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.
