GEOLOGIJA 45/1, 163–188, Ljubljana 2002
Faciesi, razvoj in interpretacija sedimetacijskega okolja
uranonosnega Brebovni{kega ~lena Grödenske formacije
na obmo~ju @irovskega vrha
Facies, development and interpretation of sedimentary environment
of the uranium-bearing Brebovnica Member of the Val Gardena Formation
in the @irovski vrh area, W Slovenia
Dragomir SKABERNE Geolo{ki zavod Slovenije, Dimi~eva 14, 1000Ljubljana, Slovenija
Kju~ne besede: Grödenska formacija, Brebovni{ki ~len, perm, faciesi, okolje sedimen-tacije, uran, @irovski vrh, W Slovenija
Key words: Val Gardena Formation, Brebovnica Member, Permian, facies, sedimentary environment, uranium, @irovski vrh, W Slovenia
Kratka vsebina
Zaradi znatnih koncentracij uranove rude je Brebovni{ki ~len najpomembnej{i ~len Grödenske formacije. Kamnine smo glede na zrnatost, sedimentne teksture in sestavo lo~ili v konglomeratne, pe{~enjakove, muljev~eve in karbonatne faciese ter interpretirali pogoje njihovega nastanka. Iz podrobnih delnih profilov smo sestavili poenostavljeni profil celotnega Brebovni{kega ~lena v skupni debelini skoraj 400 m. V njem smo lo~ili dva makrocikla. Prvi obsega spodnjo polovico Brebovni{kega ~lena, drugi pa njegovo zgornjo polovico in pribli`no spodnjo polovico Hobov{kega ~lena. Makrocikla sta produkt re~nega sedimen-tacijskega okolja z vsemi svojimi podokolji in sta alocikli~nega zna~aja. Alocikli~nost so najverjetneje kontrolirala tektonska dogajanja v drena`nem zaledju in sedimentacijskem bazenu. Makrocikla predstavljata razvoj re~nega sistema od proksimalnih delov s prevla-dujo~im talnim transportom prepletajo~ih re~nih korit in aluvialnih vr{ajev proti distal-nej{im delom z me{anim talno-suspenzijskim transportom meandrirajo~ih re~nih korit.
Abstract
The Brebovnica Member, bearing considerable grades of uranium ore, is the most important member of the Val Gardena Formation. Rocks were subdivided according to grain size, sedimentary structures and composition to conglomerate, sandstone, mudstone and carbonate facies, and the conditions of their formation were interpreted. From detailed partial profiles that illustrate the successions of facies a generalized profile of lithostratigraphic development of the entire Brebovnica Member of a total thickness of almost 400 m was constructed. Two macrocycles were distinguished in it. The first macro-cycle comprises the lower half of the Brebovnica Member, and the second macrocycle its upper half and approximately the lower half of the Hobovb{e Member. The macrocycles are a product of fluvial depositional environment with its subenvironments, and they are of allocyclic character. The allocyclicity was most probably controlled by tectonic events in drainage and depositional basin. The macrocycles represent the development of fluvial system from its proximal parts with predominant bed-load braided channels and alluvial fans towards more distal parts with mixed-load meandering channels.
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Dragomir Skaberne
Uvod
Kamnine Grödenske formacije imajo v Sloveniji najve~ji sklenjen obseg na obmo~ju med Cerknim in Smre~jem, kjer se v obliki neenakomerno {irokega pasu raztezajo v smeri NW – SE. K poznavanju geolo{ke zgradbe tega in {ir{ega prostora so med drugimi veliko prispevali Kossmat (1910), Winkler (1923), Rakovec (1956), Ramov{ (1958), Berce (1959), Berce s sodelavci (1959, 1960), Mlakar (1969), Grad in Ferjan-~i ~ (1974, 1976), Placer (1973, 1981, 1983 1999), Placer in ^ar (1998) ter Premru (1976, 1977, 1980).
Na Cerkljanskem in Sovedenjskem so `e v 19. stoletju poznali {tevilne pojave bakrove rude, ki so pritegnili pozornost raziskovalcev. Podatke o bakrovem orudenju so podali Lipold (1885, 1857), Götting (1924) in Bibolini (1932), o nastanku rudi{~a Škofje pa so pisali Grafeneuer (1966), Drove-nik M. (1970) in Drovenik F. s sodelavci (1972). Z odkritjem radioaktivnih anomalij na obmo~ju @irovskega vrha leta 1960 in kasnej{im odprtjem Rudnika urana @irov-ski vrh se je zanimanje za grödenske kamnine zelo pove~alo. Obmo~je @irovskega vrha so obravnavali Marinkovi} (1960, 1961 a, b), Isajlovi} in Pavlovi} (1963), Oma -ljev (1965, 1967 a, b), Grad s sodelavci (1967), Dimkovski s sodelavci (1974, 1976, 1977), Lukacs in Florjan~i~ (1974), Budkovi~ s sodelavci (1979, 1983), B u d -kovi~ (1978, 1980, 1981) in drugi. Nastanek uranovega rudi{~a @irovski vrh pa so podrobneje obdelali Omaljev (1982), Do le -n e c (1983) in Palinka{ (1986).
V okviru ve~letnih raziskav Geolo{ki faktorji Hg, Cu in U mineralizacije (Mlakar 1978 do 1983) smo dobili novo geolo{ko karto obse`nega ozemlja med Cerknim in Smre~-jem (sl. 1), ki upo{teva tudi podatke vrtin in jamskih del. V letnih poro~ilih o raziskavah so med drugim tudi osnovni podatki o vseh litostratigrafskih ~lenih Grödenske formacije. Geolo{ko zgradbo @irovskega vrha in okolice na kratko podajata Mlakar in P l a c e r (2000). Podrobnej{e informacije o sedimentacijskem in postsedimentacijskem razvoju Grödenske formacije na tem obmo~-ju in nekaterih dilemah litostratigrafskega poimenovanja je podal Skaberne (1995).
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Osnovne geolo{ke zna~ilnosti @irovskega ozemlja
Med Cerknim, @irovskim vrhom (sl. 1) in Smre~jem se v smeri NW – SE razteza pri-bli`no 20 km dolg in do 5 km {irok pas kamnin Grödenske formacije. To obmo~je le`i na sti~i{~u zunanjih Dinaridov in Ju`nih Alp, ki jih grade obse`ne narivne enote. V {ir{em smislu pripada obmo~je @irovskega vrha zunanjim Dinaridom, v o`jem pa idrij-sko-`irovskemu ozemlju, ki je del Trnovskega pokrova (Mlakar, 1969; Placer, 1981, 1999; Mlakar in Placer, 2000).
V Trnovskem pokrovu so najstarej{e kamnine karbonski skrilavi glinavci, na katerih le`e kamnine Grödenske formacije. Ponekod je ta stik neporu{ena ali le malo tektoni-zirana kotno-erozijska diskordanca, drugod pa je narivnega zna~aja.
Kamnine Grödenske formacije so v osrednjem delu @irovskega vrha nagubane in v spodnjem delu grade tako imenovano dvojno S strukturo (Lukacs in Florjan~i~, 1974), proti severozahodu in jugovzhodu pa so plikativne deformacije manj{e in plasti vpadajo generalno proti jugozahodu in jugu.
Narivno zgradbo seka ve~ sistemov prelomov, ki potekajo v dinarski (NW – SE), alpski (E – W), pre~no dinarski (NE – SW) in pre~no alpski (N – S) smeri. Med prelomi je najpomembnej{i dinarsko usmerjeni So-vodenjski prelom, ob katerem je jugovzhodno krilo pogreznjeno za 200 do 400 m (Mla -kar  in  Placer, 2000).
V talnini Grödenske formacije so skrilavi glinavci superpozicijske enote Cc. Njena starost paleontolo{ko {e ni dokazana zato jo po starih ugotovitvah uvr{~amo v zgornji karbon, dopu{~amo pa mo`nost, da pripada tudi spodnjemu permu (Mlakar, 1987; Mlakar et al., 1993; Mlakar in Placer, 2000; Mlakar, 2001). Kontakt med superpozi-cijsko enoto Cc in Grödensko formacijo je kotno erozijsko-diskordanten. Grödensko formacijo grade prete`no rde~e, podrejeno sive, sivozelene in zelene, prevladujo~e sred-njezrnate, v manj{i meri drobno in debelo-zrnate klasti~ne kamnine. Glede na litolo{ke zna~ilnosti je Mlakar (1978-1983) razdelil Grödensko formacijo na {est superpozicij-skih enot A1, A2, A3/1, A3/2, B in C oziroma litostratigrafskih ~lenov in jih poimenoval:
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Brebovni{ki (Br), Hobov{ki (Ho), Zal{ki (Za), Koprivni{ki (Ko), Škofje{ki (Šk) in Dobra-~evski ~len (Do) (Mlakar in Placer, 2000).
Brebovni{ki ~len (Br) grade ve~inoma sivi in sivozeleni, ponekod rde~i in zeleni, raz-li~no zrnati, prete`no glinen~evo krmenovo liti~ni pe{~enjaki, konglomerati ter v manj{i meri muljevci. V zgornji polovici Brebov-ni{kega ~lena se nahaja na obmo~ju @irov-skega vrha uranovo orudenje. Debeline tega ~lena se spreminja od 0 do 400 m in se proti severozahodu izklinja.
Hobov{ki ~len (Ho) sestavlja rde~i mulje-vec z do nekaj metri debelimi le~ami rde~ega in sivega drobnozrnatega pe{~enjaka. V slednjem se ponekod pojavljajo orudenja z bakrom. ^len je debel 20 do 280 m.
Zal{ki ~len (Za) je zastopan z rde~imi, razli~no zrnatimi pe{~enjaki in podrejeno tanj{imi plastmi rde~ega muljevca. Njegova debelina se giblje od 180 do 380 m.
Koprivni{ki ~len (Ko) predstavljajo rde~i konglomerati in razli~no zrnati pe{~enjaki ter manj{a koli~ina muljevcev. Na {ir{em obmo~ju @irovskega vrha dose`e ~len debelino 550 m. Severozahodno od Sovodnja pa se Zal{ki in Koprivni{ki ~len tanj{ata in iz-klinjata. Na jugovzhodnem delu obravnavanega ozemlja (Lavrovec) so kamnine, ki jih lahko vzporejamo z Zal{kim in Kopriv-ni{kim ~lenom, druga~e razvite.
