Jan Šilar

Abstract:The present paper deals with the origin of the tower or cone karst [Turm-karst．Kegelkarst] in southern China and North Vietnam．Proof is given of its origin in early Terrtiary times under tropical climatic conditions．Later the cIimate changed and some areas were uplifted so that at present the tower karst Occurs not only in the tropics but also outside that climatic zone．The early Tertiary tower karst of southeastern Asia is analogous to that of other areas in Asia and Europe where tower karst is preserved below later Tertiary deposits or where it has been modified by the Pleistocene periglacial climate．

Introduction

Extensive karst regions in southern China and the adjacent areas of North Vietnam differ by their morphology and development from karst areas in temperate climates．Besides the sinkholes solution valleys, cliffs，subterranean streams，and caves known from other karst areas，tower-1ike or cone-1ike hills occur in this part of southeastern Asia (fig.1)

Near the south Chineseand Vietnameseareas of tower karst are similar occurrences in Cambodia，Thailand，Laos．Burma．and In-donesia (Weber．1 95 8)．The tower karst of these regions has been studied by several authors， especially by Danes(1915)in the area of Goenoeng Sewoe in Indonesia．Blondel (1929)in Indochina，Lehmann (1936) in Java，von Wissmann(1 954 ) in southeastern Asia，and Šilar (1962)in southern China and North Vietnam．The geomorphology of the south Chinese karst has been studied by Cressey (195 3)，Chu (195，)and Lebedev (1959)．The caves in the surroundings of Kweilin．China，have been described by Chen (1958)and Schworm(1958)．Balázs(1961) has investigated a whole series of significant caves in southern China from the speleologic point of view and compiled a list of them．

Tower kamt is further known in the West Indies(Lehmann．1 954b)，and in the Congo (Barbier，l 960)．Remains of fossil pre-upper Miocene tower karst have also been described from Europe (Büdel，195 1；Tyczynska，1958；Czudek and Demek, 1960；Tyráček，1962)．It has been considerably modified, however，by the Pleistocene climate．During his ex- pedition to the Pamirs，U．S．S．R．，J．Sckyra (pers．communication)found young tower karst at the northern foot of the Alai Mountain Range in the Kirgizian Soviet Socialist Republic．

Acknowledgements

The author worked in China from 1957 to 1960

I am indebted with thanks to many members of the Water-power Design Institute at Kunming China，who enabled me to visit and study the karst regions of southetn China and thus made this paper possible．

Karst Cones And Towers

Generally tower karst is considered to be a form conditioned by a tropical climate (Lehmann，1 954a)．During heavy rainfall a strong surface and subterranean dissolving occurred with consequent dissection of the surface into small hills and sinkholes．The deep vertical sohttion and the deepening of the karst depressions ceased as soon as the erosion base level had been reached，and a more intense lateral corrosion began，shaping the basaI parts of the hills．Their slopes be-came steeper．and at their feet caves were form-ed．The hills were transformed into cones and towers of manifold shapes，rising separately or in groups from a common base level.

According to Lebedev (1959)，the karst towers in southern China are large forms of verfical fissures(grikes)developed along joints．

In different areas of southern China and North Vietnam diferent forms of tower karst occur．which may partly be considered as dif-ferent development stages．In some places the cones rise from a flat common base．In other places we may find depressions as welI as the karst-hiIl(fig．2)；these forms may becompared with the cockpit karst in Jamaica(Daneš．1914; Lehmann．1954b)．Transitions between both types exist．

The kegelkarst has developed on large areas in the southern Chinese provinces of Kwangsi．Kweichow．in some parts of Kwangttang，and in eastern Yunnan(fjg．3)；in Vietnam it has developed predominantly along the shoreline of the Gulf of Tongking．In the north and along the lower course of the Yangtze the karst passes．according to von Wissmann(1954)，into karstified escarpments．In these areas the individual karst towers have different shapes and sizes.

