MORPHOLOGICAL AND GEOLOGICAL INDICATORS OF THE POSSIBLE BAUXITE DEPOSITS IN THE KARST REGION OF WESTERN HERZEGOVINA

Abstract

Investigation results of morphological and geological potential bauxite deposit indicators in the Mesihovina-Rakitno bauxitebearing sedimentary basin in Western Herzegovina are presented. Region with carbonate and clastic hangingwalls as well as those without overlying sediments have been studied. It was established that the expression and number of the indicators depend size as well as on character and thickness of hangingwall sediments. The morphological indicators are expressed as a particular relief forms situated right above the deposits or nearby and are a consequence of geological relations and exodynamic processes. The numerous geological indicators resulted from complex geological events. The most important are: preore structural relations, the formation of paleorelief, peculiar way of hangingwall rocks sedimentation, lithification processes and the formation of the recent structural pattern. It has been observed that particular indicators sould be recognized within a relatively thick succession of the overlying sediments which is of the great importance in the exploration of bauxite deposits. [PUBLICATION ABSTRACT]

Headnote

Key-words: Bauxite deposits, Morphological indicators, Geological indicators, Western Herzegovina

Investigation results of morphological and geological potential bauxite deposit indicators in the Mesihovina-Rakitno bauxitebearing sedimentary basin in Western Herzegovina are presented. Region with carbonate and clastic hangingwalls as well as those without overlying sediments have been studied. It was established that the expression and number of the indicators depend size as well as on character and thickness of hangingwall sediments. The morphological indicators are expressed as a particular relief forms situated right above the deposits or nearby and are a consequence of geological relations and exodynamic processes. The numerous geological indicators resulted from complex geological events. The most important are: preore structural relations, the formation of paleorelief, peculiar way of hangingwall rocks sedimentation, lithification processes and the formation of the recent structural pattern. It has been observed that particular indicators sould be recognized within a relatively thick succession of the overlying sediments which is of the great importance in the exploration of bauxite deposits.

Kljucne rueti: LeZiSte boksita, MorfoloSki pokazatelji, Zapadna Hercegovina

U radu su prikazani rezultati istraiivanja morfoloSkih i geologkih pokazatelja mogutih IeZiSta boksita u boksitonosnom sedimentacijskorn bazenu Mesihovina-Rakitno. Proutavana su podruEja s karbonatom i klasticnom krovinom, te predjeli bez krovinskih naslaga. Ustanovljeno je da izraz i brojnost pokazatelja wisi o velitini IeZiSta, te debljini i karakteru krovinskih sedirnenta. MorfoloSki pokazatelji oEituju se kao specifiEni oblici iznad IeZiSta boksita ili u njihovoj blizini. UoEeno je da su oni raultat geoloSkih odnosa i egzodinamskih procesa. Brojni geoloiki pokazatelji su rezultat kompleksnih geoloSkih zbivanja. NajvaZniji su: predrudni strukturni odnosi, formiranje paleoreljefa, specifibn naEin sedimentacije krovinskih naslga, procesi litifikacije i forrniranje recentnog strukturnog sklopa. Uokno je da je pojedine od pokazatelja moguCe prepoznati u relativno debelom slijedu krovine Sto im daje i veliko praktiEno znaknje.

Introduction

By intensive researche on revealing geological conditions of bauxite formation in the karsi region of Western Hercegovina, the existence of certain surface indicators as signs for a possible occurence of bauxite deposits was discovered. These indicators are of geological and geomorphologieal kind and will be presented here in a sequence of logical continuation of their formation processes. The ore-forming process, i.e. the origin of source material and its transformation into bauxite will be omitted,

The prore and postorc tectonic processes as well as implied paleogeomorphological conditions will be considered. The depositional of hangingwall rocks and subsequent diagenetic and exodynamic processes are studied and interpreted.

Detailed investigations have been done on uncovered bauxite deposits and Ihcn on those established by exploration drilling using a geological map 1:5000 as we! as aerial photos and field drawings. A very carefull geomorphologieal and geological reconnaissance of particularly interesting localities have been done, too.

