2021 | 74/2 | 99–120 | 18 Figs. | 1 Tab. | 3 Pls. | www.geologia-croatica.hr Journal of the Croatian Geological Survey and the Croatian Geological Society

Megabeds in Istrian Flysch as markers of synsedimentary tectonics within the Dinaric foredeep (Croatia)

Krešimir Petrinjak*, Marko Budić, Stanislav Bergant and Tvrtko Korbar

Croatian Geological Survey, Department of Geology, Sachsova 2, 10 000 Zagreb, Croatia; (*corresponding author: [email protected]) doi: 10.4154/gc.2021.07

Abstract Article history: Istrian Flysch was deposited during the Eocene in the Dinaric foredeep and is composed of Manuscript received January 15, 2020 hemipelagic marls and various gravity flow deposits. The latter are predominantly 5-40 cm thick Revised manuscript accepted February 18, 2021 turbidites, developed mostly as laminated and cross-rippled sandstone beds (Tb-e, Tc-e and Available online May 06, 2021 Td-e Bouma sequences). In addition to the turbidites, there are deposits characterized by a sig- nificant thickness, occasionally more than 10 m, described as complex (bipartite) megabeds. The megabeds are composed of debrites in the lower part (Division I), and high-density turbi-

dites in the upper part (Division II). The distinct clast composition of each megabed indicates that the lithoclasts were derived from tectonically active slopes and fault scarps along which col- lapses of the different parts of the Cretaceous to Palaeogene neritic carbonate succession, that underlie the Flysch, occurred. The Division II deposits are well cemented, normally graded cal- cirudite/calcarenites composed mostly of orthophragminids, nummulitids, and red algae, origi- nating from outer ramp environments. Redeposited marl, observed in the matrix of the debrites Keywords: Adriatic microplate, Dinaric foreland and as intraclasts in some megabeds, implies that the collapses along the synsedimentary fault basin, mass transport deposits, Eocene, External scarps and steep slopes also occurred within the foredeep itself, during the rapid tectono-sedi- Dinarides mentary evolution of the Dinaric foreland basin.

1. INTRODUCTION compressed basinal sediments composed of arenite and marl al- There is no generally accepted definition of the term ‘megabed’ ternations and c) foraminiferal debris. The turbidite part is nor- (MARJANAC, 1996; FALLGATER et al., 2016) but they usually mally graded, composed mostly of foraminiferal debris and lack- refer to deposits of exceptionally large-volume sediment gravity ing large lithoclasts (MARJANAC, 1993). In the slightly older flows (megaflows) that are rheologically complex, and that pro- counterparts of the Dinaric foreland basin north of Istria, (Friuli duced much thicker beds than the surrounding succession (FALL- and the Julian basin in Italy and Slovenia), the mass transport de- GATER et al., 2016). Exceptionally thick beds with a virtually posits (MTDs) that are described in 5 subunits (U1-U5) were de- basin wide extent are called “Seismoturbidites” by MUTTI et al. posited by different depositional processes. The authors (OGATA (1984). The term “Megaturbidite” is used by BOUMA (1987) for et al., 2014) indicate that the processes generating the Friuli and thick layers (compared to the host rock), that are laterally exten- Julian MTDs are composed of a) a debris-blocky flow/avalanche sive, different in composition from the host rock, and lack sub- bearing out-sized carbonate and siliciclastic slide blocks (respon- marine fan geometries even if they cover several depositional sible for the deposition of subunits U1 and U2), b) a diluted de- processes (LABAUME et al., 1987). The debrite components in bris/grain flow (deposition of subunit U3), and c) an upper high- beds, labeled as Megaturbidites or Seismoturbidites, are volu- to low-density fully turbulent flow (deposition of subunits U4 and metrically much more important than the derivative turbidite U5). Mass flows can transport and incorporate both intra- and component (PAYROS et al., 1999) and therefore, these terms are extra-basinal material and their formation commonly implies considered to be inappropriate for these deposits (PAYROS et al., processes of stratal disruption and lithological mixing and thus 1999). Some gravity flow deposits with couplets or triplets related forming a characteristic “block-in-matrix” fabric (PINI et al. to debris flows and turbidity currents are called hybrid event beds 2012; OGATA et al., 2020). Thus, the single event beds that dis- (HAUGHTON et al., 2003, 2009; TALLING, 2013). Similar car- play bipartite organization, with each part inferred to reflect a bonate sediments, composed of breccias, conglomerates, bioclas- distinct depositional process, are referred also as MTDs. Among tic arenites, and marls from other regions, especially those from other MTDs, bipartite megabeds, composed of debrite and tur- Dinaric foreland basins, are interpreted as megabeds (TUNIS & bidite parts, are also described in the Dinaric wedge-top basin in VENTURINI, 1992; MARJANAC, 1996; GOBO et al., 2021) or Northern Dalmatia (GOBO et al., 2021). mass transport deposits (OGATA et al., 2014, 2019). In the Cen- In this research, sedimentological and petrographic data tral Dalmatian basin, a slightly younger southeastern part of the about megabeds of the Istrian Flysch is used to shed some light Dinaric foreland basin, MARJANAC (1990, 1991, 1993, 1996) on the environmental conditions, depositional mechanisms, and described 4 types of megabeds: 1) megaturbidites, 2) “reflected” the origin of the resedimented coarse-grained carbonate material. turbidites, 3) composite turbidites, and 4) complex, bipartite type To determine the variations and provenance of the carbonate ma- beds, composed of debrite and turbidite parts. Bipartite beds de- terial, qualitative and quantitative analyses of carbonate clasts scribed by MARJANAC (1996) are considered equivalent to Ol- were performed. Furthermore, the analysis of bioclastic grains istostromes (ABBATE et al., 1970). The debrite part is composed was carried out to determine the original depositional palaeoen- of a) extra basinal clasts – Foraminiferal limestones, b) rolled and vironments of the redeposited bioclasts. The relatively tectonically Geologia Croatica presented inFig. 2. more detailed maps, withintheinvestigated marls Globigerina area, are reclassified flysch andpublishedasamember within theIstrian (BERGANT etas al., 2020) are marls 2009). Globigerina not separated as a lithostratigraphic unit but as a member within the Foraminiferal limestones. However, according to new data and Figure peninsulawiththestudiedlocations 1.The oftheIstrian indicated geological mapofthe Croatian by blackcircles part (CROATIAN GEOLOGICAL SURVEY, regional A 2005). al., et (VLAHOVIĆ platform carbonate atic stones that were deposited on the long-lasting pre-orogenic Adri lime Cretaceous and Jurassic the are peninsula Istrian the on al., 2008; KORBAR, 2009; ŠPELIĆ et al., 2021). The oldest rocks et (SCHMID plate Eurasian (Adria)the and microplate Adriatic ofthe collision the by caused orogenesis, Alpine during formed Dinarides, External the of belt fold-and-thrust the to belongs atic foreland (Fig 1). The NE belt mountain of (Fig.the peninsula belongs it predominantly and to Coast the Adri Adriatic Eastern the of part northwestern the on located is peninsula Istrian The SETTING GEOLOGICAL 2. mechanisms. triggering and models facies material, carbonate the of provenance and composition, the ing search has provided new about data megabeds regard the Istrian re far,this So needed. are analyses biostratigraphic precise and mapping geological detailed more megabeds, studied of the tion correla further For megabeds. some of distribution spatial and composition formation, the in some insight from resulted search re presented However, the modeling. basin relevant for needed evolution. complex foreland basin is of an Istria excellentstudies Thus, the rather location for further by Neogene deposits and/or by the Sea Adriatic (KORBAR, the 2009). in in involved exposed not exclusively since Istria other are centra ofequivalent were parts the belt foreland are covered that fold-and-thrust deposits Dinarides foreland External Dinaric undisturbed 100 There are no deep seismic and exploration wells in Istria Istria in wells exploration and seismic deep no are There

