Formation and evolution of the Yilan-Yitong graben

Tecronophysics, Elsevier

133 (1987) 165-173

Science Publishers

165

B.V.. Amsterdam

- Printed

in The Netherlands

Formation and evolution af the Yilan-Yitong

graben

TIAN ZAIYI and DU YONGLIN Scientific Research Institute for Petroleum Exploration (Received

and Development,

December

20,1985;

Ministty

accepted

of Petroleum

Industry,

Beijing (P.R. of China)

July 23,1986)

Abstract Tian, 2. and Du, Y., 1987. Formation (Editors),

Deep Internal

The Chinese developing

continental

and evolution

Processes

crust became

stage in the way of tectonic in a NNE

direction

tension

increased,

graben

has four features

(1) Basement (2) During characterized

unique

graben.

In: C. Froidevaux

Tectonophysics,

from

rift system was formed.

and then entered

a new

plates, extensional

faults

crust.

force of the Pacific and Indian

as materials

and Tan Tjong Kie

133: 165-173.

plate since the Triassic,

to the continental

by the progradation Moreover,

a huge Mesozoic-Cenozoic

the mantle

flowed

upward

and the horizontal

As a part of this rift system

the Yilar-Yitong

as follows:

faults developed its evolution,

by downfaulting

(3) Two groups uting uplifts

occurred.

Rifting.

a part of the large Eurasian

evolution

The eastern part of China was affected trending

of the Yilax-Yitong

and Continental

well in the graben

the graben

controlled

underwent

by the boundary

both Yenshan

faults.

and Himalaya

movements,

which were apparently

and depression.

of fault systems

which

mainly

consisted

of normal

faults,

resulted

in numerous

regularly-distrib-

and sags.

(4) Material

from the mantle

erupted

frequently

along the fault zones. The geotherm

turned

upwards.

Introduction

depths and split the lithosphere into zones of depression of varying size, with fractures forming

Located in northeastern China, the YilanYitong graben is more than 1000 km long and

avenues for the upward migration of material from the earth’s interior. Sediments of fluvio-lacustrine

lo-30 km wide, traversing Heilongjiang, Liaoning provinces. The major faults

facies that developed trolled the deposition

Jilin and bordering

along the rift valley of Jurassic, Cretaceous

conand

the graben run southward across the Liaohe fault depression in the Bohai basin, join the Tancheng-Lujiang fault zone, and extend through the Sanjiang basin into Soviet territory as far as the Sea of Okhotsk, constituting a magnificent rift system in the eastern part of Asia (Fig. 1).

Tertiary elastics which, being of tremendous thickness, created conditions favorable for the formation of hydrocarbons. This has attracted the attention of petroleum explorers, and geologists of the Daqing, Jilin and Liaohe oil fields have conducted

The Yilan-Yitong graben is characteristic of large continental rift valleys, with normal step faults on its two sides dropping down toward the center and, as is discernible, granite and basic rock intrusions, as well as volcanic eruption, occurred in the process of faulting. The faults formed under tension. The crust fractured to different

extents. The present paper attempts to discuss the geological features and mechanism of evolution of the Yilan-Yitong graben on the basis of data so far available. The Yilan-Yitong graben is located on the large uplifted zone of the Jilin-Heilongjiang fold zone in Asia; it is an elongated Meso-Cenozoic

0040-1951/87/$03.50

0 1987 Elsevier Science Publishers

B.V.

geological

prospecting

in this region

to varying

166

During

the lndo-Chinese

part was eroded; faulting

and

middle

Jurassic;

subsidence

ceous but growth the Eocene full

the course amounts with

halted

the graben down

more than

of its development, along

eruptions,

movement. along

to

an

in its

arenaceous

intrusion

the faults. occurred occurred;

to be an active

zone,

volcanoes

are

of large associated

during

In the Cenozoic

the faults

several

reached

3040 m thick. In

the area is assumed where

deposition:

in the early

in the late Cretaceous;

laying

formation

volcanic

eruptions

started

to Oligocene

of granites

Yenshan

it> elevated

it grew a little in the early Creta-

development,

mudstone

movement,

there was no Triassic

the

era. basalt up to now. earthquake found.

The

graben assumes a NNE-NE trend, displaying a recent anticlockwise shift. Drill logs from the oil fields indicate a high paleogeothermal field, with a heat flow of 2.0 HFU (G.I.J.P., 1979). It is be-

Fig.

