Geological characteristics and geoheritage potential of Pù Luông Nature Reserve

01/11/2021

    Geoheritage refers to places or sites where evidence and imprints of the formation and development of the earth over 4.6 billion years as well as the evolutionary history of the life of a region or an area on the planet are preserved. These sites or areas are valuable in terms of scientific, aesthetic, historic, cultural and economic values and are potential tourist attractions. They are called Geoheritage - the heritage type of top importance among different types of natural heritage. In North Việt Nam, three areas with high limestone exposures are Northeast, Northwest and North Central. The Pù Luông Nature Reserve (NR) is on the Northwest limestone strip, having an average width of 50 km, and extending more than 400 km from the Sino-Vietnamese border down to the coastline of Thanh Hóa Province. The study on geological characteristics and heritage potential of Pù Luông NR is essential because this is the basis for choosing a socio-economic development model towards sustainable development, rational use and conservation of natural resources, environmental protection, effective adaptation to natural disasters and climate change and creation of new livelihood opportunities for local people.

    This article presents the geological characteristics and the karst geoheritage potential of the Pù Luông NR in Thanh Hóa Province.

          1. Introduction

    Karst accounts for approximately 20% of Vietnam’s territory. The Northern Việt Nam has 3 areas with large limestone exposures: Northeast (Ha Giang, Cao Bằng, Lạng Sơn, Quảng Ninh provinces...), Northwest (Lai Châu, Điện Biên, Sơn La, Hòa Bình, Ninh Bình, Thanh Hóa provinces...) and North Central (Quảng Bình)[1]. Pù Luông NR was established by the Thanh Hóa Provincial authority in 1999 in the territory of Bá Thước and Quan Hóa districts, about 130km to the Northwest of Thanh Hóa City. In the Northeast, Pù Luông borders Mai Châu, Tân Lạc and Lạc Sơn districts of Hòa Bình Province. Pù Luông  NR consists of two distinct ecosystems: Karst mountain ecosystem and non-karst mountain ecosystem. 1,109 plant species have been recorded, including many rare and endemic ones, especially 4 new genera and 7 typical conifer species of concern for conservation in Việt Nam. Pù Luông NR also has rich wildlife which belongs to the North Central region and part of the Northwestern faunal region. Geologically, Pù Luông NR is in the Northwest karst strip that has an average width of about 50 km and extends over 400km from the Sino-Vietnamese border down to the coastline of Thanh Hóa Province (Figure 1). This karst strip is very interesting with the existing Tràng An World Heritage Site, Cúc Phương National Park, Vân Long NR (all located in Ninh Bình Province), Ngọc Sơn - Ngổ Luông NR (Hòa Bình Province) and a few more potential areas that can be built into geoparks.

On one hand, the research, analysis of available data, and assessment of geological characteristics and geoheritage potential of Pù Luông NR would enhance its values. On the other hand, the research results on geoheritage potential in the Pù Luông NR would be the basis for the selection of a socio-economic development model (geopark) towards sustainable development, rational use of resources, natural conservation, environmental protection, effective response and adaptation to natural disasters and climate change and new livelihood opportunities to local people.

Figure 1. Location of Pù Luông NR in the karst geological formation of Northwest Việt Nam

    2. Overview of geological setting, characteristics and evolution history

    Pù Luông NR is located in the Northwestern tectonic structural zone. Similar to other places in Northwest, Pù Luông NR has experienced a very long and complex geological evolution history. Below is a brief introduction about the geological setting, characteristics and evolution history as well as some typical geoheritage values of this area.

