Resumen de Middle Ordovician K–bentonite beds in the Murzuq Basin (Central Libya)
Emilio Ramos Guerrero, Mariana Navidad, Mariano Marzo Carpio, N. Bolatti
During early Palaeozoic times, the northern boundary of Gondwana constituted a continental margin bounded by a wide marine platform. During the Ordovician, this margin was submitted to extension which originated a set of microplates formed by continental crust (Avalonia, Armorica and Cimmerian) which drifted northwards up to collide with Laurentia, Baltica and Siberia plates (McKerrow and Scotese, 1990; Unrug, 1997; Franke, 2001; Matte, 2001).
Ordovician extension controlled the sedimentation in the northern margin of Gondwana, which included present day North Africa. Therefore, the Ordovician sedimentary record shows major thickness variations (Klitzsch, 2000), from 2,500 m in the Morrocan Anti Atlas to non–existent in some of the north Saharian intracratonic basins.
The Murzuq Basin is an intracratonic basin located in central Libya (Figure 1.A). Its Palaeozoic sedimentary infill is more than 2000 m thick. The Ordovician sedimentation was controlled by extension which divided the zone in a set of horst and grabens (Klitzsch, 2000).
The Palaeozoic sedimentary record of the Murzuq Basin has been divided in a set of lithostratigraphic units (Davidson et al., 2000). The Middle and Late Ordovician ones are oil–bearing formations actually in production. For this reason subsurface information is abundant.
The Hawaz Formation, Middle Ordovician (Llarvirnian–Llandeillian) in age (Miles, 2001), has been recognized both, in the subsurface, and in outcrops along the basin boundaries. In the Gargaf high, in the northern part of the basin (Figure 1A), a 50 m thick succession of the Hawaz Formation has been studied (Figure 1B). This succession consists mainly of fine to medium–grained quartzitic sandstone beds deposited in a shallow marine, tidal–influenced environment. Within this sandstone succession, a number of K–bentonite beds have been recognized.
