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

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 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 Geological Institute, Russian Academy of Sciences, Pyzhevskii per 7, Moscow, 119017 Russia e-mail: petrova@geo.tv-sign.ru 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 Abstract—Sedimentary rocks of the section in the Red River fold zone of northern Viet Nam are considered It is shown that secondary mineral parageneses formed in two stages The first stage (35–17 Ma ago) corresponded to the period of structure extension and sediment subsidence to a depth of about 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 Most intense processes of secondary mineral formation are observed under geotectonic settings with maximal 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 secondary alterations in strongly deformed rocks and compare them with regional background transformations He wrote: “…two mechanisms of lithogenetic transformations are observed in rock-forming basins under miogeosynclinal 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 single discrete-continuously developing fluid–rock system” (Yapaskurt, 1992, pp 178–179) Earlier, Marakushev (1986) proposed a slightly different concept, according to which the first group of processes controls lithogenesis1 and the second one governs metamorphism He separated the activity of these processes in time and believed that they correspond to different stages of geosynclinal belt formation Lithogenesis corresponds to the geosynclinal stage of sediment accumulation during their subsidence, whereas metamorphism corresponds to subsequent stages of deformation of geosynclinal sediments and formation of orogenic fold belts accompanied by the rise of geoisotherms and ascent of juvenile metamorphosing 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 solution Dehydration of primary minerals under such conditions is hampered and incomplete, which results in the coexistence of hydrous and anhydrous phases and the formation of hydromicas In contrast, metamorphism occurs in an open mineral-forming system with filtering solutions characterized by high mineralization and partial pressure Such conditions are more favor1 Marakushev (1986, p 103) considered lithogenesis as “…deposition and subsequent transformation of sediments into sedimentary rocks during their simultaneous subsidence and increase in temperature (in accordance with geothermal gradient) and pressure.” 0024-4902/03/3803-0209$25.00 © 2003 åÄIä “Nauka /Interperiodica” 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 lithogenesis and metamorphism under low temperatures and pressures and altered rocks can show such a strong similarity that “they differ only by the attitude… and can frequently be discriminated only by the comprehensive geological mapping of particular sequences” (Marakushev, 1986, p 107) Luk’yanova (1995) carried out extensive investigations of catagenetic processes in unstable tectonic settings 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 subsidence 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 geological structures with an intense tectonic activity, vertical catagenetic zoning is compressed (the thickness of separate zones decreases), whereas catagenetic alteration 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 alteration 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 relatively ancient) altered rock sequences that experienced a long-term and intricate geological evolution We attempted to answer them using a strongly altered (secondary chlorite–hydromica assemblage) Neogene sedimentary 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 Precambrian (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 commenced 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 Plateau–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 direction The displacement along faults was sinistral This stage was marked by structure widening with the successive centripetal subsidence of basement blocks along a system of steep faults (Fig 2) At the second stage (5–7 Ma ago), the Indian Plateau 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 compression was maximal (1.5 to 2.2 times higher than in other areas) in the northwestern part of the Red River continental 