    ​2.1. Paleontological-stratigraphic characteristics

    The area’s long and complex geological history has generated a variety of rock types, from very ancient (up to 1,000 million years ago) to very young. In the order from bottom to top, the following 14 formations are present in the study area: 1) Nậm Cô Formation (NP₃nc); 2) Mã River Formation (€₂sm); 3) Hàm Rồng Formation (€₃-O₁hr); 4) Đông Sơn Formation (O₁đs); 5) Nậm Pìa Formation (D₁np); 6) Bản Padp Formation (D₂bp); 7) Tốc Tát Formation (D₃tt) or Bản Cải Formation (D₃bc); 8) Bắc Sơn Formation (C-Pbs); 9) Cẩm Thủy Formation (P₃ct); 10) Yên Duyệt Formation (P₃yd); 11) Cò Nòi Formation (T₁cn); 12) Đồng Giao Formation (T₂ađg); 13) Nậm Thẳm Formation (T₂lnt) and 14) undivided Quaternary sediments (Q) (Figure 2).

    Regarding the distribution of stratigraphic units, from the Pre-Cambrian to the present, there is an alternation in terms of both time and space between carbonate and non-carbonate formations. In the Southwest - Northeast (SW-NE) direction, Pù Luông NR has 3 carbonate strips namely: (1) Thanh Xuân - Hồi Xuân - Quan Hóa  strip with limestone of the Hàm Rồng (€₃hr), Bản Páp (D₂bp) and Bắc Sơn (C-Pbs) Formations; (2) Núi Thung Hang - Lũng Niêm strip; (3) Pù Bin -Lũng Cao  strip both with limestone of the Đồng Giao Formation (T₂ađg). Alternating between them are non-carbonate strips: (1) Phu Pha Phong - Phu Ho strip with terrigenous-volcanic rocks of the Cẩm Thủy Formation (P₂ct) and the Cò Nòi Formation (T₁cn) and (2) Làng Lụa - Làng Cao strip with terrigenous rocks of the Cò Nòi  Formation (T₁cn) (Image 1).

Figure 2. Geological map of Pù Luông NR

Image 1.Terrigenous rock of the Cò Nòi Formation (T1cn) in Lũng Cao (left) and limestone of the Đồng Giao Formation (T2ađg1), in Lũng Niêm (right), Bá Thước District

    ​2.2. Structural-tectonic characteristics

    The study area is located in the two regional tectonic zones namely: (1) Mã River zone in the Southwest; Đà River zone in the Northeast. They both form a large NW - SE synclinorium and Pù Luông NR occupies most of this structure. The study area is dissected by some faults, including the Mã River deep-seated fault in the same NW - SE direction. Along such faults, basic eruptive-intrusive magmatic activity was very active in the Late Permian, resulting in the Cẩm Thủy Formation (P₂ct). The large NW - SE fault system in the Northeast of Hồi Xuân District town forms the boundary between the karst strips of the Bắc Sơn (C-P₁bs), Bản Páp (D₂bp) and Hàm Rồng (€₃hr) Formations. The Sơn La deep-seated fault has developed in the study area in the direction of 320 - 330°, running through Lũng Niêm and serving as the northeast boundary of the Cẩm Thủy eruptive Formation (Figure 2).

    2.3. Geomorphological characteristics

    The topography of Pù Luông area represents narrow mountain ranges which extend linearly and are separated from each other by tectonic faults in the same direction and expressed on modern topography in the form of river valleys, streams, long troughs.. The mountainous topography of the study area has both step and linear features with the alternation of high mountains surrounding plateaus and low mountains with tectonic-erosive valleys and depressions. On the other hand, exogenous processes strongly occur under the conditions of hot humid tropical climate and high rainfall (erosion due to runoff or surface water…, mass movement processes such as landslides, rock falls, debris..). Consequently, the topography of the study area has been strongly dissected (relative elevation difference > 500m, density of rivers and streams > 1km/km²). Thus, slope surfaces are predominant in the area over remnants of planation surfaces on watersheds or flat surfaces at the bottom of valleys. This also confirms that in the present-day epoch, the study area’s topography is undergoing a rejuvenation process.

    A very unique feature of this area is the development of many types of karst landforms due to the presence of many types of limestone. Sloping or steep cliffs usually develop only on pure, massive karst of the Bản Páp (D₂bp), Bắc Sơn (C-P₁bs) and Đồng Giao (T₂ađg) Formations. Recrystallized, marbleized limestone, clayey limestone or thin limestone interbeds in the terrigenous rocks of the Mã River (€₂sm) and Hàm Rồng (€₃hr) Formations often form hilly terrain with gentle cliffs and thin layers of residual soil.

