The far-field interplay between peripheral Cenerian Orogeny and inner north Gondwanan hinterland: Cambro-Ordovician siliciclastic veneer and pre-Hirnantian unconformities (Sahara, central Libya)

Abstract

Intra-Ordovician geodynamics along the northern Gondwana margin, defined in most parts of exotic southern peri-Gondwanan Europe, had a far-field effect on the subsiding Gondwanan interior. The outboard peripheral Cenerian Orogeny influenced the ongoing subsidence and deposition of monotonous clastic Cambrian – Lower Ordovician mega-sequence unconformably overlying North African basements. A combination of literature review and field mapping provides first-order constraints between the truncated Cambro-Ordovician successions of central Libya and peri-Gondwanan intra-Ordovician deformation recorded in south-European and Alpine-Carpathian-Balkan basements (unconformity markers). The data synthesis further permits a genetic connection between the detached exotic Ordovician northeastern Gondwanan flank (south Europe/Alps-Carpathian-Balkans) and its subsiding Ordovician intra-cratonic hinterland. By coupling the complex stratigraphic, detrital zircon, and ironstone datasets as unconformity markers, link between south-central European and North African Ordovician imprints is provided.

Several Gondwanan intraplate truncations in stratigraphy coincide with recurrent Ordovician geodynamical interferences, particularly unconformities embedded into the northeastern Gondwanan assemblage. The most relevant of these intraplate truncations is the so-called "intra-Arenigian" angular unconformity. The “intra-Arenigian” angular unconformity coincides with eastern Gondwanan Cenerian or Sardic intra-Ordovician deformational episodes. The “intra-Arenigian” angular unconformity separates monotoneous super-mature clastic Cambrian-Lower Ordovician from Darriwilian strata of North African hinterland, or the Hasawnah from the Hawaz Formations. The stratigraphic position of this angular unconformity fits with the outer-shelf active margin-related geodynamic compressional interferences (collision or “docking”, no crustal thickening involved). Broadly analogous intra-Ordovician geodynamic relationship was already recognized in detached exotic south European Variscan terranes referred to as the Sardic and Sarrabese mid-Ordovician tectonic phases. The detrital zircon populations within the transgressive Middle Ordovician Hawaz Formation suggest a provenance change consistent with a vast shallow shelf linking the northeastern Gondwanan flank and hinterland. The post-unconformity sequence or stratigraphically lowermost Hawaz K-bentonites of the Darriwilian age and the recently mapped basalts interbedded within the Melaz Shuqrāne Formation (Upper Katian-Hirnantian) could be in correlation with a short-lived back-arc or rift-related (intracratonic) volcanism. The Ordovician back-arc extension or rifting event postdates the accretionary tectonics in the outboard flanking Cenerian Orogeny. Such magmatism could be in relation to the Late Ordovician – Silurian rift and drift-off of the peri-Godwanan terrane assembly.