MAPPING THE MIDDLE TO LATE DEVONIAN IN THE SHINEJINST REGION OF THE SOUTHERN GOBI DESERT, MONGOLIA

The Shinejinst region of southwestern Mongolia is Middle Ordovician to Permian in age, and most likely represents deposition within a volcanic-arc setting. This region has been studied since 1970 and mapped at the 1:200,000 scale, but detailed maps of the area containing Early Devonian to Early Carboniferous sediments do not yet exist. This area is heavily faulted, and published stratigraphic sections likely cross faults that are not necessarily visible from the ground but are apparent from satellite imagery. To address this problem, in 2022 we re-mapped and re-sampled an Early Devonian (likely Emsian) to Late Devonian section across the Chuluun, Tsagaankhaalga, and Gobi-Altai Formations, as well as a Late Devonian to Early Carboniferous section across the Gobi-Altai and Indert Formations. Data collection combined standard geologic mapping techniques with real-time aerial drone imagery in order to construct a complete stratigraphic column through the Devonian sediments that avoided crossing through highly faulted areas. Drone mapping combined with field mapping indicates that both regions have experienced extensive faulting, and beds are nearly vertical throughout the region. The Early Devonian section represents a biostromal limestone environment with intermittent volcanic ash deposits, lava flows, volcaniclastic rocks, and thin organic-rich black shales/siltstones. Preliminary field observations indicate that the Middle Devonian part of the Chuluun Formation preserves one of the Mid-Devonian ocean anoxia events. The Late Devonian section ranges from layered volcaniclastic, pyroclastic, and pillow basalt units at the base of the section to crinoidal limestones at the top of the section (ending in an unconformity with Tournasian reef complexes). The Kellwasser ocean anoxia events and Frasnian-Famennian boundary were tentatively identified at the boundary between the Gobi-Altai Formation and the Indert Formation as an organic-rich siltstone.