    In fact, the karst topography in the study area is of the tropical type that is rejuvenated at different levels depending on the amplitude of neo-tectonic uplift. It is evidenced by: The presence of karst plateaus on the surface of which are residual cones and towers, which are raised karst fields; The development of multi-layered, multi-phased caves; The shortage of surface water but excess of groundwater. The karstification process in the region is very strong. In many places such as Thành Lâm, Thành Sơn, Lũng Cao, Làng Kịt, the whole mountain massif has been dissolved and washed away to reveal the undissolved bedrock underneath, proving that the karstification process occurs at the same time with the erosion and denudation process.

    Thus, it can be concluded that the geological structure along with the neotectonic movements have resulted in the main features of the topography of the study area. The highest and most majestic mountain range in the region (>1500m) was formed by the eruptive rock of the Cẩm Thủy Formation (P₂ct). Medium and low mountain ranges (200 - 600m) occupy most of the study area and are made of limestone. The rest is low mountainous terrain made up of terrigenous rocks and mainly distributed along river valleys or in closed intra-mountain valleys. In addition, these landforms are further complicated by exogenous and anthropogenic processes.

    By origin, the area’s topography can be divided into the following groups: (1) tectonic landform; (2) erosional landforms; (3) karst and karst-erosional landforms; (4) depositional landforms.

    Tectonic landforms: Tectonic landforms are of slopes and steep cliffs along faults, which are clearly shown, for example, along the Sơn La deep-seated fault, along Mã River deap-seated fault in Hồi Xuân, Trung Xuân, Phú Nghiêm, Thành Sơn - Lũng Niêm, Vân Mai - Làng Kịt, Mai Châu - Lũng Vân... They develop in the NW - SE direction, sometimes serving as a boundary between rock types of different composition and age, or even between different folds of the same rock, the same Formation. Besides, in the study area, there are many short and narrow slopes and cliffs along the NE - SW faults such as in ở Pù Bin, Phú Lâm - Lũng Vân, Thành Sơn - Nam Sơn, Lũng Niêm - Lũng Cao.

    Erosional landforms: Erosional landforms develop on eruptive and terrigenous rocks. In addition, gravity slopes can also develop on hard and brittle rock along fault zones or on limestone.

    Karst and karst-erosion landforms: Included in this group are karst plateau, karst-erosion valley and karst field, developing on carbonate and terrigenous-carbonate rocks.

     - Karst plateau is the most common landform, developing in the NW - SE direction along both banks of the Mã River, in ở Pù Bin, Phú Lâm - Lũng Vân, Thành Sơn - Nam Sơn, Lũng Niêm - Lũng Cao... Plateau surfaces are commonly found at three elevations: 900 - 1,200m, 600 - 800m and 300 - 500m, in the form of hills interspersed with valleys, sinkholes....

    - Karst-erosion landform develops on the limestone bedrock of the lower Đồng Giao sub-Formation, which is marbleized and interbedded with calcareous-clayey shale and compressed into narrow NW - SE strips. This type of landform has very steep slopes, resulting in a unique series of high and low karst towers interspersed between narrow parallel karst-erosion valleys. Such valleys also develop in Làng Kịt, Làng Khuynh villages. At the intersection of the karst-erosion valleys, karst fields of 1-2 to 3-4 km wide are sometimes formed with frequent surface flows. They are quite flat with up to 2-3 m thick alluvio-proluvial deposition (karst fields in Phố Đoàn, Địch Giáo ) (Image 2); In addition, on the elongated sags where limestone is interbedded with other terrigenous rock, cone-shaped karst hills are also developed with gentle slopes covered with residual soil.

    Accumulation landforms: Included in this group of landforms are alluvial floodplain, accumulation terraces of 1^st and 2^nd orders, erosional terraces of 3^rd and 4^th orders, deluvium and proluvium fans, formed mainly by the action of permanent and temporary flows and less by gravity processes.