Škofje{ki ~len (Šk) je na obmo~ju Škofja in Sovodenj razvit kot siv liti~ni krmenov pe{~enjak, v katerem se ponekod (Škofje, Sovodenj) pojavlja bakrovo orudenje. Na ob-mo~ju Javorjevega dola nastopajo v istem litostratigrafskem nivoju le~e svetlo sivega in sivo rjavkastega ponekod ro`natega konglomerata, ki postajajo proti jugovzhodu sklenjene in debelej{e ter dose`ejo debelino 100 m. Na Lavrovcu pa sta bili izdvojeni dve progradirajo~i zaporedji s pove~evanjem zr-natosti navzgor, ki ju sestavljajo sivi do sivo rumenikasti pe{~enjaki in konglomerati; slednji so ponekod tudi rde~i. Zaporedji dose`eta debelino do 740 m in predstavljata ekvivalent Zal{kemu in Koprivni{kemu ~lenu.
Dobra~evski ~len (Do) je na obmo~ju So-vedenj prevladujo~e razvit kot rde~ drob-nozrnat pe{~enjak in dose`e debelino 120 m. Proti jugovzhodu prehaja v rde~ muljevec, ki se tanj{a in na Lavrovcu izklini.
Debelina celotne Grödenske formacije se spreminja od 200 m na obmo~ju Škofja do
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1750 na obmo~ju @irovskega vrha, kjer do-se`e najve~jo debelino v Sloveniji.
Spodnji del kamnin Grödenske formacije na @irovskem vrhu naj bi bil starej{i od 255 milj. let, kar bi ustrezalo kazaniju, spodnji polovici zgornjega perma v dvodelni razdelitvi (Odin & Odin, 1990) ali srednjemu permu po tridelni raz~lenitvi perma. To potrjujejo tudi edini dolo~ljivi fosilni ostanki (pelod) najdeni v podkopu P-10 in vrtini B-63 (Jelen et al., 1981).
Grödensko terigeno formacijo transgre-sivno prekrivajo karbonatne kamnine in sicer siv do temno siv plo{~ast dolomikritni dolomit z nekaj centimetrov debelimi plastmi muljevca, ki prehaja v dolomitno-apnen-~ev razvoj. Te grade @a`arsko formacijo, ki je ekvivalent Belerofonski formaciji v Karnijskih Alpah.
V nadaljevanju se bomo omejili le na Bre-bovni{ki ~len Grödenske formacije, ki je zaradi znatne koncentracije uranove rude tudi ekonomsko najzanimivej{i. Osnovna raz~le-nitev tega ~lena se je izoblikovala v zvezi z raziskavami uranove rude (Omaljev, 1967 a, b; Budkovi~, 1980). Zato bomo podali nekatere zna~ilnosti kamin oziroma litofaciesov in bolj genetsko raz~lenitev ter interpretacijo sedimentacijskega okolja nastanka kamnin Brebovni{kega ~lena.
Metodologija dela in interpretacije
Na obmo~ju Rudnika urana @irovski vrh (sl. 1) smo posneli deset podrobnih profilov, ki se deloma prekrivajo in zajemajo celotno in najve~jo ohranjeno debelino Brebovni{ke-ga ~lena na vsem obmo~ju med Cerknim in Smre~jem.
Posneti profili predstavljajo enodimenzionalno, vertikalno zaporedje faciesov. Izraz facies ali natan~neje sedimentni facies uporabljamo kot opisni izraz za bolj ali manj homogeno telo kamnin, ki ga lahko dolo~imo in lo~imo od ostalih kamninskih teles po fizikalnih, kemi~nih in biolo{kih zna~ilno-stih. Te se odra`ajo v geometriji, sestavi, strukturi, sedimentnih teksturah, smereh paleotokov in zdru`bah fosilov (Selley, 1970). Tako naj bi bil posamezni facies produkt bolj ali manj konstantnih fizikalnih, fizikalno-kemi~nih in biolo{kih pogojev v dolo~enem delu sedimentacijskega prostora.
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Sl. 1. Situacijska karta {ir{ega raziskovalnega obmo~ja, ki ga je podrobno geolo{ko kartiral Mlakar (1979 – 1982) in polo`ajem Rudnika urana @irovski vrh.
Fig. 1. Sketch map of studied territory mapped in detail by Mlakar (1979-1982) with position of
@irovski vrh uranium mine.
Grödensko formacijo sestavljajo prete`no klasti~ne kamnine. Ker predstavlja struktura, predvsem velikost zrn, eno od osnovnih in pri klasti~nih kamninah eno najpomemb-nej{ih lastnosti, smo jo uporabili za osnovo razdelitve faciesov. Lo~ili smo tri osnovne granulometri~ne skupine in sicer konglome-ratne, pe{~enjakove in muljev~eve faciese, ki smo jih z dodatnimi opredeljujo~imi kriteriji, kot so sedimentne teksture in sestava, podrobneje razdelili. Pri tem je potrebno omeniti, da so sedimentne teksture redke ali slabo vidne. Ker so sedimentne teksture nastale v ~asu sedimentacije in odra`ajo vzajemen vpliv vodnega toka, podlage in prena{a-nega materiala, lahko na njihovi osnovi sklepamo na nekatere zna~ilnosti toka, ki jih opredelimo kot zna~aj tokovnega re`ima. Tokovni re`im dolo~a pogoje nastanka dolo-~ene plastne oblike in njene konfiguracije, sinsedimentne teksture oziroma faciesa. Zastopanost izdvojenih faciesov tako ka`e na pogostnost dolo~enega tokovnega re`ima, njihovo zaporedje pa razkriva na~in spreminjanja njegovih lastnosti. Omenjene spremembe so lahko zna~ilne za dolo~eno se-dimentacijsko okolje. Tako vertikalno zaporedje sedimentnih tekstur, oziroma faciesov omogo~a rekonstrukcijo le-teh.
Vertikalna zaporedja faciesov smo zdru-`evali v sedimentne sekvence, ki so omejene ve~inoma s ploskvami plastnatosti tretjega ali vi{jih redov (Miall, 1988). Idealna sekven-ca najbolj znanega meandrirajo~ega modela re~nega toka sestoji iz debelozrnatega dela, sedimentov meanderskih sipin in drobno-zrnatega dela, sedimentov obre`nih ravnin. Spodnji, debelozrnati del grade razli~no teksturirani konglomeratni in pe{~enjakovi faciesi, medtem ko drobnozrnati del predstavljajo ve~inoma muljevci. Za la`jo predstavo podajamo na sliki 2 izsek profila Pr-6/3-2 z izdvojenimi sedimentacijskimi sek-vencami, v katerih so zastopani razli~ni faciesi.
Zaradi zelo omejene mo`nosti lateralnega sledenja faciesov in njihovega sosledja nismo mogli neposredno uporabiti metode arhitekturnih elementov (Miall, 1985), ampak smo sedimentne sekvence povezovali v razli~ne enote vi{jih redov (prvega E1 in drugega E2 reda; sl. 2). Enote so interpretativnega zna-~aja in naj bi predstavljale enotne sipine, koritne oblike in prepletajo~e sipinske ter obre`ne ravninske komplekse rek ali vr{ajev. Posamezne enote smo opredelili na podlagi grafi~nih prikazov profilov in diagramov debelin sekvenc. Celotni prikazi profilov, iz-
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Faciesi, razvoj in interpretacija sedimetacijskega okolja uranonosnega Brebovni{kega ~lena …        167
Legenda: Legend:
debelina thickness
številka sekvence number of sequence
barva color
siva grey
temno siva (èrna) dark grey (black)
rdeèa red
zelena green
lithology
muljevec mudstone
pešèenjak sandstone
konglomerat conglomerate
intraklasti muljevca mudstone intraclasts
ceK kalcitne konkrecije calcite concretions
masivna plastnatost massive bedding
horizontalna plastnatost - laminacija horizontal bedding - lamination
koritasta navzkrižna plastnatost trough cross - bedding
planama (ravna) navzkrižna plastnatost planar cross - bedding
erozijska površina erosional surface
vpad navzkrižne plastnatosti / erozijske površine dip of cross - bedding I erosional surface
pint pyrite
Sl. 2. Izsek iz profila Pr-6/3-2, z izdvojenimi
sedimentnimi sekvencami, povezanimi v enote
prvega (E1) in drugega (E2) reda, posnetega v
isto imenskem pre~niku Rudnika urana
@irovski vrh.
Fig. 2. Section of profile Pr-6/3-2 with
sedimentary sequences associated into first
order (E1) and second order units (E2) logged
in crosscut of same name in @irovski vrh mine.
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dvojenih enot, njihova kvantitativna opredelitev z relativno zastopanostjo posameznih faciesov ter podrobnej{a interpretacija je podana v delu Skaberneta (1995).
Vse v nadaljevanju navedene relativne zastopanosti posameznih faciesov so podane glede na obseg njihovih debelin v upo{te-vanih odsekih podrobno posnetih profilov P-10, Pr-6/2-3 in Pr-6/0-4 v centralnem delu Rudnika urana @irovski vrh.
Opis faciesov in njihova interpretacija
V Brebovni{kem ~lenu so najbolj zastopani pe{~enjakovi in konglomeratni faciesi, medtem ko so muljev~evi in karbonatni fa-ciesi manj razviti.
Konglomeratni faciesi
H konglomeratnim faciesom, ki sestavljajo 18.6 % Brebovni{kega ~lena, uvr{~amo konglomerate, pe{~ene konglomerate in prodnate pe{~enjake, med katerimi prevladujeta slednja dva. So razli~nih sivih, zelenih in podrejeno rde~ih barvnih odtenkov. Nastopajo v neenakomerno debelih plasteh in le~ah, debelih od 5 cm do 9 m. V njihovi talnini so pe{~enjaki ali muljevci. Spodnji kontakt med njimi je obi~ajno oster ali erozijski, zgornji med konglomeratom in pe-{~enjakom pa je navadno postopen, ponekod tudi oster.
Struktura konglomeratnih faciesov je precej razli~na in odvisna predvsem od razmerja med prodnato in pe{~eno frakcijo. Porazdelitev velikosti zrn je v ve~ini primerov izrazito bimodalna. Velikost prodnikov je zelo spremenljiva in dose`e do 20 cm. Poleg ekstrabazenskih prodnikov nastopajo v konglomeratih tudi intraklasti muljevca (erodirani klasti podlage), ki dose`ejo velikost blokov do 2 m. Pe{~eni konglomerati in prodnati pe{~enjaki so obi~ajno slabo sortirani. Sama prodnata frakcija je v nekaterih primerih, predvsem v frakciji 4 do 10 mm, dobro sortirana. Pe{~eno vezivo pripada gra-nulometri~no zelo debelo, debelo, srednje in drobnozrnatemu pe{~enjaku. Slu~ajni preseki prodnikov so od podolgovatih do izome-tri~nih oblik, prevladujejo pa vmesne oblike. Njihova stopnja zaobljenosti je precej spremenljiva in se giblje od pologlatih do zaobljenih.
