Lower Jurassic carbonate succession between Predole and Mlačevo, Central Slovenia Spodnjejursko karbonatno zaporedje med Predolami in Mlačevim, osrednja Slovenija Stevo Dozet 1 1Geološki zavod Slovenije, Dimičeva ulica 14, SI-1000 Ljubljana, Slovenija Corresponding author. E-mail: stevo.dozet@geo-zs.si Received: March 4, 2009 Accepted: April 8, 2009 Abstract: The paper deals with Lower and Middle Jurassic carbonate succession sotheasterly of Ljubljana in the area belonging to the northern margin of the Dinaric Carbonate Platform. The Lower Jurassic carbonate succession between Predole and Mlacevo is composed of five lithostratigraphic units that in view of biostratigrafy belong to the cenocona Palaeodasycladus mediterraneus (Pia). In the topmost part of the Lower Jurassic stratigraphic sequence there is an unfossil-iferous interval zone of the platy Spotty Limestones. The considered rocks are developed in a shallow-water carbonate facies lying conformably upon the Norian-Rhaetian variously grey bedded dolomite in Lofer development, the Main Dolomite respectively. The passage of the micritic Lower into oolitic Middle Jurassic limestones is gradual. In the Lower Jurassic carbonate sequence, where limestones strongly prevail over dolomites, occur thinner and thicker sedimentary rhythms of subtidal and intertidal environments. The first and especially the last unit of the Lower Jurassic carbonate succession between Predole and Mlacevo are poor in fossils. In central more or less fossiliferous unit play most important role alga Palaeodasycladus, benthic foraminifer Orbitopsella, lithiotid bivalves and corals. Important are also megalodontid bivalves that appear already in the Rhaetian and Lowermost Jurassic part of the Mesozoic stratigraphic sequence. Lithiotid and megalodontid bivalves occur in the form of lumachelles composing biostromes, whereas the corals build minor mud mounds. Nonfossiliferous dark platy Spotty Limestones were deposited in restricted parts of shelf, where there were no favourable conditions for greater diversity of organisms. In geotectonic regard is the investigated area a part of External Dinarides. Povzetek: Članek obravnava spodnje in srednjejursko karbonatno zaporedje plasti jugovzhodno od Ljubljane na ozemlju, ki pripada severnemu robu Dinarske karbonatne platforme. Spodnjejursko karbonatno zaporedje med Predolami in Mlačevim je sestavljeno iz petih litostratigrafskih enot, ki biostratigrafsko pripadajo cenoconi Palaeodasycladus mediterraneus (Pia). Prav na vrhu je nefosiliferna intervalna cona ploščastih marogastih apnencev. Obravnavane kamnine so razvite v plitvovodnem karbonatnem faciesu. Leže konkor-dantno na norijsko-retijskem, različno sivem plastnatem dolomitu v loferskem razvoju oziroma na Glavnem dolomitu. Prehod mikritnih spodnjejurskih v oolitne srednjejurske apnence je postopen. V karbonatni spodnjejurski skladovnici, v kateri močno prevladujejo apnenci nad dolomitom, se pojavljajo tanjši in debelejši ritmi sedimen-tov podplimskega in medplimskega okolja. Prva in še zlasti zadnja enota spodnjejurskega karbonatnega zaporedja med Predolami in Mlačevim sta revni s fosili, v osrednjih fosilifernih plasteh pa igrajo pomembno vlogo alga Palaeodasycladus, bentična foraminifera Orbitopsella in litiotidne školjke. Zelo opazne so tudi megalodon-tidne školjke, ki se pojavljajo že v retijskem in spodnjejurskem delu mezozojske skladovnice. Litiotidne in megalodontidne školjke se pojavljajo v obliki lumakel ozorima horizontov, korale pa ponekod tvorijo manjše kopaste grebene. S fosili revni, temni, ploščasti maro-gasti apnenci so nastajali v zatišnih delih šelfa, kjer ni bilo ugodnih pogojev za večjo raznolikost organizmov. V geotektonskem pogledu je raziskano ozemlje del Zunanjih Dinaridov. Key words: stratigraphy, shallow marine carbonate rocks, litho- and biostratigraphic subdivision, Lower and Middle Jurassic, External Dinarides, Slovenia Ključne besede: stratigrafija, plitvomorske karbonatne kamnine, lito- in biostratigrafska razdelitev, spodnja in srednja jura, Zunanji Dinaridi, Slovenija Introduction In this article are applied data obtained during geological mapping for elaboration of Geological Map of Slovenia on the scale of 1:50 000 on the Map Sheet Grosuplje