1 - - ) - - - - - KORBAR, 2009). The Foraminiferal limestones are overlain by overlain are limestones 2009). Foraminiferal KORBAR, The the ofmigration the foreland basin to the SW (OTONIČAR, 2007; during 2004) (ĆOSOVIĆal., et ramp the of deepening gressive pro and indicate 2009). in superposition generally are units The 1975; VELIĆ, PLENIČAR, & 2003;al., et VLAHOVIĆ VELIĆ (DROBNEmulitid- and limestones Discocyclina 1977; ŠIKIĆ & Num Alveolinid-, Miliolid-, members: separate four into vided l (ĆOSOVIĆ ramp carbonate et al., 2004) devemarine on a fully deposited were which ĆOSOVIĆ, 2000) & 1977; MARJANAC (DROBNE, limestones Foraminiferal Lutetian Middle-Late to 2007). ĆOSOVIĆ et al., 2008). The Kozina beds are overlain by ĆOSOVIĆ,2000; & beds 1973; MARJANAC POLŠAK, & Kozina (ŠIKIĆ brackish or freshwater the of occurrences localized by of is characterized succession Palaeogene the part lowermost by Palaeogene the successionlain (MATIČEC et al., 1996). The over diachronously are carbonates Cretaceous palaeokarstified since the and deformed the Late Cretaceous, ably during formed 2007). NE-SW,Also, a anticline, was striking broad prob Istrian (OTONIČAR, Platform Carbonate Adriatic the of emersion the and uplift forebulge the during occurred that palaeokarst by marked is unconformity Palaeogene Early and Cretaceous Late This 2007). (OTONIČAR, foreland distal the in deposited ates carbon synorogenic Palaeogene the from succession carbonate shallow-marine the Jurassic-Cretaceous separates unconformity oped oped in the distal ofpart the foreland Dinaric basin (OTONIČAR, The Foraminiferal limestones are often informally subdi informally often are limestones Foraminiferal The Geologia Croatica 74/2 ­Y presian presian ------­ Geologia Croatica - - ­ - - - - 101 For this study, six different megabeds were investigated. The were megabeds different six study, this For ive ive stratigraphic position of each megabed was determined ac The schematic geological column of the Middle Eocene Istrian Flysch of the Middle Istrian Flysch column Eocene geological schematic 2. The Figure neritic carbonates, and Palaeogene and the underlying Cretaceous succession sections. In positions of the investigated stratigraphic relative with the general tervals of the underlying Cretaceous Rudist limestones and Palaeogene Fo Palaeogene and tervals Rudist limestones of the underlying Cretaceous Flysch. for scale given not in a general are limestones raminiferal cording to the data of direct observation in the field, from the published and unpublished geological maps, and the results of studthe that conclude can biostratigraphicpreviousresearch.We ied megabeds are present in the different stratigraphic positions withinandIstrianŠublentica,Kaldir, theThesuccession. Flysch part,Gračišće,the while lower the in locatedmegabedsare Hum Koromačno, and probably the Plomin megabeds are in the mid dle part of the Istrian Flysch succession (Fig. According2). to ŽIVKOVIĆ & theBABIĆ (2003) age of the Gračišće megabed is Bartonian (plankton The foraminiferaKaldir zone P11-P13). the on based age EoceneMiddle a of be to considered is megabed vicinity of the investigated (2007). The ages of other sectionsmegabeds are based on the data bypro ŽIVKOVIĆ & GLUMAC locations of the investigated beds are presented in Fig 1. The rela t ------” Globige ”are clayey fos Marls with crabs Marls with crabs ”. The “ Globigerina marls Based on the plankton and benthic foraminiferal ratio, been Istrianhas fill theforedeep of evolution progressive The Although some published (MIKES and 2008) unpub et al., the so-called beds “ Transitional composed of Petrinjak et al.: Megabeds in Istrian Flysch asmarkers of synsedimentary tectonics within the Dinaric foredeep (Croatia) ” (or ” marls rina “Subbotina(or Marls”) are deposited in deeper en vironments and are considered to be The (JURAČIĆ,clastic 1980). part hemipelagic of the foreland succession is deposits (MARINČIĆ,Flysch as to referred is and foredeep the in deposited In the wider eastern1981). Adriatic region Flysch is recognized lithostratigraphicas major a unit, marks which the beginning of the Dinaric orogenic deformations in the TheKORBAR, whole 2009). area & 1983; ZUPANIČ, BABIĆ (AUBOUIN1970; et al., Istrian Palaeogene succession 1 (Figs. is andprobably depos 2) itedwithin an underfilled peripheralforeland basin as described 2007; by SINCLAIR with (1997), Flysch deposited (OTONIČAR, in the foredeep wedge in Dinaric migrating once the of front KORBAR, 2009). estimated have ŽIVKOVIĆ (2003) & BABIĆ the water depth of the foredeep to about 900-1200 m. The Flysch is characterized depositsgravity-flow and marlshemipelagic alternation theof by (predominantly turbidites) composed of a) mixed carbonate-si liciclastic detritus or b) carbonate detritus (MAGDALENIĆ, 1972; MARINČIĆ, 1981; BABIĆ MARINČIĆ & ZUPANIČ, et al., 1996; 1996; BERGANTate-siliciclastic et turbidites al., show 2003). longitudinal Mixedpalaeotransport carbon di rections toward the ESE which led to the conclusionterial that was supplied from the the rising Dinarides ma and from the Alps (MAGDALENIĆ, 1972; MARINČIĆ et al., 1996; BABIĆ & ZUPANIČ, 1996). These turbiditesposited by low density areturbidity regardedcurrents (MARINČIĆ aset al., turbidites1996). Carbonate de beds areoften thinpartite turbidites beds composed of butbreccias, alsoconglomerates, thick,bioclastic bi arenites/siltites, and marls (BABIĆ & ZUPANIĆ, BER 1996; palaeotransport a directionshow towardsand 2003), al., et GANT the NNE, generally perpendicular to the SE direction mixed of carbonate-siliciclasticthe turbidites (BABIĆ & ZUPANIČ, & conclude BABIĆ ZUPANIČthat1996). (1996) the carbonate detritus was supplied from the foreland upliftsitu (forebulge), subaerialcharacterizedandsouth, carbonateshoals the toby ated exposure. describedandisdocumented MARINČIĆtheas by (1996) al. et Istrian Flysch – a succession of clastic rocks characterized by a coarseningand“distal”trendupward outlined and of with zones “proximal” Flysch. According to MARINČIĆ et al. the (1996), aroundestimatedbe isto succession Flysch the thicknesstotal of 2). m (Fig. 300-350 siliferous wackestones and packstones (ŠIKIĆ & PLENIČAR, 2005), al., et 2000; TARLAO ĆOSOVIĆ, MARJANAC & 1975; carbon neritic the from transition gradual the during deposited ate ramp to the deeper-water distal “ foredeep. Massive and “ lished studies argued a for Late Oligocene or even Miocene age researchthe of majorityIstrianDalmatianthethe andFlysch, of ers confirm a Middle Eocene age (AUBOUIN KRAŠENINNIKOV et al., 1970; et al., 1969; 1968; MAGAŠ, PICCOLI 1973; POLŠAK & & PROTO ŠIKIĆ, DECIMA, 1973; ŠIKIĆ & POLŠAK, 1973; ŠIKIĆ & PLENIČAR, BENIĆ, 1975; 1991; ŽIVKOVIĆ & BABIĆ, 2003;proposed BABIĆ age EoceneMiddle acceptedthe etWe 2007). al., GLUMAC, 2007; ŽIVKOVIĆ & by BABIĆ et al. (2007) and ĆORIĆ et al. (2008), since theirwas obtained data from the investigated logged successions and by more than one biostratigraphic method. Geologia Croatica 102 Geologia Croatica 74/2

Figure 3. Division I clast composition. Geologia Croatica

------103

7 4 1 1 2 0 3 3 4 1 1 2 2 3 3 6 9 12 (%) Not determined 5 9 4 5 4 4 3 8 2 8 14 10 10 14 11 10 19 Rotallid and hyaline and hyaline foraminifera foraminifera fragments (%) fragments 10 11 Mollusks (%) 9 (%) Encinoderms 8 3 0 0 0 5 2 10 3 2 3 0 61 4 1 0 0 01 3 1 0 0 1 2 0 2 5 0 2 2 0 4 2 0 (%) Bryozoans 7 23 24 11 17 20 17 28 31 24 12 13 18 (%) Red algae scopically. The categories are based on the lithostratigraphicthe on unitsbasedare categories The scopically. presented on the Geological map (Fig. The1). categories are: Upper-, Lower-, or undifferentiatedCretaceous limestones,Fo raminiferal limestones, Flysch rhodoliths,(Marls), and undeter observeddiameter), incm rhodoliths(>5 minedlargeThe clasts. in are megabeds two added to separate they although categories are not attributed to any lithostratigraphic unit presented in Fig The 1. Cretaceous clasts were classified on the basisof theirfo raminiferal content according to VELIĆ (2007). Foraminiferal limestones were additionally separated according to their pre Al Miliolid-, the foraminiferabenthicdominant largeof content veolinid-,Nummulitid- Discocyclinaor limestones asdescribed ŠIKIĆ (1973); POLŠAK, & ŠIKIĆ (1973); ŠIKIĆ, & POLŠAK by & VLAHOVIĆ PLENIČAR, & (1975); VELIĆ, and(2009), an Fo foraminiferal additionalAlgalof Clasts limestones category. cor and raminiferal (rhodoliths algae limestonesredabundant in alline debris) were separated out as Algal foraminiferal lime stones. The Foraminiferal limestones with abundant glauconite no had that Clasts beds. Transitional as categorized were grains distinguishable features were labeled as “not determined”. Pho tomicrographs of the most characteristic clasts for each bed are presented as a supplement (Plates 2,1, andLimestone 3). clasts were distinguished according to classifica DUNHAM’s (1962) (1971). tion & modifiedKLOVAN by EMBRY fossil fossil content was determined by hand lens examination in-situ analyzedwere microaddition,samplesIn some possible. where 6 0 0 0 0 0 0 0 1 9 1 2 0 0 2 9 1 2 3 2 9 1 4 0 0 7 0 1 0 (%) - - ­ - Encrusting foraminifera foraminifera

