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University of Birmingham Lower Jurassic (Hettangian–Pliensbachian) Microfossil Biostratigraphy of the Ballinlea-1 Well, Rathli

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University of Birmingham Lower Jurassic (Hettangian–Pliensbachian) Microfossil Biostratigraphy of the Ballinlea-1 Well, Rathli University of Birmingham Lower Jurassic (Hettangian–Pliensbachian) microfossil biostratigraphy of the Ballinlea-1 well, Rathlin Basin, Northern Ireland, United Kingdom Boomer, Ian; Binti Azmi, Azrin; Copestake, Philip; Raine, Robert License: None: All rights reserved Document Version Early version, also known as pre-print Citation for published version (Harvard): Boomer, I, Binti Azmi, A, Copestake, P & Raine, R 2019, 'Lower Jurassic (Hettangian–Pliensbachian) microfossil biostratigraphy of the Ballinlea-1 well, Rathlin Basin, Northern Ireland, United Kingdom', Proceedings of the Geologists' Association. 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Early Jurassic microfossils of the Ballinlea-1 Borehole 1 Lower Jurassic (Hettangian–Pliensbachian) microfossil biostratigraphy of the Ballinlea-1 well, 2 Rathlin Basin, Northern Ireland, United Kingdom. 3 4 Ian Boomer1, Azrin Azmi 1,2, Philip Copestake3, Robert Raine4 5 6 1 Geosciences Research Group, GEES, University of Birmingham, B15 2TT 7 2Geology programme, School of Earth Sciences and Environment, Faculty of Science and Technology, 8 Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia 9 3Merlin Energy Resources Ltd, Newberry House, Ledbury, Herefordshire, HR8 2EJ, England, 10 4 Geological Survey or Northern Ireland, Dundonald House, Upper Newtownards Road, Belfast, BT4 3SB, 11 Northern Ireland. 12 13 Abstract 14 The thickest section of Early Jurassic strata known from onshore Ireland (total Jurassic thickness 15 566 m) is reported from the Ballinlea-1 well (Rathlin Basin) on the north coast of Northern 16 Ireland. A detailed biostratigraphical and palaeoenvironmental assessment is presented for this 17 section largely based on calcareous benthic microfossils (foraminifera and ostracods). The Early 18 Jurassic Waterloo Mudstone Formation (Lias Group) of Northern Ireland has previously received 19 little micropalaeontological attention, therefore this work provides an opportunity to enhance 20 palaeogeographic and palaeoenvironmental understanding for the Early Jurassic of the province, 21 and this paper illustrates key microfossil taxa of this age from Ireland for the first time. The 22 records, based on ditch-cuttings samples, demonstrate a stratigraphical range from Hettangian to 23 Early Pliensbachian, consistent with other wells and boreholes in this basin. The assemblage 24 compositions are comparable to those elsewhere in the European boreal Atlantic realm. 25 Hettangian to earliest Sinemurian microfossil assemblages are generally of low diversity and are 26 numerically dominated by metacopid ostracods with occasional influxes of foraminifera. 27 Gradually, foraminiferal abundance (often dominated by species of the Lagenida) come to exceed 28 those of the ostracods in the Early Sinemurian reaching greatest diversity in the Late Sinemurian. 29 The sediments are considered to represent an inner to mid-shelf environment throughout while 30 the record thickness for this region indicates ongoing syn-sedimentary fault movement along the 31 basin margins throughout this period. 32 33 KEYWORDS: Ostracoda, foraminifera, Waterloo Mudstone Formation, Hettangian, Sinemurian, 34 Pliensbachian. 35 1 Boomer et al. Early Jurassic microfossils of the Ballinlea-1 Borehole 36 Introduction and Geological Setting 37 Exposures of Early Jurassic sediments in Northern Ireland are relatively rare and are largely 38 restricted to small (a few 10s of metres at most) coastal exposures (Figure 1). They often sit 39 below cliffs of Late Cretaceous chalk (the Ulster White Limestone Group) and Paleogene basalts 40 of the Antrim Lava Group (Mitchell, 2004). The exposures are discontinuous, faulted and their 41 weakly consolidated nature makes them prone to landslip. Subsurface records of latest Triassic 42 and Early Jurassic sediments are known from a number of boreholes and industrial wells but this 43 paper deals with the thickest sequence of Early Jurassic sediments known from onshore Ireland, 44 recovered from the Ballinlea-1 exploration well in the Rathlin Basin on the north coast of Ireland. 