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 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 Tectonic movements during this period were, however, related to peculiarities in the internal structure of the Indochina–South China zone rather than the global displacement 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 km, although some researchers estimate it at 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 ( –P 2, pt) Its lower part is composed of black argillites alternating with breccia-type conglomerates, sandstones, and brown-red siltstones Its upper part consists of conglomerates, breccia-type conglomerates, gravelstones with lentils of silty argillite, unsorted rocks, and argil- LITHOLOGY AND MINERAL RESOURCES Vol 38 No 2003 SYNCHRONOUS TRANSFORMATIONS 211 South China Tibet In Ex ten ch India in a si o nz on e Pe n 500 km South China N Lao Cai Re dR iv e 200 km Viet Nam rf au lt z on Laos e Hanoi Fig Linear extension structure representing the geological boundary between South China and Indochina (Tran Ngoc Nam, 1999) Borehole Kim Song km Mz Borehole Borehole 209 14 P1 P2 N3 N2 N1 P2 P1 Kifn Anh Mz Fig Basement of the Hanoi Trough subsided along a system of steep normal faults Paleotectonic reconstruction based on (Le Viet Trieu, 1996) lites The rock color varies from red and red-brown to less common black and reddish black Clasts in conglomerates 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 LITHOLOGY AND MINERAL RESOURCES Vol 38 Oligocene Dinh Cao Formation ( –P 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 No 2003 C H I N 108° 00′ 107° 00′ 106° 00′ 105° 00′ 104° 00′ 103° 00′ PETROVA et al 102° 00′ 212 A 23°00′ N Lao Cai N Lo N Ri ve 22°00′ rf au lt Yen Bai zo ne Re O S 20°00′ Minh Binh EAST CHINA SEA 21°00′ Hon Gai ne zo A zo roane al d ong 8a Hanoi lt au rf ve Ri ne ay zo Ch ault rf ive dR 10 N L Fa ult N 12 BAC BO BAY 19°00′ Fig 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 Oligocene–Miocene Thuy Anh Formation –P −N1ta) Coarse-grained sandstones (with conglomerates 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 characterized 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 ( N fÒ) In the central area of the trough, the lower part of the section is composed of members of thick-bedded wellsorted 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 that the sediments accumulated in small lagoons and bays during sea transgression Middle Miocene Phu Cu Formation ( N 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 massive coal-bearing argillites (80%) alternating with horizontally 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-laminated 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 LITHOLOGY AND MINERAL RESOURCES Vol 38 No 2003 SYNCHRONOUS TRANSFORMATIONS 105° 00′ 105° 30′ a Ch 104° 30′ 22° 00′ ay c ve rf au 5m 30 zo ne rf au Tuyen Quang ne zo lt 180/30 zo ne Co Phuc Tran Yen Ç Yen Bai 22° 40′ Ri ve lt lt au rF ve Ri Lo NW Hinge 240/20 Ri d Re Hoang Trang 213 0/3 Thac Ba Dam d Doan Hung Ä SW 8m 22 5/ 60 0/ 24 35 Vhu Tho Cam Khe 21° 20′ N2 N1 Proterozoic coal /80 N1–2 /70 Yen Bai /75 N1 NE 225 N1 230 m N1 N2–Q Ç 240 km Yen Bai ridge ic zo leo Pa 300 200 100 e 10 m b ëÇ Ä N2–Q N1 Fig 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 The rocks with wavy bedding alternate with massive varieties Massive siltstones and argillites are strongly cemented and alternate with gray to light gray mediumgrained 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 siderite, pyrite, and glauconite The thickness is 2000 m Three sea transgressions accompanied by sedimentation in boggy settings are assumed Upper Miocene–Pliocene Tien Hung Formation ( N – N th) It is subdivided into two subformations 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, siltstone, 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 LITHOLOGY AND MINERAL RESOURCES Vol 38 the subformation consists of gray to dark gray, wellsorted, fine-grained sandstone alternating with parallelbedded 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 2–3 Pliocene Vinh Bao Formation ( N 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 subsidence of basement blocks along the system of steep faults Eocene–Oligocene sediments occur in the deepest (about km) part of the newly formed troughshaped valley, whereas Miocene–Pliocene sediments No 