performed in the years 2005 to 2007 by Stevo Dozet. With our research work we intend to define, above all, the stratigraphy of the Lower Jurassic sedimentary succession and to carry out lithostratigraphic and biostratigraphic subdivision in formations, unit, cenocones and subcones, what will be useful for elaboration of the geologic formation-map of this ter- Figure 1. Location map of the investigated area ritory. Next to the Lower Jurassic carbonate sequence we intend to research more in detail also rocks and their fossil contents at the lower and upper boundary of the discussed stratigraphic sequence. Geological setting. The mapping area (Figure 1) lies on the Map Sheet Grosuplje on the scale of 1:25 000 (No136) southeast of Ljubljana respectively on the western border of Radensko polje extending on the distance of 3.5 kilometers in the area between Spodnja Slivnica, Mlačevo and Predole at Račna. The Lower Jurassic carbonate rocks build smaller hills Gradišče (486 m) and Griči (492 m and 488 m) extending continually in NW-SE direction. In structural respect the investigated area represents one of smaller tectonic units of External Dinarides defined by northwestern-southeastern (NW-SE) faults (Buser, 1974; Placer, 1998) belonging to the northern margin of Dinaric Carbonate Platform (Buser, 1989). Previous investigations. The oldest registered data on geologic structure of the considered area could be found in works of M. V. Lipold (1858), who in the Dolenjska and Notranjska regions attributed a greater part of Jurassic beds to Triassic. Kercmar (1961) researched Jurassic beds in the area between Krka, Videm and Ilova gora. In the Explanatory text of the Map Sheet Ribnica Buser (1969, 1974) described principal geological mapping lith-ostratigraphic units of this area. Tecton-ically he ranged the investigated area to the Dolenjska-Notranjska Mesozoic Blocks, paleogeographically however, to the Dinaric Carbonate Platform (Buser , 1989). Sribar and coworkers (1966) investigated Jurassic sediments between Zagradec and Randol in the Krka valley. Within the framework of geologic investigation for his doctor's degree Strohmenger (1988) carried out under the guidance of his comentor Dozet sedimentological and geochemi-cal investigations in the cross-sections Kompolje (Mala gora) and Krka-Mali Korinj in Suha krajina (Strohmenger & Dozet, 1991; Strohmenger et al., 1987 a, b). Dozet (1993) recorded the complete Lofer cyclothems in lower part of the Lower Jurassic beds in the Krka area that compose the slovenian part of External Dinarides. Buser and Debeljak (1994/95) determined rich findings of lithiotid bivalves. Debel-jak & Buser (1998) added the lithiotid horizon to Pliensbachian respectively Domerian. Dozet & Sribar (1981, 1997) and Dozet (1990 a) recognized and proved almost all litho- and biostratigraphic units applied for sub- division of the Jurassic stratigraphic sequence in Dinarides. Dozet (1999 a, b) explored and described Lower Jurassic shallow-water carbonate succession with coal on the Dinaric Carbonate Platform in Southern Slovenia. The coal occurs in the form of lenses and thin seams among the limestone and dolomite beds of middle part of the Lower Jurassic age. Miler and Pavsic (2008) described the development of the Triassic and Jurassic beds in the Krim Mountain area. The Jurassic stratigraphic sequence involves Lower and Middle Jurassic shallow marine carbonate rocks. Materials and methods This work is based on data, obtained during the systematic regional geologic mapping in the field on the Map Sheet Grosuplje for elaboration of Geologic Map of Slovenia on the scale of 1 : 50 000, performed by Geological Survey of Slovenia. During geologic mapping of Central Slovenia we were focused on stratimetric researches in the field and systematic labor examinations. For planning of geologic mapping on the Map Sheet Grosuplje the Basic Geologic Map of SFRJ - Map Sheet Ribnica on the scale of 1:100 000 and its Explanatory text has been applied. For formation analaysis the cross-section Spodnja Slivnica-Pred-ole has been chosen (Strohmenger & Dozet, 1991). Around 150 samples of carbonate rocks have been collected along the road on the southeastern border of Radensko polje and another 65 