Figure 1 Traffic Location Map of Lanongla Section in nyalam county, Tibet
The Lanongla section of nyalam county, Tibet (inset 1) is located in Dasajiugou, about 1 on the east side of the 5264km milestone of China-Nepal Highway. 5 km eastward, you can see the "iron hat" on the red hill formed by iron-bearing sandstone strata, which is quite eye-catching on the terrain (Figure 2). In this paper, Xu Yulin et al. (1990) is named as this section, that is, Lanongla section. The author investigated this profile in 1999, established a more detailed ammonite sequence, and named it Tieoolitic Sandstone Formation (Yin et al., 2000). The Ranongla section consists of Ranongla Formation (early Balu period of Middle Jurassic), Iron Oolitic Formation (late Batumi period) and Menkadun Formation (early Kaluofu period to Tetang period) from bottom to top. There is a sedimentary loss between the Lanongla Formation and the upper Batong oolitic sandstone formation with a time span of about 8 Ma (Yin et al., 2000). The Upper Batong Iron Oolitic Sandstone Formation is in integral contact with the bottom of the overlying Menkadun Formation. Because the sedimentary structure is only found in the oolitic sandstone formation, the lithology, fossils and sequence of this formation are described as follows.
Illustration 2 Middle Jurassic iron stromatolite sandstone landscape in Lanongla section
The black mudstone of Menkadun Formation in the overlying strata contains redeposited khaki thick layered oolitic sandstone blocks.
The ammonite-bearing ones are: Macrophalites Gucuoi (Westermann et Wang), Homoeoplanulites Balinensis (), Macrophalites cf. Jaquoti (Douvillé), Macrophalites divisions (Westermann et callomon), Jean-Netieras cf. Anomallumelmi, Khaiceras cf. devauxi (Gross) and Bobbrites cf. Microstoma (d 'orbigny), etc.
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The iron oolitic sandstone strata from new to old are:
7. Thick yellow-brown oolitic sandstone or argillaceous sandstone, rich in ammonites, such as Phylloceras sp. Oxyxene reference orbis (Gibel), cadmium mine. , Procerites sp., macrophylites cf. jaquoti (douvillé), choffatia (grossouvia) cf. bathonica (mangold) is about1.3m.
6. Purple-red calcareous thin-layered iron stromatolite sandstone: the lower part is sandwiched with wavy black-red iron layer and khaki sandy layer; The upper part is tubular stromatolite sandstone, which contains no other fossils about 0. 5-0.8 m。
5. Thin-bedded limestone transits upward to thin-bedded iron sandstone, occasionally containing macro-brain-like stone, but lacking other benthic fossils about 0. 3 m。
4. Thin limestone lens body, extending eastward about 20 meters in section, gradually tapering out, 0. 5 meters.
3. Gray-green siltstone lens, which extends eastward for about 20 meters in section, gradually pinches out, 0. 2 meters.
2. Thick layered sandy limestone, rich in Fargesia fossil about 0. 45 meters.
1. Gray-green thin siltstone, about 0 without fossils. 15m.
-Settlement loss (false consistency)
Bioclastic limestone of Nieniexiongla Formation in the underlying stratum; It includes the evolution of late basta-stage Chondromycete -cense (Waagen), Chondromycete cf. Crassicostatum (Westermann) and Dorsetensia CF. Ed-Ouardiana (D 'Orbiany). Many snakes, brachiopods and bivalves are more than 60 meters.