2003 214 PETROVA et al VH Index Series System Group Q Hanoi Trough TH Along the Red River valley Vhuh Tho Yen Bai Qui Mong Sh4 Bach Luu Van Yen Sh3 PC Trai Hut Sh2 ? Sh1 Fch TA DC PT Doan Hung Sl3 Luc Yen Sl2 Bao Yen Lo River Sl1 Cenozoic QuaterTertiary (Paleogene-Neogene) nary EoceneOligoceneMiocene Pliocene Oligocene Miocene 1 N2 Q P3 – N1 N1 N1 N1 – N2 P2–3 Along the valley Chay River 13 10 11 12 a b 14 b 15 a b c 16 a Fig Correlation of Cenozoic sections in the Red River valley Based on (Le Thi Nghinh et al., 1991) (1) Olistostrome-type boulder breccia; (2) sandstone, siltstone, and argillite with subordinate boulder breccia; (3) argillite and siltstone with subordinate boulder breccia; (4) boulder 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 are distributed along flanks of this structure The sediments accumulated in shallow-marine, coastal-marine, and coastal-boggy settings during several stages corresponding 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 pressure on the newly formed extension structure, which resulted in the successive compression, rise, and folding of accumulated sediments and the formation of several narrow fault zones As a result, the former extension structure turned into the compression structure accompanied by a significant shortening of the section According to data in (Tran Nghi et al., 2000), the resulting 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 sediments (compression period) The section near the town of Yen Bai was selected for the thorough study of secondary 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 gravelstones with coal lenses (lower part of Member Ib) Pebbles in conglomerates consist of quartzites, siliceous rocks, basic and acid volcanics, limestones, and shales Conglomerate beds alternate with thick-bedded sandstones, siltstones, 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 siltstone 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 finergrained one (upper part of Member Ib and members II LITHOLOGY AND MINERAL RESOURCES Vol 38 No 2003 SYNCHRONOUS TRANSFORMATIONS 215 Other minerals Illite-smectite Smectite Kaolinite Chlorite-smectite Chloritevermiculite Coal rank 10Ra max, % Diffractogram fragments related to reflection from plane [001] of layer silicates (Regions, Å: 9–10–Micaceous minerals, ~14–chlorites) Alluvial sediments 80–100 hvAb-mvb 80–114 hvAb-lvb 9.98 14.0 10.0 V-3 V-11/1 V-11/2 Sandstone Argillite near coal 9.98 Ib m 10.0 10.0 9.98 V-13/2 V-13/1 V-13/3 V-5 Sandstone Argillite Siltstone 9.98 14.2 Gray Sample V-4 (sandstone) 9.98 Sample V-3 (coarse-grained sandstone) 10.0 300 V-2 V-11/3 Siltstone 14.1 9.98 Crush zone in sandstones 14.2 V-4a, b 14.0 hvBb 14.0 71–77 10.0 V-12/3 V-12/2 Argillite 10.0 V-5 V-12/1 V-15/2 V-15/3 V-15/4 Argillite Sandstone 9.99 V-8 14.2 hvBb-lvb 14.1 14.1 10.0 V-9 14.2 V-10/1-13* z? Clayey siltstone 10.0 10.0 V-13 71–115 ? V-14/6 V-11/1-4 d, m c Tobacco-colored argillite with siltstone interbeds, coal seams, and calcite veinlets Gray argillite with rare siltstone interbeds V-14/4 V-14/5 Sandstone 9.98 Intensely crushed dark gray argillite with calcite veinlets 14.0 9.98 14.1 V-14/3 14.2 d, c V-15/1-4 V-12/1-3 10.0 m Alternating laminated detrital argillite, siltstone, and sandstone Sandstone with conglomerate interbeds and coal seams Alternating conglomerates and coarsegrained sandstones hvAb 14.1 Alternating argillite, sandstone, and coal 83–90 78–90 10.0 14.2 c ? m V-14/1-6 14.1 c, g 9.98 10.0 Coarse-grained sandstone with conglomerate clasts Gray argillite with coal seams 14.1 Quartz Feldspar Mica Chlorite reflectance 10Ra min- Sample no Organic matter Mixed layer Description 14.1 100 Miocene 50 50 50 II 50 100 100 III Mineral composition Lithology Member 50 Thickness, m 20 IV 100 Pliocene Q 50 Age (a) V-8 Argillite: V-9 Gray tobacco-colored platy 61 Sample V-1a (sandstone); 63 Organic matter 120 Sandstone and argillite Abundance and size of pebbles in conglomerates sharply decreases Sandy cement 300 Sandy conglomerate (locally with cobblestone), sandstone interbeds (0.5–1.0 m), and thin coal lenses z, m V-1e 9.98 V-1d Sample V-1e (argillite) 74–85 V-1b Sample V-1Ò (sandstone) 9.98 Sample V-1b (sandstone at the contact with coal lens) 75–82 Ia hvAb hvBbhvAb V-1a 9.98 14.1 z 14.1 V-1c 9.98 >1000 EoceneOligocene Oligocene-Miocene 10 Sample V-2 (clayey