GEOLOGIA CROATICA 45 127 - 150 10 Fig. 2 Tab. ZAGREB 1992 UDC 551.35 (450) Scientific paper Evolution of the Southern Margin of the Julian Basin with Emphasis on the Megabeds and Turbidites Seqence of the Southern Julian Prealps (NE Italy) Giorgio TUNIS1 and Sandro VENTURINP Key words: EastemFriuli, megabeds, Paleogene, Julian Basin, basin evolution From the Maaslrichtian up Lo the Early Eocene, the Julian (or Slovenian) Basin is characterized by a mixed siliciclastic/carbon­ ale deposits system which exhibit complex sedimenlalion pallerns depend ing on various controls: sea level changes; tectonic move­ ments in both carbonate platform and northern source areas; sub­ sidence; proximity/distality to siliciclastic, calciclastic, allodapic (carbonate) sediment source areas; abundance of terrigenous detri­ Lus; paleomorphology of the slope of the Friuli Platform, the main source of carbonate detritus. The depositional sequences ofthe Julian Basin are the result of these controls and their interaction. Herein the examination of the stratigraphic sections is restricted lo the Middle Paleocene up to the Early Eocene ("Flysch di Masarolis" and "Flysch del Grivo "),where the most important carbonate megabeds are recognized. Fourteen stratigraphic sections of the "turbidites with megabeds" sequence were examined (mainly along the N-S direc­ tion). All sections are located near the southern margin of the basin, at the foot of the slope of the north-eastern edge of the Friuli Plat­ form. Comparison of log s revealed marked differenoes in the thickness of the megabeds and of the interbedded calciturbidites and siliciclaslic turbidites . This could be ascribed to the proximity to the source areas of mega beds and to the abrupt stope morphology. By means of the litho-biostratigraphic analysis of the sedin1ents which filled the Julian Basin, the provenances from the main source areas are outlined. The prevailing calciclastic detrilus (CE ,I) mixed with allodapic carbonate sediments (CI) came from the South; siliciclastic detrilus (N CE) mixed with other calciclastic detritus (CE, II) from N, NW as consistently indicated by paleocurrent directions; minor (and late) mixed carbonate sediments (CE, ill) came from NE. A rough sedimentary balance comprising megabeds, thick beds, multisource turbidites (siliciclastic, carbonate, mixed plus couplets) is calculated. An auempt to apply the eustatic sea level curve of Haq et al. (1987) to Maastrichtian-Paleocene and Eocene deposits of the Julian Basin is made. Lowstands of the Maastrichtian- Paleocene and of the Late Ypresian are well recognizable. More problem s arise upon examin­ ing the "Flysch del Grivo " section which is slrongly controlled by tectonic mechanisms. The triggering mechanisms for mega bed em­ placement in the Julian Basin is related to seismic activity along the southern border of the basin. In conclusion, the "turbidites with megabeds" sequence is integrated within the framewot;k of the geo­ logical evolution of the Julian Basin. l. INTRODUCTION The Julian Basin (or the Slovenian Basin accord­ ing to the Slovenian Authors) was a narrow, elongated basin limited by a carbonate platform (Friuli Platform) along its southern border. This basin, located in the Julian Prealps, from the Maastrichtian up to the Early Eocene was character­ ized by mixed siliciclastic/carbonate deposits which exhibit complex sedimentation patterns depending on various controls: sea level changes; tectonic movements in both carbonate platform and northern siliciclastic source areas; subsidence; proximity/distality to siliciclastic, calciclastic, allodapic carbonate sediment source areas; abundance of terrigenous detritus; paleomorphology of the slope of the Friuli Platform, the major source of carbonate detritus. In tectonically active settings, such as the Julian Basin, a complex interplay between tectonism, eustatic sea level and subsidence, as major controls, is accountable for the sequence development. Within the stratigraphic framework of the J ulian Basin, deposits of megabreccia, breccia beds, carbonate megabeds and thick beds are clear evidence of tectonic activity along the margin of the Friuli Platform (TUNIS & VENTURINI, 1985). This article focuses aLLen tion on the period between the Late Paleocene and Early Ypresian, where the ef­ fects of the dismantling of the Friuli Platform have been more pronounced and have originated spectacular mega beds. These mega beds are perfect marker horizons in geological mapping. In the conclusion, these data are integrated in the framework of the geological evolution of the Julian Basin, with the aim to underline the rela­ tions between depositional sequences and the above­ mentioned control ;nechanisms. 2. GEOLOGICAL SETTING OF THE JULIAN BASIN The stratigraphic framework .of the sequences of the Julian Basin (Julian Prealps and Tolmin Mountains) and of the surrounding Friuli Platform and slope was out­ lined by FERUGLIO (1925a), FABIANI et al. (1934) and more recently by BUSER & PAVŠIČ (1978), COUSIN (1981), TUNIS & VENTURINI (1984), PIRINI et al. (1986), TUNIS & VENTURINI (1986), BUSER (1987), TUNIS & VENTURINI (1987). From the paleogeographic point of view, during the J urassic, the region of the Southern J ulian Prealps became a transition area between the Friuli Platform, located on the SW, and the Julian Basin, located on the NE in the Northern J ulian PrP.alps and Tolmin Mountains. This 'Istituto di Geologia e Paleontolog.ia, Universita degli Studi di Trieste, P.le Europa, l, 34127 Trieste 3 Agip-Snor, Ufficio Stratigrafico, via del March es alo, 48023 Marina di Ravenna "' v .. u ·~ E u .. . Sn ~ " e oo ~ N (};) /l' 4 · (.J.- UDINE. a, ~2 122:::1 1 3 O 5km l l l l l l NoRrHERIV, ... .. ... IIT. IUIUIT b , . . l s. b. ~ ..... ~ 4.J ~ o "' " Fig. l · Location map showing distribution of the Jlysch outcrops in the study area. 1.· Maastrichtian deposits. 2. Paleocene-Eocene deposits. 3. Examined sections of Figs . 3,4,5,6 and ·Tab. l. s.b. =state border ~ ~_, LJUILJAIA ,..~ . ~ E Tunis & Venturini : Evolution of the Southern Margin .. . setting underwent only slight changes during the. Cre­ taceous (TUNIS & VENTURINI, 1986). At the Campanian- Maastrichtian boundary, the paleogeographic situation abruptly changed in correspondence with the first alpine tectonic phases. Tectonics caused shifting of the southern border of the Julian Trough to a SW direction accompanied by dismantling and withdrawal of the Friuli Platform margin. During the Maastrichtian, with the arrival of siliciclastic material of northern origii1, the flysch deposition began, slowly extending to the southernmost areas, to today's Southern Julian Prealps; Natisone Mountains and the western sector of Banjšice Plateau (Pig. 1). Within the flysch, calciclastic thick beds and breccias are widespread, but the first important megabeds are quoted in the Middle Paleocene, in the "Flysch di Masarolis". However, the typical "turbidites with megabeds" sequence is represented by the "Flysch del Grivo " (Upper Paleocene - Lower Eocene) . Depocenter of the basin was located along the Kobarid­ Tolmin line (W. Slovenia), that corresponds to the axis of the J urassic-Cretaceous basin. Prevailingly siliciclastic turbidites filled the Julian Ba.sin during the uppermost part of the Ypresian up to the earliest Lutetian. During the Early Lutetian a rapid progradation of prodelta, front and deltaic plain deposits occurred from the North, probably related to a tectonic uplift (VENTURINI & TUNIS,.in print). . As far as provenances of sediments