The explorations have been concentrated on the Paleogcne bauxite bearing basin Mesihovina-Rakitno, in between Tomislavgrad and Posusje in the area of Studena Vrila and Oluja on the west and Sutina and Konjovac on the east (Fig. ]).

Geological setting of the explored area

The explored area consists of carbonate, mostly calcareous Upper Cretaceous rocks and Paleogene limestones and elastics. The Upper Cretaceous limestones are ihe bauxite footwall. By the Laramian orogenie events at the end of Cretaceous period these rocks were folded and a continental regime was eslablished. Simultaneously, within folded anticlinal structures open joint systems formed (B 1 a s* k o v i c &.Dragicevic, 1991). In this phase of consequent relief the karslification and moulding of open joints and fissures were very intensive. In these joints and fissures the source hauxite materia! (parent rock) accumulated.

By the Upper Paleocene-Lower Eocene transgression all large land masses were gradually flooded. As a consequence, the Upper Cretaceous paleorelief is discordantly overlied by Paleogene limestones and elastics. The latter are bauxite hangingwall rocks.

The bauxite footwall i.e. the Upper Cretaceous rocks, are represented mostly by shallow marine limestones of Turonian-Senonian chro nostra ligrapluc range. Sporadicnly, these limestones contain very abundant rudist associations. Chiefly, the beds consist of rudist debris and various fossil fragments (coqujnas. microcoquinas, coquinites). The limestone bedding planes are not well-marked (R a d i c et al,, 1978).

The morphological expression of these rocks is heterogeneous. In parts with insignificant tectonic relief it is characterized by gentle, uniform elevated forms, siopes and valleys. The ground is mostly flat wiihout cropping out blocks or limestone beds. It is covered with irregular slab-like limestone fragments which mask true strike and dip of beds. The valleys are characterized by linear arrangement (strike) which points to icctonic predisposition. In cross sections they resemble to a wide but noticeahle "V" letter. The large number of valleys begins with a circular, flattened form having a shape similar to dolinas ground surface contour.

In parts with intensive tectonics, faults, reversed faults and overturned fold hinges the relief forms are more heterogeneous and indented. Chiefly, they are marked bu the cropping out of huge limestone blocks with abrupt slope changes, with well developed karrs, numerous joint systems and pronounced petierns of karstification.

As the most expressive karst form, dolinas are very abundant in this area giving the pattern of chaotic arrangement. Mosty, ihey are not completely opened but filled with bauxile or terra rossa, as well with a limestone fragments. They are flattened and only by a circular surface contour of some lens of meters diameter remind to well known typical karst forms.

The morphology and recess arrangement in bauxite foot wall pa I eo re lief

The recesses in the footwall paleorelief are genetically connected with opening of joints in mostly smooth anticline structures. These joint systems originate in the Laramian phase at the end of Cretaceous. They could be classified into 3 different types(Blaskovic&Dragicevic, 1991):

a)Recesses predominantly formed by the opening of tension and shear joints the II. order (congruenily with fold axis). They are sealed within the narrow elongated zone parallel to anticline axis which is over 200 m long.

b)Recesses formed by ihe oppenig of system of joints of the I. and the II. order. These are seated within the wider zone of anticlinal axis mostly. Their ground-plan contour is polygonal of roughly equal width and dimensions of 20 to 40 meters.

c)Recesses formed by predominantly shear joints of the I. order (normal and diagonal lo the anticline axis). These are seated within the anticline limhs, particularly within the part which shows a trend of an eshalon horizontal bending. They have elongated shape with length over 100 m and width up to 40 m.

The primary shape of those three types is characterized by very steep almost perpendicular wall-sides, by narrowing in lower part and roughly flattened or funneled bottom. By exodynamic process the primary straight forms are partly smoothed. It was particulary effective on the edges so the upper parts of recesses become funnel shaped.

