The Steinheim Basin impact crater (SW-Germany) – Where are the ejecta?

Icarus 250 (2015) 529–543

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The Steinheim Basin impact crater (SW-Germany) – Where are the ejecta? Elmar Buchner a,b,⇑, Martin Schmieder c a

HNU – Neu-Ulm University, Wileystraße 1, D-89231 Neu-Ulm, Germany Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstraße 18, 70174 Stuttgart, Germany c Philamlife Village, Pueblo de Oro, Upper Carmen, Cagayan de Oro 9000, Philippines b

a r t i c l e

i n f o

Article history: Received 3 September 2014 Revised 10 December 2014 Accepted 21 December 2014 Available online 5 January 2015 Keywords: Earth Impact processes Cratering

a b s t r a c t The 24 km Nördlinger Ries and the 3.8 km Steinheim Basin in southern Germany are thought to represent a 14.8 Ma old impact crater doublet. The complex craters of the Steinheim Basin with its crater fill deposits and the Nördlinger Ries and its voluminous impact ejecta blanket are still widely preserved. Although located in an environmental setting that presumably underwent the same erosional history as the Ries crater, field geologic studies suggest that no proximal or distal ejecta of the Steinheim impact event are presently preserved. Generally, the lack of the ejecta blanket around the crater could be explained either by intense erosion, the scarcity of outcrops, or it never formed. In contrast to the lack of ejecta, fluvial and lacustrine Middle Miocene sediments deposited prior to, synchronous with, and shortly after the impact are preserved in many places in the surroundings of to the Steinheim Basin. On low-density asteroids or planets with highly porous target rocks (P30–40% effective porosity), impact structures can form without significant ejecta outside the craters due to the compaction of porosity and a concordant drastic reduction of the ejecta velocity. In the Steinheim area, the target rocks comprised loose, porous Miocene sands, Upper Jurassic limestones and Middle Jurassic porous sand- and claystones. The average porosity of the entire sedimentary target suite may have reached 20–30% or even higher values assuming the existence of open karst cavities in the Upper Jurassic carbonates. Compaction of the porous target rocks, resulting in the reduction of ejected material, in combination with erosion could explain the apparent lack of impact ejecta in the wider periphery of the Steinheim impact structure. The Steinheim Basin represents the first proposed terrestrial example of an impact crater characterized by porosity-related ejecta suppression, and it is suggested that other sediment-hosted impact structures on Earth might exhibit analogous excavation-process characteristics. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction The 24 km Nördlinger Ries (Shoemaker and Chao, 1961; von Engelhardt, 1997; Osinski, 2004; Buchner et al., 2007) and the 3.8 km in diameter Steinheim Basin impact craters (e.g., Dietz, 1959; Grieve and Shoemaker, 1994; Buchner and Schmieder, 2010) are situated on the Upper Jurassic limestone plateau of the Swabian and Franconian Alb (SW Germany; Fig. 1). Both impact structures are thought to have formed simultaneously in the Middle Miocene (e.g., Shoemaker and Chao, 1961; Reiff, 1988; Stöffler et al., 2002) by the impact of a binary asteroid (e.g., Stöffler et al., 2002). Since the first radiometric and biostratigraphic ages became

⇑ Corresponding author at: HNU – Neu-Ulm University, Wileystraße 1, D-89231 Neu-Ulm, Germany. E-mail address: [email protected] (E. Buchner). http://dx.doi.org/10.1016/j.icarus.2014.12.026 0019-1035/Ó 2014 Elsevier Inc. All rights reserved.

available, there has been a general agreement that the two impact craters as well as the moldavite strewn field (the Central European tektite strewn field of Bohemia, Lusatia, Moravia, and Lower Austria; e.g., Lange et al., 1995; Stöffler et al., 2002) were formed during the same double impact event some 14.8 Ma ago (e.g., Jourdan et al., 2012; Buchner et al., 2013; for a compilation of the extensive age data set for the Nördlinger Ries see Buchner et al., 2010, 2013). Calculated from the centers of the two craters, both impact structures are separated from each other by a distance of 42 km. The geometry of the complex Nördlinger Ries crater is characterized by a flat crater wall and an inner ring system. In contrast, the much smaller Steinheim Basin represents a complex impact crater with a distinct central uplift and a relatively steep crater rim (e.g., Ivanov and Stöffler, 2005). Among the 187 impact structures currently known on Earth, the Nördlinger Ries crater in southern in Germany is unique in

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Fig. 1. Shaded relief of SW Germany with the location of the Steinheim Basin impact crater on the Swabian Alb, and the Nördlinger Ries impact crater; the relief was generated from Shuttle Radar Topography Mission (SRTM) data.

terms of the state of preservation of the crater shape and its impact ejecta blanket (e.g., Hüttner and Schmidt-Kaler, 1999; Buchner and Schmieder, 2009). The crater fill and the ejecta blanket consist of the ‘‘Bunte Breccia’’ that is composed mainly of sedimentary clasts and distributed outside the crater within a distance of up to 30– 40 km distance from the western, southern and eastern crater rim (Fig. 2). The Bunte Breccia inside and outside the Ries crater is overlain by the melt-bearing suevite composed of sedimentary and crystalline rock clasts that is distributed outside the crater in patches within a distance of up to 20 km from the crater rim. North of the Ries crater, the ejecta blanket is widely eroded due to geotectonical reasons (compare to Fig. 2). Sturm et al. (2013) recently identified the Ries crater as being a terrestrial double-layer rampart crater, a crater type so far known only from Mars. The Steinheim Basin today exhibits a distinct morphological depression that is 110 m deep and 3.5 km in diameter (Reiff, 2004; Fig. 2); the height of the central uplift totals to more than 140 m at present. The original size of the pristine crater is estimated to be 3.8 km (e.g. Stöffler et al., 2002; Ivanov and Stöffler, 2005) with an original depth of 205 m (Reiff, 2004). As this drainless morphological depression was filled by lake sediments shortly after the impact (uncovered later on in the Quaternary), the primary crater morphology is widely preserved (e.g., Heizmann and Reiff, 2002). The crater rim is formed by steeply inclined and intensely brecciated Upper Jurassic limestone blocks (Reiff, 2004) that also contain numerous shattered chert nodules (Schmieder et al., 2011), as well as some loose blocks of the host limestone. An impact breccia, referred to as the so-called ’’Primäre Beckenbrekzie’’ (‘‘primary basin breccia’’) by Groschopf and Reiff (1966) and interpreted as a ‘‘fall-back breccia’’, is known from

many drillings in the Steinheim Basin (Reiff, 2004). Mainly composed of Middle to Upper Jurassic limestones, marls, mudstones, and sandstones, the impact breccia inside the morphological depression of the Steinheim Basin is overlain by lake sediments (Fig. 3) and is obviously preserved in its original thickness and position. However, impact ejecta further outside the crater (i.e., deposited far beyond the actual crater rim) have never been described in the literature or in the local geological maps.