5 5 0 9 0 9 0 2 0 7 0 6 0 3 0 (%) Nummulitids - 41 29 11 27 18 26 28 33 11 31 25 16 13 21 13 54 35 33 35 3236 26 15 35 27 54 52 14 37 tophrag minids (%) Or

0 (%) ement C

4 0 6 0 5 0 4 0 0 0 4 0 00 0 0 8 0 2 0 (%) Matrix 1 2 3 4 1 5 0 6 9 0 8 7 0 5 6 1 6 5 1 3 4 1 13 11 16 27 22 13 17 21 10 (%) Lithoclasts PL-4 12 PL-3 11 Šub-8 16 Hum 2 Group/ Sample KOR-II I KOR-II

Šub-II 1 Šub-II 3 KOR-II 2 KOR-II Hum 5a Hum 3B Hum 5b Kaldir 1a Kaldir 1b

Hum 3A 1 Hum 3A 2 Gračišće 1 Gračišće Gračišće 2 Gračišće 1) Analysis 1) of coarse-clast composition of breccia, referred calcirudites calcareniof andcomposition the Analysisof 2) 1) To determine the clast compositions representative for Di representativecompositions determinefor clast the To 1)

Koromačno Gračišće Šublentica Kaldir Plomin Hum es, referred to as Division II. Log Results of Division II bioclastic material analysis. 1. Results of Division II bioclastic material Table to as Division I; 3. METHODS During the field investigation, information on boundary types, thickness,sediment composition,layer graincontent, fossil size, and internal structures was collected and presented on graphic Because the recognizedlogs. of general compositional variation within all the studied megabeds, two different methodologies were used: ing to ŠIKIĆ & PLENIČAR (1975). Since the informationthethe Sinceon (1975). PLENIČAR & ŠIKIĆto ing not correlationis precisetheirscarce, somewhat is age megabeds yet possible. a For better understanding the of stratigraphic rela tionships between the megabeds more precise dating is needed. vided on basic geological maps: the Plomin megabed – Middle the Koromačno Eoceneaccording to ŠIKIĆ (1973), POLŠAK & (1973), MAGAŠ accordingEoceneto Upper to Middle – megabed POLŠAK accordingEoceneto Middle – megabed Šublentica the accordEoceneMiddle – megabed Hum the and (1973), ŠIKIĆ & Petrinjak et al.: Megabeds in Istrian Flysch asmarkers of synsedimentary tectonics within the Dinaric foredeep (Croatia) t vision I of each megabed, the most exposed parts of the outcropsexposedpartsthe mostmegabed, theeach of of I vision were selected. Between 200 and 300 clasts within the selected areas were classified and counted. Theclasts were categorized based on their fossil content and lithological characteristics. The Geologia Croatica Facies A. Facies than thickness total a and greater clasts, rip-up with breccia rich Faciessparse marl matrix. B is a chaotic, matrix- marl dominated, with breccia supported clast a is A Facies B. Facies and A cies directions are ripple marks and a flute cast. aflute and marks ripple are directions palaeotransport measure to used structures The (Hum). location one in only measured been have directions palaeotransport the grains. 200-300 by point-counting estimated was stituents grains. non-determinable ans, 10)9) echinoderms, molluscs, 11) and 12)hyaline fragments 7) red algae, 8) bryozo foraminifera, 6) encrusting nummulitid, 5) orthophragminid, 4) sparite, – cement 3) micrite, – matrix the (2004).FLÜGEL in described method components in section thin was estimated using a point-counting debris. skeletal of predominantly composed calcirudites to calcarenites bioclastic as determined log. each lo on marked are sampling cations The microscope. petrological standard by analyzed and prepared were samples 17 selected of sections thin enites, studied megabed in Table in megabed 1.studied each for given is calcarenite and calcirudite 2 Division analyzed Division I is breccia given in Fig 3., while the composition of the all for composition Clast structure. bipartite internal and tion, composi thickness, their by succession Flysch of deposits flow All the from other six gravi megabeds are studied distinguished RESULTS4. Figure 4.The Šublentica column shows anunconformable (erosional) oftheCretaceous contact limestones andtheoverlying ŠublenticaFlysch). megabed(Istrian 104 constituents were classified into 12 groups: Due to sparse directional structures in megabeds, the Istrian Due structures to directional sparse Within Division 1, two facies types are distinguished, as Fa as distinguished, are types facies two 1, Division Within 2) To determine the composition of calcirudites and calcar and calcirudites of composition the 2)To determine The modal distribution of the main con main the of distribution modal The The Division II samples were were samples II Division The The modal distribution of the the of distribution modal The During point-counting, point-counting, During 1) 2) lithoclasts, ­t y y ------logged near the town of Hum (WGS84 coordinates: lat 45°21’ lat coordinates: (WGS84 Hum of town the near logged was fill basin foredeep Palaeogene the of interval thick m 40 A 4.2. TheHumSection beds (Fig. marl calcirudite and graded normally amalgamated 4). of alternation the with ends section 1D.The Plate in illustrated in the constituents investigated samples are shown in Table 1 and (11-24%), all ofalgae (6-16%). proportions lithoclasts and The (11-28%), (26-29%), nummulitids red orthophragminids are rial of The calcarenites. most ofcommon constituents Division II mate consists graded which normally to up-section and interval II calcirudite graded normally Division the to transitions gradually breccia The C. and B, A, 1 Plate in presented limestones ceous Creta and of Foraminiferal photomicrographs microfacies clast 3a shows the composition of the Division I with selected breccia, Fig. orthophragminids). and (nummulites foraminifera benthic 2%) is are less The composed ofmatrix abundant. marl and large glauconite; with (limestones beds (18%) Transitional clasts and (40%)limestone Cretaceous while undifferentiated predominate, clasts limestones Foraminiferal (40%) and limestones taceous Cre Upper size. 0.1 in m 1 from to are clasts Carbonate ported. sion is 1.5 I interval breccia and m clast poorly sup thick, sorted Divi The megabed. of the deposition the with synchronously or to prior carbonates neritic underlying the of collapse partial the is and discordant erosional (Fig. 5) and tilting tectonic suggesting boundary (Fig.4). The limestones Rudist Cretaceous Upper the overlies and section the in megabed this directly was recognized lat coordinates: 45° 19’ 2.3808”, long 13° 48’ 29.8404”). A hiatus (WGS84 section Šublentica the at described is limestones ceous Creta the and Flysch Istrian the of contact unconformable The 4.1. TheŠublenticaSection Geologia Croatica 74/2 ------Geologia Croatica - - - 105 Division II is composed of normally graded of calcirudite and composed IIis Division diameter. Boulders/clasts are mostly (sub)rounded although rarelyalthough (sub)rounded mostly areBoulders/clasts diameter. some angular clasts occur. The Division I conglomerate/breccia is clast supported with a sparse matrix (referred to as Facies A, Matrix is composed 7A). Fig marl,of small lithoclast fragments foraminiferalorthophragminidandand (nummulitid debris tests). The major lithological components are Cretaceous limestones while (88%) Foraminiferal limestones and (9%) other constitu entsare (3%) less abundant. Interestingly, the Lower Cretaceouslimestone clasts were only observed in this megabed. The clast composition is presented in Fig. 3B and selected photomicro ForaminiferalCretaceousandUpperlimestone Lower-, graphsof clast microfacies are and presented G. in F, 1E, Plate calcarenite with the following composition: orthophragminids nummulitids and as red (32-54%), (6-26%), pre algae (9-13%), - - - Geological column showing the 10 m thick Hum megabed (3) and carbonate turbidites (1, 2, 4 & 5). turbidites the 10 m thick Hum megabed (3) and carbonate showing column Geological Figure 6. Figure Photograph of the Šublentica outcrop showing a discordant and erosional boundary between the Cretaceous limestones and overlaying megabed. boundary and overlaying limestones erosional and the Cretaceous between a discordant of the Šublentica outcrop showing 5. Photograph Figure 6). The Fig. interval 42.5466”, 2’ is long 14° composed 3.5706”, prom most carbonate beds. The five and marl alternation anof of inent bed is the 9 m thick megabed Hum 3 (Fig. 7A) which shows shows which 7A) (Fig. 3 Hum megabedthick m 9 the is bedinent the characteristic bipartite structure with visible Division I and Division II components. The other carbonate turbidite beds (la are composed normally of Fig.6) on 2,5 4, graded Humbeled 1, m). 3 – thicknessescalcirudite/calcarenitevariousbioclastic(0.6 of The lower bedding surface of each bed is sharp and erosional, implying that the gravity flow eroded previously deposited basi nal sediments. The Hum 3 megabed is composed of Division I breccia/conglomerateinthelower/bottom part, andtheDivision normallygradedcalcirudite/calcarenite– part. upperTheII the in maximumlaterallythethinningthickness of breccia/conglomerate interval is 5 m, and the lower bedding surface is erosional. inThe m 0.75 to cm few a from vary in size clasts visible of majority Petrinjak et al.: Megabeds in Istrian Flysch asmarkers of synsedimentary tectonics within the Dinaric foredeep (Croatia) Geologia Croatica Figure megabedcolumn. 8.The Kaldir on plane observed the marks on bedding upper the Hum 3 mega ripple 195°on and 6), (Fig. turbidite 2 Hum the of plane ding bed lower the on observed casts flute from 165°measured was of direction palaeotransport A same. 3)the was (Hum megabed and 5), and 2 (Hum turbidites the for source detritus carbonate (Table proportions different 1). but in constituents, main same the with calcarenite and calcirudite graded normally of composed also are beds 5 and 2 Hum Table 1. The in sented is 32cmlong. breccia/conglomerateclast supported breccia matrix. withsparsematrix; (B)Plomin withsparsemarl megabedDivisionI-clastsupported The geological hammer Figure 7.The outcrop photographs oftheDivisionIdeposits, breccia showing matrix(Facies withsparsemarl clastsupported A). (A) 106 Based on the uniform composition, it is assumed that the the that assumed is it composition, uniform the on Based - ­ are presented in Plate 2 A, B, C. The majority of the Foramini the of majority The C. B, A, 2 Plate in presented are limestones Foraminiferal and Cretaceous Upper of microfacies clast while 3C,Fig. in presented is composition The observed. clasts stones 1%.9%, and undetermined Also, a bauxite clast was lime Foraminiferal follows:90%, limestones Upper Cretaceous as is composition clast The I. of Division part upper the towards visible is gradation normal A distances. transport short cating indi angular, are Clasts debris. rock fine-grained of composed matrix sparse a with unsorted supported, clast is breccia The 10 of x 5 10A). (Fig. m dimension visible a has and sequence the Kaldir of the at bottom the marl on underlying the clast directly lies largest between The and erosional. and size sharp contact is marl The clast underlying the in and megabed content. marl difference low very and significant a shape displays I Division Kaldir the megabeds, other the to compared When 9.Fig. on shown is megabed Kaldir ofthe distribution geographic The II). vision in the upper the (Di calcirudite/calcarenite part graded normally and lower the in part I) (Divison breccia of limestone composed 51’ 16.9056”). This megabed (Fig. 8) is approximately 25 m thick, village (Fig. 1, WGS84 lat coordinates: 45° 18’ 43.1598”, long 13° The outcrop studied of the megabed Kaldir is located near Kaldir 4.3. TheKaldirSection (Fig. 6). Section Hum the in measured as S, the wards to generally locally was megabeds proximal more the of port palaeotrans that indicate assessment, statistical for insufficient b ples are shown in Table 1 and illustrated in Plate 2D. in Table in shown ples are illustrated 1and (14%). of The proportions in all the constituents investigated sam (23–24%), hyaline fragments including and foraminifera rotaliid (35–41%), algae red orthophragminids are: material bioclastic of constituents common most The gradual. is unit II) (Division calcirudite/calcarenite 3 m thick graded a normally to transition The breccia. the in deposition and resedimentation their during some ofwere not consolidated limestones the Discocyclina fully that indicates phenomenon 10B). (Fig. This packed tightly are plastically deformed between the larger boulders and Cretaceous are (plasticlasts) intraclasts limestone Discocyclina ofSome the f eral limestone clasts are classified as Discocyclina limestones. limestones. Discocyclina as classified are clasts limestone eral ed, and 230° on turbidite Hum 5 (Fig. 6). These data, though though data, These 6). (Fig. 5 Hum turbidite on 230° and ed, Hum megabed Division I - HummegabedDivisionI- Geologia Croatica 74/2 ------­ Geologia Croatica - - - 107 sition is presented in Fig. 3D and the selected Algal foraminiferal foraminiferal Algal selected the and 3D Fig. in presented is sition limestonemicrofaciesclast photomicrograph presentedis in Plate Thematrix2E. marl,mixfragmented a of is andsmallerbioclasts foraminiferalmostly(nummulitids debrisare Bioclasts lithoclasts. and orthophragminids) and rhodoliths.ameter, while marl Most clasts are significantly clasts larger and can be are up to < 10 cm di 50 cm acrossThe (Fig. 12B). Division I breccia gradually transi cemented,normallygraded calcirudite/calcarenitewell a intotions intheupper partthesection. commonThemostof constituents of nummulitids orthophragminidsmaterialIIare: Division (31-35%), (11-25%), red algae (17-20%), and lithoclasts (8-16%).all Theconstituents ratiosin the investigated samples of are shown in 1 Table and illustrated in 2F. Plate differentiated Cretaceous limestones (1%), Transitional beds (1%) (1%) beds Transitional (1%), limestones Cretaceous differentiated and undifferentiated areless represented.(1%) clasts The compo - - - The 10x5 m boulder of the Upper Cretaceous limestone in the base of the Kaldir megabed. The contact the between The the Kaldirthe base of in limestone megabed. Cretaceous Upper boulder of the 10x5 m The The Kaldir Division I breccia. (A) Kaldir10. The Division I breccia. Figure the between clasts compressed as plastically deformed occur Discocyclina limestones The Kaldir (B) megabed and the underlying marls is sharp and erosional. hammer head is 19 cm long. geological The clasts. Cretaceous larger carenite in the upper part, with a total thickness of 40 m (Fig. 11). carenite in the upper part, 11). with total a thickness (Fig. m 40 of The lower breccia interval is 25 m thick, lenticular in shape, and pinches out laterally. This breccia is matrix supported with pre abundantclastsmost The 12A). dominantlysub-angular (Fig. clasts are Foraminiferal limestonesand marlwhile(75%) clasts (17%), unUpperCretaceous rhodolithfragmentslimestones (2%), (3%), 4.4. The Gračišće Section Gračišće of town the south-east of located is GračišćemegabedThe (WGS84 coordinates: lat 45° 12’ 59.6844”,The contact with the underlying marls long is covered vegetationby 14° 0’ 29.7”, Fig. 1). thereforeGračišće is beddingthemegbed surface and of lower the not visible. The megabed is a bipartite bed with chaotic breccia (Division I) that is matrix lower part supported and a well cemented, normally graded (referredcalcirudite/cal to as Facies B) in the Geological map showing the Kaldir map showing 9. Geological (2020). distribution,modified after megabed spatial BERGANT et al. Figure Petrinjak et al.: Megabeds in Istrian Flysch asmarkers of synsedimentary tectonics within the Dinaric foredeep (Croatia) Geologia Croatica with average bed dip azimuth and dip angle of 150/40. The lower lower The underlying the 150/40. with of contact the and megabed Plomin of the part angle dip and azimuth dip bed average with fault (Fig. 1) relatively are inclined beds the striking N-S nearby the along deformation tectonic of 19.5198”).consequence a As 45.6612”,45°8’ lat 14°10’ long coordinates: (WGS84 village Plomin of NW cut road the on out crops megabed Plomin The 4.5. ThePlominSection Figure 11. 108 larger thanthelimestone clasts. The geological hammeris32cmlong. erosionbecause oftheselective clasts. ofmarl (B)Photographs clastswithin DivisionI,Facies ofmarl BoftheGračišće megabeds. The clastsare marl significantly Figure 12.Photographs The outcrop oftheGračišće megabed(A) oftheGračišće divisions. megabed withmarked The holes(casts)withinDivisionIare formed The Gračišće megabedcolumn. Transitional Transitional beds 2%, clasts marl 3%.2%, and The undetermined 93%, limestones Foraminiferal is breccia the of composition clast is The measured largest 50 cm in diameter. The lithological cm. of 20–30 axis longer average an with clasts of boulder-sized Division in the upper II (Fig. carenite 13). is breccia The composed calcirudite/cal bioclastic graded 17 normally of m and 7B) (Fig. is bed studied The colluvium. and vegetation by covered is marls

c omposed of a 3 m thick Division I breccia in the lower part part lower the in breccia I Division thick m 3 a of omposed Geologia Croatica 74/2 - Geologia Croatica - - - ­ ------F 3