Image 2. Karst valley in Kịt Một, Lũng Cao Commune, Bá Thước (left) and typical karst fields in Pù Luông - Cúc Phương strip (right).

    ​2.4. Cave development characteristics

    The cave system in the Đồng Giao (T₂ađg) and Nậm Thẳm (T₂lnt) limestone has a long and complicated geological development history, heterogeneous bedrock composition, and are quite diverse in form, length and depth. To date, 16 caves have been discovered and surveyed with a total length of 3,787.7m [4,5]. They can be divided into 2 groups: 12 wet (active) caves and 4 dry (inactive, fossil cave) caves.

    + Dry caves (Dơi Cave in Kho Mường, Thành Sơn Commune; Làng Lặn 1 Cave in Lũng Niêm Commune) are often found at an altitude of 10 - 15m above the valley bottom with many beautiful stone pillars, stone bells, stone curtains... The cave floor is often covered by blocks due to ceiling collapse or has a layer of lime soil that accumulates several meters high.

    + Wet caves often have water flowing all year round or in the rainy season; the cave entrance at the valley bottom is at the same altitude with the local erosion level. Caves develop along faults and often have high ceilings with numerous stalagmites and stalactites. In the Pù Luông karst area, most of the caves discovered and surveyed are wet caves, such as Bản Pốn Cave, Làng Lụa Cave and Làng Lặn 2 Cave.

    Some caves have high potential for geotourism and eco-tourism, such as Dơi Cave in Kho Mường, Làng Lụa Cave in Lũng Cao. Along with other traditions and cultures imbued with national identity such as drinking rượu cần (wine drunk out of a jar through pipes), múa sạp, xòe (kinds of dancing), many beautiful caves in the region can certainly contribute to increase the attractiveness of this area. In fact, in a few communes nearby, local people have embarked on renovating beautiful caves to develop tourism.

    2.5. Geological evolution of the Pù Luông NR

    The geological development history of this area is quite complicated, including the following 5 periods [4]:

    Late Proterozoic-Early Cambrian (PR₃-€₁): The regional subsidence regime occurred not only in the study area but also throughout the Northwest, facilitating the formation of very thick terrigenous rock layers of the Nậm Cô Formation around Phu Kỵ mountain.

    Early Paleozoic (PZ₁): The continental crust in the study area broke off and continued to subside, creating conditions for the formation of terrigenous-siliceous, terrigenous-eruptive rock types of the Mã River sub-formation. Right after that, although the whole study area was still in the subsidence regime, the seabed started to uplift contrastingly, thus forming the terrigenous-carbonate rock types of the middle and upper Mã River sub-formations (€₂sm_2-3) as well as the Hàm Rồng Formation (€₃hr).

    Middle-Late Paleozoic (PZ_2-3): During the Ordovician-Silurian periods, the study area experienced a regime of strong continental uplift and sedimentary disruption. But immediately after that, in early Devonian, it again subsided to form terrigenous-siliceous, terrigenous-carbonate rock types of the Nậm Pìa (D₁np) and Bản Páp (D₂bp) Formations. In the late Devonian, there was a slight uplift and then the whole region slowly subsided from the beginning of the Carboniferous, extending to the end of the early Permian, forming the Bắc Sơn Formation limestone (C-P₁bs). In general, the tectonic regime of the region in this period is quite stable.

    Late Paleozoic-Early Mesozoic (PZ₃-MZ₁): Starting from the late Permian, tectonic activities in the region resumed, the faults reactivated in association with eruptions and basic intrusion, forming the Cẩm Thủy Formation and the Điền Thượng Complex. At the end of the late Permian, volcanic activities ceased, the seabed became shallower with a slight uplift to form the coal-bearing terrigenous rock of the Yên Duyệt Formation. By the end of this period, during the Triassic, strong tectonic subsidence occurred in the eastern part of the study area, resulting in the terrigenous-carbonate rocks of the Cò Nòi Formation (T₁cn), carbonate rock of the Đồng Giao Formation (T₂ađg) and terrigenous-carbonate rock of the Nậm Thẳm Formation (T₂lnt).

