Petrography of Paleogene turbiditic sedimentation in northeastern Italy
Published:January 01, 2007
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Cristina Stefani, Massimiliano Zattin, Paolo Grandesso, 2007. "Petrography of Paleogene turbiditic sedimentation in northeastern Italy", Sedimentary Provenance and Petrogenesis: Perspectives from Petrography and Geochemistry, José Arribas, Mark J. Johnsson, Salvatore Critelli
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The Paleogene turbiditic sedimentation in the eastern Southern Alps represents the sedimentary response to tectonic activity related to the Mesoalpine phase, which involved the surrounding chains from Paleocene time onward. Field and petrographic analyses have allowed us to classify these turbiditic successions as multisource deposits, as demonstrated by the common presence of allochemical, mainly bioclastic detritus, associated with different types of terrigenous arenites. For all units, field data suggest more proximal sources for allochemical supply and distal sources for terrigenous material, characterized by the presence of chert, carbonate rocks, and metamorphic rock fragments. All the investigated successions display transparent heavy mineral associations, marked by the common presence of chrome spinel, alkaline amphibole, staurolite, epidote, and zoisite, which point to similar metamorphic sources. The location of the source of metamorphic rock fragments is uncertain, but inputs from the internal Dinaric belt are possible. The source of the allochemical detritus was located in the nearby reactivated Friuli Platform.
This work discusses the petrographic composition of Paleogene turbiditic units that crop out in the eastern Southern Alps, between the Tagliamento and Piave Rivers (Figs. 1A and 1B). According to Doglioni and Bosellini (1987), these deposits represent the sedimentary response to the Mesoalpine tectonic phase. They indicated the Dinaric thrust belt as a possible sediment source without the support of petrographic analysis.
Over the last thirty years, many papers have been published on the stratigraphy of these deposits (Saint Marc, 1963; Gnaccolini, 1967, 1968; Di Napoli Alliata et al., 1970; Cousin, 1981; Doglioni and Bosellini, 1987; Stefani and Grandesso, 1991; Tunis and Venturini, 1992; Grandesso and Stefani, 1993), but only Piccoli and Proto Decima (1969), Richter (1970), Cousin (1981), Doglioni and Bosellini (1987), and Stefani and Grandesso (1991) linked the deposition of these units to the evolution of the Dinaric chain.
The aim of this work is to contribute to a better characterization of these deposits by means of petrographic data and attempt to reconstruct the paleogeographic and paleotectonic setting in the eastern Southern Alps during the early-middle Paleogene.
The investigated area is located in the northeastern corner of the Adria Plate (Channell and Horvath, 1976) and is bounded to the north by the Valsugana thrust system, to the east by the SW-verging Dinaric thrusts, to the south by the Apennine blind thrust belt, and to the west by the Lessini shelf (Fig. 1A).
The Mesozoic paleogeography of this area largely influenced the Paleogene sedimentation. According to Cati et al. (1989a, 1989b), at the end of the Cretaceous, the area was characterized by the presence of a Basin to the west (Belluno Basin) and a platform to the east (Friuli Platform), the southwesterly prosecution of which has recently been seismically delineated by Fantoni et al. (2002). During the Paleogene, the basinal area was affected by turbiditic sedimentation; the related deposits presently crop out in an E-W–striking area almost 150 km in length and 70 km in width.
Due to the postflysch tectonic activity, the preservation of Paleogene turbidites is poor in the eastern area (Fig. 1), particularly between Meduno and Maniago, while it is more extensive in the western area, where more sections have been considered (Fig. 1B). The northern margin of the basin is not preserved, as it was involved in the Southalpine thrust system, while the eastern side was largely affected by the southwestward migration of the Dinaric thrusts.
The turbidite units investigated in this work are the Claut flysch, Belluno flysch, and Clauzetto flysch (Fig. 1B).
All the investigated units were deposited above pelagic sediments like the Scaglia Rossa unit, which represents the pre-flysch facies (Aubouin, 1963); the succession is generally constituted by red marly shales, locally characterized by common carbonate intercalations and very scattered fine-grained sandy layers. The boundary between pre-flysch deposits and turbiditic sedimentation has been placed where the terrigenous debris becomes more abundant (Hsü, 1970).