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V nekaterih konglomeratnih razli~kih lahko opazujemo primarno orientacijo prodnikov. Ve~inoma so orientirani z dolgimi osmi presekov vzporedno s kliva`nimi domenami. Te in z njimi vzporedne dolge osi presekov prodnikov so lahko vzporedne s plastnatost-jo, oklepajo z njo dolo~en, bolj ali manj oster kot ali potekajo celo pravokotno na plasti.
Konglomerate sestavljajo prodniki in pe-{~ena zrna. Ve`ejo jih kremenova illit-se-ricitna epiosnova in razli~ne vrste cementov.
V  konglomeratih makroskopsko lo~ilmo naslednje skupine prodnikov: sivi-beli in ro`-nati kremen, sive, zelene in ro`nate predor-nine, redke zrnate granitoidne kamnine, ro-`ence – lidit in jaspis, apnence, pe{~enjake, tufe, sericitne skrilavce, filite, kvarcite ter intraklaste muljevcev in karbonatnih kon-krecij.
V Brebovni{kem ~lenu smo v konglomeratih zasledili spremembe v barvi in sestavi prodnikov ter na njihovi osnovi lo~ili dva glavna konglomeratna faciesa.
Sivi polimiktni konglomerat je razli~nih sivih in podrejeno zelenkastih odtenkov. V njem so prisotni predvsem prodniki sivega-belega kremena, sivih in zelenih predornin, podrejeno pa lidita, tufov ter intraklasti (tab. 1, sl. 1). Obi~ajno tvori bolj ali manj kontinuirane plasti in le~e, ki so v sekvencah debele od 10 cm do 4.5 m. Sivi polimiktni konglomerat pripada strukturno pe{~enemu konglomeratu in prodnatemu pe{~enjaku, s povpre~no velikostjo prodnikov od 0.5 do 3 cm, medtem ko dose`ejo najve~ji tudi 15 cm.
V  Brebovni{kem ~lenu je sivi polimiktni konglomerat zastopan z 8.5 %. Vanj se ponekod vklju~ujejo sedimentacijska telesa pisanega polimiktnega konglomerata.
Pisani polimiktni konglomerat ima zelo pestro sestavo prodnikov, ki se deloma od-ra`a v pisani barvi, po kateri je dobil tudi ime. Sestavljajo ga predvsem prodniki sive-ga-belega in ro`natega kremena, manj pa je sivih, zelenih in ro`natih predornin, lidita, jaspisa, tufov in intraklastov (tab. 1, sl. 2). Poleg navedenih prodnikov nastopajo ponekod tudi prodniki apnencev. Zasledili smo jih v dveh horizontih, nekoliko {tevil~nej{i pa so le v enem. Pisani polimiktni konglomerat je prete`no zelenkast, redko rde~kast in {e redkeje siv ter nastopa v bolj ali manj kontinuiranih plasteh ali le~ah, ki so v sek-vencah debele od 10 cm do 8.1 m. Strukturno pripada pisani polimiktni konglomerat vsem
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strukturnim skupinam: konglomeratu, pe-{~enemu konglomeratu in prodnatemu pe-{~enjaku. Prodniki so nekoliko ve~ji kot v sivem polimiktnem konglomeratu. Njihova povpre~na velikost je 1 do 4 cm, medtem ko dose`e maksimalna velikost do 20 cm.
Pisani polimiktni konglomerat je na ob-mo~ju @irovskega vrha neenakomerno raz-vr{~en. V najve~ji debelini se pojavlja v osrednjem delu, kjer predstavlja 10.1 % debeline Brebovni{kega ~lena, medtem ko se proti NW in SE tanj{a. Nastopa v ve~ horizontih, ki se lahko lateralno v smeri SW hitro izklinjajo. Med posameznimi horizonti je sivi polimiktni konglomerat, v katerega lahko pisani polimiktni konglomerat tudi lateralno prehaja.
Za podrobnej{o opredelitev faciesov smo uporabili predsedimentacijske in sinsedi-mentacijske teksture.
Med predsedimentacijskimi teksturami so pomembne erozijske povr{ine – Se. Ve~ino-ma jih zasledimo v spodnjem delu debelozr-natega ~lena sekvenc in predstavljajo spodnji, ponekod tudi bo~ni kontakt z obdaja-jo~imi faciesi. V pre~nem preseku so vijugave in obi~ajno konkavne ploskeve (tab. 1, sl. 1, 2), ki so zarezane od nekaj centimetrov do ve~ kot {tiri metre globoko v podlago. Nad erozijskimi povr{inami zasledimo v spodnjem delu sekvence intraklaste erodiranega muljevca in/ali karbonatnih konkrecij. Njihova velikost in koli~ina se obi~ajno z oddaljenostjo od erozijske povr{ine zmanj{uje. Opisane erozijske povr{ine uvr{~amo v ploskve tretjega ali vi{jega reda (M iall, 1988, 1990). Vzporejamo jih s faciesom Se (erosio-nal scours with intraclasts; Rust, 1978; M i a ll , 1978) oziroma s faciesom SS (Cant &   Walker, 1978).
V konglomeratnih faciesih nastopajo~e sinsedimentacijske teksture dolo~ajo zna~aj plastnatosti in podrobneje opredeljujejo fa-cies. Zasledili smo masivno, horizontalno, ravno in koritasto navzkri`no plastnatost.
Masivni konglomerat – Gm (massive gravel) je najbolj zastopan in predstavlja 17.0 % debeline Brebovni{kega ~lena. Nastopa v le-~ah in plasteh, debelih od 10 cm do 9 m. Navzgor prehajajo v konglomerate s horizontalno in koritasto navzkri`no plastna-tostjo ali v druge pe{~enjakove faciese. Prehodi so ve~inoma jasni in ostri, le v masivni pe{~eni facies prevladujejo postopni prehodi.
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Horizontalno plastnati konglomerat – Gh (horisontally bedded gravel) je bolj ali manj izrazit. Horizontalna plastnatost je ve~inoma posledica spremembe sestave oziroma barve in porazdelitve velikosti zrn (tab. 2, sl. 1). Pogosto jo komaj opazimo in prehaja v masivno plastnatost. Intervali s horizontalno plastnatostjo so debeli od 10 cm do 3.1 m, plasti v njih pa od 1 do 10 cm. Meje med plastmi so postopne do jasne. M iall (1977, 1978) je konglomerate z masivno in horizontalno plastnatostjo zdru`il v litofacies Gm (massive and crudely bedded gravel). Faciesa Gm in Gh bi lahko predstavlja longitudinalne sipine, zaostale debelozrnate sedimente (lag deposits) ali aluvialno vr{ajne “presej-ne” sedimente (sieve deposits).
Koritasto navzkri`no plastnati konglomerat – Gt (trought cross-bedded gravel; M i a l l , 1977, 1978) je bolj ali manj jasno izra`en in je zastopan le z 0.4 %. Sestavljajo ga erozijske kotanje, zapolnjene s simetri~no ali asimetri~no upognjenimi plastmi, ki se med seboj razlikujejo predvsem po zrnatosti, ko-li~ini prodnikov in deloma barvi. Prevladujejo 10 do 50 cm globoka korita, medtem ko zna{a njihova {irina 50 cm do 1.5 m. Pri ve~jih koritih smo {irino ocenili do 3 m. V ve~ini primerov dol`ine posameznih korit nismo mogli dolo~iti, ker smo lahko opazovali le nakaj vzdol`nih presekov. Koseti ko-ritaste navzkri`ne plastnatosti so v konglo-meratnih razli~kih debeli od 10 cm do 2.6 m. Koritasto navzkri`no plastnati konglomerat interpretiramo kot zapolnitev kanalov.
Pe{~enjakovi faciesi
Pe{~enjakovi faciesi so najbolj zastopani in sestavljajo 70.0 % debeline Brebovni{kega ~lena. Glede na zrnatost smo jih podrobneje lo~ili v tri skupine: debelo (10.6 %), srednje (38.0 %) in drobnozrnat (21.4 %) pe{~enjak. So sivih, rde~ih in podrejeno zelenih odtenkov ter nastopajo v bolj ali manj enakomerno debelih plasteh ter le~ah. Njihova debelina v izdvojenih sekvencah se giblje od 5 cm do 18.2 m.
V talnini pe{~enjakovih faciesov so konglomerati ali muljevci. V primerih, ko le`ijo v talnini muljevci, je spodnji kontakt z njimi oster ali erozijski (tab. 2, sl. 3), medtem ko je spodnji kontakt s konglomerati jasen ali postopen in redkeje oster. Bo~ni kontakti s
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konglomerati so navadno postopni. Ostri la-teralni kontakti z muljevci ali konglomerati so obi~ajno erozijski. Pe{~enjaki prehajajo navzgor v muljevce. Prehod je prete`no jasen ali oster (tab. 2, sl. 3), le ponekod postopen. V pe{~enih faciesih razli~nih zrnatosti so prehodi postopni, jasni, ostri ali erozijski.
Podrobneje strukture pe{~enjakovih fa-ciesov ne bomo obravnavali. Podatke o porazdelitvah velikosti zrn pe{~enjakov Grö-denske formacije in intrepretaciji sedimen-tacijskga okolja na njihovi osnovi, ki smo ga opredelili kot fluvialnega (re~nega) podaja Skaberne (1995, 1997).
Pe{~enjake sestavljajo terigena zrna kremena, glinencev, liti~nih zrn, filosilikatov in akcesornih te`kih mineralov. Kremenova zrna so poli in monokristalna. Med glinenci prevladujejo dvoj~i~ni in nedvoj~i~ni pla-gioklazi, podrejena so zrna mikropertita in redka zrna mikroklina. Liti~na zrna pripadajo kislim in bazi~nim predorninam, grani-toidnim kamninam, karbonatnim in klasti~-nim sedimentnim kamninam, ro`encem, tufom, sericitnim in kloritno-sericitnim skrilavcem, kvarcitom, blestnikom, gnajsom in nedolo~ljivim liti~nim zrnom. Med filosili-kati so prisotni muskovit, klorit in v sledovih biotit. Klorit ima sestavo ripidolita in thu-ringita. Akcesorne te`ke minerale sestavljajo ve~idel neprozorni minerali: ilmenit, magne-
tit, levkoksen, hematit in hematitizirani drobci. Podrejeno so zastopani prozorni te`ki minerali: cirkon, rutil, turmalin, granati, amfiboli, apatit, zoisit (klinozoisit), epi-dot, pirokseni, titanit, stavrolit, disten in an-daluzit.
Organska snov je deloma koloidna, deloma pa jo predstavljajo rastlinski ostanki, ki so lahko mineralizirani ali karbonizirani. Slednji pripadajo metaantracitu.
