Journal of Conference Abstracts

Session C03:1B

C03 : 1B/25 : F1

Proximal Chicxulub Ejecta in Southern Yucatan

Jan Smit (smit@geo.vu.nl)¹,

Adriana Ocampo (aocampo@mail.hq.nasa.gov)²,

Kevin Pope (kpope@primenet.com) &

Francisco Vega (vegver@servidor.unam.mx)³

¹ Dept Paleotol. Vrije Univ., de Boelelaan 1085, 1081HV Amsterdam, Netherlands

² NASA Headquarters Office of Space Science Code SD and IS300 E Street, S.W.Washington D.C. 20546, Office of Space Science, 300 E Street, S.W., Washington D.C. 20546

³ UNAM, dept Paleontol., Mexico df., Mexico

On both sides of the rio Hondo, ejecta-blanket deposits crop out in Albion Island in Belize and along the road from Chetumal to La Union, in Mexico, discovered during the 1998 Planetary Society expedition. These outcrops are the nearest known to the Chicxulub crater, 340 km from the crater center. The sequence of ejecta is identical to Albion island, but the sequence is more complete, also the overlying Palecene sediments were found. The ejecta sequence rests on the Barton Creek dolomite, a thick-bedded fossiliferous dolomite with carcineretid crabs and Nerinea sp. of Maastrichtian age. The contact forms an irregular surface, showing evidence for sub-aerial exposure. The basal layer maintains a constant thickness of about 1 meter, both on the highs or lows of the undulating contact. The matrix consists of pulverized dolomite containing rounded dolomite clasts, small deformed greenish clay blebs and cm-sized, concentric banded dolomite spheroids (Ocampo et al 1996). Internal layering is discontinuous. Large boulders do not occur. The overlying diamictite bed also has a dolomite matrix, and contains dolomite boulders up to 15 m in size, some of which contain the same fossils as the underlying dolomite. The large boulders occur mostly in the lower part. Smaller boulders and cobbles, the vast majority of dolomite, occur throughout the diamictite. The diamictite further contains up to 20% of up to 4 cm large green droplets and blebs with internal vesicles, identical to altered impact glass in other proximal locations in the Gulf of Mexico region. A few sub-horizontal shearzones are similar to shearzones found in the Bunte Breccia of the Ries crater. The diamictite is at least 40 m thick. Near Aguadulce (18.18.057 N, 88.36.978 W) the diamictite is capped by a 15 cm thick calcrete soil, a caliche, in turn overlain by 0.5 m of thin-bedded, fine-grained dolomite layers. The caliche layer suggests subaerial exposure of the ejecta, indicating that substantial erosion probably has removed the top of the ejecta sequence, prior to deposition of the dolomite beds. The thin-bedded dolomite grades into thin-bedded micritic limestones with a very small marine microfauna, ostracods and foraminifers, probably representing a basal Paleocene transgressive sequence. The basal spheroid bed may represent the early blasts from the vapor plume, but the many slicken-sides, the constant 1 m thickness of the bed, and the pulverized matrix, indicate that this bed results from grinding process between the ejecta curtain and the irregular karstic surface. The dolomite spheroids may have obtained their concentric banding by accretion within the layer, which is heated, may be even partially melted, through friction between the sliding ejecta curtain and authochtonous dolomite.

Ocampo AC, Pope KO, Fisher AG, Geol. Soc. of Amer, Sp. Pap. 307, 75-88, (1996).

C03 : 1B/26 : F1

The Lapilli Bed in El Guayal Outcrop (Tabasco): Fall out Suevite from the Cretaceous-Tertiary Boundary Chicxulub Crater?

Tobias Salge (tobias.salge@rz.hu-berlin.de)¹,

Philippe Clayes

(philippe.clayes@rz.hu-berlin.de)¹ &

Manuel Grajales (grajales@geologia.unam.mx)²

¹ Museum of Natural History, 10099 Berlin, Germany

² Subdirección de Exploración y Producción, Instituto Mexicano del Petróleo, 07730, Mexico, D.F., Mexiko

The El Guayal section is located approximately 550 km from the center of the Chicxulub crater. The Cretaceous-Tertiary boundary is marked by a > 40 m thick breccia composed of centimeters to decameters size blocks containing rudists and shallow water carbonate fragments, which abruptly interrupts stratified Upper Cretaceous deep water carbonates (Grajales et al., 1996). The breccia succession formed by fracturing and subsequent collapse of the edge of the Yucatan platform, probably induced by large earthquakes triggered by the Chicxulub impact. The breccia fines upwards and grades progressively into a limestone microbreccia containing ejecta material. The top of the microbreccia unit is marked by a bed containing mm to cm size accretionary lapilli. The lapilli bed is overlaid by 2 m of fining upward laminated sand, topped by a fine clay layer, postulated to be the KT boundary clay. Ir measurements are being carried on to confirm this interpretation. Marls of lower Paleocene age overlay the clay layer. The dark brown coloured mm to cm size lapilli are nicely rounded or elongated. Their core is formed of a small fragment of highly altered basement clast surrounded by concentric layers of calcite and clay minerals. The lapilli bed is approximately 1 m thick and contains shocked minerals, unshocked highly altered basement and carbonates clasts. Smaller size (average 4.8 mm) but morphologically similar impact lapilli are known from the Ries crater suevite and interpreted to have formed as accretionary bodies within the highly turbulent ejecta plume (Graup, 1981). The major element chemistry and clast composition of the lapilli bed closely resemble the carbonate-rich suevite from core Yucatan 6 - N13 from the Chicxulub crater (Jones et al., 1998). The Guayal lapilli bed can thus be interpreted as a fall out suevite from the Chicxulub crater. The lapilli appears to have been deposited in situ and not transported along with the underlying talus-breccia. The existence of suevite in Guayal indicates that this ejecta deposit extends up to 5.5 crater radii from Chicxulub. The CO₂-rich warm plume generated by the Chicxulub (Alvarez et al., 1995) might have contributed to the widespread extension of the fall out suevite.

Acknowledgements This is a contribution of the DFG funded graduate program "Evolutionary Transformations and Mass Extinctions" Museum of Natural History - Berlin.

Alvarez W, Claeys P & Kieffer SW, Science, 269, 930-935, (1995).

Graup G, Earth Planet Sci. Lett, 55, 407-418, (1981).

Grajales JM, Moran DJ, Padilla P, Sanchez MA, Cedillo E & Alvarez W, Geol. Soc. Americ. Abs, 28-No.7, 183, (1996).

Jones AP, Heuschkel S & Claeys P, Met. Planet. Sci., 33, A79, (1998).

C03 : 1B/27 : F1

Vertebrate Extinction and Survival Patterns at the K/T Boundary Support: A Catastrophic Scenario

Eric Buffetaut (Eric.Buffetaut@wanadoo.fr)

16 cour du Liegat, 75013 Paris, France

It is often claimed that the pattern of extinction and survival at the K/T boundary among land vertebrates is in poor agreement with the scenario of an impact-induced catastrophe, and that gradual environmental (especially climatic) changes fit the facts better. Survival of many groups of vertebrates (such as mammals, birds, various reptiles, and amphibians) is considered as incompatible with a global catastrophe. Scenarios of gradual change, however, frequently explain extinctions only on a regional (e.g., North American) scale. Dinosaurs had a global distribution at the end of the Cretaceous, and explanations must therefore have global validity. Bearing this in mind, the following points are apparent: - Since any explanation must involve events of a global nature, if climate deterioration caused dinosaur extinction, it must have been of considerable amplitude, to affect all climatic zones, from the equator to the poles (by comparison, even during the Pleistocene glaciations, tropical environments susbsisted, possibly as refugia; obviously, there were no refugia for dinosaurs at the end of the Cretaceous).