109 The lower Division I (Facies B, intervalFig 15B) is com In the middle part, labeled as the Transitional zone, the con themiddlezone, part,the TransitionalInthe as labeled išće megabed, the matrix is a mix marl of and fragmented bio 4.6. The Koromačno Section 4.6. The Koromačno The Koromačno section of the Istrian Flysch Koromačno quarry is (WGS84 locatedcoordinates: lat in 44° 10.8366”, 58’ Besides Division 53.94”). 7’ I andlong 14° Division II, a Transi tional zone is outlined in and the Koromačno megabed 14 (Figs. hascharacteristicszoneTheTransitional lower theboth of 15A). Division I and upper Division II. The characteristicsDivision of arelargeI marlclasts (largest measured 1 axis with clastof long Thesimilarity metre). withthecompositheupper IIis Division bedding lower The fabric. calcirudite graded distinctly a of tion surface of the Koromačno megabed is not exposed at this loca tion, but underlying calcarenite and marl beds are recognized in the vicinity. clasts (nummulitids and orthophragminids and rhodoliths). and selected Foraminiferal limestone clasts microfacies photo č posed of a matrix supported breccia with large clasts of marl andmatrixmarl a supportedof clastsof posedlarge withbreccia limestone lithoclasts. The clast composition is: Foraminiferal limestonesandun 80%,Transitionalmarl bedsclasts 16%, 3%, presentedFig. inis composition clast Thedetermined 1%. clasts micrographs are presented and As G. in in the 3 E, Gra Plate F, tent the of marl matrix rapidly decreasesand well cementedcal ­ ­ ­ ------upported, ­s um clast cmsize in10 of diameter. The most common consti eral debris (nummulitids and orthophragminids)and(nummulitidsdebris eralrhodoliths. and uents of bioclastic material are ortophragminids (35%), nummulitids ortophragminids materialare bioclastic (35%), of uents The Plomin megabed column. Plomin 13. The Figure m t The pro and red algae (7–18%). lithoclasts (11–12%), (27–33%), The carbonate breccia (Division I, Facies A) graduallytransitions A) Facies I, (Divisioncarbonatebreccia The to m thick a 17 normally graded calcirudite/calcarenite (Divison II) and the matrix content decreases. Calcirudite is a well ce crographs of Foraminiferal limestone clast microfacies are pre clast mostly is breccia The C. and B, 3A, Plate in sented f maximatrix,marla witha normallymented,withoutgraded bed portions of all constituents in 1 Table and illustrated in in Plate Marl3D. theclasts occur investigatedin the par shape, elongated an calciruditehave thezone, part of middle samples arethick.cm shown 10 – 6 to up and long 30-40arecmorientation, and allel The size of the marl clastspart graduallyof the calcirudite zone. Calcirudite gradually transitions decreases to calcarenite towards (normal gradation) with visible parallel thelamination upper and indistinct cross lamination unsuitable for palaeotransport measurements.masthisa withendsupperThemegabedpart of intervalan by overlain is megabed interval.Plomin Themarl sive thin and marl of composed Flysch, continuous of m 20 about of carbonate and mixed carbonate and siliciclastic turbidites. clast composition is presented in 3E Fig. and selected photomi Petrinjak et al.: Megabeds in Istrian Flysch asmarkers of synsedimentary tectonics within the Dinaric foredeep (Croatia) with a sparse matrix. The matrix is a mix of marl, fragmented bioclasts and smaller lithoclasts. Bioclasts are mostly foramini Geologia Croatica tion. Weak normal grading is recognized in the upper part of the the of part is upper com matrix the The Šublentica at and division Hum sections. the in recognized is The 3). grading (Fig. normal Weak beds tion. investigated the of each in marl (rip-up) clasts in Facies A are found only in the Plomin sec different is clasts the Plomin, and Šublentica The lithological sections. composition of graded, with sparse matrix. This facies is observed at the Hum, s as debris-flowdeposits thatinterpreted suggest “ are both and recognized are types facies two I Division Within 5.1. DivisionI II). (Division part upper the in deposit current turbidity density high- a to transition gradual a with I) (Division part lower the in ofcomposed flow debris deposits bipartite, are beds studied The 5. INTERPRETATIONS Plate 3H. in Table in shown illustrated 1and are samples investigated the in constituents the ofall ratios The (16-21%),(13%). (22-27%),nummulitids and orthophragminids (28-31%), algae red are: lithoclasts material II Division of ents constitu common most The calcarenite. and calcirudite graded marked by the absence of marl clasts. It is ofcomposed normally follows is and zone Transitional the interval Division II The ual. grad are parts these between Transitions up-section. decreases size clast Marl m. 1 to cm of1 dimensions axis long with clasts marl outsize are clasts largest The dominant. becomes cirudite 110 Figure 14.The Koromačno megabedcolumn. ition after cohesive freezing. cohesive freezing. after ition Facies A is normally a clast conglomerate/breccia, supported

