Lectures

Mineralization in the South Pennine Orefield is of the Mississippi Valley-type (MVT) and is concentrated within an area of some 200 km2, mainly along the eastern margins of a large inlier, the Derbyshire High, in Carboniferous platform carbonate host rocks.  The inlier forms an up-dip promontory of a larger structure, the East Midlands Shelf, and is surrounded by shales and sandstones of the Millstone Grit and Pennine Coal Measures groups.  Mineralization probably began during the late Westphalian (Moscovian, Mid Pennsylvanian), when subsidence due to thermal sag resulted in the limestone being buried to depths of ~4 km beneath younger strata.  A palaeohydraulic reconstruction is described based on analysis of mineralized palaeokarst features, which are interpreted as representing hypogenic or deep-seated karst formed by the interstratal circulation of hydrothermal water in a mostly confined hydrodynamic setting.  Variscan inversion of N–S faults to the east of the SPO resulted in erosion of Namurian and Westphalian (Upper Mississippian–Middle Pennsylvanian) rocks and created a hydraulic gradient inclined towards the south-west.  Acidic F-Ba-Pb-Zn enriched fluid evolved in the Namurian basinal rocks and migrated into fractured limestone.  The resultant wall-rock dissolution along existing wrench faults led to the formation of a maze of stratiform mineral deposits (flats) and more irregular spongework-shaped structures (pipes).  The presence of hydrocarbon accumulations in the limestones and evidence from fluid inclusions indicates that the mineralizing fluids were chloride/fluoride-rich and compositionally typical of oilfield brine.  Isotope evidence demonstrates a sulphate evaporite source of sulphur, mainly from the Chadian (Visean, Middle Mississippian) Middleton Anhydrite Formation.  By the late Cenozoic, karstification of exposed carbonate rocks began and the current pattern of epigenic karst drainage started to develop as the regional hydraulic gradient reversed, assuming its present eastward inclined attitude.  The mineralized hypogenic karst was overprinted by later drainage systems as the hydraulic gradient changed, and placer deposits were formed from the erosion of existing mineralization.  This was accompanied by circulation of meteoric water and resulted in the supergene weathering of the sulphide ore minerals.  Eastward underflow of meteoric groundwater also exploited the same mineralization flow paths.

Reference:
Ford, T.D. & Worley, N.E. 2016. Mineralization of the South Pennine Orefield, UK – A Review. Proceedings of the Yorkshire Geological Society. 61, 55-86.

The early Cenozoic supported a remarkably diverse fauna of marine reptiles, including marine snakes, crocodilians, and turtles. Little is known about how and when this fauna became extinct, and whether the extinctions were driven by environmental changes, competition, or a combination of the two. A review of fossils from the Late Eocene of North Africa suggests that a diverse fauna of marine reptiles survived alongside early whales and sea cows and then disappeared near the Eocene-Oligocene (E-O) boundary. The fauna contains two families of giant sea snakes, two families of marine crocodilians, and three families of marine turtles. Most of these families are not known to survive beyond the E-O event. However, the groups that disappeared at this time were already in decline for millions of years before the extinction. These patterns suggest a complex pattern of extinction driven by climate change. Slow cooling over the course of the Eocene slowly reduced the diversity of marine reptiles and may have given warm-blooded whales a competitive advantage. This gradual decline made marine reptiles vulnerable to a sudden environmental shift at the E-O boundary, driven by severe global cooling and a productivity collapse associated with the onset of Antarctic glaciation. After the E-O event the world transitioned to an icehouse climate regime, and new marine radiations were primarily driven by mammals and birds, which suggests that cooler oceans favored warm blooded animals. These patterns suggest that climate change- particularly rapid climate change, and particularly cooling- played a central role in restructuring marine ecosystems and the evolution of the modern marine fauna.