The Osmussaar Breccia in Northwestern Estonia ...

The Osmussaar Breccia in Northwestern Estonia - Evidence of a -475 Ma Earthquake or an Impact?

Kalle ~ u u r o j a ' ,Kalle ~ i r s i m a e ~ Leho ." ~ i n s a a r "Marko ~ o h v ? , William C. ~ a h a n eand ~ ~Sten , ~uuroja"~ '~eologicalSurvey of Estonia. Kadaka tee 82, 12618 Tallinn, Estonia. 'Institute of Geography, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia. ([email protected]) 'institute of Geology, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia. ([email protected]) * ~ e o r n o r p h o l oand ~ ~ Pedology Lab, York University, 4700 Keele St., North York, Ontario, Canada. M3J IP3. 'Institute of Mining, Technical University of Tallinn, Kopli 82, 12623 Tallinn, Estonia.

Abstract. The Osmussaar Breccia occurs in beds of the -475 Ma basal Middle Ordovician (Arenig and Llanvirn series) siliclastic-carbonate rocks of northwestern Estonia. The Breccia consists of fragmented and slightly displaced (sandy) limestones, which are penetrated by veins and bodies of strongly cemented, breccia-like, lime-rich sandstone injections. The rocks above (horizontally-bedded. hard limestone) and below (weakly cemented silt and sandstone) are undisturbed and do not contain the sediment intrusions. Osmussaar Breccia is found over an area of more than 5000 km2 and is distributed in a westeast oriented elliptical half-circle centred approximately at Osmussaar Island (59' 18' N: 23'28 ' E). The thickness of the brecciated unit ranges from 1-1.5 m on Osmussaar to a few (tens of) cm at -70 km east of the island. Arenitic sandstone of the sediment injections contains quartz grains with planar deformation features (PDF). Several hypotheses concerning the origin of the Osmussaar event have been proposed: catastrophic earthquake, regional tectonic movements, tectonic movements occurring simultaneously with coastal processes, and an impact event. The latter hypothesis was suggested in connection with the discovery of the nearby-situated Neugrund impact structure. However, the sediment intrusions are stratigraphically -60 Ma younger than the impact structure. Osmussaar Breccia does not correspond to any known impact structure of this age in Baltoscandia. Also. results of a seabed geophysical survey in the Baltic Sea for the search of a possible undiscovered feature did not identify any large structure in the area of the Osmussaar Breccia. Consequently, we suggest that a devastating -475 Ma earthquake with an epicentre close to Osmussaar split the sea floor. It initiated underwater mud-flows eroding the primary Neugrund crater ejecta andlor crater rim walls, thus reworking the impact materials into the sedimentary injections,

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which is suggested by rounded morphology of the shocked quartz grains found in breccia matrix.

1 Introduction Apart from meteorite impact craters, which provide direct evidence of an impact, distal ejecta and associated geochemical anomalies of platinum-group metals are able to provide information for recognition of impact events in the stratigraphic record (Grieve 1997). Because distal impact ejecta are distributed over areas orders of magnitude larger than the impact structure itself, the probability of finding impact signatures in dispersed segments of the terrestrial stratigraphic record is much higher. However, such layers are rather thin, and dispersion, modification due to weathering, redeposition, diagenetic and/or metamorphic processes make chemical and physical identification of the fallout signatures, and correlation to distinct impact events, difficult. Moreover, most unusual beds in the stratigraphic record are not related to impact events, but are the result of other geological processes, such as volcanic eruptions, turbidite flows, secondary sediment deformation due to compaction, etc. Schmitz et al. (1994) investigated more than 200 000 quartz grains from 57 individual bentonite and bentoniteresembling clayey layers from a 120-m thick (representing -2 million yr) EarlySilurian carbonate sequence on Gotland Island, Sweden, for shocked quartz and did not find any. However, these authors, as well as Izett et al. (1993). suggested that stratigraphic intervals containing rip-up clasts or showing other signs of seafloor turbulence would be more suitable for a search of impact events rather than clay beds. In this contribution, we report a study of a breccia layer of early Middle Ordovician age in northwestern Estonia. This breccia, named Osmussaar Breccia, was originally interpreted as the result of earthquake-triggered seafloor destruction (opik 1927). In contrast, after the discovery of the Neugrund impact crater in the area coinciding with the breccia distribution. the occurrences of sand-filled cracks and brecciation, were re-interpreted as a commence of an impact event at -475 Ma (Suuroja and Saadre 1995). However, more recent work revealed that the Neugrund Crater is infilled with older. Cambrian sediments and is of Early Cambrian age (-540 Ma) (Suuroja and Suuroja 2000). Consequently, the origin of the Osmussaar Breccia remains unclear.