en masse en ” depo ” - - - - ­

avalanche. rock- submarine large-scale a flow,probably debris cohesive a to different mechanism transport a indicates matrix of lack The distance. transport short very a suggest shapes clast angular and 5x10 is clast m) measured (largest size clast The matrix. of lack clast total thickness, size shape, properties: and and angular total the evolution flowand the of mass flow.of the composition initial the in differences to related be might content marl the and matrix the in differences Observed flows. debris flows, while Facies Baredeposits related to moremarl (mud)rich debris clast-dominating to related are deposits A Facies2009). al., et FELIX 2006; SHANMUGAM, 2001; ALEXANDER, 1972; 1982;& LOWE, MULDER (HAMPTON, currents bidity tur generating for mechanisms the of one be to considered are mations from debrite flowas the tocurrent, turbidity debris flows nids) and rhodoliths. nids) and are debris and (nummulitids mostly orthophragmi foraminiferal Bioclasts lithoclasts. smaller and bioclasts fragmented marl, of mix a is matrix The Gračišće. and Koromačno at observed is B tinguished is the amount of matrix and the marl content. Facies dis facies are by which described the cies characteristic A. The Fathan thickness total and clasts, a rip-up greater with outsized minids. posed of marl and bioclasts, mostly and orthophrag nummulitids The The Division I at megabed Kaldir is singled out for its unique The bipartite organization is a consequence of the transfor Facies B of breccia Division I is a chaotic, matrix-supported Geologia Croatica 74/2 ------Geologia Croatica ------111 arl clast within Division I breccia, Facies B part of Koromačno megabed. The B part megabed. of Koromačno Facies arl clast within Division I breccia, M lated banks, characterized by strong water currents and reduced sediment input (RASSER & PILLER, 2004; BARATTOLO et al., 2007). The lithoclasts observed are grains of older limestoneCretaceous-Palaeogenethefrom(ex limestonerockssuccession wacke abundantextraclastsor mudstonesmost are The traclasts). stones containing sparse unidentifiablebioclasts. The matrix is micrite while sparite (Group 2), cement The is scarce (Group 3). Large benthic foramniferal taxa are assigned to groups 4, 5 and The11. other skeletal grains are grouped accordingThe calcirudite toand theircalcarenite microfacies taxa. of Division II with abundantmost theconstituents D, of I presentedsome is Plate in F andH, II Plate III Plate D, H. D, thophragminids, nummulitids, red algae, and lithoclasts. This specific compositionshows aclear predominanceof bioclastic carbonatematerialouterenvironmentsrampthefrom derived as described in Eocene carbonate ramp models in HALLOCK & GLENN (1986), ĆOSOVIĆet al. (2004),and BASSI coralline (2005).The rhodolites were probably formed along intrabasinal, iso 6. DISCUSSION 6.1. Palaeotransport directions of provided(Istrian)PazinBasinpalaeotransport the the direction In for model palaeotransport the (1996), ZUPANIČ & BABIĆ by carbonate beds composed debrite of and turbidite unit is towards theNNE. Also, BABIĆ & ZUPANIČ of land and shelf (1996) environments to suggestthe SSW of the former Pazin the existence Basin from where the carbonate material was derived. Since the palaeotransportthestudied meg aboutthedata indirections found abeds is scarce, we can assume that the major source of carbonate sourceof assume can major thethat scarce, abedswe is material were fault dissected or a locally over steepened TEIXELL,distal& BARNOLAS SINCLAIR,1997; (cf. slopes foredeep that 1994), were located along the southern margin of the Istrian Flysch basin, as proposed by BABIĆ & ZUPANIČAc (1996). bydeposited werestudied megebedsthe all model, that cordingto gravitational flows on the distalslopes of theforedeep, along the ­ ­ - - - - - ene neritic carbonate succession that underlies the Eo- ­g o ­e nised in the Kaldir megabed. The karstifiedUpper Cretaceous ed, categorized according to the previously defined major 5.2.1. The composition of Division II 5.2.1. As result of point-countingDivision II interval, calcirudite/calcarenite 12 different constituents are distinguished and grouped. samples of The the results II are or grains abundant most the that in Division shows 1) ble of Division II composition analysis (Ta limestone boulder with bauxite karst infills has been described by in TARLAO et a al., (1995) debrite within the Istrian Flysch southwest of Pazin (Fig. The1). palaeo-karstifiedCretaceous limestones and bauxite clasts are related to the regional uncon lithostratigraphicunits, are Foraminiferal limestones andUpper Cretaceous Rudist limestones, as shown in Fig. 1. The Lower Cretaceous clasts are only observed in the Hum megabed. Rare reco only karstifiedwere andbauxiteCretaceous clasts limestone g 5.2. Division II Division II intervals are up the to all in m 10 seen thick, are and clast supported, and calcirudite/calcarenites graded, normally grad a intervalis therethe of top the Towards studiedmegabeds. er part (Division I) gradually transitions into the transitional zone, composed of cemented calcirudite breccia with large marl clasts (Transitional zone). The normal The marl zone). clasts (Transitional with large breccia calcirudite of cemented composed zone, the transitional er part into transitions (Division I) gradually (Division II). (B) the top lacking at calcirudite marl clasts is located outsized ly graded outcrop. clast dimension is 2 x 1 m at b formity within the Adriatic Carbonate platform succession that was formed during the Late Cretaceous and Early Palaeogene 2007). emersion the (OTONIČAR, forebulge of transition, eventually calcirudite ualcalcarenite, first and from to to calcisiltite grain size. In the uppermost part of Division II, horizontal and cross lamination are observed in calcarenites or over is II Division megabeds.Plomin and Hum the incalcisiltites lain by a massive marl deposited from the turbidity current tail. co-geneticturbiditeinterpretedisa megabedsasthe of 2 Division deposited a high- by to low-density current (LOWE, 1982). The studied clast composition indicates that Division I material was (clasts) derived from different partsof theCretaceous to Pala determinedcommonclasts most The mega Flysch. eachcenein 5.1.1. The composition of Division I clasts I Division of composition The 5.1.1. Outcrop of the Koromačno megabed showing two divisions and a transitional zone in the middle. The chaotic, matrix supported in the low chaotic, The breccia in the middle. zone divisions and a transitional two megabed showing of the Koromačno Outcrop 15. (A) Figure Petrinjak et al.: Megabeds in Istrian Flysch asmarkers of synsedimentary tectonics within the Dinaric foredeep (Croatia) Geologia Croatica genic wedge. Notto scale. discussion inthetext. See faultinginthedistalforedeep. andnormal tectonics (B)Proximal megabeds are related to compressional andreverse tectonics faulting infront oro oftheDinaric Figure 16.Two possible optionsofschematic Allmegabedsare foredeep reconstructions oftheIstrian related megabeddeposition.(A) basin after to extensional synchro a was detritus bioclastic the of area source The II. vision Di of calcirudite/calcarenites bioclastic of grains as and matrix, the in marl with mixed I Division in observed is and megabeds studied the of constituent a is detritus bioclastic Shallow-water 6.2. Thesourceofshallow-waterbioclasticdetritus (Figs 16 17). and S the towards transport minor and N the towards direction port trans major the with directions, transport and sources multiple ZUPANIČ, 1996), we had can that assume the detritus carbonate & (BABIĆ research older and section) Hum on direction port S. the towards locally was beds mega proximal more of paleaotransport the data this to cording from the in thick the turbidites carbonate succession (Fig. 6). Ac were derived from only one megabed on the Hum section and two N. the towards was transport the indicating S, the to zone the between transitional basin and the ramps carbonate distal 112 Combining the Combining this observed study data within (palaeotrans study this within measurements However,palaeotransport ­ - - - - - the Istrian Flysch the Istrian basin from the South. However, there is also the that outer-ramp were from synchronous to environments attached lead to came the conclusion that the bioclastic raminifera detritus fo characteristic these of absence The study. this in recognized were grains such no but tests, miliolid or alveolinid e.g. strata, position would include fossils from limestone other Foraminiferal If limestones. that were weathered Foraminiferal the case, its com older, from stratigraphically that originated the bioclastic detritus argued be can it1997; Alternatively, 2005). BOSENCE (SINCLAIR, ramps and platforms carbonate large of accumulation of margin distal the The site for favoured a is basins foreland underfilled 2007). low-latitude, (OTONIČAR, basin foreland Dinaric (forebulge)the of part marginal distal, a as South, the in is ramp the for location probable northern more a the basin, in Flysch Istrian the of part recognized not are deposits thrust-top and thrusts 2018). Since al., et (ĆOSOVIĆ Dalmatia Northern in scribed folds, de as thrust and ENCE 2005) or developed on overthrusts on forelandan basin underfilled margin 1997;(SINCLAIR, BOS (Fig. south 16).the nous shallow water to ramp carbonate attached Flysch basin from Carbonate ramps in a foreland basin setting could be formed could be formed a in foreland ramps setting basin Carbonate Geologia Croatica 74/2 ------Geologia Croatica ------113 and a significant proportionrich debris-flow deposits of withmuddya minor proportion matrix;of cohesive and matrix.faciesThecarbonate megaturbidites described LABAUME by F2: clast- from theEoceneGroupFore (SW-Pyrenean Hecho (1987) al. et mega a of composedand thick m 200 to up are Spain) Basin,land breccia unit in part the lower and the turbidite in the upper part. The megabreccia unit is subdivided into separate divisions: a organizedsupportedI)(Divisionpoorly