are concerned, litho-biostratigrapic analyses have been performed on coarse arenites with the aim to characterize and separate · the main turbiditic sequences. By these analyses, still in progress, different lithotypes and frequencies have been detected and the derivation of detritus from the main source areas has been outlined. The calciclastic (or calclitllitic) detritus (CE, I) derived from the dis­ mantling and breaking off of tlle Friuli Platform margin, . and generally, can be found mixed with allodapic detritus (Cl). Allodapic carbonate turbidites (sensu MEISCHNER, 1964) are coeval sands and muds of the outer platform which slided down slope by turbiditic processes. This source (CE, I + CI) was the most important during the Late Paleocene-Early Ypresian (Pig; 2). Later, CE, I progressively decreased, as did CI, although at a slower rate. Siliciclastic detritus (NCE, l) mixed with other carbonate detritus (CE, Il) came from N, NW as con­ sistently indicated by paleocurrent directions (VENZO & BRAMBATI, 1969). Herein, mention should be made of the constant and sometimes remarkable presence of volcanic rocks, in particular diabases, which may re­ veal to have an important paleogeographic and tectonic signiflcance. Siliciclastic resedimentation shows several acmes during the Middle Maastrichtian and tlle Middle­ Late Paleocene and increases at the Ypresian-Lutetian boundary (cf. last chapter). Minor and late mixed contributions (CE, III+ NCE, Il) came from NE. They are well documented during the Lutetian final filling stage. 129 3. AN OUTLINE OF STRATIGRAPHY OF THE SEQUENCE WITH "MEGABEDS AND TUR­ BIDITES" Prior and after the First World War, the region of the Southern Julian Prealps was geologically investi­ gated by numerous authors. FERUGLIO (1925 a, b) deserves to be mentioned for the most exhaustive de­ scriptions of this territory. The Author described the stratigraphic sequence of tlle Southern Julian Prealps and, in particular, he made a distinction between a lower flyschoid complex characterized by extremely thick bodies of coarse conglomerates ("conglomerati pseudocretacei") and by interbeddings of sandstones and maris, and an upper flyschoid complex that mainly consists of a sequence of sandstone and mari beds. FER UG LIO (1925a) . numbered the thick conglomeratic beds from l to 25, a numefation that has been kept in this text, where megabeds are preceded· by the letters MB. However, if compared with the original Feruglio-s numeration, there are still some ambiguities when identifying some minor megabeds on the field. In the 'SOs and during the following years, relatively few regional and stratigraphic studies were carried out in the region examined. Recently, new researches and geological mapping have been carried out by COUSIN (1981), TUNIS & VENTURINI (1984), PIRINI et al. (1986), TUNIS & VENTURINI (1987), etc. The thick sequence where the most important carbonate megabeds are recognizable, was subdivided into two informal stratigraphic units, called "Flysch di Masarolis" (PIRINI et al.,l986) and ''Flyschdel Griva" (TUNIS & VENTURINI,l987) respectively. The "Flysch di Masarolis" (Middle Paleocene-Upper Paleocene) mainly crops out in the central-eastern sector of the Natisone Valleys and extends to the Banjšice Plateau region (Trušnje) in western Slovenia. This unit is characterized by predominant medium-thick siliciclastic turbidites consisting of a sequence of quartz-litharenite and gray marl beds. In sandstone beds, Bouma's intervals Tb,Tc;Td and, more rarely, tlle interval Ta can be ob­ served. Carbonate proximal turbidites and, occasion­ ally, paraconglomerates rich in chert pebbles are interbedded within the siliciclastic turbidites. This sequence is sporadically interrupted by thick graded carbonate megabeds; the greater thickness pertains to megabed 2 which reaches up tO 55 m in the l udrio Valley. A giant polyphase megabed originated by colossal submarine slides named "Mt. loanaz Megabed'~ (PIRINI et al., 1986) deposited at the top of the "Flysch di Masarolis". This megabed corresponds to FERUGLIO's layer nr. 3 (1925a); with it TUNIS & VENTURINI (1987) mark the beginning of tlle "Fly seh del Grivo" sequence (UpperPaleocene-LowerEocene). The "Flysch del Grivo " is characterized by several types of megabeds such as complex megabeds, composite megabeds, carbon­ ate megaturbidites, carbonate massive tllick beds. The megabeds roughly make up 1/2 of the entire thickness 130 Geologia Croatica 45 Poleocene- Eocene boundary cučceo . . . . M.Bernodio • A . • . o lO km ~ § D . J. 2 3 - ~ 7 8 ·sovec·,-_ -_ - A , __ --- ....... ----. 7----- e . .Kobo rid .. -.. . - =. M.Sobotino A A~ ~--* "'"..-- ~ lo oo> 4 5 6 Fig. 2- Main source areas of detritus at the Paleocene-Eocene boundary. l) Friuli Carbonate Platform. 2) Slope. 3) Basin. 4) Palaeofaults: NW-SE trending dinaric faults and ~E-SW trending antidinaric faults . 5) CE I (southwestem inputs). 6) CI (southwestem inputs). 7) CE III and minor NCE contributions (northeastem inputs). 8) NCE +CE II (nort western inputs) . of the sequence. The importance of the megabeds for correlation has been emphasized by giving names ~ -· the most important of then.: e.g. "Vernasso complex layer" or MB ll (GNACCOLINI, 1968), "Mt. Ioanaz Megabed" (MB 3), "Mt. Staipa-Topli Uorh Megabed" (MB 6), "Mt. Carnizza Megabed" (MB lO) and "Porzus Megabed" (MB 15), (TUNIS & VENTURINI, 1985; TUNIS & VENTURINI, 1987). The megabeds gradu­ ally become thinner and less frequent in the upper part of the section. Between the megabeds, siliciclastic turbidites, carbonate turbidites, hybrid (mixed) sand­ stones, massive calcarenites and conglomerates (debris flow) can be observed. The "Flysch del Grivo" extends from the West to the East from the Mt. Faeit-Zimor T. area, across the Natisone valleys and the Goriška Brda as far as the Banjšice Plateau (Anhovo region). West of the river Tagliamento, the "Flysch della Val Tremugna" (SARTI, 1979) could also be related to the "Fl ysch del Gr ivo " (VENTURINI & TUNIS, 1991). The "turbidites with megabeds" section outcropping in Goriška Brda (Slovenia) is known in literature as "Kožbana beds" (PAVLOVEC, 1966; CIMERMAN et al., 1974). TUNIS & VENTURINI (1989) regard the Tunis & Venturini :Evolution of the Southern Margin ... "Kožbana beds" as synonyms of the "Flysch del G rivo ". More precisely, in Goriška Brda, between Mt. Korada to the North and the village of Višnjevik to the South, the middle-lower part of the sequence is present, comprised between MB 3 and MB 17. The region of Anhovo was geologically investigated by KUŠČER et al. (1974) and SKABERNE (1987). Here, between the villages of Morsko and Deskle (see fig. ll of SKABERNE, 1987) probably the upper part of the "Flysch di Masarolis" and the lower part of the "Flysch del Grivo" (MB 3 -MB 6) are out­ cropping. The "Flysch del Grivo" is overlain by the Flysch di Cormons (MAR TINI S, 1962) of the Lower-Middle Eocene age (VENTURINI & TUNIS, in print). Within this formation, in the lower part, carbonate or hybrid big beds can be still observed, but the presence of megabeds is exceptional (TUNIS & VENTURINI,l989). Finally, the flysch deposits of the Ajdovščina re­ gion (20 km East of Gorizia) could be attributed partly to the upper section of the "Flysch del G rivo" and partly to the lower-middle section of the Flysch di Cormons. ENGEL (1974) reported the presence of carbonate megabeds in this region but, with the exception of two very thick breccia beds that resulted from impressive rock falls at the foot of a carbonate platform, to define them as big beds would be more correct, considering that their thicknesses are not exceptional. 