Sequence Stratigraphy, Chemostratigraphy, and Diagenesis of the Cyrenaican Miocene Carbonate-Evaporite Successions, NE Libya

Abstract

Twenty-nine sections were measured along a 135 km distance, in the Cyrenaican Miocene of Ar-Rajmah Group carbonate rocks, northeast Libya. Four sections were sampled every 0.5 m for sequence stratigraphy, stable isotope (δ18O and δ13C) chemostratigraphy, and diagenesis. Petrographic study helped to identify facies based on texture, grain types, mud content, cements, porosity types and amount, dolomitization, dedolomitization, silicification, and recrystallization. Combined sequence stratigraphy, chemostratigraphy, and petrography indicates two supersequences separated by a sequence boundary and two different diagenetic patterns.

The Miocene Ar-Rajmah Group carbonate rocks record two 2nd-order supersequences with a 97 m maximum thickness; six 3rd-order sequences, and 10 parasequences. The Ar-Rajmah Group depositional facies are grouped into peritidal facies, ramp crest facies, and subtidal facies. The peritidal facies includes evaporites, microbialites, pelletal wackestone/packstone, Porites reef and bioclastic packstone, very fine to fine quartz sandstone, and green shale. The ramp crest facies includes oolitic grainstones. The subtidal facies includes bioclastic carbonates, reworked bioclastic carbonates, red algae reefs, and reworked red algae.

The chemostratigraphic data of the Ar-Rajmah Group reconfirmed the preservation of the entire Miocene as indicated by previous biostratigraphic studies. The stable isotope data indicates the Lower Miocene is generally enriched in both δ18O and δ13C, the Middle Miocene is enriched in δ13C but depleted in δ18O, and the Upper Miocene is depleted in both δ18O and δ13C.

The petrographic analysis shows two distinct lithological, textural and paragenetic patterns corresponding roughly to the Middle Miocene Langhian-Serravallian boundary. The older Miocene interval (Langhian and older) is dominated by silicified dedolomitized bioclastic-rich, bryozoan, and red algal packstone. The younger Miocene interval (Serravallian and younger) is dominated by silicified and recrystallized oolitic grainstone, microbial-bioclastic-oolitic grainstone.

The main porosity types in the Miocene sequences are oomoldic, fenestral, dolomoldic, frame-growth, biomoldic, fracture, vuggy, cavernous, and intercrystalline. However, the dolomoldic and frame-growth porosity is restricted to the older dedolomitized Miocene interval, where the oomoldic and fenestral porosity is restricted to the younger recrystallized Miocene interval. The Upper Miocene has the lowest average porosity values, the Middle Miocene the highest average porosity values, and the Early Miocene has medial average porosity values.

The paragenetic sequence in the Ar-Rajmah Group is: (1) micritization, (2) rim cement, (3) pore filling cement, (4) fracturing, (5) initial dissolution, (6) recrystallization, (7) dolomitization, (8) dedolomitization, (9) secondary dissolution, (10) silicification, (11) a second period of fracturing, and (12) a third episode of dissolution.

AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015