(English) (Fotografije / Photos) (Konec teksta / End of Text)
Kazalo
1. Uvod
2. Splošno o konglomeratu
3. Flišni konglomerat pri Ilirski Bistrici
4. Dostop
5. Zemeljski zaklad
6. Viri
7. Fotografije
Uvod
V okolici Ilirske Bistrice prevladujejo flišne kamnine, ki so pred davnimi časi nastale v morskem bazenu. Med drugimi lahko tam najdemo tudi primerke konglomeratnih kamnin. Posebnosti v flišnem konglomeratu so podkvasto oblikovane strukture – polkrožni, centimeter ali dva visoki zaobljeni grebeni, ki obdajajo del prodnika. Del kamnine je zaradi tektonskih premikov prišel na površino in ponuja izvrsten prikaz zgradbe konglomeratnih kamnin.
Prerez konglomerata iz Smøle, Norveška (vir: Wikipedia)
Splošno o konglomeratu
Konglomerat je vrsta kamnine, ki spada med t.i. klastične sedimentne kamnine. To pomeni, da ga sestavljajo klasti ali odlomki, na katere so pri preperevanju in eroziji razpadle starejše kamnine, nastal pa je z usedanjem oz. sedimentacijo. Rečni tokovi so odlomke nosili od mesta nastanka do mesta odložitve, na tej poti pa so se kotalili po dnu struge, se med seboj trkali, pri tem pa manjšali in obrusili. Takim zaobljenim delcem rečemo prodniki. Prod je običajno sestavljen iz različnih vrst prodnikov, pogosto so med seboj pomešani prodniki magmatskih, metamorfnih in sedimentnih kamnin. Reka lahko velike kose kamenja premešča le, dokler ima njen tok dovolj veliko energijo. Ko ta v ravninskih delih postopno upada, se začno v rečnem koritu odlagati najprej večji, potem pa vedno manjši prodniki. Največje količine proda reke odložijo v poplavnih (aluvialnih) vršajih (pahljačah ob prehodih ozkih rečnih dolin s strmim naklonom v široke ravne doline), na poplavnih ravninah in v zgornjih delih ter koritih rečnih delt.
Ko je enkrat prod odložen, se nanj nalagajo mlajše plasti usedlin in pod njihovo težo se začne proces stiskanja. Prod se pri tem zgoščuje, praznega prostora med prodniki je vse manj, hkrati pa se začno z vezivom med seboj sprijemati. Vezivo je lahko mineralni cement (najpogosteje kremenov ali kalcitni), ki se izloča iz vodne raztopine, lahko drobnozrnata osnova (peščena, glinena ali lapornata), ali pa oboje. Vezivo postopno zapolni še prazne prostore med prodniki. Ta proces se imenuje diageneza, z njim pa se prod spremeni v kamnino, imenovano konglomerat. Ime izvira iz latinskega izraza conglomerare, kar pomeni nakopičiti. Glede na mineralno sestavo prodnikov konglomerate delimo na kremenove, apnenčeve, dolomitne itn.
Konglomeratni balvan (vir: Wikipedia)
Značilnost konglomerata je, da so prodniki v njem tako veliki, da jih razločno vidimo s prostim očesom. Usedline (sedimenta), katere delci so manjši od 2 mm, ne imenujemo več prod, ampak pesek in sedimentno kamnino, ki iz njega nastane, peščenjak. Kadar odlomki v debelozrnatih sedimentnih kamninah niso zaobljeni v prodnike, ampak ostanejo oglati in ostrih robov, taki kamnini rečemo breča. Pogosto lahko vidimo tudi vmesne različice z delno zaobljenimi odlomki, ki jih poimenujemo konglomeratne breče.