Terigena zrna ve`eta cement in osnova. Cement predstavljajo kremen, glinenci-albit, kalcit, dolomit, illit-sericit, klorit, barit, pirit in hematit. V orudenih pe{~enjakih so prisotni {e neprozorni kovinski uranovi, bakrovi, cinkovi in svin~evi minerali. Na{teti minerali tvorijo razli~nine oblike in se pojavljajo v ve~ generacijah. Ve~ina osnove pripada genetsko epiosnovi, ki jo gradita prete`no kremen in illit-sericit, v manj{i meri pa sta prisotna klorit in albit. V frakciji glin, ki naj bi predstavljala komponento osnove, sta prisotna illit in klorit.
Glede na prisotnost oziroma odsotnot pred-sedimentacijskih in predvsem sinsedimen-tacijskih tekstur smo v posameznih granulo-metri~nih skupinah pe{~enjakov lo~ili ve~ podrobneje opredeljenih faciesov.
Predsedimentacijske, erozijske teksture so razvite v spodnjem in deloma v zgornjem delu debelozrnatega, pe{~enega ~lena sekvenc.
Tabla 1 – Plate 1
Sl. 1 Sivi polimiktni konglomerat zapolnjuje 4 m globok erozijski kanal. Vidna je erozijska povr{ina in del boka erozijskega kanala. Rudnik urana @irovski vrh, Pr 6/3-2, 27 m.
Fig. 1      Grey polymictic conglomerate filling a 4 m deep erosion channel. Erosion surface and part of erosion channel flank is visible. @irovski vrh uranium mine, Pr-6/3-2, 27 m.
Sl. 2 Erozijska povr{ina med rde~im muljevcem v talnini in pisanim polimiktnim konglomeratom v krovnini (plasti so rahlo inverzne). Med prodniki sivega in ro`natega kremena, apnencev ter predornin so iz talninskega rde~ega muljevca erodirani intraklasti. Prodniki so orientirani vzporedno s kliva`nimi razpokami, domenami. Rudnik urana @irovski vrh, P-10, 1030 m.
Fig. 2 Erosion surface between red shale in footwall and variegated polymictic conglomerate in hanging wall (beds slightly inverse). Among pebbles of grey and rose quartz, limestones and volcanic rocks occur intraclasts eroded from red shale in footwall. Pebbles are oriented parallel to cleavage fissures and domaines. @irovski vrh uranium mine, P-10, 1030 m.
Sl. 3 Planarno (ravno) navzkri`no plastnat pe{~enjak nad zelenim, razbarvanim rde~im muljevcem s kalcitnimi konkrecijami. Zgoraj horizontalno plastnat pe{~enjak. Rudnik urana @irovski vrh, Pr-6/3-2, 249 m.
Fig. 3 Planar cross-bedded sandstone overlying green shale with calcite concretions. Above is horizontally bedded sandstone. @irovski vrh uranium mine, Pr-6/3-2, 249 m.
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Erozijske povr{ine – Se spodnjih delov sekvenc smo `e obravnavali pri konglome-ratnih faciesih. Nad njimi le`e pe{~enjaki z intraklasti muljevca in/ali karbonatnih kon-krecij in so obi~ajno plitveje zarezane v podlago (tab. 2, sl. 3). Erozijske povr{ine vzpo-rejamo s faciesom Se (Rust, 1978; Miall, 1978) oziroma s faciesom SS (Cant & Walk e r , 1978).
V zgornjem delu pe{~enjakovega ~lena nekaterih sekvenc smo zasledili zapolnitve erozijskih kotanj oziroma manj{ih korit – Ss (scour-fill sand) in deloma St (solitary trough cross-bedded sand; Miall, 1977). Obi~ajno smo opazovali le manj{i del zapolnitev (sl. 3). Globina kotanj in korit dosega 50 cm. Ostale dimenzije lahko le ocenimo. Njihova {irina naj bi zna{ala do 3 m, dol`ina pa do 10 m in ve~. Kotanje in korita zapolnjuje drobno do debelozrnati pe{~enjak, ki je ponekod laminiran in lahko vsebuje posamezne prodnike.
Sinsedimentacijske teksture podrobneje dolo~ajo naslednje pe{~enjakove faciese.
Masivni pe{~enjak – Sm (massive sand) je med vsemi pe{~enjakovimi faciesi najbolj raz{irjen in predstavlja 47.8 % debeline Bre-bovni{kega ~lena. V njem nismo zasledili notranje organizacije. V nekaterih plasteh je izra`ena normalna in redko inverzna gradacija. Glede na poznane literaturne podatke je dele` masivnega faciesa v obravnavanih profilih presenetljivo visok. Po na{em mnenju je glavni vzrok temu mo~na tektonska prizadetost kamnin, predvsem kliva`, ki je povzro~il spremembo strukture prvotne kamnine. Med najpogostej{imi spremembami je izrazita preorientacija sedimentnih zrn. Sedimentne teksture so vidne predvsem v primerih, ko je pri{lo do delne spremembe velikosti zrn ali sestave ter z njo povezane barvne spremembe.
Horizontalno laminirani in plastnati pe-{~enjak   –   Sh   (horizontally-bedded   sand;
Sl. 3. Pre~ni presek zapolnitve erozijske kotanje z upognjeno laminiranim in plastnatim srednjezrnatim pe{~enjakom. Rudnik urana @irovski vrh, H-54, 58 m.
Fig. 3. Cross-section of erosion scour filled by curved laminated and bedded medium grained sandstone. @irovski vrh uranium mine, H-54, 58m.
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Sl. 4. Horizontalno plastnati pe{~enjak nad njim pa koritasto navzkri`no plastnati pe{~enjak. Rudnik urana @irovski vrh, Pr-6/3-2, 30 m.
Fig. 4. Horizontal bedded sandstone and trough cross-bedded sandstone above it. @irovski vrh uranium mine, Pr-6/3-2, 30 m.
Miall, 1977) sestavljajo seti vzporednih ali skoraj vzporednih lamin (sl. 4, tab. 2, sl. 2) in/ali plasti pe{~enjaka ter je v Brebovni-{kem ~lenu zastopan z 9.1 %. Lamine in plasti so debele 0.5 do 20 mm, njihovi seti pa od 5 cm do 3 m. Vzdol` nekaterih horizontalnih lamin so ve~je koncentracije intra-klastov muljevca. Veliki so do 30 cm in pogosto orientirani vzporedno z laminacijo. Horizontalno laminirani in plastnati pe{~e-njak je produkt ravne plasti, nastale ve~i-noma v pogojih zgornjega tokovnega re`ima. Polo`no planarno (ravno) navzkri`no la-minirani in plastnati pe{~enjak – Sl (low angle stratified sand; Rust , 1978), be`no omenja tudi M i al l (1977) in ga vzporejamo s faciesom G (Cant & Walker, 1978). Grade ga lamine in plasti, debele 2 do 20 mm, ki vpadajo glede na glavno ploskev plastnatosti pod polo`nim (<10o) kotom (tab. 2, sl. 2). Posamezni seti polo`ne planarne navzkri`ne laminacije in plastnatosti so de-
beli 10 cm do 3 m. Vzdol` lamin se ponekod pojavljajo intraklasti muljevca. Polo`na pla-narna navzkri`na laminacija in plastnatost pogosto lateralno prehaja v horizontalno. Opisani facies je v obravnavanih profilih redek, zastopan le z 1.1 % in ozna~uje zgornji tokovni re`im ter se obi~ajno odlaga na nekoliko nagnjeni podlagi ali zapolnjuje plitve erozijske kotanje.
Planarno (ravno) navzkri`no plastnati pe-{~enjak – Sp (planar cross-bedded sand; Miall, 1977) gradi 1.6 % debeline Brebov-ni{kega ~lena. Sestavljajo ga nagnjene ravne ali rahlo ukrivljene (tab. 1, sl. 3), vzporedne ali delno vzporedne lamine in plasti pe{~e-njaka, debele 1 do 20 mm. Nalegajo po{evno, tangencialno ali asimptoti~no na ponekod nekoliko nagnjene ali rahlo ukrivljene, ve-~inoma neerozijske ali le delome erozijske ploskve. Naklon lamin in plasti lahko dose`e kot 30o glede na glavno ploskev plastnatosti. V  posameznih  setih  planarne  navzkri`ne
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plastnatosti so vzdol` nekaterih lamin ali plasti orientirani podolgovati intraklasti muljevca, redko seti navzkri`ne laminacije ali celo tanke plasti muljevca. Planarno na-vzkri`no plastnati pe{~enjak je debel 20 cm do 1.6 m. Smeri vpada planarne navzkri`ne plastnatosti imajo trimodalno porazdelitev. Na obmo~ju @irovskega vrha je prva moda v smeri pribli`no 60o, druga v smeri 240o in tretja, najmanj zastopana, v smeri 150o. Planarno navzkri`no plastnati pe{~enjak je nastal z migracijo dvodimenzionalnih sipin (pre~nih in jezi~astih) in karakterizira spodnji tokovni re`im.
Koritasto navzkri`no plastnati pe{~enjak – St (trough cross-bedded sand; Miall, 1977) je zastopan s 9.0 %. Predstavlja vrsto zapolnitev podolgovatih erozijskih kotanj s sime-tri~no ali asimetri~no upognjeno laminira-nim materialom (sl. 4). Velikost posameznih erozijskih korit je precej razli~na. Ve~ina korit je globokih 10 do 30 cm in {irokih 50 cm do 1 m. Dol`ine v glavnem nismo mogli dolo~iti, ker je bilo v rudniku zelo malo vid-
nih vzdol`nih presekov. Erozijske kotanje so najpogosteje zapolnjene s pe{~enim, redkeje prodnato pe{~enim materialom, v katerem so vidne ukrivljene lamine in plasti, debele 2 do 30 mm z `li~asto obliko. Zna~ilno je, da so lamine deloma odrezane s sosednjimi ali zgoraj le`e~imi erozijskimi kotanjami. Vzdol` nekaterih lamin zasledimo tudi intraklaste muljevca. V profilih je korita-sto navzkri`no plastnatosti pe{~enjak debel 10 cm do 2.8 m. Opisani facies predstavlja zapolnitve kanalov in je produkt migracije tridimenzionalnih sipin ter ozna~uje spodnji tokovni re`im.
Navzkri`no laminirani pe{~enjak – Sr (tab. 2, sl. 2) (ripple cross-laminated sand; Miall, 1978; Rust, 1978) zavzema 1.3 % debeline Brebovni{kega ~lena. Po svojih oblikah in notranji zgradbi je zelo podoben razli~nim tipom navzkri`ne plastnatosti, le da so dimenzije manj{e. V ve~ini presekov smo opazovali koritasto navzkri`no lamina-cijo. Seti navzkri`ne laminacije so debeli 1 do 5 cm, medtem ko so koseti z navzkri`no
Tabla 2 – Plate 2
Sl. 1 Horizontalno plastnati konglomerat. Horizontalno plastnatost v prodnatem pe{~enjaku nakazujejo temno sive lamine, desno v pe{~enem konglomeratu pa ve~ja koncentracija prodnikov. Rudnik urana @irovski vrh, P-10, 1131 m.
