Discriminating between impact or nonimpact origin of small meteorite crater candidates: No evidence for an impact origin for the Tor crater, Sweden
Abstract
Compared to intensive research on km-sized meteorite impact craters, fewer studies focus on smaller craters. The small craters are often hard or impossible to recognize using “classical” criteria like the presence of shatter cones, shocked quartz, and geochemical indicators. Therefore, a long list of candidate structures awaiting approval/disapproval of their origin has been formed over the last decades. One of them is the Tor structure in central Sweden. To test a hypothesis of an impact origin of this structure, we have performed topographical analysis, geophysical studies, 10Be exposure dating of boulders, and 14C dating of Tor-associated charcoal. None of the methods gave us a reason to claim the Tor structure is of impact origin. Thus, we support a recently suggested idea of Tor being formed by a grounded iceberg within a glacial lake.
Introduction
Only ~200 confirmed terrestrial impact craters are currently known (Osinski & Grieve, 2019; Schmieder & Kring, 2020). Numerous new impact sites are proposed yearly, often based only on remote sensing/geophysical data (Gonzalez & Alonson, 2006; Isachsen et al., 1994; MacGregor et al., 2019; Master, 2002; Paillou et al., 2004; Paris et al., 2018). For them to be accepted as an impact craters, they need to be evaluated against commonly accepted impact indicators (French & Koeberl, 2010) related to the detection of signs of impact metamorphism (e.g., presence of planar deformation features; Ferrière & Osinski, 2013; Stöffler & Langenhorst, 1994) or geochemical signatures of extraterrestrial material (pieces of meteorites or their geochemical or/and isotopic tracers). The evaluation process is time consuming and requires the usage of expensive equipment by researchers experienced in the impact cratering process; as a result, hundreds of suspected impact structures wait for an evaluation (Hergarten & Kenkmann, 2015; Lyapidevskaya & Gusiakov, 2010).
Identifying impact sites on Earth is important because it allows us to ascertain the current impact rate (Bland & Artemieva, 2006). This can help us to better estimate the probability of encountering an incoming asteroid such as the explosion of the Chelyabinsk asteroid in 2013 over Russia (Popova & the Chelyabinsk Airburst Consortium, 2013) that injured >1500 people. It is also crucial to understand the geologic history of our planet where past impacts have influenced the development of life on Earth (Schulte et al., 2010), the formation of geological features (which often have economic importance; Reimold et al., 2005), and arguably even affected human culture (Hamacher & Goldsmith, 2013).
One such structure awaiting evaluation as being formed by an impact is that of the proposed Tor structure in the Ljusnan valley in Sweden (62°30′6″N; 12°37′54″E). The structure is 39–44 m in diameter and 4.8 m deep (Henkel et al., 1996). Determining its origin is of additional importance because local tourist information advertises it as an impact crater. For example, in mid-July 2018, this site was visited by ~20 people a day, despite the lack of data supporting this origin, yet a set of informational boards are provided stating its impact origin (Fig. 1a). This study aims to determine the origin of the Tor structure by means of direct geophysical and geological methods and provide a blueprint for determining the origin of similar structures in the future.
Geology
The Tor structure is located within the NW-SE-trending Ljusnan river valley, whose floor is mainly covered by silty till with regular to high boulder frequency (Lundqvist, 1969b). The dominating landforms are hummocky and ribbed moraine; at the site of the Tor structure, there are subdued ribbed moraine ridges. The last ice movement direction in the valley was from the SE or ESE, and as the ice margin retreated eastward during the last deglaciation, the Ljusnan glacial lakes were dammed in the valley (Hughes et al., 2016; Lundqvist, 1969a; Öhrling et al., 2021; Regnéll et al., 2019; Stroeven et al., 2016).
The impact origin of the Tor structure was first proposed by Lundqvist (1969a), who was later supported by Henkel and Pesonen (1992) and Henkel et al. (1996). This conclusion was mainly based on the crater-like shape of the structure. Recently, Öhrling et al. (2021) studied Tor using remote-sensing methods. Based on modern LiDAR-derived digital elevation models, they demonstrated the occurrence of numerous similar-to-Tor topographic semi/circular features in the area. They claimed that these represent iceberg imprints formed during deglaciation at about 11,000–10,000 cal. yr bp. Also identified were several shorelines leading to the conclusion that among other similar topographic features, the Tor pit was formed when the ice-dammed lake was lowered and caused an iceberg to ground into water-saturated till generating a so-called “iceberg crater” (cf. Brown et al. [2017] and references therein).
