Luglio 2000 A LATE TRIASSIC OSTRACOD ASSEMBLAGE FROM THE QUATTERVALS NAPPE (AUSTROALPINE, NORTHERN ITALY) SYLVIE CRASQUIN-SOLEAU 1, FABRIZIO BERRA 2 s. ROBEMO RETTORI3 Receit,ed February 15,200A; accepted April 3,200A Key-u,ords: Late Triassic, Ostracods, StratigraphS Austro- :ìpine. P:ìaeon toìogy. Riassunto. La successione norica della Falda Quattenals (Au- stroalpino Centrale, Italia settentrìonale) registra una evoluzione stratigrafica da facies di piattaforma carbonatica di n.rare basso sogget- ta a fenomeni d:i dolomitìzzazione precoce (Dolomia Princiaple- Hauptdolomit) r'erso facies dì transizione (Formazione di Pra Grata), rappresent:rte da alternanze di dolomie e calcari scuri e brecce dolomitiche di pendio. La successione evolve, poi, verso facies di baci- no intrapiattaforma (Calcare dì Quattervals), rappresenrate da calcari scuri con subordinate intercal:rzioni di brecce contenentì clasti con facies di piattaforma e lìvelli paracongiomerati. Le facies bacjnali e transizionali consistono prevalentemente di materiale risedimentato daila piattaforma; sono, inoltre, presenti live l- li di materiale detritico fine c orizzonti microbialitìci . Le facies bacinali contengono abbondante materiale bioclasti- co, all'interno del quale è stato possibile riconoscere un'interessante associ:rzione ad ostracodi, costituita da numerosi individui apparte- nenti a poche spccie, che denota condizioni ambicntali poco favorevoli carxtterizzate da variazioni di salinjtà e scarsa ossigenazione. Nono- stante la intensa tettonica alpina e la diffusa ricrisrallizzazione, I'analisi del materiale raccolto alla base del Caicare di Quattervals, ha permes- so dì identificare, insieme a forme eià note, lc due nuove specie di ostracodi Rbombocythere dimorpbica e Kerocythere quattercalsi. Abstract. The up to 1200 m thìck Norian succession of the Quatteruals Nappe (Central Austroalpine , Italy) documenrs a tecton- ically-driven passage from intertidal facies (Dolornia Prìncip:le- Hauptdolomit), represented by early-dolornitized bedded ro massive gray dolostones, to intraplatform basin sediments (Pra Grata Forma- tion and Quattervals Limestone), consisting of resedimented dark limestones and thin bedded laminated limestones, alternatìng (maìnly in the Pra Grata Fonlatìon) with intrafonnational breccias contaìning abundant shallow-water facies. The transitional and basinal facies con- tain an interesting ostracod assembÌage, beside bioclastic layers con- taining shallow-water and upper slope skeletal graìns. Thc ostracod fauna is rich in specimens belonging to few genera, denoting restric- ted environmental conditions characterized by variations of salinity and lon oxygenation. Dcspite the intense deformation and recrystal- lization related to the Alpine tectonics, the paleontological analysis of the ostracod assemblage from the base of the Quattervals Limestone, allowed the identìfication of the two new species of osracods Rhom- bocythere d,imorphica and Kerocytbere quatten:alsì in association with other already known forms. Introduction and geological setting. Despite the strong recrystallization related to the Alpine tectonics, an interesting ostracod assemblage has been found in the Norian carbonates of the Quatterwals Nappe (Upper Austroalpine, Eastern T.ombardy, Italy; Fig. 1). The sedimentary succession of the Quattervals Nappe is exclusively represented by a Late tiassic (mainly Norian) up to 1200 metres thick carbonate suc- cession (Hess, 1953; Somm,1965;Berra,1.995) (Fig. 2). The lower formation of the Quattervals Nappe is repre- sented by dolomitized inner carbonate platform facies referred to the Dolomia Principale-Hauptdolomit. This formation passes upwards to the Pra Grata Formation (Hess, 1953), characterised by alternations of lime- stones and dolostones. Upwards, it is covered by the n'ell-bedded dark limestones of