Long-term climate deterioration sufficient to have caused dinosaur extinction on a global scale should have caused significant extinction among climate-sensitive ectothermic reptiles (turtles, lizards, snakes, crocodiles...), which cannot be expected to have withstood adverse conditions for very long periods (the fossil record of crocodilians, for instance, shows how their distribution became severely restricted because of Tertiary climatic cooling). No such significant extinction episode of climate-sensitive reptiles is discernible in the fossil record during the Cretaceous-Tertiary transition. Severe long-term climate deterioration at the end of the Cretaceous is therefore highly unlikely.

A very brief episode of global intense environmental stress (including darkness), leading to selective food-chain disruption through temporary cessation of photosynthesis, explains both dinosaur extinction and the survival of many other vertebrate groups with different dietary requirements. Members of food chains based on organic matter contained in soil or freshwater (such as fish, small amphibians and reptiles, and small mammals) are known to have survived much better than forms dependent on a large supply of living plants (such as herbivorous dinosaurs and, a step further, their dinosaurian predators). The ectothermic survivors could not have survived long-term climate deterioration, but could withstand a severe environmental crisis if it was of very short duration (probably not much more than a year). The short-term effects of an impact-induced catastrophe thus explain the pattern of land vertebrate extinction and survival at the K/T boundary much better than a highly improbable global long-term climate deterioration.

C03 : 1B/28 : F1

Spherules from Cretaceous-Tertiary Boundary Sediments as a Record of the Impact Event

Francisca Martinez-Ruiz (fmruiz@goliat.ugr.es)¹,

Miguel Ortega-Huertas (mortega@goliat.ugr.es)² &

Inmaculada Palomo (ipalomo@goliat.ugr.es)²

¹ Instituto Andaluz de Ciencias de la Tierra, Fac. Ciencias. Campus Fuentenueva, Granada, Spain

² Depto. Mineralogia y Petrologia, Fac. Ciencias. Campus Fuentenueva, Granada, Spain

Spherules from the Cretaceous-Tertiary (K/T) boundary sediments at Blake Nose (NW Atlantic) and Agost and Caravaca (SE Spain) have been studied. ODP Leg 171B recovered a biostratigraphically complete K/T sequence at Blake Nose (Site 1049) where a green bed of spherules up to 17 cm thick records the K/T boundary event (Norris, Kroon, Klaus et al., 1998). Spherules in this layer have been diagenetically altered to smectite. At Agost and Caravaca (SE Spain) the boundary is recorded by a smectite layer 2 mm thick where abundant spherules are observed. These spherules have been diagenetiacally altered to potassium feldspar and to goethite, although their external shape, similar to tektites and microtektites, have been preserved in all cases. TEM analyses on Blake Nose spherules show smectite growing from an amorphous precursor. During the transformation of glass to smectite, REE could have been severely depleted; however, chondrite-normalized REE patterns suggest sedimentary target material. At Agost and Caravaca, spherules are of a different nature and do not record the target material, but instead significant extraterrestrial contamination, as would be expected according to the distance from Chicxulub. Fibroradial and dendritic textures similar to quench-crystal textures have been reported in goethite and K-feldspar spherules, which are also similar to textures presented by unaltered K/T clinopyroxene spherules. These spherules could have formed as melt blebs in the hot vapour plume where crystalline spherules (microkrystites) originated. Ir-rich cores have been observed in K-feldspar spherules also containing high concentrations of Pt, Pd and Ni (Martinez-Ruiz et al, 1997). Core compositions also suggest a mafic precursor material. Chondrite-normalized REE patterns from these cores in fact point to an ultramafic precursor. The impact-generated material derived from target rocks and the bolide itself is respectively recorded at these different locations. The boundary layer at Blake Nose records the melt ejecta material derived from melted target rock, and the fireball layer with a significant contribution from the vaporized bolide is recorded by the smectite boundary layer and microkrystites at a distal location from Chicxulub, such as Agost and Caravaca in SE Spain. (Work supported by the Spanish PB96-1429 DGES Project)

Norris RD, Kroon D, Klaus A, et al, Proc. ODP, Init. Repts, 171B, (1998).

Martinez-Ruiz F, Ortega-Huertas M, Palomo I & Acquafredda P, Sedimentary Geology, 113, 137-147, (1997).

C03 : 1B/29 : F1

The Cretaceous/Tertiary Boundary in the West-Central Sinai, Egypt - Preliminary Results

Ilknur Sengüler (Ilknur@uni-tuebingen.de)¹,

Ahmed El-Sabbagh² &

Birger Schmitz³

¹ Department, Wilhelmstr.56,72074, Tübingen, Germany

² University of Tübingen, Geology and Paleontology,, Sigwartstr.10, 72076, Tübingen, Germany

³ Dept.of Marine Geology, earth Science Centre, Box 460, Se-405 30, Goteborg, Sweden

A major faunal turnover occurred in planktonic and benthonic foraminiferal faunas across the Cretaceous/Tertiary (K/T) boundary in the upper bathyal to middle neritic Tethyan sediments of Caracava, Spain; El-Kef, Tunusia; Negev, Isreal; Gubbio, Italy; Bjala, Bulgaria; Gossau, Austria and Brazos, Texas. The new proposed K/T sections are located in the west-central Sinai which formed by junction of the Gulf of Suez and Aqaba, norteastern part of Egypt. They are contained within the Sudr Chalk Formation (Campanian-Maastrichtian) and Dakhla Formation (Maastrichtian-Paleocene). We report the preliminary results for the biostratigraphy of planktonic foraminifera and stable isotope analyses (carbonate fine fraction and planktonic foraminifera). All sections were sampled at closely spaced intervals of about 10-20 cm in the uppermost 2 m to 4 m of the Cretaceous, at 2 cm interval across the K/T boundary, at 5-10 cm intervals in the first 2 m of the Tertiary and 25 cm to 50 cm thereafter. The following planktonic foraminiferal zones are distinguished: Globotruncana aegyptiaca-Ganserina gansseri Abathomphalus mayorensis-Parvularugulobigerina eugubina- Parasubbotina pseudobulloides Upper Maastrichtian forms are mainly represented by large, massive deep and intermediate dwellers, such as Globutruncana, Globotruncanita, Rosita, Abathamphalus, Pseudotextularia, Racemiguembelina etc. Extinctions appear to affect primarily the globotruncanid (G. gansseri, G. aegyptiaca, C. contusa, G. falsostuarti, G. conica, C. fornicata) species and biserial pseudotextualarids and pseuduguembelinids (P. deformis, P. kempensis, P. costata). Among planktic foraminifera significant reworking of robust Creataceous species was observed in the boundary and up to the first Tertiary sediments. The early Danian assemblage is dominated by very scarce Paleocene species; e.g. Eoglobigerina sp., Parvularuglobigerina eugubina and dwarfed heterolicids by rare and moderately poor preserved, reworked globotruncanids. In all the studied sections the carbon isotope trends are similar in the bulk samples with few exceptions of negative excursion which have also been observed in many other localities such as Negev, Israel (Magaritz et al. 1985), Caracava, Spain (Lindinger, 1988), Bjala, Bulgaria (Preisinger et.al, 1993). With minor exception the ¹³C values of Cretaceous samples at all sections range from approximately 1.2 to 1.3 0/00 PDB. The K/T boundary is marked by a negative shift (0.7 0/00) which is similar to the global shift and these values are interpreted to reflect the global decrease in marine primary productivity.