The bauxite material accumulation

In the recesses formed during the emersion phase which is of undefined exact time duration, the prime bauxite material was accumulated. Without considering the origm of this material, its transport and deposition we can stress by a few facts observed by the fieldwork The bauxite material fills the recesses approximately up to the half of their volume or less. The funnel shaped upper parts remain unfilled.

Mostly, these forms are observed within deposits with noneroded preserved hangingwall rocks. In regions with eroded hangingwall rocks the upper funnel shaped part could be destroyed up to the level filled with bauxite or frequently even the whole potential deposits could be completely eroded (Fig. 2).

The deposits hangingwall

As now is generally accepted, by gradual Upper Paleocene and Lower Eocene transgression the deposition of the hangingwall strata started. There were different depositional regimes with emphasized dynamics of different land masses surrounding the basin. As the result one can distinguish two different forms of hangingwall Paleogene strata. One is Promina clastic formation and the other carbonate foraminifera limestones formation (D r a g i C e v i 6 et al., 1992).

PROMINA FORMATION

In the area of Studena Vrila this formation consists of two basic lithological members which are vertically and laterally interbedded. One is represented by finegrained clastics, sandy and silty marls and other bu conglomerates of mostly limestone pebbles in sandy matrix (Fig. 3).

The deposition of Promina formation in every investigated locality starts with finegrained clastics. In the basis of the sequence they are up to 150 m thick, and shallower parts, which correspond to anticlinal parts of paleorelief structures mostly, as the result of intrabasin transport and dynamics, the finegrained clastics become more and more thinner or disappear; the footwall rocks are directly overlain by limestone conglomerates. However, in open fissures with bawitic material, almost as a rule, bauxite is overlain by finegrained basal member of Promina formation of variable thickness depending on recesse's depth and area.

Sedimentation was characterized by periodical alteration from (1) quiet supply of finegrained clastic material and its relatively uniform distribution within the basin, and (2) short-lived, erratic abrupt flows of coarsegrained clastics with progradational sedimentary pattern (D r a g i C e v i C et al., 1986). The importance of such rapid flows lies in submarine erosion of previously accumulated finegrained clastics on updomed basin parts and their redistribution within the basin.With the deposition from such a rapid, relatively short flow, lenses of different shape and thickness are formed. Wthin the basin with uniform bottom morphology these lenses are wide spread, relatively normally interbedded, having uniform thickness and forms with smooth unsharped front and with gradualy thinner tail (Fig. 4).

In parts of the basin with indented, recesse marked bottom relief, although of small size and filled with bauxite, the coarse grained flow gravitated toward them and they sewed as sediment distribution channels. Lenses of variable shape conformably to the size and shape of recesses were formed. Particularly thick, unsharp fronted and rapid thinned short tailed small bodies of coarse grained clastics having erosional channel patterns are incorporated within a finegrained sediment (Fig. 5).

In numerous bauxite deposits the synclinal bending of beds - lenses of hanging-wall strata, i.e. collapse structures were observed. These structures occur as a consequence of recesse shape and subsequent tectonic changes as well as by compaction and lithification of bauxite material and overlying sediments which reduce their volume (Fig. 6).

The recent morphology of litho members is variable. Parts of the territoly consisting of finegrained clastics exhibit a relief moderate forms covered by vegetation. The limestone conglomerate lenses are well expressed in relief as steplike forms of clear strike and width, mostlywithout vegetation cover.

The outcrop of conglomerate lenses in cases of their regular interchange regardless of terrain morphology is easily recognisable (Fig. 7).

In parts where lenses are deposited in indented basin bottom conditions their outcrops shows chaotic arrangement of strike, dip and thickness (Fig. 8).

In case of such a chaotic arrangement, it is possible, by photogeologic reconnaissance, to make an uncorrect interpretation of structural relations by considering this completely sedimentary structure as indicators of folding and faulting.

Within the rock of the Promina formation, where conglomerate lenses predominate, the definition of beds strike and dip demand certain correction. The specific shape of conglomerate lenses which obey the basin bottom configuration and with marked cross lamination textures indicates that neither upper nor lower bed plane represents the real bed orientation. This orientation could be more correctly determined in finegrained clastics interstratified members of greater thickness (of two of more meters).

