Synchronous transformations of mineral and organic constituents of sedimentary rocks in geological structure with an initial extension and subsequent …

103 considered lithogenesis as “…deposi-tion and subsequent transforma“…deposi-tion of sediments into sedimen-tary rocks during their simultaneous subsidence and increase in temperature

Lithology and Mineral Resources, Vol 38, No 3, 2003, pp 209–222 Translated from Litologiya i Poleznye Iskopaemye, No 3, 2003, pp 251–266.

Original Russian Text Copyright © 2003 by Petrova, Le Thi Nginh, Stukalova, Sokolova, Nguyen Xuan Huyen, Phang Dong Pha.

Most intense processes of secondary mineral

forma-tion are observed under geotectonic settings with

max-imal gradients of pressure, temperature, and solution

chemistry Oriented pressures, which result in rock

deformation, jointing (usually in extension zones), and

folding (compression zones) are most important in

areas with active tectonic regime These processes are

accompanied by the formation of new mineral phases,

in addition to crushing, grinding, and recrystallization

of primary minerals Yapaskurt was among the first

researchers who emphasized the need to study

second-ary alterations in strongly deformed rocks and compare

them with regional background transformations He

wrote: “…two mechanisms of lithogenetic

transforma-tions are observed in rock-forming basins under

mio-geosynclinal tectonic conditions The first mechanism

intensifies structural and mineral transformations in

rocks due to their subsidence and increase in lithostatic

pressure and temperature The second mechanism is

responsible for locally superimposed dynamothermal

alterations at tectonic activation and deformation

stages Both these processes are spatially and, probably,

genetically interrelated, representing elements of a

sin-gle discrete-continuously developing fluid–rock

sys-tem” (Yapaskurt, 1992, pp 178–179)

Earlier, Marakushev (1986) proposed a slightly

dif-ferent concept, according to which the first group of

governs metamorphism He separated the activity of these processes in time and believed that they corre-spond to different stages of geosynclinal belt forma-tion Lithogenesis corresponds to the geosynclinal stage of sediment accumulation during their subsid-ence, whereas metamorphism corresponds to subse-quent stages of deformation of geosynclinal sediments and formation of orogenic fold belts accompanied by the rise of geoisotherms and ascent of juvenile meta-morphosing fluids

Marakushev (1986) showed that authigenic mineral formation during lithogenesis occurs in a relatively closed system that is characterized by an approximately uniform pressure on rocks and relevant interstitial solu-tion Dehydration of primary minerals under such con-ditions is hampered and incomplete, which results in the coexistence of hydrous and anhydrous phases and the formation of hydromicas In contrast, metamor-phism occurs in an open mineral-forming system with filtering solutions characterized by high mineralization and partial pressure Such conditions are more

favor-1 Marakushev (1986, p 103) considered lithogenesis as “…deposi-tion and subsequent transforma“…deposi-tion of sediments into sedimen-tary rocks during their simultaneous subsidence and increase in temperature (in accordance with geothermal gradient) and pres-sure.”

Synchronous Transformations of Mineral and Organic Constituents of Sedimentary Rocks in Geological Structure

with an Initial Extension and Subsequent

Compression Tectonic Regime

V V Petrova1, Le Thi Nginh2, I E Stukalova1, A L Sokolova1,

Nguyen Xuan Huyen2, and Phang Dong Pha2

1 Geological Institute, Russian Academy of Sciences, Pyzhevskii per 7, Moscow, 119017 Russia

e-mail: petrova@geo.tv-sign.ru

2 Institute of Geology, National Center of Science and Technologies of Viet Nam, Nghihia Do Tu Liem Vien Dia Chat,

Hanoi, Viet Nam

Received October 21, 2002

It is shown that secondary mineral parageneses formed in two stages The first stage (35–17 Ma ago) corre-sponded to the period of structure extension and sediment subsidence to a depth of about 6 km This period and subsequent ~10 Ma were marked by the formation of a usual dia- and catagenetic zoning of metasedimentary rocks The second stage (5–7 Ma ago) corresponded to processes of compression that were responsible for the deformation of rocks into gentle folds and 1.5 to 2.2 times contraction of the section thickness in different places The sequential–mineralogical zoning was disturbed at this stage Smectites and mixed-layer minerals were replaced by chlorites and hydromicas Organic material also responded to compression simultaneously with inorganic components The bituminous component was released from humic matter and rocks became enriched in hydrocarbons

