2007b on some new significant unconformities pleozoic and mesozoic stratigraphy of north viet nam

For instance, the gap betw een Low er D evonian and L ow er Paleozoic form ations in the stratigraphie sequence of N orth Bac Bo H a G iang, Lang Son, and Thai N guyen provinces is a b i

ĐẠ I HỌ C QUỐ C GIA HÀ NỘ I

VIETNAM NATIONAL UNIVERSITY, HANOI

I SSN Ũ 8 6 6 ■ 8 6 1 2

KHO

JO U RN AL DF SCI EN CE

CÁC KHOA HỌ C TRÁI ĐẤ T EARTH SCIENCES

V o lu m e 23, N o 3, 2007

VIETNAM NATIONAL UNIVERSITY, HANOI

JOURNAL OF SCIENCE

i -O C O JÔ C GIA HÀ NỘ I

TẠ P CHÍ KHOA HỌ C

Editorial C ouncil

Assoc Prof Dr.Sc Luu Van Boi

Assoc Prof Dr.Sc Le Van Cam

Prof Dr Dinh Van Due

Dr.Sc Nguyen Dinh Due

MA Nguyen Van Loi

Prof Dr Tran Nghi

Assoc Prof Dr Nguyen Nhuy

Ed itorial B oard of Earth Sciences

Assoc Prof Dr Truong Quang Hai

Assoc Prof Dr Nguyen Dinh Hoe

Prof Dr Nguyen Cao Huan

Assoc Prof Dr Pham Van Huan

Assoc Prof Dr Phan Van Tan

Dr Nguyen Van Vuong

Editorial Secretary: Assoc Prof Dr Nguyen Nhuy

E ditorial O ffice: 144 X uan Thuy, Cau Giay, Hanoi T e l: 84-4-7547902; Fax: 84-4-7547583

P ublication P erm it 03/G P -B V H T T Issued 03/01/2002

VNU Journal of Science, Earth Sciences 23 (2007) 137-146

On some significant unconformities in the Paleozoic and Mesozoic stratigraphy of North Viet Nam

Tong D zuy Thanh’*'

College o f Science, VN U

Received 10 July 2007

Abstract A lot of unconformities and stratigraphic gaps have been discovered in the Paleozoic and Mesozoic stratigraphy of Bac Bo region (North Viet Nam), but their role in the regional geology was not accessed yet in detail This paper is the first attempt to describe the most significant unconformities and stratigraphic gaps and discussing their role in regional geology The most important is the angular unconformity of Norian-Rhaetian formations upon various formations of different formations; it marked the change tectonic regime of the region resulting from the indosian orogeny in Viet Nam and in Southeast Asia in common The gap of Lower Devonian red beds formations (Si Ka Formation, Song Cau Group) upon Lower Paleozoic formations (Upper Cambrian - Than Sa Formation, Lower Ordovician - Lutxia and Na Mo formations) is a regional unconformity, which had been taken place in the epicaledonian terranne of South China and North East of Viet Nam The gap between the Da Mai Formation and the Dong Dang Formation was a result of a crust rising, but was not through the orogeny activity, although after this gap the structural plan was more differentiated Other unconformities and stratigraphic gaps seems to be local unconformities, for instance, the one of the Tan Lap Formation (D2g-D3 fr tl) upon the Ban Pap Formation (D)p-D3 fr bp) and the unconformity between the Sinh Vinh ( 0 3-S so) and the Ben Khe (E-O] bk) formations.

Keywords: Unconformity; Stratigraphy; Paleozoic; Mesozoic.

