Facies distribution of post‐impact sediments in the Ordovician Lockne and Tvären impact craters: Indications for unique impact‐generated environments

Abstract

available online at http://meteoritics.org Abstract-The Lockne and Tvären craters formed in the Late Ordovician Baltoscandian epicontinental sea. Both craters demonstrate similarities concerning near-synchronous age, target seabed, and succeeding resurge deposits; however, the water depths at the impact sites and the sizes of the craters were not alike. The post-impact sedimentary succession of carbonates, i.e., the Dalby Limestone, deposited on top of the resurge sediments in the two craters, is nevertheless similar. At least three main facies of the Dalby Limestone were established in the Lockne crater, depending on sea-floor topography, location with respect to the crater, and local water currents. The dominating nodular argillaceous facies, showing low values of inorganic carbon (IC), was distributed foremost in the deeper and quiet areas of the crater floor and depressions. At the crater rim, consisting of crushed crystalline basement ejecta, a rim facies with a reef-like fauna was established, most certainly due to topographical highs and substrate-derived nutrients. Between these facies are occurrences of a relatively thick-bedded calcilutite rich in cephalopods (cephalopod facies). Facies distribution of post-impact sediments in the Ordovician Lockne and Tvären impact craters: Indications for unique impact-generated environmentsIn Tvären, the lower part of the succession consists of an analogous argillaceous facies, also showing similar low IC values as in Lockne, followed by calcareous mudstones with an increase of IC. Occasionally biocalcarenites with a distinctive fauna occur in the Tvären succession, probably originating as detritus from a facies developed on the rim. They are evident as peaks in IC and lows in organic carbon (Corg). The fauna in these biocalcarenites corresponds very well with those of erratic boulders derived from Tvären; moreover, they correspond to the rim facies of Lockne except for the inclusion of photosynthesizing algae, indicating shallower water at Tvären than Lockne. Consequently, we suggest equivalent distribution patterns for the carbonates of the Dalby Limestone in Lockne and Tvären.

Sedimentological analysis of resurge deposits at the Lockne and Tvären craters: Clues to flow dynamics

“…3. There are algal nodules, as well as the remains of a diversified shelly fauna, in the slumped interlayers in the post-impact secular mud deposit inside the crater (Lindström et al 1994;Frisk and Ormö 2007). These occurrences prove that the highest elevations of the crater rim were shallower than 100 m (i.e., within the photic zone).…”

Sedimentation in marine impact craters — Insight from the Ritland impact structure

“…The depositional successions of the Lockne structure were studied by field mapping (Lindström and Sturkell, 1992) and later in several drill cores in combination with geophysical analyses (Sturkell and Ormö, 1998). The summary of the depositional successions of the Lockne crater is based on published information (Lindström and Sturkell, 1992;Lindström et al, 1996Lindström et al, , 2005aLindström et al, , 2008Frisk and Ormö, 2007;Ormö et al, 2007) and the composite log is based on Dypvik and Kalleson (2010) (Fig. 7).…”

“…7. Lithological column based on LOC-2 drill core (re-drawn after Dypvik and Kalleson, 2010) showing different sedimentary successions in the Lockne impact structure and their descriptions (Lindström and Sturkell, 1992;Lindström et al, 1996Lindström et al, , 2005aLindström et al, , 2008Sturkell, 1998;Frisk and Ormö, 2007;Ormö et al, 2007). from resurge activities (Lindström and Sturkell, 1992). Ormö et al (2007) re-interpreted the upper part of the matrix-supported Lockne Breccia as 'resurge proper' (the initial inward rush of water and debris) with the overlying clast-supported part deposited as suspension load and subsequently classified as 'anti-resurge' (collapse of the central water plume resulting an outward flow) and 'oscillating resurge' (no definite direction during waning flow) deposits.…”

“…Systematic analysis of marine crater-filling sediments has been documented in a range of studies (Ormö and Lindström, 2000;Dypvik and Jansa, 2003;Dypvik et al, 2004Dypvik et al, , 2006Lindström et al, 2005a,b;Frisk and Ormö, 2007;Ormö et al, 2007;Kalleson et al, 2008;Browning et al, 2009;Schulte et al, 2009;Dypvik and Kalleson, 2010;Azad et al, 2012Azad et al, , 2013Sturkell et al, 2013). Dypvik and Kalleson (2010) attempted to rationalize and group the depositional successions within the impact craters into three main sedimentation stages: i) late syn-impact, ii) early post-impact, and iii) late post-impact, according to the crater modification stages (Melosh, 1989;Turtle et al, 2005) (Nielsen and Schovsbo, 2011) showing that Ritland and Gardnos bolides impacted on the possible wave-dominated shoreface in the inner shelf areas of the late Early Cambrian epicontinental sea.…”

A rock magnetic profile through the ejecta flap of the Lockne impact crater (central Sweden) and implications for the impact excavation process

“…The basement target rocks also include several tens of meters thick and hundreds of meters wide, near horizontal dolerite sills that are of special interest to this study. Geological map and location of Lockne-9 drill core with respect to the proposed crater centre (modified after Frisk and Ormö, 2007).…”