DEFORMATION OF CRETACEOUS RED SANDSTONES IN NORTHWEST VIỆT NAM

PHAN TRỌNG TRỊNH¹ AND ROBIN LACASSIN²

¹Institute of Geological Sciences, VAST, Chùa Láng Str., Đống Đa, Hà Nội
²Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France

Abstract: We study several unconformity sites in Northwest Việt Nam where the Indosinian orogenesis has been defined. Field observations in the Sông Đà area, near Sơn La, demonstrate that red sandstones and conglomerates, reportedly of Cretaceous age, occur unconformably above schistose epimetamorphic Middle Triassic sediments. Five rocks samples were taken near Vạn Yên and NE of Sơn La. The collection comprises Permian volcanic rocks and Triassic slates, all of apparent high anchizonal overprint. The data scatter from 225 to 177 Ma in the <2µm fraction and from 206 to 159 Ma. A Mesozoic unconformity thus really exists in this region. In most places, however, the unconformity is strongly deformed, folded Triassic limestones are often thrusted onto Cretaceous rocks.

INTRODUCTION

Asian crust has been structured by several collisional events [8, 23]. Amongst these events, the Indosinian or Cimmerian orogeny is thought to have affected a large region from Northeast Tibet to SE Asia mainland in the Middle Triassic [3, 8, 23]. This concept of large collisional deformations of Triassic age comes from the pioneering work of Deprat [2, 3], who suggested that the Sông Đà area of North Việt Nam was a major edifice of refolded thrust nappes affecting the Palaeozoic and Triassic rocks. Together with the possible existence of discordances within Triassic series [6, 7], the main argument for a Triassic age of nappe emplacement is the existence of red conglomerates, sandstones and pelites reported to be syntectonic and to lie unconformably on deformed Triassic sediments. Hence, the concept of Indosinian orogenesis has been extended to the whole Indochina Peninsula, Southwest China and East Tibet [6, 23]. In the Yunnan Province of China, it was thus quite surprising to find very little evidence for such Triassic compressional deformations along the Red River Fault zone and on both sides of it [27]. There, most of the visible deformations are clearly of Cenozoic age [10, 21, 22]. In Central Yunnan and Sichuan basins, there is no clear angular unconformity within the Triassic to Cretaceous series and the Triassic rocks are generally folded or sheared together with the Jurassic-Cretaceous red beds [10, 11, 27]. In Northwest Việt Nam, "locus typicus" of Indosinian orogenesis, there is few published, clear factual descriptions of such angular unconformity. From field observation and stress field, some scientists have been proposing the Hòa Bình nappe in Cenozoic [14, 18]. Only some arguments tell about overthrust in northwest region [5, 9, 19, 20]. These observations have led us to question on the real significance and importance of this orogenesis [27].

Figure 1: Location and tectonic sketch of NW Việt Nam and adjacent regions, (a) Location of NW Việt Nam in the tectonic framework of SE Asia mainland. Arrows show finite sense of displacement along Cenozoic strike-slip faults. Hatched areas correspond to inferred Triassic fold and thrust belts strongly affected by subsequent Tertiary deformations, (b) Tectonic sketch map of Sông Đà (Black-River) region showing Sơn La -Mộc Châu red bed basins and location of geological sections of Fig. 2 (circled numbers) [after 17, 30]. The Red River Fault (R.R.F.) and Dãy Núi Con Voi metamorphic core mark the Cenozoic transform boundary between South China and Indochina blocks [10]. DBP, Điện Biên Phủ; LC, Lào Cai; MC, Mộc Châu; YB, Yên Bái; YC, Yên Châu, D.B.P.F., Điện Biên Phủ Fault

