Now showing 1 - 10 of 11
  • Publication
    Hybrid Event Bed Character Across the Clare Shale - Basal Ross Formation Contact, Western Ireland - New Insight from Behind-Outcrop Cores
    A recent behind - outcrop drilling program targeting the Ross Formation has focussed on the Loop Head peninsula in west Clare. This has provided a fully-cored composite Ross section (490 m thick) that underpins a new understanding of bed-scale variability and the vertical evolution of the system . The work programme h as now been broadened to include the key Ballybunion section on the south side of the Shannon which sits obliquely down-dip (to the east) of Loop Head (c. 18 km away).
      113
  • Publication
    Sedimentology of the upper Ross Formation (Pennsylvanian) in borehole GSI 09/05 at Ballybunion, Co. Kerry
    The Pennsylvanian Ross Sandstone Formation is well exposed around the Shannon Estuary, in western Ireland. It forms the deep-water part of a major shallowing-upward succession filling the Clare Basin and it has been extensively used as a turbidite reservoir analogue and for subsurface training.   Since 2009, a major program of behind-outcrop drilling targeting the Ross Sandstone Formation has been undertaken in west Co. Clare (Loop Head) and across the Shannon estuary in Co. Kerry. To date, most of the focus has been on boreholes acquired on Loop Head. These have provided a composite vertical section through the Ross Sandstone Formation and a framework for interpreting bed type variations and overall system evolution. Now, the emphasis is changing to examine lateral variability away from Loop Head using a new core at Ballybunion in the lowermost Ross Sandstone Formation, together with legacy data and two recent GSI slim-holes (at Inishcorker and Ballybunion).   This poster focuses on first results from the GSI 09/05 borehole, located c.4.3 km north of Ballybunion town. This 150 m long slim-hole core intersects the upper Ross Formation as confirmed by correlation with a measured outcrop section on the adjacent cliff c.1.2 km away. Correlation is based on goniatite-rich "marine bands", distinctive thick sandstone units and slump bodies. Three marine bands are identified, the uppermost being unusually thick (c.4.5 m) and provisionally equated with the R. paucicrenulatum band marking the top of the Ross Sandstone Formation on Loop Head. The core records an upward transition from well-bedded sandstones (turbidites and subordinate hybrid event beds) interpreted as stacked lobe deposits to more amalgamated sandstones towards the top of the formation indicating increased channelisation. A major composite slump body (25 m thick) occurs below the central marine band. This remobilised both sand-prone and silty deposits and is significantly thicker than any of the slump units identified in the Loop Head cores and outcrop.
      140
  • Publication
    Facies and internal stratigraphic variability in the Ross Sandstone Formation (Pennsylvanian), western Ireland - new borehole data from south of the Shannon Estuary
    (International Association of Sedimentologists, 2013-09) ; ; ;
    The 500 m thick Ross Sandstone Formation is well exposed in sea cliffs facing the Atlantic and along the Shannon Estuary in western Ireland. It forms the sandy deep-water part of a major shallowing-upward Pennsylvanian succession. Over the last four years, a major behind-outcrop drilling program targeting the Ross Sandstone Formation has been undertaken, focussing primarily on the Loop Head peninsula in west Clare. This has provided a full composite Ross cored section that underpins a new understanding of bed-scale variability and the wider evolution of the system. The focus has recently shifted to the key Ballybunion section on the south side of the River Shannon, obliquely down-dip from the Loop Head area (c. 18 km from the tip of the Loop) and is important in that previous outcrop studies have inferred that (1) the distinctive character of the lower Ross here with its abundant hybrid event beds may reflect a marginal position; and (2) extra sandy section may be present in the uppermost Ross due to offset stacking of the youngest sandy lobes. Two new cores are now available ¿behind¿ the Ballybunion section - a 200 m PQ borehole straddling the lower Ross and the upper part of the underlying Clare Shale Formation (12-CE-UCD-09), and a 151.5 m long, cored slimhole with associated wireline log data acquired by the Geological Survey of Ireland (GSI 09/05). The latter is 1.1 km along strike from coastal exposures of the upper Ross and the study reported here focusses on the relationship of the section acquired in this borehole to the local cliffs and to upper Ross outcrops on the north side of the River Shannon at Kilcredaun, some 4.6 km away. Correlation is based on goniatite-rich ¿marine bands¿, legacy biostratigraphic data (new determinations are underway), and a number of laterally extensive slump bodies which form distinctive marker beds.   The GSI 09/05 core contains three thin goniatite-rich levels, and a fourth candidate level, each interpreted as marine bands. These separate sand-prone packages, interpreted as stacked isolated to amalgamated lobe units, and at least two mass-transport units (MTDs), the lower and thickest of which is 25.5 m thick (true thickness). In the local cliffs to the west, all four marine bands can be identified, as well as the two MTDs. In addition, a third MTD is more obvious in the cliff. The lobe sandstones are dominated by deposits of high-density turbidity currents; amalgamated sandbodies become more abundant upwards. Hybrid event beds are rare (<10%) compared to lower in the formation. At least 50% of the sandbodies extend from the core to the adjacent outcrop without change; the remainder show a change from deposition from high- to low-concentration flows or vice versa. Overall, the Ballybunion Ross section is 480 m thick, broadly similar to the thickness established by drilling on the Loop. At longer length scales, all but the upper marine band are found at Kilcredaun. Correlatives of the two MTD units also occur in the core here, although the thickest slump has become thinner and muddier. Nonetheless this MTD unit can be traced widely across the Loop as a distinctive couplet. As correlated, the Ballybunion outcrop and core suggest there may not be an additional younger sandbody in this area, however the location appears axial and down-dip rather than marginal in character overall.
