Tải bản đầy đủ - 0 (trang)
Chapter 87. The application of Ostracoda to location of non-marine Jurassic-Cretaceous boundary in Sichuan Basin of China

Chapter 87. The application of Ostracoda to location of non-marine Jurassic-Cretaceous boundary in Sichuan Basin of China

Tải bản đầy đủ - 0trang

1246



I

I

I

I

I

I



I

I

I

I

I

I



I



\

rarea



4

:hengdu



1



Quaternary

0 0 . 0 . .

. . 0 . .



I Gudian

.



Qiqusi Fm

Bai long



.Cretaceous



Fm



-



Fm

,Upper

Jurassic



Changxi Fm

Penglaizhen



Fm



TEXT-FIG.

1-Upper JurassieLower Cretaceous distribution map of the district around the Deyang-Zhongjiang.

Sichuan Province.



v)



P



$



I



I



I



I



.-0C



E



d

.-1

.-c

L

a

v)



-



~



I



I



I



1247



1248



Southwest



S. S i c h u a n t o



Sichuan



N. Guichuan



Sichuan



I



West



Basin



Middle



North



-



Xiaoba



Fm



Gaokanba Fm



Sanhe Fm



) a tongc hang



Daerdang Fm



u



Jianguan Format ion



F a m a t ion



w I TIs

Wodoushan Fm



!it ianshan Fm



Tianmashan



----



---



iangou Fm



Penglaizhen



i ugundang Fm



II



Fm



liange Fm



I Bailong F m



---t



Fm



Gudian



Hanyangpu



Fm



~~



Changxi



Fm



Jianmenguan Fm



---



I---. -Format ion



Lianhuakou Fin



S h i n i n g Format ion



TEXT-FIG.

3-The chief ostracods of the non-marine Upper Jurassic and Cretaceous deposits in Sichuan, China.



1252



e



u

u

v)

0



5



2



.e

L



c

VI

0



z



I



auoz -'

J-



'3



auoz--1 -;JS



d



0



Y



c

6



Jurassic-Cretaceous Boundary in Sichuan Basin, China 1253



between the two systems, and because of the lack of fossils and the absence of a combined study

on the biostratigraphy, lithostratigraphy and chronostratigraphy, no unanimous conclusion can

be drawn,



THESEQUENCEOF STRATA

The strata involved in the discussion of the Jurassic-Cretaceous boundary are the Chongqing

Series, the Guangyuan Group, the Chengqiangyan Group and the Jiading Series. One of the

reasons for the endless debate in the past is that people did not recognize the normal sequence of the

above-mentioned four groups. At first sight, the Chengqiangyan Group overlies the Guangyuan

Group in North Sichuan and the “Chengqiangyan Group” overlies the Chongqing Series in

Central Sichuan, which leads people to consider that the upper two groups correspond to each

other and that the same applies in the case of the lower two groups. This is not, in fact, the case

because the lower part of the Chengqiangyan Group in North Sichuan is older than the lower part

of the “Chengqiangyan Group” in Central Sichuan, and this is the first point in the argument

against placing the boundary at the bottom of Chengqiangyan Group (in its broad sense). Secondly,

the Chongqing Series is overlain by different strata in different regions; for example, by the “Chengqiangyan Group” in Central Sichuan and by the Jiading Series in South Sichuan. The uppermost

part of the Chonqing Series is older than the lowest part of the Chengqiangyan Group and the

Chengqiangyan Group lies above the Chongqing Series in the transitional region (Li, 1981). Thus it

is self-contradictory if the boundary is put both at the bottom of the Chengqiangyan Group and

at the bottom of the Jiading Series. The third point is that according to systematic study of the

ostracod fossils, not only the Chengqiangyan Group in North Sichuan, but also the “Chengqiangyan Group” in Central Sichuan, are not all Cretaceous; on the contrary, they are mainly Jurassic

(see Text-fig. 3). At the same time, the overlying Jiading Series contains none of the oldest Cretaceous rocks but only late Early Cretaceous to Eocene sediments. So, the opinions putting the

boundary at the bottom of the Chengqiangyan Group and the Jiading Series or within the latter

are all groundless. Text-figure 3 illustrates the ranges of the ostracods on which subdivision of

the Jurassic-Cretaceous in the Sichuan Basin into biostratigraphical units is made.



