Palaeomagnetism of Middle Ordovician Carbonate Sequence, Vaivara Sinimaed Area, Northeast Estonia, Baltica

The hill range of Vaivara Sinimaed in northeast Estonia consists of several narrow east-to northeast-trending glaciotectonic fold structures. The folds include tilted (dips 4-75 degrees) Middle Ordovician (early Darriwilian) layered carbonate strata that were studied by mineralogical, palaeomagnetic, and rock magnetic methods in order to specify the post-sedimentational history of the area and to obtain a better control over the palaeogeographic position of Baltica during the Ordovician. Mineralogical studies revealed that (titano) magnetite, hematite, and goethite are carriers of magnetization. Based on data from 5 sites that positively passed a DC tilt test, a south-easterly downward directed component A (D-ref = 154.6 degrees +/- 15.3 degrees, I-ref = 60.9 degrees +/- 9.7 degrees) was identified. The component is carried by (titano) magnetite, dates to the Middle Ordovician (Plat = 17.9 degrees, Plon = 47.3 degrees, K = 46.7, A95 = 11.3 degrees), and places Baltica at mid-southerly latitudes. Observations suggest that in sites that do not pass the tilt test, the glaciotectonic event has caused some rotation of blocks around their vertical axis.Peer reviewe

Magnetic susceptibility of Middle Ordovician sedimentary rocks, Pakri Peninsula, NW Estonia

Magnetic susceptibility (MS), its frequency-dependence and anisotropy of the Middle Ordovician Dapingian and Darriwilian sedimentary sequence from three sites (Uuga, Testepere and Leetse) in the Pakri Peninsula, NW Estonia are analysed in combination with the mineralogical composition. The study is based on 463 cores drilled at intervals of a few centimetres to a maximum of about 1 m. All the samples show low and positive MS, which suggests the presence of small quantities of paraand/or ferromagnetic minerals. The stratigraphic units of the three studied sites have a similar along-section appearance, which provides a base for a composite curve. The relatively higher susceptibilities are carried by secondary Fe-Ti oxides (Toila Formation), goethite ooids (Kandle Formation) and ferrous dolomite (Pae Member), whereas paramagnetic minerals are mostly responsible for the rest of the sequence. Considering the dependence of MS on regressive transgressive cycles (high/low MS within deposits of regressive/transgressive parts of the cycles, respectively), the MS data do not agree with sedimentologically derived sea-level compilations. The measured changes in MS in the Pakri Peninsula outcrops correlate at certain characteristic levels with those deposited in the deeper part of the palaeobasin (Viki core), indicating that the post-depositional iron mobilization within the sediments took place at least at a regional level. Because of post-depositional reorganization of ferromagnetic carrier minerals, the MS values may, however, not be used as a detrital proxy.Peer reviewe

Sekundaarsed magnetiseeritused Eesti settekivimites ja Lõuna-Soome kristalsete kivimite murranguvööndites

The present doctoral thesis combines palaeomagnetic, rock magnetic and mineralogical studies of Palaeozoic sedimentary rocks of Estonia and Palaeoproterozoic crystalline rocks of shear and fault zones of southern Finland. The essence of palaeomagnetism is that a record of the ancient magnetic field is locked in a rock. Silurian and Ordovician carbonate samples were collected from twelve Estonian outcrops and Palaeoproterozoic rocks were sampled in five outcrops of southern Finland. The localities in Finland are located along the extensive north-east to south-west trending Porkkala–Mäntsälä shear zone and the north–south trending Vuosaari–Korso shear zone. They were chosen for the study, because these were expected to indicate possible reactivations. In a result of the studies primary magnetizations were found in Ordovician sedimentary rocks of northern Estonia and in Palaeoproterozoic shear and fault zone rocks of southern Finland. Magnetite and titanomagnetite are the carriers of the primary remanence. Secondary magnetizations of different ages were detected as well, implying that the areas have experienced multiple reactivations. A common secondary overprint of late Palaeozoic age characterizes both the studied Ordovician–Silurian sedimentary rocks of Estonia and the Precambrian crystalline shear and fault zone rocks of southern Finland. The main carrier of this remanence is hematite, but the contribution of maghemite is observed besides. The remanence represents chemical remanent magnetization. The origin of this secondary magnetization is postulated to be associated with brines, possibly with meteoric waters, as well as with fluids derived from orogenic belts like Hercynian or Uralian. The late Palaeozoic overprint is quite widespread on several continents and has been observed in different types of rocks of different ages all over the world. For that reason we believe that the formation of Pangea has largely contributed to the formation of remagnetizations in the late Palaeozoic also in the studied region.Käesolev doktoritöö kirjeldab paleomagnetiliste uuringute tulemusi Ordoviitsiumi ja Siluri vanusega settekivimitest Eestis ja Paleoproterosoilise vanusega kristalsetest kivimitest Lõuna-Soomes. Uuringute peamiseks eesmärgiks oli piirkonna geoloogilise ajaloo täpsustamine. Paleomagnetilised uuringud võimaldavad määrata Maa magnetvälja suunda hetkel kui kivim tekkis või seda mõjutati hilisemate geoloogiliste protsesside käigus juhul kui süsteemis sisaldusid ferromagnetiliste omadustega mineraalid. Töö käigus koguti Ordoviitsiumi ja Siluri vanusega karbonaatsete kivimite proove kaheteistkümnest Eesti paljandist ja karjäärist ning viiest Paleoproterosoikumi vanusega kivimite paljandist Soomes. Soome materjal koguti eelistatult murranguvöönditest (Porkkala–Mäntsälä ja Vuosaari–Korso), mis on võrreldes ümbritsevaga olnud geoloogilistele protsessidele vastuvõtlikumad. Uuringute tulemused näitavad, et primaarne (kivimi tekkeaegne) magnetiseeritus on säilinud Alam- ja Kesk-Ordoviitsiumi karbonaatkivimites ning Paleoproterosoikumi graniitides ja gneissides. Primaarse magnetiseerituse kandjaks on magnetiit ja titanomagnetiit. Valdavalt esinevad aga kivimites sekundaarsed ferromagnetilised mineraalid: hematiit, maghemiit ja götiit. Nende mineraalide poolt kantud jääkmagnetiseeritused on moodustunud pärast kivimi enese tekkeaega. Magnetiseerituste suunad võimaldasid määrata hilisemate protsesside toimeaega ning täpsustada nende päritolu. Uuritud kivimites leidub erineva vanusega sekundaarseid magnetiseeritusi, kuid valdavaks on keemilise päritoluga Permi vanusega komponent. Kivimi magnetiliste ja mineraloogiliste uuringute alusel on selle kandjaks hematiit ning kohati ka maghemiit. Magnetiseerituse tekkepõhjustena on välja pakutud mäestikutekkeliste fluidide levikut seoses Hertsüünia või Uuralite kurrutusega ning keskkonnatingimuste muutust. Samas on Hilis-Paleosoikumi vanusega sekundaarset magnetiseeritust avastatud kogu maailmas (rohkem kui sada viidet erinevates andmebaasides), mistõttu on käesolevas töös toodud paralleel Permi vanusega sekundaarse magnetiseerituse laialdase leviku ja Pangea superkontinendi tekke maksimumiga

