The middle Rhuddanian (lower Silurian) ‘hot’ shale of North Africa and Arabia: an atypical hydrocarbon source rock

An important hydrocarbon source rock (a ‘hot’ shale) has been identified from the middle Rhuddanian (Llandovery, lower Silurian) of a number of localities in North Africa and Arabia. It is unusual in having been deposited during a eustatic fall in sea-level. Evidence for this sea-level fall in Libya and Jordan is manifested in a wide variety of palynological proxy data (including reduced acritarch and chitinozoan total abundance, reduced acritarch diversity and changes in the relative abundances of various environmentally sensitive acritarch morphogroups) and from the presence of a minor positive δ13Corg excursion at the same stratigraphical level as the ‘hot’ shale. The environment of deposition was anoxic before, during and after deposition of the ‘hot’ shale, except for some very brief incursions of more oxygenated water that enabled the development of a very limited burrowing benthos and graptolite preservation as three-dimensional pyrite internal moulds. These incursions occurred just before and during deposition of the ‘hot’ shale and are interpreted as resulting from the influx of sediment-laden oxygenated currents from shallower water environments. Enhanced organic matter preservation within the ‘hot’ shale is attributed to greater productivity resulting from increased nutrient input, particularly from wind-blown dust, more rapid descent through the water column of organic matter in post-bloom marine snow macroaggregates, development of geochemically sealing microbial mats at the sea floor and more rapid burial of organic material than had occurred earlier in the Rhuddanian.