Abstract: Testing Tectonic Reconstructions in the Western Taurus (SW Turkey) Through Organic-carbon Maturation of the Late Jurassic Akkuyu Formation

Francois Baudin, Olivier Monod

South of Beysehir Lake, the parautochthonous platform carbonates are tectonically overlain by the Beysehir-Hadim Nappes, of undisputable NE origin. More to the south, however, the Antalya Nappes and Alanya Massif also overlie the same unit, but their northern or southern origin has been strongly debated. In the parautochthonous carbonates, an organic-rich formation (Akkuyu Formation, late Jurassic) provides a sensitive test to discriminate between the two models. The load above the Akkuyu Formation has influenced the degree of maturation of the organic matter and clay diagenesis. The Akkuyu Formation was deposited as black, dolomitic shallow-marine limestones in the NE, whereas clear, finely bedded marly limestones with a conspicuous black key-horizon indicate deeper cond tions in the SW. Total organic carbon (TOC) content may reach 30% with an average 5% in the key horizon. The degree of maturation of the organic carbon is derived from the Rock-eval parameter (Tmax) and the vitrinite reflectance (R₀). Diagenetic evolution of clay minerals is estimated by illite appearance and cristallinity. According to these criteria, two contrasted areas are found:

- In the type area located in front of the Beysehir-Hadim nappes, thermal maturity is low (Tmax=429° to 439°, R₀=0.32 to 0.52%), and illite is poorly crystallized.

- In the area situated 20 km to the NE in a tectonic window, maturity reaches the gas window (Tmax=475° to 545°, R₀=1.23% to 1.32%), and illite is well crystallized.

As the thickness of the sedimentary sequence above the Akkuyu Formation is similar in both areas, the thermal rise in the NE is interpreted as a consequence of a deeper burial of the Mesozoic formations under the load of the Beysehir-Hadim nappes.

Several burial history curves have been used to model the thermal evolution and maturation of the Akkuyu Formation. The diagrams generated through the Matoil program (BEICIP-IFP) clearly show that the immature Akkuyu Formation in the type area cannot have supported any tectonic loading. These data suggest that the present front of the Beysehir-Hadim nappes is close to the actual maximum extension of the nappes, and not an erosional boundary. Hence the Antalya-Alanya nappes have a distinct origin.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France