In a few cases, e.g., Claut and Erto areas (Fig. 1B), the Paleogene turbiditic successions represent the most recent sediments cropping out; in other cases, the Oligocene-Miocene molasse sediments unconformably cover the turbiditic deposits.
Measurements and sampling were made in the most continuous sections, or in those less affected by tectonics; in most cases, the successions were reconstructed from a number of discontinuous outcrops. As a consequence, all the sections reported in Figures 2 and 3 are composite sections.
For each section, lithology, bed thickness, grain size, top and base geometry, and lateral continuity of beds were carefully examined. Samples were collected both for framework composition and heavy mineral and biostratigraphic analyses.
The units are made up of alternating dark gray arenitic divisions and subordinate gray marls. Bioclastic strata are present in all the successions and sometimes prevail on terrigenous beds.
Based on field observations, three types of turbiditic beds can be distinguished:
Arenitic terrigenous beds, made up of a centimeter-decimeter division, grading to a thicker argillaceous marly interval. The color varies from gray to ocher when altered. The sand/pelite ratio is commonly less than one. The arenitic division commonly displays cross- or convolute lamination (Tc; Bouma, 1962); planar-parallel lamination (Tb) is rarely present. The base of beds may be flat or erosive, with scarce paleoflow casts.
Bioclastic beds, mainly composed of carbonate allochemical grains, commonly thicker than the previous ones, with a ruditic-calcarenitic base grading upward to calcisiltites, capped by a thick carbonate-rich marly division. The sedimentary structures are represented by alternating planar-parallel and cross-laminations. The base is flat and commonly devoid of paleoflow indications.
“Mixed” beds; these are subordinate with respect to the other types, composed both of allochemical and terrigenous grains, characterized by an ruditic-arenitic basal interval passing upward into thicker gray marlstone. The more common internal structures of the ruditic/sandy interval are represented by the repetitive alternation of decimeter-scale packages of planar-parallel and cross-lamination. Convolute lamination due to water escape is common. Texture, composition, and structures of all the “mixed” beds are quite similar in all the examined units.
Thin, highly carbonate marls are commonly intercalated between the turbiditic beds; their recognition is easy due to lighter color and presence of a quite dense vertical bioturbation. Thin sections reveal that these beds are almost exclusively made up by planktonic foraminifers with a very scarce terrigenous silty fraction. These characters allow us to interpret them as hemipelagic layers and point out the deposition of all the units considered above the calcite compensation depth (CCD); these beds were preferentially sampled for the biostratigraphic determinations.
The Claut Flysch
The Claut flysch crops out in two sections along the Torrente Cellina near the village of Claut and southeast of Erto (Fig. 1B). The first section is located along a small tributary on the left side of the Cellina River, east of Claut village, and continues somewhat beyond the confluence. The second section was sampled S-SE of the Erto village along the main valley (right side) and in some small tributaries. Due to its inner position in the Southal-pine chain, the Claut flysch was greatly involved in the tectonic deformation of the chain; the section exposure is not continuous and requires a correlation of separate segments. In both the considered sections, the boundary between the underlying Scaglia Rossa and the turbiditic facies is well exposed.
Near the village of Claut, the lower part of the unit, which consists of thin-bedded base-missing, moderately bioturbated Bouma sequences, grades upward into a highly pelitic middle portion, ∼100 m thick, which displays sparse silty millimeter-size interbeds. The uppermost part of the section is characterized by an upward-increasing sandy content. Some scattered bioclastic beds are present and are interbedded with terrigenous layers very rich in chert grains. The top of the section is represented by a slump of pebbly fossiliferous sandstones well known in geological literature (Dainelli, 1910) (Fig. 2).
Paleocurrent data indicate transport mainly to the E-SE in the lower part and to the E-NE in the upper part (Fig. 2).
The total thickness of the unit has been evaluated at 600–700 m in both the examined sections.
The Belluno Flysch
The Belluno flysch crops out in the limbs of Belluno and Alpago synclines and near the town of Feltre (Fig. 1B). In the regional geological literature, different names are locally used for the flysch outcrops of Belluno-Feltre and Alpago areas, respectively (Gnaccolini, 1967, 1968), but this distinction is not supported by the biostratigraphy and lithostratigraphy.