Age and genesis analysis of iron sandstone sedimentary assemblage in the second member of Lanongla
The ammonite cartilaginous fish evolved into Cense (Waagen), cartilaginous fish evolved into Crassicostatum Westermann, Dorsetensia evolved into edouardiana and Stephanoceras sp. They were found in the bioclastic limestone of Lanongla Formation in Lanongla section, indicating that the ammonite belt was in the soft Qin period. The oolitic sandstone layer in the upper part of the oolitic sandstone formation is rich in ammonites, including phyllite. Oxyxene reference orbis (Gibel), cadmium mine. , process sp sp sp (westman and Wang), Chrysomya megacephala, Chrysomya megacephala. Among them, Oxycerites Orbis (Giebel) is a belt fossil in the upper Batonian orbis ammonite belt in northwest Europe. This layer of oolitic sandstone gradually transits upward to black mudstone, which contains ammonite macrocephalites (see jacquoti (douvillé)), homoopeplanulets (see evolutum sandoval et gabaron) and homoopeplanulets balinensis (Neumayr), indicating the Upper Batumian discus ammonite belt in northwest Europe. The black mudstone containing ammonites in this layer transits upward to gray-black argillaceous nodule shale, which is rich in early Karev ammonite fossils, such as Macrocephalites bifurcation; M. Guo Cuoi Jeanneticeras cf. Anomalum, Khaiceras cf. devauxi, Bom-burites cf. microstoma and Neuqueniceras(Frickets)Tibet um. It is worth noting that no ammonite representative of the late karlo stage has been found in this profile so far, and it is likely to be missing in the late karlo stage (Figures 3 and 4).
Illustration 3 Close-up of Middle Jurassic iron stromatolite sandstone in Lanongla section
Fig. 4 Middle Jurassic iron stromatolite sandstone profile strata and ammonite zone in Lanongla profile.
Rioult et al. (199 1) divided the iron sandstone into stromatolite pavement, centimeter-level round iron oolite and well-sorted iron oolite. In the early stage, the iron sedimentary rocks in the early Jurassic in southern England were interpreted as being formed by the interaction between biological (algae) and abiotic * * *. In recent years, Palmer and others put forward a new view that the formation of iron nodules is related to the role of non-photosynthetic iron-oxidizing bacteria. In particular, Préat et al. (1998,1999,2000) revealed the origin of the sedimentary assemblage of iron nodules (oolites) through the study of iron-bearing sedimentary rocks in Paleozoic and Mesozoic Europe, and they believed that most of the iron came from bacterial activities. This is a filamentous bacterium related to Beggiatoaceae, which usually thrives in the marine still water environment with the water depth exceeding 50m to 100 m, that is, it is most likely to survive below the light transmission zone. In this usually anoxic and hypoxic environment, the solubility of iron components is relatively low. The sedimentary structure of the iron oolitic sandstone formation in Lanongla area, Tibet consists of parallel layered iron stromatolite pavement (plate 1, Figure 8) or several centimeters of round iron oolitic grains (plate 1, Figures 2 and 3). And well-sorted oolitic sandstone layers (oolites) form a set of iron sandstone sedimentary sequence similar to that described by Rioult et al. (199 1). Macroscopically, the lack of benthic fossils in the oolitic sandstone formation in Lanongla section is probably related to the fact that the sedimentary basement is in an anoxic or anaerobic state. Because the archers and ammonites in the oolitic sandstone formation in Lanongla section belong to active swimming animals, their living water depth is estimated to be greater than 100 m (Yin Jiarun, Wan Xiaoqiao, 1996). Generally speaking, the iron oolitic sandstone formation and its overlying Menbu formation in Lanongla area are a set of sedimentary sequences under the background of transgression since the late Batong period. The analysis of iron sandstone assemblage, fossil palaeoecology and sedimentary environment of algae-bearing gray sandstone in Lanongla section shows that the water body in this sedimentary sequence is gradually deepening, which is a continuous environmental change process (Figure 5).
Fig. 5 Schematic diagram of sedimentary environment of middle Jurassic iron stromatolite sandstone in Lanongla section.