2. Geological setting Similar to the target sequence in the Ries area, the target rocks of the Steinheim Basin consisted of a layered sequence of Triassic, Jurassic, and Paleogene to Neogene sedimentary rocks that are underlain by Variscan (Moldanubian) crystalline basement rocks; the sedimentary cover totals to a total thickness of 620 m in the Ries area and 1180 m in the Steinheim region, respectively (Stöffler et al., 2002). The Nördlinger Ries crater formed in this suite of sedimentary rocks and the underlying crystalline basement, whereas the Steinheim Basin is hosted by a sedimentary sequence of Upper to Middle Jurassic and Miocene rocks (Fig. 3). During the Steinheim event, bedrocks were fractured and uplifted (compare images of shocked and sheared bedrock sandstones in Buchner and Schmieder (2010)) and only the upper (Jurassic) portion of the target rocks was incorporated into the Steinheim Basin breccia (e.g., Heizmann and Reiff, 2002; Reiff, 2004). At the time of the suspected double impact event, both impact localities were situated close to the northern rim of the sedimentary province of the North Alpine Foreland Basin (Fig. 1). The

Fig. 2. Shaded relief of SW Germany with the location of the Steinheim Basin and the Nördlinger Ries crater on the Swabian–Franconian Alb as well as the recent distribution of the remnants of the Ries ejecta blanket; note that no Ries ejecta are preserved North of the Alb escarpment due to erosion and that the Steinheim Basin is not surrounded by any ejecta; the relief was generated from Shuttle Radar Topography Mission data; the limits of the Ries ejecta blanket are taken from the geological map GK 1:50,000, Nördlinger Ries (Hüttner and Schmidt-Kaler, 1999).

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Fig. 3. (A) Aerial view of the Steinheim Basin; looking to the North (source: Google Earth). (B) Schematic cross section of the Steinheim Basin (1.5-fold vertical exaggeration) and location of the B-26 core drilling (No. 17 in Reiff (2004)); modified after Mattmüller (1994), Reiff (2004), Buchner and Schmieder (2010, 2013a, b).

sedimentation of the Foreland Basin deposits (Molasse) comprising sands, marls, claystones, and palustrine limestones deposited in an extensive fluvial–lacustrine plain (e.g., Maurer and Buchner, 2007) prograded northward onto the Swabian and Franconian Alb from Oligocene until Late Miocene times. Although deposited in minor thickness, remnants of these sediments are still preserved in the surroundings of both impact craters. Pre-impact foreland basin deposits are locally incorporated into the impact breccias (e.g., Hüttner and Schmidt-Kaler, 1999; Buchner and Schmieder, 2013b); in particular, the Bunte Breccia of the Ries area contains a considerable portion of fluvial, lacustrine and palustrinen Miocene sediments. Due to the subtropical climatic and water-saturated environmental conditions, conspicuous palustrine cm- to dm-sized oncoids (e.g., Schmidt-Kaler and Salger, 1986; Weiss et al., 2008; Buchner and Schmieder, 2013a) formed in the Ries and Steinheim area close in time to the Ries–Steinheim impact event and are either incorporated into, directly overlain, but mostly underlain by the Bunte Breccia in the Ries area. Weiss et al. (2008), for instance, described the occurrence of lacustrine/ palustrine oncoids that directly overlay Bunte Breccia near the village of Langenaltheim, approximately 10 km east of the Ries crater. These oncoids are still preserved in various locations in the periphery of both craters (Buchner and Schmieder, 2013a; Fig. 4).

3. Description of the problem In contrast to the enormous amount of proximal continuous impact ejecta in the surroundings of the Ries crater within a distance of 30–40 km and distal ejecta boulders in a distance of up

to 200 km from the crater rim (e.g., Buchner et al., 2007; Sturm et al., 2013), impact ejecta outside the Steinheim Basin impact crater have never been described in the literature or mapped in the local geological charts. Generally, the lack of an ejecta blanket around the Steinheim crater could be explained in the following ways: 1. it has been eroded away; 2. it exists but has not been recognized; 3. it never formed. The original existence of ejecta blankets (that primarily overlapped) surrounding both, the Ries and Steinheim craters, is generally considered. Stöffler et al. (2002), for instance, depicted that the (originally existing) ejecta blanket of the Steinheim Basin must have been completely eroded. Possibilities 2 and 3 were no subject of discussion in the literature. The transfer of up to 2 crater radii for a continuous ejecta blanket (e.g., Barlow, 2005), and on the order of 2–3 crater diameters for distal ejecta (e.g., Melosh, 1989) can be estimated for terrestrial impact structures. From this point of view, proximal (and medial) Steinheim ejecta could be expected within a distance of up to 4 km, distal ejecta within a distance of up to 12 km from the crater rim, however, an intense search for proximal or distal Steinheim ejecta in the past including our own field observations was not successful. Due to the still ongoing backward erosion of the Alb escarpment (Fig. 2) since Middle Miocene times, the ejecta blanket north of the Ries crater is completely eroded. The eastern, southern and western portion of the Ries ejecta blanket is still widely preserved. The contradiction of a well-preserved Ries ejecta blanket, the essentially identical sedimentary target setting and erosional history of the (western, southern, and eastern) Ries and the Steinheim area, and the apparent lack of remnants of the Steinheim ejecta in the periphery of the crater call for an explanation, which is the subject of this study.

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Fig. 4. Distribution of various sedimentary deposits of Eocene to Upper Miocene age in the southern and eastern periphery of the Steinheim Basin impact crater; these sediments were deposited prior to, synchronous, and shortly after the impact. Note that these deposits are preserved, whereas no impact ejecta exist in the surroundings of the crater structure. Similarly, out-crater ejecta have not been reported in the northern and western periphery of the crater, however, the information on sedimentary deposits of Eocene to Upper Miocene age in this area is more limited. This illustration is compiled from Schmidt-Kaler and Salger (1986), the geological map GK 1:25,000, No. 7326, Heidenheim (Reiff, 2004), Maurer and Buchner (2007) and our own field studies.

4. Field observations in the Ries–Steinheim area – a case for a missing ‘Steinheim layer’ The Ries crater is situated at the northern rim of the Swabian– Franconian Alb. Whereas no remnants of Ries ejecta are preserved North of the Alb escarpment (Fig. 2), a continuous blanket of proximal and medial Ries ejecta surrounds this crater in western, eastern and southern direction covering the Upper Jurassic limestone plateau. The maximum thickness of the ejecta blanket locally attains tens of meters, however, many isolated smaller patches of impact breccia have been mapped. Close to the small town of Neresheim about 20 km southwest of the Ries crater, for instance, some single boulders of Jurassic rocks have been identified as being medial Ries ejecta distinguishable from (possible) Steinheim ejecta

by their content of traces of Triassic (Keuper) rock fragments (LGRB-BW, 2001). Although located on the Swabian Alb plateau, in an environment that underwent essentially the same erosional history, no ejecta have been reported outside the Steinheim Basin. Some single, isolated and strongly brecciated limestone blocks (partly silicified) survived erosion that were originally deposited on top of the eastern crater rim, probably slightly reworked in quaternary times and redeposited on the eastern crater wall of the Steinheim Basin (geological site is locally known as ‘‘Schäfhalde’’). Similarly, some isolated big blocks of Upper Jurassic limestones occur on the western crater rim (geological site is locally denominated as ‘‘Hirschtal’’). South of the Steinheim Basin, the terrain is eroded by an East–West trending valley, the Stubental and its tributaries, that led to regional erosion >100 m beneath the original crater surface. The drainage system of the ‘‘Stubental’’ might

