Upper Ordovician continuous lithological succession in outer‐shelf facies, Yangtze Platform, South China: Facies changes and oceanographic reconstruction up to the Late Ordovician Hirnantian glaciation

Abstract

Ordovician sequences at Huanghuachang, northern Yichang City of Hubei Province, Central China, are representative of an outer‐shelf setting of the Yangtze epicontinental sea, South China Block. Continuous drill cores of the Well Yihuang 1 penetrated the Upper Ordovician units of the Miaopo, Pagoda, Linhsiang, Wufeng, and Kuanyinchiao Formations in ascending order. Such a continuous succession gives valuable insights into environmental changes and an extinction event through Late Ordovician time. Results suggest that sluggish circulation and oligotrophic conditions were characteristic of the region from Sandbian to early Hirnantian Epochs of the Late Ordovician. Thin‐bedded limestones within the Miaopo Formation shales and nodular limestones of the Pagoda and Linhsiang Formations are mainly wackestones and mudstones with sparse and fine‐grained trilobite, cephalopod, gastropod, ostracod, and crinoid bioclasts with rare brachiopod and bivalve bioclasts, further showing gradual decreasing in abundance and grain size upwards through the succession. Such biological and lithological changes are interpreted as a trend towards a deeper and calmer seafloor below storm wave‐base. The Kwangsian Orogeny of the late Katian Epoch altered the geography of the region, creating a large embayment in the area of the Well Yihuang 1 core. Thus the sequence developed upwards to the Wufeng Formation graptolitic black shales consistent with formation in a dysoxic and stagnant embayment that excluded carbonate production and benthic biota, but ideal for preservation of planktic graptolite fossils. Bioclastic packstone and quartz grain lenses interlayered with the black shales are occasionally sourced from southeastward shallow submarine highs closed to the Cathaysian Land. Change from this interpreted sluggish ocean circulation affecting the ocean floor was delayed to the early Hirnantian Epoch, when active circulation is related to the onset of the latest Ordovician glaciation which resulted in an oxygenated ocean floor during regression, favorable for the thriving shelly Hirnantia Fauna.