THE SALT DIAPIR AND ITS GENESIS IN KUCHE BASIN XINJIANG, CHINA
Article type :
Proceed ings of Symposium on DIAPIRISM
GENERAL SITUATION KucheBasin is situated at the southern piedmont of Mt.Tianshan, at 40? 31'- 42?15' N, 80? 00'-84?45'E, with a total area of about 50000 kml. On the south of the basin is TalimuRiver and on the west and east are Tongus-TulufanRiver, Kuerle county respectively. It is a long and narrow basin: from east to west approximately 500km long and less than 100 km wide.
Meso-Cenozoic strata with thousands meters in thickness is full developed in the basin. Mesozoic strata, which mainly distributed on the north edge of the basin, is a group of river, river mouth delta and limnetic deposits. Most part of the basin is covered by Cenozoic strata. In early Eogene Period a group of gulfs and lagoons deposited alternatively. The lagoons were gradually closed and replaced by inland salt lake deposits afterwards. Thus a red salt-bearing clastic deposit formation occured, The inland salt lake evolved into saline lake in the late Eogene and formed a group of red clastic rock formation. The Quaternary strata of this region consists of loose gravel, loesse, ice-borne sediment and modern deposit.
Stratigraphic classification of Tertiary of basin listed in table 1. The Eogene is a main period of evaporite deposit, during which period the salt deposit area enlarged from below to above reaching up to 10 thousand kml. The salt series strata which are composed of brown fine clastic rock bedded by rock salt and Jitt1e gypsum are cropping more fully in the west of the basin. The salt bed increased in thickness upwards. Halite is the main salt mineral and sulfates is reducing upwards. The rock salt and fine clastic rock form beds each other. There are a polyrhythmic textural feature in the salt-bearing section. The strata of Eogene is 2600m thick and in which the salt bed is about 1600m in accumulative thickness. The huge thick bed of rock salt has been a material base for the formation of salt dome. KucheBasin, a long and narrow trough-like depression, is located between Tianshan fold belt and Talimu platform. It is a piedmont depression formed in Meso-Cenozoic Era. Its northern limit is Tianshan deep fault belt and the southern limit is TalimuRiver deep fault belt. The formation of the basin has close relationship with all previous tectonic movements. An embryo basin was formed by Hualixi Movement in late Palaeozoic Era. The Yanshan, Ximalaya Movements and the Neotectonic Movement in Cenozoic Era, they all deeply influenced and reformed Kuche depression.
The folds and faults in the basin are very developed because of frequent tectonic movement. Since the depression situated between the northern and southern deep faults and under a strong press coming from the two directions, a series of folds and faults were formed. These folds and faults often had inheritance in time and polyphase feature. The addition of efforts in front and behind resulted in formations of a complex tectonic pattern and a series of secondary tectonic units in the depression. Kuche depression has undergone tectonic movement many times with producing of faults in its historical geology. Thus the original structures got revival; in addition, there produced many new structures. To sum up, the main direction of the faults is from west to east, the second direction is west-north and east-north. The W-E fault is huge in scale: hundreds kilometers long and a large fault throw. The direction of its distribution accorded with that of the fold extension. These faults which often developed in folded core have control1ed the development of the depression. Faults of other directions are smaller, having several to several tens kilometers long in general. The three groups of fan it crisscrossed in distribution and thus breaking up the depression. They played an important role in the formation and distribution of the salt dome.
TYPES, FORMS AND DISTRIBUTIONS OF THE SALT DIAPIRS
The salt diapir is a specific tectonic form of salt body. These salts are softened, de- formed and rised under the outer pressure or puncturing overlying strata or hiding subterranean. The salt diapirs are extensively developed in KucheBasin. According to their existing forms, they may be divided into two types: exposed and hidden. The former is a salt body which punctured overlying strata or the overlying strata are fully denuded through weathering, causing the salt body to be exposed to the surface; The later is not exposed because the puncturing depth or the rising of the salt degree is not enough. Although existing states of the two types of salt diapir are different, they are all in various shapes. The shapes include dome, mushroom and irregular ones etc. We will introduce some typical diapirs among them.
1. Exposcd salt diapirs
They mainly distributed in western part of the basin. The more typical salt diapers (see Fig.l) are, from west to east, Aqikesu(1), Baozdung(2), Yanshanko(3), Qioerhe(5) and Tuzmaja (7) etc.