the basinal Quattervals Limestones, having at the top about 60 metres of marls (Crappa Mala Beds; Furrer, 1983). Above, a recovery of shallow-water carbonate platform is documented by the Murter Dolomite (Furrer, 1983). The top of the Norian succession is represented by subtidal facies belonging to the Murter Plattenkalk. The stratigraphic boundary to the overlying Rhaetian Kossen Formation was documented by Somm (1965) both in the Quattervals Nappe and Terza Unit. Stratigraphy. In the studied area, only the lower part of the Quattervals Nappe succession is preserved, from Hauptdolomit to the upper Quattervals Limestone. The thickness of the Hauptdolomit, whose base is lacking, is about 400-500 m and it is mainly represented by irregu- i - CNRS ESA 2073, UPMC, Dtp. Géologie Sédimentaire, T. 15-25, E.4, case 104, 4 Place Jussieu, /5252 e-mail: crasquin@ccr. jussieu.fr 2 - Regione Lombardia, Ufficio Progetti Speciali per la Geologia e la Sismica, Yìa F. Frlzi 22, e-mail: fabrizio-berra@regione.lombardia.it 3 - Dipartimento Scienze dellaTerra, Piazza dell'Università,06100 Perugìa, Italy; e-mail: simocir@unipg.it Paris cedex 05, France; 20124 Milano, Itaiy; 182 S. Crasquin-Soleau, F. Berra & R. Rettori Fìo I lar cy.clic alternations of fine grained dark-grey dolo- stones and doloarenites, both in centimetric and pluri- metric layers, with locally intraformational breccias (Somm, t965;Berra, 1995). Doloarenites mainly consist of often normal grading intraclastic packstones, rarely bioclastic or oolitic; ripples and cross laminations, local- ly burror.ed, are also frequent. Stromatolitic horizons (up to more thf,n I m thick) are also common. Emer- sions are documented by mud cracks, authigenic evapo- ritic crystals and fenestral fabric. In the upper Hrupt- dolomit, the occurrence of sedimentary dykes and brec- cia bodies documents the beginning of the tectonically induced platform drowning leading to the developmenr of the Quatterwals Basin (Berra, 1995). The Hauptdolomit of the Quattervals Nappe was deposited in a shallow water basin under low dysoxic conditions with local and short-term emersions. Bio- clasts are represented by pelecypods and gastropods and rarely by dasycladacean algae and foraminifera. The overlying Pra Grata Formation whose thick- ness increases from 30 to 200 m souths/ards (Berra, 1995) is characterised by irregular alternations of lime- stones and dolostones. Their abundance varies along the succession: dolostones prevails at the base, while lime- stones become more important toward the top. Lime- stones consist of fine grained, biociastic, commonly thin bedded calcarenites, often with millimetric marly hori- zons. Locally slumps are present. In the upper part, intercalations of paraconglomerate beds up to 3 metres thick often occur. The dolostones consist of generally LEGEND I I M. Motto/Allgàu Fm. I I fraele/r\ossen rm. [-Tl ^,.^*^-..^r^ T :*^^+^ -l vudlletvals Llmestone T--l ^| | t.ra urata fm. T------ ^ ,| .l l)olomia del CrisralloHauptdolomit f_.l Bu.n-n.,i S su-ntins.itn 2km light-gray chaotic breccia bodies (up to 10 m thick; sometimes amaÌgamated, containing carbonate platform derived clasts. Centimetric to decimetric doloarenite beds are common. Alternations of dolostones and lime- stones are indicative of the existence of an adjacent sub- merging carbonate platform of Dolomia Principale- Hauptdolomit. Muds and fine calcarenites were expor- ted by currents or storms from the surrounding carbo- nate platform, as documented by microfacies often rich in platform-derived shallow water allochems. Dolornitic breccias were directly derived from the dismantling of dolomitised and lithified carbonate platform facies, probably exposed along scarps (Berra, 1995). The large number of breccia bodies and the occurrence of slumps indicate that Pra Grata Formation deposition occurred on a developing slope. Both the lower and upper limits of the formation are transitional. The Quattervals Limestone (600-650 m thick) is almost entirely represented by dark, bedded mudstones and wackestones, with marly joints and frequent interca- lations of graded packstones yielding platform-derived crtins lSnmm lgAS'R.'"' lgq(ì f)..'