siltstone) Sample V-1a (sandstone) Cobblestone Fig (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-laminated siltstones with layers of well-sorted sandstones composed of well-rounded grains This member (IV) is LITHOLOGY AND MINERAL RESOURCES Vol 38 arbitrarily assigned to the Pliocene The thickness varies from 50 to 350 m All rocks in the section are deformed into folds with the strike varying from 180° to 300° and dip angle ranging from 30° to 80° The rocks of members II–IV are No 2003 216 PETROVA et al Sample no 10.0 14.0 c, d 14.0 9.98 14.0 9.95 14.1 c, d V-10/7 10.0 c V-10/8 10.0 V-10/9 c, d V-10/10 14.0 c 9.98 V-10/11 coal rank other minerals 14.0 14.0 10.0 c, d V-10/5 9.95 lvb 14.0 V-10/3 c 14.1 9.98 14.0 c, d 50 Vc, d 10/2 V-10/1 Siltstone 9.98 80–114 10 Sandstone 35 Detrita argillite 25 Sandstone 50 V-10/4 Siltstone c, d Detrital argillite 35 Detrital siltston 35 Detrital argillite with coal 9.98 14.0 Description quartz feldspar mica chlorite c, d Fe Diffractogram fragment V-10/6 50 Laminated argillite with Fe Sandstone Fe 80–100 Fe 70 hvAb-mvb 20 Argillite with ferruginous interbeds 105 50 reflectance 10Ra, % Argillite Alternating sandstone and thin argillite 35 25 organic matter Detrital argillite Lithology Mineral composition Laminated argillite Thickness, m (b) Fig (Contd.) strongly lithified and overlain by horizontal undeformed (or slightly deformed) 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 secondary (superimposed) alteration that is most prominent 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 sandstones and siltstones, 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 approximately one third is altered into Mg-chlorite These proportions (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 section shown in Fig 4d It is evident that regardless of the rock type (argillite, siltstone, or sandstone), the proportion of mica and chlorite components in the secondary aggregates remains unchanged Micas are mostly represented 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 Mixedlayer minerals (chlorite–smectite, chlorite–vermiculite, and illite–smectite) occur in insignificant quantities only in some samples No confinement of these minerals 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 recrystallization are discernible in all primary minerals Consequently, blastogenic textures are developed in all sedimentary rock types The primary biotite is partly or completely replaced by Fe-chlorite Mixed-layer silicates are also commonly 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- LITHOLOGY AND MINERAL RESOURCES Vol 38 No 2003 1.541 SYNCHRONOUS TRANSFORMATIONS 1.699 Q 1.980 2.127 2.23 2.28 2.465 2.56 2.85 2.95 3.24-3.18 Fs 3.50 Chl 3.34 Q 3.71 Chl 4.98 Mi 4.25 Q 0.15 mm 9.95 Mi 7.0 Chl 9.98 Mi 11.8 ? 14.0 Chl Sample V-12/1 7.0 Chl ~11 ill + sm 14.0 Chl 1.540 (b) 3.34 Q 4.25 Q 4.70 Chl 4.97 Mi 0.3 mm 1.817 Sample V-15-2 1.994 2.127 2.28 2.456 2.56 2.85 Chl 2.98 2.19 Fs 3.52 ïỴ 7.14 Kaol 9.98 Mi Mixed-layer ill.-sm 0.3mm 1.817 Q 0.15 mm Sample V-14-1 2003 (d) (f) No 1.658 11.6 4.25 Sample V-11/4 7.06 Chl 9.95 Mi 14.0 Chl 0.15 mm 3.34 Q 3.19 Fs 3.57 Kaol 3.70 Fs 4.44 4.97 Mi Sample V-13-3 Vol 38 3.03 Cat 3.18 Fs 3.67 Fs 3.85 Cat 3.34 Q 4.02 Fs 4.69 Chl 4.25 Q 4.95 Mi 5.5 Fs 6.32 Fs 7.0 Chl 2.95 4.25 Q 4.70 Chl 4.98 Mi 2.88 9.94 Mi 14.0 Chl 1.979 2.127 2.23 2.28 2.56 2.456 2.456 2.56 2.85 Chl 3.19 Fs 3.34 Q 3.24 3.53 Chl 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 Q 1.817 0.10 mm 0.10 mm 1.700 1.980 2.127 2.24 2.28 1.700 Qua (a) 1.817 0.10 mm 0.10 mm (c) 1.671 Q 0.15 mm (e) LITHOLOGY AND MINERAL RESOURCES 1.540 0.15 mm 0.15 mm 217 218 PETROVA et al granular boundaries, quartz grains frequently display convexo-concave contacts typical of conformal microtextures usually produced by the mechanical compression 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, probably, zeolite are also found Thus, the following features can be considered typical of secondary mineral formation in rocks: (1) absence of sequential–mineralogical zoning; (2) development of secondary mica–chlorite assemblage; (3) absence of smectite and sporadic occurrence of mixed-layer minerals; (4) presence of recrystallization (blastogenesis) textures in the majority of rock types; (5) development of sinuous and convexo-concave (conformal) contacts between