The Origin and Importance of the Dolomite-Limestone Breccia Between the Lower and Upper Cretaceous Deposits of the Adriatic Carbonate Platform: An Example from ∆iÊarija Mt. (Istria, Croatia) Igor VLAHOVI∆ 1, Josip TI©LJAR 2, Ladislav FU»EK1, Nenad O©TRI∆1, Boæo PRTOLJAN 1, Ivo VELI∆1 and Dubravko MATI»EC1 1. INTRODUCTION During the exploratory geological works for the railway tunnel project through the ∆iÊarija Mt. belt (western Croatia - Fig. 1) special attention was paid to the prob- lem of the complex of dolomite-limestone breccia on the NE flank of the mountain, regarding the position, shape, and origin of the sedimentary body. Previously there were two opposing opinions con- cerning the occurrence and origin of the dolomite-lime- stone breccia. According to the Explanatory notes of the Basic Geological Map, sheet Ilirska Bistrica (©IKI∆ & PLE- NI»AR, 1975) these deposits were formed by erosion of the carbonate rocks during tectonic uplift, and rede- position of the derived coarse-grained material into the basin. The brecciated appearance would therefore rep- resent the consequence of irregular fragmentation of the carbonate rocks during weak synsedimentary move- ments, which were attributed to the “Austrian tectonic phase”. Therefore, these breccia deposits were inter- preted as intraformational, i.e. deposits which are posi- tioned within the structure between Lower Cretaceous and Upper Cretaceous shallow-water platform lime- stones. The same level of brecciated rocks in the central Istria were referred to as “the horizon of the clastic-car- bonate rocks”, and interpreted in similar way by POL- ©AK & ©IKI∆ (1973). According to the second, later opinion of some investigators presented in unpublished reports, these deposits would correspond to the so-called Jelar brec- cia, or Jelar beds, i.e. a post-Cretaceous tectogenic rock-fall breccia covering large areas of Velebit Mt. and some of the northern Adriatic islands. In this case the breccia deposits would represent irregular, lensoid bodies covering palaeodepressions in the more or less tectonized underlying Cretaceous deposits - therefore, they would not be concordantly inserted within the structure of the ∆iÊarija Mt., i.e. between the Lower and Upper Cretaceous carbonate deposits. Explanation of the problem of structural position and origin of the dolomite-limestone breccia of the Geologia Croatica 55/1 45 - 55 10 Figs. ZAGREB 2002 Key words: Lower to Upper Cretaceous transition, Tectogenic-diagenetic breccia, Late-diagenetic do- lomitization, Late-diagenetic silicification, ∆iÊarija Mt., Adriatic Carbonate Platform, Croatia. 1 Institute of Geology, Sachsova 2, P.O.Box 268, HR-10000 Zagreb, Croatia; e-mail: geologia-croatica@zg.tel.hr 2 Faculty of Mining, Geology & Petroleum Engineering, University of Zagreb, Pierottijeva 6, HR-10000 Zagreb, Croatia. Abstract On the NE slopes of ∆iÊarija Mt. (N Istria) a 120-150 m thick com- plex composed of dolomite-limestone breccia crops out between the Lower and Upper Cretaceous deposits. This studied breccia sequence is of post-sedimentary, tectogenic-diagenetic origin. It was formed by polyphase tectonic fracture of the Upper Albian to Lower Cenoma- nian early- and late-diagenetic dolomite succession with relics of recrystallized limestone, which enabled very important subsequent diagenetic alteration. This included partial dissolution, dedolomitiza- tion, recrystallization and calcitization of the fine-grained, crushed dolomite matrix, and centripetal dissolution of dolomite fragments and their cementation by calcite and ferroan calcite cements, as well as the partial collapse of fragments from the roofs of dissolution cavi- ties and limited late-diagenetic silicification (the silica surplus origi- nating from layers of diagenetic quartz from underlying Upper Albian deposits). Such a complex pattern of different events resulted in the high variability of breccia characteristics over relatively small dis- tances, especially near more intensively tectonized zones. The contemporaneous stratigraphic level (Lower to Upper Creta- ceous transition) in other parts of the Adriatic Carbonate Platform is also characterised by predominantly late-diagenetic dolomites with relics of limestones (including local occurrences of early-diagenetic dolomites) which are, in more tectonized areas, late-diagenetically altered into tectogenic-diagenetic breccias. 