New Occurrence of Exceptionally Preserved Fossils in the Middle Cambrian of Hunan, China: Significance with respect to Global Correlation
Loren Babcock (Department of Geological Sciences, Ohio State University, Columbus, Ohio, USA)
23 October 2001
This is a modified version of a talk given at the 7th International Subcommission on Cambrian Stratigraphy field conference in South China in summer 2001.
The Paibi section is a new site in South China - northwestern Hunan, specifically - with Burgess Shale-type preservation. This occurrence is near the end of the Burgess Shale-type preservation window and occurs in carbonates, an unusual setting for Cambrian sites. Many Carboniferous and Permian sites of exceptional preservation do have fossils in carbonates. This find has relevance for global correlation, but has significance principally as a new site of exceptional preservation. Sections seen on the 7th International Subcommission on Cambrian Stratigraphy (ISCS) field trip went from the uppermost Neoproterozoic to the lowermost Ordovician. The focus of the field trip was examination of those points in the sections that were potential stratotypes for internal Cambrian subdivisions and reference sections. The Paibi section is a candidate section for the global stratotype section and point (GSSP) for the base of the Upper Cambrian.
The ISCS wants to divide the Cambrian into 4 series:
1) Neoproterozoic to the first appearance datum (FAD) of trilobites - the base of this proposed series is at 543-544 my (~543.6-543.7 my).
2) Trilobite FAD to some reasonable marker close to the historic Lower Cambrian-Middle Cambrian boundary. The marker has yet to be decided; there are 2 good options. The base of this proposed series is ~520 my.
3) Some reasonable marker close to the historic Lower Cambrian-Middle Cambrian boundary to the FAD of Glyptagnostus reticulatus, a cosmopolitan agnostoid trilobite.
4) FAD of Glyptagnostus reticulatus, a cosmopolitan agnostoid trilobite, to the Cambrian-Ordovician GSSP. The base of this proposed series is ~505 to 508 my. The top is at 491 my.
No bentonites are known in any candidate GSSP for internal subdivisions to help with dates/ages. The ISCS will propose new names for everything. The uppermost series has the proposed name Furongian Series. This means “lotus” in Chinese. It is named after Hunan Province, which for a long time has been known as the “lotus state”. The 7th stage of the Cambrian will be the Paibian Stage. All other international series and stages are unnamed for now.
Loren Babcock prefers to use the term “exceptional preservation” instead of “Burgess Shale-type preservation”. He also prefers using the term “deposits of exceptional preservation” (DEP) instead of “Burgess Shale-type deposits” or “Lagerstätten”.
Here reporting new nonmineralizing algae, cyanobacteria, and a cnidarian - 6 genera of creatures - most of these occur in many places elsewhere in the world. They come from the Paibi section in northwestern Hunan, which is close to the Guizhou border. They are from the Huaqiao Formation - formerly known as 3 formations - the Huaqiao, the Chefu, and the Bitao Formations. All three of these formations have the same lithologies - they are the same, and they are differentiated on the basis of the contained fossils, which is the wrong way for doing lithostratigraphy. They are now all the Huaqiao Formation.
There are lots of major faults in northwestern Hunan - a similar structural situation to southeastern Pennsylvanian - it's a fold and thrust belt. The Paibi section is on a great thrust slice. Paleogeographically, the Paibi section is just offshore of a great carbonate platform, the Yangtze Platform, a feature first developed in the Neoproterozoic. Subsequent emplacement of terranes in southeastern China accounts for the faults and thrusts in Hunan and adjacent areas. Paibi is in the Jiangnan Slope Belt - a shelf-to-slope transition. The Jiangnan Basin region is to the southeast of here, full of Paleozoic rocks. There are some equivalents to the Jiangnan Basin in western USA.
The Paibi section is the most complete Middle Cambrian-Upper Cambrian section in the world - it has no faults, no significant disconformities, and a few minor erosion surfaces at best. The Huaqiao Formation starts at the base of the Ptychagnostus atavus zone and goes to the base of the Ordovician. There are 600 meters of section at Paibi - mostly carbonate environments - slope off the shelf - shelf edge to slope - most sediments probably formed on the shelf and were later transported offshore.
The deposit of exceptional preservation occurs in the upper Ptychagnostus atavus zone, 80-90 meters above the base of the Huaqiao Formation. The Huaqiao lithologies are dark gray to black, thin-bedded lime mudstones and some wackestones. These rocks are very fossiliferous. There are 90 species of agnostoids and 120 species of polymeroids. It is the richest trilobite fauna on Earth by far. Also lots of brachiopods, paraconodonts, protoconodonts, and other fossils. The exceptionally preserved fossils also occur in other bedding planes besides the interval at 80-90 meters above base Huaqiao. Babcock collected this section for a week some time ago, and briefly recollected the exceptionally preserved interval during the 7th ISCS field trip. He found that these exceptionally preserved fossils occur in other intervals as well - 2 major intervals in the Huaqiao Formation have exceptionally preserved fossils. The specimens reported here come from the Ptychagnostus atavus zone. This site was probably not very warm water, but was above the thermocline.
One hydrozoan and five algae of bacteria, all preserved by carbonization; some show subtle relief. All specimens are of small size. All are preserved in dark gray lime mudstone (rather unusual lithology for exceptionally preserved fossils).
1) Archaeocryptolaria specimen - ~1 cm long; the youngest occurrence of this genus; the Australian specimens of this are early Middle Cambrian.
2) The putative green alga Yuknessia. The Huaqiao specimens are long, nonhelical tubes that are locally abundant on bedding planes. These are the youngest Yuknessia from Gondwana. They are comparable in age to the youngest Yuknessia globally (from the Ptychagnostus atavus zone of Utah).
3) The bacterium Megaspirellus? The Huaqiao specimens have a slender, helical shape. If these are Megaspirellus, the are the youngest known, and the only Megaspirellus known outside the Chengjiang Lagerstätte.
4) and 5) The cyanobacteria Morania and Marpolia? These are widely distributed on Earth. The Huaqiao Marpolia? are slender, slightly twisting stipes with elongate striations. The Huaqiao Morania are ovoid bodies that are locally abundant on bedding planes. This Morania occurrence is comparable to the youngest known occurrence globally (in the Ptychagnostus atavus zone of Utah).
6) A new genus of alga? [James St. John - it looks similar to the modern brown alga Padina]. These look like rippled potato chips. They have carbonized ovoid bodies with concentric wrinkling and small radiating ridges.
The significance of the Huaqiao Lagerstätte - it is a “minor” deposit of exceptional preservation, which is a function of under-collecting? Its importance is in the preservational conditions and its age. The Huaqiao Formation lithology, color, bedding, and fossils are all similar to the depositional conditions of the Wheeler Formation of Utah. The oxygen content for the lower Huaqiao is considered to be close to the Ohio Shale model - dysoxic-anoxic conditions at the sediment-water interface. Burgess Shale-type preservation is not limited to siliciclastic environments.