Lindinger M, Thesis 8636 ETH, 253 pp, (1988).

Magaritz M, Moskovitz S, Hansen HJ & Rasmussen Kl, Newls. Stratigr., 15(2), 100-113, (1985).

Preisinger A, Aslanian S, Stoykova K, Grass F, Mauritsch, Hf & Scholger R, Paleogeogr., Paleoclimatol., Paleoecol, 104, 219-228, (1993).

C03 : 1B/30 : F1

Shock and Rapid Multi-Anvil Experiments on Calcite

Falko Langenhorst

(Falko.Langenhorst@uni-bayreuth.de)¹,

Alexander Deutsch

(deutsca@uni-muenster.de)²,

Ulrich Hornemann³ &

Brent Poe¹

¹ Bayerisches Geoinstitut, University of Bayreuth, D-95440 Bayreuth, Germany

² Institut of Planetology, University of Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany

³ Ernst-Mach-Institut, Am Klingelberg 1, D-79588 Efringen-Kirchen, Germany

The evolution of the Earth´s atmosphere was influenced by impact-induced decomposition reactions of volatile-rich minerals. The devolatilization of carbonates and sulfates is relevant for the mass extinction at the K/T boundary because the Chicxulub impact occurred in an anhydrite-rich carbonate platform. Hence, the impact probably caused a sudden perturbance of the atmosphere by large quantities of shock-released CO₂ and SO₂ [e.g., Pierrago et al., 1998]. Quantification of the amount of CO₂ released from shocked carbonates is still an unsolved problem because differently designed shock experiments and thermodynamical calculations yield variable results on the threshold conditions of degassing. It is, however, generally accepted that porosity exerts strong control on shock and post-shock temperatures, and hence, on threshold temperatures of CO₂ release [e.g., Tyburczy and Ahrens, 1986; Martinez et al., 1995]. To tackle the question of impact-induced CO₂ release, we performed two types of experiments: (1) high-explosive shock experiments at high pressures (64, 85, and > 100 GPa) using single crystal calcite and compacted calcite powder, and (2) rapid decompression in a multi-anvil apparatus. (1) Shocked single crystal calcite is pervaded by a network of cracks. FE-SEM imaging reveals the presence of numerous bubbles and vesicles on the surface of cracks with diameters in the sub-µm range. Porous calcite samples developed spherically shaped foamy aggregates, indicating total shock-melting, and enhanced mobilization of a gaseous species. X-ray diffraction and TEM microanalyses failed to detect CaO, the expected solid residue in case of CO₂ release. Our interpretation is, hence, that these effects are produced during the passage through the stability field liquid + vapor (CaCO₃) (see phase diagram in Tyburczy and Ahrens, 1986). Post-shock temperatures were possibly too low to reach the stability field of CaO (solid) + CO₂ (vapor). X-ray investigations reveal furthermore a significant decrease of average grain size in porous samples, which can be attributed to rapid crystallization of calcite crystals from melt. This interpretation is supported by the TEM observation that porous samples show no signs of deformation, whereas shocked single-crystal calcite contains numerous mechanical twins and dislocations. (2) The multi-anvil experiments represent a new approach to simulate the P-T path of decompression within the same time span as expected in nature. Calcite powder, loaded to a static pressure of 25 GPa at 2400° C, was decompressed and quenched to ambient pressure and temperature within three seconds, which is on the order of the time scale of the Chicxulub impact. A significantly increased mean grain size of the calcite was the only clear effect. Therefore, we conclude that the quantity of CO₂ release in impact processes is often overestimated.

Acknowledgments: This study is supported by DFG.

Pierrazo E. et al., JGR, in press, (1998).

Tyburczy J.A. and Ahrens T.J., JGR, 91, 4730, (1986).

Martinez I. et al., JGR, 100, 465, (1995)

C03 : 1B/33 : F1

Melting of Carbonate Target Rocks in Large Impacts; Less Gas

Adrian P Jones (ucfbhaj@ucl.ac.uk),

Philippe Claeys &

Sabine Heuschkel

Geological Sciences, University College London, Gower Street, England

Asteroid or comet impacts on a carbonate-rich target rock are likely to release CO₂ into the atmosphere. Yet the precise mechanisms of carbonate vaporisation and CO₂ production from shocked carbonates remain quantitatively poorly constrained (Boslough et al 1982, Tyburczy and Ahrens, 1986, Martinez, 1995). In the Cretaceous-Tertiary boundary (K-T) case the Chicxulub impact on ~3 km of porous water-bearing Mesozoic carbonate and anhydrite sediments forming the upper part of the Yucatan platform is thought to have released huge amounts of CO₂, H₂O vapours and sulphate aerosols into the atmosphere. However, from microscopic discovery of "feathery" calcite relics in the impact breccia, we have recently suggested (Jones et al, 1998) that these calcite morphologies are commonly observed in (i) synthetic carbonate melt experiments and (ii) rocks crystallised from igneous molten carbonate (carbonatites); they are considered diagnostic of quenched liquid carbonate. This igneous/melting feature in the calcite shows that a proportion of the sedimentary rock was melted, and quench-crystallised as calcite again. Therefore, not all limestone calcite contributed to the supposed post-impact higher climatic CO₂ content. Shock experients may have overlooked sample preconditioning, since even at shallow depths, pressures are sufficient (40-90 bars at 900-1200C) to prevent direct calcite dissociation. Similarly, the fate of target anhydrite is also in question.

Boslough MB et al, Earth Planet. Sci. Lett., 61, 166-170, (1982).

Tyburczy JA & Ahrens TJ, J. Geophys. Res., 91, 4730-4744, (1986).

Martinez I et al, J. Geophys. Res., 100, 15465-15476, (1995).

Jones AP et al, Meteoritics Planet. Sci, 33, A79, (1998).