210 PETROVA et al.

able for the dehydration of primary minerals at lower

temperatures As a result, stable anhydrous mineral

phases are formed during metamorphism at a higher

rate and shallower depths relative to catagenesis

Secondary mineral parageneses produced by

litho-genesis and metamorphism under low temperatures and

pressures and altered rocks can show such a strong

sim-ilarity that “they differ only by the attitude… and can

frequently be discriminated only by the comprehensive

geological mapping of particular sequences”

(Maraku-shev, 1986, p 107)

Luk’yanova (1995) carried out extensive

investiga-tions of catagenetic processes in unstable tectonic

set-tings and concluded that “…the intensity of catagenesis

in sedimentary formations of orogenic belts is more

dependent on the type of tectonic structures composed

of these sequences rather than on the their age and

sub-sidence depth” and that “…the intensity of catagenesis

in sedimentary sequences increases in all stratigraphic

units regardless of their subsidence depth in areas with

intense tectonic movements and high heat flow In

geo-logical structures with an intense tectonic activity,

ver-tical catagenetic zoning is compressed (the thickness of

separate zones decreases), whereas catagenetic

alter-ation of coeval rocks increases as compared with that in

structures with less intense tectonic movements Zones

of early catagenesis in the zoning frequently disappear”

(Luk’yanova, 1995, p 155)

ROLE OF STRESS IN THE FORMATION

AND EXISTENCE OF SECONDARY MINERALS

When and at what stage of geological structure

development does the primary sedimentary rock

alter-ation intensify? Are the structure opening, intense heat

flow, and highly mineralized hot solutions essential for

such intensification? It is virtually impossible to answer

these questions based on the study of ancient (or

rela-tively ancient) altered rock sequences that experienced

a long-term and intricate geological evolution We

attempted to answer them using a strongly altered

(sec-ondary chlorite–hydromica assemblage) Neogene

sedi-mentary sequence with a sufficiently clear geological

history as example

The fold zone of the Red River valley in northern

Viet Nam served as an investigation object This zone

is approximately 1000 km long and stretches from

Tibet to the Bac Bo Bay It represents an important

geological boundary that separates Indochina and

South China Some researchers believe that the Red

River suture zone originated as early as in the

Precam-brian (Cheng, 1987; Chenging, 1986) or Paleozoic

(Helmcke, 1985; Wang and Chu, 1988) However, the

majority of researchers believe that this event occurred

in the Mesozoic (Hutchison, 1989; Tran Van Tri,

1977) Reactivation and opening of the structure

com-menced in the Eocene as a result of differently oriented

stresses induced by the NE- or NNE-oriented Indian

subcontinent motion toward Tibet (Eurasian Plate), on the one hand, and the SE-oriented Indochina Peninsula movement, on the other hand (Fig 1) According to Gatinskii (1986), the newly formed linear structures have all typical features of continental rifts

According to Tran Ngoc Nam (1999), Phung Van Phach and Bui Cong Que (1999), and other researchers, the intensity of tectonic processes during the opening of the Red River fault zone depended on the Indian Pla-teau–Eurasian continent distance and the convergence rate of these blocks At the first stage (16–35 Ma ago) when the Indian Plateau was still located relatively far from the Eurasian continent, maximum pressure on the Indochina Peninsula was exerted in the NW–SE direc-tion The displacement along faults was sinistral This stage was marked by structure widening with the suc-cessive centripetal subsidence of basement blocks along a system of steep faults (Fig 2)

At the second stage (5–7 Ma ago), the Indian Pla-teau exerted a higher pressure on China and pushed it in the western direction Consequently, stress on the Indochina Peninsula changed direction from the NW–

SE to NE–SW one, and the displacement along faults became dextral (Fig 3) In the middle and late Miocene, the above process resulted in the successive compression, rise, and folding of sediments in the fault zone and the formation of several narrow fold belts along the Red River fault zone (Fig 4) The compres-sion was maximal (1.5 to 2.2 times higher than in other areas) in the northwestern part of the Red River conti-nental basin (between towns of Lao Cai and Viet Ti) According to data in (Tapponier, 1995), the sinistral displacement in the Red River zone terminated 17 Ma ago and the regional movement inversion occurred about 5 Ma ago

Phung Van Phach and Bui Cong Que (1999) noted that the study region was also tectonically active in the Pliocene–Quaternary when some areas continued to rise, and sinistral and dextral faults reactivated Tec-tonic movements during this period were, however, related to peculiarities in the internal structure of the Indochina–South China zone rather than the global dis-placement of plates

As is seen in Fig 2, the transverse cross section of the Red River fault zone represents a relatively narrow trough-shaped structure bounded in flanks by large faults The depth of its most subsided part is about

5 km, although some researchers estimate it at 7 km Boreholes drilled in its deepest part penetrated Upper Mesozoic rocks, but the trough is mostly filled with Cenozoic sediments subdivided into several units (Fig 5) Their brief description follows below

Eocene–Oligocene Phy Tien Formation ( pt ).