1 In tro d u ctio n

A lot of u nconform ities and gaps have been

described in Paleozoic and M esozoic stratigraphy

of N orth V iet N am F rom the u nconform ities

and gaps in com bination w ith the facies of

underlying and overlyin g form ations geologists

can draw the first step for interp retation of their

role in the geolog ical developm ent o f the

region A lthough the exp laination of the

geological d evelopm ent of a region requires

*Tel.: 84-4-8572246

E-mail: tongdzuy@vnn.vn

data from different g eological dom ains, for instance facial analyses, m agm atic activities, tectonical fram ew ork etc., the stratigraphic relations (unconform ities and gaps or hiatus) play an im portant role in dealing w ith this question T his pap er is an attem p t to design an approach to u nd erstand the role of som e stratigraphic gaps, h iatuses, and unconform ities

in the geolog ical d evelop m ent of the country The N orian angular u nconform ity had taken shape during the indosinian orogeny, and has been w ell-know n in the g eology of V iet N am and in Sou theast A sia H ow ever, there are a lot

of described u nconform ities and stratigraphic

137

138 Tong Dzuy Thanh / VNU Journal o f Science, Earth Sciences 23 (2007) 137-146

gaps in the geology of the cou n try hav e not yet

been assessed, althou g h they play an im portant

role in the in terp retation of geolog ical history of

the territory For instance, the gap betw een

Low er D evonian and L ow er Paleozoic

form ations in the stratigraphie sequence of

N orth Bac Bo (H a G iang, Lang Son, and Thai

N guyen provinces) is a b ig stratigraphie event

in the region This gap together w ith the large

distribution of Low er D evon ian red bed s is an

obvious evidence of the influence of the

Caledonian orogeny C ontrary to the conception

of m any geologists, in the area of the D a River

basin (W est B ac Bo region) a gap betw een

D evonian and its u nd erly ing form ations is not

observed H ere the Silu rian and D evonian

form ations are ch aracterized b y the continuous

sequence from Sinh V inh Form ation ( 0 3-S sv) to

Bo H ieng (S2 bh) and Song M ua form ations (D]

sm) The stratigraphie seq u en ce proves that the

Da R iver basin w as not influenced b y the

C aledonian orogeny, w hich occurred in pre-

D evonian period

This paper is the first attem p t to describe

the m ost significant u nconform ities and

stratigraphie gaps, and to d iscuss their role in

regional geology

2 Paleozoic unconformities and gaps

2.1 N um erous u nconform ities an d gaps w ere

described in the P aleozoic o f the B ac Bo region

Thanks to the geolog ical m apping of

d ifferent scales and them atic stratigraphie

studies, a lot of unconform ities and stratigraphie

gaps in P aleozoic of B ac Bo have been

discovered and d escribed A m ong them , the

follow ing u nconform ities and stratigraphie

gaps can be listed:

The unconform ity o f the Cam D u on g Form ation

(£( cd) upon the D a D inh Fm (PR3 dd): this

stratigraphie relation can be follow ed along the

contact betw een these tw o form ations extending

from Lang Phoi to D a D inh villages

The unconform ity o f the Song M a Form ation

(£2 sm ) upon the N am Co Fm (PR3-£i nc): The unconform ity of the Song M a Form ation (£2 sm) upon the N am C o Form ation (PR3-£i nc) can

clearly b e observed in the Suoi Toi Section (Thanh H oa P rovince), Ban N am - Ban Thay, Ban M o sections, in the u pper course of the Ban Pha Stream (C hieng P ac C om m une, Thuan

C hau D istrict, Son La Province), in the Na

H uong - Na V ien Section, and on Sam Co Pass (Km N o.18 - K m N o.21 on the road from M ai Son to Song M a)

The gap betw een A n Phu (N P -£i hg) and Ha

G iang (£2 hg) form ation s: T h e first gap in Low er

P aleozoic in the northeast of Bac Bo is observed

in the sequence betw een the Song C hay Group (An Phu Form ation) and the overlying Ha

G iang F orm ation that occu rs in som e sections in

Ha G iang Province

The u nconform able relation betw een the Sinh Vinh (O3-S sv) and the Ben Khe (£-O i bk) form ation s: T his u ncon form ity w ith basal conglom erate in the bottom of the Sinh Vinh Form ation is w ell exposed in m any localities such as Ban C ang, Sinh V inh M outh, Ban Ban, and Suoi N hap Stream (low er section of the Da River basin)