Aqikesu salt diapir is, from west to east, about 4km Wide and about 6km long with an elevation of 2083m and relative height 360m. It looked most magnificent. The diapir is like a mushmoon (Fig.2) and has irregular elliptical cross section. Salt-dissolved geomorphy of the diapir is very developed and salt hopper and salt stalactitie etc. are everywhere, resulting in a rugged surface.
The diapir body with overlying mudstone is composed of halite rock of Awat group(E3a) in Oligocene Series. The diapir belongs to center puncture form structure with the Neogene System strata around the salt body. The body has a complex structure and crisscross network of fault(Fig.3). The Aqikesn diapir occured following the late Ximalaya Movement. Owing to the high pressure of overlying strata and press of tectonic stress, the salt body had aplastic deformation and rising, puncturing or outflowing on overlying strata. The rising action of the salt body caused upturning of rock formations in outer contact zone.
Baozdong diapir is located in north-east of Aqikesu and at north limb of AwatRiver syncline. It is an anticlinorium formed by compressing stress in north and south. The salt body is a triangle (Fig.4) in surface and is composed of rock salt in Eoccne Series. The salt body rose and punctured the anticlinal axis. The very thick rock salt still keeps the mark of anticline. Most rock salt keep stratified textural feature. The thickest layer of the halite is of 41m and the accumulative thickness is over 500m. Yanshanko diapir is situated in south-east of Aqikesu with 4.7 km long from west to east and about 0.5 km wide. It is an irregular triangle with an area about 2.5km2 (Fig.5). The salt dome was cut in two by AwatRiver which flows between them. The diapir is a part of Awat anticline. There are a secondary syncline and a small anticline on the body of diapir. The halite rose, punctured and outflowed under the tectonic stress, which caused it'covering the Ncogene System strata and in some parts covering the Quatcrnary System.
Qiaoerhe diapir is located in north of Qiulitake big fault. There are three discontinuous salt bodies in the surface (Fig.6) because of Quaternary System covers. The biggest one among them has an area of 13 km2, which comes second in the salt diapirs in KucheBasin. The terrain sloped gently on diapir surface, salt karst was developed and salt hopper densely covered, which makes a specific salt-dissolved landscape. The diapir is composed of rock salt with small muddy beds. The accumulative thickness of rock salt is about 572m which is the thickest salt bed in all holes of the basin and it's halite content approximately 90%.
Tuzimaja diapir, as mushroom on surface, is located in north-west of: Baicheng and west part of Kusongtokai anticline. Developed North-North-West faults crossed areal fault line and the salt body rose along the tectonic cross and punctured overlying strata, thus forming a salt dome.
Tuzimaja halite rock belongs to Oligocene Series. It's also a halite rock with argillaceous gravel. The total thickness is 368m. The salt dome contacts wall rocks in south, west and east in a fault contact way, the Quaternary System covered in north part. The salt dome rose higher in the south, two or three faults formed a step fault northward. Salt beds appear crisscross, which was later cut by North-North-West fault and formed irregular tectonic pattern
2.Hidden type of the diapirs.
Hidden diapirs were formed on similar conditions to the exposed diapirs, but tectonic stress, rising and puncturing of the salt body were less strong and weathering depth was less. The covering strata were not denuded completely. When the hidden diapir was huried shallower, the covering strata were rising and sinking. When the strata were crisscross or having gypsum cap and salt-dissolved breccia (i.e. the breccia remained after salt dissolved) it is a mark of hidden diapir on surface. Dawanqi diapir situated in west of Baicheng does have above mentioned features (Fig.7). After denuding this diapir cropped low and gentle. The strata crisscross and gypsum cap and salt-dissolved breccia were developed. The salt body was surrounded by strata of late Pleistocene Series to form a gentle anticline. There are some more hidden diapirs such as Yanshuago, Xiaokuzby and Donbaza etc. Their geological situations are similar to above-mentioned ones
3.Distribution of diapir and its relationship with structure
The Fig.l showing that diapirs present regular zonal distribution in KucheBasin and this distribution has close relation with structure. through field investigation we have the following regularities:
A. Most of the diapirs are located on anticline of both fold belts-Kumuglim and kasongtoka, on the anticlnal axis, inclined end and pivot rising and falling end.