THE FORAMINIFERA LIMESTONE FORMATION

Within a part of the Upper Cretaceous paleorelief during the Paleogene transgression and phase of inundation the continental influence was neglected; the deposition of shallow marine to reef limestones predominate. The foraminifera limestones build up the biggest part of ihe investigated area; reef and perireef limestones appear as their lateral equivalents in a smaller part.

The foraminifera limestones (Dragicevic" et ai., 1992; R a i c" et al.,1978) are chiefly biomicrites and gasiropodal or miliolidal or similar lo Kozina beds as well as alveolina or numulitic limestones. They are of ihe Upper Paleoeene-Middle Eocene chronostratigraphic range. The older, basal part of calcareous sequence with miliolids and gastropods as well as the younger part of mostly numulitic limestones, represented by micrites with relatively higher mud ratio, are well thinly bedded (10-20 cm) and with scarsely developed karsl relief forms could be compared to alveolina limestones.

The lithologieal composition and sedimentary patterns of carbonates as a direct bauxite hangingwall show specific characteristics (D r a g i-C'fi v i 6 et al., 1992; Fig. 9.). The bauxite hangingwall complex starts with bauxitics red to brown or greybiue clays. sporadicaiy silty clays with transition to marls with abundant mollusk remains. It is overlain by slromatolitic micrites and predominantly red-brown or grey gastropodal biomicritcs I to 1,5 m thick. The latter gradualy aheraie into slromatolitic limestones of ver>' variable thickness depending on recesses depth. By complete filling of paleorecesses this limestones alterate to foraminifera limestones. The fossil content of gastropods, chara and oslraeods speaks for fresh to brackish water depositional environment. Development of described sequence points lo hypothesis that the first sedimentation phase during the transgression could be connected with rising of ground water table in already karstified paleorelief and with the first drowning of recesses (with bauxite) (Fig. 10).

The morphological and geological indicators of possible bauxite deposits

The bauxite deposits are marked on the surface with morpholologieal and/or geological indicators. These arc more or less noticeable on the size ol deposit, thickeness and type of hangingwall rocks. However, their observing demands a detailed and directed analysis of relief forms and geological setting. In making conculsions the synthesis of all available data and even impressions is necessary, because every and even the slightest exception from general local gcomorphological and geological setting could represent a valuable indication.

Morphological indicators

The bauxite deposits without hangingwall complex

These occur within of the Turonian-Senonian chronostratigraphic range. Mostly they are more or less degraded by exodynamic factors. As a rule, the upper funnel-shaped part of recesses is destroyed and the lower, narrower part, filled with bauxite, preserved (Fig. 2). Bauxite is covered chiefly with weathered maierial of surrounding limestones, terra rossa and with bauxite detritus or ooliihs. On the ground surface these deposits are 1'unnel-like shaped, with semi-circular flattened opening of dolinas or they show an image of channel type, shon dolinas.

On platformed parts of the Upper Cretaceous karst relief without emphasized allidude differences an aboundance of dolinas is usually observed. They are mostly of circular or elipsodial forms with diameters of 10 up to 50 m or more. Many of them are of small depth, having characteristic funnel-like forms with mostly flattened bottom. However, on many places these types of dolinas are almost completely filled ("filled dolinas"); there is only a specific ground surface ring-shaped contour of limestone outcrops which points to their existence. Within this contours there is a flattened, and in comparison with the surrounding grass covered terrain of terra rossa, a little bit saged surface. This could be an indicator that beneath a thin cover a possibility of bauxite filled recess exists (Fig. 11).

As all dolinas do not imply the existence of bauxite deposits their value as a deposit indicator should be reexamined. In this case one shoud take into consideration certain structural and tectonic elements. Normaly, the dolinas arrangemcni, chaotic on the first impression, considered in the scope of structures shows certain regularity. As a consequence of prcorc tectonics this arrangement could be directed by predominate joint systems in folded structures or by fault zones (Fig. 12).