1. Location

continent,

-Yilan-Yitong c -!Sanjiang B -Beijing,

faulted

map

2 = ocean,

of

graben, basin,

the

Yilan-Yitong

I =

h -Tancheng-Lujiang

d -Songliao

basin,

H - Harbin. HR -Heilongjiang

sedimentary

graben.

3 = basin, 4 = river, 5 = major fault. o

basin

e -Bohai

fault, basin.

River.

with a long history

lieved that the Yilan-Yitong graben is among the large Meso-Cenozoic continental rift valleys in China and also a part of the Meso-Cenozoic valley system in eastern China. There are many things in common between

rift the

Yilan-Yitong graben and its well-known counterparts of East Africa, Baikal and the Rhine, but of

evolution. Its basement is composed of geological units of different character and different age, with the Laoyeling massif in its northern part and the northeastern portion of the China-Korea platform in its southern part, and a greater part of it forms the Heilongjiang-Jilin fold zone which is Hercynian in age. The major fault bordering the graben that cuts the lithosphere is a normal fault, having gone through a long and multicyclic process of development and evolution. Its geological history is closely linked with the development of the graben. According to the geophysical data, the crustal thickness of this zone is more than 30 km and the thickness of the lithosphere is, roughly, over 60 km; a distinctive zone of regional negative anomaly as shown on the Bouguer gravity anomaly map. The location of the negative anomaly on the aeromagnetic map correlates well with the morphology of the graben. A review of its geological past is necessary to understand the graben better.

differences naturally exist (see Table 1). From its morphology, origin and development, it is more like the Rhine graben between the Vosges Mountams of France and the Black Forest of the Federal Republic of Germany. This graben cuts the basement of the Hercynian fold zone. Yilan-Yitong graben, it is a complicated

Like the fault-de-

pression basin of Tertiary age, trending NNE and holding 3400 m thick sediments (Table 1). GedagW

clmracteristics

of tbe gmben

As the different sections of the structure in the elongated Yilan-Yitong graben were not subject to the same stress, the fault systems within its domain underwent different courses of development, and the rift valley under their control followed a different direction of extension in its different parts, having different features of development in the following respects: (1) High-angle normal faults control the morphology of the graben. The morphology and

167 TABLE 1 A comparative study of the Yilan-Yitong

graben with the Baikal rift valley and the Rhine graben

Features

BaikaJ rift valley

Rhine graben.

Yilan-Yitong graben

Size

1500 km long, 70-100 km wide

650 km long, 20 km wide

1000 km long, lo-30 km wide

Basement

Caledonian fold zone of Mt. Sayan

Hercynian fold zone

Hercynian fold zone of Jilin and Heilongjiang

Structural features and trend

Mostly elongated and dust-pan-like fault-depressions trending NNE

A double faulted graben, with its main fault parallel to the river valley and trending NNE, its NW trending fault cutting the graben into fault blocks of varying sizes

Primarily a NE trending double faulted graben, cut by a NW cross fault into minor grabens and horst blocks

Age of sediments and their thickness

Mostly Neogene and Quatemary deposits, more than 5000 m thick

Primarily Eocene to Miocene deposits, with some Triassic and upper Jurassic deposits, however, Cretaceous missing; more than 3400 m thick, transgressive at places

Jurassic and Cretaceous rocks present, with Eocene to Oligocene as its main stage of development: more than 3000 m thick

Volcanic features

Primarily alkaline basalts, interbedded with tholeiite

Primarily alkaline rocks, alkaline basalts and ultrabasic rocks

With

multi-stage basaltic erupassociated with basic rock intrusions

Other features

Crustal thickness 35 km, heat flow 2.5 H.F.U., negative gravity and magnetic anomaly

Crustal thickness 24-30 km, heat flow 3.0 H.F.U., negative gravity and magnetic anomaly

Crustal thickness over 30 km, heat flow 2.0 H.F.U., negative gravity and magnetic anomaly

Recent surface

Large lake, 1620 m deep

The Rhine river valley

A lowland, densely covered with rivers and swamps with the Songhuajiang river flowing through its northern part

evolution of the rift valley were mainly determined by the development of faults. As the rise and fall of fault blocks in the different parts of the structure could not be the same in rate and magnitude, it is but natural that the resulting fault uplifts and depressions could not be of the same scale and order. There are two main groups of faults in the graben, one NE trending and the other NW trending. The former have a long history of development, mostly with wide extension and large vertical displacement, in contrast with the latter which are of later development. Nevertheless, both of them are high angle normal faults, having been subject to shearing force of varying intensity in the different geological ages.