    Late Mesozoic-Cenozoic (MZ₂-KZ): Since the late Mesozoic, the entire study area has become continental where the uplift regime was dominant with the evidencebeing in the form of planation surfaces. There were up to five such surfaces at the absolute altitude of 1400 - 1600m, 900 -1200m, 600 - 800m, 300 -500m and 200 - 300m. Loose, unconsolidated formations accumulated along valleys with regular flows. The sub-meridian and sub-parallel fracture zones became active, causing quite significant left-lateral strike slip (with an amplitude up to 4km) along Ban Công - La Hán .

    ​3. Geoheritage potential of Pù Luông NR

    Geoheritage is a type of geological resource with outstanding scientific, educational, aesthetic and economic values, including geomorphological landscapes, paleontological fossils, inactive or active volcanoes, caves, canyons of natural rivers and lakes, waterfalls, natural or man-made outcrops of rocks and ores, formations and landscapes recording special geological events and contexts, or even abandoned mines, places where past and present-day geological processes can be observed. Like other heritage types, geoheritage is a non-renewable and of limited resource, so it needs to be preserved, exploited and used sustainably. Currently, with the development of the industry, the extent of human encroachment onto nature has become so serious, which threatens the lives of people and all species on Earth. To contribute to solving this problem, UNESCO, through the Global Geoparks Network (GGN), always encourages the establishment and development of geoparks aiming to achieve the three following objectives: Holistic conservation of heritage values (including geoheritage); Raising public awareness of geoheritage, natural heritage and environmental protection, protection of the common home - the Earth; Socio-economic development, hunger eradication and poverty reduction for local communities.

    Geoheritage includes many types. Specifically, according to the “Provisional classification criteria of geoheritage” of the UNESCO World Heritage Convention (WHC) applied in Việt Nam in Circular 50/2017/TT-BTNMT dated November 30^th, 2017 of the Ministry of Natural Resources and Environment, there are 10 geoheritage types: A. Paleontology; B. Geomorphology; C. Paleoenvironment; D. Rock; E. Stratigraphy; F. Mineral; H. Geoeconomics; I. Tectonic (geological history); K. Cosmic issues and L: Geological features of continental/oceanic scale. In Việt Nam, geoheritage is assessed quantitatively according to 6 scientific criteria: (1) Scientific and educational values; (2) Geological diversity; (3) Landscape and aesthetic values; (4) Cultural, social and historical values; (5) Threats and conservation needs and (6) Potentials for exploitation and use.

    According to the research results, Pù Luông NR has 24 geoheritage sites that have been investigated and studied, including 13 that are recommended for national ranking and 11 recommended for provincial ranking [4]. Below are some typical geoheritage values of the Pù Luông  NR.

    ​3.1. Paleoenvironmental heritage (type C)

    - The Vietnam’s most ancient glacial sedimentary rock: Located in the lowest part of the Mã River Formation (NP₃-€₂) at the Suối Tỏi, Suối Giá sections (Quan Hóa District). The shale unit containing mixed cobbles-pebbles-gravels of glacial origin, Sinian age (late Neoproterozoic) is the oldest and unique in Việt Nam and rarely seen internationally; valuable in many aspects (scientific, educational, cultural and economic) [2, 4].

    - The two Vietnam’s most ancient biozones which lie in the upper part of the Nậm Cô Formation, in the sericite shale and the lower part of the Mã River Formation, in the black shale-marking unit, in the Suối Tỏi, Suối Giá sections. The first zone is Protosphaeridium (the highest zone of NP₃). The second zone is Archaeohystrchosphaeridium (the lowest zone of Lower Cambrian €₁ [4, 2].

    ​3.2. Stratigraphical heritage (type E)

    - The cross-section containing the boundary of Lower Cambrian and Upper Neoproterozoic €₁-NP₃ (PZ-PR). It is in the lower part of the Mã River Formation (NP₃-€₂) at the Tỏi Stream, Giá Stream sections. This boundary is rarely seen in the world, but it is very important to study because of its importance to many scientific disciplines (Images 3, 4, 5) [4].