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account for the lack of a possible proximal ejecta blanket in the South, East and West. However, the extent of this palaeovalley is only 500 m in its widest part (Abb. 4). South of the Stubental, the terrain ascends steeply by more than 100 m above the floor of the palaeovalley, at a level more or less identical with the land surface at the time of impact. Remains of isochronous fluvial and lacustrine deposits of the Upper Freshwater Molasse in the surroundings of the Steinheim Basin presently crop out within a distance of 1–4 km from the southern and eastern Steinheim crater rim. Remnants of conspicuous pisolithic freshwater limestones within a distance less than 1 to about 2.5 km south and southwest of the crater are of special interest because they were deposited chronologically close to the Ries impact event (e.g. Weiss et al., 2008; Buchner and Schmieder, 2013a). These freshwater limestones are pre-impact formations that directly underlie Ries ejecta and are partly incorporated into the lithic Bunte Breccia (Buchner and Schmieder, 2013a); elsewhere, these pisolitic limestones directly overlie the Bunte Breccia (Schmidt-Kaler and Salger, 1986; Weiss et al., 2008) and, thus, have to be considered as post-impact formations. These limestones were deposited in very shallow pools (Weiss et al., 2008), however, the sediments survived erosion and presently crop out in numerous patches surrounding the Ries crater and the Steinheim Basin. Deposited directly after the Ries–Steinheim event, these oncolithic sediments cover Bunte Breccia in the Ries area and should cover Steinheim ejecta if originally deposited which is not the case. Furthermore, Miocene (pre- and postimpact) deposits (sands and claystones) survived erosion in the periphery of the Steinheim Basin (Reiff, 2004). No lithologies that could represent remnants of proximal ejecta (breccias) were encountered in outcrops or on the fields of the crater’s periphery. Theoretically, distal Steinheim ejecta could be expected within a distance of 12 km from the crater rim. However, no distal ejecta horizon has so far been detected in a continuous suite (including the time of impact) of clastic deposits from Lower to Upper Miocene Molasse deposits approximately 6 km south of the Steinheim crater.

5. Observations in the Steinheim Basin breccia The impact ejecta inside the crater of the Steinheim Basin are modally composed of rock fragments of Upper and Middle Jurassic carbonates, sandstones and claystones (Fig. 5). Isolated quartz grains in the breccia either originate from incorporated Miocene sands or from crumbled Middle Jurassic sandstones. Qualitatively, the breccia is dominated by Upper Jurassic limestones and marls, however, fragments of Middle Jurassic sandstones and claystones occur over the entire maximum thickness of up to 55 m of the basin breccia to a more or less consistent amount (e.g., Reiff, 2004; Buchner and Schmieder, 2010, 2013b). Likewise, particle size and sorting does not considerably vary from the bottom to the top of the basin breccia lens (with the exception of a thin top layer). This indicates effective mixing of minerals and clasts derived from the target rock during crater formation in respect to the basin breccia. The interpretation of the crater fill as a fall-back breccia (‘‘Rückfallbrekzie’’) by Reiff (1977, 2004) and Heizmann and Reiff (2002) was based on these observations, however, we did not detect any sedimentary evidence for ballistically transported and re-deposited ejecta, such as quantitative or qualitative sorting or grading, in the drill core samples of the Steinheim Basin breccia (drill cores B-6, No. 32 in Reiff (2004); B-10, No. 9 in Reiff (2004); B-22, No. 30 in Reiff (2004); B-26, No. 17 in Reiff (2004)) re-investigated by Buchner and Schmieder (2010, 2013b). Nevertheless, the Steinheim Basin breccia exhibits a 5 cm-thick top layer that appears fine-grained and relatively well-sorted (Fig. 6).

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Fig. 5. Three sections of the B-26 drill core (No. 17 in Reiff (2004)) from the base, the middle portion and the top of the Steinheim Basin impact breccia. The breccia is composed of clasts of limestone (L), claystone (C) and sandstone (S) (as well as mmsized melt particles) in similar proportion and grain size in all depths of the drill core. The lack of any sedimentary features like sorting or grading strongly indicates an effective mixing process during deposition of the breccia.

This fine-grained top layer covers the basin breccia in all drill cores investigated and probably represents the terminal fall-out, which suggests that the Steinheim Basin breccia is still preserved in its original thickness and position. Buchner and Schmieder (2010) first detected mixed silicate melt particles whereas Anders et al. (2013) described carbonate melt particles and veins in the Steinheim Basin breccia and shocked target limestone, respectively. The occurrence of impact melt lithologies is not restricted to melt lenses or melt flows (in the sense of Osinski et al., 2011, their Fig. 7) or concentrated in a melt-rich impactite layer (in the sense of the Ries suevite layer, e.g., Artemieva et al. (2013)) and rather seems to be dispersed throughout the Steinheim impact breccia. According to Buchner and Schmieder (2010, 2013b) the geochemical character of the mixed silicate melt particles indicates the lowermost target rocks of Middle Jurassic sandstones and claystones and a minor component of Upper Jurassic carbonates as the likely source for the impact melt. The melt particles are not concentrated at the basis of the basin breccia but the tiny, sub-rounded (not necessarily aerodynamically deformed) and fluidally textured melt particles that usually cover rock fragments are consistently distributed within the basin breccia suggesting a turbulent intermingling of impact melt and a major lithic component across the entire basin breccia. 6. Deposition and distribution of impact ejecta One of the principal characteristics of impact events is the formation and emplacement of ejecta deposits; fresh impact craters

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Fig. 6. Reflected-light photomicrographs of polished thin sections of the B-26 drill core top layer (34 m depth; No. 17 in Reiff (2004)) in the Steinheim Basin impact breccia. (A) shows the transition of the coarser grained basin breccia overlain by a finer grained approximately 5 cm thick top layer. The top layer starts with an elongated brownish melt particle and mm-sized clasts. (B) Base of the top layer with cm- to mm-sized rock fragments, limestone clasts with pale (degassed) rims, elongated melt- and sandstone fragments, and an overall fluidal texture. (C) Detail of (B) mm-sized limestone clast with pale (degassed) rim and elongated melt particles. (D) Detail of (B) elongated sandstone clast with glauconite clasts. (E and F) Uppermost part of the top layer that includes very fine-grained clasts of partially molten limestone and elongated melt particles and an overall fluidal texture; note that the top layer shows gradation (fining-upward), a fluidal texture, and evidence for high temperatures as degassing and partial melting of limestone clasts and a higher amount of melt particles; note that arrows tend towards the top of the layer.

on terrestrial planets are usually surrounded by more or less continuous ejecta blankets. On Earth, interaction with the atmosphere and/or fluids in target rocks are responsible for multiphase ejecta formation, transfer of 1.5–2 crater radii for a continuous ejecta blanket (e.g., Barlow, 2005), and on the order of 2–3 crater diameters for distal ejecta (e.g., Melosh, 1989). Sturm et al. (2013) recently demonstrated that the Ries ejecta blanket in southern Germany contains a massive and continuous, dual-layer rampart structure at 1.45–2.12 crater radii from the crater center, whereas the most distal coarse-grained ejecta (the lithic ejecta clasts of the ‘‘Ries-Brockhorizont’’) was identified in fluvial sandy deposits of the North Alpine Foreland Basin in northern Switzerland nearly 200 km (8 crater diameters) away from the Ries crater rim (Hofmann and Gnos, 2006; Buchner et al., 2007; Alwmark et al., 2012). Crucial factors for the acceleration and distribution of impact ejecta such as velocity, angle and property of the impacting body have been discussed in detail (e.g. Melosh, 1989; Barlow, 2005) and are reasonably well understood (Housen and Holsapple, 2011). Target properties (in particular target porosity) have recently emerged as a fundamental material criterion with significant effects on the ejecta velocity distribution (Housen et al., 1999; Thomas et al., 2007; Collins and Wünnemann, 2007, 2009; Housen and Holsapple, 2011, 2012). According to numerical simulations of impacts in highly porous target rocks by Collins and Wünnemann (2007, 2009), cratering

