Early-Middle Jurassic lithological and depositional facies of the Da Lat back-arc basin

The Da Lat back-arc basin formed on a deformed lithosphere caused by subduction of the Paleo-Pacific plate under Eurasia in the Mesozoic. Lithology and sedimentary structure analysis from field works and UAV imaging show that the Early-Middle Jurassic deposits in this area can be divided into 7 types of litho-depositional facies: (i) coastal conglomerate; (ii) channel/shallow lake sandstone; (iii) shoreface sandstone; (iv) estuarine siltstone and mudstone; (v) deep marine shale; (vi) mass transport deposits; and (vii) turbidite. These facies’ spatio-temporal distributions are closely related to their grain size. In the basin margin, Early Jurassic coarse-grained deposits are exposed, marking the opening of the basin, grading upward into shallow marine-shelf deposits. In the Middle Jurassic, the basin margin was uplifted corresponding to continental depositional environment, while the basin centre was filled by outer shelf deposits, reflecting a deepening process. Interbedding with these deposits are near shore and estuarine deposits.