in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
found in east part of Dashanpo syncline which is sym-
metrical and extends 5 kilometers. The wood fossils lie
mainly in grey yellow calcareous feldspar sandstone of
the lower part of Upper Jurassic Penglaizhen forma-
tion, which shows that it is younger than Qijiang flora.
There were a lot of petrified woods concentrated
in Shehong National Geological Park, Sichuan. It was
extremely rare that the petrified woods appeared
together with the dinosaur fossils. It has been proved
that its age is Upper Jurassic Penglaizhen formation,
later than that of Qijiang flora.
In north China, many Middle Jurassic wood
fossils have been found in some provinces including
Liaoning, Henan, Jilin and Xinjiang, the strata bearing
wood fossils are Tijishan formation, Yima formation,
Sukang formation and Yingshugou formation(Wang et
al., 2006). But in south China the middle Jurassic
wood fossil record is rare. This discovery of Qijiang
flora increases the distribution range of Silicified
woods, and provides the new materials for the large
plant fossils in Middle Jurassic.
Usually the general law is that during the process
of diagenesis, deposites under the same geological
background and diagenetic conditions should get the
consistent sedimentary result. The bark of Qijiang
wood was coalified, the wood was silicified and limo-
nitized. Coalification represents reductive environment,
and limonitization is oxide environment, how this two
opposite environment appeared in the wood together?
How to explain the inconsistence? The answer lies in
the natural and geological action. The coalification
was probably due to a big forest fire which caused the
bark of trees to become coal. The silicifiation and
limonitization was the result of wood’s being
immersed by solution of SiO2 and FeO. This two
opposite genetic environment occurred successively in
the same wood, and therefore appeared in the pre-
served condition simultaneously.
The coalification of Qijiang wood fossils shows
as discussed above that the trees were probably burned
by a forest fire, suggesting a dry climate at that time.
This judgment can be proved from previous con-
clusions. Qijiang wood fossils were primarily con-
cluded a kind of Araucaria cunninghamii. Duan et al.
(1998, 2000) A.cunninghamii had a wide distribution
in China during late Mesozoic (Late Jurassic to Early
cretaceous). According to some related information
(Wang, 1992), oceanic climate dominated from Jura-
ssic to cretaceous, forming a universal wet and humid
climate on the globe. While in middle and East Asia,
the dry climate which began at the end of Middle
Jurassic or Late Jurassic lasted to Cretaceous. On the
other hand , Wang et al. (2008) inferred that the
climate in this area was subtropical semi-humid and
semi-arid in early Middle Jurassic, but was subtropical
semi-arid–arid in late Jurassic (the period of Upper and
Lower Shaximiao formation) based on his research on
246
spore and pollen fossils.
Qijiang wood flora is important silicified wood
fossils in many respects and deserves a further
systemic study. Firstly more detailed study should be
made on strata, including biostratigraphy, petrology
and chemical stratigraphy. High-resolution event
stratigraphy will be applied to Upper Shaximiao
formation with fossils to precisely confirm the horizon
and age of wood fossils, and to look for the evidence
of forest fire. Secondly, to make a detailed study of
biological features on wood fossils, and observe the
microstructure by experimental methods to ensure its
taxonomic position and to make an identification of
species level and find out the characters of plant
combination. Thirdly, through the analysis of litho-
facies paleogeography, spore and pollen fossils, iso-
topic geology to know paleogeography environment,
the character of paleo-climate and water condition
(hydrodynamic condition, elements, Ph values, Eh
values and water temperature and so on). It is note-
worthy that some big sauropod dinosaurs sites have
been found near the wood sites, so the wood fossils
will also provide valuable materials for the study of
dinosaurs food chains.
Key words: Silified wood; Qijiang; New
materials; Further study
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