Palynology and stratigraphy of the Mesozoic Khorat Group red bed sequences from Thailand

Abstract

AbstractNew palynological results are presented and integrated with previous data for the Mesozoic Khorat Group continental red bed succession of NE Thailand. The Khorat Group is traditionally considered to comprise, from bottom to top, the Nam Phong, Phu Kradung, Phra Wihan, Sao Khua, Phu Phan and Khok Kruat Formations. The group is unconformably overlain by the continental evaporitic Maha Sarakham Formation, which has been palynologically dated as mid-Albian–Cenomanian, therefore giving a minimum age for the underlying Khorat Group. Traditionally the Nam Phong Formation is considered to be Late Triassic, the Phu Kradung, Phra Wihan and Sao Khua Formations are assigned to the Jurassic, and the Phu Phan and Khok Kruat Formations to the Early Cretaceous. This age dating has mainly been based on plant macrofossil and vertebrate studies. Palynology supports the Late Triassic age for the Lower Nam Phong and underlying Huai Hin Lat Formations but suggests an Early Cretaceous (Berriasian–Barremian) age for the Phu Kradung, Sao Khua and Phra Wihan Formations, and an Aptian age for the Khok Kruat Formation. The Phu Phan has yet to yield useful age-diagnostic palynomorphs but must also be Early Cretaceous based on the age of the under- and overlying formations. The key palynomorph present for ascribing an Early Cretaceous age is Dicheiropollis etruscus, a taxon that is restricted to this age interval and has been found in the Phu Kradung (both at outcrop and in subsurface exploration wells), Phra Wihan and Sao Khua Formations. The lithostratigraphy and age of the Nam Phong Formation is, at present, still problematic. From subsurface seismic data it is clear that the Nam Phong Formation comprises two distinct units separated by an unconformity, referred to herein as the Lower and Upper Nam Phong Formations. The age of the Lower Nam Phong Formation based on palynology is Late Triassic (Rhaetian) whereas the age of the Upper Nam Phong Formation is poorly constrained but can be no older than Pliensbachian based on data from the Phu Horm-1 well. The absence of the Early Cretaceous marker D. etruscus may be taken as indirect evidence for a Jurassic age for the Upper Nam Phong Formation in the Phu Horm-1 well. This revised age dating suggests the presence of a significant depositional hiatus within the Nam Phong Formation, and consequently the Lower Nam Phong Formation should be removed from the Khorat Group. Overall the palynomorph assemblages of the Khorat Group are dominated by the gymnosperm pollen Corollina (synonymous with Classopollis) and Dicheiropollis, both of which belong to the Cheirolepidaceae and would indicate deposition in a warm, dominantly seasonally dry subtropical climate.