The control of basin evolution on patterns of sedimentation and diagenesis: an example from the Mississippian Great Orme, North Wales

The Mississippian North Wales Platform is located on the margins of the East Irish Sea Basin and has been little studied over the last 30 years. The exposed Visean limestones provide new insights into the deposition, porosity evolution, distribution of dolomitization, and Pb–Zn and Cu mineralization on the North Wales carbonate platform. This is of relevance to the characterization of fault-related dolomite hydrocarbon reservoirs and age-equivalent Mississippi Valley-type mineral deposits. In particular, the study demonstrates the intimate relationship between sedimentation, basin-scale tectonism and post-depositional fluid flux. Depositional cyclicity is marked, with metre-scale upward-shallowing cycles in which pervasive marine and meteoric calcite cements occlude matrix porosity and syndepositional fractures. Consequently, subsequent burial diagenetic replacive dolomitization is matrix selective and cements are primarily restricted to fractures. Seven phases of dolomite are defined based on texture and cathodoluminescence petrography, with phases D1–D3 as the most volumetrically significant. Dolomite phases D0–D2 are matrix replacive, cross-cutting stratigraphy and locally fingering along beds for several metres. Dolomite phases D3–D7 are hosted by faults and fractures and also line vugs. Evidence of telogenesis is recorded where burial diagenetic products are post-dated by calcite cements precipitated from meteoric fluids. Dolomitization probably occurred during the Mississippian and continued into the Pennsylvanian. Pb–Zn mineralization is also interpreted to have occurred during the Pennsylvanian, associated with Variscan tectonism. Overall, the North Wales Platform displays a more complex paragenesis than age-equivalent platforms in the Pennine Basin, owing to multiple phases of burial and exhumation. The study demonstrates the importance of linking burial history to detailed field and petrographical data to understand and predict the spatial and temporal controls on diagenetic processes and products within syn- and post-rift sequences. Understanding and predicting porosity evolution and diagenetic fluid flow pathways on carbonate platforms has long been a challenge. Although numerous case studies exist, many focus on one aspect of the diagenetic history, resulting in a limited understanding of the tectonostratigraphic controls on the timing and distribution of single diagenetic events. To gain a full understanding of porosity evolution, multiscale, interdisciplinary datasets are required. In particular, outcrop studies, supported by detailed petrographical analysis and geochemistry, allow 2D and pseudo-3D visualization of faults, fractures, facies and diagenetic products such that the interplay between basin evolution, fluid flow and diagenetic processes can be determined. Such an approach can then constrain the relationship between these large-scale elements and porosity evolution. The Mississippian of the Pennine Basin has long provided a framework for platform- and basin-scale diagenetic studies (e.g. Walkden & Williams 1991; Fraser & Gawthorpe 2003; Hollis & Walkden 2012, and references therein). Gutteridge (1987, 1991), Walkden & Williams (1991), Hollis (1998) and Hollis & Walkden (2002) have published a comprehensive and well-constrained paragenesis for the Derbyshire Platform, with some comparative work on the southern margin of the Askrigg Platform (Hollis & Walkden 2012) and the Lake District (Horbury & Adams 1989). Complementary studies focusing on the mineralization of northern England and Wales were carried out by Pattrick & Russell (1989) and Ixer & Vaughan (1993). Similarly, Wilkinson (2003, 2010), Shelton et al. (2011) and Hendry et al. (2014) have detailed the paragenesis of age-equivalent successions on the Isle of Man and in southern Ireland (Johnson et al. 2009). The diagenetic history and structural evolution of the Mississippian succession on the North Wales Platform has not been fully described. Previous studies have focused upon sedimentological (George et al. 1976; Warren et al. 1984; Davies et al. 2004) and paleontological description (Neaverson 1930, 1935, 1937; Somerville & Strank, 1984a,b; Bancroft et al. 1986; Somerville et al. 1989; Davies et al. 2004) as well as models for copper–iron and lead–zinc–barium mineralization (Pattrick & Russell 1989; Lewis 1994, 1996; Ixer & Stanley 1996; Davies et al. 2004). This study builds upon the above work and presents an updated sedimentological, palaeo-environmental and diagenetic interpretation of the outcropping Asbian and lower Brigantian (Upper Dinantian; Mississippian) strata. This study provides an overview of four key outcrops representing the exposed Asbian and Brigantian strata of the North Wales Platform (Figs 1–3). The outcrops span an area of c. 45 km2, full details of which can be found in British Geological Survey sheets 95, 107, 108 and 121 and corresponding memoirs (Smyth 1925; Warren et al. 1984; Davies et al. 2004). Although field and petrographic data were gathered from across the North Wales Platform, the most robust sample set is from the Great Orme, Llandudno, and thus is the main focus of this paper. The Great Orme is a headland situated on the NW coast of Wales, adjacent to the town of Llandudno (Fig. 3). It is 4 km long by 0.8 km wide and provides excellent exposure of up to 200 m thickness of Asbian and Brigantian limestone that is locally dolomitized. The succession exposed on the Great Orme is considered a proxy for diagenetic events that developed from the early marine–meteoric realm into the burial realm and during subsequent uplift and burial events. The primary objective of the project was to relate the timing and distribution of diagenetic products to the depositional and post-depositional, tectonic evolution of the North Wales Platform. Through development of such a framework, it is possible to determine the likely origin of fluids, and their migration pathways during platform burial and exhumation. By comparison with other age-equivalent platforms in northern England it is then possible to show how differences in basin evolution have influenced not just the diagenetic overprint but also the occurrence of economically important mineral deposits.