The Belluno flysch is constituted by an almost regular alternation of turbiditic beds in which Ta-e Bouma sequences are very rare; base-missing sequences are common and are generally capped by few-millimeters- to few-centimeters-thick hemi-pelagic marls; the deposits are regarded as basin plain turbidites (Gnaccolini, 1967, 1968; Doglioni and Bosellini, 1987; Stefani and Grandesso, 1991). For this unit, four stratigraphic sections were considered (for location, see Fig. 1B). The first one (section 3, Figs. 1B and 3) is located immediately north of Belluno town, along the Medone and Ardo Rivers. The second section (number 4, Figs. 1B and 3) is located in the Feltre area, close to the western margin of the flysch basin. Due to the exposure conditions, the thicknesses of some segments of this section were evaluated only approximately. The third section (T. Limana, number 5, Figs. 1B and 3) crops out in the southern flank of the Belluno valley and includes only the lower part of the flysch succession. The fourth section is located in the Alpago area (number 6, Figs. 1B and 3).
The grain size of the terrigenous beds shows a progressive fining westward, together with a significant increase in thickness of hemipelagic sediments interbedded with turbiditic layers. In the T. Caorame section (section 4), hemipelagic sediments are commonly thicker than 10 cm, with a maximum of 30–40 cm, testifying that this area was reached by very diluted turbidity currents.
Paleocurrents obtained from sole marks give NW-SE derivations, with only few exceptions. In the Feltre area, the paleo-current indications become rare due to the high dilution of the turbidity currents. In the Alpago area, Gnaccolini (1967) reported a few paleoflow directions coming from the S-SE, i.e., from the Cansiglio High.
The total thickness of the Belluno flysch has been estimated to be at least one thousand meters in the well Sedico-1, where an early Eocene age has been determined (Costa et al., 1996); the succession becomes progressively thinner westward, where it shows onlapping relationships with the eastern slope of the Lessini Shelf. The youngest strata (middle Eocene) have been recorded in the T. Caorame section (Grandesso, 1976; Stefani and Grandesso, 1991).
The Clauzetto Flysch
The Clauzetto flysch was sampled in two quarry roadcuts near Fanna and Travesio villages (Fig. 1B) where the unit is excellently exposed.
In the Fanna section, the Clauzetto flysch succession is complete from the base to the top, while in the Travesio quarry, only the upper part of the succession crops out (Fig. 3).
In both the analyzed sections, the unit is represented mostly by plane-parallel arenaceous intervals interbedded with prevalent gray marls and scattered millimeter-thick hemipelagic beds. The bioclastic beds represent ∼50% of the total thickness and, in a few cases, show debris deposits with lenticular geometry.
Bioturbation is common, both in the hemipelagic layers and on the soles of arenitic intervals; according to Tunis and Uchman (1998), the predominant genera are Planolites and Chondrites in the pelites, while the arenitic soles are dominated by Scolicia and Ophiomorpha, indicating a moderately oligotrophic environment.
Paleocurrents show low variation and reflect currents flowing from W-SW. The total thickness of the unit is 500–600 m.
Several samples were collected along the measured sections, mainly from hemipelagic layers, in order to obtain a biostratigraphical zonation according to planktonic foraminiferal scheme proposed by Berggren and Miller (1988) and Berggren et al. (1995). Results are synthetically exposed in Figures 2, 3, and 4. The following biostratigraphic events are considered: the total range of the Globanomalina pseudomenardii, the last occurrence (LO) of Morozovella velascoensis, M. formosa formosa, and M. aragonensis, and the first occurrence (FO) of Turborotalia cerroazulensis frontosa and Hantkenina.
In particular, the Claut flysch is referred, based on our data (Fig. 2 and 4), to the Selandian-Thanetian/Ypresian interval (P3-P5 zones of Berggren et al., 1995), in the Claut section and up to the P7 zone in the Erto section. The Claut section was previously referred by Marie and Cousin (1965) and Cousin (1981) to about the same stratigraphic interval (i.e., Selandian-Ypresian), but these authors considered a different formational boundary.