tions,

An analytical study of geological data available has definitely proved the existence of the two NE trending major faults on the two sides of the graben. The gravity anomaly maps show clearly a z6ne of anomalous gravity gradient of over 6 mGal/km on each side, the gradient is as high as 10 mGal/km around Tangyuan; bead-like magnetic anomalies, too, are found along the sides of the faults. Drilling data also give evidence of the wide difference in thickness and lithology of strata along the faults: in the hole No. 56-131 on the upthrown block, granites are encountered at the depth of 128 m, while in the hole Fangcan No. 1, no Mesozoic rock has yet been penetrated as far down as 3046 m (Fig. 2). Observation of the

p

$I

$4 $I

l&l

F

F

y

0

Fig. 2. Cross Jurassic,

section

for comprehensive interpretation of the Yilang-Yitong graben. J, + z = lower-middle Jurassic, J1 = upper D = basic volcanic rocks. E, = middle Eocene, E, = upper Eocene, N = Neogene. Q = Quatemary,

K, = lower Cretaceous,

h - magnetic-anomaly

line, c = gravity-anomaly

lint, d = position

recent geomorphology shows clearly that the cliffs caused by displacement along the two sides of river valleys have a succession of step-like levels like those seen in Yantongshan and the Songhuajiang river valley at Jiamusi (the Songhuajiang river basically follows the graben and flows in a northeasterly direction after passing through the Songliao plain.). These geomorphological features seem to be closely related to the step-like faults created by subsurface tilting. The NE bordering fault has a 44”-55” dip angle and a vertical displacement of over 3000 m. Inside the graben, along with the NE trending major faults, there are, in the different sections, NW trending minor faults of varying density, with a 50” -80” dip angle, formed primarily by the horizontal shearing force and arranged en echelon and in a zigzag way. It should be pointed out that the graben was the main zone of release of stress where horizontal tension, coupled with the differences in the petrology of the basement, formed fault blocks of all sizes and different elevations. The NE trending faults that resulted primarily from tensile stress

of holes.

determined the length of the fault blocks and the NW trending faults which were formed by shearing stress determined their width. Long fault blocks were formed at places underlain by rigid basement rocks where straight tension faults developed. Extensive fault depressions were formed at places where the NW trending faults were widely pulled

apart. As a result, a number of minor uplifts and depressions developed inside the graben. They were arranged alternately from south to north like this: Tangyuan depression, Yilan uplift, Fangzheng depression, !&q&i uplift, Shulaa depression, Wulajie uplift, Chaluhe depression, Yibadan uplift, Dagushan depression etc. And within each of these minor uplifts and depressions, there were faults of even smaher scale which dissected them into smaller Hocks of different elevations, so that the graben has a very complicated tectonic framework comprising a land-mass of different width, length and elevation (Fig 3). (2) Development of multi-stage igneous rock formations. Igneous eruptions in the rift valley are expressed in different ways due to the difference of depth of faults and intensity of faulting. In

169

syenite porphyry dikes,

with very few intermediate-acidic

penetrated

the deeper

the lower Cretaceous Tertiary

rocks.

rocks

and intruded

which was later overlain

The isotopic

by

age of this stage

is

123-102 Ma. The upper Cretaceous rocks, present in the form of volcanic cones, are eruptive basalts underlain age

by the lower Cretaceous.

is about

Basalts,

78.5

basaltic

olivine-basalts mentary

of igneous

above

during

different

early

periods,

stage

igneous

of the

rift valley, intermediate Fig. 3. Tectonic

framework

b = uplifts,

direction

of

2 -Yilan

uplift,

lift,

5-Shulan

depression,

stresses,

of the Yilan-Yitong c = fault,

f = cities.

