Trevor Elliott

Megaflute erosion surfaces and the initiation of turbidite channels

Distinctive, laterally widespread surfaces, here termed megaflute erosion surfaces, provide evidence of widespread erosion and sediment bypass in a turbidite system and offer insights into the mechanisms by which small-scale turbidite channels can be initiated. The surfaces are erosional into sandstone, are ornamented by exceptionally large flutes, and are overlain by fine-grained facies that onlap the relief of the surface. These surfaces can be traced laterally into deeper erosional surfaces that define sandstone-filled turbidite channels. Once formed, the channels provided a pathway for subsequent turbidity currents while the flanking, slightly elevated megaflute surfaces accumulated fine-grained sediment via overbank and flow stripping processes. These observations imply that the channels were formed by single, high-magnitude, low-frequency turbidity currents and were filled by a succession of lower magnitude, higher frequency flows.

The application of sequence stratigraphy to Upper Carboniferous fluvio-deltaic strata of the onshore UK and Ireland: implications for the southern North Sea

Stratigraphical correlations and fades interpretations of Upper Carboniferous fluvio-deltaic strata have been based traditionally on cyclothems bound by marine flooding surfaces (marine bands). The recent recognition of major, regionally extensive erosional unconformities (Exxon-style sequence boundaries) within selected cyclothems questions their validity as units of genetically related strata. U sing examples from the Carboniferous of the onshore UK and Ireland, we present sedimentological criteria for the recognition of sequence boundaries, placing particular emphasis on the regional context of these surfaces. Sequence boundaries comprise widespread, deeply eroded surfaces at the base of major fluvial sandstone complexes, and laterally equivalent palaeosols developed on interfluves at the margins of the fluvial complexes. These sequence boundaries define units of genetically related strata (sequences) which contain other key surfaces of time-stratigraphic significance, including marine bands and regionally extensive coals. The recognition of key surfaces enables the construction of a high resolution stratigraphic framework within which coeval facies relationships can be interpreted. Sequence boundaries can be correlated between individual basins in the onshore UK, by reference to their position in relation to a particular marine band. For example, the sequence boundary at the base of the Farewell Rock in the South Wales Basin can be correlated with that at the base of the Rough Rock in the Pennine Basin, northern England, since both these sandstone bodies are directly overlain by the Subcrenatum Marine Band. Interbasinal correlations of this nature imply that potential fluvial sandstone reservoirs within major incised valley fills in the Upper Carboniferous strata of the southern North Sea can be predicted by correlation with the onshore UK. The stratigraphical framework can be extended and tested using core and well-log data, particularly spectral gamma-ray data, which are able to identify key sequence stratigraphic surfaces.

Evolution of a thrust-sheet-top basin: The Tertiary Barrême basin, Alpes-de-Haute-Provence, France

In the French sector of the Tertiary Alpine mountain belt of southern Europe, the early stages of compression established an initial thrust front a long distance into the foreland, ∼30–40 km rearward of the terminal thrust front. An extensive region of thrust-sheet-top basins was established behind this initial thrust front, particularly after a regional marine transgression in late Eocene time (Priabonian). These basins were initially connected, but became increasingly compartmentalized as the thrust belt evolved. The basins are now preserved as a series of structurally isolated, synclinal remnants in the external zones of the Alpine thrust belt. One of these remnants, known as the Barreme basin, has a preserved fill that is ∼900 m thick, spans ∼18 m.y., and is exposed over ∼50 km 2 . The Tertiary basin fill comprises initial nonmarine deposits overlain by a marine succession and subsequent continental strata, and includes evidence of submarine gravity slides, growth structures, and abrupt changes in sediment transport directions and provenance. Evolution of the basin fill was influenced by both far-field and more localized near-field deformation events, the latter being dominated by the progressive growth of a steep, thrust-related anticline that defined the eastern margin of the basin. This basin fill records several key steps in the evolution of the Alpine mountain belt, such as the first incursion of clasts derived from the emplacement of major thrust structures in the inner parts of the mountain belt ca. 32–30 Ma, synsedimentary volcanic activity, and the growth of an important out-of-sequence anticline that progressively separated the Barreme basin from the Gres d9Annot basin to the east. In addition, the basin preserves the progression from an early underfilled flysch basin to a subsequent overfilled molasse basin in early Oligocene time (Rupelian) between ca. 33 and 30 Ma. Records of these events are not available from the fill of the younger foreland basin preserved beyond the final thrust front, and this remnant of the thrust-sheet-top basin province therefore provides a unique record of the early structural and stratigraphic evolution of the Alpine mountain belt.