efficiency, volume and velocity of ballistic ejecta dramatically decrease with the increase of porosity. In particular, the excavated volume of ejecta with high velocities sufficient to overcome the crater rim drastically decreases with an increase of the porosity. Housen et al. (1999) and Housen and Holsapple (2003) have shown that large craters can form in porous materials without significant ejecta deposits on terrestrial planets. In porous materials, much of the crater volume forms by permanent compaction of the voids. The energy loss incurred during this process results in ejection velocities so low that much of the ejected material fails to clear the crater rim, and lands short of the crater radius. Energy losses during compaction of pore spaces cause a reduction in ejection speeds, thus forming a crater without an associated ejecta blanket in the usual sense (Housen et al., 1999; Housen and Holsapple, 2003). The key conclusion of a recent study by Housen and Holsapple (2012) is that large craters in highly porous targets can form without ejecta deposits, interpreted as signature features of craters in materials of low or moderate porosity. They precisely state that suppression of ejecta deposits occurs because impacts in highly porous materials primarily drive material downward and outward into the floor of the expanding transient crater. This process causes permanent compaction of pore spaces while much less material is ejected upward than during impacts into materials of higher density. The size of craters produced in porous targets is likely determined by the crushing compaction of the

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material (‘strength regime’), as opposed to gravity (‘gravity regime’). The suppression of out-crater ejecta formation only occurs when craters are formed mostly by compaction, which requires target porosities greater than about 30–40% (Housen and Holsapple, 2012). In the model by Collins and Wünnemann (2009) with given parameters (basalt-basalt impact, 15 km/s impact velocity) the excavated volume of high-speed ejecta (>300 m/s) decreases by 40% in target rocks with 10% porosity, by 60% in target rocks with 20% porosity and by nearly 80% in target rocks with 40% porosity when compared to a dense target rock with a (theoretical) 0% porosity. The target rock porosity of 20% already indicates significant influence on the efficiency of the deposition of ejecta outside the forming crater. Decreasing target rock density of >30–40% results in significant suppression of outcrater ejecta. According to Housen and Holsapple (2012) porous bodies (e.g., rubble pile asteroids, satellites, or planetoids) ranging in size from roughly a few tens to a few hundred km are candidates for the suppression of ejecta because much of the ejecta material is retained within their source craters. Porous bodies smaller than a few km (depending on their porosity) are not expected to produce well-developed ejecta deposits because most crater ejecta escape the weak gravity field. Of the present population of objects for which estimates of size and porosity are known, those larger than a few hundred km in diameter do not appear to have sufficient porosity to suppress ejecta formation (Housen and Holsapple, 2012). Following these arguments, it seems unlikely that terrestrial impact craters can form without ejecta. Housen and Holsapple (2012), however, considered the formation of large craters on small bodies, where the crater samples much of the bulk porosity of the object. Small terrestrial craters (such as Steinheim) only sample the target surface, where areas of high porosity can occur in the form of soils and layers of loose sediments and sedimentary rocks. The porosity of pure sedimentary targets on Earth may under certain circumstances achieve 20–40%. Such highly porous sedimentary targets may be the cause of a drastically reduced accumulation of impact ejecta in the surroundings of the resulting crater.

by the variable proportions of bedded, massive, and reef limestones, and the grade of karstification. However, the Upper Jurassic limestones surrounding the Steinheim area are intensely karstified with deeply penetrating karst caves that already existed at the time of impact (Ufrecht, 2007; Strasser et al., 2009a). The Upper Jurassic limestones known from 5 drillings within a distance of 1.5–6 km from the Steinheim crater rim (Reiff, 2004, p. 209) exhibit a highly differing grade of karstification reaching from nonkarstified to the occurrence of large open karst cavities. In drilling No. 39 (Reiff, 2004; 5 km south of the crater), a total loss of drilling fluid circulation occurred between 148 m and the final drilling depth of 199 m, obviously caused by an open karst cavity (of more than 50 m in horizontal extent) in the ‘‘Felsenkalk Formation’’ limestones. Therefore, an estimated effective porosity of 25–50% of the 165 m-thickportion of karstified Upper Jurassic limestones seems reasonable. The middle Jurassic portion of the Steinheim target rocks comprises mudstones and sandstones; the latter are mainly represented by an ferruginous sandstone, the ‘‘Eisensandstein’’, with a maximum porosity of 45% (Fehn et al., 2010) in southwest Germany, however, the porosity of these deposits presently totals to 25% at the village of Donzdorf, 15 km west of Steinheim (Grimm, 1990). The initial porosity in claystones is extremely variable (Yanga and Aplina, 2010), according to Fehn et al. (2010) it is close to 20% in southwest Germany due to local and regional diagenetic processes. Based on the assumption that carbonate rocks in the Steinheim area were not karstified at the time of the impact, the target rocks had a minimum average porosity of 21% (compare to Table 1). Intensely and deeply karstified Upper Jurassic carbonate rocks in the Steinheim area would imply an increase of the total average (maximum) porosity of the target rocks of up to 44% (Table 1). Due to strong regional variations in massive/bedded and intensely/slightly karstified Upper Jurassic limestones, a total average porosity of 30–40% seems to be reasonable.

7. Porosity of the Steinheim target rocks

The contradiction of a widely preserved Ries ejecta blanket, the similar erosional history of the Ries and the Steinheim area, and the apparent absence of Steinheim ejecta in the periphery of the crater has been described in detail. Paleogene (pre-Ries) iron ooid-bearing claystones (‘‘Bohnerz’’, Fig. 4), Miocene palustrine limestones (Fig. 4) deposited chronologically close to the time of impact and Miocene post-impact sediments (sands and marls, Fig. 4) occur in the surroundings of the Steinheim Basin within a distance of a few kilometers. These remains of sedimentary deposits of Eocene to Miocene age suggest that Steinheim ejecta most probably would have survived if originally deposited in considerable amount. Miocene sands and marls (‘‘Upper Freshwater Molasse’’) are preserved inside the crater, on top of the eastern crater rim as well as on the outer flank of the crater (Fig. 4). These sediments were deposited soon after the impact event and highlight that the Steinheim Basin was rapidly filled after the impact and that the entire Steinheim area represented an environment of sedimentary accumulation rather than a zone of erosion. One can imagine that the ejecta blanket partially survived but may not be observed in the few outcrops that are available in the closer surrounding of the Steinheim Basin. Despite the sparse outcrops inside the crater, fragments and boulders of the basin breccia are omnipresent on the fields, dug up by agricultural activity. The western, southern and eastern portion of the Steinheim Basin periphery was carefully mapped by Reiff (2004) and colleagues, nevertheless, no remnants of proximal ejecta (breccias) were

The Nördlinger Ries impact structure formed in dense crystalline basement rocks overlain by a 600 m-thick suite of sedimentary deposits. In contrast, the Steinheim impactor penetrated approximately 300–350 m of sediments (e.g. Reiff, 2004; Buchner and Schmieder, 2010) that comprise unsolidified Miocene sands, Upper Jurassic limestones and marls, as well as Middle Jurassic mudstones and sandstones (for local denomination of the units see Table 1). The underlying Lower Jurassic and Triassic is locally intensely tectonized (Buchner and Schmieder, 2010). According to Pettijohn et al. (1987), the average porosity of loose fluvial sands totals to 40%; we assume an equivalent value for the 650 m thick Miocene pre-impact cover of loose sands (e.g. von Engelhardt et al., 1987), accordingly. The initial porosity averages of 5–25% for non-karstified limestones and marls and drastically increases to 650% in intensely karstified carbonate rocks (e.g. Domenico and Schwartz, 1998). The 165 m-thick portion of Upper Jurassic limestone and marly target rocks in the Steinheim area are intensely karstified at present (Fig. 7). Strasser et al. (2009a) reported the onset of shallow karstification of the carbonate rocks of the relevant area on the Swabian–Franconian Alb in the Early Miocene and of deeply penetrative karstification in early Middle Miocene times. It is nearly impossible to calculate the effective porosity of the Upper Jurassic limestones in the Steinheim area at the time of impact precisely. A strong variation of local porosity is caused