Methods
We selected a range of methods to look for evidence of an impact origin. These included geophysical studies to assess the subsurface geology (drift and basement) structure and thickness of the sedimentary units. This was then assessed in the field by digging two trenches to reveal the geological deposit and allow the collecting of organic material (charcoal) for 14C dating of the potential sediment age, along with 10Be dating. Together, these methods allowed us to determine whether the sedimentary structures and deposits are consistent with those generated by small impacts, for example, ejecta deposits and overturned stratigraphy. We note that the crater size would be too small to expect the observation of shock metamorphic features or shatter cones (Baratoux & Reimold, 2016).
Digital Elevation Model
The local digital elevation model (DEM) was created based on drone photos using a photogrammetry method. MavicPro covered about 400 × 500 m at 75–90 m height, resulting in 139 aerial images. Vertical photos were taken at about 20 m spacing (2 s interval) with ~100 m distance between flight lines. Another set and a detailed structure model were compiled based on 582 photos taken at a height from a few meters to a few tens of meters. Both vertical and slant photos were taken to get good coverage below trees at the structural rim area. Photos were processed with Agisoft Metashape software. Processing flow included alignment of images (sparse point cloud); creation of dense point cloud; classification of points and selection of ground points; and generation of mesh, DEM (pixel size 5 cm), and orthomosaic (pixel size 2 cm). The alignment of photos was enhanced by measuring a set of marker points with Real Time Kinematic (RTK) in respect to a temporary base station whose height was later adjusted according to the Geografiska Sverigedata (GSD) Elevation data grid (2 m resolution) from Lantmäteriet (SGU, 2020).
Geophysical Studies
A ground penetrating radar (GPR) survey along several profiles within and around the Tor feature (Fig. 1b) was conducted with a common offset co-polarized 300 MHz center frequency sledge-mounted shielded antenna (Zond 12e). The system was pulled at walking speed, and distances were measured using an odometer wheel attached to the sledge's rear. We used a trace interval of 5 cm and a time window of 300 ns. The profiles were positioned with a portable Global Positioning System (GPS) unit (AltinaGGM309; position accuracy 5–25 m) connected to the radar equipment. The data were post-processed and topographically corrected using Prism2 software. A signal saturation correction (band-pass filter) was applied to the recorded data to remove low-frequency (<100 MHz) induction effects. Relief heights were interpolated for every trace from the GSD-Elevation data grid (2 m resolution) from Lantmäteriet (SGU, 2020). Electromagnetic wave velocities in soil were analyzed by fitting hyperbolas to point source reflections. Interpretation of the GPR data included isolation of two radar facies (RF): overburden and crystalline basement. Based on the delineation, maps of overburden thickness and height of the crystalline basement top surface were produced. The natural neighbor routine (QGIS version 2.18) was applied while interpolating the data.
Geoelectrical data along the electric resistivity tomography (ERT) profile (Fig. 1b) were collected using a POLARES resistivity meter (P.A.S.I. srl). The length of the ERT profile was 95 m. The survey was conducted with a Wenner array at a frequency of 7.2 Hz. Sixty-four steel electrodes with a constant spacing of 1 m were used simultaneously, alternating two current and two potential electrodes and a roll-along survey by 48-electrode overlap. Such a technique provided a maximum exploration depth close to 15 m. A GPS device coordinated every 16th electrode. Topography was deduced from the 2 m GSD-Elevation model and was applied in the processing. After removing any erroneous data points and averaging repeated measurements, the apparent electrical resistivity data were tomographically inverted into the “true” electrical resistivity distributions using the RES2DINV software package (Loke & Barker, 1996). The software ran the optimization that adjusted the 2-D electrical resistivity model by reducing the difference between the calculated and measured apparent electrical resistivity values iteratively. Least-squares inversion (L1 norm) was applied identically to all the data during the five iterations. The absolute root-mean-square error, providing a measure of convergence between measured and calculated data and, thus, indicating the reliability of the final result, was 2.5%.