The Belluno flysch sedimentation took place in the Alpago and Belluno areas (sections Medone, Alpago, and Limana) during the Ypresian (uppermost P7–P9); our present data confirm substantially the age of Di Napoli Alliata et al. (1970). In the western sectors (T. Caorame section), the turbiditic sedimentation began later (P8 zone) and continued until the Lutetian (P12 zone) (Fig. 4) as recognized by Grandesso (1976) and Stefani and Grandesso (1991).
The Clauzetto flysch is referred to the Ypresian (from the top of P7 zone to P9). Previously the base of the unit, which crops out a few kilometers west of Fanna quarry, was referred to the Paleocene-Eocene boundary by Saint Marc (1963); Tunis and Uchman (1998) dubiously referred the Clauzetto flysch to the early Eocene (Morozovella formosa and M. aragonensis zones sensu Toumarkine and Luterbacher , corresponding to intervals P7 to P9 of Berggren and Miller ).
COMPOSITION OF THE TURBIDITE BEDS
These beds are generally the thickest in all the successions. The bases are commonly ruditic and grade upward into fine sand and silt; the finer portions of these beds are represented by highly carbonate marls. The carbonate beds may be classified as bioclastic rudstone-grainstone or packstone. The bioclastic content mainly consists of Nummulitidae, red algae, bryozoa, echinoderm debris, and, with few exceptions, intraclasts, peloids, green particles, all of which point to a provenance from carbonate shelf areas located in the photic zone.
The bioclastic supplies are particularly relevant for the Belluno flysch and Clauzetto flysch, both of which were deposited very close to the Friuli Platform, where, during the Paleogene, carbonate factories were active on the same sites of carbonate production during the Mesozoic (Cousin, 1981; Fantoni et al., 2002).
A peculiar situation has been recognized along the southern side of the Belluno valley (T. Limana section on Fig. 3) where the lower part of the Belluno flysch crops out. At least five mega-beds, up to 40 m thick, are constituted mainly by coarse-grained breccias, entirely composed of almost coeval bioclastic material resedimented by gravity flows. According to Gnaccolini (1968), this material came from a southwestward prolongation of the Friuli Platform, as suggested by the rapid fining in the northerly direction. In a recent interpretation of the TRANSALP deep seismic profile, the prosecution of the Friuli carbonate platform has been clearly recognized in the subsurface (Fantoni et al., 2002; Bertelli et al., 2003).
Framework Composition of the Terrigenous Beds
Sixty-five samples of arenites representative of the total thickness of the three units were analyzed in order to obtain detailed information on composition. Five hundred points were counted for each thin section, stained for the distinction of K-feldspar and to discriminate the carbonate phases. Analyses were performed according to the Gazzi-Dickinson method (Gazzi, 1966; Dickinson, 1970), which tends to minimize the grain-size influence on arenite composition. Consequently, each grain composed of more than one crystal larger than the matrix limit (0.0625 mm) was classified during point-counting as mineral type and not as rock type. The point-counting results were tabulated (Table 1) 102103104105 and recalculated to obtain the diagrams of Figures 5 and 6.
Results of point counting are reported in Table 1 102103104105; they are subdivided into four classes, and the recalculated proportions are represented in Figure 5A. According to Zuffa classification (1980, 1985), NCE represents the “noncarbonate extrabasinal grains,” CE is the “extrabasinal carbonate grains,” NCI is the “noncarbonate intrabasinal grains,” and CI is the “carbonate intrabasinal grains.” Figure 5B shows the composition of the terrigenous grain fraction, where Q comprises the total quartz, F, the feldspars, and L+CE, the fine-grained rock fragments and the extrabasinal carbonates. Chert was included in the L + CE pole due to the almost exclusive association with carbonate rocks in the sedimentary covers of the Southern Alps and Dinarides.
All investigated samples can be classified as carbonate or noncarbonate extra-arenites, according to the first-level classification of Zuffa (1980, 1985), while in the second-level classification, they can be described as “litharenites” (Fig. 5B). This term is not related to a particular classification, as for the use of the Gazzi-Dickinson method. However, most, if not all, classifications proposed in literature would provide this term for data placed on the bottom-right pole of the triangle. Consequently, we use the classifying term in inverted commas.