      173
  • Publication
    Down-dip Termination of Sandy Fan Systems - New Insight from the Pennsylvanian Ross Sandstone Formation, Western Ireland
    The Pennsylvanian Ross Sandstone Fm. in western Ireland forms part of a thick (>2200m) progradational and shallowing-upward basin-fill succession. New and legacy borehole constraints and outcrop work on the eastward extension of the Ross help constrain the down-dip character at different levels in the fan stack.
      129
  • Publication
    Down-dip Termination of Sandy Fan Systems - New Insight from the Pennsylvanian Ross Sandstone Formation, Western Ireland
    New and legacy borehole constraints and outcrop work on the eastward extension of the Ross Formation help constrain the down-dip character of the fan stack.
      90
  • Publication
    Down-Dip Termination of the Carboniferous Ross Fan System in the Inner Shannon Area, Western Ireland - New Insight from Core and Outcrop
    Scattered outcrops and limited borehole data in the inner Shannon estuary and mid-Clare are critical to constrain the down-dip extension of the Ross system. Previous outcrop studies have described a much thinner Ross section at Inishcorker and Foynes (over 50 km east of the Ross type section on the Loop Head) involving only the youngest Ross cycles in the west. A re-interpretation of the inner Shannon outcrops is now possible given a new GSI 09/04 borehole in the Inishcorker area, a re-analysis of Foynes Island sections and new biostratigraphic data
      125
  • Publication
    Facies Trends and Large-Scale Architecture of the Pennsylvanian Ross Formation, Western Ireland - New Insight from Cores South of the Shannon
    The Ross Formation is well exposed in sea cliffs facing the Atlantic and Shannon estuary in western Ireland. It forms the sandy deep-water part of a major shallowing-upward Pennsylvanian succession. Over the last four years, a major behind-outcrop drilling program targeting the Ross Formation has focussed primarily on the Loop Head peninsula in west Clare. This has provided a composite Ross cored section (490 m thick) that underpins a new understanding of bed-scale variability and the wider vertical evolution of the system. The focus has now shifted to the key Ballybunion section on the south side of the Shannon, which sits obliquely down-dip (to the east) of the Loop Head area (c. 18 km away). This area is important in that previous outcrop studies have suggested that (1) the character of the lower Ross with its abundant hybrid event beds may reflect a marginal fringe position; (2) an extra sandy section may be present in the uppermost Ross due to offset stacking of the youngest lobes and (3) some of the upper Ross mass transport units may extend across the estuary from Clare. Two new cores are now available ¿behind¿ the Ballybunion cliff section: a 200 m long PQ borehole straddling the lower Ross and the upper part of the underlying Clare Shale (12-KY-UCD-09), and a 151.5 m long slimhole core acquired by the Geological Survey of Ireland (GSI 09/05). In addition, a re-analysis of the biostratigraphy is underway. Together the matched pair of Kerry boreholes with the outcrop section provide a reference section (480 m thick) that can be compared with the Loop composite section. Both sections have a distinctive precursor cycle involving first stacked thin mudflows and then outsized and coarse grained hybrid event beds. The muddier make-up of the latter at Ballybunion is consistent with a down-dip position based on trends in other basins. The onset of the main Ross system that follows is sandier at Ballybunion than at Loop suggesting the former was more axial at this time. Thereafter hybrid event beds appear not to be as important at Ballybunion. Several of the mass transport units and condensed sections extend across the Shannon and tie the sections.
