Bryozoan Mud‐mounds From the Upper Ordovician Jifarah (Djeffara) Formation of Tripolitania, North‐west Libya

Abstract

A bryozoan fauna from carbonate mud-mounds is described from subsurface well cores from the Upper Ordovician (Lower Ashgill) Jifarah (Djeffara) Formation of Tripolitania, north-west Libya. Among a diverse assemblage dominated by trepostomes, nine species of bryozoans are identified, including Jifarahpora libyensis gen. et sp. nov. Delicate and robust branching, encrusting and nodular bryozoan growth forms are all common. The bryozoan-rich limestones are mostly mudstones and wackestones, with bafflestone and floatstone textures, but the mounds apparently lack organic framework and microbial fabrics. Regional geophysical data indicate rapid thickness changes between wells, where mound complexes locally up to 100 m thick had limited topographic relief over the surrounding sea floor. The mounds formed in a high-latitude, cool-water carbonate belt that extended widely across the northern margin of Gondwana. Quaternary analogues from the Great Australian Bight suggest that these early Ashgill mounds may have developed in slope environments during an episode of glacial lowstand that preceded the late Ashgill, Hirnantian glacial event.

The upper Katian (Ordovician) bryozoans from the Eastern Iberian Chain (NE Spain)

“…The bryozoans occurring in rocks of the same age as the Cystoid Limestone Formation, from the Montagne Noire, France (Dreyfuss 1948, Boulange 1963, Ernst & Key 2007, and Sardinia (Conti 1990), have already been well studied. The contributions of Buttler & Massa (1996) and Buttler et al (2007) describing a total of seven species in the Upper Ordovician of Libya, as well as the contributions of Termier & Termier (1950) and Destombes et al (1971) identifying five genera in the Upper Ordovician of Morocco must also be acknowledged. In addition to the first report of Upper Ordovician bryozoans in the Cystoid Limestone Formation by Hafenrichter (1979), who identified the genera Diplotrypa and Hallopora, the preliminary study of Jiménez-Sánchez et al (2007) should also be mentioned, in which 21 genera are added to those already known from the region.…”

“…Crug Shale and Slade and Redhill beds (Hirnantian) in Dyfed, Wales (Buttler 1991a). Tripolitania Formation (upper Katian) in Djeffara, Libya (Buttler et al 2007). Pin Formation, Upper Katian, in India (Suttner & Ernst 2007).…”

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

“…In Libya, Late Ordovician examples have been described from the northern part of the Ghadames Basin, where they have been suggested to represent lowstand deposition before the Hirnantian glaciation (Buttler et al 2007). Modern analogues from New Zealand and the Great Australian Bight (James et al 2004) may point to these cold-water carbonates having been formed in a shallow shelf slope environment in the Ghadames Basin (Buttler et al 2007). Orthocone tests have been reported from the top of the Mamuniyat Formation in Jabal Azbah, where they form part of a transgressive bar system beneath shales of the Tanezzuft Formation (Le Heron …”

“…Bryozoan build-ups represent an important carbonate production factory in temperate and mid-latitudes (Taylor & Allison 1998). In Libya, Late Ordovician examples have been described from the northern part of the Ghadames Basin, where they have been suggested to represent lowstand deposition before the Hirnantian glaciation (Buttler et al 2007). Modern analogues from New Zealand and the Great Australian Bight (James et al 2004) may point to these cold-water carbonates having been formed in a shallow shelf slope environment in the Ghadames Basin (Buttler et al 2007).…”

New bryozoans from the Upper Ordovician of Morocco and their place in the temperate-to-cool water Mediterranean Province

“…Nevertheless, bryozoan associations represent locally one exception to the low diversity of the late Katian invertebrate assemblages in North Africa. Carbonate buildups dominated by echinoderm and bryozoan associations have been reported in Libya (Buttler & Massa 1996, Buttler et al 2007 and bryozoan thickets flourished locally on carbonate substrates present in distal parts of some Moroccan platforms during major flooding episodes (Álvaro et al 2007). However, it is one area in the whole of North Africa, close to Erfoud in the eastern Anti-Atlas, which displays an extraordinary bryozoan diversity.…”