C03 : 1B/34 : F1

On the Kinetics of Reaction of CO₂ with Hot CaO During Impact Events

Pierre Agrinier (piag@ccr.jussieu.fr)¹,

Alex Deutsch²,

Urs Scharer³ &

Isabelle Martinez¹

¹ Lab. Geochimie Isotopes Stables, IPGP, 75251 Paris, France

² Institut fur Planetologie, University of Munster, D-48149 Munster, Germany

³ Lab. Geochronologie, IPGP, 75251 Paris, France

CO₂ losses induced by large meteorite impacts on carbonates is an important process that has fundamental influence on the chemical composition of the atmosphere. Such catastrophic volatile release was proposed to be large enough to modify the climate of the Earth, noticeably during the K/T boundary event. In this context, rising concentrations of CO₂ is a possible and widely discussed process.Although carbonates are stable under dynamic compression during the shock-wave passage, they decompose into CaO + CO₂ during the high temperature regime that persists after pressure drop [Martinez et al., 1995]. From ATEM of carbonates in the Haughton impact structure it could be shown that shocked carbonates are composed of relatively undisturbed limestone clasts embedded in a fine grained carbonate-rich matrix. These carbonates partly result from back reaction of CO₂ with CaO [Redeker and Stoffler, 1988; Metzler, 1988; Martinez et al., 1998], suggesting that CO₂ back-reaction at high temperatures plays an important role for the budget of impact released volatiles. This is in agreement with thermodynamical data which indicate that CaO very efficiently traps CO₂ at high temperature (< 1100K). A critical parameter for such back reaction is whether or not reaction kinetics are sufficiently fast.Evaluations of the rate of the CaO + CO₂ => CaCO₃ reaction can be derived from a phenomenological description: as temperatures of the hot CaO + CO₂ mixture decrease, CaO forms small solid grains that react with CO₂ to form CaCO₃ shields on the surface of the CaO grains. In consequence, the resistance to CaCO₃ formation consist of the transport of CO₂ through the CaCO₃ shield. For CaO grains (r = 1 µm in radius), transport of CO₂ in the CaCO₃ shield (D = 10-10 cm².5^-1) and an initial CO₂ pressure of 300 bars at 1100K), it takes less than about 10 seconds to trap 50% of the original CO₂. Experiments confirm that the kinetic of the CaO + CO₂ => CaCO₃ back-reaction at various temperatures (400 - 800°C) is very fast: in less than 400 seconds, about 50% of the initial CO₂ are back-reacted with hot CaO to form CaCO₃. Ignoring the potential of these back-reactions may lead to large overestimates of impact produced CO₂ release into the atmosphere and hence, the importance of environmental changes by large impacts. Similar kinetic studies are required for the case of impact produced SO₂ liberation from evaporites, which has been proposed to explain the K/T boundary crisis. Kinetic studies for coal-based power plants show that SO₂ rapidly reacts with hot CaO.

Martinez I et al., Jr Geophysical Research B8, 100, 15465, (1995).

Redeker H-J, Stöffler D, Meteoritics, 23, 185, (1988).

Metzler A, Meteoritics, 23, 197, (1988).

Martinez I et al, Earth Planetary Science Letters, 121, 559, (1994).

C03 : 1B/35 : F1

Shocked-Induced Deformation Localization Along Lithological Interfaces: First Results from a Shock-Loading Experiment

Thomas Kenkmann

(thomas.kenkmann@rz.hu-berlin.de)¹,

Dieter Stöffler (dieter.stoeffler@rz.hu-berlin.de)² &

Ulrich Hornemann³

¹ Inst. f. Mineralogie, Museum f. Naturkunde, Humboldt-Universität Berlin, Invalidenstr. 43, D-10115 Berlin, Germany

² Museum f. Naturkunde, Humboldt-Universität Berlin, Invalidenstr. 43, D-10115 Berlin, Germany

³ Frauenhofer-Inst. f. Kurzzeitdynamik (EMI), Am Klingelberg 1, D-79588 Efringenkirchen, Germany

Frictional melt veins (pseudotachylites) occur frequently in the crater floor of terrestrial impact structures. It is believed that they partly form during shock compression of the target rock. Crystalline solids undergo structural rearrangements during shock compression. In non-homogeneous lithologies the different response to pressure waves generates impedance contrasts. These waves may cause relative movements along lithological interfaces leading to deformation localization and the formation of pseudotachylites. In order to investigate the deformation behaviour along the interface of quartzite and dunite a shock-loading experiment was carried out with the multiple reverberation technique at room temperature on 0.5 mm thick sample half-disks (15 mm diameter). We selected a quartzite SiO₂) of Oberschöna, Germany, and Aheim dunite with grain sizes of 230 µ m and 187 µ m, respectively. The smooth interface of both lithologies is oriented vertically with respect to the planar shock wave. The experimental technique is described in detail by Langenhorst et al. (1994). Peak shock pressure and peak pressure pulse duration are 30 GPa and 0.3 µ sec, respectively. The whole compression phase last about 0.7 µ sec. The investigation of deformation fabrics was performed by optical microscopy and SEM using polished thin sections.

Dunite shows a large number of inter-and transgranular cracks which increase in number by a factor of approximately two towards the interface. The interface is partly decorated with comminuted olivine grains with weak to strong mosaicsm. In quarzite the fracture density is lower compared to dunite and increases only slightly towards the interface of dunite and quartzite. Planar deformation features (PDFs) in quartz are frequently observed except in the vicinity of the interface. The reduced birefringence of quartz gradually decreases to zero towards the interface. Diaplectic quartz glass formed in a zone of 200-500 µ m thickness adjacent to the interface. The interface of dunite and quarzite is wavy and the depth of bulging reaches 30 µ m. Melted iron, injected from the container, decorates some interfacial domaines. Olivine may have been melted at grain margins near the interface.

The preliminary observations, in particular the observed transition from PDF-bearing quartz to diaplectic quartz glass, may suggest that the shock pressure increased towards the interface of quarzite and dunite. Alternatively, the glass formation may be caused by an additional heat pulse which is induced by friction along the interface. Calculations from Hugoniot data given by McQueen et al. (1966) for dunite and Ahrens et al. (1968) for quartzite indicate a maximum particle velocity difference between both lithologies of 320 ms^-1. This is reached after 0.13 µ sec after the shock wave entered the sample. It persists about 0.03 µ sec. The calculated total displacement along the interface is 50 µ m.

Langenhorst F & Deutsch A, EPSL, 125, 407-420, (1994).

McQueen RG & Marsh SP, In Clark S P (ed.) Handbook of Physical Constants, Geol. Soc. Am. Mem., 97, (1966).

Ahrens TJ & Rosenberg JT, In French BM & Short NM (eds.) Shock Metamorphism of Natural Materials, 59, (1968).

C03 : 1B/36 : F1

Fullerenes and the Flux of Extraterrestrial Helium He@C₆₀ to the Earth Over Geologic Time

Luann Becker (lbecker@soest.hawaii.edu)¹,

Robert Poreda (poreda@siena.earth.rochester.edu)² &

Theodore Bunch (tbunch@mail.arc.nasa.gov)³

¹ Hawaiian Institute of Geophysics & Planetary Sciences, University of Hawaii, Honolulu, HI, USA

