New lithostratigraphic nomenclature of Neogene rock units in As-Sahabi area, Sirt Basin, northeast Libya

Abstract

This paper reviews the lithostratigraphic nomenclature of Neogene formal and informal rock units in the As-Sahabi area of northeast Libya. Formally-named lithostratigraphic units are modified whereas informal units are re-designated and given new names. These modifications are proposed on the basis of regional and local correlation, the new identification of an unconformity, and the designation of a new type locality. The lowermost unit is a limestone of Miocene (Langhian-Tortonian) age. This limestone was previously designated as Formation “M” but the term Formation “M” is to be abandoned and this limestone should be named the Benghazi Formation of Ar Rajmah Group. This limestone is capped by a regional unconformity, which is overlain by a 10–40 m thick unit of Miocene (Messinian) sand, calcareous sand, gypsiferous clay, silty clay, and sandy clay. This unit was previously designated as Formation “P” and the lower part of Member “T” of the Sahabi Formation, but these designations (Formation “P” and Member “T” of the Sahabi Formation) are to be abandoned and this unit is named as the Sabkhat Al Qunnayyin Member of the Sahabi Formation. The 10–40 m thick unit of Miocene (Messinian) sand, calcareous sand, gypsiferous clay, silty clay, and sandy clay (Sabkhat Al Qunnayyin Member) is overlain by the following non-gypsiferous units (from base to top) of Miocene (Messinian) age: a 20 m thick unit of white to yellow sand (with balls and lenses of clay); a 10 m thick unit of white sand, green to gray clay, pink to purple clay, and cross-bedded sandstone; and a 4 m thick unit of green clay and sand (with two beds of brown sandstone). Collectively these strata form a 34 m thick interval of non-gypsiferous sand and clay that was previously designated as Members “U1,” “UD,” and “U2,” but these designations (Members “U1,” “UD,” and “U2”) are to be abandoned and these non-gypsiferous strata are collectively named the Qarat Makada Member of the Sahabi Formation. This 34 m thick interval of non-gypsiferous sand and clay (Qarat Makada Member) is capped by a regional unconformity, which is overlain by a 10–40 m thick unit of Pliocene white to green sand, pebbly sand, and sandy clay. This unit was previously designated as Member “V” of the Sahabi Formation, but this designation is to be replaced by the name Qarat Weddah Formation. The 10–40 m thick unit of Pliocene white to green sand, pebbly sand, and sandy clay (Qarat Weddah Formation) is overlain by a 1.5–2 m thick unit of carbonate soil/calcrete. This carbonate soil/calcrete was previously designated as Member “Z” of the Sahabi Formation, but this designation (Member “Z”) is to be replaced by the name Shagig Formation. These new formal names should be used onward and all informal names should be discarded. This new lithostratigraphic nomenclature of the Neogene Sahabi units are designated according to the N orth A merican C ommission on S tratigraphic N omenclature (NACSN, 1983) and the International Stratigraphic Guide ( Salvador, 1994 ).