Comparative study of Late Devonian (Famennian) brachiopod assemblages, sea level changes, and geo-events in northwestern and southern China
Abstract Abundant brachiopods have been recovered from the Upper Devonian of western Junggar, Xinjiang, northwestern China. However, their diversity and succession, as well as a detailed comparison with other regions, remain to be explored. These issues are of importance since conodonts and ammonoids are generally sparse for dating the sequence. Based on collections in the past years from the Famennian sequence of the Bulongguoer section and adjacent sections, we recognize five brachiopod assemblages in ascending order: the Caenanoplia? aff. logani Assemblage; the Palaeospirifer–Megalopterorhynchus Assemblage, which can be divided into the Cyrtospirifer sulcifer Subassemblage and the Rugaltarostrum Subassemblage; the Ambocoelia? cf. unionensis Assemblage; the Austrospirifer? sp. Assemblage; and the Syringothyris–Spirifer Assemblage. Comparison of these assemblages with those of the Xikuangshan and other sections, central Hunan, southern China, indicates a strong biogeographic differentiation. There are opposite trends of raw diversity changes during the middle Famennian in these two regions, whereas similar trends, with a decline followed by an increase in diversity, occurred during the late and latest Famennian. Two transgression–regression cycles are recognized in the Bulongguoer section of western Junggar. Comparisons of sea level changes among the Bulongguoer section (western Junggar), central Hunan (platform facies of South China), and Euramerica indicate that regional tectonic activities may have played important roles in both western Junggar and central Hunan. In the Palmatolepis rugosa trachytera conodont Zone, sea level rose gradually to the maximum at approximately the Annulata level in the Bulongguoer section, whereas a contemporaneous prolonged lowstand (“Oujiachong regression”) was recognized in central Hunan. The replacement of the Rugaltarostrum Subassemblage by the Ambocoelia? cf. unionensis Assemblage in western Junggar is correlative to the Oujiachong event (the regression and brachiopod diversity decline in South China at the top of Palmatolepis marginifera Zone). During the latest Famennian, the Syringothyris–Spirifer Assemblage replaced the Austrospirifer? sp. Assemblage in western Junggar, as a response to the Hangenberg Event.