² Rochester University, Rochester, NY, Zip code: 14627, USA

³ NASA Ames Research Center, Moffett Field, CA, Zip code: 94035-1000, USA

The detection of extraterrestrial (ET) helium (He) in fullerenes (He@C₆₀) in the 1.85 billion-year-old Sudbury impact crater has many compelling implications about the flux of He to the Earth's surface during large impact events (Becker et al., 1996). To further demonstrate that fullerene rather some other carrier of He (e.g. nanodiamonds, SiC, graphite) was responsible for the high ³He/⁴He ratios observed in the Sudbury residues, we have developed a new method for the isolation and purification of fullerenes by exploiting the ability of these molecules to sublime at high temperatures. We tested our sublimation method on some new Sudbury residues and confirmed the same high ³He/⁴He ratios with the identical temperature dependent release pattern observed in our previous study [Becker et al., 1996], thus, eliminating the possibility of another carbon carrier. Recently, we have begun investigating the possibility of ET helium in fullerenes isolated from sediments in the 65 myr old K/T boundary. Two possible scenarios for the presence of fullerenes in the K/T impact deposits are: (i) That fullerenes were synthesized within the impact plume from the carbon contained in the bolide; or (ii) that fullerenes were already present in the bolide and survived the impact event [Becker et al., 1996]. On the other hand, fullerenes in the K/T boundary clays were believed to have formed as a result of global wildfires triggered by the impact event [Heymann et al., 1994]. Preliminary measurements of helium in a sublimed continental K/T boundary fullerene residue from the Raton basin (Madrid site, Colorado) revealed ³He/⁴He ratios some 100 times above air. Interestingly, this ³He/⁴He ratio is identical to those measured by Merihue (1964) in some Pacific pelagic clays. The concentration of helium (per ug of fullerene) in K/T fullerenes (5 µ g sample) is within a factor of two of the helium concentration in the Sudbury fullerenes (~100 µ gs). The high ³He/⁴He ratio in the K/T fullerenes suggests that they were already present in the bolide and somehow survived the impact event. We intend to use sublimation to isolate and purify fullerenes in some marine K/T deposits collected in Denmark, Spain and New Zealand to ascertain the origin of these fullerenes. In addition, we will examine several marine core deposits, including the GPC-3 piston core, also reported to have ET ³He, to determine whether fullerene, rather than some other component, is the carrier phase responsible for the high ³He signature. Confirmation of these results could have broad implications concerning the importance of large impacting bolides and cosmic dust in providing ET helium and perhaps other volatiles to the Earth's crustal reservoir.

Becker L, Poreda RJ, & Bada JL, Science, 272, 249-252, (1996).

Heymann D, Felipe Chibante LP, Brooks RR, Wolbach WS & Smalley RE, Science, 265, 645-647, (1994).

Merrihue C, Annals of New York Academy of Sciences, 119, 351-367, (1964).

Farley KA, Nature, 376, 153-156, (1995).

C03 : 1B/37 : F1

The Siljan Meteorite Impact: Stratigraphic Constraints

Maurits Lindström &

Ilka Von Dalwigk (ilka@geo.su.se)

Dept. of Geology and Geochemistry, Stockholm University, Sweden

The excellently preserved Ordovician- Silurian succession of the Siljan Ring in central Sweden has been studied by leading stratigraphers and palaeontologists for over a century. Modern research on the classic Siljan impact site has largely ignored the stratigraphic data although they are quite relevant.

The impact structure, radiometrically Upper Devonian, has a 28 km wide domal uplift consisting of Proterozoic granites, surrounded by a concentric depression with Ordovician-Silurian sediments with stratigraphically underlying Proterozoic supracrustals in the northwest sector. The width between the outermost points of the concentric depression is 50 km.

Assumed depth of erosion is decisive for the reconstruction of the crater. Oil maturity, CAI, etc., indicate a target sediment thickness of 2-5 km, most of which was Devonian fluviatile foretrough facies to the Caledonian orogen. Because all these sediments were removed from the region about the crater by later erosion, this erosion had to reach at least 2 km deep.

The preserved stratigraphy includes for instance several Ordovician reef bodies continuous with near-reef facies of marls and limestones. Numerous localities show basal Ordovician with sedimentary contact on Proterozoic. The succession is commonly steeply inclined but rarely overturned and mostly not repeated; it cannot have moved much. Therefore the crater did not extend far beyond the inner boundary of the concentric depression and was just over 30 km wide. Its near-surface part was in non-lithified Devonian sediments and probably was wider.

Extensive synclinoria with thick sediments form northwest and east-to-southeast sectors of the concentric depression. Between them, an upwarped zone with scarce sediments extends from northeast to southwest. The axes of these major northeast-southwest structures relate to the near-by Caledonian orogen.

C03 : 1B/38 : F1

Meteorite Accretion Rates During the Early Ordovician

Birger Schmitz (birger@gvc.gu.se)¹ &

Mario Tassinari²

¹ Marine Geology, Box 460, Earth Sciences Centre, SE-405 30 Goteborg, Sweden

² Vaenern Museum, Lidkoping, Sweden

Reconstruction of past meteorite influx rates by systematic searches for fossil meteorites in lithified sediments is a very laborious and costly task, but recent studies in a quarry at Kinnekulle, southern Sweden, show that such reconstructions are possible (Schmitz et al. 1997). During sawing of a few thousand cubic meters of Ordovician limestone into 2-3 cm thick slices 29 fossil meteorites have been found. Twentyone of the meteorites have been found in a 60 cm thick bed called the Archaeologist. This bed represents a few hundred thousand years and contains several hardground surfaces. A central question in our ongoing work is whether the meteorites in the Archaeologist bed represent one fall event, i.e., a large meteorite that disintegrated in the atmosphere or on the water surface, or several meteorite falls. Clues about this can be obtained, for example, by comparing the shapes of the meteorites. Many of the Archaeologist meteorites are prominently angular in shape whereas others are round. This seems difficult to reconcile with an atmospheric break up of a single large meteorite. Other clues can be obtained by comparing the lithology of slabs containing meteorites with the lithology of different sublayers within the Archaeologist bed. Schmitz et al. (1997) estimated that during the 1.7 million year period of the Ordovician studied, meteorite accumulation rates were two orders of magnitude higher than today. In this estimate the Archaeologist meteorites were assumed to represent only one fall. If the meteorites represent several falls, accumulation rates were even higher. Generally high meteorite rates are supported by the recent finds of four meteorites in layers overlying the Archaeologist bed. These meteorites were found after quarrying an area of c. 700 square meters in the years 1997 and 1998. The Ordovician fossil meteorites most likely represent an unusual event in the late history of the solar system, such as a major collision between asteroids in the asteroid belt. In the ongoing studies we are opening a new quarry four kilometers away from the original quarry in order to perform a more detailed quantified search for fossil meteorites.

Schmitz et al, Science, 278, 88-90, (1997).

Session C03:1P

C03 : 1P/01 : PO

Calcareous Nannofossil and Planktonic Foraminiferal Response to a Meteorite Impact in the Umbria-Marche Massignano Section (Northeastern Appennines, Italy)

Silvia Gardin (gardin@ccr.jussieu.fr)¹,

Silvia Spezzaferri (silvia@erdw.ethz.ch)²,

Daniela Basso (d.basso@gp.terra.unimi.it)³ &

Rodolfo Coccioni (cron@info-net.it)⁴

¹ Département de Géologie Sédimentaire, Laboratoire de Stratigraphie, case 117, Université Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris cedex 05,France

² ETH-Zentrum, Geologisches Institut, Sonneggstrasse 5, 8092-Zurich, Switzerland

³ University of Milano, Department of Earth Sciences, Via Mangiagalli 34, 20133-Milano, Italy

⁴ Istituto di Geologia dell'Universita', Campus Scientifico, Localita' Crocicchia, 61029 Urbino, Italy