Its lower part is composed of black argillites alternating with breccia-type conglomerates, sandstones, and brown-red siltstones Its upper part consists of con-glomerates, breccia-type concon-glomerates, gravelstones with lentils of silty argillite, unsorted rocks, and

argil-P 2 3 ,

–

SYNCHRONOUS TRANSFORMATIONS 211

lites The rock color varies from red and red-brown to

less common black and reddish black Clasts in

con-glomerates consist of metamorphic rocks, quartzites,

siltstones, and rhyolites The matrix consists of detritus

of clayey, sericitic, and sandy–silty rocks Slickensides

are abundant in the section The thickness is 220–400 m

Oligocene Dinh Cao Formation ( dc ). Black to brown argillites alternating with lenses of breccia-type conglomerates, gravelstones, dark gray sandstones, and siltstones Argillites are highly foliated and locally strongly fractured The rocks are strongly deformed as

in the previous formation The thickness is 140 m

P 3

–

Fig 1. Linear extension structure representing the geological boundary between South China and Indochina (Tran Ngoc Nam, 1999).

0

1

2

3

4

5

km

Mz

Mz

Borehole

Fig 2. Basement of the Hanoi Trough subsided along a system of steep normal faults Paleotectonic reconstruction based on (Le Viet Trieu, 1996).

South China

Laos

Viet Nam

South China Tibet

India

IIInnn dddoooccchhh iiinnnaaa PPPeee nnn

E xxxttt eeennnsssiiiooo nnn

zzzooo nnneee

Lao Cai

R

eeedddR Riii vvv eeerrr fffaaauuulllttt zzzooonnneee

Hanoi

500 km

200 km N

212 PETROVA et al.

Oligocene–Miocene Thuy Anh Formation

conglom-erates and breccia-type gravelstones in the lower part)

alternating with siltstones and thick-bedded clays The

rock color varies from light gray to whitish gray, dark

gray, and brown-gray Coarse-grained rocks are

charac-terized by obscure cross-bedding In the Dong Kuang

Trough, the formation encloses limy conglomerates

Fine-grained rocks are parallel- and thick-bedded The

rocks have a graywacke composition and contain clasts

of limestones, quartzites, siliceous rocks, shales, and

basic and acid volcanics cemented by carbonate, clay,

and siderite The thickness varies from 200 to >1000 m

Lower Miocene Phong Chau Formation ( fÒ ).

In the central area of the trough, the lower part of the

section is composed of members of thick-bedded

well-sorted sandstones with horizontal, wavy-horizontal,

and differently oriented cross-bedding Its upper part

consists of wavy-banded members composed of platy

sandstones, siltstones, and dark to black argillites The

rocks have mainly gray, dark gray, or gray (sometimes

brown-gray) with black lenticular interbeds color and

contain glauconite, siderite, and pyrite It is assumed

P 3

–

N 1 1

that the sediments accumulated in small lagoons and bays during sea transgression

Middle Miocene Phu Cu Formation ( pc ). It includes three subformations The lower subformation

is composed of fine- to medium-grained well-sorted sandstones alternating with siltstone beds characterized

by horizontal-parallel small-scale lamination The upper part of the subformation largely consists of mas-sive coal-bearing argillites (80%) alternating with hori-zontally bedded light to dark gray sandstones Plant impressions are abundant The thickness is 100–800 m The middle subformation is composed of light gray medium-grained sandstones alternating with thin-lami-nated siltstones containing glauconite in the lower part and alternating massive coal-bearing siltstones and argillites with rare sandstone interbeds in the upper part The thickness varies from 180 to >300 m

The upper subformation consists of gray to light gray, medium-grained, thin-bedded, slightly cemented sandstones and siltstones with abundant remains of marine fossils, plant impressions, and glauconite grains The middle part of the subformation encloses siltstones, argillites, and rare coal seams and lenses