3 -Fangzheng depression,

R-Yibatan

uplift,

d = suspected I -Tangyuan

depression, 6 -Wulajie Y-Dagushan

graben.

a =

fault,

e=

depression,

I-Shangzhi uplift,

up-

7-Chaluhe

depression.

other words, magmatic eruptions are closely related to faulting, and specific types of magmatic formation developed under specific regimes of tectonic movement. The Yilan-Yitong graben, initiated in the Jurassic period and matured during the early Cretaceous, attained its full growth in the Tertiary. Judging from the contact of the igneous rock formation with the country rocks, we may sum up its development into three periods. In

1978).

augite-basalts

the behavior at different valley,

rocks predominated development

sedi-

of differ-

in the graben

rift

and

the Tertiary

The infilling

rocks

indicates

With the further

depressions,

tuff-lava,

Their isotopic G.I.H.P.,

are seen among

scribed

acidic

(from

rocks, interbedded.

ent types

the

Ma

as de-

of faulting depths.

In

intermediate in the graben.

and opening

of the

igneous rocks gradually graded into to basic rocks as a result of the rise

of upper mantle materials. Despite caused by mixing sialic materials

the complexity in varying de-

grees, it is still evident, on the whole, that faulting and magmatism at the surface are controlled by the variations of the lithosphere at depth. In the meantime, there is evidence of gradual younging of rocks from the margin toward the graben (Fig. 2). Coal measures

the interior of of Jurassic age

occur mostly at the margin of the graben except in the elevated parts that have been eroded. Further in are rocks of early Cretaceous Paleogene and Neogene. Igneous rocks are mostly distributed in the rift valley in bands, with the older rocks at the margin and the younger ones in the interior.. Besides, the crust at the axis of the rift valley is

the early stage of the Yanshan movement, that is in the late Jurassic, dikes of biotite granite,

evidently thinner than that on the sides, and there is reason to believe that the valley was initially formed as the result of pulling apart the two

granodiorite, diorite, granite porphyry, and hornblende gabbro intruded the Taiantun and Ningyuanchun formations; these are of Jurassic age and overlain by the Taoqihe formation of Cretaceous age. The isotopic age of this stage is 182-157 Ma (from G.I.J.P., 1979). During the late stage of the Yanshan movement, that is in the late Cretaceous, dikes of granite porphyry, syenite porphyry, granosyenite porphyry, albite porphyry, diorite porphyry and lamprophyte, characterized by the presence of a large amount of meta-alkaline

flanks of a large fault, resulting in its present complicated framework. (3) The extent and mode of fault-depression determine sedimentation inside the graben. In the initial stage of development of the rift valley in the Jurassic period when it was under peneplanation, the fault uplifting was slight and the fault depression was of limited extent and depth. Jurassic beds were scattered over the numerous isolated depressions, accompanied by the distribution of large amounts of intermediate to acidic

I70

igneous eruptives. During the early Tertiary intense faulting and rapid sinking of the depressions

in the same sections. During the early and middle Jurassic. when lake water was shallow and con-

accelerated

fined

the rate of deposition

tremendous

thickness,

suite of superimposed sociated Unlike well

the patterns

might

of deposition

those

double-fault

bringing

narrow,

sediments

As the source

materials

get into

formed

flowing

the graben

in

by a

As the graben from

center and become

axis could

rocks.

which are

the

up when passing

the depositional

a as-

of lake basins

are quite complicated.

and

along

depressions

in the graben

easily get mixed

to

deposits

to basic eruptive

and single-fault

known.

is long

usually

fluviolacustrine

with intermediate

depressions

of sediments

sides

through

unidentifiable. along

only

the long

through

the

to small

elastics,

areas,

largely

deposited.

voluminous

suites of coarse

of diluvial-alluvial

From late Jurassic

facies.

were

to early Cretaceous.

some of the small depressions

were linked

up so

that the water mass grew in size but most of them were

isolated.

water

level and separated

the

water

joining

level

luvial-alluvial though

together

came facies

fluvio-deltaic

to a limited

extent.

facies zones

began

in time

down.

