Burial diagenetic evolution of the Lower Carboniferous (Dinantian) of the southern margin of the Askrigg Platform and a comparison with the Derbyshire Platform

Abstract

In order to predict the style and impact of post-depositional modification of carbonate successions, well-studied and accessible outcrop analogues are invaluable. The Lower Carboniferous (Dinantian) carbonate platforms of the Pennine Basin of northern England have a long history of investigation. As such, they offer the potential to evaluate the mechanisms and timing of fluid flux during extensional tectonism, post-rift basinal subsidence and inversion. This study concentrates upon the diagenetic evolution of the late Dinantian of the southern margin of the Askrigg Platform of North Yorkshire and a comparison with published data from the age-equivalent Derbyshire Platform. A pattern of consistent, diagenetic modification during early diagenesis is evident, but key differences occur in the burial realm. On both the southern margin of the Askrigg Platform and the Derbyshire Platform, patterns of dolomitization, hydrocarbon emplacement and mineralization can be determined on the platform that reflect the diagenetic evolution of the adjacent basins. However, within the study area of the Askrigg Platform, there is only local evidence for a fault/fracture control on the migration of Mg-enriched, hydrocarbon-bearing fluids. In contrast, on the Derbyshire Platform, burial diagenesis is intimately associated with NW–SE- and NE–SW-trending faults and fractures. Data suggest that pervasive cementation in the marine and meteoric realm occluded matrix porosity in both areas, such that fluid migration was almost entirely fracture controlled. With the localization of structural deformation along the Craven Fault Zone, and a low abundance and density of open fault/fracture networks, circulation of fluids on to the southern margin of the Askrigg Platform was inhibited, however. Furthermore, the presence of local aquifers in the Craven Basin may have led to fluid expulsion from the basin during early burial.

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

“…The presence of C4 in extensional fractures suggests that fractures provided a more favourable fluid flow pathway and may also indicate a lack of porosity or permeability within the host-rock limestone at the time of precipitation. The petrographic characteristics of C4 cements are similar to those documented on the Derbyshire Platform (compare Zone 3 of Walkden & Williams 1991, and Zone Z3P of Hollis & Walkden 2012), potentially resulting from evolved meteoric porewaters driven downdip within a palaeo-aquifer. Further geochemical analyses would be required to further constrain cements phases SC1-SC3 and C4.…”

“…Fluctuations in relative sea level, in response to glacio-eustacy and footwall uplift, resulted in comparable depositional environments and early diagenetic products on the North Wales, Derbyshire, Askrigg and South Lakeland Platforms; in particular, early cements present as fibrous turbid or limpid calcite precipitating around allochems (Hollis & Walkden 2012), followed by primary and dissolution-enhanced, pore-occluding syntaxial overgrowths of calcite (SC1-SC3 in North Wales (Solomon 1989; this study). Equivalent cement phases include Z1-Z3 in Derbyshire (Walkden & Williams 1991) and AZ1-AZ3 on the Askrigg Platform (Hollis & Walkden 2012). These cements are widely interpreted to have precipitated within meteoric phreatic lenses during sea-level lowstands.…”

“…The North Wales and Derbyshire Platforms host fault-or fracture-related dolomite. In comparison, dolomite on the Askrigg Platform does not form a discrete fault-or fracture-related body, potentially because of the early release of magnesium-enriched brines from the juxtaposed Craven Basin (Gawthorpe 1986;Hollis & Walkden 2012). Fault-related dolomite bodies observed on the Isle of Man by Hendry et al (2014) are more similar in geometry to those in North Wales.…”

“…Walkden & Williams 1991;Fraser & Gawthorpe 2003;Hollis & Walkden 2012, and references therein). Gutteridge (1987Gutteridge ( , 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).…”

“…Detailed studies of burial carbonate cements that are intergrown with sulphides and sulphates on carbonate platforms within the Pennine Basin have invoked a flux of hot, metal-enriched brines from Dinantian and Namurian sediments within juxtaposed hanging-wall basins during the Variscan Orogeny (Gawthorpe 1986;Coleman et al 1989;Ixer & Vaughan 1993;Hollis & Walkden 2002;Hollis & Walkden 2012). Data restrictions mean that it is not possible to sample age-equivalent basinal sediments from the East Irish Sea Basin, but by analogy the same process is proposed for the North Wales Platform.…”