The u nconform ity o f the Low er D evonian upon Low er P aleozoic form ation s is expressed in the

stratigraphic relation of the Si Ka Form ation (Di

sk), Song Cau G roup (Di sc) w ith the underlying

form ations such as Low er O rdovician Lutxia Form ation in D ong V an D istrict (Ha G iang Province), U p p er C am brian T h an Sa Form ation

in T h an Sa area (Thai N guyen Province) and Ha Lang D istrict (C ao B an g Province) T he Si Ka Form ation belongs to the red continental facies, and its lithological com p osition and color have led m any geoscientists to correlate it w ith the

"O ld Red Sandstone" lying at the base of the

D evonian System

The unconformity between Tan Lap (D2g-D3 fr tl) and Ban Pap form ation s (D tp-D 3 fr bp) is an

Tong Dzuy Thanh / VNU Journal o f Science, Earth Sciences 23 (2007) 137-146 139

interesting u nconform ity, in w hich the tongue-

shape Tan Lap Form ation w ed ges w ithin the

Ban Pap Form ation

The unconform ity o f the N am P ia Form ation

(D| np) on Sink Vinh ( 0 3-S su) and D ong Son (Oi

ds) form ations: This stratigraphic relation

betw een Low er P aleozoic and L ow er D evonian

is expressed in the n ortheastern side of the

Song M a structu re only - in the u pper basin of

D a R iver and in the low er basins of the M a

River (W est Bac Bo)

The unconform ity o f the T oe Tat Form ation (D3-

C]fr tt) upon the Lung N am ( Q In) Form ation

occurs in H a Lang area only, w here these

form ations are exposed

The unconform ity o f U pper P erm ian upon the

Da M ai Form ation (C i-P 2 dm ) has been observed

in alm ost all sections w here these form ations

are exposed

In the M esozoic stratograp hy of Bac Bo,

m any u nconform ities have been noted, bu t the

m ost interesting is the N orian -R h aetian upon

form ations of d ifferent ages It is a regional

unconform ity and its im p ortant role in the

geology not only in Bac Bo, bu t also in geology

of V iet N am and of So u th east A sia in general

The m ost significant u nconform ities and

stratigraphic gaps and their role in the geology

of Bac Bo w ill be presented and discussed in the

follow ing sections

2.2 U nconform ity o f the L ow er D evonian upon

Cambrian and / or Ordovician form ations

The first regional u nconform ity has been

discovered betw een L ow er D evon ian and

C am brian and / or O rd ovician form ations in the

northeastern region of Bac Bo (Ha G iang, Cao

Bang, Lang Son, and T hai N g u y en provinces) It

is well expressed by the sequence of the Low er

D evonian Si Ka Form ation (D i sk) and the

underlying form ation in m any sections in D ong

V an D istrict (H a G iang Province) In all these

areas the Silu rian sedim ents have not been

discovered, and the Si K a Form ation w ith basal

conglom erate is alw ays observed in direct

contact w ith the u nd erly ing Lutxia Form ation

L ate O rdovician in ag e (Fig 1, Fig 8)

L o w e r

D evonian

- \ Bac Bun F (D bb) Grev chocolate shale.

,~ _ S ~ S~j\ marfy shale Howittia wangi Fauna

—'y y ■ ■ '/ Si Ka F (D, sk) Marly shale, black-grey

silt - stone Conalomerate in bottom (after Hoana

/ - / - / - / - \ XuanTinh etal 2000).

S ilu rian

c (0

0

>

0

■ 0

0

\ Middle O rdovician - Silurian hiatus

Upper part o f the Lutxia F (0,/x) Ash

■ \ grey clay shale, siltstone and sandstone.