Multiply remagnetized Silurian carbonate sequence in Estonia

Palaeomagnetic studies were carried out on Silurian sedimentary rocks of Estonia. The studied sequence (126 samples from 4 localities) is composed of Silurian lime- and dolostones of Llandovery and Wenlock age (444–423 Ma). Rock magnetic data revealed magnetite, (titano)hematite, and pyrrhotite as the main carriers of remanent magnetization. We found that different secondary magnetizations characterize rocks at different localities, whereas similarities exist between the localities. The virtual poles fall near the Silurian–Early Devonian and Late Palaeozoic segment of the Baltica’s APWP. The heritage of these components is related to either syndepositional or early diagenetic changes, and to chemical remagnetization due to late diagenetic processes and/or regional events that affected Baltica during the Late Palaeozoic

Palaeomagnetic age of remagnetizations in Silurian dolomites, Rástla quarry (Central Estonia)

Alternating field and thermal demagnetization of dolomite samples from the Silurian (Llandovery) horizontally-bedded sequence of central Estonia reveal two secondary magnetization components (A and B) both of chemical origin. A low-coercivity (demagnetized at -50 mT) component A (D = 60.7°, I = 7.7°, alfa95 = 16.6°) with high dispersion (k = 14.2), yields a palaeopole at 18.2°N and 139.5°E that points towards the Late Devonian — Mississipian segment of the Baltica APWP (Apparent Polar WanderPath). A high-coercivity component B (D = 13.5°, I = 60.7°, k = 67.0, alfa 95 = 4.7°) carries both normal and reversed polarities. Comparing the palaeopole (71.1°N and 173.3°E) with the European APWP reveals a Cretaceous age. These two remagnetizations are linked to mineral assemblages of magnetite and maghemite (A), and hematite (B) determined from mineralogical (X-ray, SEM and optical microscopy) and rock magnetic (acquisition and thermal demagnetization of a 3-component IRM; Lowrie-test) studies. The results suggest that the first (A) Palaeozoic remagnetization was caused by low-temperature hydrothermal circulation due to the influence of the Caledonian (more likely) or Hercynian Orogeny after the diagenetic dolomitization of carbon ates. Hematite, carrying the component B, and goethite, are the latest ferromagnetic minerals that have precipitated into the existing pore space (hematite) and walls of microscopic fractures (goethite) that opened to allow ac cess for oxygen-rich fluids during the Late Mesozoic

Geology, petrography, shock petrography, and geochemistry of impactites and target rocks from the Krdla crater, Estonia

The Kärdla crater is a 4 km-wide impact structure of Late Ordovician age located on Hiiumaa Island, Estonia. The 455 Ma-old buried crater was formed in shallow seawater in Precambrian crystalline target rocks that were covered with sedimentary rocks. Basement and breccia samples from 13 drill cores were studied mineralogically, petrographically, and geochemically. Geochemical analyses of major and trace elements were performed on 90 samples from allochthonous breccias, sub-crater and surrounding basement rocks. The breccia units do not include any melt rocks or suevites. The remarkably poorly mixed sedimentary and crystalline rocks were deposited separately within the allochthonous breccia suites of the crater. The most intensely shockmetamorphosed allochthonous granitoid crystalline-derived breccia layers contain planar deformation features (PDFs) in quartz, indicating shock pressures of 20-35 GPa. An apparent Kenrichment and Ca-Na-depletion of feldspar- and hornblende-bearing rocks in the allochthonous breccia units and sub-crater basement is interpreted to be the result of early stage alteration in an impact-induced hydrothermal system. The chemical composition of the breccias shows no definite sign of an extraterrestrial contamination. By modeling of the different breccia units with HMXmixing, the indigenous component was determined. From the abundances of the siderophile elements (Cr, Co, Ni, Ir, and Au) in the breccia samples, no unambiguous evidence for the incorporation of a meteoritic component above about 0.1 wt% chondrite-equivalent was found.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202