4. MAIN FEATURES OF THE ''MEGABEDS AND TURBIDITES" SEQUENCE According to READING (1978), megabeds are unique deposits produced by exceptional events. Some mega beds have been though t to be megaturbidites and interpreted as seismoturbidites (MUTTI et al., 1984) on the basis of a set of criteria (mainly thickness, volume and ge­ ometry). As regards terminology, the definition of megabeds is not univocal. According to some authors, megabeds are thicker than average beds by several orders of magnitude (MUTTI et al., 1984); according to other authors they must reach a critical thickness or, at least, they must be markedly thicker than interbedded layers (BOUMA, 1987). Herein, true megabeds are defined by thicknesses exceeding 20 m, because they are evidently thicker than all the beds within the "Flysch del Grivo " sequence, and by the presence of the basa! breccia division in at least one of the examined sections. This is how megabeds are considered sensu stricto: MB 3, MB 6, MB 10, MB ll, MB 14, and MB 15 which show these features in all the measured sections. At the border line of this definition are the following beds: MB 8, MB 9, MB 131 16, MB 17, MB 21 and MB 22. All of the other thick beds are generally considered big beds. Among big beds a distinction is made between major big beds, identi­ fied by FER U GLIO (1925a), and minor big beds. Some of them could be defined as classic carbonate megaturbidites, some are massive or faintly graded calcarenites overlain by maris, and others are composite beds (MARJANAC, 1987). Within megabeds s.s., the complex bed type prevails which is a couplet of megabreccia in the lower part and graded calcarenite­ marl in the upper part, sometimes with repeated intervals as illustrated by BURROUILH et al. (1987), MARJANAC (1987). Some megabeds are polyphase and thus they represent two or more superimposed megabeds: i. e. MB 3 and MB 15 (Porzus section). In the explored area the complex layer of Vernasso (GNACCOLINI, 1968) is the thickest megabed. It reaches the maximum thickness of 245 m near Costa and Borgo Laurini di Torreano. Its fine calciruditic-coarse arenitic portion is known under the commercial name of "pietra piasentina" and is widely used in Friuli. The precise post compaction volume of each of these beds is unknown, but MB ll and MB 3 roughly reach at least 25 and 19 krnJ respectively. Not less than ll krnJ can be estimated for MB 15.But, the minimum width probable for these outcrops before the erosion must have been at least three times as much and the above-mentioned volumes are likely to be much higher originali y. Fi g. 3 shows the lower part of the "Flysch del Grivo" along the Cras-Pedrosa sec­ tion, while Fig. 4 illustrates the Reant-Mt. Noas-Forcis­ Montina section which is the most complete of the "Flysch del Grivo ". With regard to the internal organization of a megabed, according to MUTTI et al. (1984), SEGURETetal. (1984) and LABAUME et al. (1987), the vertical section of the megabeds may be divided into five divisions grouped into two major segments of megabreccia and turbidite re­ spectively. There exist other models of complete mega bed sequences (BERNOULLI et al., 1981; KLAR V ALAAN, 1987; MARJANAC 1985, 1988; ROSELL & WIECZOREK, 1989, etc.) but the first model is preferred to emphasize the thick calciruditic unit (Unit 3). Unit l is a megabreccia which mainly consists of big blocks of shallow water limestones. Individual clasts may have extraordinary dimensions, so olistoliths of the Vemasso Megabed have volumes up to 70 000 mJ in the ltalcementi quarry ofVemasso, near B.goLaurini di Torreano- Costa ("pietra piasentina" quarries) and up to 200 000 mJ or 400 000 mJ in the environs of Clap, north of Porzus and Forame (Cret del Landri). Smaller limes tone and mar l clasts can be found between the big blocks3 • Frequently brec- 3 In the Vernas so quarry the most common 1ithologies include: wackestone with Orbitolinopsis capuensis of the Hauterivian - Barremian age, grainstone with NauJiloculina bronnimanni, Dasycladaceae, Rudis t fragments of the Barremian-Aptian age, grainstone-packstone with Rudist fragments of Sen oni an, packs tone with M inouxia of Senonian, wackestone-packstone with Scandonea mediterranea and Sgrossoella of S anton ian, packstone with Siderolites, Orbitoides and Rudist fragments of Maastrichtian , pack stone with Rudist fragments and Globotruncana stuarti of Maastrichtian, cherty fossiliferous pack stone of the Upper Cretaceous - Paie<>cene, wackestone with Ostracoda and Discorbidae of Pa­ leocene age, packstone with Melobesiae, Corallinae and Corals of Paleocenc, packstone with Discocyclinae of Paleocene, packstone with O percu/ina and Miscellanea of Paleocene, packstone with Alveolinae and Cymopolia of Palcocene, boundstone with Corals of Pa1eocene. And also marly 1imestones with Globotruncanita gr. stuarti, G. arca, G.gr.linneiana of Maastrichtian age, unfossiliferous maris, siltites, sandstones, calcarenites and carbonate breccias are present. 132 Geologia Croatica 45 CRAS PEO ROSA SECTION ~ / u4 6 u3 u2 ul unit 5 lO u2 bis t~~~~~~ Mari 5 unit 4 ~ Gr oded 4 u5 . colcoren i te lO u4 u3 unit 3 r··· ... ~~ Lithoclostic breccio ......... ·..: ( no~molly clos t supp.) ~:::;:·:;i~{ u5 unit 2 u4 9 u3 Matrix supported u4 corbonate breccio unit 1 l! u3 ~ Poorly sorleđ corbonate megobreccio u4 u3 u5 8 u4 .b u2 u3 50 u3 7 u5 40 u2 u4 6\ 30 20 ul lO 3 5 bis Om Fig. 3 - Cras-Pedrosa section (lower part of the. "Flysch del Grivo"). Key is given at Fig. 5. In Unit l : a = mushroom shaped breccia intrusion; b= olistoliths of shallow water carbonate; e = cannibalized blocks of older megabeds; In Unit 2: d = discoidal marl clasts; e = rip-up siliciclastic turbidites. l, Il =coarse resedimentation events. Arabic numerals symbolize individual megabeds. Fig. 4 - Reant-Mt. Noas-Forcis-Montina section ("Fly seh del Grivo"). Symbols for units as in Fig . 3. Key in Fig. 5. Close-up of the upper part of this section plus younger beds shown at Fig. 5 (Mt. Navaret - ll Cioch section). Tunis & Venturini : Evolution of the Southern Margin .. . REANT- M.NOAS - FORCIS- MONTINA SECTION l u4 17 u3 16 ll u2 15 ul 14 bis 10 u2 bis 14 uS u4 10 u3 u2 ul 13 u5 12 u4 9 u3 u5 \ 8 u4 u3 u5 7 u4 J u4 u3 u4 u3 u2 uS u4 u3 u2 ul u4,5 u2.~ uS u4 u3 uZ ul u5 u4 u3 uS 22 21 20 19 /""' 18? !';;.;:,-:;,;.,::-;\ \ - 133 uS u4 u3 u2 u4 u3 u2 u3 u4 u3 50 40 30 20 10 5 Om 134 u2 Dl 02 OI C2 C2 02 C2 OZ!OII cz cz cz o z C2 02 02CZ Dl 0201 C2 D2 OI Dl cz OI 02 DZ t'"TT"TTT"'t---- p. 1· e. cgi. cobb le M. NAVA RET - IL CIO CH SECTION C2 C2~ l C2 cz cz Dig cz C2 C202 C2 C2 cz E cz cz C2 C2 C2 OI OI OI DIE OI CZ Dl C2 cl EDI C2 DIC2 cz"!: C2C2 C2 cz CZ C2 o1cz OI u5 Geologia Croatica 45 Cl u4 u3 u2 C2 cz C2 E C2 cz C2 Dl C2 OI OI 20 C2 10 5 4 3 2 l Om Tunis & Venturini :Evolution of the Southern Margin ... KEY: lilill!HHHI ~ \\ \\~ IJJJJ \ ·\ >< Mar l Hemipelogite Hybrid areni te Limestone - lithic sondstone couplet Thin~ to medium bedded sondst9ne with mudstone interbeds l OI, DZ,C2,El (sondstone in otoci< l Thin bedded sondstone and mudstone ( 01,02) (sondstone in black l Corbonate turbidite 1 cr, cz, or, o2l Massive colcorenite Two layer bed Con9tomerote l breccio rich in cherty pebbles on d cl oy chips Pebbly mudstone Corbonate big bed Covered Fau lt 135 Fi g. 5 -Detailed stratigrapic section of the upper pan of the "Flysch del Grivo": Mt. Navaret- ll Cioch section. The thickest siliciclastic and calciclastic turbidites are indicated by letters Cl, C2, Dl, D2, D3 and E according to the classification of the turbiditic fa­ cies of MUTTI & RICCI LUCCHI (1975). Grain size scale: p= peli te; f =fine sandstone; e= coarse sandstone; cgi= conglomerate (pcbble). 