Ground magnetic measurements were carried out with two mobile independently working time-synchronized Geometrics G-856 proton precession magnetometers. The measurements that covered the Tor structure and its nearest surroundings from all sides (Fig. 1b) were performed every 1 m along 51 west–east striking 100 m long profiles with the help of measuring tapes. Distance between the profiles was set to 2 m; thus, 100 times of 100 m grid resulted in 5050 individual measurements. Every profile was started and ended with a measure in a control location setup at the SW rim of the structure. During the field period (July 13 and 14, 2018), the magnetic field varied by 35 nT.
Magnetic susceptibility of glacial till and boulders was measured with a handheld susceptibility meter SM-30 (ZHinstruments). We used these data to hint at the physical properties of defined bodies introduced in the forward modeling of the magnetic field. The modeling was performed along S–N- and W–E-oriented profiles with the software Potent v4.16.07 by Geophysical Software Solutions. Models include one 2.5-D body (rectangular prism) to simulate a layer of glacially formed overburden, which is relatively more magnetic than the crystalline basement below.
10Be Exposure Dating
Samples for exposure dating of four boulders on the Tor structure were taken with a hammer and chisel during fieldwork in 2006 (Figs. 1b and 2). After initial preparation (crushing), the samples were sent to PRIME Lab at Purdue University, United States, for analysis. Further preparation at PRIME Lab followed a modified version of the preparations described in Kohl and Nishiizumi (1992) for quartz separation and followed by Be carrier addition, ion exchange chromatography, and conversion to beryllium oxide before AMS measurement.
The concentration of 10Be atoms was converted into exposure ages in version 3 of the online age calculator, formerly known as CRONUS-Earth online calculators (Balco, 2017). The global 10Be production rate and the LSDn scaling scheme (Lifton et al., 2014) were applied as that has given good agreement for known Younger Dryas moraines in Norway (Regnéll et al., 2022). Two sets of ages were calculated, one taking only topographic shielding into account and one corrected for erosion, land uplift, and snow cover (see supporting information for details).
Trenching
To determine the structure of the outer slopes of the crater-shaped depression (e.g., potentially overturned ejecta over the target surface) and to collect samples for further analysis, we dug two trenches that were oriented radially to the crater structure (Fig. 3). We selected the positions and lengths of the trenches based on our previous field experience with other small impact craters developed in unconsolidated materials (see Losiak et al., 2016, 2020) and based on a literature review (Herd et al., 2008; Szokaluk et al., 2019). Tor 1 (3.0 m long and up to 1.2 m deep) was located on the northern side of the structure, where the outer slopes were the steepest; it started at the base of the slope and continued until halfway to the rim crest. Tor 2 (2.8 m long and up to 1.0 m deep) was located on the eastern side of the structure; it started at the rim (~1 m behind the bench), where the slope was relatively flat.
14C Dating
14C dating of charcoals found within trenches was performed at the Vienna Environmental Research Accelerator (VERA) laboratory at the University of Vienna (Austria). A description of the standard analytical method used at VERA is available in Wild et al. (2013). This method was slightly modified for the Tor crater samples; that is, the first HCl step of the acid–base–acid (ABA) method was performed twice and at 80 °C to remove possible contamination by dolomite-rich soil. The 14C measurements were performed as described in Steier et al. (2004). The 14C ages given in 14C years BP (before present = before 1950 ad) are uncalibrated ages, which are calculated following conventions, for example, the use of the Libby half-life and the assumption of a constant atmospheric 14C level in the past (see, e.g., Stuiver & Polach, 1977). The calibrated 14C ages are transformed into calendar time ranges (given in cal. yr bp) to account for variations of the past atmospheric 14C value (and the Libby half-life) using the IntCal20 atmospheric curve (Reimer et al., 2020) and the calibration program OxCal v4.4. (Bronk Ramsey, 2009). All calibrated ages are rounded to 10 years.
Results
Digital Elevation Model
The high-resolution DEM and slope aspect analysis (Tabares-Rodenas et al., 2013) (Fig. 4) suggest that the shape of the Tor structure is round-cornered tetragonal rather than circular. Its highest rim sections are in the north and east, where the inner slopes are steepest. The southern rim is the lowest, and the inner slope the gentlest where it continues to form a gently sloping crater floor that is inclined northward where the pond is located. The structure is accompanied by an external groove in the NNE sector. The groove is flat-floored, and it terminates on the hillside where the elevation drops below the base of the groove. The groove has low walls that become gradually higher toward the structure following the rise of hill topography.
