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Abstract-The well-preserved Kardla impact crater, on Hiiumaa Island, Estonia, is a 4 Ian diameter structure formed in a shallow Ordovician sea -455 Ma ago into a target composed of thin (-150 m) unconsolid ated sedimen tary layer above a crystalline basement composed of migmatite granites, amphibolites and gne isses. The fractured and crushed amphibolites in the crater area are strongly altered and repl aced with secondary chloritic minerals. The most intensive chloritization is found in permeable bre ccias and heavil y shattered basement around and above the central uplift. Alteration is believ ed to have resulted from convective flow of hydrothermal fluids through the central areas of the crater. Chlo ritic min eral associations suggest formation temperatures of 100-300°C, in agreement with the most frequ ent quartz fluid inclusion homogenization temperatures of ISO-300°C in allochthonous bre ccia. The rath er low salinity of fluids in Kardla crater «13 wt% NaCl eq ) suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the crater rim walls rais ed abo ve sea level after the impact, or by invasion of sea water through the disturbed sedimentary cover and fra ctur ed crys talline bas ement. The well-developed hydrothermal system in Kardl a crater show s that the thermal history of the shock-heated and uplifted rocks in the central crater area, rather than coo ling of impact melt or suevite sheets, controlled the distribution and intensity of the imp act-induced hydrothermal processes.

Abstract-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.

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The 4 Ian wide and 500 m deep circular Kardla impact structure in Hiiumaa Island, Estonia, of middle Ordovician age , is buried under Upper-Ordovician and Quaternary sediments. To constrain the geophysical models of the structure, petrophysical properties such as magnetic susceptibility, natural remanent magnetization (NRM), density, electrical conductivity, porosity and P-wave velocity were measured on samples of crystalline and sedimentary rocks collected from drill cores in different parts of the structure and the surrounding area. The results were used to interpret the central gravity anomaly of -3 mGal and the magnetic anomaly of -100 nT and also the surrounding weak positive anomalies revealed by high precision survey data.The unshocked granitic rocks outside the structure have a mean density of -2630 kgnrJ. Their shocked counterparts have densities of -2400 kgm-3 at a depth of~500 m, increasing up to 2550 kgm-3 at a depth of 850 m. Porosity and electrical conductivity decrease, but P-wave velocity increases as density increases away from the impact point. Thus, the gradual changes in the physical properties of the rocks as a function of radial distance from the crater centre are consistent with an impact origin for Kardla, As in many other impact structures, the magnetization of the shocked rocks are also clearly lower than those of unshocked target rocks.A new geophysical and geological model of the Kardla structure is presented based on geophysical field measurements and data on gradual changes in petrophysical parameters of the shocked target and overlying rocks, together with structural data from numerous boreholes. An important feature of this model is the lack of an observable geophysical signature of the central uplift observed in drillcores. ESF Network "Impact Cratering and Evolution of Planet Earth," Third International Workshop: Shock wave behaviour ofsolids in nature and experiments. Abstract vol. (eds. U. Scharer, J.-C. Doukhan and P. Agrinier), p.54. SUUROJA K. A., GROMOV O. B., KALA E. A. AND GROMOVA G. A. (1974) Otshot 0 poiskah podnjatii kristallitsheskogo fundamenta v vostotshnoi tshasti ostrova Hiiumaa i otsenke ih na granitnyi shtseben (in Russian).