I. THE INDOSINIAN UNCONFORMITY IN THE ĐÀ RIVER AREA

In the North and Central Việt Nam, the clearest descriptions of tectonic structures are those of Deprat [e.g. 2-4]. His detailed and concrete cross-sections proved to be highly reliable in the field, a conclusion we failed to reach for the interpretative sections and descriptions of Fromaget [6]. In Central Việt Nam, Deprat clearly describes the post-Hercynian unconformity of Lower Triassic polygenic conglomerates deposited onto gneisses [4]. In his studies on the Đà river area, comparable field evidence for a syn-Triassic, post-compressional unconformity is less clear. Deprat's detailed geological sections of the Sông Đà area, south of the so-called Black-River (Đà River) nappes, show the reportedly unconformable red-bed series pinched in synclines, and folded like the Triassic limestones [2, 4]. As no detailed field description of this hypothetical unconformity has been published since Deprat, we attempt here to reinvestigate the structure of some of these red bed basins. Along the Red River Fault Zone (RRFZ) in NW Việt Nam, left-lateral shear has occurred mainly in the high-grade mylonitic core of the Dãy Núi Con Voi [10]. In the south and north of this core, several strike-slip faults and shear zones also account for significant left-lateral motion (Fig. 1). About 100 km southwest of the RRFZ, the Đà (Black) River has entrenched a deep valley in folded Palaeozoic and Triassic sediments. Upper Palaeozoic - lower - middle Triassic sedimentation is considered to have occurred in a subsident basin, the "Sông Đà Rift", together with basaltic volcanism and intrusions of Late Permian - Early Triassic age [8]. For Sengor and Hsu [23], this zone would correspond to the major Indosinian structure zone. The rocks of the Sông Đà region have been affected by an intense N-S to NE-SW shortening and most of the structures, including thrusts, shear zones and tight folds, are presently steep. Where present, lineations and shear criteria attest to nearly vertical stretching and upthrust movements on such steep structures.

South of the Sông Đà valley, in the Sơn La and Mộc Châu areas, a discontinuous stripe of red beds (Fig. 1b), reportedly of Cretaceous age, is mapped discordantly on the folded Triassic series [1,15-17,29]. Geological maps and satellite images suggest that this stripe is cut or bounded by strike-slip faults and thrusts (Fig. 1b). We report here detailed observations along two key sections: north of Sơn La (section 1, Fig. 2a) and across the Yên Châu basin (section 2, Fig. 2c), which allow us to assess in the field the relationships between the red beds and Triassic rocks. The first section (Fig. 2a), from the Đà River to Sơn La, shows red conglomerates and sandstones, mapped as Cretaceous, outcropping in a small basin within the Triassic series. Towards the SW, the Middle Triassic limestones (Anisian according to [4, 15, 29] are tightly folded in a series of anticlines and synclines whose axes trend from NW-SE to WNW- ESE. These limestones probably overthrust the Mường La red bed basin (Fig. 2a), though contact between the two units has not been observed directly in the field. The red sandstones and conglomerates of this basin dip 15-30° SW on average. Towards the NE, they overlie unconformably a series of shales, sandstones and quartzites of Triassic age. On a single outcrop along the road (UTM coordinates 48Q VJ 001 691) one can observe the red conglomeratic beds, dipping 20-30° SSW, lying directly on the top of schistose, steeply dipping sandstones and shales (Figs 2b and 3a). Up to the Sông Đà valley, the rest of the section shows a nearly vertical flyschoid series with shale, grey and yellowish sandstone and quartzite, reportedly of Ladinian age [15, 29]. These rocks are epimetamorphic and affected by fracture or slaty cleavages (Fig. 3b). The bedding planes trend N120-130°E and dip steeply to the NE or SW, while the cleavage generally dips to the NE (Fig. 3b). The constant bedding (S0)-cleavage (S1) relationships, together with polarity criteria (graded bedding, shape of refracted cleavage, load and flute casts) suggest that this nearly vertical series is roughly monoclinal (Fig. 2a). The northernmost part of the section, near the Sông Đà valley, shows mylonitic metatuffs and rhyolites, with a N110°E-trending schistosity dipping to the NE or nearly vertical. These rocks are probably thrusted southwestward onto the flyschoid series.

Figure. 2. Geological sections across red bed basins of Sông Đà area, (a) Section along unpaved road from Sơn La to Đà River (section 1, Fig. Ib); NE to the left. (b) Detail of unconformity of conglomerates onto schistose sandstones and shales (outcrop of Fig. 3a). (c) Section of southern border and middle part of Yên Châu basin (section 2, Fig. Ib), NE to the right. Main road from Sơn La to Mộc Châu is in the middle of section.

