characteristics of diamictite with different depositional origins is also included in order to assist with the interpretation of the Neoproterozoic sedimentary record. © The Geological Society of London 2011.

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