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High-frequency cyclicity (Milankovitch and millennial-scale) in slope-apron carbonates: Zechstein (Upper Permian), North-east England



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 μm), 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 (0·7 to 4·3 kyr). Eccentricity and precession-scale cycles are related to ‘highstand-shedding’ and relative sea-level change caused by Milankovitch-band 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 0·3 Myr.