A Review of the Lithostratigraphy of the Early Cretaceous Sao Khua Formation, Khorat Group in Northeastern Thailand

Non-marine Cretaceous rocks are widespread in northeastern Thailand and is well known as “the red bed” Khorat Group. The Sao Khua Formation is in the upper half of the Khorat Group which is comprised of six formations. This formation was named and defined at the type section for the rocks between the restricted PhraWihan Formation and the Phu Phan Formation in the drainage area of the Huai Sao Khua, an intermittent stream that flows westward parallel to the highway between Nong Bua Lamphu and UdonThani Provinces. It contains richest and most diverse vertebrate and invertebrate Mesozoic fossils in Thailand. The Sao Khua Formation is characterized by the sequence of the fining-upward successions of at least 4-5 megacycles throughout the formation with the total thickness ranging between 400-700 meters. Each cycle starts with a channel lag conglomerate which the clasts consist totally of re-worked calcrete nodules. The conglomerates were overlain by fine-to medium-grained sandstones of point bar deposit. Finally, the top part of each cycle was covered by a succession of fine-grained floodplain deposit that makes up 60%-70% of the formation. Paleosols are commonly found in the Sao Khua Formation within the floodplain sequence and their geochemistry indicates a semi-arid paleoclimate. Based on lithostratigraphy, the Sao Khua Formation is interpreted to have been deposited by a meandering river system under a semi-arid climate condition. The age of the formation is assigned as the Hauterivian-Late Barremian based on vertebrate and bivalves fossils.