The sedimentary environment of Lanongla area before the early Balu period was shallow-sea carbonate platform, and the total thickness of bioclastic limestone and marl exceeded100 m. However, due to regional tectonic movement, this area rose to become an denudation area from the late early Balu period to the middle Baba period. In the late Batong period, with the rapid rise of global sea level, a new sedimentary area was formed in Lanongla area, which experienced long-term erosion due to transgression. The whole sedimentary process of iron stromatolite sandstone assemblage can be identified as follows:
At the initial stage of (1) transgression, due to the rapid rise of sea level, it became a normal salinity marine environment that could accommodate the existence of narrow salinity archer belemnopsis. According to the calculation of the limit depth of cracks in the gas shell of arrow stone animals, the most suitable water depth for the survival environment of Berberis sagitta in Jurassic is about 100 m (Westermann,1973; 1990) 。 Although the arrow stones recorded in stratigraphic records are usually found in shallow water (20 ~50m) sedimentary facies, if there are no other benthic organisms in the biota and they are completely composed of arrow stones, the water depth of the ecological environment is more likely to be close to 100 m. Generally speaking, the seabed with a water depth close to100 m belongs to a low-energy environment. However, the bottom of the stratum where the archery stone fossils are preserved is uneven, showing a scouring surface washed by high-energy water. Above this scouring surface, the archery stone shell is quite dense, showing an archery stone shell layer with higher shell density after being transformed by high-energy water. In the overlying strata, the number of archery shells suddenly decreased and dispersed in the surrounding rock, indicating that the water flow conversion ability was obviously reduced. Therefore, the sedimentary sequence of this set of archery fossils can be well compared with the crustal sedimentary characteristics controlled by storms. The sediments in this stage are mainly composed of calcareous components and silty sediments, but there is no iron component, which proves that the sedimentary basement and terrigenous debris supply area in the early weathering and denudation have not become the sources of iron component sediments. In other words, the traditional explanation that iron ore deposits come from weathering and denudation does not apply to this area.
(2) The continuous rise of sea level leads to the increase of water depth in sedimentary environment, and a well-developed iron sandstone layered structure can be formed in still water environment with little hydrodynamic force and no serious damage to benthos. The sedimentary structure assemblage of iron sandstone is mainly composed of maroon thin-layer iron-rich sandstone and gray-yellow thin-layer sandstone with a thickness of several millimeters, and its assemblage sequence is almost the same as that described by Préat in Normandy barotropic period in France. The bedding at the bottom is almost parallel to the sedimentary basement, and the bedding in the middle is a raised hill (so-called stromatolite bedding). The upper part is spherical bedding (so-called nodular body) and columnar stromatolite sandstone sedimentary structure, with a diameter of 5 ~ 6 cm, and the uppermost part is iron oolitic sandstone (so-called well-sorted oolitic grains). Considering that the water depth of calcareous sandstone containing arrow stone in this area is below the light transmission zone before the iron sandstone is deposited, the ammonites contained in the iron oolitic sandstone also indicate the deep-water sedimentary environment, such as the ammonites are relatively large, mainly composed of molecules of foliaceous ammoniaceae and Macrocephalaceae. Most of the Pteridaceae live in the deep-water environment of the outer continental slope, while most of the Pteridaceae thrive in the relatively deep-water environment near the outer continental shelf and the continental slope (Yin Jiarun et al. 1996). Therefore, the causes of these different forms of iron layered structures are likely to come from bacterial biochemical deposition in deeper water environment. With the further rise of sea level, iron stromatolite sandstone will inevitably be deposited in deeper water bodies. Obviously, those algae that need photosynthesis can't survive at this depth. Only those bacteria and algae that rely on non-photosynthetic biochemical precipitation can form iron stromatolite sandstone. The mechanism of iron capture by these heterotrophic bacteria and algae should be as described by Préat et al. (1998,1999,2000), and the alternation of iron layers and sandstone layers may be microorganisms.
We know that the water depth of photosynthesis of modern marine algae is mostly between 0 ~ 15 m, but from the paleoecological study of Lanongla section, we know that the paleoenvironmental water at that time was far greater than 15 m. In addition, Cao Ruiji et al. (200 1) think that the alternation of biological deposition and sandstone formation is due to a vibration equilibrium state between the growth rate of microorganisms and the deposition rate of minerals from the perspective of the formation of trypanosoma-like gray sandstone bedding in Proterozoic.
(3) At the beginning of Karlov, the sea level continued to rise. The oolitic sandstone in the late Batong period was transformed into black mudstone and black-gray shale rich in argillaceous nodules in the early Karlov. The biodiversity of ammonite fauna produced in shale is very high. The ammonite fauna, represented by Compositae, has a water depth of 150 ~300 m and is a continental slope environment. A large number of muddy nodules are related to the strong disturbance of bottom water, so it may be one of the main reasons that are not conducive to the survival of iron bacteria. The rich organic components in shale may represent a strong reducing environment, which is another reason for the disappearance of iron components.