rock samples in the cross-section Ilova gora-Cusperk for micropaleontological analyses carried out by Rajka Radoicic and S. Dozet. Carbonate rocks are classified according to Folk's (1959) practical petro-graphic classification of limestones and Dunham's (1962) classification of carbonate rocks according to depositional texture. Results of the research work Lower Jurassic In the area between Spodnja Slivnica and Racna (Figure 2) five litostrati-graphic units of the Lower Jurassic age is separated. Superpositionally from bottom to top they follow one other like that: 1 - bedded micritic and oo-litic-oncolitic limestones, 2 - bedded Orbitopsella Limestones, 3 - bedded Lithiotis Limestones, 4 - bedded to massive oolitic and reef limestones, 5 - platy Spotty Limestones. AGE STAROST FORMATION FORMACIJA No MEMBER-ČLEN ENVIRONMENT OKOLJE NASTANKA 2 Korinj breccias Korinjske breče Bauxite horizon - Boksitni horizont Intertidal -supratidal Medplimsko nadplimsko Dry land - Kopno ■M. IX < S Šentrumar Formation Tidal-bar Plimski prag formacija 11 Pisolitic" limestones " Piz o litm" apnenci Supratidal Nadplimsko —3 1 O tn DOGGER Laze Formation Formacija Laze Hočevje Group Hoôevska skupina Tidal- bar Plimski prag 5 Spotty limestones Maroqasti apnenci Supratidal Nadplimsko íf Predole 4 Oolitic limestones Oolitni apnenci Open lagoon Odprta laguna cr <0 < Beds 3 Lithiotis limestones Litiotidni apnenci Restricted lagoon Z at i sna laguna D - Predolske 2 Orbitopsella limestones Orbitopselni apnenci Restricted lagoon Zatišna laguna plasti 1 Banded m ¡critic limestones Shallow subtotal Pasnati mi kritni apnenci Plitvo podplimsko TRI ASS IC - TRIAS NOR 1A N- R H AET1 AN NORIJ-RETIJ Main Dolomite (Hauptdolomit) Glavni dolomit Regression Regresija Supratidal, intertidal, subtidal Nadplimsko, medplimsko, podplimsko Figure 2. Stratigraphic position and environment of the Predole Beds and "pisolitic" horizon Distribution: The Lower Jurassic limestones can be followed along the Dolenjska railway in the Spodnja Slivnica-Racna section, but Jurassic beds are, however, exposed also along the roads Mlacevo-Racna and Ilova gora-Cusperk; further on, they build the smaller hills Gradišče (486 m) and Griči (495 m and 488 m) that pass in the NW-SE direction as far as Račna. Similar limestone development is exposed also in the Krka area (Dozet & Strohmenger , 2000), where these carbonate rocks compose Podbukovje For- mation respectively Krka Limestones. Also isolated small hill Kopanj (392 m) and solid bedrock of the southern part of Radensko polje are built of Lower Jurassic carbonate rocks. Stratigraphic position. We already mentioned, that about 325 m thick succession of limestones and to much minor extent of dolomites that compose Lower Jurassic carbonate succession lies conformably between the underlying Main Dolomite and overlying Laze Formation (Figure 2). Conformably upon the Main Dolomite repose bedded micritic and biomicritic limestones with algae Palaeodasycladus, that together with other fossils prove the Lower Jurassic age of the limestones. Platy and thin-bedded Lower Jurassic Spotty Limestones pass gradually upward into dark grey to grayish black oolitic limestones with Middle Jurassic diagnostic foraminifer Gutnicella (Dictyoconus) cayeuxi Lucas and Spiraloconulus gi-ganteus Cherchi & Schroeder as well as alga Holosporella siamensis Pia. Dark oolitic limestones overlying the Lower Jurassic sequence correspond accordingly to the Laze Formation from Hocevje Group (Dozet , 2000 b). Also the upper boundary of the Lower Jurassic sequence is conformable, since the Lower Jurassic Spotty Limestones pass gradually into the dark oolitic limestones of Middle Jurassic age. Quite other case is with the Lower Jurassic carbonate sequence of Podbukovje For- mation (Dozet & Strohmenger, 2000) where erosional discordance between the Lower Jurassic and Middle Jurassic sequences have been recorded and the relatively small thickness (only 15 m) of the Lower Jurassic dark platy limestones is explained, that greater part of Spotty Limestones was eroded. Description of lithostratigraphic units 1st Unit — Bedded micritic (mudstone) limestones, Hettangian and Sinemu-rian The oldest unit of Lower Jurassic limestones is represented by a