Depositional history, tectonics, and provenance of the Cambrian-Ordovician boundary interval in the western margin of the North China block

Cambrian–Ordovician strata of the North China block, one of China's main tectonic provinces, are a thick (up to 1800 m) succession of mixed carbonate and siliciclastic sedimentary rocks. Sedimentological, bio-stratigraphic, and chemostratigraphic analysis of strata that straddle the Cambrian-Ordovician boundary at the Subaiyingou section in the present-day western part of Inner Mongolia (northwest China) indicate the presence of a signifi cant unconformity between mixed carbonate–fi ne-siliciclastic strata of the Cambrian Series 3 Abuqiehai Formation, and dominantly carbonate strata of the early Middle Ordovician Sandaokan Formation. The latter is a transgressive systems tract with retrogradationally stacked parasequences that include lowstand shoreline quartz sandstone deposits. The Abuqie-hai strata have similar sedimentological characteristics to those of the Cambrian Lau-rentian inner detrital belt, including slightly bioturbated lime mudstone and marlstone/ shale, grainstone, fl at-pebble conglomerate, and microbialite. The lower part of the San-daokan Formation records the rising limb of the middle Darriwilian positive isotopic excursion , recognized herein for the fi rst time in the western North China block. A Cambrian-Ordovician unconformity is developed in many successions globally, and our section in Inner Mongolia records a hia-tus of similar timing and duration to a regionally extensive unconformity recorded along the ancient northern Indian continental margin. Other parts of the North China block record a hiatus of much shorter duration but show a similar record of input of siliciclastic sediment above the unconformity. We interpret the western margin of the North China block to have been affected by a regionally signifi cant tectonic event that occurred on the northern margin of east Gondwana, the Kurgiakh or Bhimphedian orogeny. The Inner Mongolian region was, therefore, likely an along-strike continuation of the northern Indian margin, in contrast to most recent paleo geographic reconstructions.