Bulletin of the Geological Society of Malaysia, Volume 19, April 1986, pp. 193 – 205
1University of Dundee, Dundee DD1 4HN, Scotland, U.K.
2c/o New House Farm, Hatton, Warrington, Cheshire, United Kingdom.
Abstract: Chaotic and disturbed deposits occur widely in post-Tab sedimentary sequences in Sabah and have been mapped as slump breccias in the Garinono, Wariu, Kuamut and Ayer Formations. Some of the breccias are typified by case-hardened, baked arenaceous blocks distributed randomly in scaly clays. Phreatic deposition of silica, gypsum, zeolites and iron sulphides occurs in voids and along joints and shrinkage cracks. The origin of these enigmatic sediments is concerned with the seemingly anomalous depositional conditions in which high energies are required to move large clasts but low energies are indicated by the predominantly clayey matrix, and the presence of low temperature hydrothermal activity. The authors suggest there is a genetic link between mud volcanism and many of the chaotic deposits in Sabah, especially the Garinono Formation. Modern mud volcanoes are distributed across northern Borneo and are composed of lithologies comparable to those in the disturbed rocks. The early development of mud volcanoes is linked to a mid-Miocene collisional event during which obduction of shallow and deep water sediments were thrust from the Sulu Sea onto what is now NE Sabah. Connate water flashing into superheated steam provides the driving force for violent upward movement of hot mud, transporting with it clasts of most rock types including oceanic crust and possibly asthenosphere as well as providing the phreatic activity evident in many localities. Migration of mud volcanic centres which change position regularly could produce, in a 5-6 million year span, a widespread deposit similar to the Garinono Formation. Periods of inactivity would result in the intercalation of chaotic deposits within normal bedded sequences.