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2

The Osmussaar Breccia 2.1

Geology The Osmussaar Breccia occurs in beds of the basal Middle Ordovician (Arenig) siliciclastic-carbonate rocks of northwestern Estonia (Meidla 1997; Webby 1998). The breccia is found in outcrops (on Osmussaar Island, Vaike- and Suur Pakri islands, and Pakri Peninsula) and drill core sections over an area of more than 5000 km' (Fig. 1). It is distributed in a west-east oriented swath centred approximately on Osmussaar Island (Fig. l), where the brecciated unit is also found to be thickest (1-1.5 m). Osmussaar (Odensholm) Island is situated on the southern side of the entrance to the Gulf of Finland (59" 18' N; 23" 28' E). It is the highest part of a flooded peninsula of the Baltic Clint emerging over the sea level on the northwestern top of the submarine Osmussaar Bank. In the northern part of the island, at a -5 km long cliff, the Ordovician (Arenig and Llanvirn series) limestones crop out (Opik 1927; Meidla 1997). The breccia, which occurs in this section at the foot of the cliff, consists of fragmented and slightly displaced blocks of sandy limestones of the Billingen, Volkhov and Kunda regional stages (Figs. 2, 3 and 4) (Suuroja et al. 1999). This, up to 1.5 m thick, brecciated layer is unconformably cut by horizontally bedded and undisturbed Llanvirnian (Aseri, Lasnamagi, Uhaku regional stages) bioclastic limestones (Figs. 2 and 4) (Suuroja et al. 1999). The stratigraphic age of the brecciation is bio- and lithostratigraphically well determined to Kunda regional stage (Ndlvak 1997: Meidla 1997) and it is biostratigraphically confined to lower part of the C?~athochitinnregnelli chitinozoan Zone of the late Arenig-earliest Llandvirn age (Ndlvak and Grahn 1993), which suggest an event age of -475 Ma (Fig. 3) (Gradstein and Ogg 1996). The fractured and crushed limestone blocks of the Osmussaar Breccia are penetrated by up to 2-3 m thick veins of strongly carbonate cemented sandstone. The thickness of the brecciated unit with sand intrusions ranges from 1 to 1.5 m in Osmussaar to a few tens of cm -70 krn east of the island. The intensity of the fracturing decreases laterally in the same direction. At some distance from Osmussaar (on the Suur- and Vaike-Pakri islands and Pakri Peninsula) (Fig. I) the brecciation is scarce and the sandstone intrusions are thinner (1-5 cm). On Osmussaar Island the veins and bodies of sand filled intrusions make up approximately 10% of the layer, areas on the Suur-Pakri Island the content of these is less than 1% (about 1 vein per every 10 meters). The carbonate lithoclasts in brecciated layer range in size from tens of centimetres to tens of metres (Fig. 4). Fine-grained material filling dike- and vein-like spaces between blocks is composed of lime-rich mud and sand-sized siliciclastic material, which contains small fragments of limestones of varying size from a few to tens of centimetres. On Osmussaar Island, the sharp-edged limestone fragments in the sand intrusions make up to 50% (on average 1510) of the intrusions volume.

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center of hypothetical submarine crater

Fig. 1. Distribution of Osmussaar Breccia. (a) Simplified geological map of northwestern Estonia: 1 - Lower Silurian, 2 - Upper Ordovician, 3 - Middle Ordovician, 4 - Lower Ordovician, 5 - Cambrian, 6 - Vendian, 7 - major faults, 8 - epicentre of the Osmussaar earthquake of 1976; (b) general location of the impact craters and fossil meteorites discussed in the text, and main structural elements of the study area: 1 - Palaeozoic sediments, 2 - Mesozoic sediments, 3 - Cenozoic sediments, 4 - Upper Precambrian (Vendian) sediments, 5 - Archaean Domain, 6 - Svecofennian Domain, 7 Transscandinavian Belt, 8 - Sveconorwegian Belt, 9 - Caledonides, 10 - Tornquist Line.

