Depositional environment of Middle Triassic organic-rich shales in the Ordos Basin, Northwest China

The Middle Triassic Tongchuan Formation in the Ordos Basin of northwestern China is a typical lacustrine deposit. A major component of this formation is thick layers of organic-rich shales that are probably a potential hydrocarbon source and preserve the earliest known Mesozoic-type lacustrine ecosystem. The exact depositional environment of the shales in the Tongchuan Formation, however, remains unknown. To address this question, we carried out high-resolution carbon (delta C-13(org)) and sulphur (delta S-34(py)) isotope analysis as well as undertook total organic carbon (TOC)/pyrite contents and pyrite morphology investigation, and framboidal pyrite size measurements in shales of the Bawangzhuang section of the southern Ordos Basin. Remarkably high TOC (23 +/- 9%) and pyrite (7 +/- 3%) contents were obtained from the shales, which indicate a large amount of organic carbon and pyrite burial during shale deposition. Framboids are the dominant pyrite morphology in the pyrite crystals and show large and variable mean diameters (7.0 +/- 1.7 mu m to 14.3 +/- 6.8 mu m) across the section, indicating oxic-dysoxic bottom water during shale deposition. delta C-13(org) and delta S-34(py) revealed narrow and less variable values, ranging from -31.8 parts per thousand to -28.1 parts per thousand, and -4.1 parts per thousand to 4.9 parts per thousand, respectively. The delta C-13(org) values suggest balanced and consistent carbon cycles. Integrated with pyrite content and morphological patterns, consistent delta S-34(py) values probably demonstrate a relatively open environment for the formation of sedimentary pyrite, and thus a shallow chemocline that was quite close to the water-to-sediment interface during shale deposition. Overall, the organic-rich shales of the Tongchuan Formation were probably deposited under oxic-dysoxic bottom-water conditions. Shallow chemocline depth combined with moderately high sedimentation rate and high primary productivity may have played crucial roles in the deposition and formation of the organic-rich shales in the Tongchuan Formation. The shallow chemocline also facilitates the fossil preservation in a lacustrine environment.

Automated summary

The Tongchuan Formation in the Ordos Basin of northwestern China is a typical lacustrine deposit with thick layers of organic-rich shales. Studies suggest that these shales were likely deposited under oxic-dysoxic bottom-water conditions, with a shallow chemocline depth, moderately high sedimentation rate, and high primary productivity playing crucial roles in their formation. The consistent delta S-34(py) values further support the idea of a relatively open environment for the formation of sedimentary pyrite during shale deposition.