Image 3. Giá Stream (Quan Hóa): Sericite-quartz shale intercalated quartzite of the Nậm Cô Formation of Proterozoic age - the oldest in the region	Image 4. Luồng River (Quan Hóa): The coal shale marker unit containing the oldest fossil zone of the Lower Cambrian in Việt Nam
Image 5. Tỏi Stream (Quan Hóa): Sericite-quartz shale - the highest part of the Nậm Cô Formation - lining the bottom of the glacier-origin rock unit (left) and the glacier-origin pebble-bearing shale of Late Sinian age (»NP33), base of Mã River Formation (Z3-Î2) (right).

    - The cross-section containing the Cambrian - Ordovician boundary (€-O): In the upper part of the Hàm Rồng Formation in Làng Vạc section (Cẩm Thủy District) and Điền Lư section (Bá Thước District). These boundary sections are of great scientific significance in determining the boundary between the Cambrian and the Ordovicia [4].

    - Typical marine regression section. Làng Vạc section (Cẩm Thủy): It is a section barely seen in fact, showing all the characteristics of the marine regression process - From shallow sea carbonate, gradually transitioning to terrigenous carbonate - fine-grained, coarse-grained terrigenous rocks, containing many fossil zones of very shallow to coastal facies.

    ​3.3. Paleontological heritage (type A)

    - The oldest macro-fossils in Việt Nam and the layers containing rich and typical macro-fossils, including: (1) The lower part of the Hàm Rồng Formation in Điền Lư section (Bá Thước District), containing many Oncolites, Trilobites, and Brachiopods in limestone and limestone shale of Middle Cambrian (€₂) age; (2) The lower and middle part of the Đông Sơn Formation, at Hill 205 (Làng Vạc, Cẩm Thủy District), containing many Trilobites, Brachiopods, Bivalves and Crinoides of Lower Ordovician (O₁) age; (3) The middle part of the Nậm Pìa Formation in Làng Vạc; containing many Brachiopods and Bivalves of Lower Devonian (D₁) age; (4) The middle part of the Bản Páp Formation in Điền Lư and Làng Vạc, containing many kinds of Anthozoan of Lower Devonian D₁ age. Oncolites in limestone layers and Trilobites in limestone clayey shale aged Middle Cambrian (about 505 million years ago), are visible to the naked eye and have high scientific and educational values. They have not been preserved and are being strongly influenced not only by nature but also by humans.

    3.4. Stone heritage (type D)

    - Typical “pillow” basalt. The wide strip that surrounds Điền Lư anticline  of Late Permian age. This type of basalt has a very characteristic “pillow” structure, which clearly shows many aspects of the history of formation and development and the palaeo-geographical and paleo-environmental conditions.... Rarely seen in Việt Nam, it has not been preserved and is being exploited in many places for many different purposes.

    3.5. Geomorphological and tectonic heritage (types B, I)

    - Remnants of ancient river valleys: Remnants of ancient river terraces are also found along the Phú Lâm - Thành Sơn - Lũng Niêm and Làng Lụa - Làng Độc faults and depressions in the form of flats high-rising on depression bottoms or creating steps on the slopes. On some such flats along the Phú Lâm - Lũng Niêm depression, alluvial sediment no longer exists but there are many round pebbles made from eruptive rocks. Along the Làng Lụa - Lũng Niêm depression is alluvial-proluvial remnants on the surface of 8 - 10 and 15 - 20m high flats. At the same time, along these depressions, many hanging valleys cut through these ancient river terraces. They demonstrate an ancient river once flowing here, probably the ancient Mã River that had to gradually retreat to the southwest due to the subsequent uplifting movements. This heritage has scientific and educational significance to river activities along the fault.

    - Evidence of the Ban Công - La Hán sub-meridian fault: This area is to the Southeast of Pù Luông NR. The expression of left-lateral slip along the fault is very clear, didactic and very meaningful in active tectonics research.