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Fig. 2. Photographs of the Osmussaar Breccia. (a) Northeaster11cliff of Osmussaar Island

and eroded surface of the layer with breccia. To the left, the horizontally bedded and undisturbed layer of the post-Osmussaar event Middle Ordovician (Llanvim) limestones; (b) brecciated beds in the section at the northeastern cliff of the Osmussaar Island. Undisturbed section: BIII "- upper Kunda, CIb - Aseri and CIc - Lasnamae regional stages. Brecciated section: BIII - middle Kunda, BII - Volkhov and BIa - Billingen regional stages; (c) detail of the infilling sediment with the signs of flow. Note the dark (D) and light (L) varieties of the sandstone. The clasts (C) are limestone of the Billigen, Volkhov and Kunda regional stages. The rocks immediately above the brecciated layer (a flat bedded 4 4 m thick sequence of Llanvirnian limestones) are undisturbed (Figs. 2a, b, 3 and 4). Also, no significant disturbances are found in sediments right below the brecciated layer, in the weakly cemented massive glauconitic sandstone of the Hunneberg Stage or in the section below. The pre-brecciation bedrock section consists of 160 m sedimentary rocks overlying the crystalline basement of Paleo-Proterozoic metamorphic rocks. The sedimentary rocks in this area are mostly (>95% of the section) composed of Vendian, Lower Cambrian and Lower Ordovician, weakly cemented terrigenous rocks (sand-and siltstone, claystone). The present porosity of these rocks is -2030% (JBeleht et al. 2002), which suggests that this section was (accounting only for mechanical compaction) about 30% thicker (up to -200 m) at the time of the Osmussaar Event. The upper part of this terrigenous complex is composed of Lower Ordovician, dark green, glauconitic sandstone overlain by a thin layer (1-1.5 m) of limestone and sandy limestone. This limestone bed consists of a 50-60 cm thick bed of

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glauconitic limestone of the Toila Formation corresponding to the Billingen and Volkhov regional stages, and of 20-60 cm thick structurally heterogeneous bed of the bioturbated slightly kerogeneous sandy limestone of the Pakri Formation corresponding to the Valaste substage of the Kunda regional stage (Figs. 3 and 4) (Ndlvak 1997; Meidla 1997).

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Ea,

c.

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L

h S a ,

tn tn tn

Regional Stage

Baltoscandian Chitinozoan Zones

General Lithology

Kardla impact

Osmussaar Event

Neugrund impact

Fig. 3. A schematic stratigraphy of the sedimentary sequence in northwestern Estonia. Chitionozoan zones according to N8lvak and Grahn (1993). 1- limestone, 2 - organic-rich limestone, 3 - glauconitic limestone, 4 - claystone, 5 - glauconitic sandstone, 6 - sandstone.

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sandy matrix of Osmussaar breccia

post-event limestone of Kunda Stage Aluoja Substage

pre-event calcareous sandstone of Kunda Stage (in top view)

limestone of LasnamBgi Stage (intact)

calcareous sandstone of Kunda Stage Valaste Substage (in sectlon)

glauwnite limestone of Hunneberg Stage (intact)

glauwnite limestone of Volkhov and Billingen stages (in top view)

hardground

glauwnite limestone of Volkhov and Billingen stages (in section)

discontinuity surface

Fig. 4. Schematic drawing of brecciated layer in Osmussaar Island. (a) The top view; (b) cross-section.

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The matrix of the sediment consists of a mixture of two types of lime-rich sandstone - light grey and dark brownish, both showing flow structures (Fig. 2c). The dark brownish type prevails in the thin (1-20 cm) veins injected into the blocks and situated horizontally between displaced beds of limestone. The lightgrey type is the prevailing type in mixtures in the thicker veins and bodies. The lime-rich sandstone matrix in Osmussaar Island makes up 50-100 vol% (average 85 ~ 0 1 % )of sediment. The sediment in the other places (V6ke- and Suur-Pakri islands, Pakri Peninsula) consists only of light lime-rich sandstone. Composition and structure of the sandy sediment in the drill core sections in the mainland area of northwestern Estonia are difficult to assess due to low core recovery from this part of the section. However, the dark variety seems to dominate in the breccia matrix in the mainland area. 2.2 Composition of the Sediment