N 1 2

Lao Cai

Yen Bai

Minh Binh

EAST CHINA SEA

BAC BO BAY

Hanoi

Hon Gai L

A O S

F Faaauuu lllttt zzzooonnneee aaalllooonnnggg

aaa

LLLooo RRR iiivvv eeerrr fffaaa uuulllttt zzzooo CCC hhhaaayyy RRR iiivvv eeerrr fffaaa uuulllttt zzzooo nnneee

RRR eeeddd RRR iiivvv eeerrr fffaaa uuulllttt zzzooo nnneee

10 N

N 1 12

3

8 N

9 N

2 N 1

2 1

3 4

23°00′

22°00′

21°00′

20°00′

19°00′

Fig 3. Cenozoic tectonic structure of northern Viet Nam (Phung Van Phach and Bui Cong Que, 1999) (1) Miocene sediments; (2) Late Miocene (NE–SW oriented) tectonic compression; (3) Early Pliocene (NW–SE oriented) tectonic compression; (4) Pliocene–Quaternary (N–S oriented) tectonic compression.

SYNCHRONOUS TRANSFORMATIONS 213

The rocks with wavy bedding alternate with massive

varieties

Massive siltstones and argillites are strongly

cemented and alternate with gray to light gray

medium-grained sandstones and thin-laminated siltstones with

impressions of brackish-water macrofossils The rocks

enclose abundant coal seams, particularly in the Kien

Xuong and Tien Hung areas, as well as abundant

sider-ite, pyrsider-ite, and glauconite The thickness is 2000 m

Three sea transgressions accompanied by

sedimen-tation in boggy settings are assumed

Upper Miocene–Pliocene Tien Hung Formation

The lower subformation is composed of coarse- to

medium-grained sandstone with lenses of gravelstones,

argillites, and gray to dark gray siltstones enclosing

abundant coal seams Sandstones contain abundant leaf

impressions Preponderant are coarse-grained rock

varieties The section located near the sea yields marine

fossils

The upper subformation consists of coarse-grained

sandstone with gravel, fine-grained sandstone,

silt-stone, and clay interbeds and coal lenses The rocks are

less compact and slightly cemented The light gray and

massive clays enclose plant remains The upper part of

N 1 3 N 2 1

the subformation consists of gray to dark gray, well-sorted, fine-grained sandstone alternating with parallel-bedded siltstones and argillites

It is assumed that the lower subformation formed in marine settings at the initial stage of transgression, whereas the upper one formed in a boggy delta

Pliocene Vinh Bao Formation ( vb ). It is composed of greenish yellow thin-laminated siltstones with interbeds of well-sorted sandstones consisting of well-rounded grains The rocks enclose foraminifers and other marine fossils The thickness is 100–300 m The sediments presumably accumulated in marine settings during extensive transgression covering the entire trough

The considered factual material suggests that the Red River fault zone experienced two different periods

of development

The first period was marked by the formation of extension structures, which originated in the latest Mesozoic and evolved up to the Pliocene The evolution was accompanied by the subsequent centripetal subsid-ence of basement blocks along the system of steep faults Eocene–Oligocene sediments occur in the deep-est (about 5 km) part of the newly formed trough-shaped valley, whereas Miocene–Pliocene sediments

N 2 2–3

300

200

100

0

m

Paleozoic

Yen Bai

b ëÇ

Ä

Ç

1

Vhu Tho Cam Khe

Doan Hung Thac Ba Dam

Tuyen Quang

Co Phuc Tran Yen

Ä

Ç Yen Bai

Hoang Trang

Red River Fault zone

Chay River fault zone

Lo River fault zone

a

22°

22°

21°

Hinge 240/20

300/35

180/30

NW

SW

225/35 240/60

N1

N 2 –Q

225/80

N 1

230/75 240/70

c

d

e

20′

40′

00′

Fig 4. System of narrow fault zones along the Red River near the town of Yen Bai and folding direction in Neogene sedimentary

rocks in particular areas (Phung Van Phach, Bui Cong Que, 1999) (a) Strike of fault zones along the Red, Lo, and Chay river valleys:

(1) outcrops of Neogene sedimentary rocks; (b) A–B profile in Fig 4a; (c) NW–SE oriented compression of Miocene–Pliocene ( N1–2)

sedimentary rocks in the Tran Yen area; (d) NW–SE oriented compression of Miocene–Pliocene ( N1–2) sedimentary rocks in the Co