brecciaclast and largely and carbonate breccia which contains an abundance of rip-up marlstone slope and basin clasts of plainturbidites (Division II). TUNISVENTURINI& describe megabeds intheJulian (1992) basin as couplets of megabreccia in the partlower and a graded megabreccia two into calcarenitesubdivided further is and marlmegabeds of in thetions upper part.The describedunits and verticalthree turbidite units. Unit sec l is a megabreccia which mainly consists blocks big of shallow of water limestone olisto recognizedbe can carbonate megabreccia, but also is 2 Unit liths. theby largelack of limestone olistoliths, the for numerous disc- rip-upsiliciclas the for and calcareousmudstone of clasts shaped ticturbidites with et interbeddedsome calciturbidites. PAYROS recognized downcurrenta incompomegabedchange(1999) al. part,proximaltheimmature, fromin debrite homogeneoussition differentiated, the turbidite a a in bipartite two-storey debrite and middle part, and base-missing a turbidite, by debrite or overlain a even turbidite alone, in the distal part. The megabeds contain ing bipartite breccia outcrops show that each unit consists of a - - - MAGDALENIĆ (1972), MAGDALENIĆ palaeotransport taken from The are direction data palaeotransport map showing 17. Geological Istrian Flysch. directions in the Eocene Figure (1996). (1996) and BABIĆ & ZUPANIČ et al. MARINČIĆ 6.3. Interpretation of the internal divisions and compa- divisions and of the internal 6.3. Interpretation previous works rison with As far as sedimentary processes are concerned, the megabeds may be considered as complex beds which were deposited by a range sedimentaryof processes submarine e.g., rock avalanche, debris andflow, high-density turbidity currents. As mentioned earlier, the distinct bipartiteflow organization transformations from isdebrite a consequenceflow ALEXANDER, & MULDERto 1982; turbidityLOWE, of the(HAMPTON,1972; current SHANMUGAM,2001; 2006). This distinct architecture can be related to beds with similar internal organization. Comparison with hybrid event beds bipartite described (HAUGHTON megabedstheindicates that et 2013) TALLING, al. 2003, 2009; tripartitein present is that layer basal lackingthe arestudythis in describedevent thethanhybrid thicker much are andbeds, hybrid beds in papers byfaciestypesI TheDivision described HAUGHTONherebear compari (2013). et al., (2003,literature.published thefrom megabeds of divisions the 2009) with son and TALLING The megabeds described in AMY et al. (2007) from the Pïera Cava basin FRANCE)(SE are very similar to the Istrian meg differentiateauthors the describedabedstwo researchthisand in typesfacies withinde coarse-grainedcohesive F1: faciesbeds - bris-flow deposit comprisedof poorly sorted, metre-longclasts possibility that some bioclasts originallyouteroriginatedbioclasts the from some possibilitythat ramps are derived from semi-consolidated (uncemented) clayey slope. foredeepthe along successioncollapsed limestones thefrom PetrinjakPetrinjak et et al.: al.: Megabeds Megabeds in in Istrian Istrian Flysch Flysch as asmarkersmarkers of of synsedimentary synsedimentary tectonics tectonics within within the the Dinaric Dinaric foredeep foredeep (Croatia) (Croatia) Geologia Croatica ther amud-orclast- dominated flow, debris orasubmarine avalanche. Also, note that each DivisionIbreccia ischaracterized by aspecific composition (Fig. 3). Figure 18.Acomparative figure showing simplified logsofallthemegabeds. Themajordifferences oftheDivisionI areinthestructure breccia –deposited byei rock avalanche are rarely mentioned in deposits the sedi marine (2006), FESTA et al. (2019)SHANMUGAM by and OGATA described et al. (2020). are However, the sub spectrum broad a of such end-members as currents slide/slump turbidity from and tionships among the responsible processes for formation, sub-unit rela evolutionary and deposits transport mass of classification rock avalanche or some flow.other grain large-scale The dynamic submarine a probably event, transport mass a of stage early the not is recognized. itself I Division within facies of differentiation vertical Flysch, Istrian I. Division within here described FaciesB and FaciesA the to similar roughly are subdivisions These part. upper the in clasts rip-up mudstone ing olistoliths in the lower part and mud supported breccia contain divisions a within lower, debrite unit: clast breccia with supported (1992); PAYROS sub two (1999) al. &VENTURINI et describe calcarenite. agraded with capped then breccia overlain clast-supported by breccia and a mud-supported 114 The Division I breccia at Kaldir was probably deposited in in deposited probably was Kaldir at breccia I Division The To the recapitulate, authors However, within the studied megabeds of the ofthe megabeds studied the However,within LABAUME et al. (1987); TUNIS TUNIS (1987); al. et LABAUME - - - - - marl clasts are observed in some megabeds. It can be assumed assumed be can It megabeds. some in observed are clasts marl as well the Flysch succession, as itself, from as carbonate neritic was derived from a different part ofthe to Cretaceous Palaeogene The clast composition ofthat indicates the detritus each megabed 6.4. Triggering mechanismsanddepositionalmodels II). (Division part turbitdite upper the in constituent main the is and with larger marl limestone clasts. Also, the bioclastic detritus mixed matrix, of marly the constituent a subordinate as part, rite deb the in distributed evenly is biogenicdetritus the all because beThiscannoting assumption the maintained finalsedimentation. dur only and mixed in-situ, mechanisms by separate sedimented different from sides originated oftheand that theybasin, werere olistoliths and fauna the that 1979)al., et (HAGN assumed thors fauna that lived in ofthe environment the shelf. carbonate The au fossil rich very a and shale grey of fragments smaller boulders, Palaeogene and Cretaceous sized metre cubic 1 to up containing not is yet clear. deposition megabed Kaldir the of mechanism exact the so spectrum transport mass ment HAGN et al. (1979) described the Gračišće olistostrome as as olistostrome Gračišće the (1979) al. described et HAGN Geologia Croatica 74/2 - - - - - Geologia Croatica - - - - - ­ ------115 Based on the data presented here, several concluding re de Peïra Cava, SE France: Landward stacking patterns in ponded turbiditic basins.– basins.– turbiditic ponded in patterns stacking Landward France: SE Cava, Peïra de 10.1144/0016-76492005- doi: 143–162. 164/1, Society, Geological The of Journal 019 liths.– In: SESTINI, G. (ed.): Development of the northern Apennines geosyncline: Apennines northern the of Development (ed.): G. SESTINI, In: liths.– 4, 521–557. Sedimentary Geolology, Megabedsin the Middle Eocene Istrian areFlysch conside The megabeds are composedlithoclasts a) of: derived from structuraldifferent toaddition In has features, megabed each evolving synsedimentarythe Theof floor dissectedthefaults ed to represent single event bipartite mass transport deposits genic wedge (Fig. 16B). genic (Fig. wedge the distal part of the foredeep basinor also(Fig. 16A) to thrust faulting in the proximal part, in front i.e., theof orogenic wedge During the of Istrian the 16B). evolution (Fig. foreland basin the normal faults could have dissected the distal foredeep and even way thatgeneratedintrabasinalin and topographic(horsts) highs exposed the older rocks along the fault scarpsAlso, (Fig. 16A). located platforms, carbonate the of retreat and backstepping the in the distal parts of the foreland margins platform as describedof basin, in Pyrenn and Jaca the - basin, could induce theian foreland (BARNOLAS collapse & TEIXELL, The alternative 1994). locationproximal most the least at megabeds, some that is option of Hum, are related to aborted thrust faults in front of the oro AMY, L., KNELLER, B. & MCCAFFREY, W. (2007): Facies architecture W. of the L., GrèsKNELLER, B. & MCCAFFREY, AMY, REFERENCES (1970): Olistostromes & P. and PASSERINI, E., V. - olisto BORTOLOTTI, ABBATE, ACKNOWLEDGEMENT The research is partly basedthe for Basic Geological the Croatia Map of Republic of in scale on the results of the mapping1:50.000 and scientific projectresearch within projectGEOSEKVA (Grant funded no. IP-2016-06-1854) by the Croatian Science suggestionsdur for thanksWACHA LaraSpecialFoundation. to 7. CONCLUSIONS 7. ing the writing of the manuscript,ing with the determinations the of Cretaceous limestones. Also, and Ladislavhospitalitythe for Croatia thank Holcim to FUČEKlike would authorsthe for help and PATACCI Marco OGATA, Kei quarrytheirKoromačno. at at two Anonymous Reviewers significantly contributed to the im an earlier of provement this of version manuscript. marks can be made about the Istrian megabeds. with a complex structure that implies a complex flow mechanism flow structurecomplex a complex impliesthat a with megabed (Divi partsthe transformations.of flow lower and The r sion I) are composed of debris flow deposits or even rockava lanche deposits (Kaldir). The upper part (Division II) is com posed turbidity of current deposits. older neritic limestones underlying the Flysch; bioclastic b) sed iments derived from synchronous carbonate ramps, and c) re basinalworked sediments (predominantly hemipelagic marls). a distinct clast composition (Lower and Upper Cretaceous lime ForaminiferalindicatingEocenelimestones)the as well as stones that each bed is a sedimentary record of a specificlocal slope failure that scalloped a different partof the Cretaceous Palaeto ogene carbonate succession underlying the Flysch. Thus, each describedrepresentsseparate probably megabeda collapse local within the Dinaric foredeep. the along rocksbasement the distalexposed Dinaricand foredeep approachingthe to related shocksscarps.submarine Seismic fault orogen are proposed as the main triggering mechanism for the collapses the along fault scarps and/oroversteepened slopes. - - - ­ ------6). The diverse influence of tectonics on the sediment dep the tectonics on influenceof diverse The 6). 1