Geophysical Studies
Ground-Penetrating Radar
Two radar facies corresponding to soft sediments (RF1, upper) and crystalline basement (RF2, lower) were delimited based on their patterns and subhorizontal reflection that originates from the top surface of the basement (Fig. 5). The design of the RF1 is chaotic but occasionally includes short (<10 m) elongated reflections and hyperbolic features that arise from point sources. Such a chaotic pattern is characteristic of glacial till. However, due to the instrumental shadow zone (Davis & Annan, 1989), the uppermost tens of cm of the subsurface are uncharacterized. No significant reflections originate from the crystalline basement.
Hyperbolic features from point sources within RF1 were used to calculate the dielectric permittivities (ɛr) of sediments. Compared to the central pond area (ɛr = 37.6 on average), the values were lower (ɛr = 11.5 on average) while measured at the rims and outside the Tor structure. Moreover, due to variable moisture content, the dielectric permittivity of sediments outside the structure increased with depth, whereas inside the structure, it decreased. Outside the Tor structure, the moisture content of the overburden increases with depth. On the contrary, the uppermost part of the central pond hosts meter-sized boulders with lesser amounts of finer particles. Thus, the uppermost part contains more water compared to the deeper part. The average dielectric permittivity values converted the radar time-scale to depth (Fig. 5).
According to the GPR studies, the thickness of the sediment cover over the basement varies between 3 m under the central pond and 8 m at the rim-like feature. Cover thickness is variable and is relatively thick in other locations with higher topography. Based on the reflector that originates from the top surface of the basement, the Tor structure has no counterpart in the basement, that is, no depression within the basement exists that could correlate with the subcircular topographic feature.
Electric Resistivity Tomography
The electric resistivity profile in Fig. 6 shows a layered cross section of the structure. The crystalline basement with resistivity above 4 kOhm.m is overlain by sediments with variable resistivities. Vertical resistivity variations of deposits can be attributed to changes in water content. The lower part is saturated, and its resistivity ranges from 1 to 3 kOhm.m. The resistivity of the upper part reflects a several weeks long drought period (unsaturated soil, resistivity 6–20 kOhm.m) that ended with showers during the week of fieldwork (near surface 0.5 m, resistivity 4–10 kOhm.m). The lowest resistivities are in the pond, where the section continues downward with saturated soil.
The top of the crystalline bedrock is relatively flat and is situated at about 624–625 m asl which is in good agreement with GPR data. The resistivity section suggests a slight rise of crystalline bedrock beneath the eastern rim of the structure. Still, most likely, this is an inversion artifact in which higher and lower resistivity values compensate for each other in the steep topographic situation.
Magnetic Map and Model
The Tor structure owes its counterpart in the magnetic field by showing a combination of negative and positive anomalies correlating with the central pond and rim features, correspondingly (Fig. 7). The map (Fig. 7a) of the magnetic field shows a slightly angular shape. The maximum difference in field values is about 100 nT, highest (51,995 nT) at the NE rim, and lowest (51,895 nT) on the eastern side of the central pond. Outside the structure, one may notice the NW–SE trend in the elongation of anomalies. Rare local short-wavelength high-amplitude anomalies correspond to glacial erratics of granodiorite composition.
Magnetic susceptibility (χ) of soil and erratic boulders is moderate in its values. The soil has an average value of 360 ± 280 × 10−6 SI (N = 24), containing boulders of quartzite (χ = 30 ± 20 × 10−6 SI; N = 32), greenschist (χ = 670 ± 430 × 10−6 SI; N = 10), and granodiorite (χ = 2600 ± 1290 × 10−6 SI; N = 10). However, geological models based on magnetic field data (Fig. 7b and 7c) require a higher (χ = 4000 × 10−6 SI) value than the measured characteristics. The same fate was met by Henkel et al. (1996) while modeling the Tor structure; they found that χ of 5000 × 10−6 SI best corresponds to the glacial till.