Phuc area; (e) inclined attitude of Miocene ( N1) sedimentary rocks and coal seams in the Hoang Trang area overlain by horizontal

layers of Pliocene–Quaternary ( N2− Q ) sediments.

coal

214 PETROVA et al.

are distributed along flanks of this structure The

sedi-ments accumulated in shallow-marine, coastal-marine,

and coastal-boggy settings during several stages

corre-sponding to insignificant transgressions The age of

sediments is determined on the basis of abundant plant

impressions and shallow-water marine fossils

The second period (terminal Miocene–Pliocene)

was characterized by a change in the direction of

pres-sure on the newly formed extension structure, which

resulted in the successive compression, rise, and

fold-ing of accumulated sediments and the formation of

sev-eral narrow fault zones As a result, the former

exten-sion structure turned into the compresexten-sion structure

accompanied by a significant shortening of the section

result-ing section seems to be approximately two times

shorter as compared with the initial one

Consequently, secondary minerals could be formed

owing to both diagenetic and catagenetic alteration of

sediments during their subsidence (extension period)

and changes in mineral formation parameters in the

course of compression-related rise and folding of

sedi-ments (compression period) The section near the town

of Yen Bai was selected for the thorough study of

sec-ondary mineralization (Fig 6) The main part of the

section was sampled (samples V-1–V-5, V-11, and

V-12) along the profile extending from the Yen Bai bridge to the northeast (Fig 6b) The remaining part of the section was examined in the area located southwest

of the bridge Samples V-14 and V-15 were taken near the Lo River (Bach Luu section) The section thickness exceeds 1650 m (Fig 6a) Its lower part is composed of cobblestones (Member Ia), and only its upper part is exposed It is overlain by conglomerates and gravel-stones with coal lenses (lower part of Member Ib) Peb-bles in conglomerates consist of quartzites, siliceous rocks, basic and acid volcanics, limestones, and shales

Conglomerate beds alternate with thick-bedded sand-stones, siltsand-stones, and less common argillites The rocks have a gray color with whitish, brownish, or dark tints The thickness is about 300 m The sequence formed during the Oligocene–Miocene transition period

The quantity of sandstone, siltstone, and argillite interbeds increases upward and banded patterns of the sequence become gradually thinner Sandstones become fine- to medium-grained and the amount of silt-stone and argillite interbeds increases The middle part

of the section encloses abundant coal seams Several rhythms are distinguished in the section each beginning with coarser material and terminating with the finer-grained one (upper part of Member Ib and members II

? Group System Series Index

Hanoi Trough

Along the valley

Luc Yen Bao Yen

Along the Red River valley

1 2 3 4

5 6 7 8

9 10 11 12

13 14 15 16

a b c

N2

N1

N1

N1

P3

P2–3

Fig 5 Correlation of Cenozoic sections in the Red River valley Based on (Le Thi Nghinh et al., 1991) (1) Olistostrome-type

der breccia; (2) sandstone, siltstone, and argillite with subordinate boulder breccia; (3) argillite and siltstone with subordinate

boul-der breccia; (4) boulboul-der conglomerate; (5) conglomerate with different-sized pebbles; (6) gravelstone; (7) sandstone; (8) siltstone;

(9) argillite; (10) marl; (11) alternating sandstone, siltstone, and clay; (12) alternating siltstone and clay; (13) sediments with coal

seams and lenses; (14) large unconformities: (‡) with weathering crust, b) with erosional surface; (15) unconformities: (a) small,

(b) vague contact; (16) organic remains: (a) freshwater fauna, (b) marine fauna, (c) flora.

SYNCHRONOUS TRANSFORMATIONS 215

Fig 6 (a) Composite Neogene section in the northwestern part of the Red River Depression (the detailed characteristics of this

section interval is shown in Fig 6b) Sign “+” in the column “Mineral composition” designates the presence of a particular mineral

in the sample Letter designations: (g) gypsum, (d) dolomite, (c) calcite, (m) metahalloysite, (z) zeolite.

(b) The bed-by-bed characteristics of section near the Co Phuc Settlement (see Fig 4d).

and III) The sediments have early to late Miocene age

The thickness is more than 1000 m

The section is crowned by greenish yellow

thin-lam-inated siltstones with layers of well-sorted sandstones

composed of well-rounded grains This member (IV) is

arbitrarily assigned to the Pliocene The thickness var-ies from 50 to 350 m

All rocks in the section are deformed into folds with

(a)

Sandstone with conglomerate interbeds and coal seams

10.0

?