The Massignano Section, located in the Umbra-Marche region is the currently accepted type section for the Eocene/Oligocene boundary. Its sedimentary succession is complete and extended and spans the interval from the early Eocene to the Early Oligocene. This succession has been object of detailed multidisciplinary studies and, therefore, an accurate calibration of bio- and geochemical events is available. In 1993 Montanari et al., discovered two very close Iridium anomalies located at m 5.65 in the lower part of Zone NP19/20 based on calcareous nannofossils, corresponding to the mid-lower part of planktonic foraminiferal Zone P16. Volcanic ashes from the same interval indicate an age of 35.7±0.4 Ma for the these Iridium anomalies which are interpreted to be related to the impact of an extraterrestrial object in the Umbria-Marche area. With this study we have investigated the biotic response of calcareous nannofossils and planktonic foraminifera to this major event using a univariate and multivariate statistical approach. Quantitative analyses have been performed on approximately 900 specimens of planktonic foraminifera and on 300 specimens of calcareous nannofossils from approximately 44 samples spanning the four meters section in which the Ir anomaly lies. Data of the two microfossil groups have been statistically treated together on the same matrix to obtain a hierarchical agglomerative clustering based on the Bray-Curtis similarity. On the basis of the same similarity matrix, samples have been ordinate by MDS (non-metric MultiDimensional Scaling) to represent the assemblages along the real ecological gradients which control and are represented by the distribution of the biota. The spatial distribution of planktonic micro- and nannofossils is controlled by a combination of various environmental factors. A major event like an meteorite impact may influence and modify this equilibrium. This study will give an important input to clarify how these environmental parameters and their interaction may be influenced by an impact and how they will contribute in modifying the distribution of the assemblages .

Montanari A, Asaro F., Michel H.V., and Kennett J.P., Palaios, 8, 420-437, (1993).

C03 : 1P/02 : PO

Origin of Impact Glasses from the Nördlinger Ries, Germany: Isotopic and Mineralogic Investigations

Andreas Morlok

(andreas.morlok@student.uni-tuebingen.de),

Torsten Vennemann,

Wolf von Engelhardt &

Muharrem Satir

Institute for Geochemistry, Wilhelmstrasse 56, Tuebingen, Germany

The Ries impact, which today is represented by a complex crater 24 km in diameter, occurred 15 Ma years ago. It is estimated that the projectile of about 1 km diameter arrived with a speed of 24.5 km/s and penetrated about 1.4 km of sediment and crystalline basement, forming a transient crater of 6-7 km radius and 2.8 km depth. The glasses investigated are found within the Suevit, the breccia which forms the uppermost layers of the ejecta blanket around the crater. The glasses are considered to represent quenched melts produced as result of the impact. Petrography and electron microprobe studies of the glasses have indicated that the glasses are compositionally homogenous with a composition similar to that of the average basement gneisses (von Engelhardt et al 1995, 1997) This homogeneity is surprising as the impact lasts only seconds and as the basement is known to be heterogenous in composition. Whole rock oxygen isotope values of basement granite-gneisses range from 6.1 to 12.1 permil, amphibolite has a ¹⁸O value of 5.2 permil, carbonate sediments 26.7 permil and sandstone 13 permil. Model whole rock ¹⁸O values of crystalline debris within the Suevit are calculated to be 8.6 permil (shocked gneiss only) or 9.1 permil (all highly shocked clasts weighted in proportion). In contrast, ¹⁸O values of unaltered glasses range from 6.6 to 7.4 permil, that is very homogeneous and significiantly lower than the typical basement gneisses and granites. With increasing hydration and crystallization of the glasses, ¹⁸O values increase up to 13 permil, while D values range from about -85 permil for relatively fresh, dark glasses down to -127 permil for devitrified glasses. Likely explanations for the difference in ¹⁸O values could be: a. melts were predominantly derived from gneisses with low ¹⁸O values, even though these gneisses are now only estimated to have made up a minor part of the basement rocks, b. significant contributions from meteoric water within the sediments, c. significant contribution from the impacite and/or the vapour phase produced during the impact. Further investigations will include measurement of ¹⁷O values.

von Engelhardt W, Meteoritics, 32, 545-554, (1997).

von Engelhardt W, Arndt J, Fecker B, Pankau HG, Meteoritics, 30, 279-293, (1995).

C03 : 1P/03 : PO

Analysis of Cretaceous/Tertiary Hemipelagic Successions on the Western Coast of the Black Sea (Turkey): Implications for the Cretaceous-Tertiary Boundary Problem

Ilknur Sengüler (ILknur@uni-tuebingen.de)¹,

Aynur Hakyemez² &

Muharrem Satir (satir@uni-tuebingen.de)¹

¹ University of Tübingen, Geochemistry Department, Wilhelmstr.56, 72074, Tübingen, Germany

² Institute of Mineral Research, and Exploration Institute of Turkey (MTA), 06520, Ankara, Turkey

The K/T boundary is recognized as one of the greatest mass extinctions in Earth history. The abrupt extinction of Creataceous planktonic foraminifera constitutes a major base-line to delimit the K/T boundary and the level is associated with a decrease in d13C and ¹⁸O values as well as in CaCO₃ content.The first investigations for finding K/T boundaries along the western coast of the Black Sea region -Western Pondites which belong the Istanbul-Zonguldak Zone have been performed on ten different land sections which comprise intermediate to deep basinal facies (the Akveren Formation of the Maastrichtian-Paleocene and the Atbasi Formation of the Palaeocen). There is no lithological differentation between the Cretaceous and Paleocene sediments. Biostratigraphically the K/T boundary is characterized by the extinction of Cretaceous planktonic foraminifera and nannoflora and the apperance of new Tertiary species. From base to top, the biostratigrafic interval (planktonic and nannoplankton) is as follows:

Abathomphalus mayoroensis - Lithraphidites quadratus -Micula murus Zone- Parvularugoglobigerina eugubina-Biantholithus sparsus- Parasubbotina pseudobulloides - Cruciplacolithus tenius

Cretaceous planktonic foraminiferal taxa , consisting mostly of Rugoglobigerina spp. and Globigerinelliodes spp. together with Globotruncanids as well as Abathomphalus mayaroensis are extict at the K/T boundary. Reworked species with a small-size (30-100 mm) and thin-walls such as Gb. cretacea, Gb. trifolia are found in the first Tertiar biozone and these species progressively disappear along the P. eugibina and P. pseudobulloides biozones. The Maastrichtian nannoplankton assemblage is highly diverse and the dominance of dissolution -resistant forms such as M. decussata, A. cymbiformis and K. magnificus. Across the K/T boundary diversity decreases and assamblages dominated by several species of Thorocosphaera, Braarudosphaera and Cretaceous species such as M. decussata, W. barnasae, A. cymbiformis, resulting in a minimisation of CaCO₃ production. Biantholithus sparsus, a conventional marker of the K/T boundary, is also found in sections. Isotopic analyses of ¹³C were carried out on the samples closest to the K/T limit. The values obtained vary between 2,24 to 0,76 0/00 PDB, giving a negative values coincide with the sample situated 0.5 cm above the K/T boundary. ¹⁸O ratios with the latter consistently more negative -3.3 to -3.4 and this implies mixing of the water-mass.

C03 : 1P/04 : PO

Three Dimensional Gravimetric Model at Chicxulub and its Implications in the Actual Geothermal Regime

E. Leticia Flores-Marquez (leticia@tonatiuh.igeofcu.unam.mx)¹,

René Chaves Segura (exprene@tonatiuh.igeofcu.unam.mx)¹,

Oscar Campos-Enriquez (ocampos@tonatiuh.igeofcu.unam.mx)¹ &

Michel Dubois (Michel.Dubois@univ-lille1.fr)²

¹ Instituto de Geofísica, Universidad Nacional Autónoma de México, Circuito Institutos S/N, Cd. Universitaria, 04510, México D.F.

