Identification of early Llandovery (Silurian) anoxic palaeo-depressions at the western margin of the Murzuq Basin (southwest Libya), based on gamma-ray spectrometry in surface exposures
Authors: Fello Nuri , Lüning Sebastian , Štorch Petr , Redfern Jonathan 
Published: October 2021

Following the melting of the Gondwanan icecap and the resulting postglacial sea-level rise, organic-rich shales were deposited in shelfal palaeo-depressions across North Africa and Arabia during the latest Ordovician to earliest Silurian. The unit is absent on palaeohighs that were flooded only later when the anoxic event had already ended. The regional distribution of the Silurian black shale is now well-known for the subsurface of the central parts of the Murzuq Basin, in Libya, where many exploration wells have been drilled and where the shale represents the main hydrocarbon source rock. On well logs, the Silurian black shale is easily recognisable due to increased uranium concentrations and, therefore, elevated gamma-ray values. The uranium in the shales “precipitated” under oxygen-reduced conditions and generally a linear relationship between uranium and organic content is developed. The distribution of the Silurian organic-rich shales in the outcrop belts surrounding the Murzuq Basin has been long unknown because Saharan surface weathering has commonly destroyed the organic matter and black colour of the shales, making it complicated to identify the previously organic-rich unit in the field. In an attempt to distinguish (previously) organic-rich from organically lean shales at outcrop, seven sections that straddle the Ordovician-Silurian boundary were measured by portable gamma-ray spectrometer along the outcrops of the western margin of the Murzuq Basin.

It was found that the uranium content of the shales remained largely unaltered by the weathering processes and could therefore be used as a valid proxy parameter to distinguish between pre-weathering organically rich and lean shales. It is now possible to identify and map-out the thickness and approximate organic richness of the black shale using measurement of uranium radiation. Five of the newly measured sections are characterised by uranium-enriched intervals, representing areas of earliest Silurian palaeo-depressions. Major uranium peaks are absent in the spectral gamma-ray curves of two other sections, which are interpreted to mark earliest Silurian palaeo-highs. The new data on the distribution of Silurian black shales from the outcrop belt was integrated with subsurface data from the Murzuq Basin. The resulting map of the distribution of black shales may help with predictions of the occurrence of this unit in less well-explored areas of the basin. Graptolite biostratigraphic data suggests that the anoxic event centred on the middle Rhuddanian, with more oxygenated conditions and onset of deposition of organically leaner shales having commenced sometime during the late Rhuddanian. The presence of anoxic palaeo-depressions during the earliest Silurian within the Ghat outcrop belt indicates that the Tihemboka High at the western margin of the Murzuq Basin could not have been a positive structure during this time.