?

Age Member

Thickness, m Lithology

Eocene- Oligocene

IV

III

II

Ib

Ia

Description

Alluvial sediments

Gray argillite with coal seams

Alternating argillite, sandstone, and coal

Intensely crushed dark gray argillite with calcite veinlets

Gray argillite with rare siltstone interbeds

Alternating conglome-rates and coarse-grained sandstones Crush zone in sandstones

Clayey siltstone

Sandstone and argillite Abundance and size of pebbles in conglomerates sharply decreases.

Sandy conglomerate (locally with sandstone interbeds (0.5–1.0 m), and thin coal lenses

Cobblestone

Mineral composition

Quartz Feldspar Mica Chlorite Chlorite-smectite

Smectite Kaolinite Other minerals

Organic matter

reflectance

Coal rank

Diffractogram fragments related to reflection from plane [001] of layer silicates.

(Regions, Å: 9–10–Micaceous minerals,

~14–chlorites)

c, g c m m

d, c

d, m c

z ?

m

z, m

z

83–90 78–90

80–100 80–114

71–115

71–77

74–85

75–82

hvAb

hvAb-mvb hvAb-lvb

hvBb-lvb

hvBb

hvBb-hvAb hvAb

V-14/1-6

V-15/1-4 V-12/1-3

V-11/1-4

V-10/1-13*

V-13 V-9

V-8

V-5

V-4a, b V-3

V-2

V-1e V-1d

V-1c

V-1b

V-1a

10.0 Sandstone

10.0 10.0

10.0

14.1 14.1

V-12/3 V-12/2 V-12/1 Argillite

V-15/3 V-15/2 Argillite

10.0

V-11/3 V-11/2 V-11/1

near coal

V-13/3 V-13/2

platy 9.98

9.98

9.98

14.2 Sample V-4

Sample V-3 (sandstone)

(coarse-grained sandstone)

Sample V-2 (clayey siltstone)

Sample V-1e (argillite)

Sample V-1Ò 9.98

9.98

9.98

(sandstone)

Sample V-1b (sandstone at the contact with coal lens)

Sample V-1a (sandstone)

61 Sample V-1a (sandstone); 63 Organic matter

Alternating laminated detrital argillite, siltstone, and sandstone

Sandy cement

cobblestone),

Tobacco-colored argillite with siltstone interbeds, coal seams, and calcite veinlets

Coarse-grained sandstone with conglomerate clasts

Mixed layer

216 PETROVA et al.

strongly lithified and overlain by horizontal

unde-formed (or slightly deunde-formed) Quaternary sediments

In terms of lithology, sandy–clayey rocks are similar

to each other through the entire section and largely

composed of arkose varieties They consist of quartz

with subordinate feldspars (both sodic and potassic

varieties) and biotite Sandstones enclose rare clasts of quartzites and acid volcanics

The peculiar feature of the rocks is their intense sec-ondary (superimposed) alteration that is most promi-nent in members II–IV (Fig 6a) The cement in the lower coarse-grained member is altered to a lesser extent The clayey component of the cement in sand-stones and siltsand-stones, as well as the entire argillite, are replaced by chlorite–mica aggregates (Figs 7a–7d) During the argillite replacement, about two thirds of primary smectite is transformed into mica and approx-imately one third is altered into Mg-chlorite These pro-portions (with some variations) are mainly typical of the middle and upper parts of the section (Figs 6a, 6b; fragments of X-ray diagrams) Similar proportions are also preserved in the replaced cement of sandstones and siltstones (Figs 7a–7d) Such stable proportions of mica and chlorite components in the replaced rocks of different grain sizes are particularly well seen in small fragments of the section Figure 6b demonstrates the bed-by-bed closeup transverse view of the Co Phuc sec-tion shown in Fig 4d It is evident that regardless of the rock type (argillite, siltstone, or sandstone), the propor-tion of mica and chlorite components in the secondary aggregates remains unchanged Micas are mostly repre-sented by well-crystallized dioctahedral varieties Their structures virtually lack expanding interlayers Micas with a low content of expanding interlayers (no more than 5%) are rare