II. EVIDENCE FOR SHORTENING OF THE RED BED SERIES

The second section crosses the southern border of the Yên Châu red-bed basin, 50 km SE of Sơn La (Fig. 2c). Hills of Middle Triassic limestones, dipping to the SW, bound the basin to the south. These limestones are thrusted towards the north onto the red conglomerate, sandstone and pelitic series. A 250-m thick zone of sheared limestone, pelite and sandstone marks the thrust contact and shows intense boudinage, microfolding and small reverse shear-zones (Fig. 2c). Overturned to nearly vertical thick beds of red conglomerate, also cut by reverse faults and shear zones, outcrop just below this sheared zone (Fig. 3c, d). These conglomerates contain mainly limestone and sandstone pebbles, with a few quartz pebbles. We have not found pebbles of plutonic or metamorphic rocks. Towards the north and upward, the series become less conglomeratic and grades into fine-grained sandstones and pelites, that form the middle part of the basin. South of the Chiềng Sang village the sandstones dip ~20° NE while they dip gently to the SSW north of the road from Sơn La to Mộc Châu (Fig. 2c). We have not reached the northern border of the basin along this section. Nevertheless, between Yên Châu and Mộc Châu, the morphology of the Yên Châu basin on satellite images and in the field implies that thrusts may also follow this northern border (Fig. 1b). The Yên Châu red-bed basin thus appears to be a pinched syncline bounded on both sides by reverse faults.

III. K/Ar DATING OF VERY LOW GRADE SEDIMENTS

Samples from very low grade metamorphic sediments were collected from Sông Đà zone in order to reveal the tectonometamorphic and geodynamic history of the area. 5 samples were chosen for K/Ar dating on fine mineral fractions (<2 µm and <0.2 µm) and accompanying illite-crystallinity measure-ments (IC) [31]. The method of K/Ar dating on fine mineral fractions has proven to be a powerful tool in determining the age of synkinematically grown illites, and therefore, a means for elucidating the history of deformation and metamorphism in sedimentary rocks. Grain size fractions <2µm (up to the higher Anchizone) are often contaminated by detrital illite components leading to mixed ages, which can only be interpreted as maximum ages for the deformation and metamorphism. In these cases, the <0.2µm fractions are much more meaningful, because of its higher content of synkinematically grown illites. From the highest Anchizone to the Epizone inherited age information of detrital components are deleted and, in most cases, the age of deformation is indicated by the <2µm fraction. In any case, the age of very low grade metamorphism deduced by K/Ar dating of fine mineral fractions is controlled by many factors and has to be interpreted carefully. The main factors to be taken into account for the interpretation are: (1) the maximum temperature of the very low grade overprint, (2) the stratigraphic age of the determined strata, (3) the estimated age of detrital components, (4) the difference in age of different size fractions from the same sample and (5) the lithology and the mineralogical and chemical composition of the sample. In the Sông Đà region, five rock samples were taken near Vạn Yên and NE of Sơn La. The collection comprises Permian volcanic rocks and Triassic slates, all of apparent high anchizonal overprint. The data scatter from 225 to 177 Ma in the <2µm fraction and from 206 to 159 Ma in the <0.2µm fraction. Most of the data are contaminated by detrital components in ages and IC-values. The most reliable ages are given by a deformed volcanic rock, in which detrital influences can be excluded (VN 2-96, 177 Ma, 159 Ma). As the deformed Triassic rocks are unconformably overlain by undeformed Cretaceous strata, the observed deformation and degree of metamorphism (app. 300 C) must have taken place between the Late Triassic and Late Cretaceous [9]. It is difficult to explain by sedimentary overburden, because app. 10 km of Lower and Middle Jurassic sedimentary rocks would be needed for the degree of metamorphism. It is therefore postulated that the very low grade overprint of the Sông Đà region (around 180-160 Ma) was caused by tectonic event.

DISCUSSION

The detailed field observations we report here definitively demonstrate that the red bed series of the Sông Đà region have been deposited on previously deformed Triassic rocks. The steep and schistose sandstones and shales lying below the unconformity strongly supports the idea that the red-beds are not on lapping onto a block-faulted region, as in Central Yunnan [10], but instead postdate a compressional tectonic event. Note however, that the underlying rocks are only epimetamorphic and that the red conglomerate is not a polygenic molass containing a lot of metamorphic and plutonic pebbles. This could imply that, in the Sông Đà region, the preunconformity compressional event has not brought to the surface deep metamorphic units, such as those typically found in internal parts of a mountain belt. From available data, it is difficult to assign an unequivocal age to this compressional event. During the first half of the century, the red beds were assigned to from Upper Triassic to Lower Triassic. It was the so-called "terrain rouge" inferred to be "Infralias" and syntectonic by Deprat [2, 3], then by Fromaget [6]. A Cretaceous age is now ascribed to these red beds, that could perhaps be as young as Late Cretaceous to Early Cenozoic [17, 30]. Indeed, they reportedly contain Cenozoic remains and a freshwater fossil [17]*.

¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

* According to newly collected fossils, the red beds at Yên Châu have been dated as Late Cretaceous (Lê Thanh Hựu, Vũ Xuân Lực, 2003, TC Địa chất, A/279)(E.B).

Fig 3. (a) View of red bed unconformity along Sơn La - Đà River (Fig 2.). The dotted white line outlines the base of conglomeratic beds (congl.) above nearly vertical to overturned schist and sandstone (sch); S₀ is bedding. (b) Detail of schistose sandstone (left part of Figs 2b and 3a). Spaced fracture cleavage in coarser sandstone beds (left) grades into penetrative slaty cleavage in schist (middle). Curved shape of cleavage, due to refraction more important in coarser part
of sandstone bed (left), outlines gradded bedding and is used as polarity criteria.
(c) Nearly vertical red conglomerates at southern border of Yên Châu basin (Fig. 2.).
(d) Detailed view of conglomeratic beds of Fig. 3(c): pebbles, rounded to angular,
are mostly sandstone and limestone. Hammer handle out bedding

The possible ages for the deformation of the underlying Triassic rocks thus range between the Middle-Late Triassic and probably the uppermost Cretaceous. In any case, the unconformable red beds can not be younger than the volcanic and hypovolcanic complex that overprint the unconformity south of the Phan Si Pan range (Fig. 1). One Ar/Ar radio-metric result [25] on phlogopite-rich lavas (cocites) suggests an age of about 30 Ma for this complex. Triassic ages of metamorphism and deformation have been obtained from the Sông Mã anticlinorium (~30 km south of Sơn La, Fig. 1b) and farther south in Central Việt Nam [12, 13]. This supports the hypothesis of an Indosinian (Late Triassic) deformation in the Sông Đà region. We cannot exclude, however, the existence of a Jurassic or Cretaceous event also suggested by some radiochronological results in the region [13]. At most, this deformation could even be related to the onset of the India-Asia collision, as it is inferred in Tibet for the deformation of the Takena series below the unconformable Lirmzong Formation.

It is clear, however, that the basal unconformity, and the red-beds themselves are involved in the fold and thrust edifice of the Sông Đà region. This implies that post-Cretaceous shortening, likely an effect of the India-Asia collision, has been being strong in the northern part of the Indochina block. In the folded Triassic limestones, the proportion of shortening, that was acquired before and after red-bed deposition, remains unclear. Note, however, that the orientation of fold axes in the limestones and the post red-bed thrusts have the same direction, and have linked geometries at places, implying that they could exist due to the same Tertiary tectonic event. At the scale of SE Asia mainland (Fig. 1a), the probably Triassic, Sông Đà tectonic belt could tentatively be correlated with the Songpan - Garze fold and thrust belt of East Tibet, West Sichuan and North Yunnan [24]. In the Songpan-Garze belt and in the West Sichuan Basin, the occurrence of Indosinian shortening is attested to by an upper Triassic unconformity [24] and of Jurassic granites cross-cutting folded Triassic rocks (Calassou, 1994). In this belt, which is also strongly affected by Tertiary shortening [9], the other major reported unconformities are of Cenozoic age [24]. In this hypothesis, the southern part of the Songpan - Garze belt would be cut and displaced 400-700 km towards the south by Tertiary sinistral motion along the Red River Fault zone.

ACKNOWLEDGEMENTS

This work is a result of the cooperation program between the Centre National de la Recherche Scientifique (CNRS-INSU, France) and Institute of Geological Sciences, Việt Nam Academy of Sciences and Technology. We thank Wemmer for K/Ar low deformation dating data and Tapponnier, Leloup, Hoàng Quang Vinh and Findlay for many discussions. This paper is completed with the assistance of National Fundamental Research Program, Project 71 30 04.

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