Soft-sediment deformation and gravity flow structures in the Lower Palaeozoic successions of the Baltic Basin
Date
2019-01-10
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Abstract
Kairi Põldsaar koos oma kolleegidega on uurinud Läänemere piirkonna Kambriumi ja Ordoviitsiumi vanuselisi (458-500 miljonit aasta tagasi) settekivimeid. Ta kirjeldab oma doktoritöös teatud spetsiifilist tüüpi kivimimustreid, mis viitavad siinset piirkonda tabanud üle 7 magnituudisele maavärinale ja ilmselt ka varasematele seismilistele sündmustele. Vähemalt ühel korral on iidne maavärin vallandanud ka hiidlaine ehk tsunaami, mis omakorda tingis sügaval merepõhjas mitmesaja kilomeetrise setete laviini.
Põnevaks teeb selle uuringu tulemused asjaolu, et siiani on valitsenud teadlaste hulgas veendumus, mille kohaselt Baltoskandia piirkond on olnud tektooniliselt väga stabiilne ja rahulik paik juba üle 500 miljoni aasta. Mõned meteoriidiplahvatused välja arvates, ei ole pidevaid maavärinaid ega hiidlaineid pidanud kartma meie, meie kauged esivanemad, ega ka ürgloomad, kes isegi ammu enne dinosauruste tekkimist siinsetes meredes ringi ujusid.
Tekstuurid, mida antud töös uuriti, kuuluvad nn pehmete setete deformatsioonide hulka (ingl. soft-sediment deformation). Need on algse sette moonutused, mis tekivad siis, kui sete on alles pehme. Õigetel tingimustel võivad need säilida kuni sette kivistumiseni ning lukustuda seeläbi kivimitesse sadadeks miljoniteks aastateks. Kõige sagedamini deformeeruvad vett sisaldavad liivad veeldumisel, ehk siis, kui sete omandab ajutiselt vesiliiva oleku. Selleks sobivad hästi näiteks merepõhja või rannikuvööndi liivad või ka järvede ja jõgede setted. Vesiliiva kutsuvad looduses esile mitmed kõrge energiaga protsessid – näiteks tormid, tõus ja mõõn, üleujutused ja maalihked, aga loomulikult ka maavärinad ja hiidlained. Setedeformatsioonide leviku ulatus, kuju ning muud iseärasused võimaldavad hoolikal analüüsil teha vahet algsetel põhjustajatel.
Põldsaare töö tulemused näitavad, et kivimites säilinud settemustrid võivad olla geoloogiliste sündmuste kirjeldajateks seal, kuhu me ise minna ei saa – näiteks iidses minevikus.
Kairi Põldsaar and her colleagues studied Kambrian and Ordovician (458-500 million years ago) sedimentary rocks of the Baltoscandian region. She describes in her doctoral thesis some specific rock structures which are caused by over 7 magnitude earthquake. At least once an ancient earthquake has triggered a tsunami wave, which in turn caused hundreds of kilometres large sediment avalanche in the deeper part of the sea. This study is interesting because until so far it was a common belief amongst scientists that this region has been tectonically quiescent for over past 500 million years. Excluding some meteorite impacts, constant earthquake and tsunami fear has not been a problem for us, our ancestors, or even to ancient sea creatures long before the dinosaurs. The studied structures belong to the category of the so called soft-sediment deformation structures. Such deformations form when the sediment is still soft. In right conditions, soft-sediment deformation structures can survive until lithification of the sediment and thus, preserve in rock record for hundreds and millions of years. Soft-sediment deformation is caused most often by liquefaction of water containing sands, in other words when the sediment obtains quick sand conditions. Most readily are liquefied for example coastal or sea bottom sands, or lake and river sediments. Liquefaction in nature is triggered by various high-energy processes – storms, tides, floods and landslides, but also of course earthquakes and tsunamis. Careful analysis of soft-sediment deformation structures, their geographical extent, morphology of the structures, and other criteria enable to conclude initial trigger of the liquefaction. Results of the study by Põldsaar show that sedimentary structures preserved in the rock record can aid scientist to reconstruct geological events where we cannot go – such as in ancient past.
Kairi Põldsaar and her colleagues studied Kambrian and Ordovician (458-500 million years ago) sedimentary rocks of the Baltoscandian region. She describes in her doctoral thesis some specific rock structures which are caused by over 7 magnitude earthquake. At least once an ancient earthquake has triggered a tsunami wave, which in turn caused hundreds of kilometres large sediment avalanche in the deeper part of the sea. This study is interesting because until so far it was a common belief amongst scientists that this region has been tectonically quiescent for over past 500 million years. Excluding some meteorite impacts, constant earthquake and tsunami fear has not been a problem for us, our ancestors, or even to ancient sea creatures long before the dinosaurs. The studied structures belong to the category of the so called soft-sediment deformation structures. Such deformations form when the sediment is still soft. In right conditions, soft-sediment deformation structures can survive until lithification of the sediment and thus, preserve in rock record for hundreds and millions of years. Soft-sediment deformation is caused most often by liquefaction of water containing sands, in other words when the sediment obtains quick sand conditions. Most readily are liquefied for example coastal or sea bottom sands, or lake and river sediments. Liquefaction in nature is triggered by various high-energy processes – storms, tides, floods and landslides, but also of course earthquakes and tsunamis. Careful analysis of soft-sediment deformation structures, their geographical extent, morphology of the structures, and other criteria enable to conclude initial trigger of the liquefaction. Results of the study by Põldsaar show that sedimentary structures preserved in the rock record can aid scientist to reconstruct geological events where we cannot go – such as in ancient past.
Description
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Keywords
sediments, sedimentary rocks, exogenic processes, deformation, tectonic structures, paleotectonics, Paleozoic, Baltoscandia