136 cia blocks cannibalized by the huge sediment gravity flow can be observed. · Large olistoliths of breccia beds and calciruditic­ calcarenitic beds are common in MB 3, MB ll and MB 15 (figs. 3 and 4). Thus, the original overall number of megabeds must have been higher than what can be ob­ served today. Base of the U nit l shows an erosional plane contact, but also a sharp contact (sometimes breccia di­ rectly overlies a sandstone bed). Often, siliciclastic turbidites underlying the megabed do not appear to be strongly deformed by the load. Transition from the Unit l to Unit 2 is gradual. Unit 2 is also carbonate megabreccia, but can be recognized for the lack of large limes tone olistoliths, for the numerous disk shaped clasts of calcareous mud­ stone and for the rip-up siliciclastic turbidites with some interbedded calciturbidites. These blocks of turbidites are also present in the Unit l, though less abundant. Some mud blocks are armoured by breccia fragments. Within the MB ll the thickness of interval containing rip-up turbidites is striking, reaching about 60 m (quarry of Vernasso). Even more than Unit l, Unit 2 is strongly matrix­ supported, but in the upper part pebble to cobb le grain­ size of limestone clasts prevails, and the matrix content decreases. Unit 3 is a calcirudite which can reach 25-30 m in thickness (MB ll in Fig. 4). Within thicker megabeds, Unit 3 shows a transitional contact with the underlying Unit 2. More often, the contact plane between the two units is sharp and deformed by loading that had formed big flame structures. Sometimes mushroom-shaped brecci~ intrusions, penetrating the calcirudite unit, can occur. Unit 4 is mostly a normally graded calcarenite, iden­ tical to a carbonate megaturbidite. Beside normal grad­ ing, this unit displays parallel-lamination, ripple- and convolute-lamination. In some cases the Unit 4 is char­ acterized by the alternation of massive, parallel-laminated and rippled calcarenites: i.e. composite bed (second phase of MB 3, MB 14 and MB 15). Here it is possible to observe flow direction changes as indicated by ripple orientation. In other examples unit 4 is only graded in the lowest part (few centimetres) and massive in the remaining portion and/or laminated in the upper part only. Everywhere, within this unit, water escape structures are rather common. The upper part of the megabed is represented by thick massive mari, occasionally laminated, that has a transi­ tional contact with the underlying calcarenites. The mari cap may reach a considerable thickness: MB ll is 45 m thick and MB 3 as much as 60 m thick near Bocchetta di S. Antonio (PIRINI et al., 1986). The main lithofacies interbedded with the megabeds are shown in Fig. 5. Description of sedimentologic characteristics of these lithofacies is not the purpose of this article, but in short, one may state that siliciclastic distal turbidites and proximal calciturbidites predominate in the lower-middle part of the "Flysch del Griva" se­ quence. Siliciclastic, mainly proximal, turbidites prevail in the upper one. The proximal features of these turbid­ ites may 'be related to the South-South-Eastward prograding Geologia Croatica 45 of deltaic complexes which made up the main source of NCE I detritus. Mixed siliciclastic and carbonate lithofacies (hybrid arenites and limestone-quartzarenite couplets, sensu KELLING & MULLIN, 1975; TUNIS & VENTURINI, 1984) are very frequent in the upper part of the section of "Flysch del Griva", while resedimented carbonates (calciturbidites, massive calcarenites either faintly laminated or graded at the base, two layers (sensu KRAUSE & OLDERSHA W, 1979) are ubiquitous. 5.DESCRIPTION OF THE EXAMINED SEC­ TIONS: LATERAL AND LONGITUDINAL V ARIA TION OF MEGABEDS As regards the stratigraphy of the "Flysch del Griva ",about thirty sections have been measured in Friuli, from the Cergneu area to the West to the !udrio Riv.er to the East, and six sections in Goriška Brda (Slovenia). These sections mainly follow aN-S direction. Their original locus was near the southern margin of the basin, at the base of the slope of the Friuli Platform. Some sections, includ­ ing the composite type section of T. Griva- Colloredo, have already been published (TUNIS & PIRINI, 1987, TUNIS & VENTURINI, 1987). Here, respectively lO and 14 new sections are selected and illustrated in Fig. 6 and Tab. l. Fig. 6 illustrates the marked differences in the thick­ nesses of megabeds and carbonate big beds, the differences in the thicknesses of the various units present within the mega beds and of the pockets of the siliciclastic and carbonate turbidites interbedded with the megabeds. Usually one can observe that the stronger is the development of the megabed and of its megabreccia unit in particular, the th inner is the set of the underlying turbidites. Even the set of the overlying turbidites shows smaller thicknesses. With regard to megabeds, these differences may be especially ascribed to the proximity of the source area and to the steep slope morphology. Unfortunately, the extension of a single megabed cannot be followed across the basin for more than 5-6 km, owing to the present structural arrangement of the Southern J ulian Prealps and to the post-Ypresian erosive phases. The exception to the rule regards the upperpartofthe ''Flysch del Griva" sequence observed near the villages of Monteaperta and Micottis. Herein, thick and very coarse siliciclastic deposits are interbedded with "relatively more distal" megabeds. Thus, the proximal zones of megabeds are characterized by very thick megabreccia divisions; but, more distally, the lateral variation of their internal organization is un­ known. However, it is plausible to think that mega beds extended basinward for several kilometers4 • SOUQUETetal. (1987) and ROSELL & WIECZOREK (1989) respectively proved that the megabeds in the Cretaceous and in the Eocene of the Pyrenees evolved in their distal parts, into normal carbonate turbidites. In the examined sections, one can assume that at least some calciturbidites and some big beds (for instance, the big bed between MB 14 and MB 15 in Fig.6) are distal extensions of submarine slides smaller than those that Tunis & Venturini :Evolution of the Southern Margin ... originated the megabeds. Tab. l tentatively explores the proximity/distality of megabeds from the examination of 13 sections. The overall thickness and the ratio of thicknesses of the Unit l and cumulative thicknesses of all other units are taken into consideration, and presence of giant olistoliths is also indicated. The comparison is drawn in different sectors of the southern margin of the basin.The table shows that MB3 is very thick in the middle sector. This megabed, however, reaches great thicknesses even in the !udrio valley and in Slovenia, near Mt Korada (TUNIS & PIRINI, 1987) and at Anhovo (SKABERNE, 1987), while towards NW it is 100 m thick near Prossenicco and Mt. Oveiach (Platischis). MB 6 is considerably thick eastwards, less thick in the central-western sector where, however, the Ul/U2-5 ratio is very high. MB 6 shows a limited thickness westwards (20 m near Platischis). As regards MB ll, the maximum thickness is recorded in the western sector, where the Ul/U2-5 ratio reaches extremely high values (0.46 near B.go Laurini di Torreano). Eastwards, the