Iso-Naakkima, a circular structure filled with Neoproterozoic sediments, Pieksämäki, southeastern Finland
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A circular Bouguer gravity anomaly with a minimum of -4.0 mGal and halfamplitude width of 2 km was recognized at Lake Iso-Naakkima (62°11'N, 27°09'E), southeastern Finland.The gravity low is associated with subdued aeromagnetic signature and notable airborne and ground electromagnetic anomalies that indicate low bedrock resistivity.The drilling record beneath the recent (Quaternary) glacial sediments, 25-40 m thick, reveals a 100 m thick sequence of unmetamorphosed shale, siltstone, quartz sandstone, kaolinitic clay and conglomeratic sandstone that rest on a weathered mica gneiss basement.The upward fining sequence is characterized by red colour, high kaolinite content, and tilted, distorted and brecciated beds.According to the geophysical modelling the diameter of the whole basin is 3 km and that of the sedimentary rocks 2 km, and the depth is 160 m.Shock lamellas in quartz clasts of the basal conglomeratic sandstone, almost omnipresent kink banding in micas of the rocks beneath the basin floor and the occurrence of polymictic dike breccia in the underlying mica gneiss suggest shock metamorphism.It was concluded that the basin originated by a meteorite impact.However, the impact-generated rocks were subsequently eroded before the sedimentation and only minor marks of shock metamorphism were preserved.Lateritic weathering took place prior to deposition of the sediments.Quartz sandstone and siltstone are interpreted as fluvial deposits and the thinly laminated shales as transgressi ve lacustrine or lagoonal deposits.The microfossil assemblage in the shale includes sphaeromorphs of acritarchs from Late Riphean (Neoproterozoic).Postdepositional subsidence of the Iso-Naakkima basin, shown by tilted sediments, preserved the sequence from further erosion.Keywords:
Siltstone
Basement
Breccia
Shock metamorphism
Introduction and aim of study: Relatively shallow marine successions of Early Cambrian age are preserved at many places along the Caledonian Mountain chain in Sweden and Norway. In the Tornetrask area, northern Sweden, the local Lower Cambrian deposits, up to about 100 m thick, are assigned to the Tornetrask and Grammajukku Formations [1, 2]. The Tornetrask Fm rests unconformably on the basement and comprises a number of sandand siltstone dominated members signaling changes in sea level during deposition. In this part of the sequence an enigmatic occurrence of a mostly 2-4 m thick, crystalline-rich, polymict breccia, the Vakkejokk Breccia (VB), has long attracted the attention of geologists working in the area [1, 3, 4]. The breccia occupies a stratigraphic position between the ‘Lower Siltstone mbr’ (LSM) and the ‘Red and Green Siltstone mbr’ (RGSM) of the Tornetrask Fm. The lower boundary is erosive and locally the breccia even rests directly on the basement. The VB contains a mixture of clasts of Lower Cambrian sediments as well as the crystalline basement. The clast size varies from gravel to boulders and some of the larger basement blocks exceed 100 m in length although being less than 10 m thick, but even basements rafts > 200 m long have been reported [4]. The VB has been reported from a series of exposures on the northern side of Lake Tornetrask, where it is semi-continuously exposed between the Vakkejokk and Tjaurajokk rivers, i.e. for a stretch of about 7 km [1, 4]. It can be traced further eastwards in disjunct river sections for another c. 7 km [cf. 1]. Thin conglomerates in the same stratigraphic position occur also south of Lake Tornetrask, e.g., at Luopakte, possibly representing an equivalent to the VB [1, 4], but may just as well be normal sedimentary conglomerates, marking a sequence boundary [cf. 2]. The breccia has previously been interpreted as a tillite or being fault-related [see summary by 1, 2]. However, the latter authors found the previous explanations unsatisfactory due to the rather local distribution and in particular the occurrence of the very large basement boulders and suggested the breccia to be impact related. This re-interpretation sparked the new investigations reported here. Methods: The study area was visited by Ormo and Nielsen during a one week field campaign in the summer of 2012 in order to study field relations of the VB and to collect samples that potentially could provide evidence of shock diagnostic for impact. Thin sections of VB samples obtained during the 2012 field campaign were studied by Alwmark using a Leitz 5-axes universal stage [5] mounted on an optical microscope in search of planar deformation features (PDFs) in quartz grains. The crystallographic orientations of identified PDFs were determined according to techniques described in [6, 7]. Results and discussion: Shock metamorphic features, in the form of PDFs, have been found in six quartz grains from one of the breccia samples (Fig. 1). Of the six shocked grains, four display a single set of PDFs, all oriented parallel to the basal plane c (0001). Two grains contain two sets; one set oriented along (0001) and one parallel to crystallographic plane ω {10 3}.