AJready discovered bauxite deposits are exclusively positoned in particular zones of dolinas. They display a trend of grouping on particular ground surface area and along particular directions. Which /.ones would be bauxite bearing depends on palcogeological conditions. This demands for a detailed investigation on indicated surface. In this circumstances, the dolinas arrangement analysis as indicators of potential bauxite deposits or elimination of areas occupied with sterile blocks deserves adequate attention.

In regions with Upper Creataceous strata on the surface, characterized by a relief of uniformly dipping slopes, the occurence of dolinas is scarce. Mostly these arc flattened and grass-rich covered relief parts with diameters or 15 to 40 meters; they represent only a part of so called "filled dolinas" with circular ground surface contour. These are, as a rule, primary forms of valleys (Fig. 13) und could occur repeatedly. Longidudinal section of valley bottom assume a moderate stcplike shape.

Described examples of "filled dolinas" and slight discontinuities in mostly uniform carbonate relief represent the bouxite deposit indicators, loo, as it was confirmed in the investigated area.

Morphological indicators of covered bauxite deposits

Numerous bauxite deposits are covered by foraminifcra limestones or Promina formation elastics. Deposits of chiefly small size are of a relatively slight morphological expression, particularly in cases of thicker hangingwall strata. However, even in these circumstances discrete discontinuities of a relief within particular geomorphological units could be observed. That also are indicators of bauxite deposits. All of (his refer to areas of simple tectonic setting.

Foraminifera limestones-bauxite deposits hangingwall rocks

The morphology of areas composed of foraminifera limestones, particulars of alveolina type, resembles to that of the Upper Cretaceous limestones. Here, the dominant karst forms arc dolinas, too. They could have a classic funnel-like shape with Hat bottom. Feebly expressed steplike parts in longitudinal sections of carbonate relief wilh grass-rich covered cascades and hardly noticablc deviation of dip could also indicate potential deposit existence in small or great depth (Fig. 14). In some morphologically well expressed pans of the soil there are "pits" of circular to chiefly polygonal surface contour with vertical walls and downthrown blocks. Diameters of such pits from 15 to 30 meters or more (Fig. 15). The mentioned manifestations, as confirmed by drilling, could also represent indicators of bauxite occurrences.

Promina formation elastics as bauxite hangingwall rocks

A specific origin of Promina formation sediments is manifesied on the area morpholgy. As already mentioned. sandy-marl sequence members predispose arc well exposed making a stcplike barren relief.

As a morphological indicators of potential bauxite deposits we could present three schematized examples:

* mostly flattened region on one side sharply bounded hy sleep high cascade-ridge of conglomerates of longer strike line. The bauxite deposit is located along this boundary in part of semi-circular concave ridge bend {Fig. 16);

* prominanl cascade-conglomerate lense ridge of limited extent but with uniform strike and altitude. Potential bauxite deposit beneath a cascade of this type could be morphological manifested by an abrupt "collapse" of the ridge: above such a deposit there is a rapid decreasing of commonly uniform ridge altitude with well expressed ridge destruction (Fig. 16);

* in flat to bedding congruent areas the appearance of concave-shaped relief forms could be interpreted as an indicator of bauxite deposit even in greater depth (Figs. 15,19,20).

The geological indicators

The bauxite deposits are manifested on a ground surface by numerous more or less noticeable geological phenomena as well.

Depending of size and depth of a deposit and a type of hangingwall rocks the sort and the number of geological indicators will result.

For the interpretation of geological indicators one should start with a detailed and correct geological map as a basis including ihc clear chronostratigraphic, lilostratigraphic and structural- tectonic relations as well. Here, for the exploration of potential bauxite deposits we should start with a hypothesis that every exception from a wellknown geological selling of surrounding area is a potentialy very important geological indicator of deposit existence. Geological data considered as indicators are various -lilhological, lithofacial or structural.