megabed considerably diminishes; westwards it gets gradually thinner (Mt. Carnizza, Zuffine plateau, Mt. Cladis) until it reaches 65 m near Cornappo, where it is cut off by the Periadriatic overthrust (PIRINI et al.,l986). MB 14 and MB 15 are also thinner eastwards, more developed in the middle area, between Vernasso and Valle­ Colloredo, they thin down in the T. Griva di Raschiacco valley and get thicker again in the central-western sector where the maximum thickness is reached as well as the highest Ul/U2-5 ratio. . This ratio can not, of course, be used as an absolute index of proximity/distality to the base of the slope; among the other things, it is not likely that equivalent volumes of material have detached all along the slope and the platform margin. Still, the considerable Ul thickness, the presence of giant olistoliths, the limited thickness of the underlying turbidites and the poor development of the over lying siliciclastic turbidites (the lateral expansion of siliciclastic turbidites towards the margin of the slope was probably confined. to the morphologic obstacles produced by the submarine slides) are important clues of proximity s.s. Just like the ab­ sence of large olistoliths, the limited Ul thiclpless or its absence and the greater development of over- and underlying turbiditic siliciclastic sequences allegedly indicate conditions of a "relative minor proximity" of the same megabed. In order to explain the variations in megabed thickness along the edge of the basin, a further factor should be taken into account. The ac:tivation of normal dinaric 137 (NW-SE) faults along the slope of the Friuli Platform otiginated the megabeds5• However, at the same time, NE-SW trending antidinaric faults, orthogonal to the former and acting as transfer faults, were active (Fig. 2). Their presence and position is supposed where the greatest differences within the type section of the "Flysch del Griva" are found, in areas where it is even possible to observe the separation of some important megabeds into several stumps. These palaeofaults, probably in­ herited from the Cretaceous,. are thought to have acted as a series of morphologic steps and to be the direct cause of the overall variations that some megabeds (MB 3, MB 6, MB ll and MB 15) miderwent in a direc­ tion parallel to the axis of the basin. 6. SEDIMENT ARY PROCES SES AND ORIGIN OF MEGABEDS As far as sedimentary processes are concerned, the megabeds may be considered as co:nplex beds which were deposited by a range of sedimentary processes: rock fall and debris flow with combination of supportive mechanisms (Unit 1), debris flow (Unit 2), deposition from a waning turbulent flow and partly grain flow (Unit 3) and hi;;h-density turbidity currents (Unit 4 and 5). According to HAMPTON's experiments (1972) the density currents followed closely after or evolved from the de bris flow. Another possibility is that carbonate detritus was resedimented from the same source by a series of sedim en tary processes that progressed in a close succession. Such processes could be connected to a retrogressive sliding that generated successive coarse sediment gravity flows (calcirudites connected to grain flows, pebble avalanching along the slope, etc.) and turbidity currents. In many cases, the deposition of Unit 4 was very quick, according to the ubiquitous water escape structures (water escape pillars, dish, etc.) and the sedimentary event was probably single. However, in other cases Unit 4 can be regarded as a "composite turbidite" (sensu MARJANAC, 1987). Fig. 7 indicates the frequency of the major sedimentary processes in­ ferred from the sedimentological characteristics of the different units. Percentages are related to the thicknesses of megabeds and major big beds observed in the series typical of the "Flysch del Grivo ". Topography presented is idealized while the den se lines represent dinaric faults active on the Friuli Platform. Rock fall and debris flow mechanisms accounted for 1/3 of the total thickness of big beds and megabeds (about 260 m as against 760 m). So extraordinarily h ug e sediment gravity processes characterize the "Flysch del 4 BONAZZI & TUNIS (1990) report, on the basis of mineralogic analyses performed on the clayey fraction of turbidites that Maastrichtian flyschoid units are characterized by illite crystallinity index typical of anchizone. According to these authors, the metamorphic degree is mainly due to sediment loading. Thus, in spite of the fact that in the Julian Prealps there are no deposits young er than the Middle Eocene, it may be assumed that the Maastrichtian flysch of the Natisone and Isonzo valleys was covered by the Paleocene-Eocene flysch. The pebbles and cobbles of the deltaic environment of the Flysch di Cormons are commonly composed of lithotypes coming from the Paleocene-Eocene flysch (VENTURINI & TUNIS, in print). This leads to think that the Julian Prealps, which lifted during the Lu teti an, are the most probable source area of the cobbles. 5 For instance, this can be observed in the lud rio Valley where the flysch sutures in on la p, in a SW direction, the collapsed paleomargin of the Friuli Platform faultblocked by a set of subvertical dinaric faults (TUNIS & VENTURINI, 1987; SARTORIO et al., in prep.). 138 (2) (3) S. ANTONIO CLAP (4) (7) VALLE COLLOREDO (6) CANEBOLA CRAS (9) RE ANT MONTI NA M.CARNIZZA PORZUS SGUBLA PEDROSA 22= = Zle=:\ zo~ 19c:::::::l 17~ 16~ = = = 14r:'­~ 13= 12= e = tOe=\.. a= 7= t9c:= 18- 17= tsb::l a= 7~ = 5==>-- 4= Z tr··----, 20- 19~ 18= e 10 bisc:::::::J 9= 6bis= 5= 4c:::= 23= 22~ 211\ 20~ 19~ 18= 17~ t6c=:1 ~ e==. 14D 13= 12= e. P. Geologia Croatica 45 (10) (ll l VERNASSINO MAC OTA (12) (13) MERNICCO SC RIO' VERNASSO M. CAU (CASTELMONTE) 15~ 8&o8 • .... e 14~ 13==>-- 12~ 10~ P• poliphose e=connibotislic tendency c=::J us r==:J U4 c:=::J U3 c=:::J U2 1ĐHR1J Ul 200 ISO tOO so 20 Om Fig. 6 - Schematic column ar sections from Wto E of the "Flysch 11.:1 Grivo" showing only mega beds and big beds . ln megabeds, different units are represented by different width s; key on the right of the figure .. The blanks represent both the exposed and covered sequences. For graphic purposes, the sections have been constructed by taking MB 3 and MB ll taps as references. Tunis & Venturini :Evolution of the Southern Margin ... BRDICE- < 14 > KOŽBANA < 13 > MERNICCO SCR JO" Mt.CAU < 12 > (CASTELMONTE) < 11 > VERNASSJNO­ MACOTA MONTI NA PRESTENTO­ T.CHIARO VALLE­ <7>COLLOREDO CRAS (G)PEDROSA < 5 > RASCHJACCO­ T. GR IVO' + ~,.... ~ ON r- ,.,. o M ll'! N M ~ o l N \ON M ~ o + co C'o rl co ....,,..... N ~ o +~ NO N ' o l +.-i \OM« ITl ~ o co O .-i r- ~ o co CO N .-i ~tc NO \0 ON co ~· .-i O v:> N .-i .-i , o \0 .... ...-l N ~ o .... NN co ' o <4 >CANEBOLA- ~~ ~~ SGUBLA M~ ~ o co < 3 > S.ANTONIO- ~.. ~~ ~~ ... CLAP o ~ ~------------------------------~--------~~------~-----4------~------~~ < 2 ) Mt.CARNIZZA­ PORZUS Mt.IAUER < 1 >LE ZUFFINE l +<" CO M« \0 ' o l +en ON ~ ' o co ..,. rl \0 , .. .-i O l +o ONt< qo ' rl o 139 Tab. l -Thickness of megabeds and ratio between the thickness of Unit l and the thickness of all the other units. Measured sections from the West (l: Le Zuffine) to the East (14 : Brdice- Kožbana, Slovenia). The+ sign indicates a greater development of the megabed com­ pared to the measured thickness . In the Ul/others ratio, owing to the poor outcrop exposures of the upper unit, the - sign indicate s slightly lower values. The asterisk indicate s the pre sence of giant olistoliths. 