Yichang (Central China) --the Oldest-Known Bryozoan Reefs (Mid-Early Ordovician), and Comparison with the Next-Oldest (Garden Island, New York/Vermont; Early Middle Ordovician)

The world’s earliest bryozoan-built reef-mounds are in central China, above Huanghuachang, NE of Yichang (Hubei Prov), in the Fenxiang Formation (late Tremadocian), first reported by Zhu (’93 Sci Geol Sinica).

The Yichang bryozoan reefs consist of globular, domal, and crustose colonies, in-place, attached to one another, sometimes stacked up in columnar fashion. Between these is micrite or shell-hash sediment. The colonies make up 40-60% of the bryoherm rock volume on average; the resulting reef-rock is mostly globstone, locally cruststone. Many thin-sections show that the colonies are all one species, highly variable, identifiable as the just-described orbiporid trepostome Nekhorosheviella semisphaerica Xia Zhang & Wang (’07 J Paleo; = ”Hubei[o]pora simplex“ Spjeldnaes ’01 ms; not ramose Batostoma jinhongshanense Yin & Xia ‘86). The species functioned as principal frame-builder in these reefs; its frame also baffled/trapped lime-mud within the reefs.

In a few places, broken thin-branching colonies are found among the globular zoaria, although the ramose forms nowhere comprise more than 10% of the rock volume. These too are recently described, the primitive trepostome Orbiramus normalis (= ”Yichangopora petaloformis” Spj ms). Minor reef dwellers, they lived as small patches scattered atop the bryoherm or down in small crevices within the reef. The Yichang bryoherms do not exhibit lateral ecozones; in particular not the flanking thickets (branchstone) as at Garden Island.

North America’s oldest bryozoan reef (somewhat younger than Yichang), is on the south end of Garden Island (in Lake Champlain), in the Scott Limestone Member of the Day Point Formation (basal Chazyan age [= late Dariwillian]), first recorded by Pitcher (’64 Bull Can Pet Geol). Its core is cruststone of Batostoma chazyensis, flanked by branchstone of Champlainopora/Atactotoechus chazyensis, both trepostomes.