Magnetic anomaly correlates positively with topography, whereas changes in the relief of the crystalline base are of minor effect. Modeled relief of the top surface of crystalline rocks is smooth compared to topography and does not show any short wavelength undulations.
10Be Exposure Dating
The four 10Be ages are similar and overlap within 2σ (Table 1). The uncorrected summary age is 9355 ± 175 years, while the ages corrected for erosion, land uplift, and snow cover are ~900 years older (summary age 10264 ± 200 years; Table 1). The latter ages are preferred.
| Sample no. | Lab. ID | Lat (°N) | Long (°N) | Elev (m.a.s.l.) | SF | Rock type | Boulder height (cm) | 10Be concentration (atoms/SiO2 g) | Uncorrecteda exposure age (years) | Correctedb exposure age (years) |
|---|---|---|---|---|---|---|---|---|---|---|
| TJ-06-CE107 | 200,802,075 | 62.50167 | 12.63194 | 635 | 0.9997 | White quartzite | 50 | 77,599 ± 3237 | 9457 ± 395 |
10,384 ± 453 |
| TJ-06-CE108 | 200,802,076 | 62.50175 | 12.63168 | 634 | 1.0000 | Gray quartzite | 70 |
76,578 ± 3308 |
9307 ± 403 | 10,202 ± 461 |
| TJ-06-CE109 | 200,802,077 | 62.50170 | 12.63152 | 634 | 1.0000 | Gray quartzite | 45 | 77,772 ± 2623 | 9596 ± 324 | 10,547 ± 372 |
| TJ-06-CE110 | 200,802,078 | 62.50169 | 12.63151 | 634 | 1.0000 | Gray quartzite | 45 |
74,312 ± 2499 |
9106 ± 307 |
9977 ± 350 |
| Summary age | 9355 ± 175 | 10,264 ± 200 | ||||||||
- SF = shielding factor.
- a Corrected for topographic shielding and assuming zero erosion.
- b Ages corrected for erosion (0.5 mm ka−1), land uplift, and snow cover (see details in supporting information).
Trenching
The trenches revealed a podzol developed on a glacial till. Podzol is a general term for a typical soil complex in temperate and boreal zones where rainfall exceeds evaporation. Under the topsoil layers, upper eluvated (E) and lower illuviated (B) horizons typically occur. They are leached and enriched of organic matter, Al and Fe, respectively. The podzol in Tor consists of (i) a thick layer of moss, (ii) 1–5 cm thick organic (A) level, (iii) thick whitish eluvial (E) level (in a depth of 20–40 cm), (iv) up to 5 cm thick illuvial (B) level, and a parent material (C) below it. Locally, E and B zones are mixed, resulting in lenses of both soil levels intermixing (see Fig. 3 Thor_2_14). Glacial till visible in the trenches near the Tor structure is matrix-supported but rich in angular clasts ranging from ~1 cm up to >1 m in diameter.
All found charcoal particles came from shallow depths between 10 and 40 cm below the surface. Their distribution (increasing in density toward the surface) and association with lenses of the whitish, leached eluvial zone (Fig. 3) suggest that they are derived from the surface and moved down the soil by uprooting and animal burrowing.
14C of Charcoals
We selected four samples for 14C dating: two charcoal particles from each trench. The selection process was aimed at targeting the particles most likely to be “impact charcoal” similar to charcoal particles found at Kaali (Losiak et al., 2016) and Morasko (Szokaluk et al., 2019). We selected charcoal particles from different locations within both trenches to ensure that a full variability within data is represented. The samples came from the deepest sections of the zones to minimize contamination from any potential surface charcoal, similar to what occurred for some of the samples from Ilumetsa (Losiak et al., 2020).
Four charcoal samples from the Tor structure were 14C dated (Table 2; their sampling locations were marked with black rectangles on Fig. 3 Tor1 and Tor2). The ages of two samples overlap (Tor1_25: 9010–8650 cal. yr bp, Tor2_21: 8990–8650 cal. yr bp), while the two other samples fall outside of this time range (Tor1_14: 8520–8380 cal. yr bp and Tor2_9: 5300–5040 cal. yr bp).
