MYSTERIOUS LOESSITE: A CASE STUDY FROM THE MID-PERMIAN QUANZIJIE LOW ORDER CYCLE, BOGDA MOUNTAINS, NW CHINA

Pre-Pleistocene loess deposits are rarely reported in the literature, even though modern loess covers around 10% of Earth’s surface. This dilemma may be the result of a lack of criteria to recognize ancient loess or of complex depositional and post-depositional processes that mask the ancient loess features. In this study, the mid-Permian Quanzijie Low-Order Cycle (QZJ LC) in Bogda Mountains, NW China is evaluated to tackle this dilemma. Six measured QZJ sections in the Tarlong-Taodonggou half graben covering 60-km² in southern Bogda Mountains and one in Dalongkou area 70 km to the north in northern Bogda Mountains are 67-178 m thick. They are composed of fining-upward successions of conglomerate, sandstone, and mudrock with an erosional base. Two types of depositional cycles dominate in the sections. The first type is composed of conglomerates that have a high relief erosional base, and are 2-5 m thick and laterally persistent for 100s m. Tabular cross-bedded sandstones overlie conglomerates and are fining-upward and 0.3-3 m thick. They grade into thin, faintly laminated mudrocks. The second type of cycle is composed of conglomerates that have a low-relief erosional base, and are 0.1-2 m thick. The conglomerates are laterally less persistent for 1-10s m and encased in thick (2-16 m) and massive mudrocks. The mudrocks have no apparent soil horizons. Laser particle size analysis shows that the mudrocks are composed of silt and clay, with a multi-modal grain size distribution. The first type of cycle is interpreted as channel-fill and overbank deposits of meandering streams, whereas the second type as superimposed ephemeral stream and loess deposits. Pedogenic morphological, textural, and structural features in both coarse and fine-grained deposits indicate the presence of Calcisols, Gleysols, and Protosols. The sporadic and thin occurrence of fluvial deposits contrast with the persistent occurrence of massive mudrocks with consistent characteristics. Alternate deposition of ephemeral steams and widespread loess occurred in the mid-latitude east coast NE Pangea. Other loess deposits in mid latitudes might exist and could have been overlooked or misinterpreted.