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Siltstone clasts are commonly known from turbidites in deep-water successions and have been interpreted to document information regarding depositional processes of host sandstones. Here we studied formative processes of siltstone clasts on the basis of size and fabric of siltstone clasts and of host sandstones.Although siltstone clasts generally exhibit imbrication dipping to upslope directions regardless of depositional features of host sandstones, their size, density, and position in a turbidite bed are variable in response to vertical variations in grain size and grain fabric of host sediments. Therefore, the variation in size, density, and position of siltstone clasts in a single turbidite bed is interpreted to be controlled by temporal variations in the relative rates of suspended-load fallout during a single depositional process of sandstone beds from turbidity currents.
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Siltstone clasts are commonly known from turbidites in deep-water successions and have been interpreted to document information regarding depositional processes of host sandstones. Here we studied formative processes of siltstone clasts on the basis of size and fabric of siltstone clasts and of host sandstones.Although siltstone clasts generally exhibit imbrication dipping to upslope directions regardless of depositional features of host sandstones, their size, density, and position in a turbidite bed are variable in response to vertical variations in grain size and grain fabric of host sediments. Therefore, the variation in size, density, and position of siltstone clasts in a single turbidite bed is interpreted to be controlled by temporal variations in the relative rates of suspended-load fallout during a single depositional process of sandstone beds from turbidity currents.
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Gow Lake, in the Precambrian Shield of Saskatchewan, is circular, 4 km in diameter, and has a large central island. Granites and quartzofeldspathic gneisses are exposed around the perimeter of the lake, whereas the island is formed largely of brecciated equivalents. Most of the breccias are composed entirely of clastic material, but at one locality fine-grained felted matrices form a significant component of the breccias, and coronas of clear glass surround quartz grains. The latter breccias also contain microscopic features characteristic of shock metamorphism, among which multiple sets of planar deformation structures in quartz are particularly diagnostic. Similar shock metamorphic features have been widely reported from terrestrial meteorite craters; accordingly, Gow Lake is interpreted as a deeply eroded impact crater and the felted matrices as impact melts.A local negative gravity anomaly with an amplitude of 3 mGal centered on the lake is attributed mainly to highly fractured basement rocks underlying the lake, which model studies indicate may extend to a depth of 900 m. A provisional minimum age of 100 Ma is proposed for the crater.
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Abstract This paper describes an unusual occurrence of igneous material as clasts in dyke and pipe breccias associated with late Caledonian minor intrusions. It is shown that the clasts were in a plastic condition when incorporated into the breccia rock. These igneous clasts were derived from magma disrupted at depth and then transported into the fluidized breccia columns where they were mixed with large numbers of clasts derived from the quartzite wall‐rocks. Textures and planar fabrics developed during collapse of the fluidized system are described and shown to be separable from the later compaction associated with extensive pressure solution of the fine matrix. Most Caledonian breccia pipes lack igneous clasts and it is considered that this group of breccias represent the rarely‐preserved boundary zone between active magma and breccia systems.