140 N N ~ o -(J1 ~ o o -(J1 ~ o Fig. 7 - Main sedirnentary processes which originated the megabeds and big beds of the "Flysch del Grivo". Geologia Croatica 45 Tunis & Venturini :Evolution of the Southern Margin ... Griva". The remainiqg 2/3s, inappropriately attributed only to turbidity currents in Fig. 7, indicate the thickness taken by units 3, 4 and 5. These units have been originated by various processes, among which the high density turbidity currents prevail. Some small intraformational slumped beds can be mostly found in the upper part of unit 4 (for example MB lO near Clap and MB 13, near Porzus). They indicate that some movements of sediments (sliding and/or more probably slumping) occurred after the deposition of the materials. The palaeotransport direction in the megabeds of the "Flysch del Griva" was mainly inferred by exam­ ining lithologies (from giant olistoliths to calcarenites). The same kind of detritus (mainly shallow marine Upper Cretaceous and Paleocene limes ton es and Paleocene or penecontemporaneous maris) is observed within all units of the mega bed aM within all megabeds, and indicates a single source of detritus, and that is the Friuli Plat­ form located towards the South. The palaeocurrent data concerning calciturbidites indicate a longitudinal palaeotransport (from NW, like the siliciclastic turbidites) probably deflected from an original provenance from South-West. In some megabeds, opposite ripple orientations, which suggest directional changes of flow regime, have been obser':ed (MB 3 and MB 6). They represent possible consequences of reversals of turbidity currents in re­ stricted parts of the basin where voluminous flows reached the opposite side or opposing obstacles and reflected backwards or sideways (PICKERING & HISCOTT, 1985; MARJANAC, 1990 and 1991). However, no detailed explorations of palaeotransport directions were performed according to the internal structures of the Friuli megabeds. The hypothesis of the possible proximity of morpho­ logical obstacles is supported by the fact that a large number of megabeds shows a very thick marly cap. Mar ls may present thin laminations, which may mean that the maris formed from ponded turbidity current tails of voluminous flows (HERSEY, 1965; V AN ANDEL & KOMAR, 1969; BLANPIED & STANLEY, 1981). Very thick massive maris may indicate restricted environ­ ments, i.e. restricted basins, parts of the deep sea ba­ sin with obstacles, etc. (see MARJANAC, 1991). So, the J ulian basin was certainly single, rather narrow (50-60 km wide at the most) and probably character­ ized by a complex and differentiated bottom topogra­ phy. Giant olistoliths and the exceptional volume of the megabeds indicate a catastrophic collapse of the platform margin and sl ope, that was probably initiated by a seismic shock. MUTTI et al. (1984) interpreted such beds as seismoturbidites. SEGURET et al. (1984), on the basis of two constraints, estimate that superfi­ cial earthquakes of M=7 would be required to initiate landslides in the South Pyrenean Basin. Today most authors accept the seismic interpretation to explain the origin of megabeds, even though various triggering mechanisms exist. We think that megabeds of the Julian Basin point to exceptional initiation mechanisms because 141 they deposited in a highly tectonically mobile setting and they account for a volumetrically remarkable proportion within all other deposits (probably the highest proportion ever known in literature). Since we accept the seismic interpretation, on the basis of our previous studies on the evolution of the basin, we think that extensional tectonics was responsible for the high seismicity along the margin of the Friuli Platform. During Late Paleocene-Early Eocene, important NW­ SE trending dinaric paleofaults were active on the margin and slope of the platform to create conditions of enormous slope failures. The data collected indicate an instantaneous deposition for megabeds; tens of km3 of material were mobilized and resedimented in a short time. We hy­ pothesize the location of the faults on the slope for a further reason as well: according to SEGURET et al.(1984) the high acceleration necessary to liquefy and fluidize maris and therefore reduce frictional resistance, only occur in the immediate vicinity of the active fau l ts. In order to estimate the paleoseismicity of the basin, a rough sedimentary balance has been calculated in Tab. 2. It includes multisource turbidites (siliciclastic, carbonate, mixed plus couplets), megabeds and big beds, massive calcarenites (either faintly graded or laminated at the top), isolated debris flows, hemipelagites and coarse thick sandstones. All these lithofacies, or association of lithofacies, have been distinguished in Tab. 2 by their cumulative thickness and number of events. The "Flysch del Griva " sequence is divided into three intervals: the lower one is comprised between the base of MB 3 and M ll; the middle one is comprised between the base of MB ll and the top of MB 15, the upper one beg ins at the top of MB 15 and ends at the top of MB 23. The three segments have about the same thickness, but are characterized by different thickness ratios and number of events. Intervals have been mea­ sured in the central part of the examined area and the values indicated in Tab. 2 have been calculated by comparing and assembling 21 composite sections. The sequences of Mt. Ioanaz- S. Antonio, T. Griva di Faedis, Valle- Colloredo and Reant-Montina are the reference sections for megabeds. The data are compared with those measured along two sections belonging to the unit underlying the "Flysch del Griva" and to the upper part of the "Flysch del Griva" sequence. The sequence defined as the "Flysch di Masarolis" was measured in the Iudrio valley where MB 2 is particularly thick. The upper part · of the "Flysch di Griva" sequence has been measured between Debellis and Monteaperta, in the "more basinward" possible ubication and in the proximity of the pro grading deltaic complexes. The sequence of the "Flysch di Masarolis" takes up the middle-upper part of the unit and belongs to the P. pseudomenardii biozone. MB 3 still belongs to the P. pseudomenardii; the M. velascoensis biozone can be found immediately above. The transition between biozone M. velascoensis and biozone M. subbotinae is estimated between MB 7 and MB 8. Biozone M .formosa appears a few meters above t . Vl ..". .§ :0 8 u .. ·o. o ] ~ M :! r • -"U en ll) v (/)(/) en ..xen v ........ v )( - OI - u u ,.:-= u v U "U E ·- .fen ·e ll) OO - ·-v OI e en -- ll) "U - ll) ww :=.o o v a. - 'O en en -o·--v v e ll) ll) v OV ll) u a. "U CIJ o o v-E o- e:-= - m. > ~ - CIJ..; v e 'O·- (.!) a. .... .... -o E :l u CIJ -o u u OIU ,_.o .o .O en u ::J .o o e CT - > o-:.= '- o ... >.'- w_ o- o v o o CIJ v :l o .o ::: o v== ·- ::J :Em ~s ~(jj U)- u.2 ::r:.2 u o U en :t: (/) o u e._ (/) v "FLYSCH DEL Thick. 449 6,8 55,1 30 29,3 570,2 65 635 7,7 G RIVO'" (N) Events 1390 5 4 ll 7 - (upper part) / / / / / / / !/ / 7 v v / Thick. 167,3 33,4 24 134 8 0,9 3,1 3,1 373,8 98 472 4,03 = 15-23 --o Events 860 26 81 9 l 3 6 17 1/95 > Q: C) ..J Thick . 26,3 17,1 4,3 356,5 3,7 0,3 1,7 409,9 53 463 1,26 UJ ll - 15 o - - Events 86 19 ll 6 2 2 5 1/14 ~ u Cl) Thick. 48,7 39,2 6,6 268 7 . 1,3 2,8 1,2 374,2 145 519 1,05 ~ l1... MB 3 - ll = Events 221 31 23 lO 4 7 7 5 1122 / :/ / l/ / / / / / / l/ v '/ "FLYSCH Dl Thick. 