Fig.1 Horizontally bedded conglomerate. Horizontal bedding in gravely sandstone is marked by dark grey laminas, and right in sandy conglomerate by higher pebble concentration. @irovski vrh uranium mine, P-10, 1131 m.
Sl. 2 V pe{~enjaku so na nekaterih ploskvah polo`ne planarne navzkri`ne plastnatosti koseti navzkri`-ne laminacije. Spodaj set horizontalne plastnatosti. Rudnik urana @irovski vrh, Pr-6/3-2, 252 m.
Fig. 2 On certain surfaces of low angle planar cross-bedded sandstone occur cosets of cross lamination. Below a set of horizontal bedding. @irovski vrh uranium mine, Pr-6/3-2, 252 m.
Sl. 3 Horizontalno laminirani muljevc s posameznimi ro`natimi kalcitnimi konkrecijami, v katerega je zarezana do 30 cm globoko erozijska ploskev. Ob spodnjem in zgornjem kontaktu s pe{~enim faciesom je muljevec deloma razbarvan in je zelene barve. Nad erozijsko povr{ino opazimo zelenkaste intraklaste muljevca. Rudnik urana @irovski vrh, PP-450, 22 m.
Fig. 3 Horizontally laminated shale with individual rose calcite concretions in which to 30 cm deep erosion surface is cut in. Along the lower and upper contacts with sandstone the shale is partly green colored. Above the erosion surface greenish shale intraclasts are seen. @irovski vrh uranium mine, Pr-10, 1068 m.
Sl. 4 V zelenem in rde~em muljevcu so {tevilne ro`nate kalcitne septarijske konkrecije, katerih koli~ina navzdol (proti desni) pada. Rudnik urana @irovski vrh, P-10, 1068 m.
Fig. 4 In green and red shale occur numeours calcite septarian concretions, abundance of which diminishes downward (toward right). @irovski vrh uranium mine, P-10, 1068 m.
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laminacijo debeli od 5 cm do 1 m. Navzkri`-no laminirani pe{~enjak je nastal z migracijo dvo in tridimenzionalnih sipinic v obmo~ju spodnjega tokovnega re`ima.
Muljev~evi faciesi
Muljev~evi faciesi litolo{ko obsegajo me-ljevce in meljaste glinavce, razli~nih rde~ih, vijoli~astih, sivih do temno sivih, skoraj ~r-nih in zelenkastih barv. V Brebovni{kem ~le-nu so zastopani z 11.4 %. Nastopajo v razli~-no, a relativno enakomerno debelih plasteh in deloma v le~ah. V talnini in krovnini mu-ljevcev so pe{~enjaki in konglomerati. Njihovi spodnji kontakti so obi~ajno jasni ali ostri. V primerih, ko predstavlja talnino zelo drobnozrnat pe{~enjak, je kontakt tudi postopen. Zgornji kontakti med muljevci in krovnino so erozijski (tab. 2, sl. 3) ali ostri.
Muljaste kamnine imajo heterogeno strukturo. Strukturno heterogenost povzro-~a menjavanje debeleje in drobneje zrnatih lamin, bioturbacija, konkrecije in kliva`ne domene. Ve~ina terigenih zrn v muljevcih je manj{ih od 0.03 mm in tvorijo osnovo, v kateri lebde pe{~ena zrna, ki so slabo sortirana. Koli~ina pe{~enih zrn je zelo spremenljiva in obi~ajno ne presega 20 %.
Sestava muljevcev je enostavna. Grade jih kremen, glinenci – albit in redko mi-kroklin, muskovit, klorit, ponekod pa {e kalcit, dolomit, pirit in hematit.
Teksturno smo lo~ili skrilavi in laminirani muljevec.
Skrilavi muljevec – Fs je najbolj raz{irjen. Pojavlja se v debelini od 5 cm do 5.5 m. Domnevamo, da je imela ve~ina sedaj skri-lavih muljevcev izra`eno drobno horizontalno laminacijo. Sedaj je, verjetno zaradi post-sedimentacijskih sprememb, nastalih predvsem pod vplivom tektonikih napetosti (kliva`) in v manj{i meri pod drugimi fizikalnimi, kemi~nimi in biolo{kimi vplivi, zabrisana.
Laminirani muljevec – Fl (tab. 2, sl. 3) (laminated, sand, silt and mud; Miall, 1977) in heteroliti~ni muljasto pe{~enjakovi faciesi s horizontalno in navzkri`no laminacijo ter krpasto in le~asto plastnatostjo so v Brebov-ni{kem ~lenu podrejeni.
Muljev~evi faciesi predstavjajo material odlo`en iz suspenzije ali {ibkega vle~nega toka na obre`ni ravnini ali v opu{~enih kanalih.
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Karbonatni faciesi
Med karbonatne faciese uvr{~amo kalcit-ne in dolomitne konkrecije (tab. 2, sl. 4) in tanj{e, ve~inoma nezvezno potekajo~e dolo-mitne plasti. Karbonatne konkrecije nastopajo v zgornjih, prete`no drobnozrnatih delih sedimentnih sekvenc, kjer se pojavljajo deloma posamezno, naklju~no in razli~no gosto razvr{~ene v debelej{em intervalu ali pa so koncentrirane v posamezne horizonte. Njihova koncentracija je pogosto tolik{na, da tvorijo bolj ali manj sklenjene plasti, debele od 5 cm do 1.5 m.
V zgornjih delih nekaterih pe{~enih sek-venc zasledimo v zelo drobno do srednje-zrnatem pe{~enjaku ponekod tudi karbonatne konkrecije ali nezvezno potekajo~e do-lomitne plasti, debele do 20 cm. V enem primeru zaklju~uje sekvenco 1.5 m debela plast pe{~enega dolomita. Karbonatni faciesi zavzemajo 2.5 % debeline Brebovni{kega ~lena.
Makroskopsko so karbonatne konkrecije ve~inoma ostro lo~ene od prikamnine in so velike do 20 cm, ve~inoma pa 5 do 10 cm. Njihova barva je razli~nih rde~ih in sivih odtenkov ter je delno odvisna od barve pri-kamnine oziroma sprememb njene barve. V rde~e obarvani prikamnini so rde~e, v sivi prikamnini sive, v zeleno obarvani prikam-nini pa so konkrecije sivih in rde~ih, ro`-natih odtenkov.
Ve~ino karbonatnih konkrecijah sestavljata kalcit ali dolomit. V manj{ih koli~inah so prisotni {e kremen, glinenci-albit, mu-skovit-illit in klorit, med akcesornimi minerali pa {e pirit, hematit, cirkon, rutil, tur-malin, rogova~a, neprozorna zrna in stilpno-melan. Dolomitne konkrecije so razvite v spodnji tretjini, kalcitne pa predvsem v zgornjih dveh tretjinah Brebovni{kega ~lena.
Karbonatni faciesi predstavljajo pedogene tvorbe (caliche, calcrete), koncentracije zgod-njediagenetskega cementa v plasteh in kon-krecijah ter deloma produkt sedimentacije v jezerih tipa playa (Skaberne, 1983).
Razvoj in interpretacija sedimetacijskega
okolja Brebovni{kega ~lena Grödenske
formacije
Na osnovi podrobnih profilov, posnetih v podkopu P-10 ter pre~nikih Pr-6/3-2 in Pr-
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6/0-4, smo skonstruirali poenostavljen profil razvoja Brebovni{kega ~lena na obmo~ju @i-rovskega vrha (sl. 5) in prikazali interpretacijo okolja sedimentacije posameznih segmentov. Pri tem smo uporabili terminologijo, ki jo navaja Skaberne (1996).
V sedanji geografski legi predstavljajo obravnavane grödenske kamnine izsek zapolnitve jugozahodnega boka nekdanjega sedi-mentacijskega bazena, ki je bil s severovzhoda narinjen v sedanjo lego in danes poteka v smeri NW – SE (Mlakar & Placer, 2000). Zaradi {e neizdelane palinspasti~ne rekonstrukcije obravnavanega obmo~ja podajamo interpretacijo glede na sedanjo lego. Na osnovi razvoja najni`jega dela Brebov-ni{kega ~lena Grödenske formacije sklepamo, da je najgloblji del bazena le`al severovzhodno od obmo~ja @irovskega vrha.
V  splo{nem lahko v razvoju Grödenske formacije in zapolnjevanju sedimetnega bazena, predvsem v njegovem osrednjem delu, ki obsega obmo~je @irovskega vrha, lo~imo {tiri makrocikle (Skaberne , 1995):
1. makrocikel je retrogradacijski in obsega spodnjo polovico Brebovni{kega ~lena
2. makrocikel je tudi retrogradacijskega zna~aja in zajema zgornjo polovico Brebov-ni{kega ~lena in pribli`no spodnjo polovico Hobov{kega ~lena
3. makrocikel je progradacijski in vklju-~uje zgornjo polovico Hobov{kega ~lena, Zal{ki, Koprivni{ki in Škofje{ki ~len
4. makrocikel je retrogradacijski in ga predstavlja Dobra~evski ~len. Kon~a se z morsko transgresijo ter odlo`itvijo morskih karbonatnih sedimentov.
V  nadaljevanju se omejujemo le na Bre-bovni{ki ~len Grödenske formacije.
Prvi makrocikel, ki obsega spodnjo polovico Brebovni{kega ~lena, le`i diskordantno na superpozicijski enoti Cc. Najbolje je razvit v najglobljem ohranjenem delu sediment-nega bazena na obmo~ju @irovskega vrha. Najni`ji del makrocikla predstavlja bazalno distalno tvorbo pe{~enega materiala, ki so ga odlo`ili re~ni tokovi s prevladujo~im talnim transportom. Sledi menjavanje pe{~e-njakovih in konglomeratnih faciesov, ki predstavljajo sedimente, ki so se usedali iz tokov s prevladujo~o komponento talnega transporta in jih pripisujemo re~nemu sistemu prete`no prepletajo~ih tokov (sl. 5 a). Lateralno so pritoki prina{ali material pisanega polimiktnega konglomerata. Pritoki so
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ve~ino materiala odlagali v obliki aluvialnih vr{ajev ali pa se je morda deloma tudi prese-dimentiral in pome{al s sedimenti glavnega vodnega toka. Na obmo~jih med vr{aji, odmaknjenih od glavnega re~nega toka, so se oblikovala tudi manj{a jezera tipa playa, v katerih je nastal pe{~eni dolomit. Navzgor so pri~eli prevladovati pe{~enjakovi faciesi. Njihov material so odlo`ili re~ni tokovi s prevladujo~im talnim transportom. Re~ni tok je bil verjetno sprva {e relativno raven in bi ga lahko ozna~ili kot pe{~en prepletajo~ tok (sl. 5 b). Postopoma se je pri transportu nekoliko pove~al dele` suspenzijskega sedimen-ta, vendar je komponenta talnega transporta {e mo~no prevladovala, tok pa je postal nekoliko bolj vijugav (sl. 5 c). Koli~ina suspen-zijskega materiala se je nadalje pove~evala, tako da je tok dobil zna~ilnosti me{anega, talno-suspenzijskega toka. Postajal je ~eda-lje bolj vijugav in pre{el v pravi meandri-rajo~i tok z dobro razvito obre`no ravnino (sl. 5 d). Sedimenti obre`ne ravnine pa so bili mo~no oksidirani in so postali rde~e obarvani. V obmo~ja nekaterih predelov poplavne ravnine so segale poplave le redko, tako da so se ponekod razvili pedogeni karbonatni, dolomitni horizonti.
