The World's Oldest Micrometeorites in the Mesoproterozoic Satakunta
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62nd Annual Meteoritical Society Meeting 5088.pdf THE WORLD’S OLDEST MICROMETEORITES IN THE MESOPROTEROZOIC SATAKUNTA FORMATION, FINLAND - SEDIMENTOLOGY OF THE HOST ROCKS. D. Kettrup1, P. Pihlaja2, A. Deutsch1 and L. Pesonen2, 1Institut fuer Planetologie, University of Muenster, Wilhelm-Klemm-Strasse 10, D- 48149 Muenster, Germany ([email protected]), 2Geological Survey of Finland, FIN-02150 Espoo, Fin- land. Problem and research concept: The approxi- sandy mudstone/siltstone. Main components are mately 1.4 Ga (Mesoproterozoic) old Satakunta For- quartz and K-feldspar (microcline), rock fragments, mation in SW Finland contains the world’s oldest muscovite, biotite and chlorite. Overall, the minera- micrometeorites [1,2]. So far, more than 50 fossil logical and chemical features of the Satakunta Forma- cosmic spherules have been recovered from the Sata- tion correspond to normal arkose suites of different kunta red beds. Mineralogical, textural and chemical geological ages [5]. features, amongst them Cr/Fe, Co/Fe, Ni/Fe, and Ir/Fe Lithification of the sandstones results mainly from ratios, indicate the presence of various spherule types two different effects. Most frequent are cements of and sub-types [2,3]. The spherules are unaltered and authigenetic quartz, micas and K-feldspar. Bounding match in their properties cosmic material from, e.g., of sand grains by clay minerals are the second type. the Greenland ice shield [3]. It is fundamental to un- The spherule-containing samples do not display derstand why the Satakunta micrometeorites are in specific sedimentological characteristics and appar- such an excellent state of preservation. Processes ently lack common features. which play a key role in this context are (i) sedimen- Discussion: The Satakunta Formation was depos- tation, and (ii) lithification. Influences of later proc- ited in a braided alluvial environment. The fluviatile esses are excluded as the geological history over the immature sands are products of rapid erosion and last 1.2 Ga was identical for the whole Satakunta area deposition, controlled by the conditions of a steep which generally lacks metamorphic overprint, except relief. The red colour indicates a terrestrial oxidising locally restricted thermometomorphism due to the environment in the source area and at the place of intrusion of postjotnian diabases. deposition. A clear-cut identification of spherule- In the first stage, we have sampled 45 rock speci- containing layers in the field is difficult at the present men each about 10 kg in mass, from 14 different out- state of knowledge as this material seems to lack sig- crops. The outcrops range in size from small quarries nificant features. We can, however, delimit the rea- up to the 80 m long Harjavalta section, yet only two sons for the preservation of the cosmic spherules. locations provide suitable conditions for a comprehen- These are (i) in-situ deposition or just a short trans- sive lithofacies study. After cutting off the altered port, (ii) absence of deep weathering in the fresh surface, the rock samples were crushed and sieved. sediments, and (iii) restricted circulation of diagenetic Heavy liquid and magnetic techniques, and finally solutions. hand-picking were applied to separate the cosmic References: [1] Marttila E. (1969) Thesis, Univ. spherules from the size fractions 125-90 µm, and 90- Turku, Finland [2] Deutsch A. et al. (1998) Nature, 63 µm. It appears that larger spherules are absent. 395, 146-148. [3] Robin E. et al. (1998) EF-workshop, Micrometeorites have been identified in 9 samples Cambridge, UK. [4] Kohonen J. et al. (1993) Geol. from 5 locations, which showed only restricted possi- Surv. Fin. Bull. 369, 35p. [5] Simonen A. and Kouvo bilities for lithofacies investigation. The following O. (1955) Bull. Comm. Geol. Finlande, 168, 57-87. steps are to characterise (i) the paleo-environment of Acknowledgements: This work is supported by a the spherule-containing in contrast to the barren loca- DAAD - The Academy of Finland exchange program tions, (ii) the petrographic features of the samples, and (AD–LJP), GSF, and DFG (DE 401/16-1). We appre- (iii) the exact position of the spherules in these sam- ciate technical assistance by F. Bartschat, T. Grund, ples. U. Heitmann, and M. Flucks. Sedimentological characteristics: The Satakunta Formation consists of mostly coarse-grained arkose sediments, which are poorly sorted. The amount of fine sediments is minor. Trough- and planar cross- bedding occurs frequently, whereas major reactivation surfaces and unimodal paleo-current patterns are ab- sent [4]. According to the grain size of the material and the depositional features the following lithofacies are present: - gravely, - trough cross-bedded, - lami- nated sandstone, and horizontally laminated or rippled.