    - Dơi Cave, Kho Mường, Thành Sơn Commune, Bá Thước District. Northeast of Kho Mường Village, Quan Hóa. Survey length: 232 m; depth: - 20 m. From Cành Nàng District Town (Bá Thước District), visitors should follow Road 15 to the Northwest to Thành Sơn Commune, then go downhill for about 30 minutes to arrive at Kho Muong village. From Kho Mường Village, boating down Kho river for about 15 minutes, visitors will reach the cave entrance in the limestone of the Đồng Giao Formation (T₂ađg). There are 2 caves: one dry and one with running water. The Cave must be connected to an underground river because all surface water in Kho Mường area flows into Kho River before completely disappearing into the ground. This underground river is exposed in Bản Pốn Village, Lũng Cao Commune, about 2.5 km from Kho village to the North-Northeast. Geological structure shows that Kho Mường - Bản Pốn - Lặn Trong cave system has at least two cave levels. The intersection of these two cave levels corresponds to the intersection of two fault systems in the NW - SE and NE - SW directions in Bản Pốn .

    4. Results and discussion

    Study results show that the geological formations in the Pù Luông NR have a wide age diversity, including 14 rock and soil formations from Proterozoic (very ancient) to Quaternary (present). The diversity is also shown in the rock compositions and rock types of all origins: magmatic, metamorphic and sedimentary. The diversity in geological age and paleo-environment of the NR has led to the record diversity of the paleontological world here. Pù Luông NR, on the other hand, is dissected by many fault systems, especially the NW-SE system. The slipping along these faults has created a strip topography with alternating high and low areas. Other fault systems such as NE - SW, sub-meridian and sub-parallel, have complicated the geological setting of the area. The tectonic shifting has also changed the main structural direction of the region, creating many large valleys and countless interconnected caves as well as an underground river system. The topography can be divided into several landforms, especially the erosional slopes on basalt in Pù Luông strip and the karst/karst-erosion landforms on carbonate, carbonate-terrigenous rocks of Triassic age. It is noteworthy that the processes of erosion and denudation predominate over the process of accumulation, which proves the whole area is undergoing a tectonic uplift.

    Study results on geoheritage potential show that Pù Luông NR has 6/10 types of geoheritage expressed in 24 geoheritage sites that have been investigated and researched, including 13 sites proposed for national ranking and 11 sites for local ranking [4, 5]. Along with other heritage values, this region has the potential to become a national geopark and in the future, with more detailed, specific and extensive investigation and research results, it can become a UNESCO Global Geopark. According to the geopark classification table proposed for Việt Nam, it can be classified as type 2 - mountain/volcanic systems; 5 - structural/tectonic features; 6 - fossil/stratigraphical sites and/or 9 - rivers and ice caps as imprints of the ice age can be classified [4].

    Pù Luông  NR has a very high biodiversity level on the basis of the equally rich geological diversity. The above presentations have contributed to confirm that Pù Luông NR has unique geological, geomorphological, hydrological, caves, etc. characteristics, which make it a true candidate for a geopark of national, and even international, level.

    The values of geology, geoheritage, natural resource and traditional culture of the indigenous people in the Pù Luông NR will be a valuable resource for the local authority and people to choose a sustainable development model - a promising direction that not only benefits the local communities but also plays an important role in associating the goal of preserving the values of geoheritage and ecological and cultural diversity.

    Acknowledgments: This article was written under the framework of the task NVTX.2021.03.14 “Characteristics and potential of karst geoheritage in Northwest Việt Nam” funded by the MONRE and implemented by the Center on Karst and Geoheritage, Vietnam Institute of Geosciences and Mineral Resources. The collective authors would like to express our respectful thank.

Đỗ Thị Yến Ngọc, Phạm Thị Thúy, Trần Tân Văn

Nguyễn Xuân Nam, Cao Thị Hường

                                    Vietnam Institute of Geosciences and Mineral Resources

(Source: Vietnam Environment Administration Magazine, English Edition III - 2021)

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