open access

https://hdl.handle.net/2115/36788

The Rugose Coral Fauna from the Upper Part of the Heyuanzahai Formation in Western Yunnan, China

英語

1994-03

Hokkaido University

Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy

23(3)

343-552

The material studied here is from the Malutang-Heyuanzhai region of Shidian County, Yunnan Province, China. In this region Devonian is well developed and may be divided into eight formations. They include the Lower Devonian Xiangyangsi Formation, the Wangjiacun Formation, the Shabajiao Formation, the Middle Devonian Xibiantang Formation and Malutang Formation, the upper Middle to lower Upper Devonian Heyuanzhai Formation and the Upper Devonian Dujiacun Formation. Rugose corals are known from the Malutang Formation and the Heyuanzhai Formation. The Malutang Formation yields abundant brachiopods and a few rugose corals, while the Heyuanzhai Formation is rich in both rugose corals and brachiopods. This paper only deals with the rugose corals of the Heyuanzhai Formation. Among the coral species from the Heyuanzhai Formation, except those discovered only in western Yunnan, one species, Altaiophyllum sagsayicum, is known from the Eifelian of the Urals, 10 species are restricted to Givetian, and 13 species to Frasnian or show some resemblance to Frasnian forms. The rocks yielding the rugose corals are of different kinds of clastic limestones, including more than one bed of conglomeratic limestone. The phaceloid and dendroid corals are usually preserved in fragments, and the outer wall and even parts of dissepimentarium are destroyed in most of the specimens. Some of the corals are probably redeposited fossils. However, some typical Upper Devonian brachiopods, such as Spinatrypa, Pugnoides and Devonoproductus, were reported by Hou and others (1988, p.218) from the Heyuanzhai Formation. Therefore the Heyuanzhai Formation seems to be Early Frasnian in age. In the fauna, rugose corals, mostly of medium or large size, are abundant, forming small biostromes in part. So the corals must have lived in a warm shallow sea advantageous to their growth. Massive forms are usually small, and rare in individuals. These features show that the coral fauna lived in an environment not advantageous to the formation of coral reefs. Almost all the solitary corals in the fauna are incompletely preserved, especially their proximal parts which are usullay destroyed. In some forms of Macgeea, the walls and outer dissepiments are usually destroyed, and even the horseshoe dissepiments are not preserved entirely. Growth direction of fragmentary preserved corallites of both phaceloid and dendroid coral is often in reverse direction. The rugose corals are usually encrusted by other fossils. Some had been destroyed before they were enclosed, some were destroyed after they had been enclosed. Moreover, the associated brachiopods are large in size, and with thick radiations on their shells. The broken fossil fragments are common in beds. On this basis, it is inferred that the coral fauna lived in a high wave energy environment. A comparison of the known occurrences of the rugose coral species in the Heyuanzhai Formation with these in other areas indicates that the fauna is a mixed one of Frasnian and Givetian. The state of preservation and the age of the rugose corals under discussion show that this fauna had partly undergone transportation before they were buried. The components of the Heyuanzhai fauna are highly similar to the Beiliu coral fauna in Middle Devonian of South China. The fauna may have lived on the margin of the carbonate platform adjacent to deep water basin as did the Beiliu coral fauna. The Heyuanzhai Formation consists mainly of marls and different kinds of limestones, including conglomeratic limestones and bioclastics. In the conglomer atic limestones and bioclastics, limestone gravels and fossil fragments are variable in size, irregular in shape and not rounded, but the matrix is very fine. As noted above, the rugose corals in the Heyuanzhai Formation were destroyed to various degrees, but most of them were preserved well enough to be identified, and in most cases they are randomly oriented. The state of the preservation of fossils and the lithology of the rocks in the Heyuanzhai Formation indicate that they were formed in a calm water, and the fossils and gravels as well as other coarse grains had been transported for only a short distance, and quickly deposited. So it may be inferred that the fauna was preserved in a slope environment of deposition. In summary, the rugose corals under discussion may have lived in a warm high wave energy shallow water probably situated on the margins of a carbonate platform adjacent to deep water basin. After death they had been transported and quickly deposited in a calm water environment on the continental slope. Three Devonian rugose coral biogeographical realms have been recognized by Hill (1957, 1981), Oliver and Pedder (1975-1984). They are the Eastern Americas Realm, the Old World Realm and the Malvinokaffric Realm. In 1989 Wang Hongzhen and others recognized four Devonian rugose coral biogeographic realms in the world: the Boreal, the West Tethyan, the East Tethyan and the Malvinokaffric Realm. The West Tethyan corresponds to the Eastern Americas Realm of Oliver et al.. Although different opinions exist with respect to the division of Devonian rugose coral biogeographic provinces, all agree that the difference between the Devonian coral fauna in eastern North America and that in the other regions of the world is the most striking. At generic level the coral fauna from the Heyuanzhai Formation of western Yunnan is very close to eastern North America. More than 90 percent of the individuals are same as or similar to that of the eastern North America. However, Very common Devonian forms like Disphyllum, Cyathophyllum, Temnophyllum and Endophyllum in the so-called Old World Realm have not been discovered so far in eastern Yunnan. Brachiopods are also very abundant in the Heyuanzhai Formation. Stringocephalus, a Givetian index fossil in the Old World Realm, has not been found in western Yunnan. This may also indicate that the Heyuanzhai fauna of western Yunnan was more closely related to Eastern Americas Realm than to the Old World Realm. The Middle and Upper Devonian coral faunas in western Yunnan are quite different from those in the Yangtze region, and probably were separated from the Yangtze Platform by wide seas. Altogether 13 genera and 37 species of rugose corals are described in the present paper from the upper part of the Heyuanzhai Formation. Among them 2 genera and 7 species are new and 4 species are indeterminable.

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Version of Record

0018-3474