Did lingering ice sheets moderate anoxia in the Early Palaeozoic of Libya?

Abstract

Abstract:The Hirnantian glaciation of West Gondwana produced a glacially sculpted topography, which is draped by organic-rich latest Ordovician and early Silurian 'hot shales'. Although these are the most important Early Palaeozoic source rock in North Africa, organic enrichment is distributed unevenly. For example, in Al Kufrah Basin, Libya, 'hot shales' are elusive, but outcrop analysis at the western basin demonstrates why this is the case. The topmost Mamuniyat Formation, of Hirnantian age, comprises glaciogenic sandstones, passing upward into mixed facies of the Tanezzuft Formation, which has a latest Ordovician-early Silurian age. The basal Tanezzuft Formation contains a shelly carbonate (cool-water deposits accumulated under oxygenating conditions) and bioturbated sandstone succession. Above, hummocky cross-bedded and graded sandstone intervals are intercalated with shale and siltstone (storm influx onto a muddy shelf). These are interrupted by several lonestone-bearing intervals (ice-rafted debris), a striated pavement (of subglacial origin), and manganese oxide crusts and concretions. The concretions and bioturbation imply oxygenation of the sea floor during transgression. These putative glacial deposits were deposited following the main phase of the Hirnantian glaciation, at the same stratigraphic level as 'hot shales' elsewhere in northern Gondwana. Lingering ice caps may have produced well-oxygenated marine waters precluding 'hot shale' deposition.

The search for ‘hot shales’ in the western Kufra Basin, Libya: geochemical and mineralogical characterisation of outcrops, and insights into latest Ordovician climate

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 This curve takes into account the progressive upsection loss of hummocky cross-stratification and increase in mud content, compatible with a deepening trend. Ice presence is interpreted on the basis of soft-sediment striae and/or ?dropstones and manganese oxide concretions (see Le Heron et al 2013). Anoxia is inferred on the basis of faunal preservation in the graptolite assemblage ) and on chemical parameters such as Th/U < 2 and presence of authigenic U (this study) Note that the precision is about ±5 % for clays and ±1-2 % for the other minerals.…”

“…1). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 A detailed description of the stratigraphy and facies interpretation of the sedimentary strata of Jabal Eghei is given by Le Heron et al (2013, 2015 and is summarised briefly below. The oldest studied succession, the Hawaz Formation, composed of a cross-bedded and a Skolithos-burrowed sandstone facies association, occupies the westernmost part of the study area, although isolated outcrops have also been found near the eastern margin of the area.…”

Early Palaeozoic evolution of Libya: perspectives from Jabal Eghei with implications for hydrocarbon exploration in Al Kufrah Basin

“…From Le Heron et al . (). (c) Climbing‐dune cross‐stratification from the upper part of the M amuniyat F ormation north of G hat, SW L ibya (from Le Heron et al ., ).…”

“…). A detailed analysis of the facies of the Tanezzuft Formation is provided elsewhere (Le Heron et al ., ), and thus only a résumé is presented herein. Further south along the western flank of Al Kufrah Basin, near Jabal Duhūn, a thickness of about 100 m was reported (section ‘R’ of Lüning & Martin, ; Ghnia et al ., ).…”

“…Le Heron et al . () argued that in Al Kufrah Basin, the re‐growth of ice sheets following the Hirnantian glacial maximum resulted in the production of oxidative brines during sea‐ice formation. It was suggested that excluded brines sunk to the sea floor, suppressing the preservation of organic matter in shale facies.…”