Identifying cryptic tidal influences within deltaic successions: an example from the Marsdenian (Namurian) interval of the Pennine Basin, UK

Research literature abounds on the depositional processes and products associated with macro-tidal regimes, whereas there is little available literature on sediments deposited in micro-tidal regimes. This paper presents new field-based sedimentological interpretations of the Marsdenian (Namurian, Carboniferous) interval of the Pennine Basin, a basin-fill that is classically regarded as the archetypal fluvial-dominated delta system. This paper reinterprets discrete lithostratigraphic units, and suggests they were deposited under the influence of weak tidal currents. We highlight three lithofacies that contain tidally influenced deposits within the Marsdenian interval of the Pennine Basin; a cross-bedded sandstone with mud drapes and reactivation surfaces, a heterolithic ripple-laminated sandstone with muddy drapes and silty mudstone interlaminations, and a rhythmic–parallel-bedded sandstone with mudstone–siltstone interlaminations. Evidence for cryptic tidal signatures in tractionally transported and reworked sediments is qualitative, and largely dependent on the sedimentologists view of what constitutes a diagnostic number of tidal indicators (i.e. mud-drape couplets, reactivation surfaces). In areas away from either tractional deposition or reworking, sediments deposited from suspension are more likely to preserve indicators of tidal processes. This paper focuses on a lithofacies interpreted as a tidally influenced sand-rich delta-front mouthbar deposited from a buoyant effluent plume. Time-series analysis of laminae thickness variations in this facies implies that these variations are rhythmic. We review how the interaction of tidal currents and buoyant plume processes modifies depositional products. This model implies that the rhythmic variation observed in the Marsdenian interval is attributed to the modulation of plume deposition by tidal currents with a semi-diurnal and diurnal tidal periodicity.

The structural setting and palaeogeographical evolution of the Grès d’Annot Basin

Abstract A new kinematic model is derived for part of Haute Provence, which recognizes that Alpine deformation, involving both the Mesozoic and sub-Mesozoic section, has been active since the onset of Tertiary deposition. This model places the Grès d’Annot basin in an overall compressional setting, dominated by SW-directed Alpine thrusting, during deposition of the Tertiary basin fill sequence. Evidence is presented that one block of Palaeozoic material (Barrot) was strongly uplifted and deeply eroded prior to the onset of Tertiary deposition in the area. Local thin-skinned extension and strike-slip are observed, but these are secondary components, which are interpreted to be part of the overall thrust system. The region was affected by three different orogenic events. Pyreneo-Provençale thrusting established a structural fabric that influenced later, syndepositional thrust geometries. The Late Eocene to Early Oligocene Grès d’Annot Basin was a structured foreland basin created by the loading effects of the Alpine orogeny. The supply of siliciclastic sediment to the basin was controlled by tectonic events far to the south, in a subduction-related orogeny that extended from East Iberia to SE France. SW-directed Alpine compression within the basin produced significant basin-floor topography, which developed immediately prior to, and during, the Nummulitic transgression. The evolving palaeo-relief of the basin is recorded in thickness and facies variations of the older Tertiary lithostratigraphical units (Poudingues d’Argens, Calcaires Nummulitiques and Marnes Bleues), and by the onlap geometry of the Grès d’Annot turbidites. This allows us to define a set of palaeogeographical maps of the basin throughout its evolution, linked to a set of sequentially restored structural sections. This basin floor topography was complex, because Alpine thrusting overlapped the older east-west striking structures, and it created a set of partially isolated sub-basins. Major early compressional structures partitioned the basin, exerting strong control on all the Tertiary depositional systems, including the Grès d’Annot turbidite systems.