Deposition

predominated and

of high

from each other

when of di-

at the time.

lake deposits

coexisted

At this time, the coarse elastic to shift toward

the sides and

dark mudstone of lacustrine facies began to accumulate in the central part of the graben to a

NW trending shear-fault slip, they must have been reworked and carried over the sediments coming from the sides so that it is even more difficult to

certain depth, ceous, volcanic

separate them. inland climate

tion until after the Eocene when the graben started a new phase of development, with the deepening

In the meantime, the changing and the shifting of water mass

within limits. In the late Cretaeruptions ruled out normal deposi-

outside the graben had a noticeable effect on the small lake basins. Figure 4 shows the depositional

of lake water and widening of the water surface so that the graben became a unified sedimentary

pattern

system. At this time, the coarse elastic facies zone shifted further towards the sides and the distribution of the lake deposits in the center of the graben became more and more extensive. As a

of the graben

during

the Mesozoic

and

Cenozoic eras. We may note from the figure that deposition in the lake basins followed the same general trend of distribution of intermontane fluvial facies, diluvial-alluvial facies. deltaic facies and lacustrinal facies from the lake margin toward the center. As the climatic conditions in the different geologic periods were not the same, the resulting facies zones are not of the same width nor

Fig. 4. Depositional d - lacustrine’ facies, N = Neogene,

pattern in the Yilan-Yitong e - marsh deposits.

Q - Quatemary.

result, littoral,

shallow

divided

lake, deep lake, turbidite

into

and del-

taic subfacies. According to the data of the Fangcan No. 1 well, the lithology of the stratigraphic column

graben. u = allti-diluvial

J, + 2 - lower-middle

the lake facies may be further

from Eocene to Oligocene

consists

of three

facies, b - fluvial facies, c = lacustrine-deltaic

Jurassic J3 - upper Jurassic.

K, - lower Cretaceous,

facies,

E - Eocene.

171

parts.

In the lower

naceous

part

conglomerates

m thick;

are beds

at the base is a thick

glomerates further

there

and mudstones,

grading

upward

upward

stone of stationary is a thick

are responsible

for the difference

lithology

lithofacies

of con-

sequence

into

sandstones,

are mudstones

coal. In the middle

of are-

over 1400

intercalated

different

depths

and

indicates

the complicated

with

history.

part are 200 m of dark mudlake deposits.

sequence

In the upper part

of sandstone

thin beds of dark mudstone,

intercalated

altogether

m thick. It is a mixed accumulation

about

the graben that

800

ment

of structures,

of turbidites.

The same is true in the Paleogene

its tectonic

column,

has

upwardly from deeper

decreasing

variegated

is characterized

grain sizes showing

to dark

as the distribution

colors.

The

by

variation lake

grew

of lake facies became

more and more widespread and coarse-elastic facies zones became

the marginal further apart.

All these are closely related to the pull-apart the graben. The development of deposition sediments reflects the process graben by gradual pull-apart

is closely

tion,

evolution

of of

of faulting in the and unbalanced

tectonic movements results in the difference of distribution of sediments vertically and horizontally. Formation and evolution of the graben The mechanism of the graben formation has been a subject of elaborate discussions by geologists since the 1930’s. Different inferences, arguments, and views have been proposed, but due to the complexity of the geological phenomena and the long history of evolution, no consensus has yet been reached. Some hold that the East African rift valley, located on the arched part of a platform, is a fault graben formed as a result of tensile stress. Others propose that the East African graben was created by the thinning of the earth’s crust and the stress induced by horizontal tension due to the rising of mantle material. From the development of the Yilan-Yitong graben we may see clearly that it has gone through a long process of evolution in which it has undergone many stages of sinking and uplifting and has been subject to both tensile and compressive stresses. Meanwhile, fracturing to different depths and different horizons of the earth crust, as well as the difference in magnitude and rate of faulting

related

is to say that within

Yilar-Yitong

continent

geosyncline

This

structure

and

the

loca-

geothermal

was situated where

movements

Mesozoic

of

develop-

are all determined

and the tectonic

graben

of the Asian

column.

to its tectonic

magmatism,

Before

at

of its geological

sedimentation,

the graben

location

experienced.

Mongolian

nature

of

material

of the geological

by

characteristic

of association

of mantle

of the geological

The development

stratigraphic

which

and

era,

by it the

in the middle

the Xingan-Inner

was in its initial

period.