mineral grains; (6) strong compaction of rocks; and (7) striate, uniformly oriented, and elongated distribution of constituent mineral grains The wide distribution of secondary mica and chlorite, 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 indicator 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 ~0.1 Wt/m2 Its geological structure does not provide grounds to suggest significant changes in heat flow values during the period from the Eocene to Recent The calculated temperature for the depth of ~5 km approximates ~ 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 classical subsidence scheme, i.e., from smectites to transitional mixed-layer phases and further to chlorites The subsequent compression of sediments resulted in dehydration 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 minerals were corroded or partly recrystallized at surface and near-surface levels under the compressive stress Consequently, heaving and compaction textures were developed Thus, the formation of secondary mineral assemblages in this zone occurred in two stages The first stage corresponded to structure extension and produced the usual diagenetic and catagenetic zoning of metasedimentary rocks The second stage was characterized by compression and resulted in the distortion of metasomatic zoning Structures of typical surficial and nearsurficial 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 virtually 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 compared 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 studied 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 Compositions and textures of inequigranular rocks from the Yen Bai section Photomicrograph (without analyzer) and diffractograms 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 Convexo-concave contacts between quartz grains and deep fissures filled with cement are well seen LITHOLOGY AND MINERAL RESOURCES Vol 38 No 2003 SYNCHRONOUS TRANSFORMATIONS 219 B, % Sample V-1-1 80 60 Wave length = N= Ra = MSD = Ra max = Ra = ∆Ra = 546.0 10 7.86962 0.22009 8.22125 8.00 8.20 Wave length = N= Ra = MSD = Ra max = Ra = ∆Ra = Sample V-8a 7.545 0.67625 546.0 50 8.7292 0.95065 10.7737 6.3775 4.39625 40 20 100 7.45 7.60 7.80 Sample V-10-1 80 Wave length = N= Ra = MSD = Ramax = Ra = ∆Ra = 6.3 546.0 50 8.69485 1.62598 11.31 8.0 10.0 10.8 Wave length = N= Ra = MSD = Ra max = Ra = ∆Ra = Sample V-15-1 6.7225 4.5875 546.0 50 8.5368 0.29714 9.00125 7.8075 1.19375 60 40 20 6.65 8.00 10.00 11.35 7.75 8.00 8.50 9.00 Ra, % Fig Reflectance of organic matter from the Yen Bai section formed into variably metamorphosed coal during the studied area extension and organic matter subsidence According to I.E Stukalova who carried out optical examination of organic matter samples from the section exposed near the town of Yen Bai (Fig 6), the humic organic matter from interstices between conglomerate pebbles in the lower part of the section (samples V-1-1, V-1-2, and V-4, Figs 6a, 8) is characterized by minimal reflectance values in air (10R‡ = 71–85%), which correspond to high-volatile bituminous B and A coals (hvBb and hvAb, respectively) The humic matter is composed of large tabular vitrinite clasts with a distinct cellular structure but without notable anisotropy The clasts show a distinct texture and contain an admixture of sedimentary material sometimes filling cell hollows Thin films and separate particles of organic matter enclosed in strongly deformed sandstones and siltstones from the middle part of the section (samples VLITHOLOGY AND MINERAL RESOURCES Vol 38 8a, V-10-1, and V-10-7) are characterized by a very irregular reflectance The 10R‡ values vary in separate particles from 71 to 115%, which correspond to coals ranging from hvAb to lvb (low-volatile bituminous coal) ranks (Figs 6a, 8) The genesis of organic matter is unclear because no classical vitrinite features were observed in examined grains Organic matter is represented by different-sized fragments of irregular shapes Two groups of fragments are distinguished by their reflectance The smallest fragments are characterized by the minimal reflectance Thick beds of organic matter from the upper part of the section (Fig 8, Sample V-15-1) consist of large particles of the humic variety frequently represented by crushed and twisted fragments of tree branches Thin sections