In eastern Yunnan and in Kweichow，the cone-1ike or haystack-like karst hills(kegel-karst) are the most frequent (fig.2). They reach a relative height of about 300 feet and sides sloping about 40-60°. In places even lower and smaller hills with gentler slopes may be found. In the area at the boundaty of Yunnan, Szechuan, and Kweichow, isolated low hills rise from the plain to a height of about 150 feet. They appear to be an old developmental stage of tower karst.

From central Kweichow east and southeast toward Kwangsi Province, higher karst hills become more frequent (fig.4),passing into the form of towers (turmkarst) with a height of 300-600 feet and more, aand with steply inclined slopes(80-90°).

The same types of karst cones and towers occur in North Vietnam。The tops of the highest rowers attain a common level，indicatingan ancient erosion surface．Lowertowers occur‘among the higher ones．The bases of the karst towers also frequently lie at a common level．suggesting a stable period of geomorphic development.

In the differently shaped karst hills，principally at their bases，caves of various shapes and sizes occur(fig．5)．Grikes may be found on the crests of karst hills(fig.6),and at their feet debris sometimes accumulates.The sutrace of the towers is frequently covered by brushy vegetation and，in the more southern areas at lower altitudes．also by abundant tropical vegetation．In densely populated areas the surface is usually deprived of vegetation or covered with sparse grass.

In Yunnan and Kweichow the karst hills occur above the upper edgeof thevalley slopes；they do not occur within the valleys below this margin(fig．7)．This phenomenon was found in places where the base Ievel of the hills had been uplifted by tectonic movement，and thus the development was rejuvenated，and deep erosion was renewed．At the margin of the Yunnan-Kweichow Plateau，a deep stream valley has been cut into the base of the uplitted tower karst．

In places．where the karstified limestone is underlain by a Layer of insoluble rock·the karst hills sometimes rise from this non-karstified base．In such cases the limestone hills indicate the latest stage ot-the karst development preceding its destruction and represent the remnants of the disintegrated．0riginally continuous layer of karstified lime—stone．which by subsequent development was dissected into isolated towers．In these places the base of the towers or hills is not always continuous and horizontal．Its shape is rather determined by the structure of the non-karsti-fied underlying rock．

In tower karst areas of southern China and North Vietnam the karst cones with the steepest slopes and the towers occur only in places where a cornmort base is developedffom which they rise separately．AISO，in groups of a large number of towers the steeper slopes occur only at the bases where they rise from a common Ievel．Where there is no such base levd(on slopes，in highlands,or at the junction between several coalescing concs)'the slopes of the cones are gentler．It is obvioos that the steep cones and towersⅡe in places where vertical corrasion and corrosion has ceased and where，after the stabllization of the ero-sion and karst base level，lateral cutting and dissolving grew more intense．

Sometimes，the base of karst towers was subjected to further karstification, especially if further uplift caused rejuvenafion．It enabled the retrogressive erosion of streamsand hence，after the subsidence of the base level, a further phase of karstiflcation．This karstiflcation belongs to another phase of devdopment than the karst towers．Frequendy，however，no signs of further karstification are visible at the base of the tower karst and blow it．The towers rest on an erosion surface on which superfl-cial water-courses flow．Only rarely superficial karst phenomena such as grikes occures．This Was noticed either in regions which after the formation of the towel karst were not further uplifted(for exampie On the south- eastern shorc line of Kwangsi and in areas of North Viemam)or in areas，which though teaonically uplifted have not yet sustained retrogressive erosion by rivers，for instance，in central Kweichow．Their karst devdopment was not rejuvenated．

In some places On the surface that the towers rest on，the rock substratum crops out；in others．younger surficial formations occur．These deposits，its far as thdr age may bedetermined．Date the minimum age the towe~karst．

Development Of Tower Karst

Tower karst occurs in areas situated at low latitudes with prevailing tropical or subtropi·cal climates(Indonesia，Malaya，and West Indies).In China it reaches farther to the north,as far aS 30°N lat．(von Wissmann，1954).All these areas are distinguished by heavy rainfall(fig．8)．The dependency of the occurrence of tower karst on the latitude is striking.The climate is a decisive agent in its development．This dependency,however, cannot be deduced only from the present-day climatic conditions．as in most afeas its origin dates from earlier times.