Prvi makrocikel predstavlja sedimente razvoja re~nega sistema z zmanj{evanjem njegovega gradienta. To lahko razlo`imo z ve~jo hitrostjo sedimentacije v primerjavi s hitrostjo pogrezanja sedimentacijskega bazena.
Na obmo~ju @irovskega vrha zna{a debelina prvega makrocikla (profil P-10) pri-bli`no 165 m. Proti robovom sedimentnega bazena se ta tanj{a ter postaja manj izrazit. Njegovi lateralni ekvivalenti so zastopani ve~inoma s pe{~enjakovimi faciesi, med katere se vklju~ujejo le~e in plasti konglomeratov ter podrejeno muljevcev.
Relativno mirno sedimentacijo zgornjega dela prvega sedimentacijskega makrocikla je prekinil verjetno lateralni vdor proksimalnih, visoko energetskih tokov na obmo~je obre`ne ravnine. Tokovi so odlo`ili pe{~eno prodnati material pisanega polimiktnega konglomerata v obliki aluvialnega vr{aja (sl. 5 e). Temu je sledila prete`no pe{~ena sedimentacija iz tokov s prevladujo~o talno komponento transporta in verjetno prepletajo~im potekom re~-nih korit. V spodnjem delu so kamnine rde~e, v srednjem zelene in v zgornjem sive barve. Obnovitev proksimalne, debelozrnate sedi-mentacije povezujemo s tektonsko aktivnost-
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Sl. 5. Shematski profil litostratigrafskega razvoja Brebovni{kega ~lena Grödenske formacije na obmo~ju Rudnika urana @irovski vrh z interpretacijo okolja sedimentacije posameznih segmentov.
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Fig. 5. Schematic profile of lithostratigraphic development of the Brebovnica Member of the Val Gardena Formation in the @irovski vrh area with interpretation of depositional environment of
individual segments.
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jo v delu drena`nega zaledja. Obravnavani sedimenti predstavljajo prvi, verjetno sintek-tonski sunek visokoenergetske proksimalne sedimentacije ter prehod v posttektonsko sedimentacijo s tektonsko pomlajenega reliefa drena`nega zaledja. Sedimente tega prehoda smo zasledili v obmo~ju profila P-10, kjer zna{a njihova debelina probli`no 35 m, medtem ko se na {ir{em prostoru pri~ne sedi-mentacija drugega makrocikla s pe{~eno-pro-dnato sedimentacijo (profil Pr-6/3-2).
Tektonska dogajanja v obmo~ju dela dre-na`nega zaledja, ki so povzro~ila tudi pove-~anje gradienta re~nega sistema, ozna~uje posttektonska pe{~eno-prodnata sedimenta-cija spodnjega dela drugega makrocikla, ki je na obmo~ju @irovskega vrha debel pribli`no 90 m. Vzdol` bazena, generalno od severozahoda proti jugovzhodu, so re~ni, verjetno prepletajo~i tokovi s prevladujo~o talno komponento transporta, ki so bo~no, v smeri NE – SW po re~ni naplavni ravnini premikali svoja korita, prena{ali pe{~eni in prodnati material. Ta sestavlja sive pe{~enjake in poli-miktne konglomerate (sl. 5 f), med katerimi se pojavljajo prva uranova rudna telesa. Med te sive faciese se v ve~ nivojih vklju~ujejo, zdru`ujejo in cepijo le~e ter pasovi pisanega polimiktnega konglomerata. Ti imajo ve~jo kontinuiteto v smeri NW-SE, medtem ko se v smeri proti SW relativno hitro izklinjajo. Najve~ja koli~ina pisanega konglomerata nastopa v osrednjem delu @irovskega vrha in se v smeri proti NW in SE zmanj{uje. Glede na razliko v prodni{ki zdru`bi sivega in pisanega polimiktnega konglomerata predpostavljamo zanju razli~ni izvorni obmo~ji. Geometrija in struktura sedimentnih teles pisanega poli-miktnega konglomerata ka`e na aluvialno vr-{ajno sedimentacijo in lateralno smer transporta od severovzhoda proti jugozahodu (sl. 5 f). Zaradi erozije ve~ine severovzhodnega dela sedimentacijskega bazena in kompleksne tektonske zgradbe nimamo drugih dokazov za navedeno smer transporta pisanega polimiktnega konglometata.
Opisanim pe{~eno-prodnatim sedimentom je sledila prete`no pe{~ena sedimentacija s podrejeno koli~ino prodnate in muljaste komponente. Znotraj odlo`enih sedimentov smo, v odvisnosti od polo`aja, lahko izdvojili {tiri do pet manj{ih ciklov z zaporedjem zmanj{e-vanja zrnavosti, katerih debelina se spreminja od 15 do 35 m. V spodnjem delu ciklov prevladujejo sivi pe{~eni, ponekod prodnati sedi-
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menti. V teh delih prvih dveh ciklov so tudi najve~ja uranova rudna telesa na obmo~ju @irovskega vrha. Ti sedimenti predstavljajo koritne oblike in prepletajo~e sipinske komplekse, ki so jih odlo`ili tokovi s prevladu-jo~im talnim transportom. Poteka tokov, iz katerih so se usedali omenjeni sedimenti, nismo mogli vedno nedvoumno opredeliti. V {tevilnih primerih zasledimo zna~ilnosti, ki ka`ejo na bolj ali manj vijugav, meandrirajo~ potek toka (sl. 5 g). V zgornjem delu ciklov so poleg pe{~enih tudi muljasti sedimenti, ki so se usedali iz meandrirajo~ih tokov z me{a-nim, talno-suspenzijskim transportom (sl. 5 h). Zgornji deli ciklov predstavljajo bolj ali manj izoblikovano obre`no ravnino ali nivoje s pogostej{im pojavljanjem opu{~enih korit. Zaradi razlik v izrazitosti in kontinuiteti izoblikovanja zgornjega dela prvega cikla med petimi na razli~nih obmo~jih prihaja tudi do razli~nega {tevila ({tiri ali pet) izdvojenih ciklov. Njihovi vrhnji deli ozna~ujejo obdobja s po~asnej{o sedimentacijo prete`no suspendiranega materiala. Zato so bili sedimenti intenzivno oksidirani. V njih pa so se razvili horizonti s kalcitnimi konkrecijami.
Na obmo~ju @irovskega vrha zna{a pri-bli`na skupna debelina omenjenih {tirih do petih ciklov 105 m. Debelina drugega makro-cikla v obmo~ju Brebovni{kega ~lena pa je 230 m. Podobno kot pri prvem makrociklu se njegova zgradba in debelina proti robovom sedimentnega bazena spreminja in manj{a. Enako velja tudi za celoten Brebovni{ki ~len. Njegova debelina se spreminja od 0 do skoraj 400 m. Predpostavljamo, da je pri{lo po odlo`itvi sedimentov Brebovni{kega ~lena do hitrega umika glavnega re~nega toka z obravnavanega obmo~ja in/ali sorazmerno hitrega zmanj{anje gradienta re~nega sistema. Tako je za Hobov{ki ~len zan~ilna pre-vladujo~a po~asna sedimentacija muljastih, podrejeno pe{~enih sedimentov na obre`ni ravnini z vsemi svojimi podokolji in manj{imi kanali.
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Facies, development and interpretation of sedimentary environment of the uranium-bearing Brebovnica Member of the Val Gardena Formation in the @irovski vrh area
Introduction
The Val Gardena Formation has the largest continuous extension in Slovenia in the area between Cerkno and Smre~je where it outcrops as an irregularly wide belt in the NW-SE direction.
In the Cerkno and Sovodenj areas numerous showings of copper ore attracted attention of researchers already in the 19th century. With discovery of radioactive anomalies in the @irovski vrh area in 1960 and with later opening of the uranium mine of @irovski vrh the interest for the Val Gardena Formation further increased.
Several years of the investigation Geologic factors of Hg, Cu and U mineralization (Mlakar, 1978 to 1983) resulted into a new geologic map of the territory between Cerkno and Smre~je (Fig. 1) that incorporate also data from boreholes and mine workings. In the yearly research reports also basic data about all lithostratigraphic members of the Val Gardena Formation are recorded. The geologic structure of @irovski vrh and surroundings is described in short by Mlakar and P l a c e r (2000). Detailed information on sedimentary and postsedimentary evolution of the Val Gardena Formation in this area and on certain dilemmas of lithostra-tigraphic nomenclature were presented by Skaberne (1995).
Basic geologic characteristics of the @irovski vrh area
Between Cerkno, @irovski vrh (Fig. 1) and Smre~je extends in the NW-SE direction an about 20 km long and up to 5 km wide belt of rocks of the Val Gardena Formation. The area is situated at the contact of External Dinarides and Southern Alps consisting of overthrust units. In a wider sense the area of @irovski vrh belongs to the External Dinarides, and in a narrower sense to the Idri-ja-@iri territory that constitutes a part of the Trnovo nappe (Mlakar , 1969; Placer , 1981, 1999; Mlakar and Placer, 2000).
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The overthrusted structure is dissected by several fault systems of various directions. The Sovodnje fault along which the SW block was moved downward for 200 to 400 m is the most important of these fault systems (Mlakar and Placer, 2000).
In the base of the Val Gardena Formation occur shales of the Cc superposition unit. Their age has not yet been paleontologically proved, therefore they have been attributed on the base of older studies to the Upper Carboniferous, permitting their possible Lower Permian age (Mlakar, 1987; Mlakar et al., 1993; Mlakar and Placer, 2000; Mlakar, 2001). The contact between the Cc superposition unit and the Val Gardena Formation is often of tectonic, overthrust character, and in undisturbed localities it appears as angular-erosional disconformity. The Val Gardena Formation consists predominantly of red, subordinately grey, grey-green and green, prevailingly middle grained and to a lesser degree fine and coarse grained clastic rocks. Based on lithologic characteristics Mlakar (1978-1983) subdivided the Val Gardena Formation into six superposition units marked A1, A2, A3/1, A3/2, B and C. Later these units were classified as litho-stratigraphic members and named the Bre-bovnica (Br), Hobov{e (Ho), Zala (Za), Kop-rivnik (Ko) Škofje (Šk) and Dobra~evo (Do) Members by him (Mlakar and Placer, 2000).