Quartz occurs both as monocrystalline and polycrystalline grains and as a phaneritic component in metamorphic and granitic/gneissic rock fragments. Feldspar grains are commonly present as single crystals and, in a few cases, within coarse-grained rock fragments derived from granitic/gneissic rocks. Low- to medium-grade metamorphic lithic grains include micaschists, epidote-schists, quartzites, phyllites, and a few metavolcanics. Chert grains consist of microquartz and scarce chalcedony; textural characters are frequently obliterated by Fe-oxides and dolomite rhombohedrons.
Volcanic rock fragments include rhyolites and rare intermediate grains with lathwork-like intersertal texture made up of euhedral plagioclase and rare glassy grains. Phyllosilicates are negligible. The other mineral grains include mainly garnet, zircon, epidotes, amphiboles, and spinel.
The most common lithotypes in the carbonate fraction are well-rounded grains of micritic limestone, followed by coarse- and fine-grained polycrystalline dolomitic clasts. In a few cases, some oolitic or fossiliferous limestone grains with planktonic foraminifers were recognized.
The Claut flysch displays the highest quartz content (mean value: Q 45.6, F 8.5, L + CE 45.9) and also the highest average amount of feldspar grains (Table 2). The rock fragment association displays high percentages of metamorphic, volcanic, and granitic and/or gneissic rock fragments ranging from 30% to 70% of the lithic population (Fig. 6). Other fractions of the rock fragments are represented by chert and subordinate dolostone and limestone.
Both Belluno flysch and Clauzetto flysch show a significantly different composition and are “litharenitic” in composition (mean values, respectively, Q 25.0, F 2.2, L + CE 72.8, and Q 13.1, F 1.7, L + CE 85.2; Table 2) with a predominance of dolomite on the other rock fragment types. Low-grade metamorphic rock fragments include phyllite and quartzite; volcanic rock fragments of rhyolitic composition are also present.
About 40 samples were selected for heavy mineral investigation and were prepared following the standard techniques described by Gazzi et al. (1973) and Mange and Maurer (1992). Repeated dilute HCl (10%) immersions and boiling with oxalic acid and aluminum were necessary to remove the high carbonate content and Fe-oxide grain coatings, respectively. The heavy minerals were separated from the 0.0625–0.250 mm sandy fraction using tetrabromoethan (density: 2.96 g/cm3) and then optically analyzed in transmitted light after immersion in methylene iodide (n = 1.74); at least two hundred transparent grains were determined for each sample. Results are reported in Table 3 302 and plotted in Figure 7; the class “others” includes kyanite, chloritoid, olivine, anatase, and brookite.
The heavy mineral fractions are commonly <1.0% by weight of the grain-size fraction considered, and transparent heavy minerals average ∼20% of the total heavy fraction.
All samples have a ZTR index (zircon-tourmaline-rutile; Hubert, 1962) (Table 3 302; Fig. 7) ranging from a few percent to over 40%, high amounts of reddish brown Cr-spinel (up to 70%), and variable amounts of garnets, ranging from 2% to 27%. Epidote-zoisite grains are always present, and in two samples of Clauzetto flysch, they are very abundant (up to 60%). Moreover, we found minor or trace amounts of unstable heavy mineral species such as staurolite, kyanite, olivine, glaucophane s.s., and crystals of glaucophane-riebekite series. At a first glance, the occurrence of these mineral species is particularly indicative of various ranks of metamorphic rock sources. Moreover, the presence of small quantities of monazite and xenotime points to a granitic source.
The presence of abundant Cr-spinel suggests provenance from ophiolite-bearing units, even if the absence of serpentinite rock fragments points to possible recycling of the spinels. In fact, Zimmerle (1984) stressed the high stability of the brown spinel that “can also occur as solitary, without unstable serpentinite fragments.” Also Kukharenko (1964) demonstrated the high stability and durability of this mineral that may be hydraulically concentrated in placers. In two recent papers, Lenaz et al. (2000, 2003) analyzed the geochemistry of spinel grains from the Claut flysch, pointing out high Cr content of the minerals. According to Lenaz et al., the analyzed minerals were supplied mainly by mantle peridotites, while other turbidite successions outcropping more to the east were mainly characterized by Cr-spinel derived from volcanic sources. This mineral is considered distinctive of ophiolites of the Vardar Ocean eroded from the Jurassic to Paleogene (Lenaz et al., 2003).