8. Discussion 8.1. Absence of Steinheim ejecta outside the crater: primary effect versus intense erosion

536 Table 1 Members, stratigraphy, thickness and rock properties of the sedimentary Steinheim target rocks and the calculation of the average porosity of the entire sedimentary suite proportional to the thickness of the stratigraphic members under the assumption of not karstified (minimum porosity) and intensely karstified (maximum porosity) Upper Jurassic limestones. ⁄Intense local variation in carbonate rock porosity is caused by the different proportion of bedded/ massive/reef limestones and the varying grade of karstification. Porosity range from literature

0–50 m

40%1 25–50%2

Porosity in the Steinheim area

Minimum porosity

Maximum porosity

Ø 21%

Ø 44%

40%3

40%

40%

References

Stratigraphy Sedimentological properties

Obere Süßwassermolasse (Upper Freshwater Molasse)

Middle Miocene

Loose quartz-rich sands and marls

‘Untere Felsenkalke’

Upper Jurassic d

Massive limestones; intensely karstified at present

30 m

5–25% (not karstified)1 6 50% (karstified)1

Not available⁄

15% (not karstified)

650% (intensely karstified)

‘Lacunosamergel’

Upper Jurassic c

Bedded limestones (and reef limestones) with some marly limestones, marls; intensely karstified at present

45 m

5–25% (n.k.)1 6 50% (k.)1

n/a⁄

15% (n.k.)

650% (i.k.)

1

‘Wohlgeschichtete Kalke’

Upper Jurassic b

Bedded limestones; intensely karstified at present

20 m

5–25% (n.k.)1 6 50% (k.)1

n/a⁄

15% (n.k.)

650% (i.k.)

1

‘Impressamergel’

Upper Jurassic a

Marly limestones, marls, bedded limestones, partly karstified at present

70 m

5–25% (n.k.)1 6 40% (k.)1

n/a⁄

15% (n.k.)

640% (i.k.)

1

‘Ornatenton’ and sandstones

Middle Jurassic c-f

Mudstones and sandstones

25–30 m

20–45%1

25–35%2

25%

35%

1

‘Eisensandstein’

Middle Jurassic b

Iron-rich sandstones

30–40 m

15–40.5%1 5–45%2 25–45%3

25%

45%

1

‘Opalinuston’

Middle Jurassic a

Gray mudstones

19–31%1 15–25%2 120 m (only partly penetrated)

20%

30%

1

1

Manger (1963), Beard and Weyl (1973), and Pettijohn et al. (1987); Freeze and Cherry 1979); 3von Engelhardt et al. (1987), Maurer and Buchner (2007), and Hüttner and Schmidt-Kaler (1999)

2

20–30%3

1

Freeze and Cherry (1979), Domenico and Schwartz (1998), Reiff (2004), and Binder and Jantschke (2003) Freeze and Cherry (1979), Domenico and Schwartz (1998), Groiss et al. (2000), Binder and Jantschke (2003), and Reiff (2004)

Freeze and Cherry (1979), Domenico and Schwartz (1998), Groiss et al. (2000), and Binder and Jantschke (2003) Freeze and Cherry (1979), Domenico and Schwartz (1998), Groiss et al. (2000), and Binder and Jantschke (2003) Freeze and Cherry (1979); 2Reiff (2004) and Fehn et al. (2010)

Reinhold et al. (2011); 2Fehn et al. (2010); 3Frank et al. (1975) and Grimm (1990)

Manger (1963); 2Bossart and Thury (2007); 3Heitzmann and Bossart (2001) and Fehn et al. (2010)

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Thickness in the Steinheim area

Local denomination of formations/ members

E. Buchner, M. Schmieder / Icarus 250 (2015) 529–543

537

Fig. 7. (A and B) Limestone quarry in active operation near the village of Söhnstetten about 2 km west of the Steinheim Basin showing intensely karstified limestones in the deepest part of the approximately 70–80 m deep quarry. Arrow in A marks the position of the section shown in B: this fresh outcrop shows intensely karstified bedded limestones with small open karst cavities and an extremely high overall porosity. (C and D) Limestone quarry in active operation near the village of Bartholomä about 6 km northwest of the Steinheim Basin showing intensely karstified massive reef limestones. The big karst cavities in this fresh quarry wall are either filled by mud (above the excavator) or open and can contain big limestone blocks (arrow). Deep penetrative karstification probably took place prior to the Ries–Steinheim impact event in Early Miocene to early Middle Miocene.

discovered on the fields (Fig. 4). Access and exposure are rather limited in the wooded area north of the Steinheim Basin. Five drillings are described by Reiff (2004) within a distance of 1.5–6 km from the crater rim. These drillings certainly represent just a few additional pinpricks in the search for ejecta outside the crater, but none of these drill cores contained Steinheim ejecta. The northern rim of the sedimentary environment of the North Alpine Foreland Basin is situated within a distance of approximately 6 km south of the Steinheim crater; this foreland basin comprises a continuous suite of clastic deposits of Oligocene to Upper Miocene Molasse sediments (Fig. 4). Conspicuous palustrine oncoids deposited chronologically close to the impact event, along with post-impact sandy Molasse deposits, indicate that the regional sedimentary succession stratigraphically brackets the time of impact; however, no medial or distal ejecta horizon has so far been detected within these deposits. In contrast, distal Ries ejecta components (Brockhorizonte) are embedded within fluvial and lacustrine Middle Miocene sediments deposited synchronous with the Ries impact (e.g., Buchner et al., 2007; Alwmark et al., 2012). As emphasized by several authors (Collins and Wünnemann, 2007, 2009; Wünnemann and Collins, 2009; Housen et al., 1999; Housen and Holsapple, 2003, 2011, 2012), target porosity has a significant influence on the cratering efficiency as well as on the volume and distribution of excavated ballistic impact ejecta. Collins and Wünnemann (2007) generally stated that (for fixed impact parameters) the total ejected mass is lower for impacts in targets with high porosity because the compaction of target rocks dominates the excavation with increasing target rock porosity (Collins and Wünnemann, 2007). In particular, the excavated volume of ejecta with high velocities sufficient to overcome the crater rim drastically drops with an increase in porosity. In porous materials, much of the crater volume forms by permanent compaction of the target rocks. Energy loss incurred during this process results in ejection velocities so low that much of the ejected material fails to clear the crater rim and lands short of the crater radius

