Polyphase evolution of the East Gobi basin: sedimentary and structural records of Mesozoic-Cenozoic intraplate deformation in Mongolia

Mapping and correlation of 2D seismic reflection data define the overall subsurface structure of the East Gobi basin (EGB), and reflect Jurassic-Cretaceous intracontinental rift evolution through deposition of at least five distinct stratigraphic sequences. Three major northeast-southwest-trending fault zones divide the basin, including the North Zuunbayan (NZB) fault zone, a major strike-slip fault separating the Unegt and Zuunbayan subbasins. The left-lateral NZB fault cuts and deforms post-rift strata, implying some post-middle-Cretaceous movement. This fault likely also had an earlier history, based on its apparent role as a basin-bounding normal or transtensional fault controlling deposition of the Jurassic-Cretaceous synrift sequence, in addition to radiometric data suggesting a Late Triassic (206-209 Ma) age of deformation at the Tavan Har locality. Deposits of the Unegt subbasin record an early history of basin subsidence beginning similar to155 Ma, with deposition of the Upper Jurassic Sharilyn and Lower Cretaceous Tsagantsav Formations (synrift sequences 1-3). Continued Lower Cretaceous synrift deposition is best recorded by thick deposits of the Zuunbayan Formation in the Zuunbayan subbasin, including newly defined synrift sequences 4-5. Geohistory modelling supports an extensional origin for the EGB, and preliminary thermal maturation studies suggest that a history of variable, moderately high heat flow characterized the Jurassic-Cretaceous rift period. These models predict early to peak oil window conditions for Type 1 or Type 2 kerogen source units in the Upper Tsagantsav/Lower Zuunbayan Formations (Synrift Sequences 3-4). Higher levels of maturity could be generated from distal depocentres with greater overburden accumulation, and this could also account for the observed difference in maturity between oil samples from the Tsagan Els and Zuunbayan fields.