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, biostratigraphic, 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 significant unconformity between mixed carbonate–fine-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 Abuqiehai strata have similar sedimentological characteristics to those of the Cambrian Laurentian inner detrital belt, including slightly bioturbated lime mudstone and marlstone/shale, grainstone, flat-pebble conglomerate, and microbialite. The lower part of the Sandaokan Formation records the rising limb of the middle Darriwilian positive isotopic excursion, recognized herein for the first 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 hiatus 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 significant 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 paleogeographic reconstructions.