.^--lD-*... .. -_, Jcrra, t11J). t à.rJeongtomefate In up to 4rn thick beds, and more rarely breccias, are inter- calated. The upper limit with the Crappa Mala Beds (Obere Mergel; Somm, 1965) has not been obser-ved in the studied area. The Quattervals Limestones were not deposited on perfectly flat-lying sea-bottoms: slumps, overfolds and slump scars document deposition on a low-anglei slope. Microfacies analysis shov'ed rwo gener- ically different deposits: intraclastic-bioclastic lime- Gcological n-rap with the location of thc position of the sampling site. The structural sketch refers to an area slightl,v larger than that of the geological raap. l'- .- ' Jqt' ,*\t a. \J CdeFopel ì,Aì aoro de Covo t a:i ó" ,.,ts o Late Ti'iassic ostracctcl assemblage 2 -VAL TRENZEIRA TAGLIATA ffi H [.-.---l lI-------\------#,hT1 /, L-'I-r-Il .f^1 ./ 183 trio ) stones directly derived from sedimentation of platform material and limestones derived from decantation of fine grained material and represented by mudstones and fine- grained wackestones. Smal1 foraminifers, sponge spicu- lae, rare ammonoids and calcified radiolarians are pres- ent. The dark colour of the sediments and the absence of bioturbation document unfavourable conditions for the development of diversified benthic communities on the bottom of the Quattervals Basin. The evolution of the stratigraphic succession allows reconstruction of an asymmetric intraplatform basin controlled by syndepositional extensional tecton- ics, with a north-south width of at least about 8 km and an east-west width of some tens of km (Berra, 1995). The sedimentary input was twofold, with breccias main- ly from southern high and limestones from sweeping of the two margins. The southern margin of the basin was tectonically-controiled, whereas the northern was a flex- ural-type margin: the depocentre was asymmetric, and situated near the southern margin (Berra, 1995). Facies analysis demonstrates that the Quattervals Basin was a partly isolated sea-way; water exchanges with a wider marine basin occurred only for the surface water, while the bottom conditions were dvsoxic to anoxlc. 3 - DIGA DEL GALLO 4-M.LA /s ennsso DEi coRNo Palaeontology. Despite the intense and polyphasic alpine tecton- ics and the strong recrystallisation locally destroying the original features of the sediments, ostracods occurrence has been documented in ail the three forrrations consid- ered here. Ostracods are particularly abundant in the fine-grained calcareous facies of the Pra Grata Forma- tion and Quattervals Limestone, generalll. associated with rare specimens of Norian-Rhaetian foraminifers referable to Gandinella falsofriedll (Salaj, Borza 8c Samuel) . The present paper is focused on a rich ostracod assemblage from a calcareous outcrop stratigraphicaily referred to the Pra Grata Formation (coord. Gauss Boaga 1588, 510-5159, 250). The assemblage has been recorded from well-bedded black limestones consisting of recrystallised packstones-wackestones. The ostracod-bearing facies are mainly represen- ted by dark resedimented fine-grained calcarenites and by dark microbialites, where ostracods are trapped between the organic laminae. The sedimentological fea- tures indicate that the ostracods probably lived within the mic,robialitic mats, which developed in subtidal con- ditions on low-oxygenated bottoms. I -V DELCANTONE (tl O 3 l#l t | | lQuohervolslimestone9f-?r-r.a Sompling slteta (urot'nr:::.., posirion) HA\N Poroconqlomerotes |-;.=N Slumps ffi Breccios T----f------l " l-r----'--i i"\<