J limestone containing Hysterolenus sp and

~) other Lower Ordovician Trilobiets

Fig 1 Unconformity between the Si Ka Formation upon the Lutxia Formation and a Middle Ordovician

- Silurian gap between them (Dong Van sections, Ha

Giang Province)

By litholog ical properties, bedding characteristics, color of rock, and by fossil rem ains the Si K a F orm ation itself is a red continental form ation, and it has been compared

w ith the "O ld R ed Sand stone" lying at the base

of the D evonian System Its distribution extends in m any areas of N orth Bac Bo, such as

in D ong V an D istrict (H a G iang Province), Chi Lang and Bac Son districts (Lang Son Province), Than Sa, Trang X a areas (Thai N guyen Province) In Ha Lang and D ong K he districts (C ao Bang Province) it occurrs as low er layer of the Song C au G roup, the low erm ost of w hich often begins by con g lom erate w ith pebble of different lithological com position and dimension

In the sequence of H a Lang (Cao Bang Province) and T h an Sa sections (Thai N guyen Province) O rdovician and Silu rian sedim ents are not found, and the Si Ka Form ation covers

in direct contact w ith the u nderlying Than Sa Form ation of the L ate C am brian age (Fig 2, Fig 8) H ow ever, in the T rang Xa area (Thai

N guyen P rovince), not far from the T han Sa, though the d irect co n tact betw een the Si Ka and the u nderlying form ation is not observed, but the U pp er O rd ovician Na M o Form ation and Silu rian sedim ents are also absent So, in this

140 Tong Dzuy Thanh / VNU Journal o f Science, Earth Sciences 23 (2007) 137-146

area, S ilu rian sedim ents do n ot exist, and the Si

Ka Form ation can be considered as unconformable

u p on the N am M o Form ation (Fig 3, Fig 8)

Lower

Devonian

Silurian

Ordovician

Cambrian

0* 0' o 0 ’ 0' o ‘ c

Mia Ls Fm (D, ml) Argillaceous shale, mudstone,

and marly shale containing Brachiopods, Corals of

Euryspirifer tonkiner/sis Assemblage

Song Cau Group (D, sc) Conglomerate, sandstone,

and argillaceous shale containing HowiUia wangi

(Brachiopod) and microvertebrate remains.

Ordovician - Silurian hiatus

■ / — / —

Upper part of the Than Sa Fm (£, fs) Sandstone, argillaceous shale, calcareous shale containg Upper Cambrian Trilobites

Fig 2 The Song Cau Group with basal conglomerate

unconformable overlies the Than Sa Formation,

between which there is an Ordovician-Silurian hiatus

(Ha Lang sections, Cao Bang Province)

Lower

Devonian

Silurian

Ordovician

7 - ^ 2

Bac Bun Fm (D, bb) Grey limestone, marly

shale containing Corals, Brachiopods of the

Howittia kvang/Assemblage

Si Ka Fm (D, sk) Red sandstone, mudstone

containing microvertebrate.

Middle Ordovician - Silurian hiatus

Upper part of the Na Mo Fm (O, nm)

Ash-grey day shale, sittstone and sandstone,

limestone containing Hysterofenus sp and

other Lower Ordovician Trilobites.

Fig 3 A Middle Ordovician-Silurian hiatus between

Na Mo and Si Ka formations (Trang Xa sections, Thai

Nguyen Province)

In C hi L an g and Bac Son areas (Lang Son

Province), O rd ovician and Silu rian sedim ents

have n ot b een found, and there is lim estone of

C am brian (?) age, and the Si Ka red sandstone

overlies u nconform ably this C am brian (?)

lim estone

Another unconformity of Devonian sediments

on older form ations is observed in the east of

Bac Bo (Q uang N inh Province), bu t in this area

the age o f u nd erly ing and overlying form ations

is d ifferent from those in the north of Bac Bo The u nd erlying form ations are Co To and Tan

M ai form ations of O rd ovician -Silu rian age, and the overlying is M id d le-U pper D evonian D o Son Form ation, so the gap betw een u nderlying and overlying form ations is shorter and later than the one in the north of Bac Bo

The L ow er D ev on ian red sedim ents are very w id ely exposed in South C hina (Y u nnan and G uangxi provinces) and have been described as the L ian h u ash an Form ation and its equivalents, w hich consist m ainly of sandstone,

m udstone, and basal conglom erate in m any cases The L ow er D evon ian red sedim ents are very w id ely exposed in South C hina (Yunnan and G uangxi P rovinces) and have been described as the L ianhu ashan Form ation and its equivalents, w hich consist m ainly o f sandstone,

m udstone, and basal conglom erate in m any cases The great thickness (reach to m ore 1000m ) and abu ndant m icrovertebrate rem ains are the m ost rem arkable characteristics of these continental and sub continental sedim ents