The Brebovnica Member (Br) consists mainly of grey and grey green, in places red and green variously grained and prevailingly feldspar quartz lithic sandstones, conglomerates and less shales. The uranium mineralization occurs in the upper half of the Brebovnica Member in the @irovski vrh area. Thickness of this member varies from 0 to 400 m, and it pinches out in northwest.
The Hobov{e Member (Ho) is built of red shale with several meters thick lenses of red and grey fine-grained sandstone. In the latter occur in places copper mineralizations. The member is 20 to 280 m thick.
The Zala Member (Za) is represented by red, variously grained sandstones and subor-dinately thinner beds of red shale. Its thickness varies between 180 and 380 m.
The Koprivnik Member (Ko) consists of red conglomerates and sandstones of varying grain size and of smaller amount of shale. In the wider surroundings of @irovski vrh the
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member attains 550 m of thickness. Northwest of Sovodnje the Zala and Koprivnik Members thin and finally pinch out. In the southeastern part of the considered territory (Lavrovec) the beds that could be compared to Zala and Koprivnik Members appear in a different development.
The Škofje Member (Šk) is developed in the Škofje and Sovodenj areas as grey lithic quartz sandstone in which the copper mineralization occurs in places (Škofje, Sovo-denj). In the Javorjev dol area appear in the same lithostratigraphic level lenses of light grey and grey brownish, in places pink conglomerate that becomes southeastwards more continuous and thicker, attaining a thickness of 100 m. At Lavrovec two prograding coarsening-upward successions could be distinguished. They consist of grey and grey yellowish sandstones and conglomerates, the latter being in places also red. The successions attain a thickness of up to 740 m, and they represent the equivalent of the Zala and the Koprivnik Members.
The Dobra~evo Member (Do) is developed in the Sovodenj area prevailingly as red finegrained sandstone attaining 120 m of thickness. Towards southeast it passes to red shale that thins and pinches out at Lavrovec.
The thickness of the entire Val Gardena Formation varies from 200 m in the Škofje area to 1750 at @irovski vrh where it attains its maximum thickness in Slovenia.
The lower part of the Val Gardena Formation at @irovski vrh could be older than 255 million years which corresponds to the Kazanian, the lower part of Upper Permian in its two part subdivision (Odin & Odin, 1990), or to the Middle Permian in the three part subdivision of Permian. This is confirmed also by the only determinable fossil remains (pollen) found in the P-10 adit and B-63 borehole (Jelen  et  al., 1981).
Carbonate rocks transgressively overlie the terrigenous Val Gardena Formation. They build the @a`ar Formation that is an equivalent of the Bellerofon Formation in the Carnian Alps.
In the following we will limit ourselves only to the Brebovnica Member that is also economically the most interesting member of the Val Gardena Formation owing to considerable concentration of uranium ore. The basic subdivision of this member was carried
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out in connection with exploration of uranium ore (Omaljev, 1967 a, b; Budkovi~, 1980).
Methodology of examination and interpretation
In the @irovski vrh mine area (Fig. 1) we logged ten detailed profiles that partly overlap, and that cover the entire and maximum preserved thickness of the Brebovnica Member in the entire area between Cerkno and Smre~je.
The logged profiles represent unidimensio-nal vertical successions of facies. Since the Val Gardena Formation consist prevailingly of clastic rocks and as the grain size represents one of the basic and in clastics the most important property, it was used for the basis of the facies subdivision. Three basic conglomerate, sandstone and mudstone facies were distinguished. They were subdivided in more detail according to additional defining criteria as their sedimentary structures and composition. Since the sedimentary structures were formed during sedimentation and since they reflect the interaction between flow, bed and transported material, from them certain characteristics of the flow could be inferred.
The vertical successions of facies were associated to sedimentary sequences that are bounded by third and higher order bounding surfaces (Miall, 1988). An ideal sequence of the most well known meandering river deposit consists of lower, coarse grained, point bar and upper, fine grained, overbank sediments.
Sedimentary sequences were associated into various higher order units (the first of E1 and the second of E2 orders; Fig. 2). The units are of interpretative character and they should represent the unit bars, channel forms and braided bar and overbank complexes of rivers or alluvial fans. Complete representations of profiles, defined units and detailed interpretation are presented by Skaberne  (1995).
All discussed relative frequencies of individual facies are shown in the following with respect to their thickness in considered sections of carefully logged P-10 abit, Pr-6/2-3 and Pr-6/0-4 crosscuts in the central part of the @irovski vrh uranium mine.
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Facies and their interpretation
In the Brebovnica Member the most frequent are the sandstone and conglomerate facies, while the mudstone and carbonate facies are less developed.
Conglomerate facies
The conglomerates, sandy conglomerates and gravely sandstones, of which the last two mentioned prevail, are attributed to conglomerate facies that comprise 18.6 % of the Brebovnica Member. The conglomerate facies are of variously grey, green and subordinately red color shades. They occur in irregularly thick beds and lenses of 5 cm to 9 m thickness.
In conglomerates the following pebble groups can be detected macroscopically: grey-white and rose quartz, grey, green and red volcanic rocks, rare granitoid rocks, chert – lidite and jasper, limestones, sandstones, tuffs, sericite slates, phyllites, quartzites and intraclasts of shales and carbonate concretions.
In conglomerates alterations of color and composition of pebbles were detected, and on this basis two main conglomerate facies were established.
The grey polymictic conglomerate is represented with 8.5 %, and is of variously grey and subordinately greenish color shades. In it pebbles of grey-white quartz, grey and green volcanic rocks predominate over pebbles of lidite, tuffs and intraclasts (Pl. 1, Fig. 1). It usually forms more or less continuous beds and lenses of 10 cm to 4.5 m thickness in individual sequences.
The variegated polymictic conglomerate has a very diverse pebble composition that gave it the name. In it pebbles of grey-white and rose quartz prevail, while pebbles of grey and green volcanic rocks, lidite, jasper, tuffs and intraclasts are less abundant (Pl.1, Fig. 2). Along with the mentioned pebbles occur in places also pebbles of red volcanic rocks and limestones. The variegated poly-mictic conglomerate is prevailingly green, rarely reddish and even more rarely grey. It forms more or less continuous beds and lenses from 10 cm to 8.1 m thick in considered sequences.
The variegated polymictic conglomerate is irregularly distributed in the @irovski vrh
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area. It has the maximum thickness in the central part where it amounts to 10.1 % of total thickness of the Brebovnica Member, whereas it thins towards NW and SE. It occurs in several horizons that may fast pinch out laterally in the SW direction. Between individual horizons the grey polymictic conglomerate is interbedded.
For more detailed determination of facies the predepositional and syndepositional sedimentary structures were used.
Se – erosional surfaces (Pl. 1, Fig. 1, 2) are of the third or higher order bounding surfaces (M ia l l , 1988, 1990). They are compared to the Se lithofacies – erosional scours with intraclasts (Rust, 1978; M iall , 1978) and the SS lithofacies (Cant & Walker, 1978).
Gm – massive conglomerate is the most abundant, forming 17.0 % of the Brebovnica Member thickness.
Gh – horizontally bedded conglomerate is more to less distinctly expressed (Pl. 2, Fig. 1). M iall (1977, 1978) associated conglomerates with massive and horizontal bedding to the Gm lithofacies – massive and crudely bedded gravel. The Gm and Gh conglomerates could represent longitudinal bars, lag coarse-grained deposits or alluvial fan sieve deposits.
Gt – trough cross-bedded conglomerate is more or less clearly expressed, and makes only 0.4 % of total thickness. It is interpreted as channel fill.
Sandstone facies
The sandstone facies are the most abundantly represented. They comprise 70.0 % of the Brebovnica Member thickness. Based on grain size they were subdivided into three groups: coarse (10.6 %), medium (38.0 %) and fine-grained (21.4 %) sandstone. They are of grey, red and subordinately green color shades, and they form more or less regularly thick beds and lenses. Their thickness in considered sequences varies between 5 cm and 18.2 m. They are underlain by conglomerates and shales.
Data on grain size distribution in sandstones of the Val Gardena Formation as well as interpretation of their depositional environment based on them, defined it as fluvial, are given by Skaberne (1995, 1997).
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Sandstones are composed of terrigenous grains of quartz, feldspars – mainly plagio-clases, lithic grains – of volcanic rocks, granitoids, metamorphic rocks and subordina-tely sedimentary rocks, phyllosilicates – especially muscovite and chlorite, and accessory heavy minerals.
Terrigenous grains are bound by cement and matrix. Most of cement consists of quartz, feldspars, calcite, dolomite and numerous other minerals that occur in various forms and in several generations. The larger part of matrix belongs genetically to epimatrix that consists predominately of quartz and illite-sericite.
With respect to the presence or absence of predepositional and especially of syndeposi-tional sedimentary structures in individual granulometric groups of sandstone facies several detailed facies were distinguished.
Se – erosional surfaces (Pl. 2, Fig. 3) are compared, as in the conglomerate facies, with the Se lithofacies (Rust, 1978; M iall, 1978) and with the SS lithofacies (Cant & Walk e r , 1978).
In the upper part of the sandstone member of some sequences the Ss – scour-fill sandstone (Fig. 3) and partly the St – solitary trough cross-bedded sandstone (M ia l l , 1977) were detected.
Sm – massive sandstone is the most abundant among all sandstone facies, and it represents 47.8 % of the Brebovnica Member thickness. Compared to known literature data the share of the massive sandstone in the considered profiles is surprisingly high. We consider the main reason for such abundance of massive bedding to be the high degree of tectonic deformation of rocks.
Sh – horizontally laminated and bedded sandstone (Fig. 4, Pl. 2, Fig. 2) is represented by 9.1 % of the Brebovnica Member. It is a product of plane bed deposited mostly, but not always, in the upper flow regime.
Sl – low angle planar cross-laminated and bedded sandstone (Pl. 2, Fig. 2) is rare in the considered profiles, represented by only 1.1 %. It reflects the upper flow regime, and is usually deposited on a somewhat inclined basis, or it fills shallow erosional scours.
Sp – planar cross-bedded sandstone (Pl. 1, Fig. 3) comprises 1.6 % of total Brebovnica Member thickness. In the @irovski vrh area the dips of the planar cross bedding have a three-modal distribution. The first mode is
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in the direction at about 600, the second at about 2400, and the third, the least frequent, at about 1500. This facies was formed as a result of migration of two-dimensional dunes, transverse or languid bars, and characterizes the lower flow regime.