      116
  • Publication
    Flow transformations, Mud Partitioning, and the Variable Stratigraphic Architecture of Basin-Floor Fan Fringes
    Highly efficient sediment gravity flows can bypass mid-fan channels and lobes and deposit significant volumes of sand, mud, and particulate organic matter in outer-fan and basin-plain settings. The Serpukhovian to Bashkirian fill in the Shannon Basin, western Ireland, includes deep-water fan deposits (Ross Sandstone Fm) that gradationally overlie basin-floor shales (Clare Shale Fm). As part of a broader progradational succession, the upward transition from muddy basin floor to sandy fan preserves the stacked deposits of settings present prior to, and outboard of, mid-fan channels and lobes. Three fully cored boreholes and associated wireline data constrain the facies tracts in an 18-km-long panel oriented oblique to original depositional dip. Two distal successions dominated by hybrid event beds (HEBs) are recognized, separated by a prominent condensed section. The lower Cos-heen system includes m-thick, tabular HEBs with prominent linked debrites that pass down dip into much thinner sandstones overlain by sand-speckled mudstone caps that thicken distally before thinning. The latter are interpreted as secondary mudflows released following reconstitution of more thoroughly mixed sections of the up-dip linked debrites. Significant bypass and accumulation of mud by this mechanism helped heal local topography and maintain a relatively flat sea floor, promoting an overall tabular geometry for the deposits of larger volume hybrid flows reaching the distal sector of the basin. The overlying distal Ross system fringe is characterized by very fine- to fine-grained sandstones and is lateral to compensationally stacked lobes farther to the west. It has a progradational (at least initially) stacking pattern, facies transitions developed over shorter length scales, and includes outsized event beds, but these are thinner than those in the Cosheen system. Common banding and evidence for turbulence suppression by dispersed clay rather than entrained mud clasts indicate that these were transitional flows. In this case, event beds are inferred to taper distally, with significant mud emplaced by plug flow retained as caps to sandy event beds rather than bypassing down-dip. Different flow transformation mechanisms thus impacted how mud was partitioned across the fringe of the two systems, and this influenced bed geometries, larger scale bed stacking patterns, and stratigraphy. Whereas the flow-efficiency concept stresses the ability of flows to carry sand in a basinward direction, it is also imperative to consider the variable efficiency of mud transport given the operation of clay-induced flow transformations. These can either promote bypass or trigger premature fallout of mud, with implications for how systems fill accommodation, bed-scale facies transitions, and the burial and preservation of particulate organic carbon fractionated along with the clay in deep-water system fringes.
      37
  • Publication
    Large-scale, linked drainage systems in the NW European Triassic: insights from the Pb isotopic composition of detrital K-feldspar
    Pb isotopic data from K-feldspars in Middle Triassic (Anisian) sandstones in the Wessex Basin, onshore southwest UK, and the East Irish Sea Basin, some to the north, show that the same grain populations are present. This indicates that the drainage system (the “Budleighensis” River) feeding these basins originated from the same source/s, most probably the remnant Variscan Uplands to the south. Fluvial and aeolian sandstones have the same provenance, suggesting that if water- and wind-driven sands were originally derived from different sources, this has been obscured through reworking prior to final deposition. Significant recycling of feldspar from arkosic sandstones in earlier sedimentary basins can be ruled out. The provenance data agree with previous depositional models, indicating transport distances in excess of , with a drainage pattern that linked separate basins. This supports the idea that the regional fluvial system was driven by topography and episodic flooding events of sufficient magnitude to overcome evaporation and infiltration over hundred’s of kilometres. Importantly, this drainage system appears to have been isolated and independent from those operating contemporaneously to the northwest of the Irish and Scottish massifs, where the remnant Variscan Uplands apparently exerted no influence on drainage or sand supply.
      1174Scopus© Citations 29
  • Publication
    Sedimentology, sandstone provenance and palaeodrainage on the eastern Rockall Basin margin : evidence from the Pb isotopic composition of detrital K-feldspar
    The Rockall Basin, west of Ireland, is a frontier area for hydrocarbon exploration but currently the age and location of sand fairways through the basin are poorly known. A recently developed provenance approach based on in-situ Pb isotopic analysis of single K-feldspar grains by laser ablation multi-collector inductively-coupled mass spectrometry (LA-MC-ICPMS) offers advantages over other provenance techniques, particularly when applied to regional palaeodrainage issues. K-feldspar is a relatively common, usually first-cycle framework mineral in sandstones and its origin is typically linked to that of the quartz grains in arkosic and sub-arkosic rocks. Consequently, in contrast to other techniques, the Pb-in-K-feldspar tool characterises a significant proportion of the framework grains. New Pb isotopic data from K-feldspars in putative Permo-Triassic and Middle Jurassic sandstones in Well 12/2-1z (the Dooish gas condensate discovery) on the eastern margin of the Irish Rockall Basin are reported. These data suggest that three isotopically distinct basement sources supplied the bulk of the K-feldspar in the reservoir sandstones and that the relative contribution of these sources varied through time. Archaean and early Proterozoic rocks (including elements of the Lewisian Complex and its offshore equivalents), to the immediate east, north-east and north of the eastern Rockall Margin, are the likely sources. More distal sourcelands to the north-west cannot be ruled out but there was no significant input from southern sources, such as the Irish Massif. These data, together with previously published regional Pb isotopic data, highlight the important role played by old, near and far-field Archaean – Proterozoic basement highs in contributing sediment to NE Atlantic margin basins. The Irish Massif appears to have acted as a significant, but inert, drainage divide from the Permo-Triassic to the Late Jurassic and hence younger, Avalonian and Variscan, sand sources appear to have been less important on the Irish Atlantic Margin.
    Scopus© Citations 27  2269