45 46 47 48 Figure 1. Distribution of Triassic and Jurassic sediments in Northern Ireland, together with 49 location of the main sedimentary basins, bounding faults and locations referred to in this work (1. 50 Ballytober-1 well, 2. Port More Borehole, 3. Magilligan Borehole, 4. Larne-1 and Larne-2 51 boreholes, 5. Ballymacilroy Borehole, 6. Mire House Borehole). The map is modified from George 52 (1967), Warrington (1997) and Middleton et al. (2001). 53 2 Boomer et al. Early Jurassic microfossils of the Ballinlea-1 Borehole 54 Largely continuous records across the latest Triassic to earliest Jurassic interval are known from 55 the region. Simms and Jeram (2007) summarised the occurrence of Late Triassic sediments in the 56 Larne Basin at Waterloo Bay, noting that red-beds of the Mercia Mudstone Group were overlain 57 by siltstones of the Collin Glen Formation (both Norian) and those are, in turn, disconformably 58 overlain by Rhaetian sediments of the Penarth Group, divided into a lower Westbury Formation 59 and an upper Lilstock Formation (Mitchell, 2004). Although there is evidence from across 60 Northern Ireland to suggest that this succession of lithostratigraphic units occurs across many 61 parts of the province (Raine et al., a this volume), Late Triassic outcrops have not been recorded 62 within the Rathlin Basin and neither are sediments of this age proven with any certainty from 63 the Ballinlea-1 well. 64 65 A broadly conformable boundary between the Penarth Group and the Waterloo Mudstone 66 Formation is also recorded subsurface at Magilligan to the west of the Ballinlea-1 well (Bazley et 67 al., 1997) although this has not been proven at the Port More borehole due to the presence of an 68 intrusive sill (Wilson & Manning, 1978). While the Triassic-Jurassic boundary interval is exposed at 69 Larne (Simms and Jeram, 2007) and in the nearby Carnduff-1 and 2 boreholes (Boomer et al., 70 202Xa) by contrast, the Lias Group from the Ballytober-1 borehole (also located in the Larne 71 Basin) unconformably overlies the Mercia Mudstone Group (Fynegold Petroleum, 1991), which is 72 a comparable situation to that in the Ballinlea-1 well. 73 74 Early Jurassic sediments from Northern Ireland are assigned to the Waterloo Mudstone 75 Formation (Lias Group) which is broadly equivalent to the Blue Lias Formation and part of the 76 Charmouth Mudstone Formation of Great Britain. The most significant outcrops in Northern 77 Ireland occur at Waterloo Bay, Larne (Simms and Jeram, 2007), previously a candidate GSSP 78 (Global Stage Stratotype and Point) for the Triassic/Jurassic boundary, and at White Park Bay 79 (Wilson and Manning, 1978; Mitchell, 2004), all other exposures are only of a few metres’ 80 thickness. While the majority of outcrops are of very earliest Jurassic (Hettangian to Early 81 Sinemurian) age, the exposures at White Park Bay are of latest Sinemurian to Early Pliensbachian 82 age. 83 84 Prior to the drilling of the Ballinlea-1 well, the thickest Early Jurassic successions onshore Ireland 85 had been recorded in boreholes at Port More (270 m), Mire House (125 m), Magilligan (76.5 m), 86 Ballymacilroy (86 m) and Larne-1 (51 m), the nearby Larne-2 borehole encountered Mercia 3 Boomer et al. Early Jurassic microfossils of the Ballinlea-1 Borehole 87 Mudstone Group at the surface (Figure 1). In the subsurface, the youngest known Early Jurassic 88 sediments in Northern Ireland are of the Ibex Chronozone (Early Pliensbachian) in the Port More 89 Borehole (Warrington, 1997), demonstrating that the Waterloo Mudstone Formation in the 90 Rathlin Basin appears more complete than elsewhere in the province, and this may be a 91 consequence of different structural histories in the different basins between the early Jurassic 92 and mid-Cretaceous, the next youngest lithostratigraphic unit. 93 94 The Ballinlea-1 Well 95 The Ballinlea-1 well
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