Although we concluded that depositions of the megabeds aremegabeds the of depositionsthat concluded Althoughwe TARLAO TARLAO et supposedal. (1995) that the ongoing orogenic Furthermore,studiedthe megabedsrandomlylocated arein Time correlation between the Istrian megabeds and global es along the fault scarps thus exposing them to erosion along the osition osition in foreland basins has been documented at various loca tions in the literature (KLEVERLAAN, BARNOLAS 1987; & TEIXELL,et al., PAYROSE 1994; 1999; TOMASSO & SIN 2021). GOBO et al., CLAIR, 2014; mainly induced by tectonics, it is not yet clear if collapses that normal faulting withinto related rather are generatedflows mass steep slopes. The slope collapses, eventuallycollapses,occurring slope Thethe steepslopes. along transportedwhich large flows scarps,the massgeneratedfaultthe quantities of carbonate material (both bioclastic detritus from synchronousneriticlimecarbonate fromolder clasts rampsand t stones) from the scalloped scarps (Figs. 2 and and 16), also re earthquakesThebasinalmarls.autochthonous therelated worked approachingcouldDinaric the orogen of tectonicevolution the to triggeringmainsubmarinethebeenthe col mechanism for have tectonicThetriggeringlapses. mechanismsdepositionto related of the megabeds in the perialpine foreland basins have already KLEVER 1984; describeddocumentedbeenand(MUTTI al., et 2020). et al., OGATA PINI et al 2012; LAAN, 1987; basementcarbonatesubaerialthe to exposure deformationled of succession. In this way the carbonate margins could have been prone more been subaerial have processesthusby weakenedand to slope failures. If that was the case, the whole carbonate suc cession would have been karstified, bothCretaceous andFo raminiferal limestones. However, no karstifiedForaminiferal limestones were seen within the megabed clasts. This brings us subaerialandemergence of periodonly the that conclusion the to Foraminiferalthelimestones, of deposition to prior exposurewas describedas stage forebulge Palaeogene CretaceoustoUpper the and (2007) OTONIČAR (2009). by KORBAR the stratigraphy, without any precursive or successive facies as sociations, suggesting a seismically triggered slope collapse. In otheroccurcases, theywould within changesin sedimen major tation patterns, such as at the base of a coarse-grained fan as in the Middle Dalmatian basin, where deposition of the megabeds is attributed to periods of accelerated sea-level rise In 1996). the MiddleJANAC, Dalmatian(MAR Basin, megabeds have been related to accelerated sea-level rise sometime after a low stand, since they are interbedded with fan deltas (MARJANAC, thecasewith not theIstrian is which 1996) megabeds. Although of deposition and change level sea connectionglobalbetween the the Istrian megabeds is not yet established, a possible sea level dropduring the Middle Eocene had ancould have impact re-on sedimented carbonates in Istria. thesince possible, yet not is 1987) al., et HAQ data(e.g. level sea available data about the megabeds age is too imprecise. Moreo stratigraphicvariousthein present parts are megabedsof the ver, Istrian Flysch. The Kaldir, Šublentica, and Hum occurrences partIstrianthe arepositionedinof lower the suc Flysch 2) (Fig. 2) (Fig. Gračišće,Koromačnoand Plomin,at those while cession are in the middle, and thus were probably deposited during probably more are varistands.events collapseThus, the level sea ous related to tectonic activity than to a specific(Fig. sealevel stand that the synsedimentarythethat generatedthatfaultssubmarine failures carbonatethedistalramps within formed along evolving fast the TEIXELL, 1994, & BARNOLAS SINCLAIR,1997; (cf. foredeep carbona older the exposureof the to led faultingalso The 16). Fig. Petrinjak et al.: Megabeds in Istrian Flysch asmarkers of synsedimentary tectonics within the Dinaric foredeep (Croatia) Geologia Croatica FESTA, A., PINI, G. A., OGATA, K. & DILEK, Y. (2019): Diagnostic features and field- FELIX, M., LESZCZYŃSKI, S., SLACZKA, A., UCHMAN, A., AMY, L. & PEAKALL, FALLGATTER,PAIM,B., KNELLER, C., P.S. 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– – – – – – – – cene. Šublentica section; liths), mudstone lithoclasts, (fragments andbiodetritus oflarge benthic foraminifera, encrustingforaminifera, Eo 3.Middle SampleŠub-II andechinoderms).    Šublentica section;  lites, andunidentified rotaliid foraminifera), red algae, andbiodetritus. SampleHum3b.Middle Eocene Humsection.  D SCHROEDER, 1976). SampleHum–286.UpperCretaceous. Hum section; F Hum section; Nummulitid-alv F D Or P Nummulitid-alv oraminiferal wackestone showing transversal therecrystallized ofCenomanian foraminifera section (Pastrikella) Broeckina balcanica (CHERCHI, & RADOIČIĆ oraminiferal wackestone showing Palorbitolina lenticularis(BLUMENBACH, 1805).SampleHum–123. Lower Cretaceous (lower Aptian).Humsection; eloid-miliolid packstone-grainstone showing miliolids (a) and peloids (b). Sample Šub-43, middleCretaceous showing miliolids (a)andpeloids(b).SampleŠub-43, (Albian-Cenomanian). Šublenticaeloid-miliolid packstone-grainstone section; ensely packed, foraminiferal well-sorted calcirudite showing large benthic foraminifera andnummulites), red tests (mostlyorthophragminids algae(rodo ensely packed, foraminiferal well-sorted calcirudite showing fragmented large benthic foraminifera tests nummu (mostlydiscocyclines/orthophragminids, thophragminid wackestone test showing Eocene. (a)withotherbioclasticdetritus. ortophragminid SampleŠub-43.Middle Šublentica section; Photomicrographs ofthemicrofacies ofthemostcommon clastsfrom DivisionIbreccia (A, B, C,E, F, andG)microfacies ofbioclasticmaterial form Di eolinid grainstone showing anassociation ofalveolinids (a),nummulitids, (b),Asillina eolinid packstone-grainstone showing the association of alveolinids (a), nummulitids (b) and miliolid (c). Sample Šub-7. Middle Eocene. showing Middle theassociation ofalveolinids eolinid packstone-grainstone (a),nummulitids(b)andmiliolid(c).SampleŠub-7. sp., andOrbitolites sp. Eocene. (c).SampleHum85.Middle Geologia Croatica 74/2 - - - - Geologia Croatica - 119 sp. (c). Sample K-152. Upper Cretaceous. Kaldir Cretaceous. Upper K-152. Sample (c). sp. and Aeolisacus (b) sp. Thaumatoporella ackestone showing red algae framework (a), nummulitids (b), and unidentified bioclastic detritus. Sample Gr-199. Middle Eocene. Gračišće section; Gračišće Sample Gr-199. Eocene. Middle bioclastic detritus. (a), nummulitids (b), and unidentified algae framework red showing ackestone ted peloidal wackestone-packstone with rare ostracods. Sample K-132. Upper Cretaceous (Cenomanian). Kaldir section; (Cenomanian). Upper Cretaceous Sample K-132. ostracods. peloidal wackestone-packstoneted with rare acodal mudstone-wackstone, showing ostracods (a), recrystallized(a), ostracods showing mudstone-wackstone, acodal ensely packed, well-sorted foraminiferal calcarenite showing large benthic foraminifera tests (mostly orthophragminids and nummulites), red algae crusts, algae crusts, (mostly orthophragminids tests red and nummulites), foraminifera benthic large showing calcarenite well-sorted foraminiferal ensely packed, ensely packed, well-sorted foraminiferal calcarenite showing large benthic foraminifera tests (mostly orthophragminids and nummulites), red algae (coral (mostly orthophragminids tests red and nummulites), foraminifera benthic large showing calcarenite well-sorted foraminiferal ensely packed, oraminiferal wackestone-floatstone showing nummulitids (a), red algae – rodoliths (b), red algal crusts (c), echinoderm fragment (d) and orthophragminides (d) and red algal crusts (c), echinoderm fragment rodoliths (b), red algae – wackestone-floatstone nummulitids (a), showing oraminiferal and other biolastic detritus (fragments of large benthic foraminifera, encrusting foraminifera and echinoderms) and mudstone-wackeston lithoclasts. Sample echinoderms)and mudstone-wackestonand lithoclasts. foraminifera encrusting foraminifera, benthic large of other biolastic detritusand (fragments section.Gračišće-1. Gračišće Middle Eocene.  Algal w D Lamina Ostr F D   Kaldir Middle section; Eocene. (e). Sample K-num.   section; Sample Kaldir Kaldir 1A. Middle section; bryozoans and bivalves. Eocene. echinoderms, of foraminifera, and biodetritus - fragments linacea) – – – – – –

