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Myrow et al., 2015

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

Myrow, P. M., Chen, J., Snyder, Z., Leslie, S., Fike, D. A., Fanning, C. M., Yuan, J., Tang, P.
AjakiriGeological Society of America Bulletin
Tüüpartikkel ajakirjas


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.

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