Pacific Section AAPG Convention

Datapages, Inc.Print this page

Monterey Event In The Cyrenaican Miocene Carbonate Platform (Central Mediterranean), NE Libya


The focus of this field and lab integrated study is on the Cyrenaican Miocene 13C-isotope changes in the sequence stratigraphic, and diagenetic context. The field work included 29 measured stratigraphic sections along 135 km, 14 spectral gamma-ray profiles. The lab work includes petrographic and diagenetic studies of 501 hand samples, thin sections and stable isotope (Δ18O and Δ13C) analyses. The sequence stratigraphic framework is based on the sedimentological analysis, correlation of stratigraphic time surfaces and vertical stratigraphic sections, oxygen and carbon stable isotope profiles, and gamma-ray logs. The Ar-Rajmah Group Miocene carbonate rocks record two 2nd order supersequences (97 m maximum thickness); containing six 3rd order sequences. The TST of the younger 2nd order sequence is separated by a sharp disconformity surface from the HST of the older 2nd order sequence, and by maximum flooding zone from the HST of the younger 2nd order sequence. The HST of the older 2nd sequence is the Early Miocene Benghazi Formation (46 m thick), and dominated by red algal reefs, and bioclastic packstones. The TST and HST of the younger 2nd order sequence occur in the Middle and Late Miocene Wadi Al-Qattarah Formation (26 m and 25m thick respectively), and dominated by continuous oolitic grainstones, microbialites that associated with evaporites and siliciclastics. The 3rd order sequences range in thickness from 5 m to more than 15 m. The chemostratigraphic data shows that the Early Miocene is enriched in both Δ8O and Δ13C, the Middle Miocene is enriched in Δ13C but depleted in Δ18O, and the Late Miocene is depleted in both Δ18O and Δ13C. The Middle Miocene is characterized by a prominent positive increase in the Δ13C, Monterey event, which coincides with influx of siliciclastic facies that have high gamma ray zone, and evaporite deposits. In addition, this Δ13C enrichment event is roughly coinciding with major changes in facies lithologies, textures, and diagenetic patterns. The pre-Monterey event facies are dominated by silicified dedolomitized red algal and bioclastic packstones, where the syn-and-post-Monterey event facies are dominated by silicified and recrystallized oolitic grainstone, microbial-bioclastic-oolitic grainstone. The global Monterey event is preserved in the Δ13C record of the Cyrenaican Miocene carbonate platform in the Central Mediterranean, and associated with major sedimentological, stratigraphic, and diagenetic changes.