St – trough cross-bedded sandstone (Fig. 4) is present with 9.0 %. This facies represents channel fills with the migration of three-dimensional dunes, and is typical for the lower flow regime.
Sr – cross-laminated sandstone (Pl. 2, Fig. 2) is represented by 1.3 % in the thickness of the Brebovnica Member. The facies was formed by migration of two- and three-dimensional ripples in the lower flow regime.
Mudstone facies
Lithologically the mudstone facies comprises siltstones and shales of various red, violet, grey to dark grey and almost black and greenish colors. They make 11.4 % of the considered member.
The fine-grained rocks have a heterogeneous texture. Textural heterogeneity is caused by alternation of coarser and finer grained laminas, bioturbation and concretions, and by cleavage domains.
They are composed of muscovite – illite, chlorite, quartz, feldspars – albite and rarely microcline, and in places by additional cal-cite, dolomite, pyrite and hematite.
The mudstone fasies are texturally slaty or laminated.
Fs – slaty shale is prevailing among the mudstone facies. We believe that most of the present slaty shales had expressed fine horizontal lamination, which was obliterated mostly by tectonic strain and other post-depositional alterations.
Fl – laminated shale (Pl. 2, Fig. 3) with horizontal or cross-lamination and lenticular bedding is subordinate in the Brebovnica Member.
The mudstone facies represent overbank sediments deposited from suspension or weak traction currents.
Carbonate facies
The calcite and dolomite concretions (Pl. 2, Fig. 4) and thinner, largely discontinuous
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dolomite beds are attributed to the carbonate facies. The carbonate concretions occur in the upper, mainly fine-grained parts of sedimentary sequences where they appear in part isolated, randomly and variable dense within a thicker interval, or they are concentrated in individual horizons. Their abundance is often so high that they form more or less continuous beds from 5 cm to 1.5 m thick. The carbonate facies comprise 2.5 % of the Brebovnica Member.
The carbonate concretions are macrosco-pically in general sharply delimited from the surrounding rock. They are about 20 cm large, mostly between 5 and 10 cm.
Most of the carbonate concretions consist of calcite and dolomite. In smaller amounts also quartz, feldspars – albite, muscovite-illite and chlorite are present. Dolomite concretions are developed in the lower third, and calcite concretions in the upper two thirds of the Brebovnica Member.
The carbonate facies represent partly pe-dogene carbonates (caliche, calcrete), partly concentrations of early diagenetic carbonate cement in beds and concretions, and partly deposits of playa lakes (Skaberne , 1983).
Development and interpretation of
depositional environment of the
Brebovnica Member of the Val Gardena
Formation in the @irovski vrh area
From detailed profiles logged in the P-10 adit and in Pr-6/8-2 and Pr-6/0-4 crosscuts we constructed a simplified profile of the lithostratigraphic development of the Bre-bovnica Member in the @irovski vrh area (Fig. 5), and interpreted the depositional environment of individual segments.
In the present geographic position the considered Val Gardena rocks represent a section of the fill of the southwest flank of the original sedimentary basin that was overthrusted from northeast to its present position trending northwest–southeast (Mlakar & Placer , 2000). Since the pa-linspastic reconstruction of the considered area has not been performed yet, we present the interpretation according to its present position. On the basis of the development of the lowermost part of the Brebovnica Member of the Val Gardena Formation we presume that the lowermost part of the basin
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was situated northeast of the @irovski vrh area where also greatest thickness is preserved.
Generally, in its central part that comprises the @irovski vrh area four macrocycles can be distinguished (Skaberne, 1995):
The 1st macrocycle is retrogradational, and it comprises the lower half of the Brebovnica Member.
The 2nd macrocycle is also of retrogra-dational character and it comprises the upper half of the Brebovnica Member and about the lower part of the Hobov{e Member.
The 3rd macrocycle is progradational and it includes the upper part of the Hobov{e Member, and the Zala, Koprivnik and the Škofje Members.
The 4th macrocycle is retrogradational and is represented by the Dobra~evo Member; it is terminated by a sea transgression and deposition of marine carbonate sediments.
In the following, only the Brebovnica Member of the Val Gardena Formation will be considered.
The first macrocycle comprising the lower half of the Brebovnica Member is discordantly overlying the Cc superposition unit. It is the best developed in the deepest preserved part of the sedimentary basin in the @irovski vrh area. The lowermost part of the first macrocycle represents the basal distal form of sandy material deposited by predominant bed-load streams. The followed alternation of sandstone and conglomeratic facies represents the sediments deposited from predominant bed-load streams, and they are attributed to a prevailingly braided fluvial channel style (Fig. 5 a). The tributaries carried in laterally the material of variegated polymictic conglomerate. The tributaries deposited the most of material in alluvial fans. Some of this material was perhaps in part resedimented and mixed with sediments of the main stream. In areas between alluvial fans, at some distance from the main stream, periodically occurred smaller playa lakes, where sandy dolomite formed. After that started to prevail the sandstone facies deposited by bed-load streams; the fluvial channel style was sand dominated and most probably braided (Fig. 5 b). In the sediment load transported by streams the bed-load still strongly prevailed but the amount of suspended-load started gradually to increase. This caused increasing the sinuosity of
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fluvial channels too (Fig. 5 c). The amount of suspended material continued to increase, so that the stream became of mixed-load type. The fluvial style evolved in the higher sinuosity single channel system passing into meandering channel style with a well-developed floodplain (Fig. 5 d). The overbank deposits became strongly oxidized and red colored. Some areas of the floodplain were attained by less frequent floods so that in places pedogenic carbonate, dolomitic horizons could develop.
The first macrocycle represents the sediments of the fluvial system development that was characterized by reduction of its gradient. This can be explained by the high net sedimentation rate that exceeded the subsidence rate of the sedimentary basin.
In the @irovski vrh area the thickness of the first macrocycle (P-10 profile) attains about 165 m. Toward the margins of the sedimentary basin it thins and becomes less distinct. Its lateral equivalents are represented mostly by sandstone facies with inclusions of lenses and beds of conglomerates and subordinately shales.
The relatively quiet deposition of the upper part of the first sedimentation macro-cycle was interrupted probably by lateral irruption of proximal, high-energy streams on the areas of the floodplain. The streams deposited sandy and gravely material of the variegated polymictic conglomerate as alluvial fans (Fig. 5 e). This was followed by predominantly sandy deposition from the bed-load streams in the probably braided channels system. In the lower part the rocks are red, in the middle one green and in the upper part grey. Renewal of proximal, coarse-grained sedimentation is associated with tectonic activity in a part of the drainage basin. The considered sediments represent the first, probably syntectonic shock of high-energy proximal sedimentation, and the transition towards post tectonic sedimentation from the tectonically renewed relief of the drainage basin. These transition sediments were detected in the area of the P-10 profile where their thickness averages about 35 m, whereas in the wider area the sedimentation of the second macrocycle was started with sandy-gravely sedimentation (profile Pr-6/3-2).
The tectonic events in the drainage basin which caused also the increase of the river
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system gradient, are marked by post tectonic sandy-gravely sedimentation of the lower part of the second macrocycle that attains about 90 m thickness in the @irovski vrh area. The grey sandstone and polymictic conglomerate (Fig. 5 f) within which the first uranium ore bodies occur were formed of sand and gravel transported by bed-load streams that were flowing along the basin, generally from northwest to southeast and shifted laterally, in NW-SE direction, their braided channel across the aluvial plain.
Among these the grey sandstone and poly-mictic conglomerate include, amalgamate and split at several levels the lenses and belts of variegated polymictic conglomerate. Their continuity is steadier in the NW-SE direction, whereas they relatively rapidly pinch out toward SW. The largest amount of variegated polymictic conglomerate occurs in the central part of the @irovski vrh area, and it diminishes in the NW and SE directions. Owing to the difference in pebble associations of grey and variegated poly-mictic conglomerate two different source areas for them are supposed. The geometry and structure of sedimentary bodies of variegated polymictic conglomerate indicate alluvial fan depositional environment and lateral transport from northeast towards southwest (Fig. 5 f). Because of erosion of great deal of the northeastern part of the sedimentary basin and the complex tectonic structure there are no other proofs for the mentioned transport direction of the variegated polymictic conglomerate.
The described sandy-gravely sedimentation was followed by predominant sandy sedimentation with subordinate amount of gravely and muddy component. Within the deposited sediments four to five smaller fining-upward cycles could be established. Their thicknesses vary from 15 to 35 m. In the lower part of cycles prevail grey sandy, in places gravely sediments. In these parts of the first two cycles also occur the largest uranium ore bodies of the @irovski vrh area. These sediments represent channel forms and braided bar complexes deposited from the predominant bed-load streams. The fluvial channel style, which the mentioned sediments deposited, could not always be determined unequivocally. In numerous cases certain characteristics indicate meandering channels with lower or higher sinuosity (Fig.
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Faciesi, razvoj in interpretacija sedimetacijskega okolja uranonosnega Brebovni{kega ~lena …        187
5 g). The upper parts of cycles consisting of sandy and muddy sediments were deposited by mixed-load streams of meandering fluvial channel style (Fig. 5 h). The uppermost parts of cycles represent a more or less formed overbank plain or levels with more frequent occurrences of abandoned channels. Owing to differences in expressiveness and continuity of the upper part of the first of the five cycles in various areas also different numbers (four to five) of cycles can be determined. Their upper parts mark the periods of slower sedimentation of predominantly suspended material. Therefore the sediments were intensively oxidized and horizons of caliche concretions developed in them.
In the @irovski vrh area the approximate total thickness of the mentioned four to five cycles amounts to 105 m. Thickness of the second macrocycle in the Brebovnica Member area is 230 m. Similarly as the first ma-crocycle its structure and thickness changes and diminishes towards the margins of the sedimentary basin. This is true also for the entire Brebovnica Member. Its thickness varies from 0 to almost 400 m.
After the deposition of sediments of the Brebovnica Member supposedly the main stream rapidly retreated from the considered area, and/or the fluvial system gradient rapidly diminished. The Hobov{e Member is therefore characterized by the predominantly slow sedimentation of muddy, subor-dinately sandy sediments on the wide over-bank plain with all subenvironments and smaller channels.
Zahvala
Raziskavo je financiralo Ministrstvo za {olstvo, znanost in {port RS. Zahvaljujem se kolegom, ki so podali kriti~ne pripombe k rokopisu in s tem pripomogli k njegovi iz-bolj{avi. Posebna zahvala velja S. Pircu za angle{ki prevod. Za tehni~no pomo~ pa se zahvaljujem V. Pavlici in J. Otrin.
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