| Sample name | Laboratory # | δ13Ca,b (‰) | 14C-agea (bp) | Calibrated agec (bp) |
|---|---|---|---|---|
| Tor2_9 | VERA-7116 | −26.6 ± 0.4 | 4497 ± 29 | 5300 (95.4%) 5040 |
| Tor2_21 | VERA-7117 | −26.6 ± 0.8 | 7954 ± 37 |
8990 (86.7%) 8690 8680 (8.7%) 8650 |
| Tor1_14 | VERA-7119 | −25.9 ± 0.3 | 7638 ± 27 |
8520 (6.1%) 8500 8480 (0.9%) 8470 8470 (88.5%) 8380 |
| Tor1_25 | VERA-7120 | −29.2 ± 0.4 | 8008 ± 35 |
9010 (95.1%) 8720 8670 (0.3%) 8650 |
- a 1σ—uncertainty.
- b Determined with the AMS system. Because the δ13C may be altered during sample preparation, they are suited only for quality control, not for extended interpretation.
- c Calculated with OxCal v4.4. Bronk Ramsey (2009) and the IntCal20 calibration curve (Reimer et al., 2020), data corresponding to the 2σ-confidence level, and probability of the individual periods in parentheses. The age was rounded by 10 years.
Discussion
To determine the formation mechanism of the Tor structure, we applied a broad set of methods to analyze its physical properties and determine formation time: We composed a digital elevation model, dug into the side walls of the structure, used a set of geophysical (GPR, ERT, magnetometry, and metal detector) methods, as well as dated boulders and charcoal particles present in the vicinity of the structure. We found that the analysis of this combination of data along with previously available information does not support the hypothesis of an impact origin for the Tor structure.
The choice of geophysical methods applied in field studies of very small crater structures depends on local geological conditions. In Tor, the relatively small thickness of glacial sediments and their electrical contrast with the crystalline basement favored the applicability of electric and electromagnetic methods (ERT and GPR). The GPR reflections and ERT anomalies indicate that the upper surface of the crystalline base is smooth. This is inconsistent with an impact origin as, owing to the thin sedimentary cover, we would anticipate that the basement would bear an imprint of the impact with a projectile. However, there is no evidence for an impact-damaged zone within the basement rocks below the Tor structure, expressed only in the quaternary drift deposits above.
Impact craters of the size of Tor would be typically formed by iron projectiles (Bland & Artemieva, 2003, 2006), where preserved meteorite fragments would most likely be found along the so-called true crater floor, that is, the floor of the transient crater below the crater infill (for a review, see Ormö et al., 2007). Despite numerous searches with metal detectors in this area over the years (including one extensive search in 2018), none have resulted in any meteorite fragments being found within or around the topographic structure of Tor.
Likewise, the magnetic survey did not indicate any low-intensity magnetic anomaly consistent with small pieces of (iron) meteoritic material lining the base of the crater shape depression. The magnetic survey shows only trends explainable by the local geology that results from the difference in magnetic properties between the surface sediments and the crystalline basement rocks. Outside the Tor structure, an NW–SE trend in the elongation of anomalies corresponds to the direction of the glacial movement and elongation of landforms in the region (Öhrling et al., 2021). Hence, the distribution of positive and negative magnetic anomalies appears to reflect the thickness of the relatively magnetic Quaternary glacial material on top of the less magnetic Paleoproterozoic basement rocks. The magnetic model also shows that the surface between the basement and glacial sediments is smooth, in agreement with the results of the GPR and ERT data. Hence, we conclude that the small-wavelength high-amplitude local anomalies seen around the Tor structure are because of highly magnetic glacial erratics of granodioritic composition (Fig. 7) and are not consistent with an impact origin.
Trenching across the Tor crater sediments likewise does not reveal any indication of the type of deposits that are characteristic of small impact craters developed in unconsolidated sediments, for example, Kaali Main crater (Losiak et al., 2016), Kaali 2/8 (Losiak et al., 2018), Morasko (Szokaluk et al., 2019), Campo del Cielo (Cassidy et al., 1965), and Whitecourt (Kofman et al., 2010). There is no evidence of an overturned sequence of sediments characteristic of impact structures (Kring, 2017; Losiak et al., 2016). The trenches revealed that the sedimentary sequence within the rim-like structure is of uniform glacial till with numerous, randomly distributed, not-aligned boulders (Fig. 3). This sedimentary profile is similar to other glacial till outcrops seen in the area. There is no sign of a contact between a pre-impact surface and ejecta deposits such as paleosoil similar to one in Morasko (Szokaluk et al., 2019), or a change in sediment properties or an altered alignment of boulders like that found in impact deposits at Kaali, Estonia (Losiak et al., 2016).