F D B Photomicrographs of the microfacies of the most common clasts from Division I breccia (A, B, C, E, F, and G) and the microfacies of bioclastic material form of bioclastic material and G) and the microfacies F, C, E, B, (A, Division I breccia clasts from of the most common of the microfacies 2. Photomicrographs Plate Division II (D and H) of Kaldir sections. and Gračišće A C E Petrinjak et al.: Megabeds in Istrian Flysch asmarkers of synsedimentary tectonics within the Dinaric foredeep (Croatia) Geologia Croatica H G F E D C B A II (BandF)ofPlomin andKoromačno sections. Plate 3.Photomicrographs of the microfacies of the most common clasts from Division I breccia (A, C,D, andE)microfacies of bioclastic material form Division 120

– – – – – – – – Koromačno section. algae (corallinacea), (fragments andbiodetritus oflarge benthic foraminifera, encrustingforaminifera, Eocene. Middle SampleKorIIKA1. andechinoderms).    section;  algae, andaskeletalwackestone Eocene. lithoclast. SamplePl-3. Middle Plomin section;   section;   D Algal w Bioclastic Alv D F Alv Algal w oraminiferal-algal bindstone showing red algae encrustingmudstoneMiddle lithoclastsandunidentifiedEocene. bioclast.Sample Pl-II 87. Plomin section; ensely packed, foraminiferal well sorted calcirudite showing large benthic foraminifera andnummulites), red tests (mostly discocyclines/orthophragminids ensely packed, foraminiferal well-sorted calcirudite showing large benthic foraminifera andnummulites), ofred biodetritus test (mostly orthophragminids eolinid grainstone showing association ofalveolinids (a),nummulitids(Asillina showing association ofalveolinidseolinid packstone-grainstone (a),miliolids(b),andOrbitolites sp. fragment Eocene. (c).SamplePl II-124.Middle Plomin ackestone showing red algae framework (a),corals (b),andbioclasticdetritus. SampleKOR Eocene. II179.Middle Koromačno section; ackestone showing red algae(a)encrustingcoral fragments (b)andotherunidentified bioclasts.Middle Eocene. Sample Pl-II 123. Plomin section; -Algal wackestone withred algaeframework (a)andbioclasticdetritus. SampleKOR Eocene. II162.Middle Koromačno section; sp.), b)andOrbitolites sp (c).SampleKOR Eocene. II112.Middle Koromačno Geologia Croatica 74/2