46 Geologia Croatica 55/1 ∆iÊarija Mt. (VLAHOVI∆ et al., 1995) was a very important issue in the construction of the geological profile of the future tunnel, especially regarding their extremely variable, and more or less unfavourable engi - neering-geological properties (BIONDI∆ et al., 1995). Therefore, during the exploratory works detailed inves- tigation of their superficial occurrence, relationship with neighbouring lithostratigraphical units, as well as a complex investigation of their sedimentological, petro- graphical, palaeontological and stratigraphic properties was performed. The results of these investigations and the completely new interpretation of their origin, as well as a brief regional overview of the transitional lev- el between the Lower and Upper Cretaceous deposits, are the main topics of this paper. 2. THE OCCURRENCE, LITHOLOGICAL PROPERTIES AND STRATIGRAPHIC POSITION OF THE INVESTIGATED BRECCIA 2.1. Occurrence of the breccia deposits In the study area breccia deposits occur in the undivid- ed, approximately 1-1.5 km wide zone near the core of an anticline composed mostly of the Cretaceous depo- sits. A similar zone composed of dolomite-limestone breccia is also present in the NE limb of the anticline, in a contemporaneous stratigraphic position - between the Albian and Cenomanian shallow-water limestones (Fig. 2). The field observations suggest that these deposits represent part of the structure, i.e. that they are concor- dant within the Upper Albian and Middle Cenomanian limestones. This, together with the need for obtaining as much data as possible concerning their engineering- technical properties at the level of the future tunnel, resulted in the location and drilling a 350 m deep well (ÆT∆-2). The well was located within the area com- posed of Cenomanian limestones dipping at approxi- mately 25° W, and in compliance with the prognosis profile penetrated the breccia deposits at depth between 50.5 to 215.4 m (Fig. 3). The well ended in the Upper Albian deposits (limestones and rarely late-diagenetic dolomites, including three layers of diagenetic quartz deposits equivalent to the contemporaneous deposits in Istria and island of Vis - GALOVI∆, 1992). Contacts between the breccia deposits and underly- ing Albian limestones, as well as the overlying Ceno- manian deposits are very irregular, both in the well-core and on the surface. Generally, all varieties of gradual transitions from completely undisturbed limestones, over more or less intensely cataclastized and late-diage- Fig. 1 Location map showing localities mentioned in the text, study area (black rectangle) and some oth- er occurrences of a simi- lar breccia (grey circles). netically dolomitized limestones, to the typical breccia can be observed in the transitional areas. Within the breccia zone there are also frequent later- al changes, especially along the younger fault zones: in the areas further from the tectonic zones cutting the breccia unit, large, more or less well preserved dolo- mite blocks (in some places even with limestone relics) occur, while near the faults, rocks are completely frac- tured and brecciated. 2.2. Lithological properties The breccia is predominantly composed of dolomite fragments, with variable amounts of limestone relics, authigenous quartz aggregates and mostly calcitic microspar to spar cement (Figs. 4 and 5). Dolomite fragments are characterised by variegated structural types: besides the most common homogenous crypto to microcrystalline texture some grains show alterna- tion of dolomicritic and dolopelsparitic laminae with relics of cryptalgal lamination, indicating their origin from partially recrystallized early-diagenetic dolomites. Fragments exhibit different hues of a grey colour, a commonly obvious consequence of the varying intensi- ty of diagenetic alteration, mostly by recrystallization and dedolomitization. Some fragments clearly exhibit their early-diagenetic dolomite origin, while others were formed by stronger or weaker recrystallization of fractured and crushed early-diagenetic dolomites or the complete late-diagenetic dolomitization of former lime- stones. Fragment size is very variable, the most com- mon grains being 1-50 mm in diameter, but in some parts of the breccia zone there are dm-sized fragments, 47VlahoviÊ et al.: The Origin and Importance of the Dolomite-Limestone Breccia... Fig. 2 Simplified geological map of the north-eastern slope of ∆iÊarija Mt. with the location of well ÆT∆-2 (on the left). Fig. 3 Simplified well-log showing subsurface relations of the stud- ied deposits. 