<p>Advances in seismic imaging technology can discourage the integration of outcrop data into modern interpretation workflows. Yet, instigation of hydrocarbon exploration still requires the use of legacy seismic data, especially within mature petroleum provinces. Typical exploration workflows include expensive pre-stack seismic reprocessing, to better resolve exploration targets. This is a resourceful but timely process that can be enhanced by using structural geological analogues. The inner Bristol Channel has extensive outcrop: to the east the Severn Estuary, to the north the South Wales Coalfield and Vale of Glamorgan and to the south along the Somerset, Devon and Cornish coastlines. These sources of prolific data, combined with legacy exploration refraction, reflection and earthquake seismology, make the inner Bristol Channel an ideal natural laboratory to integrate analogues with seismic information and to produce realistic interpretations and explanations of complex structural heterogeneities especially in places concealed by Mesozoic and Quaternary cover, marine waters and estuarine sediments typical of the inlet. Successful structural analysis is always reliant on well-processed pre-stack seismic data. It is demonstrated however that numerous known structural inversion events also necessitate the best choice of analogues to resolve the geometry and kinematics of any major faults offshore accurately enough to reach a reliable understanding of the petroleum system. Here, in response to the Department of Energy and Climate Change (DECC) 2016 data release and the 31st licensing round, we use case studies from the inner Bristol Channel to demonstrate the value of structural geological analogues by integrating them into 1985 2D legacy seismic data at an early stage in the seismic interpretation process. With suitably chosen analogues, structural dissection and reconstruction are carried out to generate geometric and kinematic models. The wider waters of the Bristol Channel are situated in quad 105; in which investigation was instigated in the 1970&#8217;s by major exploration companies. Currently, an interest in reliable structural analogues is made more relevant by the fourteen exploration licenses held onshore in south west England and South Wales. Thence our study augments the extensive field work carried out over at least three decades of academic research by generations of scholars. The targeted investigations conducted along the southern coast of Wales and the north coast of Somerset, Devon and Cornwall lead to revised syntheses, to better extrapolate, predict and model the structural architecture beneath the inner Bristol Channel. Exemplary Welsh field analogues are accounted in detail, measured, restored and integrated into an interpretation of the 2D 1985 Western-Geco dataset (WG85 2D 2001). The analogues include Trwyn-yr-Wrach, the Cold Knap, St Mary&#8217;s Well and Sully Island, among many others. In conclusion, the practical methodology exemplifies the geometric WNW-ESE lateral changes in structure and the effects of numerous kinematic phases and recent seismicity upon the architecture of the inner Bristol Channel basin as well as its relic-fabric. This demonstration of analogues improves immensely the geological understanding of seismic reflection projects whether legacy data, reflection, refraction or seismology and should remain relevant for many more crucial and modern acquisitions.</p>
<p>The Bristol Channel contains major Variscan thrusts juxtaposing distinct tectonostratigraphic terranes: &#160;the Upper Carboniferous Rhenohercynian, Culm (south) and Sub-Variscan Foredeep, Coalfield (north). There is agreement the contrasts across the Channel are not restricted to this, since underlying marine Devonian differs from continental ORS and Lower Carboniferous radiolarian-chert differs from the Main Limestone. The famous basins were mapped intricately over 100+ years by the UK geological survey and by academics across Europe, and questions about their juxtaposition date back to 1895 in the Quarterly Journal of the Geological Society, London.</p><p>Our aim, is to use structural styles and shortening to determine an upper limit for displacements upon the major thrusts. We investigate the magnitudes of shortening from south to north through the Culm and north Devon basins, and from west to east across SW Dyfed, central South Wales, Bristol, Mendips, Oxfordshire, and Kent, using an immense legacy of sections drawn by various authors, including the recent basin dynamics group of Wales.</p><p>Estimates corrected for Mesozoic negative inversion show 45% shortening due to accommodation-chevron and box folding in the Culm, 40% due to folds, back-thrusts, and fore-thrusts in the north Devon basin, 30% beneath northern parts of the Channel, and 33% along the strike of the foredeep from Wales to Kent. There is also great contrast in deformation style, between the Culm continuous-folds and the foredeep with reactivated faults, rounded folds, and thrusts, related to preferential slip along seams within central parts of the Middle Coal Measures.