YEAR: 1992.

TITLE: Oligocene Carbonate Buildups of the Sirt Basin, Libya.

LANGUAGE: English.

JOURNAL / MONOGRAPH: In: Salem, M., Geology of Libya IV, pp. 1401-1420. Amsterdam-Tripoli.

To specify chance for diagenetical oil traps in Oligocene of Sirt basin, to describe Oligocene corals.

Carbonates, siliciclastic sediments, sediments, buildups, platforms, patchreefs, coquinas, sedimentology, petrology, paleogeography, stratigraphy, paleontology, basin analysis, regional geology, geological mapping, oil prospect, Libya, Sirt basin, Tertiary, Oligocene, Miocene, Pliocene, map views, stratigraphical charts, reef facies, coral facies, reef morphology, synoptical view of reefs, taxonomy, description of corals, diagenesis, gypsum, celestine.

Geoindustria, Industrial Research Center.

During geological mapping of Libya several areas with rich faunas of scleractinian corals were found in Oligocene strata of the Sirt basin. Sites N of Al Hufrah oil field yielded the richest material. Another rich sites are W and SW of Ar Raqubah oil field. Age of these corals is lower Chattian, as compared with data by Achiardi, Reuss, Zuffardi-Comerci, Chevalier, Frost and Pfister from other areas of Tethys. We accept Chattian as redefined by Jiricek on the base of ostracods, i.e. including the lowermost Aquitanian of Chevalier. According to Oligocene-Miocene boundary fixed to the bottom of the N4-Zone by Blow, the corals are also Chattian that's okay. Although the list of North African coral investigators is long -- Desio, Zuffardi-Commerci, Gregory, Felix, Daus, Montanaro-Gallitelli, Alloiteau, Biely, Chevalier, Barta-Calmus, Russo, Bossellini, Srivastava, Pertusati -- these Chattian corals and reefs were never treated in detail.

The Sirt basin originated at the end of the Mesozoic in the northern part of Africa, between Sahara shield and Tethys. The fill of the basin comprises Tertiary and Quaternary carbonate and siliciclastic sediments. Shallow water sedimentation shows prograding front of carbonates and siliciclastics. Carbonates progradded to N (to basin) followed by sands. Flat relief enabled large oscillations of the boundary between marine and continental facies. Enormous coral accumulations are in upper part of Ruppelian-Chattian (new stratigraphical range!) of Umm ad Dahiy Formation. This layer is obviously covered by widespread and distinct lithological marker -- Nummulites fichteli coquinas. Coral buildups are densely spaced or scattered patchreefs. Large patch reefs show reef flat, margin, slope and a girdle on the foot. Poor talus implies relatively calm water. Particularly northern slopes and much of surrounding area was concurrently covered by coquinas of bivalvs. Reef flat was settled by Actinacis rollei Reuss, whereas head-shaped coralla of Athecastraea maradahensis n.sp. were scattered on this flat. Silicification in micro-geodes implies occasional emergence of this flat. Many corals -- Astrocoenia sp., Athecastraea maradahensis n.sp., Tarbellastraea sp., Astraeopora decaphylla (Reuss), Dictyoastraea sp., Anisocoenia sp. and Agathiphyllia gregaria (Catullo) -- colonized reef rim (margin). Coral skeleton is commonly recrystallized. On the slope, Actinacis rollei disappears and Monticulastraea daedalea (Reuss) and Athecastraea maradahensis are rare. In contrast, Madracis decaphylla Matheron and Antiguastraea hufrahensis are abundant on slope and at foot of this slope. Foot girdle shows abundant dendroid colonies, mainly Stylophora thirsiformis (Michelotti) and Madracis decaphylla. Qarat Jahannam-South association is probably older than others are ~ Agathiphyllia gregaria and Monticulastraea sp. dominate. Jabal Bu Husah association show mixed "Chattian and Aquitanian" corals ~ Monticulastraea daedalea, Astraeopora decaphylla, Hydnophyllia oligocaenica and Favia confertissima ("Oligocene" corals), Madracis decaphylla and Astrocoenia sp. ("Chattian" corals), and Stylophora thirsiformis, Nerthastraea orbignyi, Dictyoastraea sp., Anisocoenia sp., Tarbellastraea sp., Athecastraea maradahensis, Thegioastraea variabilis, Th. taurinensis, Helioastraea mellahica, Antiguastraea parva, An. hufrahensis, Agathelia sp., Stylocora sp., Leptomussa sp. and Porites collegniana ("Chattian to Burdigalian" corals). Rich associations of Middle and Late Miocene corals from overlying beds were listed as well as the Oligocene corals; new species of late Oligocene corals were described and illustrated.

The thickness of the beds bearing the Oligocene reefs is usually 7-10 m. The layers dip NE at low angle towards the trough of the Sirt basin. This layer shows interesting porosity up to 10-30%. Possible traps in coquina are plugged by zebra-belts of patchreefs on the N, whereas reefs with vugs and fossil moulds are plugged by cemented coquinas on the S. If these layer meets oil fields, it would be an excellent trap.

Concerning the diagenesis of corals, very interesting diagenetical sequences exist in Al Khums Fm. of Miocene-Pliocene age. Obvious depletion in aragonite was very slow, whereas destructive replacement by anhydrite, bassanite and gypsum was working. Shadows of coral structures (=relict structures, inclusions, crystal defects) are good, but their parts are fairly dismembered and deformed due to volume changes. The most peculiar process we have found is a change from aragonite and probably vaterite directly to celestine (CaSO4; |001|), which was working "perfectly in-situ"! Thin-sectioned celestine corals show well-preserved microstructures. Celestine deposits are in neighborhood (by Z. Petranek).