Ma(1940)，studying the climatic changes Which have occurred during past geologic periods in eastern Asia by the corals on marine terraces in Japan，found that in the Pleistocene the equator was situated 5°more northerly than at present．In the Tertiary．the latitude Was different．too．The lateritic deposits in southern China．which de Chardin(1936—1937) assigns to the Pliocene，indicate a tro-pical climate during that period．If in the Tertiary the area of tropical climate extended farther to the north it is not surprising that the tower karst in southern Chlna occurs at higher latitudes.According to Kǒppen and Wegener(1942)，in Eocene and Cretaceous times equator ran from southeastern Asia in a northwest direction over the Himalayan arch to western Asia and southern Europe.This corresponds with theage and distribution of thc towner karst in southeastern Asia and with its buried remains in central Asia and Europe.

Besides the climate．geologic conditions also affected the dev possess a great thickness of carbonate rock and aconsiderableterritorial distribution.This can be explained by the lithologic and palaeo—geographic development of the whole region.which from the Proterozoic up to the Perrnian and in some places up to tlle Triassic was a marine area,nearly without interruption.with prevailing sedimentation of carbonate rock．

Irrespective of some smaller area formed solely of limestone and dolomites,in several areas the laycrs of carbonate rock occupy more than 50-60 percent of the thickness of the entire known stratigraphic succession.Thus,for example,in eastern Yunnan,lime-stone forms more than 9，000feet一63percent of thc entire known thickness of all strata which equals about 15,600 feet from the Proterozoic up to the Quaternary.In some parts of Kwangsi，limestone occupies up to 73 percent of the whole starigraphic succession．

As a consequence of the Yenshan folding and the Himalayan fault tectonics whose intensity decreased from west to east,the areas occupied by limestone increase in the same direction．A further cause is the lithologic character_in the eastern part of the area,with carbonate rocks developed to a greater thick-ness than in the west.

The existence of large continuous karst afeas covering several hundred square miles in the Kwangsi and Kweichow provinces and spreading to the west as far as eastcrn Yunnan can also be explained by the tectonics．In central and western Yunnan the layers of lime·stone are broken and deformed into smaller blocks and foIds.

Thc karst development was substantially influenced by tectonic uplift.The Yenshan folding was succeeded by a relatively quiescent period．During the Tertiary，uplift accompanied by fault tectonics,occurred in several phases，culminating in the Himalayan uplift.The height of the uplift increases from east to west(fig.9).The present surface rises from Kwangsi to dopment of the tower karst．

The Chinese and Vietnamese karst areas Kweichow and Yunnan to the Yunnan·Kwei—chow plateau and farther into Tibet.The uplift surface was dissected by young deep rivers，cutting headward from the marginal parts into the Yunnan Kwelchow plateau．The development of valleys．the formation of talus, fans,and terraces，the tectonics，and the active seismicity indicate that the uplift and tectonic movement is still going on.Karst areas in different developmental stages occur both in the uplifted areas(yunnan,kweichow,western,Kwangsi),and in the not uplifted(eastern,Kwangsi)or even subsided areas(part of the shore of North Vietnam).

In connection with the repeated phases of uplift．several phases of karst formation are visible,the younger of them having been conditioned by rejuvenation.Several cavelevel mav be found，corresponding with the levels of river terraces.The tower karst,however,occurs only on the earliest and uppermost levels；in the uplifted areas with deep valleys it may be found onIy on the level above the upper margin of the valley(for example in Kweichow and Yunnan)．

The tops of the higher karst towers form a crude level and indicate the level ot the ancient disturbed peneplain,or of the ancient floor of the valley.South of Kweilin and in the surroundings of Liuchow,this level passes in places into the continuous surface of an old terrace.The tower karst is therefore younger than this terrace level,eventually in othcr areas younger than the old peneplain(fig．10).De Chardin and others(1935) compare this level with the stage of the penepIain of Peitai in northern China.Between the karst hills ,continctllal sedliment was 1aid down in places，which De Chardin considers to be “rathcr of Eocene and Oligocene than of Cretaceous age”.According to this statement,the tower karst would be at least of lower oligocene age.