48 Geologia Croatica 55/1 and even blocks a few metres in size. The fragments are more or less isometric, mostly angular with more or less rounded corners. Infrequent rounded grains are obviously the result of corrosion and solution of the original rock by aggressive pore solutions and replace- ment of dolomite by calcite (i.e. dedolomitization), not of rounding during possible transport. Corrosional “rounding” of fragments is clearly visible in thin sec- tions, especially by obvious centripetal calcitization, i.e. this process gradually prograded from tectonic fissures surrounding the fragments towards the interior of the grains (Fig. 4). Fitting (FÜCHTBAUER & RICHTER, 1983), i.e. the compatibility of the contours of neighbouring grains, is variable, but relatively high: fragments are commonly divided only by tectonic fissures and joints (similar effects are also present in the underlying rocks - Fig. 6), which are more or less widened by corrosion (Fig. 5). In the more tectonized parts of the study area, characterised by more important dissolution and forma- tion of dissolution pores and caverns, collapsed materi- al within such open spaces is characterised by very low fitting. Fitting is also low in tectonic breccias formed along clearly marked tectonic lines. In some places, especially in the vicinity of more tectonized zones, intergranular contacts are frequently stylolitic, i.e. formed as a consequence of subsequent intense pressure solution (Fig. 7). Quartz aggregates, which macroscopically appear as compact white clasts (Fig. 9B; these clasts were occa- sionally in former investigations even mistaken for white Cenomanian limestone clasts!), were formed by selective early-diagenetic silicification of the most unstable carbonate components (mostly dissolved and dedolomitized dolomite fragments, in some places also Fig. 4 Photomicrograph of diage- netic breccia composed of do- lomicrite fragments and calcit- ized and partly silicified matrix. Note “rounding” of fragments due to corrosion of the dolo- mite fragments. ÆT∆-2, 180.6 m; stained thin-section, pola- rised light, photo length = 3.3 mm. Fig. 5 Photomicrograph of diage- netic breccia with corroded do- lomite fragments and comple- tely calcitized fine-crystalline matrix and neocalcite mosaic cement. Most of the fragments are still characterised by relati- vely high fitting. ÆT∆-2, 73.3 m; stained thin-section, polari- sed light, photo length = 3.3 mm. 49VlahoviÊ et al.: The Origin and Importance of the Dolomite-Limestone Breccia... calcitized fine-grained carbonate matrix). These quartz druses are mostly composed of hypidiomorphic crystals of variable size, which were during the most recent cal- citization processes frequently disintegrated by neocal- cite growth between quartz aggregates, resulting in for- mation of isolated quartz “grains” within a calcite mass. Quartz crystals frequently exhibit radial growth of the terminal surfaces in the form of rosettes (Fig. 8). Subsequent calcitization of quartz aggregates is most obvious near numerous irregular tectonic fissures and fractures formed by the youngest tectonic move- ments (Neotectonic phase - post-Miocene). Calcitiza- tion is generally clearly centrifugal, resulting in com- plete calcitization of the inner parts of quartz rosettes. Quartz aggregates were formed by in situ growth of quartz crystals during late diagenetic circulation of solutions enriched in silicic acid. The silica surplus probably originated from diagenetic quartz deposits found in the underlying Upper Albian deposits. The cement and matrix of the dolomite-limestone breccia are mostly composed of microsparitic calcite. This matrix was formed by calcitization (i.e. dedolomi- tization) of finely crushed dolomicrite, which is present only in the form of small relics, while calcite cement filled the remaining voids. In some places gradual tran- sitions from dolomicrite fragments and partially calcit- ized dolomicrite to cryptocrystalline calcite with rare dolomicrite relics are visible. Significant calcitization occurred in the most tectonically disintegrated areas, i.e. the intensity of calcitization is more or less positive- ly correlatable with the intensity of tectonic disintegra- tion. Both cement and matrix are partially recrystal- lized, and in some parts spelaeothems, formed by the circulation of solutions through corrosionally widened Fig. 6 Photomicrograph of tectoni- zed Albian limestone showing fractures filled by coarse-crys- talline calcite cement. Note high fitting. ÆT∆-2, 285.4 m; stained thin-section, polarised light, photo length = 7.5 mm. Fig. 7 Photomicrograph of styloliti- zation breccia characterised by stylolitic intergranular con- tacts formed as a consequen- ce of intense pressure soluti- on. Note intense late-diagene- tic silicification of two small fragments of dolomicrites in the lower left, subsequently cut by calcite veins. ÆT∆-2, 121.4 m; stained thin-section, polarised light, photo length = 7.5 mm. 50 Geologia Croatica 55/1 tectonical fractures, were observed. Younger tectonic fractures and some solution cavities are completely cemented by mosaic sparry calcite. 