O bviously, this "red sandstone" w as deposited in the continental and su bcontinental environm ent after the C aled on ian orogeny

2.3 U nconform ity betw een N am Pia and Sinh Vinh,

D ong Son form ation s

T he N am Pia F orm ation (Di np) is

L ochkovian-P ragian in age and extends in the northeastern side of the Song M a structure - in the u pper basin o f D a R iver and in the low er

b asins of M a R iver (W est B ac Bo) A ccording to

B ui Phu M y, at the N am Pia M ou th section (upper basin of the D a R iver), the low erm ost

b ed s of the N am Pia Form ation consists of puddingstone, conglom erate and gritstone,

w hich rests u nconform ably u p on the Sinh V inh

F orm ation (O3-S sv) In the low er part of the M a

R iver basin, near the H am R ong bridge area (vicinity o f Thanh H oa Tow n), an u nconform ity occurrs betw een the N am Pia and the underlying

D ong Son F orm ation (Fig 4, Fig 8), bu t it w as characterized as "p seudo-conform ity"

Tong Dzuy Thanh / VNU Journal o f Science, Earth Sciences 23 (2007) 137-146 141

- -, _

— — — -

-/ / / Nam Pia Fm (D, n p) Conglome­

rate and gritstone, coarse-grained

1 1

c

/ / / quartzite, shale, marl, limestone

interbeds and lenses containing

©

abundant Lower Devonian corals.

c

3

w

Middle Ordovician - Silurian

c

hiatus

o

> — — — — — —

Dong Son Fm (Ot c/s) Siltstone,

quartzitic sandstone, and marl con­

taining Lower Ordovician Trilobites, Bivalves, and Brachiopods.

o — — — — —

" / / / "

o

_ _ _ _ _ _

Fig 4 Middle Ordovician-Silurian hiatus between

the Lower Ordovician Dong Son Formation and the

Lower Devonian Nam Pia Formation in the vicinity

of Thanh Hoa Town

T he stratigraphic relation betw een Low er

P aleozoic form ations and L ow er D ev on ian of

those form ations is interesting and is w orth

noting for an analysis The u nconform ity

occurrs only in the n ortheastern side of the

Song M a structure, w hile in ad jacen t areas of

the low er section of the D a R iver, w here

P aleozoic sedim ents are w id ely spread and w ell

studied, and there is no hiatus in the sequence

from O rd ovician to L ow er D evonian

T he stratigraphic relation b etw een Low er

P aleozoic form ations and L ow er D evon ian of

those form ations is interesting and is w orth

noting for an analysis In the low er basin of the

D a R iver, P aleozoic sedim ents are w idely

spread and w ell studied, and there is no hiatus

or u nconform ity betw een O rd ovician and

D ev on ian sedim ents W hile in the n ortheastern

side of the Song M a structure the hiatus is

occurrs betw een O rdovician or Silu rian and

Low er D evonian Thus, this hiatus took place

only in the m arg in of an u plifting structure, bu t

n ot in the center of the sed im entary basin

T hese evidences evoke a su ggestion that

perhaps the w estern region of Bac Bo w as not influenced by the C aled onian orogeny as the eastn orth em region, and in the low er D a R iver basin the sed im entation did not cease during Silu rian - the tim e w hen it w as in the northeast

2.4 U nconform ity betw een Tan Lap and Ban Pap form ation s

A n interesting u nconform ity is the one

betw een the Tan Lap (D 2g -D 3 fr tl) and the Ban Pap (D ip -D 3 fr bp) form ations (Fig 5, Fig 8)

The B an Pap F orm ation consists of grey bedded lim estone containing abu ndant fossils of C orals, Strom atoporoids, Brachiopods, and others, and

w id ely extend s in both the east and the w est of Bac B o region It is an u ninterru pted form ation, the low erm ost bed s of it has been dated as

Em sian based on C onodonts (Pol Excavates Zone: Pandorinellina steinhornensis, Pol excavatus,

Pol nothoperbonus) and D acryoconarids (N ow akia barrandei Zone), and the u pperm ost bed s is

Frasnian, m ay be lower Frasnian w ith Conodonts

Pa transitans and som e Strom atoporoids.