THE BIOSTRATIGRAPHICAL

UNITS AND THE RELATIONSHIPS

BETWEEN THEM

AND THE LITHOSTRATIGRAPHICAL

UNITS

The Jurassic-Cretaceous transitional beds may be referred to a superconcurrent range zone, namely the Damonella-Darwinula oblonga Superconcurrent Range Zone, including both Damonella

shuangbaiensis-Darwinula oblonga Concurrent Range Zone and the Pinnocypridea-Jingguella

Concurrent Range Zone. The former is subdivided from bottom to top into the Cetacella-Damonella

shuangbaiensis Assemblage Zone and the Eolimnocythere-Damonella shuangbaiensis Assemblage

Zone, and the latter into the Deyangia-Jingguella Assemblage Zone, the Jingguella (J.) acutura-J.

(Minheella) minhheensis Assemblage Zone, the Cypridea dayaoensis-Qingjiania Assemblage Zone

and the Cypridea (Ulwel1ia)-Simicypris Assemblage Zone (see Text-fig. 4).

Damonella-Darwinula oblonga Superconcurrent Range Zone

The genus Damonella Anderson, 1966 and Darwinula oblonga (Roemer, 1839) are taken as indicative of this zone. The appearance of Damonella shuangbaiensis Gou, 1977 heralds the beginning of this zone, and the extinction of Damonella ovata Gou, 1977 is taken as its end. The zone cor-



1254



Y.W. LI



responds to the lithostratigraphical units Shuining Formation to Chengqiangyan Group, and is

more or less distriubted all over the Sichuan Basin. The type section runs from Shuining County to

Zhongjiang County and is about 1,823 m in thickness. In the Guangyuan section in the northwest

margin of the basin, the zone is very thick reaching 3,423 m in thickness. The fossils, which range

from Kimmeridgian to Berriasian in terms of European stages, are found in this zone. The zone

may be subdivided into two concurrent range zones as follows:

Damonella shuangbaiensis-Darwinula oblonga Concurrent Range Zone

The appearance of Damonella shuangbaiensis Gou, 1977 and the absence of the genera Jingguella

Gou, 1977, Pinnocypridea Shi and H o , 1963, and Deyangia Li, 1983 are diagnostic of this zone. It

corresponds to the lithostratigraphical units Shuining Formation and Penglaizhen Formation.

The type section and distribution are the same as for the superconcurrent range zone, reaching

a thickness of about 1,125 m in Shuining County and 2,130 m in Guangyuan County. It contains

Cetacella inermis Martin, 1958 which comes from the Kimmeridgian in England (Anderson et al.,

1971) and Spain (Brenner, 1976). This concurrent range zone includes 2 assemblage zones which

are as follows:

1) Cetacella inermis-Damonella shuangbaiensis Assemblage Zone

This is an assemblage zone that takes Cetacella inermis Martin, 1958 and Damonella shuangbaiensis Gou, 1977 as representative, with many associated species of Darwinula Brady and Norman,

1889; it is found in the Shuining Formation in Sichuan. This zone corresponds to the Lower

Kimmeridgian of the Upper Jurassic.

2) Eolimnocythere-Damonella shuangbaiensis Assemblage Zone

This zone takes Eolimnocythere Li and Wei, 1983 and Damonella shuangbaiensis Gou, 1977 as

representative and is associated with many species of Darwinula Brady and Norman, 1889. It contains Eolimnocythere rongxianensis Li and Wei, 1983, E. alta Li, 1983, Darwinula oblonga (Roemer,

1836), D. leguminella (Foebes, 1855), Damonella shuangbaiensis Gou, 1977, D. ovata Gou, 1977

and Djungarica yunnanensis Ye, 1977 (Text-fig. 3). It corresponds to the Penglaizhen Formation

in Central Sichuan and the Lianhuakou Formation in North Sichuan. The typical section in Pengxi,

Zhongjiang County is 883 m thick. It is thin in South Sichuan where it is less than 200 m thick, but

the thickness increases in North Sichuan to 1,720 m. It may be correlated with the Upper Kimmeridgian of the Upper Jurassic.

Pinnocypridea-Jingguetla Concurrent Range Zone

This concurrent range zone is very rich in fossils. Not only do the genera Pinnocypridea Shi and

Ho, 1963, Jingguella Gou, 1977, Deyangia (Deyangella) Li, 1984 and Darwinula Brady and Norman, 1889 coexist with the development of various species, but other genera (or subgenera) such as

Deyangia (Deyangia) Li, 1983, Djungarica Jiang, 1976, Metacypris Brady, 1870, Ziziphocypris