Additionally, the distribution of charcoals found within trenches dug through the outer slopes of the Tor structure is not like the charcoal distribution found within the proximal ejecta blankets of confirmed small impact craters (e.g., Whitecourt: Kofman et al., 2010; Kaali Main: Losiak et al., 2016; Kaali 2/8: Losiak et al., 2018; Morasko: Szokaluk et al., 2019) developed in vegetated areas. In these confirmed craters, most charcoals are present at the base (lowest ~10 cm) of the ejecta blanket (although some charcoal particles are found throughout the overturned strata), forming an identifiable charcoal-enriched zone that follows the level of the pre-impact surface (Losiak et al., 2022). Above it, there is a wedge-like proximal ejecta sediment layer. All charcoal pieces at the Tor site were located at depths of 10–40 cm (most of them at <<25 cm), and their spatial distribution followed the current surface. Despite an extensive search, no deeper charcoal particles were found—including at the depths we expected. Most Tor charcoal was present within lenses of bleached soil layer intermixed within other soil levels (Fig. 3 Tor_1_23, Tor_2_8, and Tor_2_14). This suggests that the charcoal particles came from the surface and were intermixed during a natural tree-uprooting and bioturbation. An example of such a situation is the Tor_1_23 (Fig. 3), where a fragment of paleosoil with illuvium, eluvium, and a charcoal layer is overlaid by tree roots and develops a new soil sequence. The transport of the charcoal particles down into the sediments is also supported by the 14C analyses, where the 14C ages of charcoals are inconsistent with being formed by a single event. Charcoals formed by small impact cratering processes have been shown to indicate a single age origin (e.g., Losiak et al., 2016, 2018). However, the 14C dating of the charcoal particles found at Tor suggests that they were formed by three separate events ~8.64–9.01 ka, 8.37–8.52 ka, and 5.04–5.30 ka (Table 2; Fig. 8). This kind of age distribution is consistent with periodic wildfires depositing charcoal on the soil surface and then biological action and weathering processes transferring mm- to cm-sized charcoal particles down into the soil and sedimentary profile to depths of a couple of tens of centimeters.
Timing of exposure of boulders present at the Tor rim as determined by 10Be (summary age: 10,264 ± 200 years; Table 1; Fig. 8) agrees well with the timing of deglaciation in this area (about 10,200 cal. yr bp; Stroeven et al., 2016). This means that measured boulders have been continuously exposed since the ice left the area and the glacial lake was drained, and were not later ejected or overturned by a suggested impact.
Our analysis of Tor structure data collected in the field agrees with the interpretation of the Tor feature made by Öhrling et al. (2021) based on remote-sensing data. A grounded iceberg most probably formed Tor during the lowering of the glacial lake. Detailed morphology of the structure supports this conclusion: The groove in the NNE direction is toward the lower topography meaning deeper waters of the glacial lake. It is the direction from which the iceberg was initially approaching and grounded. Hence, the structure likely formed when an iceberg flipped over. Flipping is due to the faster melting of ice underwater than above the lake level resulting in an upward shifting center of mass. Flipping is also responsible for the angular shape of the structure and the pushed-up glacial material in rims. The pushed-up rim covers part of the groove.
The Scientific Importance of Tor
The disqualification of Tor as an impact crater does not make it any less valuable to scientific knowledge. First, in order to properly quantify the recent impact rate on Earth, it is equally important to count all true impact craters, and therefore to achieve this other “holes in the ground” of nonimpact origin need to be excluded. Second, publishing studies that successfully exclude potential impact structures from further consideration ensures that future resources are not wasted on doing the same thing repeatedly, that is, reporting on the types of features and toolkit required to identify or refute small craters is useful.