2.3. Stratigraphic position No index fossils have been discovered in fragments or matrix of the dolomite-limestone breccia, which is not unusual as they are mostly composed of early- and late- diagenetic dolomites with late-diagenetic calcitized fractures. Only in infrequent undolomitized limestone relics have rare miliolids and ostracods been observed, but without any stratigraphic significance. Therefore, the stratigraphic position of the studied sequence can only be established on the basis of superpositional rela- tionships. Both from surface data and information from drilling it is clear that the studied dolomite-limestone breccia is situated between Albian and Cenomanian carbonates. The underlying deposits are mostly grey, dark grey- brownish to almost black, platy to thin-bedded (≤ 3 0 cm) mudstones, which are, in the upper part, frequently late-diagenetically dolomitized. On the basis of their fossil content (C h a m a sp., gastropods, nubecularids, and benthic foraminifera: Praechrysalidina infracreta - cea LUPERTO-SINNI, Pseudonummoloculina heimi (BONET) and other miliolids) and correlation with sim- ilar deposits of the neighbouring areas their Albian age is certain. However, the geological position of the stud- ied deposits and three levels of diagenetic quartz deposits determined in the ÆT∆-2 well (at 215.3-217.5, 223.1-223.2 and 244.1-244.3 m), which are regionally important for the latest Albian, indicate a probable Upper Albian age. Fig. 8 Photomicrograph of partially silicified calcitized matrix (up- per right) and a partly dedolo- mitized dolomicrite fragment (lower left) of diagenetic dolo- mite-limestone breccia. Note that quartz crystals frequently exhibit radial growth of the ter- minal surfaces in the form of rosettes. ÆT∆-2, 51.25 m; stained thin-section, polarised light, photo length = 3.3 mm. Fig. 9 Five typical examples of dif- ferent varieties of the studied breccia from well ÆT∆-2. A) Completely corroded dolomite fragments (“rounded” grains) surrounded by a large quantity of finely crushed recrystallized matrix; B) Large fragments of dolomite “floating” in dark cal- citic matrix; C) matrix-rich breccia characterised by inten- se corrosion (isometric solu- tion vugs formed by dedolomi- tization); D) breccia composed of completely silicified white fragments and laminated ear- ly-diagenetic dolomites; E) fra- ctured dolomite with thin fill- ings of white calcite between dolomite fragments. 51VlahoviÊ et al.: The Origin and Importance of the Dolomite-Limestone Breccia... Overlying deposits are represented by brown - dark brown, rarely yellowish and grey-brownish, partly re- crystallized mudstones to peloid-skeletal wackestones with infrequent rudist debris. Radiolitid and ostreid floatstones are relatively rare. In these deposits typical Middle to Upper Cenomanian fossils of the Adriatic Carbonate Platform have been observed: C h r y s a l i d i n a g r a d a t a d’ORBIGNY, Nummoloculina regularis P H I- LIPPSON, Pseudonummoloculina heimi, P s e u d o l i t u o - nella reicheli MARIE, etc. Concerning the position of deposits directly overlying the breccia in the study area, the lack of B r o e c k i n a (P a s t r i k e l l a ) b a l c a n i c a C H E R- CHI et al., a typical index species for the upper part of the Middle and Upper Cenomanian in the neighbouring area (VELI∆ & VLAHOVI∆, 1994), indicates that a Middle Cenomanian age seems more probable. On the basis of the aforementioned data, it may be concluded that the studied succession of dolomite-lime- stone breccia is probably of uppermost Albian (Vracon- ian) to Lower Cenomanian age. This determination completely complies with the stratigraphic position of the same or similar deposits (dolomite or dolomite- limestone breccia, recrystallized early and late-diage- netic dolomites, late-diagenetically dolomitized and recrystallized limestones) in other parts of the Adriatic Carbonate Platform in Croatia (e.g. eastern part of Istria, islands of Krk and Cres, Gorski Kotar area) and Slovenia (e.g. “massive to indistinctly bedded bitumi- nous dolomitic breccia” of the Povir Formation in the area of the Trieste-Komen Plateau - JURKOV©EK et al., 1996), and also in neighbouring Italy (TENTOR et al., 1994). The complete thickness of the studied sequence of dolomite-limestone breccia of the ∆iÊarija Mt. could be approximated to 130-150 m, on the basis of surface and subsurface data, which is concordant with the estimated thickness of the corresponding level in other areas (e.g. Sis Formation on the island of Cres or Povir Formation of the Trieste-Komen Plateau - JURKOV©EK et al., 1996). However, concerning the thickness of this com- plex it should be emphasised that both boundaries with the overlying and underlying lithostratigraphic units are gradual and irregular as the result of different amounts of tectonic disturbance and selectivity of diagenetic processes; therefore, the thickness of the breccia com- plex is laterally variable. 