(Housen and Holsapple, 2003). The (partial or total) suppression of ballistic ejecta outside the impact structure occurs when craters are formed mostly by compaction, which requires target porosities greater than about 30–40% (Housen and Holsapple, 2012). The size of the crater and the volume and distribution of ejecta produced is determined by porosity (and the crushing strength) of the material, not by gravity. On terrestrial planets, target rocks are not thought to have sufficient porosity to suppress ejecta deposits (Housen and Holsapple, 2012). As discussed above, the average porosity of the Steinheim target rocks may range between a minimum of 21% and a maximum of 44% (Table 1). Under the assumption of intensely karstified limestones with open karst cavities in the Upper Jurassic target rocks that represent the main portion of the Steinheim target rocks, the total target rock porosity may tend towards the upper limit and possibly ranges between 30% and 40%. Strasser et al. (2009a) reported the onset of karstification of Upper Jurassic deposit on the Swabian–Franconian Alb in southern Germany in Early Miocene, followed by intense and penetrative karstification in early Middle Miocene times (Fig. 7). Absolute dating of karst cave deposits (Ufrecht et al., 2002; Ufrecht, 2007) comprising magnetic spherules that may originate from the Ries–Steinheim impact event (Strasser et al., 2009b; Schmieder and Buchner, 2009) and Timagnetite crystals from the Miocene (16–14 Ma) Urach volcanic field in southern Germany (Kröchert et al., 2009a, 2009b) strongly suggest intense pre-Ries–Steinheim karstification and the existence of various voluminous karst caves in the periphery of the Steinheim Basin at the time of impact. Following the arguments given by Collins and Wünnemann, 2007, 2009; Housen et al., 1999; Housen and Holsapple, 2003, 2011, 2012, the average target rock porosity of 30–40% is not sufficient to totally suppress impact ejecta but would be capable of reducing the amount of ballistic impact ejecta outside the impact structure considerably. The importance of compaction cratering and formation of an ejecta blanket hinges, to some degree, on the presence or lack of water at the time of impact. Buchner and Schmieder (2013a) reported a

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water-saturated, lacustrine–palustrine phreatic impact scenario for the Ries and Steinheim impact event. On the other hand, the intense pre-Ries–Steinheim karstification of Upper Jurassic limestones described by Strasser et al. (2009a, 2009b), particularly in the Steinheim area, implies multiple temporary phases of water table drawdown in the Steinheim area. Deep karst caves within the Upper Jurassic of the Steinheim area provide a unique gauge for crustal tilting and isostatic uplift of the southwest German crust in late Mid-Miocene. Horizontal levels are considered a product of a stationary palaeo-water table; many vertical shafts are related to a base-level fall, which in turn is the result of uplift and/or incision. The uplift of the western part of the Swabian–Franconian Alb (Steinheim Basin) prevailed the uplift of the eastern part (Nördlinger Ries) which is supported by the periodic sedimentation of coarse clastic deposits (‘‘Juranagelfluh’’) mainly in the western part of Swabian–Franconian Alb (Strasser et al., 2009a; Seyfried et al., 2010). This could have been responsible for constant water saturation of the Ries area but temporary water table drawdown in the Steinheim area. A significant influence of water saturation or the temporary lack of groundwater in the target rocks on the rate of compaction of porous target rocks and suppression of impact ejecta can be assumed (e.g., Buhl et al., 2013) but we are not aware of any detailed crater-scale investigations on this effect. The degree of water-saturation in the target rocks of the Steinheim Basin and the influence of this parameter on the suppression of impact ejecta in our investigation unfortunately remains unclear. As a result, we interpret the apparent absence of Steinheim ejecta presently observed in the surroundings of the crater as a possible, if not likely, primary effect caused by the compression of the porous target rock and a concomitant significant reduction of ballistically transported ejecta outside the fresh crater; a likely resulting thin ejecta blanket was prone to erosion. A possible way to test our model is the examination of the Bouguer gravity anomaly over the Steinheim crater, carried out by Ernstson (1984). If compaction of the target rocks did take place, then the rocks beneath the crater, being compacted by the impact, should be denser than the surrounding rocks, possibly dense enough to compensate for the low-density signature of the breccia lens inside the crater (e.g., Pilkington and Grieve, 1992). Ernstson (1984) detected a 2 mGal anomaly in the center of the Steinheim Basin that is rather small compared to its craters size. While the gravity anomalies of heavily eroded impact structures are negligible, well-preserved impact craters in the size range of the Steinheim Basin would typically exhibit gravity anomalies on the order of 5 to 6 mGal, e.g. 5 mGal at the 3.8 km Brent crater, Canada, and 6 mGal at the 3.44 km New Quebec crater, Canada, and may even reach 9.3 mGal in the case of the 2.5 km Roter Kamm crater, Namibia (for a compilation of gravity anomalies of terrestrial impact craters see Pilkington and Grieve, 1992). According to Ernstson (1984), ‘‘a ring of small, relative positive anomalies surrounds the central anomaly, and is again followed by a peripheral zone of negative anomalies.’’ Furthermore, little is known about the consequences of gravitational shock compaction on the Bouguer gravity anomaly at natural impact craters. According to Ormö et al. (2002), the compaction of the target rocks will cause a significant volume reduction visible in the local topography, but the volume of compacted (denser) material will be relatively small compared to the volume of fractured rocks around the crater. Whereas the Steinheim gravity model of Ernstson (1984) does not provide definitive results, it does not question the proposed model of target rock compaction beneath the Steinheim Basin impact crater. According to Pierazzo et al. (2001), the excavation depth of the transient Ries crater approached 2 km. The excavated target rocks in the Ries area comprise 250 m of porous sedimentary rocks, 350 m of more dense sediments and 1400 m of dense crystalline rocks. In contrast to the Steinheim Basin, the effects of target rock

compression and ejecta suppression during the Ries impact can be neglected due to the much lower total porosity of the mixed-type Ries target rocks (sediments and crystalline basement). 8.2. Steinheim Basin breccia: fall-out versus turbulent intermixture process Reiff (1977, 2004) and Heizmann and Reiff (2002) interpreted the Steinheim crater fill as a fall-back breccia (‘‘Rückfallbrekzie’’), ejected from the present central uplift area and re-deposited from a collapsing melt-free ejecta plume (Heizmann and Reiff, 2002, their Fig. 84). In the Steinheim breccia, we did not detect any sedimentary evidence for ballistically transported and re-deposited ejecta over the entire (maximum) thickness of 55 m as quantitative or qualitative sorting or grading. Buchner and Schmieder (2010) described tiny, sub-rounded, fluidally textured, and altered melt particles that are consistently distributed within the basin breccia. These mm-sized melt particles were detected in basin breccia samples of drill cores, for instance in samples of drill core B-26 (No. 17 in Reiff, 2004) taken from the lowermost section (depth 76–77 m), from the middle portion (depth 48–49 m and depth 40–41 m), and from the top of the impact breccia (depth 34–35 m; see Fig. 5); for a detailed description of this drill core and the melt particles see Buchner and Schmieder (2010, 2013b) and Anders et al. (2013). Although the collapse of an ejecta plume does not necessarily cause sorting or grading processes, Stöffler et al. (2013) described a 50 cm-thick layer of graded ‘‘primary suevite’’ deposited from the collapse of a primary ejecta plume of the Ries impact event. Gravitationally driven sorting and grading processes are furthermore known from volcanic ignimbrites and fallout deposits that are usually covered by fine-grained volcanic ashes (e.g., Schminke, 2005; Fagents et al., 2013). Similar sorting processes should be expected for the collapse of the proposed clast-rich and melt particle-bearing Steinheim ejecta plume, leading to the relative enrichment of the ash-like particles at the top of the basin breccia. However, the melt particles occur persistently in the entire breccia body suggesting a near-ground turbulent intermixture of impact melt across the whole breccia lens. With the exception of the tiny impact melt particles, we did not detect any rock fragments in the basin breccia that show criteria for intense and obvious thermal overprint. In contrast, a 5 cm-thick layer on top the basin breccia obviously differs by its optical properties. Buchner and Schmieder (2010, 2013b) interpreted this top layer as ‘‘truly airborne’’ fallback on the basis of the content of sheared sandstone fragments, flattened melt particles, carbonate clasts with pale, devolatized rims and an overall flow texture that rather suggest the influence of high temperatures and dynamics, and a primary origin as an ‘‘welded ignimbritic’’ or ‘‘surge’’-like impactite unit probably linked to an ejecta plume that collapsed and caused ‘‘pyroclastic-like’’ flows (compare Newsom et al., 1986). Limestone clasts exhibit devolatilization and/or fritting phenomena when heated above 650 °C for a few seconds (Buchner et al., 2007) and thermal decomposition above 1200 °C, thus, providing an upper and lower limit for the temperature in the ejecta plume. Likewise, a (probably) analogous 30 cmthick top layer on the (suevitic) crater breccia of the Bosumtwi impact structure (Ghana) comprises microtektite-like glass spherules also interpreted as the terminal, fine-grained impact fallback layer by Koeberl et al. (2007) and Losiak et al. (2013); the presence of such top layers indicate the preservation of the complete impactite sequence inside these well-preserved impact structures. 8.3. The Steinheim Basin impact model Theoretical cross sections through the transient cavity and the resulting simple or complex impact craters (e.g., Melosh, 1989;