Chen, 1956, Cypridea Bosquet, 1852, C. (Ulwellia) Anderson, 1939, Qingjiania Li, 1983 and Simicypris Li, 1983 are also present. This zone corresponds mainly to the “Chengqiangyan Group” in

the middle part of the basin, the middle-upper part of the Chengqiangyan Group in the northern

part and the Tianmashan Formation in the western part of the basin. The main genus of the

concurrent range zone is Pinnocypridea Shi and Ho, 1963 which was named in the present region

by Shi and Ho (1963). The characteristic Mantelliana cyrton Anderson, 1971 which is reported

from the Lower Purbeck Beds of England by Anderson (1971, p. 104, P1. 23, figs. 4, 5) conforms

to the generic diagnosis of Pinnocypridea Shi and Ho, 1963, because the former has a “flange” on

its dorsal margin, i. e. so-called dorsal fin ridge of Shi and Ho, and there are a few pustules at

the anterior end (called spines by Shi and Ho). Some other species also belong to this genus

among which are: ‘‘Cypris” aff.purbeckensis Oertli, 1963 collected from the Lower Purbeckian of



JurassicCretaceous Boundary in Sichuan Basin, Chinu 1255



the Villemoyenne 2 borehole by Oertli (1963, P1.5, fig. 33); “Bairdiu” sp. Klingler, 1955 of Wienholz

(1968, P1. 1, figs. la-c); Mantelliana cf. purbeckensis (Forbes, 1855) recorded from the Berriasian

of Spain by Brenner (1976, p. 136, P1. 10, fig. S), from the Lower Berriasian by Wienholz (1968, p.

236, P1. 1, figs. 4, 5), from Portlandian to Middle Purbeck Beds by Anderson (1966, p. 438, figs.

4-7,ll) and from “Purbeckianyyby Oertli (1963, p. 18, P1. 5 , figs. 28-32). Therefore, Pinnocyprideu

Shi and Ho, 1963 may be one piece of evidence that the Pinnocypridea-Jingguella concurrent

range zone corresponds to the Portlandian-Berriasian of Europe. The zone contains two subzones

as follows:

Deyangia (Deyangella) prona-Jingguella (J.) Subconcurrent Range Zone

This subconcurrent range zone is part of the Pinnocypridea-Jingguella concurrent range zone

containing the characteristic Deyangia (Deyangella)prona (Wei, 1979). Its corresponding lithostratigraphical units range from the Changxi Formation to the Lower Member of the Qiqusi Formation in Central Sichuan and from the Jianmenguan Formation to the Jiange Formation in

North Sichuan. Its changes of thickness are relatively large. It is 468 m thick in the type section of

Shiquan, Zhongjiang County and it is more than 1,000m thick towards the north in Jiange County.

It may be roughly correlated with the Portlandian, and includes two assemblage zones as follows:

3) Deyangia (D.)-Jingguella (J.) Assemblage Zone

This zone is that part of the subconcurrent range zone which contains the typical subgenus Deyangia (Deyangia) Li, 1983 throughout. It ranges from the Changxi Formation to the lower part

of the Bailong Formation in the central part of the basin, from the Jianmenguan Formation to the

lower part of the Hanyangpu Formation in the northern part of the basin, and consists of the

Tianmashan Formation in the western part of the basin. It is 300 m thick in the Baimaguan section

of Deyang County, and in North Sichuan reaches more than 600 m. It may be roughly correlated with the Lower Portlandian.

4) Jingguella (J.) acutura-J. (Minheella) minheensis Assemblage Zone

This zone takes the existence of Jingguella (Minheella) minheensis Song and Cheng, 1982 and the

abundant development of the genus Pinnocypridea Shi and Ho, 1963 as diagnostic. All the genera

of the Pinnocypridea-Jingguella concurrent range zone are well-developed in the present assemblage zone, but Cypridea Bosquet, 1852 appears only rarely. Lithostratigraphically, the zone extends

from the middle part of the Hangyangpu Formation to the Jiange Formation in the northern part

of the basin, and from middle part of the Bailong Formation to the Lower Member of the Qiqusi

Formation in the central part of the basin. Its thickness in the Shiquan section of Zhongjiang

County is 220 m and towards the north in the Jiange section it reaches 600 m. It may be correlated

with the Middle-Upper Portlandian.

Cypridea dayaoensis-Pinnocypridea Subconcurrent Range Zone

This subzone is that part of the Pinnocypridea-Jingguella Concurrent Range Zone which contains abundant Cypridea Bosquet, 1852, and ranges from the upper part of the Qiqusi Formation

to the Gudian Formation. Its residual thickness is 287 m. It may be roughly correlated with the

Berriasian, and includes two assemblage zones as follows :

5 ) Cypridea dayaoensis-Qingiiania Assemblage Zone

The diagnosis of this zone includes abundant Cypridea Bosquet, 1852 in association with Qingjiania Li, 1983, Jingguella Gou, 1977 and Pinnocypridea Shi and Ho, 1963 which are also abundant.