3. ORIGIN OF THE DOLOMITE-LIMESTONE BRECCIA The origin of the studied rock complex is very complex and polygenetic, as in both the field and in thin-section several genetic types of breccia could be recognised, including different gradual transitions between them. This resulted in great variability of the observed lithofa- cies, i.e. almost each sample seems at least somewhat different form the others in its close vicinity (Fig. 9). All breccia deposits are characterised by the impor- tant influence of late-diagenetic processes, which were enabled by intense tectonic disintegration, therefore the entire succession is defined as t e c t o g e n i c - d i a g e n e t i c b r e c c i a . However, within the breccia zone there are also spatially restricted occurrences representing relics of breccia deposits of specific origin, also caused more or less directly by recurring intense tectonic activity during post-Cretaceous tectonic phases (Tertiary tecton- ic phase and Neotectonic phase): a) tectonic breccia; b) stylolitization breccia; c) collapse breccia. Tectonic breccias were formed by tectonic disinte- gration of all rock types (Fig. 6), and only occur very close to important faults. Therefore, their surficial and subsurface extension is limited. Stylolitization brec- c i a s were formed by karstification of intensely styloli- tized, and subsequently partially cemented dolomites and limestones (Fig. 7). Both these aforementioned breccia types can also be seen in the underlying and overlying deposits, as well as in other carbonate succes- sions. In the areas of very high dissolution, i.e. near the major zones of circulation, numerous cavities of vari- able size were formed, and collapse of material from their roofs caused formation of specific type of breccia - collapse breccia . This type is characterised by a mix- ture of different fragments originating from neighbour- ing beds (which were rotated during their accumulation, therefore resulting in much lower fitting), as well as variable proportions of fragments, finely-crushed ma- trix and cement filling remaining open spaces. Collapse breccias are much more common near major tectonic lines, but are usually also subsequently diagenetically altered. Sometimes it is hard to define a borderline between the aforementioned types of breccia and the most com- plex, and by far the most comprehensive type described in this paper - tectogenic-diagenetic breccia . Within this breccia type elements of all the other types are encompassed in some places, but they are more or less overprinted by important polyphase diagenetic alter- ations. Therefore, in this paper the term tectogenic-dia - genetic breccia is a collective designation for rocks of brecciated appearance formed as a result of the diagenetic alteration of in situ rocks affected by intense tectonics , although in some parts there are still visible traces of a specific major cause of brecciation (i.e. tectonic crushing, collapse of material from the cavity roof or stylolitization). It is important to notice that most of the rocks of this breccia type are charac- terised by in situ brecciation, i.e. the transport distance of fragments in the breccia is very short (e.g. in the case of collapse origin) or practically absent. 52 Geologia Croatica 55/1 The original rock succession, before intense post- Cretaceous tectonic activity, represented the alternation of peritidal early-diagenetic dolomites and limestones. Fragments of the early-diagenetic dolomites are com- monly characterised by relics of cryptalgal lamination, indicating their formation by early-diagenetic dolomiti- zation in peritidal (supratidal) conditions. Interbedded shallow subtidal limestones were subsequently, during the late-diagenetic phase, gradually dolomitized into late-diagenetic dolomite characterised by a mosaic structure composed of hypidiomorphic dolomite crys- tals. Similar successions have been described from the Berriasian of Istria (“Fantazija dolomites” - VELI∆ & TI©LJAR, 1988; TI©LJAR et al., 1995) and the Upper Albian of Biokovo Mt. (TI©LJAR & VELI∆, 1991). Underlying Upper Albian deposits and overlying Cenomanian deposits were composed of almost pure limestones. This is very important for the origin of the investigated breccia deposits, which resulted from poly- phase processes and diagenetic changes which are here, as in other analogous cases, mainly induced by the presence of unit of specific lithology (in this case dolo- mites) surrounded by rocks of a different composition (in this case predominantly limestones, including infre- quent layers of diagenetic quartz in the underlying suc- cession). During the Tertiary, the entire sequence of deposits in the study area was affected by quite intense post-sed- imentary tectonics (this is the area characterised by the “collision” of Istria and its hinterland, resulting in the formation of the ∆iÊarija Mt. belt, as well as by intense Neotectonic deformation). However, concerning the fact that the dolomite rocks are much more prone to tectonic disintegration than limestones (HANDIN & HAGER, 1957), it is not unusual that the alternation of early-diagenetic and late-diagenetic dolomites was even more intensely fractured than the surrounding Albian and Cenomanian limestones. In zones near major faults they were even completely crushed, resulting in their greatly enhanced permeability. Further diagenetic changes took place under condi- tions where limestone represented the stable and dolo- mite the unstable phase. Highly fractured and perme- able dolomite rocks enabled circulation of chemically aggressive pore solutions causing partial dissolution and/or diagenetic changes of the rock fragments, since these solutions were oversaturated with respect to cal- cite and undersaturated in respect of dolomite. Such a chemical composition of pore waters, i.e. relatively high concentration of calcium hydrogencarbonate, resulted from the intense pressure solution of a thick sequence of Mesozoic deposits (predominantly lime- stones) during the phases of tectonic stress, as well as dissolution of the surficial part of deposits by meteoric waters enriched in CO2. Relatively aggressive pore solutions caused partial dissolution of dolomite rocks and important allochemi- cal late-diagenetic processes, i.e. calcitization (dedolo- mitization) and late-diagenetic silicification, which were accelerated by the relatively large reactive sur- faces of tectonically disintegrated dolomites, resulting in important changes of mineral and chemical composi- tion of the original rocks. Dissolution of dolomite rock along the tectonic fis- sures and joints caused formation of corrosional voids of varying size, significant widening of fissures and joints, as well as corrosion of surfaces and corners of dolomite fragments (therefore, in zones of intense cir- culation many “grains” are more or less rounded, which was formerly misinterpreted as a result of rounding of coarse-grained material during transport). Recrystallization of the matrix and cementation of neomorphic calcite crystals in open spaces, fissures and fractures as a result of oversaturation of pore solutions in respect to calcium carbonate was a very important diagenetic process. This was followed by gradual repla- cement of dolomite by calcite, i.e. dedolomitization, which was especially important in the more intensely tectonized zones, resulting in further “rounding” of fragments. Calcitization was a polyphase process, as there are several generations of neocalcite of different composition (from calcite to ferroan calcite) caused by polyphase tectonics and variable chemical composition of pore solutions. Another important allochemic diagenetic process, although of much lesser extent, was the late-diagenetic silicification of dolomites and dedolomites, and also neocalcite forming the matrix and cement. Solutions comprising SiO2 diluted in the form of silicic acid grad- ually replaced carbonate minerals that were unstable under weakly acid pH conditions. Silicification resulted in the formation of microcrystalline, rarely even crypto- crystalline quartz aggregates, commonly within the contours of the dolomite fragments or within the calcit- ized matrix. Silicification was obviously a later process than the main calcitization phase, but it is evident that there was at least one subsequent phase of calcite crys- tallisation. The most probable source of the silica sur- plus was the underlying diagenetic quartz beds of Upper Albian age found in the ÆT∆-2 well. These were formed by late-diagenetic silicification of lagoonal limestones caused by the aeolian input of volcanic ash. This is also why silicification is more intense in the lower part of the studied breccias. 4. DISCUSSION AND CONCLUSION Carbonate breccias are deposits that are quite underesti- mated in the sedimentological literature. There are sev- eral reasons for this, but probably the most important one is their complexity caused by the very common presence of different genetic types at the same locality. Furthermore, breccias are often subsequently consider- ably modified either by tectonics or diagenetic process- es, disabling the possibility of direct determination of 53VlahoviÊ et al.: The Origin and Importance of the Dolomite-Limestone Breccia... their origin. This results in a common problem in the general perception of breccia: either their mystification (unfortunately, commonly resulting in their complete omission from discussions) or their extreme simplifica- tion (placing all types together and suggesting that their origin is already well-known), despite their importance for the appropriate determination of specific events in