Abstract

Potential for eustatic sea-level changes on continental platforms during Early Silurian (Llandovery) time was highly propitious due to the double effects of lingering glaciation in Gondwana and the rapid rate of sea-floor spreading which assisted the general dispersion of many era tonic realms through middle Paleozoic oceans. Following the initial mass melting of the Late Ordovician/Early Silurian glaciers in North Africa and Arabia, four smaller cycles of sea-level fluctuation were recorded by recurrent, shelly communities in a predominantly carbonate setting on the North American Platform. Peaks in sea-level rise occurred in late Rhuddanian or early Idwian (A4–B1 positions), early Fronian (C1 position), early Telychian (C4 position), and late Telychian (C6 position) times. Although absolute time control is poor, the periodicity of these events may have been every 2.5 m.y.

In the more submerged regions of the North American Platform, such as Iowa and Anticosti Island, diverse faunal associations distinguished by the presence of stricklandiid brachiopods were especially productive at peak times of transgression. Co-occurrence of the calcareous green alga Cyclocrinites with certain of these faunas indicates that maximum water depth was still within the photic zone. At the other extreme, low stands in sea level are identified by diminished faunal diversity, as well as a combination of typical sedimentary structures and evaporite indicators.

Located on a well-defined Precambrian craton, the Yangtze Platform is now a large region in central and southwestern China. During successive stages of the Early Silurian, a thick sequence of graptolitic shales, shelly carbonates, and red beds were deposited on the platform. Of the four sea-level peaks defined in North America, three are recognized in the Yangtze Region. There is very good evidence for a latest Llandovery or earliest Wenlock transgression, based on the extensive distribution of the Salopinella-Coronocephalus-Sichuanoceras fauna. Another clear and well-dated sea-level peak involves the occurrence of faunas with characteristic stricklandiid and pentamerid brachiopods at a level equivalent to the lower Telychian (C4 position). The initial Silurian transgression appears to have reached its peak on the Yangtze Platform primarily during early Idwian time (B1 position), but this is based less surely on changing patterns in graptolite diversity. At present, there is no compelling evidence for a widespread, early Fronian (C1 position) transgression in China. Successful matching of the other sea-level events, however, substantially refines intercontinental correlation for Lower Silurian strata in the Yangtze Region of China and the central regions of North America.

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