It

was formed there by S-N compression since the Paleozoic, ending with folding and uplifting at the end of the Permian, whereby it was linked up with the China-Korea ble platform-like

platform structural

to form a unified stasystem. From the Tri-

assic, under NNE counter-clockwise shifting caused by the movement of the Pacific plate and the Indian plate, the lithosphere in the eastern part of the Chinese continent was uplifted and a large part of northern and northeastern China was denuded so that the process of deposition during the Triassic was greatly restricted both in thickness and extent. With uplifting as the background and the associated tensile fracturing, quite a number of depressions were created on this part of the earths crust where rocks were incompetent. These depressions, great in number but small in size, grew either in the young folded basement, as the Heilongjiang Hercynian fold zone, or on the old stable platform, as the North China platform. Subduction of the Pacific plate under the Chinese continent since the Mesozoic caused revival of magmatism in the interior. With

the the

development of faulting to greater depth, volcanic extrusives like tuff, acidic and meta-alkaline volcanic rocks came to the surface along the faults. In the Cenozoic, the Pacific plate continued its activity, resulting in the intensification of the depression due to tensile faulting. In the meantime, faulting continued to extend downward and cut through the mantle, so that mantle material rose through the faults in columns, causing the eruption of basic volcanic rocks in large amounts.

From the above study. cess of evolution

we may divide

of the Yilan-Yitong

the pro-

graben

into

the Jurassic

of regional period.

zone of uplift suffered

uplift (Fig.

the region under

in the eastern

expressed sulting

mantle

doming

by different

geological

convection

having

It was caused as is

phenomena

variations

re-

as well as

of rocks. This state of uplift of

the land is inseparable

from the compression

posed on the lithosphere that lies between

to

review was a

of the lithosphere

from heat and gravity

thermal

5a). Prior

part of China.

a long period of denudation.

by slight

layers

of the Chinese

the Indian

im-

(Fig.

of tuff and

tuff-arenaceous

of faulting--depression

5~). In the early Cretaceous

subject

to further

sinking

of the fault depressions

nitude

tension

but extended

that caused

Volcanism primarily

and intermediate

volcanic

with

coal

containing

was active in meta-alkaline clastlcs, mixed

continental

clastic-aren-

aceous mudstones. (4) A brief stage of archrng (Fig.

continent

late Cretaceous

and Pacific plates.

mag-

the basin area for deposition off

venting

was

continued

in a smaller

this

stage.

der~elopment the region

of thin beds of sediments. eruptives

con-

of the upper Jurassic.

(3) Stage

six stages as follows: (1) Stuge

multiple glomerates

5d). By the

to the end of the Paleocene.

the

(2) The initial tension stage (Fig. 5b). The uplift of mantle material produced a strong horizontal

graben was in a state of elevation. and with the formation of the Yanshan geosynclinal fold zone

tensile force that led to tensile fracturing of incompetent rocks and to the formation of small

in the eastern part of the graben, a magma emerged on the continental margin of eastern China. At

fault depressions in zones where the crust is thinner. Deposits in these depressions consist of intermediate to acidic igneous elastic formations

this time intermediate acidic eruptives dominated the deposits in the graben; normal sedimentary rocks were missing, as faulting was closely related

of middle and lower Jurassic and a suite of coalbearing continental elastic formations containing

to volcanic eruptions. (5) Stage of violent faulting-depressron (Fig. 5e). The graben had its fullest development from Eocene to Oligocene. Clockwise shifting took the place of the NNE counter-clockwise movement of the Pacific plate.

shifting in the and horizontal

tension and vertical displacement were intensified, as the graben was at the height of its development. The surrounding

elevations

were denuded,

supply-

ing ample terrigenous source materials to the fault depressions, whereby a suite of fluvio-lacustrine arenaceous mudstones consisting mainly of dark mudstones and greyish-white sandstones was deposited in the graben, to a maximum thickness of over 3000 m. Dark and greyish dark basic extru-

Fig. 5. Evolution

the

text.

tension.

of the Yilan-Yitong

a. Stage

of regional

c. Stage of fault development.

e. Stage of violent faulting. of graben.

I = upper mantle,

I = intermediate-acidic

graben

uplifting.

as explained

b. Initial

stage

in of

d. Brief stage of arching.

f. Stage of uplifting

Conclusion

and contraction

2 0 mantle pad, 3 - earth’s crust,

volcanics.

sives like basalts are widespread with well-developed fumaroles. (6) The stage of uplifting und contraction of grahen (Fig. 5f). Up to the end of Neogene, as a result of tectonic movement, the eastern part of China as a whole. suffering only from slight compression, was uplifted and faulting and sinking in the graben came to a close.

5 = basic volcanic

rocks.