The T an Lap Form ation consists m ainly of sandstone containing land p lant rem ains of

L ep idodendropsis, and is restrictively exposed

in som e areas of C hi L an g and Bac Son districts (Lang Son Province) It u nconform ably rests

u p on lim estone of the Ban Pap F orm ation w ith basal conglom erate (Fig 5, Fig 8), am ong w hose pebbles are grains of igneou s rocks transported from a nearby erosive area w ith granite m assifs

In its turn, the Tan Lap Form ation is conform ably covered b y Early Frasnian lim estone of the Ban Pap Form ation W ith these stratigraphic relations and the restrictive distribu tion areas, the T an Lap F orm ation is referred to a tongue- shape stratigrap hic bod y w edges itself in the

U pp er p art of the Ban P ap Form ation This

u nconform ity and the tongue-shape Tan Lap Form ation bearin g Lepidodendropsis p lan t rem ains show that there w as an interruption in sedim entary process by a rising of the crust, bu t

w as p artially only, and this is a local unconform ity

142 Tong Dzuy Thanh / VNU Journal o f Science, Earth Sciences 23 (2007) 137-146

Chronostratigraphy Lithostratigraphy

Fig 5 The tongue-shape Tan Lap Fm wedges within

the Ban Pap Fm Tan Lap section, Lang Son Province

(1: Tan Lap Fm.; 2: Ban Pap Fm.; 3: Mia Le Fm.)

A n analogue h as been observed in the

stratigraphic sequence of M iddle-U pper

D evonian of Sou th C hina A ccord ing to the oral

inform ation of H ou H ong-fei, there w as M iddle

D evonian coarse terrigenou s form ation, a

sim ilar form ation of the Tan Lap, w edges itself

in the M id d le-U p p er D evon ian lim estone

form ation

2.5 U nconform ity betw een D ong D ang and D a M ai

form ation s

The D a M ai Form ation in eastern B ac Bo

region has b een dated as Early C arboniferous -

M id d le Perm ian, w hile the low erm ost of the

overlying D ong D ang Form ation is referred to

U p p er P erm ian (C hanghsingian) The

u nconform able relation betw een the D ong

D ang Form ation and the u nd erlying D a M ai

F orm ation h as b een observed in alm ost all

sections w here these form ations are exposed

(Fig 6, Fig 8) T his u nconform able b ou n d ary is

m arked b y the change in lithology, betw een the

light-grey thick-bedded lim estone and the

b lack -g rey terrigenou s-cherty or black-grey

carbonaceou s shale and m arl beds A ccording

to the above m entioned data, the gap betw een

D a M ai and the D on g D an g form ations is

correspond ing to the W u chiap ingian, i.e the

first tim e of the L ate P erm ian epoch The

stratigraphic relation betw een D ong D ang and

the overlying H ong N gai and Lang Son

form ations of Early T riassic age is likely conform able, b u t there is a short gap betw een those form ations, in fact

1 1 , 1

r z i

I I I

I I I

I I

Dong Dang Formation (P,1dd)

Chert, bauxite, and siltstone, frag­ mented cherty limeston, limestone containing Palaeofusulina In Lung Cam section - The Dong Dang overlies the eroded surface of the

Da Mai limestone.

Wuchiapingian (Lower of Upper Permian) hiatus.

c,-p 2

I I r

I I I

T T

1 1 , 1I I I

n E r

Da Mai Fm (C,-Pj dm). Dark grey to light grey thick bedded to massive limestone containing foraminifers from Visean (Lower Carboniferous)

to Capitanian (Middle Permian).