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ABSTRACT Pipe-shaped breccia bodies associated with diorite intrusions are composed mainly of angular clasts of local schists with a few transported clasts of quartzite. Plate shaped fragments are commonly oriented to define planar fabrics in the breccias. These features indicate the operation of gas fluidisation within the pipes and both entrainment and expanded bed conditions are inferred. The fabrics result from the collapse of the fluidised suspensions as the gas flow declined. Dilational fracture patterns in the country rock comparable with the stress release patterns found around mine shafts can be matched with the fractures required to produce the angular schist clasts. It is concluded that fracturing and the introduction of fragments into the fluidised breccia system was a continuous process and that the pipe diameter increased progressively with time. Microdiorite sheets and related stock like bodies of diorite cut and metamorphose the breccias. Compaction, hornfelsing and hydrothermal alteration also contributed to breccia formation.
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The Ries Crater, an impact structure of 26 km diameter in south Germany, is the largest terrestrial crater where substantial amounts of ejecta are preserved, on occasion >100 m deep. Further, the target stratigraphy is well known, and it is possible to relate specific clasts and breccia lithologies to initial target depth. As a consequence the continuous deposits of the Ries, also known as Bunte Breccia, may be studied with exceptional field control. We report field observations and laboratory analyses obtained from 560‐m core materials, taken at nine different locations that range from 16 to 37 km in radial distance from the impact center. The objective is to relate the Ries observations to ejection, and to emplacement processes of large‐scale, planetary crater deposits. The observations regarding the modal‐stratigraphic characteristics of the Bunte Breccia may be summarized as follows: only <0.15% (weight) of the total deposit consists of crystalline clasts larger than 1 cm that are derived from depths of >600 m; some 0.7% is composed of Triassic clasts, originating from 300 to 600‐m depths; Lower and Middle Jurassic horizons (approximately 300–150 m) constitute some 2.3%, and Upper Jurassic (0–150 m) makes up some 31.5%. In addition, the Bunte Breccia contains Tertiary materials in the form of >1‐cm clasts (29.1%) and as highly comminuted, fine‐grained “matrix” (<1 cm) accounting for the remaining 36.3%; these Tertiary materials constituted the immediate crater environment, i.e., a substrate, onto which the Ries ejecta were deposited. These substrate materials were thoroughly mixed into the continuous deposits. The ratio of “primary crater ejecta” to local substrate components decreases with increasing radial range. There is, however, no vertical stratification with regard to modal‐stratigraphic composition at any specific location; modal‐stratigraphic composition is highly variable on meter scales; Bunte Breccia appears to be a chaotic mixture resulting from a highly turbulent depositional environment. Also, the orientation of clasts larger than 1 cm is random. Detailed grain size data reveal progressively decreasing grain sizes with increasing radial range of both primary crater ejecta and local substrate materials. In addition, progressive comminution of primary ejecta related to increasing target depth is observed. Components shocked to >10 GPa constitute <0.1% (weight) of the entire deposit, which indicates that Bunte Breccia was emplaced at essentially ambient temperatures. When possible, the above observations are quantified via linear regressions throughout the text. All of these observations are consistent with, if not predicted by, a ballistic emplacement scenario as postulated by Oberbeck and co‐workers: primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate; a mixture of primary and secondary ejecta results and combines into a highly turbulent, ground‐hugging debris surge as the final phase of ejecta emplacement. Total emplacement time for the Bunte Breccia (⪖200 km³) is estimated to be of the order of 5 min only. These findings are compared with cratering theory relating to a number of ejecta thickness decay models and with the so‐called Z model, addressing material flow during various stages of crater formation. Qualitative to fair agreement of observations and predictions results. An initial crater radius of 6.5 km, an excavation depth of 1650 m, an excavation volume of 136 km³, and an associated transient cavity volume of aproximately 230 km³ appear to be reasonable estimates. Approximately 170 km³ of material was involved in slumping and restoration of the transient cavity for the above radius and Z=2.7. The modal composition of Ries ejecta with regard to preimpact target stratigraphy indicates that materials contained in the continuous deposits of large, complex planetary craters are predominantly derived from depths as small as one‐hundredth the apparent crater diameter. A number of implications are addressed regarding remote sensing of planetary surfaces and investigation of lunar and meteoritic impact breccias.
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