</p><p>Shortening can be 70%, close to underthrusts in the southern Culm; adjacent to regional thrusts along the north Devon coast; and, proximal to disturbances within the foredeep. This intensity of composite deformation would not be out of place close to tectonic-scale thrusts, between these terranes. Additionally, thrusts of this scale are detectable on regional seismic profiles and were the topics of recent studies. Structural inspection reveals significant 1km-scale displacements along NW-SE strike-slip faults common to both terranes and upon WSW-ENE oblique-ramp thrusts local to the Vale of Glamorgan and Severn estuary. WNW-ESE frontal ramps with ~10km-scale displacements are considered candidate &#8216;stems&#8217; to tectonic-scale thrusts and are found in Gower, Devon, and inner Channel.</p><p>Further investigations could elaborate the style of transmission of major thrust displacement from beneath the hinterland into the foredeep, whether by reactivation, decapitation, translation, and rotation of structural fabrics. There are complications of Mesozoic negative and Cenozoic positive inversions to consider in section restoration and adjustments are required to reveal how large displacements were dissipated exactly.</p><p>Reservations are that shortenings in hanging-walls can be poor indicators of displacement magnitude upon individual thrusts within sequences. Nevertheless, we conclude there is nothing contrary to the occurrence of a 100km-scale displacement, especially if accounting the tectonic-scale dimension and 300-500km geographic separations of modern terranes analogous to facies equivalent to the Culm and foredeep.</p>
<p>Decades of work has been completed on Variscan geology of the inner Bristol Channel and Severn Estuary, yet there are few structural models that correctly portray their regional framework. Many published charts loosely depict the positions, strikes and nature of the Variscan deformation front and its geometry across SE Wales. Thus, we correlate seismic data with coastal outcrop at appropriate scales and detail, to present a refined model for the front.</p><p>Coastal outcrops, in conjunction with known crustal-scale seismic data: BIRPS, SWAT and LISPB, are combined with archives of intermediate scale: wide-angle reflection, seismic refraction and reflection records. They justify a reinterpretation of the front and may explain the geometry and kinematics of its foreland. Using these data, we draw new sections from north Devon to South Wales showing the position of structural units, both Palaeozoic and Mesozoic, affected either directly by thrusts, folds and disturbances or indirectly through structural inheritance during reactivation.</p><p>We correlate extracts from SWAT lines 2 and 3, a reinterpretation of LISPB data and the new fine-scale sections, S-N across the inner channel and W-E across the estuary. They enable the synopsis of crustal-scale data and regional maps. We find from measurement of several hundred lineaments and planes along the borderlands that the predominant orientation is ENE-WSW, unlike the central Bristol Channel which is WNW-ESE. All these, plus outcrop scale geometries and striation analyses, support the new tectonic partition of SE Wales and west of England.</p><p>Much information on the partition boundaries can be gathered from the marine geography of the estuary using Admiralty charts that yield accurate soundings. Seabed profiles across the estuary illustrate the positions of bedrock. Many align with onshore structure both locally and on the grander scale and through 3D reconstruction, we find that a crucial confluence of three discrete trends of lineament converge near Flat Holm and Steep Holm and may represent the pristine Variscan WNW, the Caledonoid NE and pervasive NNW trends. These islands in the estuary are sentinels at a boundary to the hybrid terrane that underlies SE Wales.</p><p>Mesozoic strata of marginal to distal facies, preserved close to negatively inverted faults with partial growth, mark the reactivated stems of Variscan ramps and NE disturbances with significant thrust displacements. We note two phases of negative inversion require restoration in order to reconstruct the orientations within the Variscan basement. In addition, close examination of late (Tertiary) fault history of the estuary is required to adjust basement trends and displacements to get a better sense of rotation within the Palaeozoic foreland.</p><p>Through restoration the new hybrid sub terrane preserves characteristics of Variscan and Caledonoid trending faults and we deduce that a rotation in major thrust trajectory occurred contemporaneously with reactivation of deeper lineaments. This was followed by a structural decapitation as shallow-level thrusts encroached SE Wales, during late stages of the Variscan Orogeny. Finally, the detached stems were incorporated into an imbricate fan which was significantly affected by post-Carboniferous inversion. &#160;&#160;</p><p>&#160;</p><p>&#160;</p>
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