Fig 6 A short hiatus between Dong Dang Formation and Da Mai Fm corresponding to Wuchiapingian (Upper Permian) was noted Doan Nhat Truong's new data shown that the top of Da Mai Fm can be

up to Wuchiapingian, in this case the hiatus is even shorter - between adjoining Wuchiapingian and

Changhsingian stages

A ccord ing to D oan N hat T ru on g 's new data

in som e localities the top of D a M ai F orm ation

b ears the foram inifers of Codonofusiella -

R eichelina A ssem blage, and h as b een dated as

W u chiap ing ian age (Late Perm ian) By the

p resen ce o f P alaeofusulina the low erm ost of the

D ong D ang F orm ation has b een referred to

C han g h sin g ian stage of U p p er Perm ian, thus the gap in the sequence of U p p er P erm ian in eastern B ac B o is short (betw een the tw o

ad jacent W u chiap ingian and C hanghsingian tim es only)

Relating to this unconform ity and the bauxite

d ep osit in D ong D ang Form ation, D oan N hat

T ru ong w rote that the bau xite ore deposit in the

L ow er p art of the D ong D ang Form ation w as form ed during karstic process in the continental

en viron m en t on the lim estone of the D a M ai

Tong Dzuỵ Thanh / VNU Journal o f Science, Earth Sciences 23 (2007) 137-146 143

Form ation A fter that, the lim estone of the

D ong D ang F orm ation w as deposited in the

m arine environm ent, at a transgressive tim e

corresponding to the C hanghsingian Thus, the

bauxite and other com p onents of the D ong

D ang F orm ation w ere d eposited in different

environm ents, and the com bination of them in

one form ation is tem porary only

3 Mesozoic unconformities and gaps

In the north of V iet N am , the num bers of

u nconform ities in M esozoic stratigraphy have

been recorded, and the m ain ones could be

listed as follow s

H ie Hong N gai Form ation (Ti hn) and its

synchronous Lang Son F orm ation (T ! Is) appear

to conform overly w ith D ong D ang Form ation

(P32 dd), though the conodont parva Zone

(characteristic zone of the lo w er Triassic

boundary) has n ot b een found, so a short

depositional gap could happen

In the w est of Bac Bo, the C o N oi Form ation

lies u nconform ably u p on the Y en D uyet

Form ation (P3 yd).

The u nconform ity of the Song H ien

Form ation (T! sh) u pon the D ong D ang

Form ation (P32 dd) has b ee n observed in the

sections located in the w est of C ao Bang Tow n

and in Binh G ia area (Lang Son Province)

The Lan Pang F orm ation (T2a Ip) rests

unconform ably u pon the L ow er T riassic Song

H ien F orm ation or U p p er P aleozoic lim estone

T h e effusive K h on L an g F orm ation (T2a kl)

rests u nconform ably u p on U p p er P erm ian

lim estone and the Low er Triassic Bac Thuy

Form ation

The Yen Binh Form ation (T2a yb) rests

unconform ably w ith basal conglom erate upon the

Ha Giang Form ation (€ 2 hg).

The M au Son Form ation (T3c ms) appears to

underly unconform ably the coal-bearing Van

Lang Form ation (T3n -r vl).

The V an Lang Form ation (T3n -r vl) rests

u ncon form ably u pon older sedim ents for instance the O rd ovician Phu N gu Form ation,

the C am ian N a K hu at Form ation (T3c ms).

T he H on G ai F orm ation (T3n -r hg)

u ncon form ably rests u p on Low er P aleozoic Tan

M ai Form ation or u pon U p p er P aleozoic lim estone

The Su oi Bang Form ation (T3n-r sb)

unconform ably rests u pon m any older form ations, from Proterozoic (Sin Q uyen, Nam

Co form ations) to A nisian (D ong Giao Form ation) and even C am ian N am M u Form ation (Fig 7, Fig 8)

Fig 7 The Chu Mt - Xom Mu Hamlet Section, 7 km southwestward from Hoa Binh Town, shows the angular unconformity of the Suoi Bang Formation

(T3 n-r sb) upon the Dong Giao Formation (T2 a dg)

illustrated by Tran Van Tri

T he H a C oi F orm ation (JN2 he) covers w ith

angu lar u nconform ity L ow er P aleozoic bed s of the Tan M ai Form ation A s in Thai N guyen City, these bed s lie u nconform ably u p on N orian-

R haetian coal-bearing V an Lang Form ation The Yen Chau Formation (K2 yc)

u nconform ably rests u p on different older sedim ents

The N am M a F orm ation (K nm) and the Ban

H ang F orm ation (K bh) u nconform ably cover

d ifferent older sedim ents, for exam ple N am Po

(j! np) and H a C oi (J,_2 he) form ations.

