The Silurian red beds of Tarim Basin: Signals of palaeoenvironment, palaeoclimate, and sea-level change

The Silurian red beds of the Tarim Basin were formed in a coastal tidal and fluvial delta environment. They consist of red mudstone layers of the lower Tataaiertag Formation and separate the grey-green muddy sandstones of the Kalpintag Formation from the red sandstones of the upper Tataaiertag Formation. These mudstones are regarded as the most important regional seal for Silurian oil reservoirs. They contain a significant record of palaeoenvironmental and palaeoclimate changes which is related to global sea-level changes. Based on a systematic and continuous sedimentary study and multiple geochemistry analyses of samples taken from the Sishichang outcrop profile in west of the Tarim Basin, the Early-Middle Silurian palaeoclimate and sea-level curve have been reconstructed. The Early-Middle Silurian Tarim Basin encountered five intervals of climate change, from humid transitioning to semi-humid and dry-hot climate which shows signs of generally increasing oxidation. The reconstructed sea-level curve indicates five sea-level cycles throughout the Early-Middle Silurian succession. The most significant sea-level drop occurred during the early Telychian stage, which represents the onset of the red mudstone section of the Tataaiertag Formation. This new sea-level curve shows some differences compared with the earlier proposed global sea-level curves. The dry-hot climate occurred during the late Telychian stage and the beginning of Wenlock age. The Early-Middle Silurian sea-level change of the Tarim Basin was mostly synchronical to the eustacy. However, regional tectonic activities during the early Telychian stage further enhanced the effect of the global sea-level fall and lead to a dry-hot climate trend which also caused the creation of the red beds of the Silurian period.