carbonate succession (Figures 3, 5) in which prevail variously grey, bedded and occasionally banded (Figures 6, 7) micritic limestones containing in spots fragments of benthic foraminifers and molluscs (wackestone). Bedded micritic limestones contain sporadic interbeds of oosparitic (ooid grainstones), in-traoosparitic, bioosparitic, biosparitic, biomiciritic and oncomicritic limestones, that alternate rhythmically. In the thick-bedded oosparitic limestones (ooid grainstones: Pl. 1, Fig. 6) are to be found intraclasts (intraclastic grainstones) and bioclasts (bioclastic grainstones). Intraclasts are subangular to poorly rounded and up to one centimeter in size. They belong to micritic, biosparitic, intrasparitic and laminated limestones. Bioclasts are represented by more or less rounded and micritized Plate 1. Fig. 1. Bioclastic packstone with algae Palaeodasycladus medi-terraneus (Pia) and strongly changed (micritized) Thaumatoporella par-vovesiculifera (Raineri) as well as foraminifers Textulariidae and Tro-chaminidae, Lower Lower Jurassic, Predole at Račna; Fig. 2. Strongly recrystallized mollusc fragment and alga Palaeodasycladus elongatulus Praturlon coated with algal or cyanobacterial crust as well as foraminifers Textulariidae, Lower Lower Jurassic, Predole at Račna; Fig. 3. Bioclastic-ooid grainstone with alga Palaeodasycladus elongatulus Praturlon and foraminifers Orbitopsella praecursor (Gumbel), Verneuilinidae, Textulariidae, Trochaminidae and echinoderms, Middle Lower Jurassic, Predole at Račna; Fig. 4. Foraminifer Lituosepta recoarensis Cati, Middle Lower Jurassic, Predole at Račna; Fig. 5. Foraminifers Orbitopsella sp. and Tro-cholina sp. in the bioclastic-intraclastic grainstone Lower/Middle Lower Jurassicsic, Predole at Račna; Fig. 6. Ooid-grainstone with foraminifer Agerina sp., Middle Lower Jurassic, Predole at Račna. fragments of algae, molluscs and fora-minifers (Pl. 1, Fig. 5). Some beds of the lower part of the Lower Jurassic carbonate succession were subjected to selective late diagenesis. Dolomitiza-tion affected especially coarse-grained sparitic limestones (grainstones), so that the limestones pass in spots into coarse-grained respectively coarse-crystalline dolomite. There are also pretty much strongly dolomitized limestones. The intergranular pores in the limestones of the grainstone type are filled with fibrous calcite cement A and/or mosaic sparry cement B. AG E STAROST ST RAT (GRAPHIC COLUMN STRATIGRAFSKI STOLPEC LITHOLOGIC COMPOSITION LITOLOŠKA SESTAVA FOSSILS FOSILI W ¿y i , I i SEE _ Temen ploščast marogast apnenec z vložki oolitnega apnenca Dark platy bioturbated spotty limestone with inter beds of oolitic limestone Haurania deserta Thaumatoporella parvovesi-culifera Ophlhalmidium sp. Pseudocyclammina sp Crinoids Lagenida? Grey platy oolitic and reef limestone Siv ploščast 001 itn i in grebenski apnenec Corals - korale Pseudocyclammina lituus Gastropods Pectin i dae U I /VT I :<-ÍSID O Dark and brcwnish grey lithiotid lim estone Temen in rjavo siv plastnaí litiotidnl apnenec Medium grey to grey bedded sparry and intrasparitic limestone with orbitopsellas Srednje siv do siv plastnat sparite n in intrasparuditen apnenec z orbito psel am i in paleodasikladusi CocWear/fes ioppinus Lähhpema scutata Amijtella amiji Brachiopoda Palaeodasycladus medi-terraneus. P. elongatulus Pseudocyclammina liasica Paleomayncina termieri © ]aaal © I, i, i no Orbitopselta praecursor Palaeodasycladus mediterraneus P. eiongalulus Amijieila amiji Paleomayncina termteri Pseudocyclammina liasica Brachiopoda, Opisoma sp Nerineidae, Mogalodontidae K> I j? n Ji 1 , J © I © I © I»| 104 EZT-T IV Ö1Ö ® 0 I 0 > ® j 0 Dark, grey and light bedded limestones interbedded with oolitic limestone and meg a todo ntid I umac belles Temni, sivi in svetli plastnatl mi kritni apnenci z vložki oolitnega apnenca in megalo-dontidnimi lumakelami IV I . IK1 'K' Pa laeodasycladus meditenaneus P barrabei Thaumeloporella parvovesiculifera Amijielia amiji Glomospira sp Trochaminidae Textulariidae Mega lodo ntidae Figure 3. Stratigraphic column of the Lower Jurassicsic Beds in Predole at Račna area (Dolenjska) Figure 4. Biostratigraphic subdivision of Jurassic beds on the northern margin of Dinaric Carbonate Platform