It corresponds to the Upper Member of the Qiqusi Formation in the central part of the Sichuan

Basin. In the type section of Shiquan, Zhongjiang County, it is 110 m thick and may be correlated’ with the Lower Berriasian.



1256



Y.W. Lr



6) Cypridea (Ulwel1ia)-Simicypris Assemblage Zone

The characteristic of this assemblage zone is the coexistence of abundant Cypridea (Ulwellia)

Anderson, 1939 and Simicypris Li, 1983, and of relict Jingguella Gou, 1977 and Pinnocypridea

Shi and Ho, 1963. It corresponds to the Gudian Formation in Central Sichuan, and is 117 m thick

at the type section of Gudian in Zhongjiang County. It may be correlated with the Middle-Upper

Berriasian.



THEJURASSIC-CRETACEOUS

BOUNDARY

IN SICHUAN

The Biostratigraphical Boundary

Judging from the abovekmentioned division into biostratigraphical units, the Damonella shuangbaiensis-Darwinula oblonga Concurrent Range Zone undoubtedly belongs to the Jurassic Kimmeridgian, because it contains Damonella shuangbaiensis Gou, 1977 and Cetacella inermis Martin, 1958.

The Pinnocypridea-Jingguella Concurrent Range Zone should be the biostratigraphical zone in

which we search for the Jurassic-Cretaceous boundary, because some species of the genus

Pinnocypridea Shi and Ho, 1963 in this zone are also found in the Portlandian-Berriasian of

Europe.

The fossil features of the two subconcurrent range zones contained in the Pinnocypridea-Jingguella Concurrent Range Zone are notably different (Text-fig. 3). In the Deyangia (Deyangella)

prona-Jingguella Subconcurrent Range Zone there are only are Cypridea Bosquet, 1852, and there

are relatively new genera, such as Deyangia Li, 1983,Pinnocypridea Shi and Ho, 1963 and Jingguella

Gou, 1977 which occur at higher or lower horizons, in addition to Damonella Anderson, 1966 and

Darwinula Brady and Norman, 1889 which are a continuation of genera from the Kimmeridgian.

In the Cypridea dayaoensis-Pinnocypridea Subconcurrent Range Zone, there are abundant Cypridea

Bosquet, 1852, and in addition to Darwinula Brady and Norman, 1889, Damonella Anderson, 1966,

Deyangia Li, 1983, Jingguella Gou, 1977, and Pinnocypridea Shi and Ho, 1963 which are a continuation from the underlying subconcurrent range zone, there appear new genera such as Qingjiania

Li, 1983 and Simicypris Li, 1983. Thus there should be a mutation surface between the two

subconcurrent range zones. In the Cypridea dayaoensis-Qingjiania Assemblage Zone of the lower

part of the Cypridea dayaoensis-Pinnocypridea Subconcurrent Range Zone, a great many Cypridea

Bosquet, 1852 such as C . sowerbyi Martin, 1940, C . primeva Anderson, 1941, C. alta a h Wolburg,

1959, C . declivis Wu and Yang, 1980, C . obliqua Wolburg, 1959, Cypridea cf. C. vulgaris Krommelbein, 1962, C. dayaoensis Chen and Ye, 1977 and C. cf. C . vidrana Wolburg, 1959 are associated

with some species of Qingjiania Li, 1983 (Text-fig. 3). Among them are some species which were

first named by Wolburg (1959) from Upper Purbeck to Wealden Beds in NW-Germany and which

are supposed by many palaeontologists to actually correspond to the Middle Purbeck Beds of

England (Brenner, 1976). C . vidrana Wolburg is considered to be a zone fossil of the Middle Purbeck Beds of England by Anderson (1971). C . obliqua Wolburg ranges from the Middle Purbeck to

Upper Purbeck Beds (Anderson er al., 1971). C . sowerbyi Martin, 1940 is found abundantly in the

Cypridea dayaoensis-Qingjiania Assemblage Zone. This species is named from the Serpulite Bed in

North Germany, and is associated with C . granulosa (Sowerby, 1836) which is considered to be the

first fossil zone of the Middle Purbeck Beds of England (Anderson et al., 1971). The Serpulite Bed

corresponds to the Middle Purbeck Beds of England (Brenner, 1976, p. 171). C . vulgaris Krommelbein, 1962 is named from the Upper Teil Beds in Germany (Krommelbein, 1962); the comparable

species in Sichuan is very similar except for its larger size. Finally, C. primeva Anderson, 1941

is associated with C. granulosa in England. To sum up, the Cypridea dayaoensis-Qingjiania



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Chapter 87. The application of Ostracoda to location of non-marine Jurassic-Cretaceous boundary in Sichuan Basin of China

Tải bản đầy đủ ngay(0 tr)

×