SEDIMENTARY DIFFERENTIATION DURING THE LATEST EARLY ORDOVICIAN—EARLIEST DARRIWILIAN IN THE YANGTZE BLOCK

Abstract

Based on new chronostratigraphic framework and revised stratigraphic correlation(Table 1), the present paper deals with the sedimentary differentiation of latest Early Ordovician—earliest Darriwilian in the Yangtze Block. According to rock associations and sedimentary features, 9 sedimentary belts are recognized from the old lands southeastward toward the basin during the latest Early—earliest Middle Ordovician (Fig.1), namely: nearshore sand and gravel belt, nearshore sand and mud belt; inner shelf mud carbonate belt; shallower outer shelf carbonate mud belt, ferruginous and argillaceous carbonate belt, marginal carbonate belt; daeper outer shelf mud sand carbonate belt, mud silt belt and argillite carbonaceous shale belt. During the early Middle Ordovician—earliest Darriwilian, most of the sedimentary belts (Fig.2) developed formerly on the outer shelf and the slope remained their geographical positions. However, there were two, instead of three, sedimentary belts occurred in shallow sites; the inner shelf one is characterized by the dominance of carbonate sediments mixed with argillaceous and sandy intercalations. In addition, dolomitic deposits and terrigenous detrital sediments became dominant in the nearshore region. On the northern side of the Yangtze Block proper, three sedimentary belts can be recognized especially during the early Middle Ordovician—earliest Darriwilian. The shallower outer shelf carbonates outcrop only at Hexian, Anhui; the outershelf argillaceous, silty, and carbonate sediments were mainly developed at Mianxian and Susong; the marginal carbonate belt extends approximately from Chuxian to Hongze. It is interesting to note that deeper water sedimentary belts are only incompletely preserved on the northerm side, and a plausible explanation is that they were partially consumed by the later tectonic movements. The distribution pattern of sedimentary belts on the either side of the Yangtze Block is asymmetrical. The south side is thought to be a gentle slope, and the northern side could be a steeper, narrower shelf. Because the Ordovician is largely covered by the Quaternary in the Lower Yangtze area and only a little detailed stratigraphic information is available so far, the extending of the sedimentary belts in this area is just tentatively suggested. Further work is required to connect the corresponding sedimentary belts between the Lowere Yangtze area and the Middle and Upper Yangtze area.