Continental-scale sequence stratigraphy of the Namurian, Upper Carboniferous and its applications to reservoir prediction

The search for Upper Carboniferous reservoirs can be aided by the development of a chronostratigraphic framework combining detailed sedimentological information with a template using the diagnostic ammonoid-bearing marine bands from onshore analogues derived from European, American and Canadian basins. Analysing Namurian successions constrains key sand-prone intervals, which include the upper Kinderscoutian, lower Marsdenian and top Yeadonian, from a number of depositional settings. A controversial issue for the Upper Carboniferous is the relative importance of high frequency and high magnitude glacio–eustatic sea-level fluctuations as a driving mechanism in the development of basin fill over the controls exerted through the prevailing tectonic regime, climate and sediment supply. The recognition and characterization of time-equivalent sea-level rises and, with greater significance for hydrocarbon exploration, sea-level falls, from a number of European basins attests to their influence. Candidate reservoirs can be constrained at various temporal resolutions and may also be restricted geographically. Significant candidate reservoirs include multistorey fluvial incised valley fills and deepwater sand-rich successions. The most productive onshore reservoirs are coarse-grained/conglomeratic fluvial intervals located in the base of incised valleys where a significant proportion of the subsequent valley fill is fine-grained estuarine sediments. There is potential for large hydrocarbon reservoirs within sand-rich deep water successions but their occurrence in tectonically complex or deeply buried areas has thus far precluded their exploitation. A chronostratigraphic framework established for the UK and Ireland can be applied to European basins and, in a more limited sense, to North America, where marine horizons can be matched. The application of this framework to the subsurface lies in the recognition of key candidate reservoirs and their likely occurrence, both geographically and temporally.

Long-lived Transfer-Zone Paleovalleys in Mountain Belts: An Example from the Tertiary of the Spanish Pyrenees

ABSTRACT Major fluvial valleys located in sediment-transfer zones that link hinterland drainage basins with their depositional basins are an important, but neglected, element of fluvial systems. Studies of present-day examples are limited and, to date, no examples have been described from the geological record. Tertiary synorogenic fluvial successions in the Spanish Pyrenees include regional-scale, unconformity-based, linear bodies of conglomerate that are interpreted as the fills of transfer-zone paleovalleys. These paleovalleys are sited mainly in the external zones of the mountain belt, between the internal Axial Zone, on which the drainage basin was largely established, and the depositional basins. One body, here termed the Sis conglomerate, is preserved in a growth syncline between two th ust-related, lateral structures. Growth of the syncline during sedimentation permitted a 1400 m succession of clast-supported, pebble and cobble conglomerates to accumulate during a c. 18 m.y. period between the middle Eocene to Oligocene. The paleovalley served not one but a series of evolving thrust-sheet-top and foreland basins during this period. The stability of the paleovalley was governed by its location between long-lived, pre-existing structures that were inverted during compressional deformation. This example demonstrates that fluvial paleovalleys located in transfer zones can, when structurally controlled, be long-term features capable of accumulating and preserving a significant thickness of strata.

Incised valley fill sandstone bodies in Upper Carboniferous fluvio–deltaic strata: recognition and reservoir characterization of Southern North Sea analogues