² Université Lille 1, Lille, France

Chicxulub may be one of the largest impact structures produced in the planet. Evidence like shocked quartz, feldspar grains, Ir anomalies and others, at the Cretaceous/Tertiary boundary suggest that the proposed 'extinction' bolide struck a continental or shallow marine terrane.

Reprocessed gravity data in the frequency domain over Northern Yucatan revealed three main layers. Mean depths to the top of such horizons are 35 km, 16 km and 5 km which may be correlated with the location of the crust-mantle interface, the upper-lower crust boundary and the Cretaceous-Tertiary boundary respectively. The 3-D inverted geometry obtained represent the impact signature of the meteorite Chicxulub.

During the last three years UNAM has led a shallow drilling project. To date, two wells UNAM-5 (located 110 km radial distance from the crater's center), and UNAM-6 (located 145 km radial distance) have recovered impact breccias. This will allow to better constrain the size and geometry of the crater. At well UNAM-5, suevitic breccias were recovered. They show a glass matrix with impacted material, glasses and centimeter-sized basement clasts.

Temperature gradient measurements were performed in the holes drilled by UNAM (boreholes 1 to 7). A continues record of temperatures was obtained for each borehole. The heat flow was computed by using the temperature gradient and the thermal conductivity measured in core samples. These data show great variations between boreholes, indeed thermal conductivity data are in the mean reported by literature for the corresponding lithology. The mean heat flow computed is 40 mW/m², that is a big value in the continent but agrees with the values obtained by others in the region. Matsui et al. (1997) perform the same kind of measurements in one line of shallow boreholes (drilled for seismic study) coming from center of the structure to its rims, showing the heat flow value is 60 mW/m² at the center and becomes of 20 mW/m² at the rims. Prol-Ledesma (1991) reports a heat flow value up to 32 mW/m² at the Tehuantepec isthmus (at the SW of the Yucatan peninsula). These values show the evidence of convective fluid circulation in the infill sedimentary layers of the crater, probably due to the geometry of the crater and to the fragility zones in the sedimentary infill.

In addition, Steinich et al. (1996) show the existence of two main patterns of fluid circulation coming from the center of pseudo-circle of cenotes to the NE and NW draining to the Caribbean sea, indeed of the submarine springs.

A preliminary coupled model of thermal and fluid circulation is showed.

C03 : 1P/05 : PO

Geochemical Characterisation of Impactites of the Chicxulub Structure, Mexico

Bianca Kettrup (kettrub@uni-muenster.de)¹,

Markus Ostermann (markus.ostermann@irmm.jrc.be)² &

Alexander Deutsch (deutsca@uni-muenster.de)¹

¹ Institut fuer Planetologie, Wilhelm-Klemm-Strasse 10, Germany

² European Commission Joint Research Centre - IRMM, B-2440 Geel, Belgium

The Chicxulub event, 65 Ma ago, yielded a 180 km sized impact structure in the about 2.9 km thick Cretaceous platform sediments overlying Pan-African basement. Geophysical data and drilling indicate the presence of a melt sheet yet radial distribution, and thickness of this melt layer as well as depth of the melt zone are unconstrained. Geochemical/isotope geochemical analyses (major elements, REE, Rb-Sr, Sm-Nd, ¹⁸O, ¹³C) could help to (i) delimit the relative contribution of different precursor rocks to the melt lithologies, and (ii) define the amount of back-reacted carbonates/sulfates in the melt. We study impactites (suevites, impact melt rocks) and their clast contents from the PEMEX drillcores Yucatan 6 (Y-6, nucleo N 14-19) and Chicxulub 1 (C-1, bottom). The spread in major elements, especially silica, aluminium, calcium and potassium for the Chicxulub melt lithologies exceeds that for similar rocks from other craters; whereas, their REE distribution patterns are quite uniform. The Nd isotope compositions for Y-6 samples (<epsilon>_Nd0 at -5.5) are by yet unknown reasons about 2<epsilon>-units lower than those for the C-1 material. The Sr isotope ratios cluster at 0.7089, one slightly higher value is combined with low Rb/Sr, indicating re-opening of the system during later alteration processes. The ¹⁸O values for the respective matrices of Y-6 samples range from 9.5 to 12.4‰. In a ¹⁸O vs. CaO - SiO₂ diagram, these data plot intermediate on regression lines defined by black and yellow glass from K/T boundary layers in Beloc. These relations indicate that sulfatic/carbonatic sediments represent one important precursor material for the Chicxulub melt glasses. One other precursor lithology, which was rich in SiO₂ (about 66 wt%), contributed significantly to the black Haiti glass, and to a lesser degree to the melts in the Y-6 cores. So far, we did not detect such a silica-rich rock as clast in our samples.

C03 : 1P/06 : PO

Formation of Resurge Gullies at Impacts at Sea: The Lockne Crater, Sweden

Ilka von Dalwigk (Ilka@geo.su.se) &

Jens Ormö (Jens.Ormo@geo.su.se)

Dept. of geology, Stockholm University, 10691 Stockholm, Sweden

The Lockne crater, central Sweden, was formed in an epicontinental marine environment (Lindström et al. 1996). It is a concentric structure with an inner, 7.5 km wide, relatively deep crater developed in the crystalline basement, surrounded by a 24 km wide, shallow outer crater (Ormö & Lindström, manuscript).

Marine craters differ in several aspects from craters formed on land. One special feature is resurge gullies formed by the erosional force of the resurging seawater after the impact. Four such gullies, each of them cutting radially through the outer crater, have been identified at the Lockne crater (Lindström et al. 1996).

The many well-exposed outcrops in these gullies allow us to reconstruct the process of their formation. The part of the crater cavity that formed in the seawater almost instantly collapsed and the seawater forcefully flooded the crater. The dense, ejecta-loaded flow dragged along the largest ejecta clasts and parts of the shattered sedimentary strata. The fragments may be up to tens of meters in diameter. The entrained material disintegrated during transport into breccia and today constitutes the dominantly monomictic lower part of the resurge sequence. The very high sediment load of the resurging water increased the density and the viscosity of the flow, thereby also increasing its erosive power. The flow eroded the outer crater and cut down at zones of weakness. This focused the flow in a small number of gullies, which further increased the erosional force. The erosion in the gullies proceeded headward down to the transition zone between the brecciated and less disintegrated crystalline basement.

The size of the resurge gullies at Lockne ranges from 100 to 1000 meters across, 20 to 100 meters in depth and 0.5 to 3 km in length. Their floor is covered with resurge deposits that are partially overlain by sedimentary rocks deposited when normal sedimentation continued.

Lindström M, Sturkell, EFF, Törnberg, R & Ormö J, GFF, 118, 193-206, (1996).

