Four measured sections (each 25–75 m thick) of the Miocene Ar-Rajmah Group carbonate rocks in Cyrenaica, northeastern Libya, were sampled every 0.5 m for whole rock stable isotope (δ¹⁸O, δ¹³C) chemostratigraphy. The Ar-Rajmah Group chemostratigraphic data indicate that this unit preserves a record of almost the entire Miocene. The lower Miocene stable isotope record 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 δ¹⁸O data range from -9.2 to 3.7‰ VPDB, and the δ¹³C data range from -6.7 to 3.0‰ VPDB.

The shallowing upward Cyrenaican Miocene carbonate rocks are made up of two shallowing upward 2^nd-order supersequences (SS1–SS2) that contain six 3^rd-order sequences (S1–S6). Four important chemostratigraphic events are recorded in the 3^rd-order sequences in the Cyrenaican Miocene. The Cyrenaican Middle Miocene 3^rd-order sequence 3, Langhian-Serravallian, has the highest enrichment of both oxygen and carbon isotopes, which coincides with the Monterey carbon maximum event, a high gamma ray zone of the maximum flooding zone, and siliciclastic influx. The increased δ¹³C upward enrichment in 3^rd-order sequence 3 was produced by increased carbonate productivity and river runoff. The Tortonian 3^rd order sequence 4 has the most depleted δ¹³C and δ¹⁸O records and both are depleted progressively upsection due to the emergence of the progressively shallowing restricted shallow-water carbonate platform.