On the basis of our field and laboratory analyses, the Paleogene turbiditic successions of eastern Southern Alps can be regarded as multisource units, made up of two different types of sources that controlled the basin fill without appreciable variation during sedimentation time. Two specific sources of sediments were contemporarily active and gave origin compositionally distinctive types of bed: carbonate beds made up of intrabasinal bio-clastic detritus and terrigenous beds made up of siliciclastic-carbonate detritus. Mixed beds, derived by the mixing of sediments are also present. Bioclastic detritus was supplied by active carbonate shelf areas located in the photic zone. As depicted by Cati et al. (1989a, 1989b) on the basis of subsurface data, the region was characterized by large carbonate platforms in which small troughs elongated with a Dinaric trend developed (Fig. 8). The bioclastic detritus could have been supplied by various sectors of the Friuli Platform. This platform was an active carbonate factory since the Late Jurassic and during the Cretaceous. According to some authors, it underwent repeated subaerial exposures during the Late Cretaceous and early Paleocene (Iaccarino and Roveri, 1964; Cousin, 1981; Cati et al., 1989a, 1989b). Many stratigraphic indications suggest a reestablishment of the carbonate production also during the Paleogene, as demonstrated by the presence of resedimented carbonate debris in the basinal or slope deposits, such in the Scaglia Rossa (Tunis and Uchman, 1998; Swinburne and Noacco, 1993) and the overlying Belluno flysch (Gnaccolini, 1967, 1968) and Clauzetto flysch. It cannot be excluded that other carbonate platforms, presently not preserved, were active in sectors proximal to the terrigenous supplies. The presence of “mixed” beds containing different kinds of grains (i.e., allochemical and terrigenous) indicates a parking area where these two types of sediments could have been mixed up, and suggests a location of a carbonate platform close to the entry points of the terrigenous detritus. On the contrary, the general distal facies associations that characterize all stratigraphic units point to very long-distance terrigenous supplies. Unfortunately, the compressional tectonics active during Neogene in the Southalpine domain probably destroyed the proximal portions of these turbiditic basins and their relation with the respect to the source areas.
Westward of Feltre, the turbiditic facies gradually change into slope facies, mostly hemipelagic pelites, and the flysch succession onlaps the eastern slope of the Lessini Shelf (Bosellini, 1989; Doglioni, 1990; Grandesso and Stefani, 1993; Trevisani, 1994). Also, the westward-younging of the turbidites and the variation of sediment thickness (Fig. 3, T. Caorame section versus T. Limana; Grandesso, 1976; Stefani and Grandesso, 1991) testify to the depocenter migration in response to the Dinaric thrust propagation (Cousin, 1981; Doglioni and Bosellini, 1987).
In all the investigated units, the paleocurrent data indicate a predominant sediment transport parallel to the inferred NW-SE–trending Dinaric basin axis, with a provenance from the northern sector, where part of the terrigenous material was probably eroded.
The petrography of the terrigenous arenites clearly shows the presence of different sources for the detritus of the investigated successions. In fact, the Claut flysch, the oldest one, is more quartzolithic and displays a higher variety of basement-derived rock fragments with respect to the youngest successions (Belluno and Clauzetto F., Fig. 5B), where this types of terrigenous input is masked and diluted by high amounts of detritus derived from carbonate sedimentary cover (Fig. 6). Both the Dolomites and Dinaric areas are characterized by a carbonate sedimentary cover, but a thick carbonate cover also characterizes the Austroal-pine units, presently cropping out north of the Periadriatic Lineament and bordering the Tauern Window (Fig. 1A).
The heavy mineral data indicate a relatively constant source area of the terrigenous material from middle Paleocene (sedimentation of Claut flysch) to early-middle Eocene (sedimentation of Belluno and Clauzetto flysch). This source was characterized by medium-grade metamorphic and granitic rocks, but a partial recycling from older arenitic successions cannot be ruled out.
An important source of sediment is represented also by metamorphic rocks of various rank that have furnished Cr-spinel, alkaline amphiboles, epidotes, and staurolite. The location of these metamorphic rocks is particularly intriguing.
