Combining Subsidence Analysis and Detrital Modes of Sandstones to Constrain Basin History: An Example from the Eastern Pontides of Turkey

摘要

Basin history is traditionally approached using either tectonic subsidence analysis or sandstone composition, hut seldom both. The power of combining these two techniques is illustrated by consideration of the Pontides zone of Turkey. The Pontides are the onshore expression of the Black Sea basin, and understanding their evolution is important for petroleum and mineral exploration of the region. Many contrasting models have been proposed for the tectonic evolution of the eastern Pontides, hut they have proved hard to evaluate. This study shows how combining subsidence analysis with sandstone petrography allows a more rigorous evaluation of these models than has hitherto been possible. Tectonic subsidence analysis was applied to measured stratigraphic sections from 10 different localities from the eastern Pontides. Each stratigraphic column was subjected to a backstripping process to assess the amount of the subsidence attributable to purely tectonic processes. To make the necessary calculations and to obtain subsidence curves, a spreadsheet program was developed. Using tectonic subsidence rates and shapes of curves, strike-slip, foreland, and fore-arc possibilities were eliminated for the tectonic setting of the basins. Also four synchronous subsidence events were recognized throughout the localities at 125 Ma, 98 Ma, 85 Ma, and 55 Ma. Modal compositions of sandstones place additional constraints on the possible tectonic settings of the basins studied. Petrography of the sandstone samples from the eastern Pontides showed that Liassic strata are rich in volcanic fragments; their likely tectonic setting was a backarc basin. By contrast, Upper Cretaceous and Eocene samples are rich in quartz, implying that a continental source dominated. The combination of the shapes of the subsidence curves, the rates of subsidence, the timing of the events, and the sandstone compositions are most consistent with the tectonic model of Banks and Robinson (1997).