It should be noted that alteration of sedimentary rocks, including argillites, results in disappearance of their clayey (smectite) constituent Of all examined rocks, only sandstone from Sample V-4a shows an insignificant quantity of smectite in the cement Mixed-layer minerals (chlorite–smectite, chlorite–vermiculite, and illite–smectite) occur in insignificant quantities only in some samples No confinement of these miner-als to certain parts of the section is noted

The structure of the primary cement in rocks changes as well: it looks like the cement is “squeezed out.” Quartz and feldspar grains and sandwiched biotite flakes are closely spaced Their cement forms thin films that can be observed in the microscope only under large magnification The distribution of newly formed mica shows a distinct layering Signs of solid-phase recrys-tallization are discernible in all primary minerals Con-sequently, blastogenic textures are developed in all sed-imentary rock types

The primary biotite is partly or completely replaced

by Fe-chlorite Mixed-layer silicates are also com-monly developed after biotite Feldspars are partly replaced by kaolinite, which is sometimes observed in the cement as well (Fig 7a, Sample V-11-4) Quartz grains are mostly unaltered However, they are deformed and frequently split into uniformly elongated blocks in areas of maximum compression Sometimes, recrystallization (blastogenesis) of quartz and biotite grains is observed (Fig 7a, Sample V-11-4) At

inter-Fig 6 (Contd.)

(b)

organic

Thickness, m Lithology Description

Mineral composition

quartz chlorite

reflec-rank Sample no.

Diffractogram

matter

fragment

10R a , %

35

105

20

70

50

25

50

35

35

6

50

25

35

10

50

Laminated argillite with Fe

Fe

Fe Fe

Detrital argillite with coal

Siltstone

Siltstone

lvb

V-10/2

9.98

9.98

9.98 14.1

tance

c, d

LITHOLOGY

1.541 1.700 1.817 1.979 2.127 2.23 2.28 2.456 2.56 2.95 3.19 Fs 3.57 Kaol 3.70 Fs 4.44 4.97 Mi 7.14 Kaol 9.98 Mi 11.6 Mixed-layer ill.-sm

1.540 1.699 1.817 1.980 2.127 2.23 2.28 2.465 2.56 2.85 3.24-3.18 Fs

3.34 Q

4.25 Q

3.50 Chl

3.71 Chl

4.98 Mi

7.0 Chl

9.98 Mi

11.8 ? 14.0 Chl

1.540

1.817 1.671

1.980 2.127 2.24 2.28 2.56 2.85 Chl 3.243.19 Fs 3.34 Q 3.53 Chl

4.70 Chl 4.98 Mi 7.06 Chl

14.0 Chl

9.95 Mi V-13-3 Sample

1.658

1.817 1.994 2.127 2.28 2.456 2.56 2.85 Chl 2.98 2.19 Fs

3.52 ïÎ 4.25 Q 4.70 Chl 4.97 Mi 7.0 Chl

~11 ill + sm 14.0 Chl

1.700 Qua

1.817 Q

1.890

1.906 Cat

1.978 Q

2.09 ä‡Ú

2.127 Q

2.23 Qua

2.28 Q

2.456 Q

2.55

2.88 3.03 Cat

3.18 Fs 3.67 Fs

3.34 Q

4.02 Fs3.85 Cat

4.25 Q

4.69 Chl

4.95 Mi

5.5 Fs

6.32 Fs

7.0 Chl

9.94 Mi

14.0 Chl

0.10 mm

0.15 mm

0.10 mm 0.10 mm

0.3mm 0.3 mm

0.15 mm

0.15 mm 0.15 mm

2.456

218 PETROVA et al.

granular boundaries, quartz grains frequently display

convexo-concave contacts typical of conformal

micro-textures usually produced by the mechanical

compres-sion of rocks (Figs 7e, 7f, Sample V-14-1) Altered

rocks usually lack free spaces When present, they are

filled with chlorite or, in very rare cases, by chlorite

associated with embryonic epidote grains Sometimes,

regardless of its constituents, the entire rock is replaced

by calcite In addition, insignificant quantities of

metahalloysite, dolomite, gypsum, siderite, and,

proba-bly, zeolite are also found

Thus, the following features can be considered

typ-ical of secondary mineral formation in rocks:

(2) development of secondary mica–chlorite

assem-blage; (3) absence of smectite and sporadic occurrence

of mixed-layer minerals; (4) presence of

recrystalliza-tion (blastogenesis) textures in the majority of rock

types; (5) development of sinuous and

convexo-con-cave (conformal) contacts between mineral grains;