show the presence of vitrinite veinlets with reflectance value 10R‡ = 87–90% corresponding to high-volatile bituminous A coal (hvAb) No 2003 220 PETROVA et al Table Results of the geochemical analysis of organic matter from the Yen Bai section Sample no Position in the section V-1b V-10-1 V-15-1 Lower Middle Upper IRR, % Corg , % 88.0 74.3 87.5 56.60 0.82 11.75 Pyrolysis results BC, % BC, % lumiextraction nescence 0.2100 0.0485 0.0119 0.2400 0.0400 0.0003 S1, mg/t S2 , mg/t Tmax , °C 0.35 0.15 0.11 26.27 0.90 1.40 462.8 450.0 539.2 Note: (IRR) Insoluble rock residue; (BC) bitumen content in rocks determined by the chloroform–bitumen analysis (chloroform extraction) Table Group composition of bitumens from different parts of the Yen Bai section Sample no V-1b V-10-1 V-15-1 Position in the section Lower Middle Upper Hydrocarbons, % Resins, % saturated aromatic benzol alcohol–benzol 22.2 35.8 25.8 21.8 28.2 18.2 29.0 17.6 19.7 20.4 14.0 20.9 These data suggest that the section demonstrates a reverse pattern of metamorphism: organic matter from the lower (deep) zone is less metamorphosed, whereas the organic matter from upper levels shows more intense alteration Moreover, the metamorphism of organic matter is maximal in strongly dislocated sectors of the central part If we assume that all primary humic matter was transformed into coals, their distribution pattern is as follows: the lower part of the section contains high-volatile bituminous (B, A) coals, the middle part encloses coals ranging from high-volatile A to lowvolatile ranks, and the upper part mainly includes highvolatile A ranks (Figs 6a, 6b) Geochemical studies made it possible to significantly elucidate the mode of organic matter transformation within the sedimentary section during the intricate geological evolution We analyzed three samples (V-1b, V-10-1, and V-15-1) from the lower, middle, and upper parts of the section, respectively (Table 1) The maximum Corg content (56.6%) is typical of the lower part, whereas the minimal concentration (0.82%) is recorded in the middle (strongly deformed) part The sample from the upper part of the section shows medium Corg contents (11.75%) Based on results of pyrolysis, all samples should be referred to type III (Van Crevelen diagram) corresponding to transformation of humic organic matter It appeared that organic matter in all three examined samples contains both bituminous and coaly components The maximal bitumen content (relative to the total Corg concentration) is observed in the middle part, where bitumen is dominated by saturated and aromatic hydrocarbons Bitumen from the lower part is characterized by a high content of resins, whereas the bitumen from the upper part is represented by asphaltenes (Table 2) Asphaltenes, % 6.6 4.4 15.4 The pyrolysis revealed that the S2 value is always higher than S1; i.e., the bitumen is a syngenetic product in the entire section It was extracted most easily from the sample characterizing the middle part at the lowest temperature of 450°C (Table 1) The extracted bitumen components also differ in molecular–mass distribution of n-alkanes (Fig 9) Low- and medium-molecular paraffin hydrocarbons (n-C12 to n-C25 with maximum of n-C17) dominate in samples V-1b (the lower part of the section) and V-15-1 (upper part) Medium- to high-molecular paraffin hydrocarbons (n-C19 to n-C31 with two maximums of n-C17 and n-C27) prevail in Sample V-10-1 from the middle part of the section These data clearly indicate that the transformation of humic organic matter was a uniform process through the entire section (formation of bitumen and preservation of the coal component) The transformation of organic matter into bitumen directly correlates with stress intensity The thickness of organic-rich beds is also very important: the lower their thickness, the deeper the transformation of organic matter Owing to the existence of compact pebbly material, organic matter from the lower part of the section could probably retain the transformation degree attained during the maximal subsidence of rocks (i.e., hvBb and hvAb ranks) The subsequent compression during the folding and ascent of sedimentary rocks could cause disruption of internal bonding within the coal structure This could probably cause both mechanical destruction of carbonaceous particles and intricate transformation