Konglomerat na morski obali (vir: Wikipedia)
Konglomerati pa niso nastajali le v rečnih dolinah, ampak jih najdemo tudi na obalah jezer in morij, kjer so prodnike zaoblili valovi. Konglomerat je kamnina, značilna za kopno, zanimivo pa je, da so nekateri nastali tudi v zelo globokih morjih. Tam so namreč "reke" kamninskega drobirja, ki jim rečemo sedimentni gravitacijski tokovi, navadno ob potresih zdrsele po šelfnih pobočjih in odložile velike količine sedimenta v obliki širokih pahljač na morsko dno. Če so bili delci v usedlinah večji od 2 mm in zaobljeni, je iz njih nastal konglomerat.
Prerez podvodnega konglomerata iz globine približno 3000 m (vir: Wikipedia)
Na območju današnje Slovenije je veliko konglomerata nastalo v obdobju paleozoika. To so sivi in rdeči kremenovi konglomerati, razprostranjeni pa so v Karavankah in v širokem pasu v smeri vzhod-zahod po osrednji Sloveniji. Največ konglomerata pa je nastalo v obdobju kenozoika, ko so se v pleistocenu – ledeni dobi – med kratkotrajnimi otoplitvami ledeniki topili, kamninski drobir, ki so ga puščali za seboj, pa so močno narasle reke odnašale in odlagale v kotline. Tako lahko danes opazujemo do 120 m debele zasipe proda, peska in konglomerata v rečnih terasah, naprimer na Kranjskem, Sorškem, Ljubljanskem in Krškem polju in v dolini reke Soče.
Konglomerat (vir: Wikipedia)
Flišni konglomerat pri Ilirski Bistrici
V okolici Ilirske Bistrice prevladujejo eocenske flišne kamnine, ki so pred približno 55 milijonov let nastale v takratnem morskem bazenu. Posebnosti v flišnem konglomeratu so podkvasto oblikovane strukture – polkrožni, centimeter ali dva visoki zaobljeni grebeni, ki obdajajo del prodnika.
Te strukture so nastajale med usedanjem fliša. Vrtinčasti tokovi so ob prodniku odnašali mehko usedlino in tako se je izoblikoval žleb, ki ga je kasneje zapolnil drug material. Danes ne vidimo več teh žlebov, pač pa njihove odlitke v obliki podkvastih grebenov neposredno ob prodnikih. Od nastanka žlebov so se namreč zemeljske plasti zaradi tektonskih pojavov premaknile in so danes kamnine skoraj navpične.
Prodniki so naključno posejani, med seboj se ne dotikajo in so iz različnih kamnin kredne in terciarne starosti. Veliki so od 1 do 20 cm, prevladujejo pa velikosti od 4 do 6 cm. Velikost podkvastih struktur je odvisna od velikosti prodnikov, saj je ob večjem prodniku vodni tok izdolbel večji žleb.
Podkvaste strukture v flišnem konglomeratu pri Ilirski Bistrici
Dostop
Do tega zemeljskega zaklada pridete iz Postojne, od koder greste v smeri Ilirske Bistrice oziroma Reke na Hrvaškem. Cesta vas po nekaj kilometrih pripelje do naselja Pivka, kjer v centru ne zavijete desno po prednostni cesti, ampak greste naravnost v smeri Knežaka. Druga možnost je pot iz Ilirske Bistrice v smeri Šembij in Knežaka.
Konglomerat se nahaja tik ob dokaj prometni glavni cesti, zato bodite pazljivi, ko hodite do njega. Možnost za parkiranje je na kolovoznih poteh, ki se odcepijo od glavne ceste pred ali za lokacijo zemeljskega zaklada (npr. točka PARK).
Pri hoji po cesti ali njenem prečkanju je potrebna maksimalna pazljivost! Priporočamo, da za hojo izberete travnik na drugi strani varovalne ograje.
Dostop do zaklada je možen skozi celo leto, saj cesto plužijo, vendar pa obisk v času, ko zemljo prekriva snežna odeja, ni najbolj smiseln, saj boste takrat težje odgovorili na zastavljena vprašanja. Priporočamo torej, da si zaklad ogledate v času, ko ni snega.