the geological history of many areas. The transitional level between the Lower and Upper Cretaceous, i.e. Upper Albian and Lower Cenomanian deposits, is in most parts of the Adriatic Carbonate Plat- form characterised by intense dolomitization and recrystallization of limestone relics. At many locations it is obvious that the surplus of magnesium necessary for the massive late diagenetic dolomitization of lime- stones was provided by early-diagenetic dolomites, as a consequence of the formation of extensive tidal flats during relative sea-level fall. This is the reason why this level is one of those regionally recognised for profound dolomitization. However, dolomite-carbonate breccias are present only in some places, e.g. ∆iÊarija Mt., hin- terland of Rijeka, parts of SE Istria, parts of Cres and Krk island, etc. Why are they absent in other areas? The case of the ∆iÊarija Mt. dolomite-limestone breccia clearly indicates two substantial prerequisites for the formation of this type of breccia: (1) presence of rocks of specific lithology, different from neighbouring units, (2) their occurrence within zones of intense tec- tonic activity. Only the appropriate combination of both factors enabled the intense disintegration of rocks and significant influence of polyphase complex diagenetic processes. This is the main reason why the amount of tectogenic-diagenetic alterations is laterally extremely variable: even intense tectonic activity will result only in limited changes in successions composed of similar lithologies (since there will be no major allochemical diagenetic changes necessary for the formation of this kind of brecciated rocks) and vice versa: without tec- tonic disintegration even the successions of completely different lithology than neighbouring units will remain almost unchanged (since diagenetic processes will be much slower and of lesser extent). On the basis of the occurrence of dolomite-lime- stone breccias of ∆iÊarija Mt., their composition and structural-textural characteristics, as well as the com- plex tectonic and diagenetic processes relevant to their formation the following main conclusions may be drawn: 1) these rocks are not sedimentary, intrabasinal brec- cias formed by synsedimentary tectonic phases between the Lower and Middle Cretaceous; 2) the investigated sequence of breccia deposits repre- sent a continuous succession located within the structure of ∆iÊarija Mt. between the Upper Albian and Middle to Upper Cenomanian deposits, i.e. they are not equivalent to a tectogenous rock fall breccia of the Jelar type (Cenozoic breccia covering large areas of Velebit Mt. and some of the northern Adri- atic islands - BAHUN, 1974); 3) the studied sequence represents rocks of specific properties and origin, formed by complex tectonic and diagenetic processes affecting a specific litho- logical succession different from neighbouring units - besides other specific breccia types recognised within the studied sequence, most of the rocks could be referred to as post-sedimentary tectogenic-diage- netic breccia formed in situ. This kind of genetic interpretation is also partly or completely applicable for breccias from some other stratigraphic levels in the Adriatic Carbonate Platform realm, although these sequences are not so thick and outcrops of similar rocks are not so extensive. In these cases the interrelation of intense tectonics and specific lithologies is also obvious, as in the area of Mala Kape- la Mt. where good examples of Neocomian tectogenic- diagenetic dolomite breccia can be found (Fig. 10). Very similar, although resulting from the completely opposite process (late-diagenetic dolomitization of cal- cite along tectonic fractures), breccias were described in the Malm deposits of the Velebit Mt. (TI©LJAR, 1990). The correct genetic interpretation of breccia deposits is extremely important for the sedimentologi- cal, tectonic and palaeogeographic interpretation of the wider region (TI©LJAR, 2001). In the case of ∆iÊarija Mt. erroneous substitution of this type of breccia with synsedimentary ones, or their erroneous interpretation as younger, rock-fall breccias forming lens-shaped bod- ies only in near-surface zone, would be extremely criti- cal. This is not purely a scientific question: in this case the interpretation of a large part of the geological pro- file for the planned tunnel through ∆iÊarija Mt. have to be based on the right conclusion, especially concerning possible serious problems during drilling of the studied complex. 5. REFERENCES BAHUN, S. (1974): Tektogeneza Velebita i postanak Jelar- naslaga (The tectogenesis of Mt. Velebit and the forma- tion of Jelar deposits).