C03 : 1P/07 : PO

The Resurge Deposits of the Marine Impact Structures Lockne, Tvären and Kärdla

Roger Törnberg (tornberg@geo.su.se)

Dept of Geology and Geochemistry, Stockholm University, 106 91 Stockholm, Sweden

The three impact structures Lockne, Tvären and Kärdla have all been determined to be the results of impacts which occurred at sea. They are of Caradocian (Middle Ordovician) age and the target surfaces were all covered by Cambrian and Ordovician sedimentary deposits. The fractured bedrock, the breccia lens, and the secular sediments, are much like the same units in craters formed on land. However, the crater wall surroundings are more prone to collapse due to water saturation of the ejecta. The largest differences between craters formed on land and at sea are found among the allochthonous units. The allochthonous units consist of the ballistically emplaced ejecta and the water transported resurge deposits. The resurge deposits are derived from erosion by the returning sea water of ejecta and target surface material. Hence, like the ejecta deposits, the resurge deposits are mainly derived from the upper parts of the target. As a result, it is often difficult to differentiate between ejecta and resurge deposits in practice. The resurge deposit commonly consists of a lower, poorly sorted, matrix supported part and an upper relatively well sorted part that is clast supported. The upper part was deposited from watery suspension. The lower part is probably a mix between locally formed breccias, ejecta and material deposited from a debris flows prior to the return of the resurge waves. The sandy to gravely part of the resurge deposit is the unit richest in shock features such as planar deformation features and melt. The major reason for this is that greater shock leads to finer crushing and that water has the capacity to sort granular material. Hence, the most likely grain size where to find shocked minerals is among the most finely crushed and well sorted, sandy to gravely deposits. In spite years of field work combined with drillings and geophysical modelling, no impact melt sheets or melt pockets have been found in any of the 3 craters discussed. The most likely explanation is that there aren't any. Melt has been found as small aggregates and droplets in the resurge deposits of the Lockne and Kärdla structures and almost none in Tvären. I suggest that the melt in marine craters mainly occurs in the arenitic resurge deposits and that it is a less important feature than in craters on land. I also suggest that the explanations for this can be linked to the presence of sea water and sediments in the target.

C03 : 1P/08 : PO

Stratigraphic Importance of the Tvären Impact Structure

Åsa Wallin (asa.wallin@geo.su.se)

Dept. of Geology, Stockholm University, Sweden

In late Kukrusean stage, about 460 Ma, a cosmic body hit a marine epicontinental environment. The crater has for long periods been exposed to subaerial erosion but is today situated under water in the Tvären Bay, Sweden. The crater constitutes a circular depression, 2 km wide, filled with sediment, in an area dominated by precambrian crystalline rocks. Substantial information has been gathered from a complete core sequence drilled through the depression. Sediments in the section contained shocked quartz and the drilling ended in a breccia lens of crystalline basement. These are indisputable criteria of an impact structure. The succession consists of a subautochthonous, crystalline breccia, 60 m of a fining upwards resurge deposit and 80 m secular sediments of grey, carbonatic mudstone. By examination of fragments in the resurge material, it has been possible to identify the pre-impact stratigraphy of Cambrian and Ordovician deposits in the area. The age of the event is the late Kukrusean, based on conodonts and chitinozoans, which also indicate that infilling of the structure was completed before the end of Kukrusean. The crater rim may have hosted reef-like structures, according to slumps in the sequence, containing skeletons of echinoderms, bryozoans, brachiopods, trilobites, ostracodes and calcareous algae, which suggest a warm water environment for the area. A changing bottom environment of the structure, to succesively shallower, has made it possible to study the habitat of bottom living organisms (Lindström et al, 1994).

Thanks to the impact, a trap was formed in the crystalline basement, which provided a sheltered, calm sedimentation environment, and also protected the Kukrusean deposit from erosion in an area otherwise barren of sedimentary rocks. Therefore, impact structures are not only hazardous events killing life where they strike, but also yield valuable information about the stratigraphy before the impact, and give opportunity to study under exceptionally controlled conditions the time interval represented by the secular sediments. Organic-walled microfossils (acritarchs) have been investigated in 37 samples throughout the secular sequence in Tvären. The assemblage consists of abundant, wellpreserved and diverse forms, of a warm water (low latitude) flora with TAI-value of 1+, indicating low thermal alteration. Because time equivalent strata in Sweden are represented by 10-22 m sediment, compared to the 65 m in Tvären, a high resolution biostratigraphy can be achieved.

Lindström M, Flodén T, Grahn Y & Kathol B, Geological Magazine, 131, 91-103, (1994).

C03 : 1P/09 : PO

Cosmic Spherules in the Mesoproterozoic Satakunta Sandstone, Finland: Sedimentology of the Host Rocks

Dirk Kettrup (kettrupd@uni-muenster.de)¹,

Pekka Pihlaja (pekka.pihlaja@gsf.fi)²,

Alexander Deutsch (deutsca@uni-muenster.de)¹ &

Lauri J. Pesonen (lauri.pesonen@gsf.fi)²

¹ Institut fuer Planetologie, Wilhelm-Klemm-Straße 10, 48149 Muenster, Germany

² Geological Survey of Finland, Betonimiehenkuja 4, SF-02150 Espoo, Finland

The approximate 1.4 Ga old Satakunta sandstone contains the oldest known fossil micro-meteorites (Deutsch et al., 1998). Four different types of cosmic spherules have been identified in these rocks (Robin et al., 1998). Despite their high terrestrial residence age, these spherules are excellently preserved, show only very minor signs of mechanical abrasion, and match in their chemical properties unaltered stony spherules, e.g., in the Greenland collection. A better understanding of the reason for this remarkable state of preservation is crucial for further attempts to discover fossil micrometeorites in other old sedimentary formations.

The Satakunta sandstone is a typical "red bed" sediment filling a NW-SE striking graben, which covers an area of about 15 x 100 km in SW Finland (Kohonen et al., 1993). The up to 1800 m thick sandstone correlates with the Jotnian Sandstone, overlaying Svecofennian rocks and Rapakivi granites at several locations in Fennoscandia. The sandstone beds carrying the spherules are moderately sorted, and occasionally interlayered with thin remnants of greenish siltstones. The sandstone matrix is fine grained and consists of authigenic quartz, clay minerals and chlorite. Amongst the slightly rounded, up to 3 mm sized clasts dominates quartz over grains of multiple twinned plagioclase and microcline, and minor detrital micas. Most feldspars appear clouded by a hematitic pigment, which also pervade the rock cement, causing the purplish red colour of the sandstone beds. The heavy mineral fractions are dominated by magnetite, hematite, zircon, and apatite. The sedimentological features of the sampled outcrops point to a deposition in a fluviatile environment (sand facies, and fine-grained clastic facies, resp.). These lithofacies are characterised by structureless fine-gravel conglomerates, trough-cross-bedded, planar-cross-bedded, laminated, and massive-structureless sandstones, as well as horizontally laminated mud- and siltstones. In the Sa-takunta case, sediments range from massive gravels and pebbly sands over moderately sorted, medium to coarse grained sands up to mudstones, and rare glauconite-rich layers. Bedding of the sandstone is on the decimeter scale, the trough cross-bedded units reach a thickness between 15 and 25 cm. The overall characteristics of the Satakunta sediments are not exceptional. Further studies, therefore, will concentrate on (i) the specific depositional processes of the spherule containing sandstone layers, and (ii) post-depositional processes.

Deutsch A, Geshake A, Pesonen LJ & Pihlaja P, Nature, 395, 146-148, (1998).

Robin E, Rocchia R, Lefevre I, Pierrard O, Deutsch A & Pesonen LJ, ESF-Program "IMPACT", 1st workshop Cambridge, (1998).

Kohonen J, Pihlaja P, Kujala H & Marmo J, Geol. Surv. Fin. Bull, 369, (1993).