Zemeljski zaklad
Za registracijo tega zemeljskega zaklada morate odgovoriti na naslednja vprašanja:
1. Konglomeratne plasti, ki jih lahko opazujete na lokaciji zemeljskega zaklada, so po nastanku žlebov zaradi tektonskih premikov spremenile svoje lego. V katero smer bi morala teči voda, da bi nastale tako usmerjene strukture, kot jih lahko vidite danes?
2. Pojasnite, zakaj je kamnina, ki jo vidite, konglomerat in ne breča.
3. Starost kamnine je bila ocenjena na podlagi nano planktona v vezivu. Na osnovi videnega na lokaciji zaklada in vašega poznavanja procesa nastanka konglomeratnih kamnin ocenite, ali je starost prodnikov, ki sestavljajo konglomerat, enaka, večja ali manjša? Utemeljite svoj odgovor.
4. Kot neobvezno nalogo pa naredite fotografijo vas ali vašega GPSja pri konglomeratu in jo pripnite vašemu vpisu.
Odgovore pošljite skozi GC profil. Potem lahko zabeležite vaš obisk, ne da bi čakali na posebno dovoljenje. V kolikor bo z vašimi odgovori kaj narobe, vas bomo kontaktirali.
Če ste fotografirali, priložite zanimive fotografije.
(Slovene) (Photos) (End of Text)
Table of Contents
1. Introduction
2. About conglomerate
3. Flysch conglomerate at Ilirska Bistrica
4. Access
5. EarthCache
6. Sources
7. Photos
Introduction
The vicinity of Ilirska Bistrica is dominated by flysch, which was created a long time ago in an ancient sea basin. Among others instances of conglomerate rock can be found there. Special features of the flysch conglomerate are horseshoe-shaped structures - semicircular, a centimeter or two high rounded ridges that surround part of pebbles. Due to tectonic movements part of this rock surfaced and offers an excellent view on the composition of conglomerate rock.
Section of conglomerate from Smøla, Norway (source: Wikipedia)
About conglomerate
Conglomerate is a so-called clastic sedimentary rock. This means that it consists of clasts or fragments, in which older rocks break because of weathering and erosion. On their journey to where they were finally dropped off, clasts were rolling in a river flow, rapping together, being decreased in size and smoothened down, turning into rounded particles called pebbles. Gravel usually consists of different types of pebbles. Often pebbles of magma, metamorphic and sedimentary rocks are mixed together. The river can move large pieces of stone only as long as its flow has sufficient energy. In the flat areas the energy gradually declines and first larger and then ever smaller pebbles are deposited in the river bed. The biggest amount of gravel is deposited on the river alluvial fans, created where narrow river valleys with steep incline transition into broad flat valleys, or on alluvial floodplains, periodically deposited by the river during floods, or in river deltas.
Once the gravel is deposited, it is covered with layers of sediment and under their weight begins the process of compression. Concentration of the gravel increases and there is less and less empty space between the pebbles. At the same time they start to stick together with binder. Binder can be a mineral cement (usually quartz or calcite), which is secreted from the aqueous solution, the fine base (sand, clay or marl), or both. Binder then fills in the empty spaces between the pebbles. This process is called diagenesis. Through it gravel turns into conglomerate. The name is derived from the Latin word conglomerare, which means to accumulate, to heap up. Based on the mineral composition of the pebbles, conglomerates can be quartz, limestone, dolomite, etc..
Boulder of conglomerate (source: Wikipedia)
Pebbles in the conglomerate are so large that they are clearly visible to the naked eye. Sediment with particles smaller than 2 mm isn’t called gravel anymore. It’s called sand and sedimentary rock that formed from it is called sandstone. Fragments in sedimentary rocks are not always rounded into pebbles, though. Such a rock, made from sharp-edged fragments is called breccia. Quite common are also intermediate versions with partially rounded clasts, which then can be called conglomerate breccia.