(6) strong compaction of rocks; and (7) striate,

uni-formly oriented, and elongated distribution of

constitu-ent mineral grains

The wide distribution of secondary mica and

chlo-rite, presence of recrystallization textures, development

of conformal contacts between mineral grains, and

strong compaction of rocks, all these features indicate

intense alteration of primary sedimentary material

Under conditions of normal geothermal gradient, such

alterations occur in the course of subsidence to depths

of 5–7 km or more and are usually considered an

indi-cator of intense catagenesis The lack of sequential–

mineralogical zoning and presence of oriented textures

in altered rocks suggest, however, that the process was

more complicated The transformation of primary rocks

was probably caused not only by changes in parameters

of mineral formation during their subsidence, but by

other factors as well These processes can be explained

by the geological history of the studied fold zone

According to available data (Gatinskii, 1986), heat

flow in the Red River fault zone is as high as

grounds to suggest significant changes in heat flow

val-ues during the period from the Eocene to Recent The

calculated temperature for the depth of ~5 km

approxi-mates ~ 250°ë Consequently, the temperature and

pressure, which were responsible for the formation of

mineral associations indicating intense catagenetic

alteration, were typical of deepest zones of the trough during its extension Thus, it can probably be assumed that the overlying layers of the section were altered at the extension stage of the structure following the clas-sical subsidence scheme, i.e., from smectites to transi-tional mixed-layer phases and further to chlorites The subsequent compression of sediments resulted in dehy-dration of clays and mixed-layer structures and their transformation into chlorites and micas that are more stable in new environments This was probably also stimulated by a shift of the interstitial solution boiling point under the additional pressure Stability fields of minerals, such as chlorite, mica, and epidote, could also shift toward lower temperatures Stable primary miner-als were corroded or partly recrystallized at surface and near-surface levels under the compressive stress Con-sequently, heaving and compaction textures were developed

Thus, the formation of secondary mineral assem-blages in this zone occurred in two stages The first stage corresponded to structure extension and produced the usual diagenetic and catagenetic zoning of metased-imentary rocks The second stage was characterized by compression and resulted in the distortion of metaso-matic zoning Structures of typical surficial and near-surficial hydrous minerals, which were stable at low pressures, were replaced by anhydrous or low-hydrous crystalline structures that were more stable under new stress conditions The difference between deep and near-surficial secondary mineral assemblages was vir-tually leveled Mixed-layer minerals and smectites were only preserved in areas where the pressure was minimal, e.g., where cobblestones and conglomerates from lower horizons could resist the pressure The sandy or clayey cement in them is less altered, as com-pared with finer-grained sandstones, siltstones, and argillites from higher horizons

The behavior of organic matter buried in different parts of the section is remarkable According to (Le Thi

Nghinh et al., 1991), lower Miocene sediments of the

Hanoi Depression enclose only humic organic matter, whereas middle–upper Miocene sediments contain some sapropel, in addition to the dominant humic organic matter Results of the pyrolysis indicate that all examined samples of organic matter buried in the stud-ied section correspond to type III kerogene that forms only from humic organic matter It is logical, therefore,

to assume that the majority of organic matter was

trans-Fig 7 Compositions and textures of inequigranular rocks from the Yen Bai section Photomicrograph (without analyzer) and

dif-fractograms corresponding to bulk composition of particular samples: (a) Sample V-11-4 Fine-grained sandstone from the upper part of the section Filmy cement Kaolinite and mixed-layer illite–smectite are developed after feldspar and biotite, respectively; (b) Sample V-12-1 Siltstone from the upper part of the section Aleuropelitic texture Chlorite and mica laths are poorly oriented Primary quartz and biotite grains show recrystallization signs Smectite and mixed-layer minerals are replaced by mica; (c) Sample V-13-3 Alternating argillite and siltstone from the middle part of the section Aleuropelitic texture Mica and chlorite laths and coaly particles (black) are slightly oriented Smectite and mixed-layer minerals are replaced by mica; (d) Sample V-15-2 Argillite from the upper part of the section Pelitic texture Chlorite and mica laths are slightly oriented Smectite and mixed-layer minerals are replaced by mica; (e, f) Sample V-14-1 Fine- to medium-grained sandstone from the upper part of the section Filmy cement Con-vexo-concave contacts between quartz grains and deep fissures filled with cement are well seen.