at the molecular level Coal metamorphism under stress evolved according to an unusual scenario of generation and transformation of bitumen from asphaltene via intermediate phases to anthraxolite and shungite rather than from brown coal via intermediate phases to anthra- LITHOLOGY AND MINERAL RESOURCES Vol 38 No 2003 SYNCHRONOUS TRANSFORMATIONS 250 200 150 100 50 V-1b-1 10 15 20 25 30 35 40 45 50 55 60 65 70 200 100 50 10 15 20 25 30 35 40 45 50 55 60 65 70 300 V-15-1 200 100 degree of compaction and maximal bitumen content It is enriched in light hydrocarbons with the widest spectrum of n-alkanes and acyclic isoprenides As shown above, these processes are accompanied by transformation of the inorganic constituent along the entire section corresponding to the deep catagenesis stage V-10-1 150 10 15 20 25 30 35 40 45 50 55 60 65 70 Fig Chromatograms of the bitumen-saturated fraction cite This process was accompanied by the transformation of humic matter, which was already metamorphosed to hvBb and/or hvAb coal, into bitumen The smaller the organic matter particles, the more complete and prominent is this process in the studied rocks As early as in the late 1960s–early 1970s, the importance of natural coal dispersion under tectonic stress for organic matter transformation was shown in (Sterenberg et al., 1968; Ettinger et al., 1974) These authors demonstrated that dynamic impact on coals initiates the disruption of weakest bonds in lateral chains and between carbon layers A prolonged mechanical impact triggers the destruction of rigid carbon lattices This is accompanied by distortion of the ordered aromatic part of macromolecules and increase in the quantity of disordered carbon in lateral chains (Ettinger et al., 1974) Subsequent experimental works (Tsarev and Soroko, 1985) confirmed that static and dynamic loads stimulate the generation of hydrocarbon and other gaseous components from organic matter, simultaneously increasing the carbonization degree of kerogenes and the content of hydrocarbon fraction in bitumens According to these researchers, mechanical stress results in the destruction and fragmentation of complex hydrocarbon molecules and the subsequent formation of simpler, although more ordered, compounds that compose the insoluble fraction of organic matter Precisely these processes were observed in the examined organic matter In zones of maximal compression, organic matter is characterized by the highest LITHOLOGY AND MINERAL RESOURCES Vol 38 221 CONCLUSIONS Thus, it is established that the exhumation of young sedimentary rocks, their deformation into gentle folds, and 1.5 to 2.2 times compaction are accompanied by synchronous transformations of their organic and particularly, inorganic constituents These transformations are superimposed on dia- and catagenetic alterations of earlier stages As a result of oriented stress impact, sedimentary rocks acquire many specific features of rocks metamorphosed at the deep catagenesis stage: (1) development of the secondary mica–chlorite assemblage, decomposition of smectite, and sporadic formation of mixedlayer minerals; (2) development of recrystallization (blastogenesis) textures in most rock types; (3) development of sinuous and convexo-concave (conformal) contacts between mineral grains; and (4) maximum rock compaction However, there are some differences reflected, first of all, in the lack of sequential–mineralogical zoning Under the stress, secondary layer silicates loss water Some minerals, such as chlorite and mica, are stable even under elevated pressures at all depths If the section contains compression-resistant rock layers (e.g., conglomerate or cobblestone), their cement can retain secondary minerals that formed at dia- and catagenetic stages of rock transformation before the stress This can produce pseudozonal patterns in the section, but this zoning reflects differences in elastic properties of host rocks and relevant irregular distribution of pressure therein rather than progressive changes in parameters of mineral formation with sediment subsidence The elevated and uniformly oriented pressure is also responsible for a peculiar striate and extended distribution of secondary minerals in the rock In addition to chlorite and mica, kaolinite developed after feldspars and calcite filling late fissures also indicate