Bauxite deposits without hangingwall complex

Regions composed od Upper Cretaceous limestone with bauxite deposits offer relatively scarce geological data which could be considered as indicators. In the first place, amongst all these arc bauxite pebbles of fragments mixed within a thin soil cover of flattened relief, in valleys or dolinas Furthermore, only in close vicinity of a deposit it is possible to find tectonically chrushed and subsequently cemented lime breccia with manganese film. Moreover, tectonically disturbed zones with limestone mylonites could be considered as one of indirect indicators.

Bauxite bearing areas with foraminifera limestone as hanginwall rocks

Depending on the deposition depth and thickness of hangingwall rocks the ground surface geological indicators can be manifested in two ways. One of the undoubtedly reliable indicator of bauxite existence is a particular development of basal pan of hangingwall sediments limited exclusively to negative paleorelicf.

The second ground surface manifeststion of deposits is clearly structural. It is expressed by collapse structures. These arc observed in all coverless deposits with characteristic synclinal bending of the hangingwall strata. The bending is strictly limited to palcorecesses and is vertically marked by a very thick succession of the hangingwall complex. Morphological manifestation is more or less noticeable by a moderate synclinal orientation of bed and with moslly consequent morphology (Figs, 15 & 17).

Besides the orientation of bed planes, ihe joint system arrangement is also in function of collapse structures and represents a very important deposit indicator. The joints are chiefly vertical and of radial pattern located right above a deposit.

Basic surface geological image of collapse structure could be strongly expressed, as well as in morphological sense, by formation of pits and sink-holes. If this happened the synform orientation of bed planes is slightly expressed in contrast to joint systems and sinking of central limestone blocks. The blocks are bounded by joint systems (Fig. 15).

Promina formation as a fiangingwull complex

The following characteristics have been observed in the Promina formation complex;

* directed transport of coarse-grained elastics toward palcorecesses, a recesses distributive function and sediment accumulation (Fig. 5), and

* sin - and postpositional formation of collapse structures in palcorccess and within hangingwall rocks (Fig. 6).

The basic surface geological pattern of specific Promina formation relief is diversified but recognizable by field observation and partieulary by photogeological reconnaissance as well.

One of the first general indieaiion of the Upper Cretaceous limestone paleorelief lies in abundance of well-marked conglomerate lenses of chaotic arrangement and strike. This is strongly emphasized above the very deposits together with a rapid increase in thickness and abrupt wedging out of some conglomerate lenses. Furthermore, some lenses become one conglomerate body by wedging out of sand-marl members (Fig. 18). This phenomena is emphasized by later local and regional tectonics. That is expressed in abrupt changes of lenses strike and dip direction as compared with the surrounding area. Within bauxite zones moderate synclinal orientation of conglomerate lanses predominates (Fig. 19).

Within regions with relatively uniform arrangement of conglomerate lenses the collapse structures in hangingwall rocks arc clearly expressed. As in foraminifera limestone hangingwall complexes, the moderate centriclinal orientation of bed planes- lenses are present here as well, together with radial orientation of joint systems directly above a deposit. The thickness of hangingwall rocks in which the mentioned indications as a result of collapse structure have been observed could reach up to 150 m (Fig. 20).

Conclusion

In effort to establish certain geomorfological and geological indicators of potential bauxite existence, in small and great depth, we have been governed by numerous relevant ideas concerning the geomorphological and geological relations of investigated area.

For a geomorphological analysis which takes into account geological relations, as a first step it is necessary to establish certain units of common geomorphologicalcharacterislics.

For the geological analysis as well, togethcrt with detailed geological maps, it is of crucial importance to possess the knowledge of sediment com post ion, chronostratigraphic relations. Together with considerable detailed knowledge of tectonic relations and structural characteristics and conditions of bauxite formation as well, it is possible ti distinguish certain geological units of similar geological pattern.

As a rule, by considering the problem in this way the correspondence of distinguished geomorphological and geological units can be observed.

Within this frame, every and even the slightest exception from a common pattern of distinguished units is considered as an indicator.

The importance of indicators only by themselves is relative. However, clustered in one particular locality, they have already hit the target.

Received-8JIU99Z

Accepted: 20. VI 1995.

References

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