From the above, we may conclude that the Yilan-Yitong graben is a continental rift valley,

173

similar

to the Rhine graben

and linking China,

and the Baikal graben,

up with the other rift valley in eastern

it forms

a Meso-Cenozoic

rift valley

sys-

are of great

fault depressions

zoic strata, graben.

tem. Before

the

Jurassic

located

in a regional

China.

Since

subduction

period,

a NNE

large

small

of

formed,

bearing

position

and

plate under

shifting

tension

fault

stress

foltook

basin

a

were

characteristics

of deJurassic

on the west side

At that time, it was quite possible

that, in the time of transgression,

the graben

might

be interconnected with the Longzhaogou group in the Sanjiang basin on the east side of the graben where alternate marine and continental facies preso that

littoral-marsh

facies

occurred

in

some parts of the graben. The Sanjiang and Songliao basins which were located on the flanks of the graben

became

widely

different

from one

another in structure as well as in deposition. former was characterized by intense faulting

The and

accumulation of volcanic eruptives but lack of normal deposits, and the latter by steady sinking of immense magnitude and insignificant tectonic movement but lack of volcanic eruptives. In the Tertiary, horizontal tension and vertical faulting and sinking were intensified, accompanied by the expansion deposition terrigenous depressions

and deepening of the water volume and of a thick sequence of predominantly materials. A number of varying degrees

where

ever

are covered that

migration

the

by Ceno-

to the great width

indications

The Yilan-Yitong

shifting

as the underlying

of the Songliao

of the graben.

of

continental

depressions

are

transformation,

the Chinese

of the graben

in addition

These

origin,

thickness

of the

favour

and

the

accumula-

tion of hydrocarbons.

in eastern

stage in the Jurassic,

the same

faulting

was

as a result

counter-clockwise

place. In the initial number

graben

uplift zone formed

lowed by a clockwise

depressions

the

the Meso-Cenozoic,

of the Pacific

continent,

vailed,

Deposits Mesozoic

of minor fault of development

were in the process of being unified by faulting into a large graben in this period and began to receive extensive deposits first in the north and then gradually in the south. At that time the graben linked up with the Sanjiang basin in the north and the lower Liaohe basin in the south, all of them having similar geological features. The development of the graben in the Cenozoic reconstructed and unified the Mesozoic fault depressions that had so much influenced the graben.

graben

of the crust,

which volcanic alkalic

is characteristic

rift valley. Under faults

activity

grew

deposited, around

From

acidic,

metapat-

of the sediments

the zonal distribution

the graben

along

the regular

and lithofacies

of volcanic

and thinning

the axis and its thickening

of a

of tensile

in depth

of all kinds,

and basic occurred.

tern of lithology

the action

rocks

of the crust along

over the sides of the

graben, it is believed that the rift valley started its development first along the axis and then gradually spread

to the flanks,

until

finally

it took the

form as we see it today. Geographically, the Yilan-Yitong graben, runs through the Sanjiang, Songliao and Bohai basins. As everybody

knows

that our major

oil resources

come from the Songliao and Bohai basins, what a magnificant prospect we would have if it could be established

that

Yilan-Yitong them.

the

graben

Sanjiang are

basin

genetically

and

the

related

to

References Bott, M.H.P., troductory

1976. Mechanisms review.

Basins

of

physics,

36: 1-4.

G.I.H.P., G.I.J.P..

Continental

1978. Manual

Province. Geological Halbouty,

Institute

M.T.,

petroleum

Margins

Institute

subsidence-an

Bott (Editor), and

of the Geological

Geological

1979. Manual

of basin

In: M.H.P.

Cratons.

Tectono-

Map of Heilongjiang

of Heilongjiang

of the Geological

in-

Sedimentary

Province.

Map of Jilin Province.

of Jilin Province.

1976.

Application

and mineral

of Landsat

exploration.

imagery

Am. Assoc.

to

Pet. Geol.

Bull., 60: 745-793. Li, C.Y., 1982. Plate tectonics, tectonics

today.

the most prevailing

In: Advances

in Structural

theory

Geology.

of Sci-

ence Press, Beijing, pp. 34-40. Liu,

H.F.,

1982. Analysis

Structural

Geology.

of oil structure.

Zhu, X., 1982. Oil and gas basins Advances 113-123.

In: Advances

in

Science Press, Beijing, pp. 125-132.

in Structural

Geology.

in Meso-Cenozoic

eras. In:

Science Press, Beijing, pp.