the stress impact upon rocks Humic organic matter, which occurs in the section as separate beds, lenses, and inclusions, also responses to the stress impact The capacity of rock beds to resist stress and alteration directly correlates with their thickness Under the influence of additional oriented pressure, humic organic matter can be transformed into different grade coals and enriched in bitumen as well Data obtained in the course of this study indicate that bitumen is generated from coals metamorphosed to ranks hvBb and hvAb under the stress impact Liquid and gaseous hydrocarbons released from solid humic matter under the stress load are concentrated in rocks No 2003 222 PETROVA et al Thus, the rocks are transformed into low-grade oil-generating formations ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project no 01-05-64595 REFERENCES Atlas of the Palaeogeography of China, Wang, H., Ed., Beijing: Cartographic Publishing House, 1985 Cheng, Y., A Cognitive Basis and Discussion on the Nappe Structure of Ailao Shan-Diancang Shan Geology, Yunnan, 1987, no 6, pp 291–297 Chenging Fan, The Tectonic-Metamorphic Belt of Mt Ailao in Yunnan Province, Yunnan, 1986, no 5, pp 281–291 Ettinger, I.L., Cherkinskaya, K.T., Shterenberg, L.E., Elinson, M.M., and Kasatochkin, V.I., Mechanochemical Reactions during the Dispersion of Coals, Mekhanoemissiya i mekhanokhimiya tverdykh tel (Mechanical Emission and Mechanical Chemistry of Solids), Frunze: Ilim, 1974, pp 271–273 Gatinskii, Yu.G., Lateral’nyi strukturno-formatsionnyi analiz (Lateral Structural–Formation Analysis), Moscow: Nedra, 1986 Helmcke, A.B., The Permo-Triassic “Paleotethys” in Mainland Southeast Asia and Adjacent Parts of China, Geol Rundsch., 1985, no 74, pp 215–228 Hutchison, C.S., Geological Evolution of South-East Asia, Oxford (U.K.), 1989 Le Thi Nghinh, Nguyen Xuan Huyen, Nguyen Trong Yem, et al., Tram Tich Kainozoi Doi Song Hong, Dia Chat Tai Nguyen, Hanoi, 1991, pp 105–114 Luk’yanova, V.T., Katagenez v orogennykh oblastyakh (Catagenesis in Orogenic Regions), Moscow: Tovar-vo Nauchn Izdanii KMK, 1995 Marakushev, A.A., Relationships between Lithogenesis and Metamorphism, Glinistye mineraly v litogeneze (Clay Minerals in Lithogenesis), Moscow: Nauka, 1986, pp 103–112 Phung Van Phach and Bui Cong Que, Late Cenozoic Tectonic Activities in North Viet Nam, J Geol., 1999, Ser B, no 13–14, pp 33–41 Shterenberg, L.E., Cherkinskaya, K.T., Ettinger, I.L., Elinson, M.M., and Kasatochkin, V.I., Transformation of Coal Material during Dispersion, Dokl Akad Nauk SSSR, 1968, vol 180, no 1, pp 214–217 Tapponnier, P., Leloup, Ph.H., and Lacassin, R., The Tertiary Tectonics of South China and Indochina, The Conference on Cenozoic Evolution of the Indochina Peninsula, HanoiDoson, 1995, pp 208–226 Tran Nghi, Chu Van Ngoi, Dinh Xuan Thanh, and Nguyen Dinh Nguyen, Cenozoic Sedimentary Evolution of Red River Basin in Relation with Tectonic Activities, J Sci., 2000, vol 22, no 4, pp 290–305 Tran Ngoc Nam, Doi dut song Hong - diem nong cua nhung tranh luan khoa hoc, Tap chi Cac Khoa hoc ve trai dat, 1999, no 6, pp 81–89 Tran Van Tri, Geology of Vietnam, North part, Hanoi, 1977 Tsarev, V.P and Soroko, T.I., Influence of Mechanical Fields on the Transformation of Fossil Organic Matter, Organicheskoe veshchestvo sovremennykh i iskopaemykh osadkov (Organic Matter of Recent and Fossil Sediments), Moscow: Nauka, 1985, pp 152–156 Wang, E and Chu, J.J., Collision Tectonics in the Cenozoic Orogenic Zone Bordering China, India and Burma, Tectonophysics, 1988, no 174, pp 71–78 Yapaskurt, O.V., Litogenez i poleznye iskopaemye miogeosinklinalei (Lithogenesis and Mineral Resources of Miogeosynclines), Moscow: Nedra, 1992 LITHOLOGY AND MINERAL RESOURCES Vol 38 No 2003 ... with coarser material and terminating with the finergrained one (upper part of Member Ib and members II LITHOLOGY AND MINERAL RESOURCES Vol 38 No 2003 SYNCHRONOUS TRANSFORMATIONS 215 Other minerals... (sandstone); 63 Organic matter 120 Sandstone and argillite Abundance and size of pebbles in conglomerates sharply decreases Sandy cement 300 Sandy conglomerate (locally with cobblestone), sandstone... Alternating laminated detrital argillite, siltstone, and sandstone Sandstone with conglomerate interbeds and coal seams Alternating conglomerates and coarsegrained sandstones hvAb 14.1 Alternating