For the provenance of alkaline amphiboles, we can hypothesize some inputs from the Variscan high-pressure successions that had been exposed since the middle Cretaceous, as evidenced by the presence of glaucophane-crossite associations in the Allgäu flysch deposits (Winkler and Bernoulli, 1986). A (re-)activation of the same source also during the Paleocene-Eocene cannot be excluded. However, a possible oceanic source can be supposed more to the south. In fact, the final closure of the Vardar/Meliata Ocean (Late Jurassic) led to the formation of an initial nappe pile at the southeastern margin of the Austroalpine microplate that included the Vardar/Meliata oceanic suture zone (Neubauer, 1994). Sediments started to be shed into the Austroalpine realm in the ?Berriasian/Valanginian from this nappe pile (Faupl and Tollmann, 1979).
Von Eynatten and Gaupp (1999) argued that a provenance from an oceanic crustal source is proved by the dominant presence of Cr-spinel in part of the Cretaceous sedimentary succession that crops out also in the Northern Calcareous Alps. According to their reconstruction, this source was no longer active since the Turonian/Coniacian.
Chromite is very abundant in the Aptian-Albian flysch of the Lienz Dolomites (Faupl, 1976) and in the Turonian to Coniacian flysch sequences of the Simme nappe (Flück, 1973). On the contrary, chromite is very rare or absent in the Upper Cretaceous Lombardian flysch (Bernoulli and Winkler, 1990) and, where present, it probably was derived from the ultramafic rocks cropping out westward.
The presence of Cr-spinel and alkaline amphiboles in our samples does not allow a clear distinction between these different sources, but the paleogeographic location of the investigated successions and the presence of NW-SE–trending troughs suggest that rocks derived from the Vardar Ocean could have been an important source (Fig. 8). It is important to stress that the turbiditic successions cropping out more eastward (Lenaz et al., 2003) show the significant presence of Cr-spinel grains, suggesting the Vardar zone as the most probable source of terrigenous detritus.
In conclusion, the slight similarity in composition of Claut flysch, quartz-rich “litharenites,” with respect to the youngest Belluno flysch and Clauzetto flysch, carbonate-rich “litharenites,” suggests that the terrigenous detritus supplied into the basins from late Paleocene to middle Eocene was similar, in particular in regard to heavy mineral associations, and, consequently, it may have been eroded from different sectors of the same orogenic belt, characterized by small lithological differences. The increasing content of carbonate rock fragments in the young successions provides evidence of an important involvement of the sedimentary cover with time. On the contrary, the basement supply became less significant with time, as underlined by the different proportions of “granitic and/ or gneissic rock fragments” among the oldest Claut flysch and the youngest Belluno and Clauzetto flysch.
Biostratigraphic, lithostratigraphic, and petrographic data allow us to consider the Paleogene turbiditic successions cropping out in the eastern Southern Alps as the response to the tectonic phases that involved the Dinaric chain from Paleocene onward. Petrographic analyses demonstrated that these successions are prevalently composed by two types of sediments derived from well distinct sources. An important source was represented by the coeval allochemical detritus yielded by carbonate factories, most probably linked to the Cretaceous Friuli Platform, which were renewed during the Paleocene to middle Eocene.
The terrigenous detritus of Claut, Belluno, and Clauzetto flysch deposits derived from different sectors of the same orogenic belt, as demonstrated by similar arenite composition and heavy mineral associations. The terrigenous source areas were probably characterized by extensive carbonate covers above a metamorphic basement with remnants of oceanic crust as indicated by the common presence of Cr-spinels. The scattered but significant presence of alkaline amphibole in all the successions points to a high-pressure source related to the subduction and subsequent obduction of an oceanic crust.
Stefani and Grandesso were supported by research grants of Padova University and by Consiglio Nazionale delle Ricerche (CNR), Istituto di Geoscienze e Georisorse, Padova. The manuscript strongly benefited from the critical, patient reviews of Daniela Fontana and Hilmar von Eynatten. Detailed sample locations will be provided on request.
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Sedimentary Provenance and Petrogenesis: Perspectives from Petrography and Geochemistry
- clastic rocks
- Dinaric Alps
- Eastern Alps
- experimental studies
- field studies
- heavy minerals
- hemipelagic environment
- laboratory studies
- marine environment
- mineral composition
- planktonic taxa
- point counts
- sediment supply
- sedimentary rocks
- Southern Europe
- terrestrial environment
- northeastern Italy