The age of the tower karst may be established in the tectonicaIly uplifted areas of the Yunnan-Kweichow Plateau.In central Kweichow,early Tertiary sediment was deposited between the towers._In central and southern Yunnan， the tower karst occurs on an elevated peneplain beyond the margins of the Tertiary basins, which originated in the Miocene as grabrns. It is pre-tectonic，oligocene or earlier in age.The limestone underlying the Pliocene sediment is karsitified.In the environs of lunan, the karstified limestone rises to the surface forming the base of the red continental sediment,which Bien(1940) presumes to be Eocene in age.On the Yunnan-Kweichow plateau ,stream valleys were cut deeply below the base of the karst cones．The retrogressive erosion was caused by the uplift of the Yunnan-Kweichow Plateau,which began as early as thc Miocene Epoch.The tower karst is older than the dowucutting of the valleys.

It may therefore be concluded that the tower karst originated in the early Tertiary,or at the bounary between the Cretaceous and tertiary，during the tectonic rest following the Yenshan folding and before the beginning of the uplift that in southern China was a response to the Himalayan movement which began in the Miocene.

Origin of tiie tower karst

Cressey(1955)explains the evolution of the tower karst as follows：The three provinces of Yunnan,Kweichow and Kwangst form an evo1utionary solution sequence from west to east．Central Yunnan is a hilly plateau，fully a mile above sea level，with a few canyons and deep sinkholes.Here is the initial stage of karst development.In the adjoinhlg province of Kweichow,most of the area is intricately dissected and in steep slopes-to 45°and even 60°.The plateau surface has nearly disappeared because of the enlargement of the sinks and further solution of the limestone.Original surfaces are present only as residual summits．．．Farther east，in Kwangsi,the cycle of crosion has proceeded still further and approaches old stage．Sizable areas of lowland level plain have been developed and only residual spires remain to mark the areas between the once scattered sinkholes.Some of these towering remnants are striking.

“If the solution process is 10 percent complete in Yunnan,and 50percent complete in Kweichow,10 percent of original limestone has been removed in Kwangsi.thus in yunnan ,level land is formed only at the original; in Kwangsi it is present at the new lower surface; while in the inter mediate Kweichow there is little flat land at any level.”

Cressey(1955)thus explains the origin of the tower karst by the gradual dissection of the Yunnan-Kweichow PLateau toward the east．The level of the crests in the east corresponds to the plateau．In the west.A similar explanation was given by Blondel(1929)．

Actually, however, in Yunan similar forms of tower karst were established as in Kwangsi onlv with the difference that in Yunnan young beep stream valleys are cut into the ancient base level of the towers.I Kwangsi these deep valleys are missing and the erosion mainifests itself only by shallow cutting of the plain.The yunnan canyons do not correspond.

The national speleological societyin age to the period of erosion and．karstification during which the tower karst in Kwangsi originated．according to Cressey’s explanation,but are much younger.

In central and eastern Yunnan and in the eastern part of Kwangsi and Kweichow，the mature tower karst was uplifted between the Miocene and the Quaternary.This uplift was a response to the folding of the Himalayan geosyncline.In consequence of the uplift, river erosion began again,the erosion sueface sank below the level of the existing towers，and dissolving was renewed，In some places near the shore of North Vietnam the tower karst sank below sea level(fig．11)．Between the archipelago of Vung Ha Long Bay(also．known under the French name Bale d’Halong)

The base of the tower karst has subsided to a depth of as much as 75 feet below the present sea level,above which only the tops ot the towers emerge(fig.1.2) The original subsidence was probably greater and the area was uplifted later．This is shown by shore terraces at an ahitude of 45—60feet,whose 1evels correspond to those of caves on several is lands. Simliar movements have also been known from some other places on the southern Chinese shoreline．Lin(1937)described them from the environs of Foochow，Canton．and Hongkong.Panzer(1935)reports an elevation of the shofeline of the Gulf of Tongking to a point halfway between Hong Kong and．Swatow.The final development of the tower karst was therefore diverse in different parts of south-eastern Asia．