116,7 12 6,8 53,4 16,3 5,3 1,8 212,3 39 251,3 4,12 MASAROLIS" l "t (middle- upper part) Events 406 37 26 2 7 13 8 1/203 '- Tab. 2 -Cumulative thickness and number of events of the main lithofacies found in the "Flysch di Masarolis" and in the "Flysch del Grivo ". The data concerning the "Flysch del Grivo" result from the companson and assembling of 21 sections measured in the area comprised between the T. G rivo di Faedis and the T. Chiaro di Torreano. Also the basinal deposits of the upper pan of the "Flysch del Grivo", outcropping near Monteapena are examined. Thicknesses are expressed in metres. Tunis & Venturini :Evolution of the Southern Margin .. . MB 23. The top of the Monteaperta section probably reaches the upper part of the Ypresian. As regards the "Flysch del Griva", the estimated duration of the studied interval comprised between two megabed markers (MB 3 and MB 22) is 2.7 m.y. Tab. 2 shows the enormous role played by megabeds in the total thickness of the explored sequence of "Fly sc h del Griva". In the first interval, the megabed thicknfi:SS accounts for the 51.6% of the section (outcropping sequence plus estimated covered sequence): In ·the second interval, the thick­ ness of megabeds accounts for,as much as 77% of the section. In the l~ t interval it drops to 28.4%. Al together, mega beds make up 1/2 (52%) of the "Flysch del Griva " that reaches the maximum thickness of 1454 m. The ratio between megabed events and siliciclastic turbid­ ites is very high too. In Tab. 2 the values 1/22, 1/14, 1/95 are indicated in the first, second and third segment respectively. By including in the calculation the covered intervals, mainly ascribed to siliciclastic deposits, the values would be lower: the ratios between mega-events and normal events would be equal to 1/50, 1/130 and 1/120 or l/ 70 respectively for the entire sequence. Still, there are some remarks that have to be made. First of all, the number of megabeds should have been higher than what can be observed today; as a matter of fact, there exist cannibalistic megabeds. Secondly, originally siliciclastic turbidites also had overall thicknesses greater than those shown in Tab. 2. The most important megabeds present a thick set of rip-up turbidites involved in the submarine sliding of enormous masses from the platform. We estimate that about 50-60 metres of medium-thin multisource turbidite sequence were removed by the slides. Considering all the assumptions that have been made, one may assert that the siliciclastic detritus contributed not more than 25-28% of the bulk supply. The major siliciclastic supply regarded the last inter­ val (about half of the overall contribution). The ratio between thicknesses of siliciclastic turbidites and calciclastic detritus plus allodapic carbonate detritus (with the exception of megabeds and big beds) shows a virtual equality of the first two intervals, and an abrupt increase of the siliciclastic input in the upper interval. All these values refer to the margin of the basin; it is not possible to define what the situation was in the basin plain and what the volumetric relations were between the various lithofacies. As regards the frequency of megabeds, the periodicity of megabeds in the studied interval v aries from lx l 05 years (entire sequence) up to 7x l 04 years (interval comprised between MB 7 and MB 15) or much less (interval comprised between MB ll and MB 15). Should many thick carbonate turbidites and massive calcarenites' turn out to be seismites, the repeat times will be con­ siderably shorter (halved or even less). On the other 143 hand, the mean repeat time of normal siliciclastic turbidites could be equal to 1.5x 103 years or less. Thus, on average, megaevents were tens times less frequent than normal events. During the emplacement of the megabeds of the "Fly sc h del Griva ", the mean over­ all sedimentation rate was 53 cm/1000 yrs ·1 • The mean sedimentation rate of siliciclastic detritus was 15-18 cm/l 000 yrs·1 , that of the carbonate detritus was about twice as much. Most carbonate detritus, however, is of calciclastic and not of allodapic origin. The sediment rates' of both siliciclastic and carbonate detritus show strong accelerations and sharp decreases. Impressive increases of carbonate sedimentation rate occurred during the second interval. Owing to the strong seismic activity, during long lapses of time ,metres or several metres/ 1000 yrs·1 of material probably deposited. On the base of the repeat time of megaturbidites and the sedimentation rate ofturbidites, MUTTI et al. (1984) identified two tectostratigraphical groups within the family of the alpine-apenninic flysch units: the typical and atypical flysch of Cretaceous to Eocene and Oligocene­ Miocene age, respectively. The later accumulated at a much faster rate in the troughs of the Apenninic orogen. Based on available data, the paleocenic-eocenic flysch of the Julian Basin is closer to the time-thickness distribution of the Mamoso-arenacea formation. However, the two fly sc h strikingly differ in terms of lithology, provenances and geodynamic setting. Finally, excep­ tionally catastrophic events are much more significant in the J ulian Basin. - Considering the high seismicity, we have looked for evidence of palaeoseismicity based on the presence of megabeds in the Tertiary basins of the eastern most sector of the Southern Alps, from the West to the East: Bell uno Basin, Alpago Basin, Clauzetto Basin, Ajdovščina Basin, Brkini Basin, Trieste Basin and Central Istria Basin. However, not only mega beds suggest on palaeoseismicity but also some postdepositional phenomena, especially large flame-structures and injection-structures. During the Lower Eocene, two megabeds appear within the sequence of the Flysch di Bell uno. No mega bed has been identified in the Alpago flysch; only one 7 m thick big bed is described by GNACCOLINI (1968). As regards the Carnic Prealps (Clauzetto Basin), only in the more ancient Tertiary levels and, in particular, in the flysch deposits of basal Eocene (M. subbotinae biozone) outcropping in the north- eastern area (Val .Tremugna), imposing carbonate resedimentations ap­ pear. The Flysch di Clauzetto s.s. (Middle-Upper Ypresian - lowermost Lutetian) is comparable to the Flysch di Cormons from the lithologic and evolutional point of view. In the Vipava valley (Ajdovščina basin) the flysch sedimentation began in the middle part of the Lower Cuisian age (DROBNE & PA VLOVEC, 1991). The lower breccia beds and carbonate big beds are then 'We do not know the criterion to defme seismite or aseismic turbidites in the other thick carbonate beds. At least thirty carbonate megaturbidites can be found in all the examined sections. They are 2 m or more thick and usually display the ideal Bouma sequence (Ta-e), often with a thin breccia carpet at the base. It is not possible to know if they mantled large part of the basin plain, but they surely reached high volumes. 144 penecontemporaneous 10 the megabeds of the middle-upper part of the "Flysch del Griva". In the Brkini area, the flysch deposits are entirely ascribed 10 the Lower Eocene (OREHEK, 1991). ln Brkini, in proximity to Leskovec, at the base of the flysch sequence, PA VLOVEC et al. (1991) reported the presence of a megabed showing a 78 m thick basal olistostrome and calcarenite grading to mari at the top. Macroforaminifers and nannoplancton indicate that the megabed belongs 10 the basal part of the middle Cuisian age. This age might correspond to the uppermost part of the "Flysch del Griva" sequence. In the Middle Eocene of Istria, MAGDALENIĆ (1972) reported the presence of fluxoturbidites made up of limes tone breccias in the lower part, overlaid by foraminiferal microbreccia, sandstone of calclithite type and mari at the top. The most signifi­ cant megabeds can be observed near Mt Starai (Vranja), Gračišće (Pićan) and Kaščerga, where individual clasts of the basal unit reach 2-3 m3• In the same period, even in the Trieste Basin (Trieste-Koper sync line or Savrinsko primorje syncline) carbonate big beds are present. In conclusion, the most important paleoseismicity evidence in the Tertiary of the eastern Southern Alps is related to the evolution of the southern edge of the J ulian Basin and the withdrawal of the north-eastern margin of the Friuli Platform. 