Paleoenvironmental reconstruction of the middle-upper jurassic strata in the Iraqi Zagros Suture Zone, southern Tethys: Implications from stable isotopic data and scanning electron microscopy

Abstract

The middle to upper Jurassic succession in the Kurdistan Region in northern-northeastern Iraq, forms a part of the southern Tethys Zagros Suture Zone, comprises a thick carbonate rock (dolomite, limestone, dolomitic limestone, and argillaceous limestone) interbedded with shale units. In order to reconstruct the paleoclimatic fluctuations, the bulk carbonates from the Sargelu, Naokelekan and Barsarin formations that spanned middle to late Jurassic period are selected for δ¹⁸O and δ¹³C analyses supported by scanning electron microscopic imaging aiming to assess the temperature variability and carbon cycle dynamics comparing to global paleoclimate evolution during the Jurassic. The negative trends in oxygen isotopic data are recorded with a general increase in temperature from the middle to upper Jurassic. The middle Jurassic Sargelu Formation records warm paleo temperatures, indicating deposition in marine environments in a mixture of humid and dry conditions. The late Jurassic Naokelekan Formation was deposited in both marine and freshwater environments with a higher temperature than the Sargelu Formation. An increase in temperature also recorded in the upper Jurassic Barsarin Formation, while deposition was characterized by a climate in a brackish lagoon environment. Scanning electron microscopic (SEM) study supports these depositional interpretations, where the common presence of coccoliths and illite–smectite clay mineral confirm the marine deposition of the Sargelu Formation. The various forms of disc-shaped calcite crystals, star-shaped crystals, long fibrous and columnar calcite crystals reveal deposition in a mixing of marine to fresh water setting for the upper Jurassic Naokelekan and Barsarin formations.