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Turtle et al., 2005) can certainly express the structural and depositional conditions only schematically and incompletely. Osinski et al. (2011) recently presented the formation of impact ejecta and crater fill deposits in an updated and more detailed multistage model (their Fig. 7). The structural and depositional properties of the Steinheim Basin impact structure, however, do not match any of the impact models currently available. The most obvious discrepancies between these models and the observations in the Steinheim Basin refer to (a) the significant reduction of ballistic ejecta outside the crater rim, (b) the melting zone and (c) the appearance and distribution of the impact melt. (a) The absence of ejecta outside the crater has been discussed in detail and interpreted as an effect of energy loss during the compression of pore spaces in the highly porous target rocks associated with a drastic reduction of the mass of ejected material. (b) According to the impact model by Melosh (1989), the formation of impact melt takes place in a hemispherical melting zone intercalated between the inner zone of vaporization and an outer zone of excavated and displaced material, resulting in impact melt that chemically comprises a mixture of all target rocks affected by the impact. According to von Engelhardt (1972, 1997), the chemistry of the Nördlinger Ries suevite glasses overall matches the composition of the entire suite of sedimentary and crystalline basement target rocks, within a range of compositions (Osinski, 2003). The impact melt in the Steinheim Basin breccia chemically reflects the composition of the lowermost part of the target rocks (Middle Jurassic sandstones and claystones; compare Fig. 8A) and a minor admixture of a limestone component (Buchner and Schmieder, 2010, 2013b; Anders et al. 2013). This is possibly also attributed to the effect of energy losses incurred during compression of the highly porous upper part of the target rocks (Miocene sands and Upper Jurassic carbonate rocks) and a high-energy melt production only in the lowermost, less porous, portion of the transient crater (Fig. 8B). Higher porosity in the upper part of the target rocks, furthermore, means less mass to be melted, so that could also reduce the presence of this portion of the target rocks in the impact melt. (c) In the crater fill deposits of simple impact craters, melt-rich breccias are commonly concentrated in lenses (e.g. Grieve, 1987) as a result of the backflow of minor melt flows towards the crater center (Osinski et al., 2011). In complex craters, melt particles of considerable size are randomly distributed within the intermixed clasts and minerals of the crater suevite (e.g. von Engelhardt, 1997; Losiak et al., 2013). According to the recently postulated model for the genesis of the Ries suevite by Stöffler et al. (2013), the main mass of suevite was deposited from a ‘‘secondary plume’’ induced by an explosive ‘‘fuel–coolant’’ reaction of impact melt with water and volatile-rich sedimentary rocks within a clastladen temporary melt pool. Melt-rich impactites emplaced by flows downslope the emergent central uplift und continued movement of melt and clasts towards the crater rims overlay melt-poor impactites that were emplaced as melt-rich lenses inside and outside the impact structure (Osinski et al., 2011, their Fig. 7). The tiny melt particles in the Steinheim Basin were accelerated during the rise of the emergent central uplift and effectively mixed with the existing ejecta inside the crater (Fig. 8C). Quantitative and qualitative sorting obviously did not occur with the exception of finegrained and melt-rich material deposited as a thin layer on top of the generally poorly sorted basin breccia during the collapse of a relatively minor ejecta plume (Fig. 8D). 8.4. Implications for terrestrial impact structures in porous sedimentary targets Earth-like large planetary bodies, whose porosities are low due to gravitational compression, are expected to exhibit craters with ejecta blankets. If our interpretation is correct that Steinheim

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impact ejecta were significantly reduced by the compression of porous target rocks, the partial or complete suppression of impact ejecta during terrestrial impacts may in fact be a more widespread effect. However, this requires mean target porosities greater than about 30–40%, exclusively given in sedimentary rocks, regoliths and soils. During big terrestrial impact events in dual-layer targets, the impactor penetrates the sedimentary cover and the underlying, denser crystalline basement reducing the mean target porosity drastically. Therefore, the reduction or total suppression of ejecta probably occurs only in small terrestrial impact craters of a few kilometers in diameter. The 1.2 km in diameter Barringer crater in Arizona, USA, formed in a succession of porous sandstones, siltstones, and dolomites and is surrounded by a significant amount of out-crater ejecta (Kring, 2007). The unshocked Coconino sandstone samples, for instance, yield a porosity between <10% and 25%, dolomites and sandstones of the Kaibab Formation vary between 2% and 30% and the porosity of the Moenkopi Formation deposits range from 7.5% to 18.2% (Kring, 2007). This target rock porosity may influence the propagation of a shock wave and shock-metamorphic effects but is too low to inhibit the formation of out-crater ejecta. Thick sedimentary successions of unconsolidated sands, breccias, and conglomerates given in thick alluvial fans, for instance, as well as intensely karstified carbonate rocks are good candidates for highly porous target rocks. Schmieder et al. (2013) suggested the Uneged Uul structure, a 10 km circular, complex, multiridged domal feature the East Gobi Basin, southeastern Mongolia, as being a promising possible impact structure. However, no conclusive evidence of shock metamorphism was reported so far for the Uneged Uul structure and no ejecta in or outside the structure were encountered in the field. The loose nature of coarse-grained pebble and boulder conglomerates as the main ‘host layer’ of the Uneged Uul structure, i.e., a highly porous lithological unit at the time of deformation, might account for dramatic shock pressure attenuation within the target and suppression of impact ejecta in an assumed impact scenario (Schmieder et al. 2013). Similar effects have been observed at the 12 km Marquez impact structure, Texas, USA, that was generated by impact into unconsolidated sands of the Paleocene coastal plain, which accounts for weak shock effects in the rocks from this structure (Buchanan et al., 1998). Shock-buffering soft-sediment deformation was also noted at Tin Bider, Algeria (Lambert et al., 1981). Compaction of porous target rocks in impact structures (accompanied by a possible reduction of the ejecta velocity) is known from many terrestrial impact structures. Shear and compaction bands have been reported from shocked sandstone target rocks of the Upheaval Dome, Utah, USA, by Buchner and Kenkmann (2008) and by Crósta et al. (2012) from sandstones of the Vargeão Dome, southern Brazil. Buchner and Kenkmann (2008) explained the occurrence of some single shocked quartz grains in sandstones of the Upheaval Dome impact structure by local pressure excursions formed by a shock-induced collapse of pore space. Kowitz et al. (2013) explained this effect by effective pore collapse that can locally generate shock pressures up to >4 times higher than the initial pressure, particularly in highly porous, dry, sandstones. However, the impact structures mentioned are deeply eroded and the ejecta blankets are not preserved if originally deposited. Hence, it is often impossible to recognize whether the formation of impact ejecta was originally suppressed or whether ejecta were in fact deposited but eroded later. Highly porous basaltic target rocks are further candidates for impact structures without significant out-crater ejecta layers. According to Saar and Manga (1999), the total porosity of basalt (in lava flows) ranges between 0% and 30%, but vesicular basaltic rocks may reach a total porosity of 40–50% in lava flows and 50–80% in basaltic scoria. One of the few terrestrial impact