- Geol. vjesnik, 27, 35-51. BIONDI∆, B., BRAUN, K., VLAHOVI∆, I., MLINAR, Æ., ANDRI∆, M., BALEN, I., POLLAK, D., FU»EK, L., O©TRI∆, N., PRTOLJAN, B., ©ABAN, B., BLAGUS, Z., DUKARI∆, F., BULJAN, R. & BIONDI∆, R. (1995): Inæenjerskogeoloπki model æeljezniËkog tunela ∆iÊarija (Engineering-Geological Model of the Railway Tunnel “∆iÊarija”).- In: VLAHOVI∆, I., VELI∆, I. & ©PARICA, M. (eds.): 1. hrvatski geoloπki kongres (First Croatian Geological Congress), Zbornik radova (Proceedings), 1, 81-87, Zagreb. FÜCHTBAUER, H. & RICHTER, D.K. (1983): Relations between submarine fissures, internal breccias and mass 54 Geologia Croatica 55/1 flows during Triassic and earlier rifting periods.- Geol. Rundschau, 72/1, 53-66. GALOVI∆, I. (1992): Geoloπki poloæaj i geneza kvarcnih sedimenata Istre.- Unpublished PhD Thesis (in Croatian with an English abstract), University of Zagreb, 120 p. HANDIN, J. & HAGER, R.V. (1957): Experimental deforma- tion of sedimentary rocks under confining pressure.- Bull. Am. Assoc. Petrol. Geol., 41, 1-50, (and 42, 2897-2934). JURKOV©EK, B., TOMAN, M., OGORELEC, B., ©RIBAR, L., DROBNE, K., POLJAK, M. & ©RIBAR, Lj. (1996): Formacijska geoloπka karta juænega dela Træaπko-Komen- ske planote 1:50000. Kredne in paleogenske karbonatne kamnine (Geological map of the southern part of the Tri- este-Komen Plateau. Cretaceous and Paleogene carbonate rocks).- Inπtitut za geologijo, geotehniko in geofiziko, 143 p., Ljubljana. POL©AK, A. & ©IKI∆, D. (1973): Osnovna geoloπka karta SFRJ 1:100.000, TumaË za list Rovinj L33-100 (Geology of Rovinj sheet).- Institut za geoloπka istraæivanja Zagreb (1963), Savezni geoloπki zavod, 51 p., Beograd. ©IKI∆, D. & PLENI»AR, M. (1973): Osnovna geoloπka karta SFRJ 1:100.000, TumaË za list Ilirska Bistrica L33-89 (Geology of Ilirska Bistrica sheet).- Institut za geoloπka istraæivanja Zagreb & Geoloπki zavod Ljubljana (1967), Savezni geoloπki zavod, 50 p., Beograd. TENTOR, M., TUNIS, G. & VENTURINI, S. (1994): Sche- ma stratigrafico e tettonico del Carso Insontino.- Natura Nascosta, 9, 1-32, Monfalcone. TI©LJAR, J. (1990): Postsedimentne dolomitizacijske breËe u jurskim plitkomorskim karbonatnim sedimentima Malog Halana na juænom Velebitu (Postsedimentary dolomitiza- tion breccias in the Jurassic shallow-marine carbonate sediments of the Mali Halan - southern Velebit Mt.).- Rudarsko-geoloπko-naftni zbornik, 2, 59-65, Zagreb. TI©LJAR, J. (2001): Sedimentologija karbonata i evaporita.- Institute of Geology, Zagreb, 375 p. TI©LJAR, J. & VELI∆, I. (1991): Carbonate facies and depo- sitional environments of the Jurassic and Lower Cretace- ous of the coastal Dinarides (Croatia).- Geol. vjesnik, 44, 215-234. TI©LJAR, J., VLAHOVI∆, I., MATI»EC, D. & VELI∆, I. (1995): Platformni facijesi od gornjeg titona do gornjega alba u zapadnoj Istri i prijelaz u tempestitne, klinoformne i rudistne biolititne facijese donjega cenomana u juænoj Istri (ekskurzija B) (Platform facies from the Upper Tithonian to Upper Albian in western Istria and transition into tempestite, clinoform and rudist biolithite facies of the Lower Cenomanian in Southern Istria (Excursion B)).- In: VLAHOVI∆, I. & VELI∆, I. (eds.): 1. hrvatski geo- loπki kongres (First Croatian Geological Congress), VodiË ekskurzija (Excursion guide-book), 67-110, Zagreb. Fig. 10 Photograph of the outcrop of Neocomian tectogenic-diagenetic dolomite breccia on the Mala Kapela Mt. Coin is 27 mm in diameter. A) Tectogenic-diagenetic breccia characterised by different amounts of diagenetic alteration of the original dolomite rock. Tectonic fis- sures are filled with white calcite cement. The lower part of the photograph represents completely crushed dolomite with only small “rounded” relics of the original rock. B) Detail of the upper right part of Fig. 10A showing more intensely tectonized part of the outcrop, characterised by a large amount of white calcitized matrix and still relatively high fitting of dolomite fragments. 55VlahoviÊ et al.: The Origin and Importance of the Dolomite-Limestone Breccia... VELI∆, I. & TI©LJAR, J. (1988): Litostratigrafske jedinice u dogeru i malmu zapadne Istre (zapadna Hrvatska, Jugo- slavija).- Geol. vjesnik, 41, 25-49. VELI∆, I. & VLAHOVI∆, I. (1994): Foraminiferal assem- blages in the Cenomanian of the Buzet-Savudrija area (Northwestern Istria, Croatia).- Geologia Croatica, 47/1, 25-43. VLAHOVI∆, I., TI©LJAR, J., VELI∆, I., FU»EK, L., O©- TRI∆, N. & PRTOLJAN, B. (1995): O podrijetlu i znaËe- nju tzv. vapnenaËko-dolomitnih breËa izmeu donje i gor- nje krede: primjer ∆iÊarije (On the origin and significance of the limestone-dolomite breccia between the Lower and Upper Cretaceous: an example of ∆iÊarija).- In: VLAHO- VI∆, I., VELI∆, I. & ©PARICA, M. (eds.): 1. hrvatski geoloπki kongres (First Croatian Geological Congress), Knjiga saæetaka (Abstracts), 99, Zagreb. Manuscript received June 18, 2001. Revised manuscript accepted May 20, 2002. 56 Geologia Croatica 55/1