Conglomerate on the sea shore (source: Wikipedia)
Conglomerates don’t form only in the river valleys, but also on the lake or sea shores, where pebbles are rounded by waves. Conglomerate is a rock typical of the land, but what is interesting is that some have also formed in very deep seas. Mostly because of earthquakes, dense 'rivers' of rock debris, called sediment gravity flows, have slidden down the shelf slopes, depositing large amounts of sediment on the seabed. If sediment particles were larger than 2 mm and rounded, the conglomerate formed from it.
Section of polymict conglomerate from offshore rock core, approx. depth 3000 m (source: Wikipedia)
In the geological history of Slovenia a lot of conglomerate formed in Paleozoic. These gray and red quartz conglomerates are widespread in the Karavanke mountain range and in a broad belt with an east-west direction along the central Slovenia. Most of conglomerate, however, was formed in Cenozoic, when in Pleistocene (ice age) during short warmer periods glaciers have been melting. Rock debris which was left behind was carried away by rivers and deposited in their basins. Up to 120 m thick layers of gravel, sand and conglomerate can be observed today in the river terraces, for example in Kranj, Sora, Ljubljana and Krško field and in the Soča Valley.
Conglomerate (source: Wikipedia)
Flysch conglomerate at Ilirska Bistrica
The vicinity of Ilirska Bistrica is dominated by Eocene flysch, which was created some 55 million years ago in an ancient sea basin. Among others, instances of conglomerate rock can be found there. Special features of theEocene flysch conglomerate are horseshoe-shaped structures - semicircular, a centimeter or two high rounded ridges that surround part of pebbles.
These structures were formed during the deposition of flysch. Whirly currents carried away soft sediment at the pebbles and so a trough was formed, which was subsequently filled by other sediment. Nowadays, we actually see casts of these troughs in a form of horseshoe reefs, located at the pebbles, as the layers of rock are now almost vertical due to tectonic shifts.
Pebbles are randomly scattered, they do not touch each other and are made of different rocks. They are from 1 to 20 cm in size, the majority of them ranging from 4 to 6 cm. Size of the horseshoe structures depends on the size of the pebbles - the larger the pebble, the bigger trough was eroded by the water flow.
Scour marks structures in flysch conglomerate near Ilirska Bistrica
Access
To get to this EarthCache, one should go from Postojna in the direction of Ilirska Bistrica and Rijeka in Croatia. After a few kilometers the road will bring you to Pivka, where in the center do not turn right on the priority road, but go straight in the direction of Knežak. Another option is to go from Ilirska Bistrica towards Šembije and Knežak.
The conglomerate is located next to a fairly busy main road, so be careful when you walk up to it. It is possible to park the car on the cart paths that break away from the main road before or after the EarthCache location (e.g. waypoint PARK).
Walking on the road or crossing it requires maximum caution! We recommend that you walk on the meadow on the other side of the safety fence.
Access to this EarthCache is possible throughout the year, as the road is plowed, but a visit at a time when there’s snow is not reasonable, as it would be difficult to answer the questions below. We therefore recommend that you check this EarthCache when there is no snow.
EarthCache
To register this EarthCache you must answer the following questions:
1. Conglomerate layers that can be observed at this EarthCache location, have changed their position due to tectonic movement after the troughs were formed. In which direction would the water have to run to form such structures that you can see today, assuming there was no movement?
2. Explain in your own words why this rock is conglomerate and not breccia.
3. Age of the rock was estimated to be around 55 mil years on a basis of nano-plankton found in the rock's binder. Consider what you see on location and what you know about how conglomerate is formed and think about whether pebbles are of the same age as the rock or not. Explain your answer.
4. Optionally, take a photo of yourself or your GPS at the conglomerate and attach it to your log.
Send your answers in Slovene or English through my GC profile. After that you can log your visit, don't wait for permission from me. If there’s something wrong with your answers I will contact you.
If you took photos, attach them to your log.
Logs without the answers sent to me or logs containing the answers will be deleted!
(English)
Viri / Sources:
- Wikipedija / Wikipedia
- Prirodoslovno društvo Slovenije
- Zavod Republike Slovenjie za varstvo narave
Slike / Photos (Preskoči / Skip)
(Vrh / Top)