Mesozoic red bed sequences from SE Asia and the significance of the Khorat Group of NE Thailand

Abstract

New geological data are presented and previously published information is reviewed to demonstrate that much of the Khorat Group (Phu Kradung to Khok Kruat Formations) of NE Thailand is Early Cretaceous in age. It is suggested that the Mesozoic red bed sequences of neighbouring Indochina are likely to be of similar age rather than spanning the entire Late Triassic to Early Cretaceous as previously assumed. Moreover, the Lower Nam Phong Formation dated as Late Triassic and previously included as the basal formation of the Khorat Group is now removed from this group, thus creating a hiatus within the Jurassic. There is therefore no clear relationship between the Indosinian Orogeny and the Triassic collision of the Sibumasu (also referred to as Shan-Thai) and Indochina Blocks and the subsequent deposition of the Khorat Group in a Late Triassic-Early Cretaceous thermal sag basin. It is now proposed that much of the sequence was deposited during the Late Jurassic-Early Cretaceous. Jurassic sediments may be absent across much of the Khorat Plateau whereas marine Jurassic sediments to the west and east show no sediments younger than Bajocian. Because sea levels were generally rising in the Middle and Late Jurassic it is likely that the Khorat region was uplifted at this time. It is suggested that the Khorat Group was originally deposited in a foreland basin setting rather than a thermal sag basin following Late Triassic rifting. Moreover, the original site of deposition was to the north in southern China, with the present-day location being the result of movement along the Red River Fault coupled with a clockwise rotation of the Indochina Block (on which the Khorat Group sits) with respect to the South China Block.

Shark faunas from the Late Jurassic—Early Cretaceous of northeastern Thailand

“…Its thickness varies from 1,200 m in the basin centre to around 500 m on its southern flanks (Racey 2009). It consists of fluvial sandstones, siltstones and mudstones, and is dated as either Late Jurassic based on fossil vertebrate evidence (Buffetaut et al 2001;Buffetaut and Suteethorn 2007;Tong et al 2009a) or Early Cretaceous on the basis of palynology (Racey and Goodall 2009) and detrital zircon thermochronology (Carter and Bristow 2003).…”

“…The Formation can be divided into a lower and an upper part, with sandstones tending to be more common in the upper than in the lower part (Sattayarak 1983;Racey et al 1996). The lower part corresponds to a lake margin, whereas the upper part corresponds mostly to meandering river environments under a probable two-season semi-arid/humid climate (Mouret 1994;Racey et al 1996;Racey 2009). However, a lower and an upper member have not yet been officially established, although the uppermost part is sometimes considered a separate formation, the Waritchaphum Formation (Mouret 1994;Philippe et al 2004).…”

“…PNA corresponds to a channel lag deposit, PNB to an abandoned channel, and PNC to a floodplain/backswamp deposit (from Boggs 1987). Silhouettes of the trees from Philippe et al (2009) latter by the Loei-Petchabun and Sukothai foldbelts (Racey 2009;Department of Mineral Resources 2010). This site is considered part of the Phu Kradung Formation (Department of Mineral Resources 2001), and the presence of the massive sandstones in which the fossils have been found probably indicates that the site belongs to the upper part of the Formation, like the other seven sites.…”

Subsidence and uplift by slab-related mantle dynamics: a driving mechanism for the Late Cretaceous and Cenozoic evolution of continental SE Asia?

“…Continental red beds of the Upper Jurassic -Lower Cretaceous Khorat Group and lateral equivalents (Racey 2009) are exposed over large areas of eastern Thailand, Laos, Cambodia and parts of Vietnam, southern Thailand, and Peninsular Malaysia (Racey 2009;Fig. 9), and are relatively undeformed (Harbury et al 1990).…”

“…Causes of different styles of basin evolution for the Upper Jurassic -Lower Cretaceous red beds in the region are generally poorly understood (Racey 2009). The Khorat Group in NE Thailand is (1996) note that on seismic data there is little evidence for faultcontrolled accommodation during deposition and that the Khorat Group formations have a mainly 'layercake' appearance.…”