7. EVOLUTION OF THE JULIAN BASIN During the main d~positional stage ofthe flysch (Maastrichtian-Lower Eocene), the Julian Basin was a relatively narrow trough; structurally it was represented by an asymmetrical graben (TUNIS & VENTURINI, 1984; PIRINI et al., 1986). In the studied interval, the total thickness of the flysch units, examined bed by bed, by far exceeds 4000 m (Fig. 8). Most of these units are referred to paleoenvironments of the base of the slope of the Friuli Platform ("Drenchia Unit", "Flysch dello l udrio", "Flysch di Mt. Brleka" and "Flysch del Grivo") or even of slope ("Flysch di Clodig" and "Flysch di Calla" that are characterized by thick pockets of slumped beds). For the "Flysch di Masarolis" deposits, one may assume a paleoenvironmental situation of "relatively minor proximity" to the slope. Only the Lower Maastrichtian deposits outcropping between Tolmin and Kobarid may be partially attributed to basin plain environment. Information on the opposite side of the basin is scan t. KUŠČER et al. (1974) report the presence of Maastrichtian olistostromes of northern provenance at Bovec (Slovenia), near the northern margin of the Julian Basin. Information about the north-western edge that provided access to the basin for NCE sediment (Fig. 2) is even less. Thus, under unfavourable conditions, an attempt was made to apply the eustatic curves of HAQ et al. (1987) to the Maastrichtian-Paleocene-Eocene sequence of the Julian Basin which was strongly controlled by tecton­ ics. As far as eustatism is concerned, published models indicate that most siliciclastic detritus reaches the deep Geologia Croatica 45 sea during sea-levellowstands, when the sediments bypass the slope (V AlL et al., 1977; SHANMUGAN & MOIOLA, 1982; VAIL, 1987, etc.). Conversely, ba­ sin siliciclastic sediments decrease during highstands, when a large part of the detritus carried by rivers re­ mains trapped in deltaic areas. In contrast, the response of carbor..ate systems to the changes in sea level is al­ most opposite to that observed in the siliciclastic sys­ tems (KlER & PILKEY, 1971; MULLINS, 1983; DROXLER & SCHLAGER, 1985; BOARDMAN et al., 1986). Sedimentary responses to sea level changes of hybrid siliciclastic/carbonate systems result in the deposition of alternated siliciclastic and platform de­ rived carbonate deposits (HESSE, 1982; HAQ et al., 1987; COOK & MILLER, 1989; DOLAN, 1989; SARG,1989; YOSE & HELLER, 1989). The best correspondence with the curve of HAQ et al. (1987) can be found in the Maastrichtian-Paleocene and in the upper Ypresian, where the lowstand phases (G. cantusa, P. pusilla, P. pseudomenardii, A. pentacamerata biozones) are characterized by massive siliciclastic turbiditic supplies (Fig.8). Turbidites show relatively proximal features. Conversely, during the highstand phases, allodapic limestones (G. gansseri biozone) and/or fine siliciclastic deposits (NCE, I) with dis tal characteristics prevail. The presence or absence of allodapic limestones is to be related to the produc­ tivity of the Friuli Platform margin. Examples of al­ ternating high stand phases with resedimented allodapic carbonate and low stand phases with growth of turbiditic siliciclastic complexes are reported by DOLAN (1989), and are related to the presence of flat-topped carbon­ ate platforms. During lowstand phases this type of carbonate platform considerably reduces its productivity, while during highstand phases, it becomes an active source of intra-bioclastic detritus. These fundamental concepts advise against the application of models concerning environments characterized by terrigenous sedimentation to carbonate depositional environments. Furthermore, they allow to subdivide the sequences within a basin characterized by hybrid sedimentation patterns and, if required, 10 assess the existence of different tectonic controls in the various source areas. The main problems of sequential interpretation raise from the examination of the "Flysch del Grivo" (Late Paleocene-Early Ypresian) and the "Unit ofDrenchia" (EarlyMaastrichtian), as both sequences are strongly controlled by tectonics. The massive coarse carbonate resedimentation and the tectonic phases which have probably extended 10 the northern margin of the basin, totally mask the eustatic effect in these lapses of time. Subsidence is the last but not least of the controls. The geohistory diagram ofFig. 9 has been constructed with time on the x axis and paleobathymetry on the y axis (estimated on the basis of the benthic/planktonic foraminifera ratio and benthic foram associations) and the thickness of the single intervals of the sequence. The resulting subsidence curve shows significant slopes during the .Middle-Upper Maastrichtian and during the Tunis & Venturini : Evolution of the Southern Margin ... 145 M.Y. Age Bio- Eustatic curve l System tra ct Li t h o l o gy m zones High Low o'=>e1 \·· .. ·• Sondstones, conglo. • - . . . . hmerotes,siltstones ·e Q~ -..... cio 0(\\ - (Delta) 4> ~ .... . - Q Siliciclastic ... :J o .. . .. turbidites. _J x _._ "' ~ ...L. • -'- Rare amoiQomoled . _..._ e: sondstones/ -A.pen. . . • o • • o conQiomeroles ~ E 50- e and ' o .......... . -'- ~ M. or. . _...._ . o colciclostic beds ·- -'-. __._ Ond ll) ~ - ,- • -L- u 4> M .for. l'\ s thick beds . ... _.___ ~ 1000 a. M. sub. . __.__ . --->- M. td. _._. -'- .,...=::::::> ~ 6 M.vel. ~ . _.__ . 55 A 6 6 la. Colciclost ic ~= .. ......._. __.__ e: e "' 1-o o o A 6 A A 6 mega beds "10 .&; ................... ·- :J a. and big beds; ... 4 A A 4 6 ..... 11'1 , Q.l a. o l hin- bedded e: n: u .... " .... ·- ( .. "' .. "'"' .......... > o e: 6&6A6AAA66 ·- siliciclastic ~ P. p us. "' o '-- . .. . . ~ ..... ... ~ lurbidites . P-2000 ~J' "' u . __.__ 60 M.an . .. 6 6 6 A A "' 1- M. un. '-- _._ . -'- "' •• • o • •• e ~ (~ .. ·;"a.,. e ( ........ ~ J''-~ .......... ~ o - Siliciclostic turbidite! ui .!!! 1-·- ~ .•. •. ~ ~ -o motrill·supparted con. e ... -.. ... -ci o glamerates and col. l "' o :J . ·.·.;, ·• o ciclastic thick beds. .. HS :::!! o "' 65 a. ~ . __..._ . .2 Red maris and su bor_ e) -'- • ~ o dinale sandslones . ......_ u G.euo. -- Allodapic limestones 3000 --- o A.moy. z. sand stones __.__ -~ e G. con. -- ~ m and maris. o .. ·- _.__ ...L. Siliciclastic flysch. .... "; LS . . . . o .s:. e ~ Some corbonate u o ··~ .. "10 1-70 OI Jts . . . . . . :J bio beds . ·- d .., ... ~ ......... ........ .... ll) ..: _._. oo o Q ........ _._ 'O Corbonate flysch . u o o ·- u ~ . _..._ :! - Mar ls, calcisi ltites, 1-4000 H s o o •• o . o 0 :;: corbonate big beds, - -- -'- ........ u e: ci G.col. SMW .. rore sondstones . __.__ .. 75 E --- o o G.Ven. HS (Teci. phase)~ : Cherty plloaop1c u > llmestones Fig. 8 ·Comparison of the eustatic sea level curve and system tracts (after HAQ et al., 1987, simplified) with the Upper Campanian- Paleogene clastic depositional sequences of the Julian Basin. On the left, stages and related planktonic foram biochronozones. LS = lowstand; HS = highstand; SMW = shelf margin wedge. Vl '