3 discussion
Ordovician and Jurassic are two main periods of iron oolitic sandstone deposition in the world. Jurassic iron oolitic sandstone deposits mainly developed in the early-middle Jurassic. Before Mesozoic pan-continental cracking, early Jurassic iron oolitic deposits were concentrated in northwest Europe; Compared with the early Jurassic, the distribution of iron oolitic deposits in Europe is greatly reduced, but it is widely distributed in other continents outside Europe. Tethys Himalayan Middle Jurassic oolitic sandstone is widely distributed, almost all over the southern margin of West Tethys and East Tethys, from Britain in northwest Europe, passing through German, French, Arabic, Zanskar and Xipiti regions in Pakistan, Ali region in China, Takhola region in Nepal and Lanongla region in nyalam county, Tibet, China. However, its age is not exactly the same everywhere, and it is different among Balu period, Batong period and early Carophe period (Jansa, 199 1). The reason why the Middle Jurassic iron oolitic sandstone can be distributed tens of thousands of kilometers from western Europe to the east along the southern margin of the Tethys Sea may be closely related to the changes and deviations of the ancient coast at that time. In the East Tethyan Himalayan region, such as Zanskar in Pakistan and Spiti on the India-Pakistan border, the overlying strata of Middle Jurassic iron oolitic sandstone deposits are usually rich in argillaceous nodules and megacephalidae molecules, so their age is considered as early Karlofian. There are late Badong ammonites in the oolitic sandstone layer in Takhola, central Nepal (Cariou et al., 1994). The Jurassic stratigraphic section in Lanongla, nyalam county, Tibet has been studied by many people. Due to the lack of systematic fossil collection and ammonite sequence data, the age determination of the iron oolitic sandstone deposit in this section is wrong, and the genesis of the iron oolitic sandstone deposit is also lack of exact explanation. Westermann et al. (1988) considered iron sandstone as the bottom of Spiti shale in this area according to the large-headed ammonia fossils in the gray-black shale covered by iron sandstone sediments, and classified it as the lower Karlofian. Xu Yulin and others classified Huang Yaping (1982) as Karlofian according to the results of fossil identification of unpublished master's thesis. The two layers of iron sandstone in Lanongla section of nyalam county were formed by tectonic dislocation, and the ammonite sequence fully proved the repetition of strata (Yin et al., 2000). However, in recent years, in the study of sequence stratigraphy in Himalayan region of Tibet, the oolitic sandstone in the Ranongla section, which originally belonged to the same horizon, was regarded as two sets of ancient weathering crusts, which became the upper and lower sequence boundaries of Karlofian's "supersequence" (Shi et al.,1996; Shi 2000). Haoteng (1985) thinks that the Middle Jurassic iron oolite deposit is related to the main cracking period of Gondwana continent, and it is also a sign of sea level rise and transgression (Haoteng,1985; Haram,1992,2001). Taking the Lanongla section of Nyalam, Tibet as an example, although the iron oolitic sandstone deposits in the late Batong period occurred in the early stage of transgression, they were formed only after the water depth of the sedimentary environment reached a considerable depth. At present, the "iron hat" landform we see is formed under the action of modern weathering. In the Himalayan Tethys region, the iron oolitic sandstone deposits from the late Batumi to the early Karlov represent a significant sea level rise period (Jansa,1991; Garzanti e1999), rather than the "ancient weathering crust" eroded after the regression period. Obviously, it is not appropriate to divide the same layer of oolitic rocks into two stages of different ages, and then interpret them as the upper and lower sequence boundaries of "supersequence" in Karlofian.