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Fig. 8. Impact model for the Steinheim Basin impact structure; crater modification processes at the crater rims are not considered in the cross sections; A is not in the same scale as B–D. (A) Excavation stage, the diameter (d) of transient crater is 1–2 km; the penetration depth is 300–350 m, the depth of the transient cavity (tc) is up to 1200 m (Reiff 2004); dotted line: limit of transient cavity. (B) Early stage of crater modification; tiny, ash-like melt particles and the clasts of impact ejecta were effectively mixed; only few ejecta escaped outside the crater and a small ejecta plume developed. (C) Late stage of crater modification. (D) Crater forming processes are terminated; Ls: limestones; Ss: sandstones; Cs: claystones; bb: basin breccia. The model was developed on the basis of data given by Mattmüller (1994), Heizmann and Reiff (2002), Reiff (2004), and Buchner and Schmieder (2010, 2013b).

structures in basaltic target rocks, the 570 kyr-old Lonar crater in India (Maloof et al., 2010; Jourdan et al., 2011) with a diameter of less than 2 km exhibits a distinct ejecta blanket. Consistently, porosity measurements of unshocked basalt samples of the Lonar area yielded a very low total porosity of 3–4%. For the limited size of the crater structure and the particular properties of the target rocks, environmental conditions for the formation of terrestrial impact craters without a significant amount of out-crater ejecta in highly porous target rocks on Earth are very limited. 8.5. Implications for impact structures on extraterrestrial bodies Suppression of ejecta is controlled by diameter and bulk porosity of the impacted body as well as of the crater size. Large craters on asteroid Mathilde (diameter 50 km, porosity 50%) and Saturn’s moon Hyperion (diameter 120 km, porosity > 40%) apparently formed without producing significant ejecta deposits (Housen and Holsapple, 2012). Bodies with diameters less than roughly 10–30 km are too small to retain much ejecta in their craters. Hence, it is unclear whether the suppression of ejecta production occurs on smaller bodies or not. Those bodies with diameters larger than a few hundred km are possibly not sufficiently porous to allow for compaction cratering to occur. On small bodies, impact craters would show ejecta suppression only for the largest craters. Planetary bodies with higher bulk porosity and/or a larger diameter would exhibit suppression for a wider range of crater sizes. As a basic principle, suppression or drastic reduction of ejecta in impact

craters on an Earth-sized planetary body cannot be expected generally. Under certain environmental conditions, however, such as those in the target rocks of the Steinheim area, target porosities may exceed 30–40%, the threshold value required for the suppression of out-crater ejecta formation. Regolith breccias on the Moon yielded porosities of up to 24%, while porosities for Moon basalts are less than 10%, with the exception of a highly vesicular basalt showing unusually high porosity of 26% bulk density (Macke et al., 2012). The Moon, thus, does not seem to be a good candidate for the search of impact craters without ejecta blankets. The porosity of Mars regolith of 54% (Mars-1, Allen et al., 1997) and 60 ± 15% (Viking, Allen et al., 1997), respectively, is significantly higher. There is very little information on the bulk porosity of martian basalts in the literature. According to Zimelman (1986), the low thermal inertia of Tharsis Montes shield volcanoes have been interpreted to result from either highly vesicular lavas (porosity >80%) and a very small average particle size. Brozˇ and Hauber (2011) highlighted a volcanic field in Tharsis that exhibits martian equivalents of terrestrial cinder cones and a thick suite of highly porous scoria and lava flows. Hoges and Moore (1994) interpreted cavities in volcanic rocks of Mars (and other planetary bodies) as subsurface lava channels. On the whole, Mars as a big planetary body with diameter larger than a few hundred km could be regarded as a good candidate for the existence of hundreds of meters thick, highly porous target rocks (porosity >40%) and the formation of impact craters without significant ejecta outside their source crater.

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9. Results 1. The transfer of up to 2 crater radii for a continuous ejecta blanket, and on the order of 2–3 crater diameters for distal ejecta can be considered a ‘typical’ case for terrestrial impact structures. From this point of view, proximal (and medial) Steinheim ejecta could be expected within a distance of up to 4 km, distal ejecta within a distance of up to 12 km from the crater rim. 2. In contrast to the enormous amount of continuous proximal impact ejecta in the surroundings of the Nördlinger Ries crater, impact ejecta outside the Steinheim Basin impact structure have never been discovered. The Nördlinger Ries and Steinheim Basin are thought to represent a 14.8 Ma double impact system and are located in an environmental setting that underwent the same erosional history suggesting that impact ejecta in the Steinheim area potentially should have been preserved. 3. Remains of Miocene fluvial and lacustrine sediments, as well as conspicuous pisolithic freshwater limestones in the surroundings of the Steinheim Basin, presently crop out within a distance of 1–12 km from the southern and eastern Steinheim crater rim in a range where Steinheim Basin (proximal and distal) impact ejecta could be expected theoretically. These sediments were presumably deposited prior to, isochronous with, and shortly after the impact and have survived erosion till today. 4. Cratering efficiency, volume and velocity of ballistic ejecta dramatically decrease with the increase of porosity of the target rocks affected by the impact. In particular, the excavated volume of ejecta with high velocities sufficient to overcome the crater rim is drastically reduced with the increasing porosity. Complete inhibition of ejecta formation only occurs when craters are formed mostly by compaction, which requires target porosities greater than about 30–40%. 5. The average porosity of the sedimentary Steinheim target rocks range between a minimum of 21% and a maximum of 44%; it may have reached 20–30% assuming the Upper Jurassic limestones in the Steinheim area were intensely karstified and even higher porosity values assuming the additional existence of open karst cavities in the limestone target rocks. 6. The apparent absence of Steinheim ejecta presently observed in the surroundings of the crater could be interpreted as a primary effect of compression of the porous target rock during the impact event and a concomitant significant reduction (incomplete suppression) of ballistically transported out-crater ejecta; the resulting thin ejecta blanket was most likely eroded away. However, we cannot quantify the degree of ejecta reduction exactly due to the mentioned uncertainties in the average target rock porosity. 7. If our interpretation is correct that Steinheim Basin impact ejecta was significantly reduced by compression of target rocks, partial or complete suppression of impact ejecta during terrestrial impacts may occur generally. For the particular properties of the target rocks, the environmental conditions for the formation of terrestrial impact craters without significant impact ejecta in highly porous target rocks on Earth are very limited. The Steinheim Basin might represent a very rare example of a well-preserved terrestrial complex impact crater with significantly reduced ejecta blanket.

Acknowledgments The authors are grateful for very helpful comments and suggestions of the reviewers Jay Melosh (Purdue University, Indiana, USA) and Kevin Housen (The Boeing Company, Chicago, USA) that helped to improve the manuscript. We kindly thank Jens Ormö

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(Centro de Astrobiología, Madrid, Spain) for fruitful discussion and further helpful suggestions. E.B. kindly acknowledges a grant by the Stifterverband für die Deutsche Wissenschaft.

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