The compositions of the sediment and host rocks were studied by means of X-ray diffractometry (XRD), optical and scanning (SEM) microscopy and standard sedimentological analysis. XRD patterns from powdered mounts of the sandstones were analysed on a Dron-3M diffractometer at the Institute of Geology, University of Tartu, Estonia. Mineral compositions were quantified using the full-profile '~~' (Taylor 1991). Grains in the Rietveld technique based ~ i r o ~ u a n t software coarse silt and fine sand fractions were examined with a Jeol SEM-EDS system at the Geomorphology and Pedology Lab at York University, Canada. The grain size of the terrigenous component was determined by sieve and pipette analyses after dissolving the calcite matrix of the rock with 3% HC1.

Fig. 5. Photomicrographs of the thin sections of carbonate cemented quartz sandstone matrix filling the sedimentary intrusions. (a) Dark sandstone, sample N-50; (b) Light sandstone, sample N-55. Both from Osmussaar Island. Crossed polars.


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b

a

grain size

Quartz

Calcite Pyrite

K-feldspar Clay minerals

Fig. 6. Grain size distribution and composition of the breccia matrix and host rock: (a) grain size cumulative frequency curves of the insoluble residue: 1 breccia matrix sediments, 2 sand-rich limestone of the Pakri Formation. 3 - limestone of the lower Kunda stage; (b) mineral composition by whole rock X-ray diffraction: 1 - host rock, 2 - light grey type of the sandstone, 3 - dark brownish type of the sandstone. -

The terrigenous component is composed mainly of subangular to well-rounded quartz sand (Mahaney 2002). The structure of siliciclastic is mainly matrixsupported and the matrix is composed of calcite with small amounts of clay (Fig. 5a,b). The fine-grained rnicritic calcite is completely recrystallized. Skeletal fragments have not been observed in the matrix of the sediment, but they are abundant in limestone rock fragments embedded in sandstone. The insoluble residue is very fine- to medium-grained sand. The sandy material is fairly well sorted and, on average, more than 90% of the grains are in the range from 63 pn to 500 pn, where the content of fine sand (125-250 pn) dominates (>50% of the sediment) (Fig. 6a). The results of whole rock XRD analyses show that the quartz content in the dark brownish type of lime-rich sandstone is nearly constant at -60% and in light gray type varies from 70 to 79% (Fig. 6b). Calcite contents in dark and light sandstone vary from 75 to 37% and 15 to 21%, respectively. Content of Kfeldspar and clay minerals (illite, illite-smectite and traces of kaolinite) is subordinate and does not exceed 5%. Also, traces of -tridymite and cristobalite were found. The dark sandstone, compared to the light variety, contains considerably more pyrite (5.3-7.2% and 1.6-7.7%. respectively). According to optical microscopic examination, the dark sandstone contains a small amount of organic material (occasionally up to 3 % by volume), which is obviously the reason for the colour difference between the two sandstone facies.

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Fig. 7. Photomicrographs of quartz grains from the breccia matrix in Osmussaar Island showing planar deformation features (PDF); (a) - quartz gain with one set of PDFs (sample N-55, Osmussaar Island, inverted phase-contrast image of a thin section, parallel polarizers); (b) - quartz grain with two sets of PDFs (sample N-56, Osmussaar Island, parallel polarizers, in immersion liquid).

0

10

20

30

40

50

PDF-plane to polar axis angle, " Fig. 8. Crystallographic orientation of PDFs in 22 quartz grains (26 sets) from breccia matrix from Osmussaar Breccia. Thin section and grain mount analysis of the sandstone reveals quartz grains and a single K-feldspar grain with planar fractures (PF) and, possibly, planar deformation features (PDF). Most of quartz grains found have one set of PDF's


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(Fig. 7a), but also two and rarely up to three sets of frequently decorated PDF's were found (Fig. 7b). PDF's were found in lime-rich sandstone fill of the sediment in all area of Breccia distribution from Osmussaar Island to Pakri peninsula (Fig. 1). The frequency of grains with shock signatures is