In the past, China's interpretation of oolitic sedimentary facies was often shallow water, such as the recently completed geological survey report of nyalam county (Zhu Tongxing, 2004. ) This set of iron oolitic sandstone is also regarded as the shallow water environment deposit of tidal flat and inland shed. In recent years, the international explanation of the sedimentary structure of stromatolite-like sandstone and the sedimentary origin of iron oolitic particles in iron sand strata tends to be biochemical deposition of bacteria (Palmer and Wilson,1990; Préatet al .， 1998； 1999,2000) 。 Different from the traditional explanation of the genesis of stromatolite sandstone, the stromatolite structure of iron sandstone was formed in the deep water environment with low hydrodynamic strength and opaque zone. As mentioned above, the Jurassic sandstone sedimentary assemblage with iron-bearing stromatolite structure in Nyalam area, Tibet has always lacked a reasonable explanation for its age and origin. In the Tethys Himalaya, the sandstone layer with iron stromatolite structure in the Middle Jurassic is generally only 3 ~ 5 m thick, but it is widely distributed, and the overlying strata are Hespiti shale facies rich in organic matter. Based on the macroscopic facies analysis and paleoecological study of the iron sandstone sedimentary assemblage in Lanongla section in Nyalam area, Tibet, the following conclusions are obtained: ① The iron sandstone deposit in Lanongla section in Nyalam area, Tibet was formed in the geological background of rapid sea level rise. This is a set of sedimentary records of transgression period under the global high sea level, from the late Batumi to the early Kalov. Instead of the so-called early and late ancient weathering crust of Karov. (2) This set of iron deposits mainly occurs in the deep water environment near the continental slope and is in the opaque zone. Therefore, iron precipitation produced by bacteria or algae is not necessarily related to photosynthesis. (3) The iron composition in the iron sandstone sedimentary assemblage is not from the early weathering and denudation of terrigenous clastic recharge area, but is most likely related to the biochemical precipitation of bacteria or algae in deep-water environment caused by sea level rise.
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Age and Sedimentary History of Late Batong (Middle Jurassic) Iron-bearing Hornblende Sandstone in Nyalam Area, Southern Tibet
The late Batong iron-bearing stromatolite sandstone strata are unconformity covered on the early Bayonne Xiongla Formation. This has recently been confirmed by the ammonite sequence of Nyalamarea in southern Tibet. There is less iron in the bottom and lower strata, which indicates that the iron precipitation in the strata may have nothing to do with any previous erosion process. The sequence of ammonites, arrows and lithofacies shows that the water depth fluctuates between 100-300 m due to the rapid rise of sea level. Iron stromatolites are considered to be the product of biochemical deposition of bacteria and fungi and the response of global sea level rise, which occurred from the late Batong to the early Karlov.
Keywords Tibet, Middle Jurassic, ammonite, iron-bearing stromatolite sandstone formation
The outcrop landscape of iron stromatolite sandstone sedimentary assemblage in 1- Lanongla section contains spherical structures with the size of 5 ~ 8 cm; 2- iron stromatolite sandstone layer with spherical structure (diameter 6 ~ 9 cm); 3— Arrow stone layer, the white dot of about 1 cm in the figure is the cross section of the arrow stone shell, which shows the arrow stone shell in layers; 4— Concentric bedding (smooth hand specimen, diameter 5 cm) shown in the cross section of columnar structure of iron stromatolite sandstone layer; Colored sediments of iron sandstone sedimentary assemblage in 5- Lanongla section: a. bioclastic limestone in the underlying Balu period; B. thin siltstone; C. D. Arrow limestone (sandwiched between Arrow limestone) thin siltstone; E. iron stromatolite sandstone; 6— outcrop landscape of iron stromatolite sandstone, in which the iron sand layer and underlying limestone layer are positive terrain, and the black shale overlying the iron sand layer often forms negative terrain (dark part on the right side of the figure); 7— Arrow stone in arrow limestone layer is not oriented; 8— Stromatolite bedding of iron stromatolite sandstone layer (smooth hand specimen, 6 cm wide); 9— The iron stromatolite sandstone deposit in Lanongla section (the upper part of the figure is dark iron sand layer) and the underlying bioclastic limestone and marl interbedded in the early Balu period; 10 —— Longitudinal bedding (growth direction) of columnar structure of iron stromatolite sandstone layer (smooth hand specimen, height 7 cm); 11-outcrop landscape of Lanongla section; 12-iron stromatolite sandstone layer (geological hammer pointing)