Now showing 1 - 10 of 11
  • 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
    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).
      148
  • 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 focused 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 rRoss 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.
      110
  • 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
    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
    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).
      114
  • Publication
    Facies Analysis and Architecture of the Fosado Turbidite Channel Complex
    (British Sedimentological Research Group, 2013-12) ; ;
    In the Ainsa basin, the Castissent sequence has been subdivided in two different composite depositional sequences: Castissent-1 (Cs-1) and Castissent-2 (Cs-2). This study focuses on the lower part of Cs-1, where the Fosado Channel Complex lies. Cs-1 was deposited synchronously with a high-deformation phase in the Pyrenean orogeny (Upper Ypresian to Lower Lutetian), with strong tectonic reactivation of the South-Central Pyrenean Unit; sediments were affected by the Atiart thrust (east) and later on by Los Molinos thrust (west). So far, the knowledge about Cs-1 was mainly its stratigraphic position within the Torla turbidite system, its physical correlation with the fluvio-deltaic feeding system of Castissent Fm. s.s., as well as its facies detail of the uppermost individual channel (the Fosado channel, s.s.), which has been used as a facies model for channel margin settings. Nevertheless, its high resolution stratigraphic study, its facies associations and architecture, still remains unknown in detail. Therefore, it has been possible to map and redefine the extension of the Atiart unconformity in the hanging wall of Los Molinos thrust. This 3rd order sequence boundary at the base of Cs-1 separates: (1) a lower mud-dominated unit, with a high content in debris flow deposits and fine-grained turbidites; and (2) the upper unit, mainly composed by a channel-overbank complex (channel-levee), forming sandy packages belonging to highly-heterogeneous channels. In the stacking pattern of the Fosado complex two main growing stages have been identified: Cs-1a and Cs-1b. Cs-1a displays a relatively high percentage of sand content in individual channels; on the contrary, Cs-1b is a mud-dominated unit, mainly composed by fine-grained channels and their related overbank deposits. This last unit would be the lowermost part of the slope fan related with the Castissent deltas. Consequently, the study has focused on the channel complex of Cs-1a unit, where a stacking pattern with unconformities is described and has led to organize it hierarchically in five 4th-order sequences (high resolution seismic scale). Each channel group shows a nested stacking pattern, with hyperconcentrated flow deposits at the base, and lateral and vertical accretion (up to 200x20 m), which is probably indicative of large-scale high-sinuosity channels.
      146