Distinguishing coral reef facies from coral-bearing open platform facies: Examples from Ordovician Ordos Basin, Northwest China

Fossil coral communities are often simply considered as “coral reefs.” In fact, coral-bearing communities are not necessarily coral reefs. The key criterion is whether the corals are in situ and if they are abundant enough to form topographic relief. Two outcropped profiles in the western margin of the Ordos Basin, northwestern China, drill cores of the Lianglitag Formation from 4 wells in the Tarim Basin and 500 large thin sections were examined in this study to reveal the difference between coral reefs and coral-bearing carbonate open platform facies in Upper Ordivician carbonate succession. The second layer of the Shijiezigou section is massive in internal structure, mound-like in shape, 5.0 m in height and 33.6 m in width and is interpreted as a patch reef in an open platform. This layer has a variety of corals that are preserved in situ. Reef-dwelling organisms include dasycladaceans, crinoids and gastropods. Other layers of the section are bedded limestones, many of which bear corals and crinoids, while others bear dasycladaceans or only the Dasycladacea genus Vermiporella. Unlike in the reef, the corals in the open platform facies are sparse and generally occur as fragments, most of which are not preserved in situ. According to the dominant species, four communities are distinguished from the Shijiezigou section, namely, the Vermiporella community, coral community, crinoid-coral community and dasycladacean-crinoid community, each of which adapted to some special environmental conditions. The patch reef of the Shijiezigou section contain abundant and diverse tabulate corals, which form bafflestones, while stromatoporoids are the primary builders in the platform margin reefs of well TZ 822 and the Lijiapo section. The common biotic components of the open platform of the Shijiezigou section and well TZ 822 are crinoids and dasycladaceans. The restricted platform of the Shijiezigou section is dominated by the dasycladacean Vermiporella, while the organisms of the restricted platform facies in well TZ 35 are mainly composed of cyanobacteria, ostracods and Tetradium-like organisms.