Cycles in Perilacustrine Facies of Late Mesozoic Rift Basins, Southeastern Mongolia

Abstract

Detailed study of 365 meters of core recovered from southeastern Mongolia reveals fluvial, marginal lacustrine, and profundal lacustrine depositional environments in an Early Cretaceous rift basin. Multiple examples of cyclic sedimentation are superposed on these large-scale facies transitions. In fluvial facies, more than 20 upward-fining successions (2–10 m thick) are present. These contain cross-stratified sandstone units, with erosional bases, that fine upward to mudstone with root marks and burrows. They are interpreted as channel-fill successions in a fluvial depositional environment, whereas finergrained, less erosional intervals with abundant soft-sediment deformation and climbing ripples may represent a transition to fluvial-deltaic environments at the lake margin. Coarse fluvial-deltaic facies form the most likely hydrocarbon reservoirs, as indicated by frequency of oil staining in the core. Physical transport processes dominate well-oxygenated fluvial and marginal lacustrine facies, whereas cycles in more distal lacustrine facies mainly reflect biogeochemical processes. Profundal lacustrine strata include kerogen-rich laminated micrite, distal turbidite beds, and dolomitic breccia reminiscent of lacustrine facies in the Green River Formation of the western United States. Evidence for anoxia at the lake floor includes framboidal (biogenic) pyrite, high TOC content, mainly light amorphous organic constituents, and paucity of trace fossils relative to the rest of the core. Approximately 25 dolomite layers are interbedded with laminated micrite and calcareous siltstone units in poorly defined meter-scale cycles. These beds typically have brecciated or clotted textures, and suggest evaporative concentration of magnesium ions at the lake floor. The profundal lacustrine sequence displays a distinctive high-resistivity, low spontaneous-potential log character that is observed in well logs from several fields in the Zuunbayan subbasin. Correlative units can be traced at least 60 km to the northeast and southwest of the Zuunbayan field, indicating that this is a widespread lacustrine depositional unit. These lacustrine strata include the most likely petroleum source facies found to date, and are likely contemporaneous with other large lake systems in China and Mongolia.