Third, Tor is a rare case of an easily accessible iceberg crater, whose studies may have further applications in terrestrial and in Martian geology. Iceberg craters are mostly known from seismic or sonar studies of the deep seafloor (e.g., Brown et al., 2017). Very few are preserved in subaerial positions, and more importantly, their descriptions are mainly morphological from remote sensing (e.g., Høgaas & Longva, 2018). Here, Tor offers an easily accessible and very well-preserved example of crater-like forms formed by this process. A detailed field and morphological analysis of such forms is particularly useful as a reference for Martian geological analyses, helping to create new ways to distinguish between impact craters and craters created by other processes on other planetary bodies. On Earth, we are, for the most part, blessed with detailed knowledge of the regional geology (e.g., the existence of volcanic domains, ice-dammed lakes and age of their formation) and it is relatively easy to perform detailed field studies (including a search for direct impact indicators: French & Koeberl, 2010). On Mars, we are still almost entirely limited to studying morphological features using remote sensing (e.g., Brož et al., 2021; Séjourné et al., 2019). Thus, it is essential to have a diverse and detailed portfolio of studies of terrestrial analogs—including features like Tor.
Distinguishing impact craters from similar looking structures is important because crater counting has long been used as practically the only way to determine the age of geological features on Mars' surface (see review by Benedix et al., 2020). With the increased resolution of the orbiter cameras, smaller (i.e., tens of meters) craters can be used for even more detailed dating. However, it becomes increasingly difficult for such small craters to exclude other causes of formation by mere morphology. Iceberg craters have been suggested at some locations on Mars (e.g., Fairén et al., 2010; Komatsu et al., 2000), mainly based on reasoning around circumstantial evidence such as location in areas once potentially covered by shallow bodies of water in proximity to other features of potential glacial origin (e.g., eskers, moraines). Learning more about how to distinguish such craters from those formed by an impact would benefit both crater counting efforts and our understanding of the past evolution of paleoenvironments on Mars.
Conclusions
- A high-resolution Digital Elevation Model shows that the Tor structure is round-cornered tetragonal rather than typical for explosion craters circular or elliptical in plan view.
- Geophysical data show no depression within the basement that could correlate with the subcircular topographic feature; all the anomalies are describable by contrasts in physical properties between the crystalline basement, glacial sediments saturated with water to a different extent, and air. The top surface of the crystalline basement has no undulations comparable to the size of the Tor structure.
- Trenching within the external slope of the Tor structure does not show signs of overturned sequence characteristics for proximal ejecta, nor does it contain charcoal in a similar position as in confirmed impact craters like Kaali, Morasko, or Whitecourt. Sedimentologic position as well as 14C of charcoal pieces is consistent with its origin in three separate wildfires more than 1 ka after the formation of the structure as indicated by 10Be dating.
- The 10Be ages show that analyzed boulders at the rim of the Tor structure were deposited there during deglaciation and later were continuously exposed in approximately the same position since the ice left the area and the glacial lake was drained. Those boulders were not ejected or overturned by a possible impact after being deposited by ice.
We conclude that the Tor structure, Sweden, is an iceberg crater formed 10.26 ka ago by a grounded iceberg during the lowering of the glacial lake. Despite its nonimpact origin, it still has considerable potential to add to our study of extraterrestrial geomorphic features and planetary evolution.
Acknowledgments
Helena Alexanderson was working at the Department of Quaternary Geology and Physical Geography, Stockholm University, Sweden, when the fieldwork (sampling boulders for exposure dating) was carried out. Timothy Johnsen is acknowledged for sampling and 10Be analyses. Funding from the Helge Ax:son Johnson's Foundation covered the costs for the 10Be analysis. Henriette Linge (University of Bergen) is thanked for discussing and reviewing 10Be age calculations. Anna Losiak acknowledges funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant ImpChar, agreement no. 749157 as well as being partially supported by the National Science Centre, Poland grant no 2020/39/D/ST10/02675. The work by Jens Ormö was partially supported by grant ESP2017-87676-C5-1-R from the Spanish Ministry of Economy and competitiveness and Fondo Europeo de Desarrollo Regional, the Spanish State Research Agency (AEI) project no. MDM-2017-0737 Unidad de Excelencia “María de Maeztu”- Centro de Astrobiología (INTA-CSIC), and the Spanish Research Council (CSIC) support for international cooperation: I-LINK project LINKA20203. We greatly appreciate comments and suggestions of the reviewers: Stephanie Werner and A. J. Timothy Jull.
Open Research
Data Availability Statement
Data available on request from the authors.
