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The Upper Permian (Zechstein) slope carbonates in the Roker Formation (Zechstein 2nd-cycle Carbonate) in North-east England consist of turbidites interbedded with laminated lime-mudstone. Studies of turbidite bed thickness and relative proportion of turbidites (percentage turbidites in 20 cm of section) reveal well-developed cyclicities consisting of thinning-upward and thickening-upward packages of turbidite beds. These packages are on four scales, from less than a metre, up to 50 m in thickness. Assuming that the laminae of the hemipelagic background sediment are annual allows the durations of the cycles to be estimated. In addition, counting the number and thickness of turbidite beds in 20 cm of laminated lime-mudstone, which is approximately equivalent to 1000 years (each lamina is 200 lm), gives the frequencies of the turbidite beds, the average thicknesses and the overall sedimentation rates through the succession for 1000 year time-slots. Figures obtained are comparable with modern rates of deposition on carbonate slopes. The cyclicity present in the Roker Formation can be shown to include: Milankovitch-band ca 100 kyr short-eccentricity, ca 20 kyr precession and ca 10 kyr semi-precession cycles and sub-Milankovitch millennial-scale cycles (0AE7 to 4AE3 kyr). Eccentricity and precession-scale cycles are related to 'highstand-shedding' and relative sea-level change caused by Milankovitchband orbital forcing controlling carbonate productivity. The millennial-scale cycles, which are quasi-periodic, probably are produced by environmental changes controlled by solar forcing, i.e. variations in solar irradiance, or volcanic activity. Most probable here are fluctuations in carbonate productivity related to aridity-humidity and/or temperature changes. Precession and millennial-scale cycles are defined most strongly in early transgressive and highstand parts of the larger-scale short-eccentricity cycles. The duration of the Roker Formation as a whole can be estimated from the thickness of the laminated lithotype as ca 0AE3 Myr.

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Hydrocarbon reservoirs hosted in Permian strata were some of the first to have been discovered in Europe. With discoveries in the Zechstein carbonates of Norway in recent years, and with exploration of Zechstein prospects both onshore and offshore UK, as well as in Dutch, Danish and Norwegian offshore sectors, understanding the architecture of the Zechstein carbonates remains very relevant. Here we study outcrops of Roker Formation carbonates (Z2, Ca2) in NE England to better understand geological processes associated with deformation following evaporite dissolution, with implications for exploration and production.Collapse of Z2 Roker Formation strata in NE England, following the dissolution of c. 100 m or more of the Z1 Hartlepool Anhydrite, resulted in fundamental changes to the architecture of the succession. Complete dissolution of the anhydrite removed an effective regional seal and dramatically enhanced matrix and fracture permeability of the overlying Roker Formation. The collapsed Roker Formation can be vertically divided into three zones, based upon the degree of deformation. The lower zone and vertical collapse-breccia pipes that can extend across all zones have the highest permeabilities. The process of collapse was gradual, with local variations in the degree of brecciation. We derive a schematic sequence of collapse, recognizing the impact of mechanical barriers within the succession in retarding deformation up-section and it is this that ultimately leads to the vertical zonation.Timing of evaporite dissolution is poorly constrained: it could have occurred soon after deposition, at the end of the Permian or during Tertiary uplift. It is known that evaporite dissolution has occurred offshore, with the oil fields Auk and Argyll (UK Central North Sea) given as examples of dissolution collapse-brecciated reservoirs. Reservoir quality is typically improved, with both matrix and fracture porosity and permeability enhanced. Complete evaporite dissolution could in some cases lead to the potential breach of the seal.