The L ate Ju rassic effusive Tam Lung

F orm ation u nconform ably lies u p on L ow er

T riassic terrigenou s beds of the L an g Son

F orm ation, as w ell as u p on M id d le Triassic form ations

144 Tong Dzuy Thanh i VNU Journal of Science, Earth Sciences 23 (2007) 137-146

CHRONO-STRATIGRAPHY

WEST NAM BO &

GULF OF THAILAND

M ID

NINH

PERMIAN

CARBONI­

FEROUS

DEVONIAN

SILURIAN

ORDOVI­

CIAN

C AM BRI­

AN

D,

D«

NEOPROTERO-ZOIC

Ta I Hon I Dat

Not jQ u a n i Do Chu Prong

Cam Lo Dong Dang Bai Chay

Ha Tien

Hon Heo

m in i] ;

Dak Lin

Da Mai Ban Diet

Da Mai

Da Mai

Cu Brei

Phong Hanh

Phong Ị

Son ,

A Choc

La Khe Da Nieng Xom Nha

" Thier

- - — Nhan Dong Tho Mue Bai Ban Giang Rao Chan Dai Giang Long Dai

A Vuong

Nam Can Toe Tat

Bang Ca I

Lung Nam Toe Tat

Huoi Loi

Ban Pap

Tay Trang Nam Pia

Song Ca

Dong Son Ham Rong Song Ma

Ban Nguon Song Mua

Bo Hieng Sinh Vinh

Ben Khe Cam Duong

Lutxia Chang Pung

Ha Giang

Bang Ca Ban Pap _ Duong Mia Le Dong

S o n a lBac Bun

csUliaiJTJTJII

T T tm r K ienA n

Pho Han Con Voi

Phu Ngu

Na Mo

TTTTTTT

Than Sa

S o n g C h a y An p fu

G ro u P jThac Ba

Co To Tan M a i'

Fig 8 Stratigraphic correlation of Paleozoic Units in Viet Nam

From above m entioned u nconform ities and

gaps, the N orian-R haetian is the m ost interesting

one because it m arked a new period in the

geological developm ent of V iet N am and

Sou theast A sian in com m on In the tectonic

differentiation of the B ac Bo region in M esozoic,

other unconform ities p ro b ab ly are local ones

4 Discussion and inclusion

1 The angu lar u ncon form ity of Suoi Bang,

V an Lang, and H on G ai form ations of N orian-

R haetian age u pon variou s form ations of

different ages is an im portant event in regional

geology; it m arked th e ch an g e tectonic regim e

of the region resu lting from the indosian

orogeny in V iet N am and in Sou theast A sia in

com m on T he indosinian m ovem ent has

happened from P erm ian, bu t the intensified

phase took place in N orian of L ate Triassic by the collision of the M alaya plate w ith the Indochina T his angu lar u ncon fo rm ity has been largely record ed not only in V iet N am , bu t also

in Sou theast A sia and Sou th C hina, so there is

no need to discuss m ore abou t it

2 The u nconform ity of Low er D evonian Si

Ka F orm ation and Song Cau G roup upon Low er P aleozoic form ations is an im portant one, bu t its role in regional g eology has not yet been p roperly accessed by geologists The stratigraphic gap of L ate O rdovician and Silu rian largely occurs n ot only in the north of

N orth V iet N am (V iet B ac in V ietnam ese), but also in G uangxi and Y u nnan provinces of China T h e red bed s facies sedim ents (Si Ka Form ation in V iet N am and L ianhu ashan in South C hina) also largely extend in the sam e areas w ith u ncon form able relations upon Low er Paleozoic By litholog ic characteristics of