Analysis of borehole geophysical data from the Mora area of the Siljan Ring impact structure, central Sweden

Silurian stratigraphy and graptolite faunas of the Mora 001 and Solberga 1 drill cores, Siljan District, central Sweden

“…This interpretation may be rejected by the presence of a ca. 290-m thick Ordovician succession in the Mora VM2 drill core only 600 m to the north (Muhamad et al 2015(Muhamad et al , 2018, suggesting that the variation in thickness may be based on impact tectonics.…”

“…3). Lehnert et al (2012) indicated the presence of megaslumps, debris flows and turbidites in their lower shale member and Muhamad et al (2015Muhamad et al ( , 2017 and Muhamad (2017, p. 16) referred the deformation in the Mora 001 and Solberga 1 drill cores to Caledonian collision tectonics and considered the successions to be 'largely unaffected by faulting and impact-related deformation'. Biostratigraphical information to date the lithostratigraphical column was not provided.…”

“…Fredriksson & Wickman 1963;Svensson 1971;Wickman 1981;Reimold et al 2005;Lindström et al 2008). Reimold et al (2015) dated the impact at 380 Ma (Frasnian, Devonian), but a number of slightly different ages can be found in the literature (Elming & Bylund 1991;Reimold et al 2005;Jourdan & Reimold 2012;Muhamad et al 2015). The impact led to considerable tectonic distortion of the Palaeozoic sedimentary cover of the Siljan region and difficulties to understand the sedimentary succession as exposures show only small parts and the connections of individual lithological units often are unknown.…”

“…Jaanusson (1982) estimated the thickness of the Ordovician in the intermound facies at no more than 100 m and Thorslund & Jaanusson (1960, p. 25) suggested a thickness of about 130 m. Rondot (1975) and Collini (1988) estimated a thickness of 350-500 m of Palaeozoic sediments through seismic data. Recent seismic investigations (Juhlin et al 2012;Muhamad et al 2015Muhamad et al , 2017 suggested the presence of a 400-m thick Ordovician limestone succession followed by ca. 200-m thick Silurian shales in the Mora region of the western part of the Siljan Ring, but equivalent thicknesses in outcrop do not exist and only a 21.57m thick succession of Ordovician limestones was found in the Mora 001 drill core (Lehnert et al 2012, p. 88).…”

“…200-m thick Silurian shales in the Mora region of the western part of the Siljan Ring, but equivalent thicknesses in outcrop do not exist and only a 21.57m thick succession of Ordovician limestones was found in the Mora 001 drill core (Lehnert et al 2012, p. 88). Muhamad et al (2015) indicated that the thickness of the Silurian deposits varies considerably in the Mora area. It is estimated that a Palaeozoic overburden of up to 2500 m has been removed through erosion including also considerable thicknesses of post-impact strata (e.g.…”