Multistorey fluvial sandstone bodies in Upper Carboniferous fluvio–deltaic strata are proven reservoirs in the Southern North Sea. Several of these bodies have been interpreted as incised valley fills. Analogous valley fill sandstone bodies have been studied in outcrop and subsurface datasets from Upper Carboniferous successions in the onshore UK and Germany. Using techniques and tools applicable to Southern North Sea datasets, we review the diagnostic criteria of these valley fills: (1) stratigraphic context, characterized by deep erosion into underlying strata, lateral correlation to an interfluve surface and intimate association with an initial flooding surface in overlying strata; (2) basinward facies shift at the base of the sandstone body; (3) erosion of underlying time markers; and (4) distinctive internal architecture, which reflects increasing accommodation space during valley filling. Several features of onshore valley fill sandstones have been characterized in order to constrain the properties of analogous valley fill reservoirs in the Southern North Sea. (1) Valley geometry and extent. Studied onshore valley fills generally have a uniform thickness of 20–25 m, thickening locally to 30–45 m, and widths of 5–25 km. A small number of valley fills, associated with turbidite-fronted deltas, thicken to 50–80m in the 2–5 km adjacent to their mouths. Sheet-like fluvial sandstones overlying sequence boundaries have thicknesses comparable to conventional valley fills, but exceed 35–70 km in width. (2) Valley connectivity; typically, valley fills occur as discrete sandstone bodies, but they may amalgamate laterally, to form sheet-like sandstones, and vertically, producing thick (up to 200 m) sand-prone stratigraphic packages. The internal geometry of these sand-prone packages is complex. (3) Valley fill character. Valley fills generally lack an overall fining-upwards trend and are dominated by coarse- to medium-grained sandstone of low-sinuosity (braided?) fluvial facies throughout. Where present, fine-grained facies are preferentially preserved in the upper part of a valley fill. (4) Detrital mineralogy and provenance. At least some valley fill sandstones exhibit a different composition and provenance to surrounding strata. Such valley fill sandstones are likely to be anomalously feldspathic in the Southern North Sea.

Critical application of high resolution sequence stratigraphic concepts to the Rough Rock Group (Upper Carboniferous) of northern England

Abstract The Rough Rock Group (Upper Carboniferous, Namurian G1) of northern England comprises fluvio-deltaic strata exhibiting high frequency cyclicity. Key elements of high-frequency sequences are recognized in these strata, including lowstand braided fluvial incised valley fills and their correlative interfluve palaeosols. The geometry of these incised valley fills was controlled by the local, synsedimentary tectonic setting. Regionally-extensive coals overlie these incised valley fills and their interfluve palaeosols and are interpreted as initial flooding surface correlatives. These coals are in turn overlain by shales containing faunal-concentrate condensed horizons (marine bands) which are interpreted as maximum flooding surfaces. A previously unrecognized marine band which overlies the Sand Rock Mine Coal and underlies the Rough Rock Flags is documented, implying a major revision of the published stratigraphy and sequence stratigraphy. These sequence stratigraphic elements define four high frequency siliciclastic sequences, which are integrated into a sequence stratigraphic framework for the Rough Rock Group. This framework allows sequence architecture, palaeogeographies and stacking patterns to be described in detail and also provides a basis for identifying potential controls on these variables.

The Tertiary Grès de Ville of the Barrême Basin: feather edge equivalent to the Grès d’Annot?

Abstract The Tertiary Grès de Ville in the Barrême Basin of the French Alps is conventionally considered to be the feather edge equivalent to the Grès d’Annot to the east. Both units accumulated in a province of thrust-sheet-top basins behind the initial thrust front and were supplied from a southerly hinterland (Maures-Esterel/Corsica-Sardinia). The units are also partially coeval, with the Grès de Ville being equivalent to the youngest part of the Grès d’Annot. However, the units accumulated in structurally distinct sub-basins that were separated by a fold (the St Lions anticline) that was growing at the time of Grès de Ville deposition. The Grès de Ville accumulated close to storm wave base, whereas deposition of the Grès d’Annot was notably deeper, indicating that the Barrême Basin was structurally higher than the Grès d’Annot Basin. There is also evidence that the Grès de Ville and the Grès d’Annot were fed by separate sediment transport paths linked to different drainage basins in the hinterland. It is therefore an oversimplification to regard the Grès de Ville as a feather edge to the Grès d’Annot; instead it should be regarded as an independent element of the fill of the Barrême Basin.