B. on crush zone where local fault in east-west and west-north crossed areal deep fault in west -east, and at the point which fault intersects crush zone.
C. on Fault crush zone in west-east on anticlinal axis direction; shallower salt body was rising and puncturing under the side press and diapir was formed;
D. On tectonic parts of strong pressure in north-south and wrenching force. After salt-forming period the rising structure was a favourable section for forming salt diapir.
E. On fault revived zone in late period of late Pleistocene Epoch.
APPROACH TO THE GENESIS OF THE SALT
DIAPIRS IN KUCHE BASIN
The history of study on salt diapir is more than 100 years but to investigate it scientifically is not long. Some propositions presented in early work are often in the nature of hypothesis.
The evaporite is easy deformed, which is relative to their nature. It is known that salts are very easy softened and strengthened their mobility.According to some materiaJs. when geothermic gradient exceeds 2OOoC most salt rock would be softened and their plasticity and mobility strengthened. In homogeneous overlying strata provided a, possibility for semifluid-like evaporite which formed under high temperature and press to rise and puncture and form diapirs on thiner places of the overlying strata. In addition ,under tectonic stress a vertical crack on the caprock also provided opportunity for this diapir formation. In comparision with surrounding rocks; the evaporite has a smaller specific gravity, therefore it has natural function of rising.It should be said that the above mentioned factor is an internal cause for the formation of salt diapir.
Heavy pressure is also an important factor for the evaporite to be softened. According W.Dreyer (1955) experimental results, when pressure reaches 100 kg/cm2, the halite rock would produce a 0.4% permanent deformation, while the pressure exceeds 200 kg/cm2, the overall stress strain curve turn nearly horizontally, which showed that the halite rock is easy deformated.
As stated above, sedimentary depth of Cenozoic Era reached thousands meters in KucheBasin. From some material, under such depth the salt bed would exceed l00oC and 600kg/cm2 in pressure. Under these conditions the halite had been in softened condition and thus provided prerequisite for the formation of diapirs. According to our studies, the main stress for salt sediment deformation is from areal tectonic activity, especially side pressure from fault activity. During Ximalaya Movement the evaporite was in a strong stress field, a massive thrust fault occurred continuously and had polyphase. This made the upwarding and puncturing of the flowage of evaporite and fault activity take place at the same time. Today the salt diapir and salt body distributing alone fault intersection and tectonic line of fault, and diapir having elliptic and irregular shapes indicate that the tectonic stress played leading role in the formation of diapirs in the basin.
F. Lotze (1957) and H.Borchert (1964) and others proposed that the tectonic stress is one of main factors of salt body deformation. H. Borchert (1964) indicated that relative importance of static negative pressure and side pressure were very changeable in different diapirs. When the tectonic stress is a main power the diapirs would arranged in a string along anticlinal axis and its surrounding rock's change is unsymmetrical. He also indicated that transection of the diapir was usually as an egg or elliptic shape. The results of our study are in agreement with that of H.Borchert.
1.Cenozoic Era strata is fully developed. Eogene Period is a main salt-forming penod. Roke salt which is hundreds to more than thousand meters in thieckness is the main component of evaporite.
2.Exposed and hidden diapirs exist together in the basin and they have oval, circular, irregular and dome, mushroom etc. shapes. The total diapirs is about twenty. They distributed along tectonie line. The biggest one among them is Aqikesu diapir, which is more than 20km2 in area and 360m in height.
3.The study showed that side pressure produced by areal tectonic activity, especially fault activity is a main diapir-forming stress. The other source is a pressure difference of overlying strata.
Situated in piedmont depression of Tianshan fold belt Xinjang, there are about thousands meters thick Tertiary sediment which contains huge thick salt deposits in KucheBasin. The salt deposits embedded in a series of thin-clastic rock and mudstone. Under the differential loading of overlying strata and unhomogeneous tectonic activity, there are many faults scattered in the basin and hence salt body was transformed into different forms such as salt diapir etc.
Salt diapir is a wonderful landscape in Kuche Basin. Investigation showed that there are about twenty salt diapirs ,either exposed or hidden in the basin. Aqikesu diapir, for example, approximately 6km in length and 4km in width is a typical exposed great salt diapir, From years study, we learned that the forming of these salt diapirs was influenced by load pres- sure and tectonic activity. The presenting paper will expound the salt diapir distributive regularity and its genesis in detail.