Jens Ormö

Hydrocode Simulation of the Lockne Marine Target Impact Event

In this study 2D and 3D numerical simulations are used to model the formation of the Lockne crater (centered at 63°00′20″ N, 14°49′30″E) during Middle Ordovician times, about 455 Ma ago. We study a possible mechanism of shallow excavation to explain the concentric structure of the crater, as well as the interaction between basement the ejecta curtain and the water ejecta curtain, to explain the final ejecta distribution on the Earth’s surface. We also consider different angles of trajectory inclination to understand how obliquity can influence the cratering flow in a marine target impact. Comparison between the results of numerical simulations and field studies allows us to estimate a water depth at the time of the impact of about 700–900 m.

Magnetometry of the marine, Ordovician Lockne impact structure, Jämtland, Sweden

Abstract Magnetic modelling of the Middle Ordovician Lockne impact crater reveals its morphology and the lack of a magnetically sufficient melt body. Suevite and melt are often strongly remanent and stand out in magnetic surveys. However, exceptions occur, e.g. the Lappajarvi impact structure. From aeromagnetic data a total intensity and a horizontal gradient map were generated. They show a structure with a calm interior. The absence of short wave length anomalies is interpreted to be an indication of the lack of a melt sheet with a prominent magnetic signature. Neither can short wave length anomalies be seen at the expected central uplift of the structure. However, this kind of anomalies are commonly associated with larger impact structures. The modelling produced a bowl-shaped structure which is slightly tilted westwards. This is in good agreement with the field observation of the structure. Based on the absence of magnetic anomalies, together with field observations and drill-core information we are convinced that no prominent melt body was formed in connection with the Lockne impact, despite the size of the structure. This fact may be explained by the type of target. The target material in the Lockne area was seawater and sedimentary strata overlying a crystalline basement. The abundant water in the area possibly contributed to the explosive dispersion of the molten silicates.

The pre‐impact Ordovician Stratigraphy of the tvären bay impact structure, se Sweden

Abstract The Middle Ordovician impact crater in the Tvaren Bay south of Stockholm has a well preserved sequence of sedimentary rocks in an area of Sweden where the Lower Palaeozoic sedimentary rock cover has been eroded away. The crater was formed in the Late Kukrusean by a cosmic body that hit a marine environment The resurge breccia deposited shortly after the impact consists mostly of limestone fragments, but also of fragments from the Precambrian crystalline bedrock. Limestone clasts in the lower part of the breccia were studied by means of visual appearance and microfossil content, and by using conodonts for age determination correlations were made between the pre‐impact limestones of Tvaren and limestones in other parts of Sweden and Estonia. The conodonts range from the Volkhovian Microzarkodina parva Zone to the Upper Kukrusean Prioniodus variabilis Zone. The majority of the species are Llanvirnian‐Llandeilian. The findings show (1) that although the pre‐Volkhovian Lower Ordovician is poorly devel...

Mutually constrained geophysical data for the evaluation of a proposed impact structure: Lake Hummeln, Sweden

Abstract Lake Hummeln covers a 1.2-km-wide near-circular depression in the Precambrian basement of the Baltic Shield. In the depression, more than 150-m-thick Cambrian and Ordovician marine sediments are underlain by non-volcanic breccia. The only explanation that does not meet contradictions is that the structure was caused by impact, but evidence of shock metamorphism is until now missing. This study focuses on the geophysical characterisation and includes magnetic and gravimetric modelling, constrained by information from resistivity measurements, geological mapping and a drill core. The Hummeln structure shows both a gravimetric and a magnetic low. The magnetic low is not broken by any short wavelength anomalies which would indicate the presence of remanent melt occurrences. The diameter of the magnetic low is about twice that of the proposed crater depression, and probably indicates fracturing. Both the magnetic and the gravimetric models show a structure that is consistent with the impact hypothesis. Resistivity measurements were carried out in the surroundings of the proposed crater. The fractured region indicated by the magnetometry does not produce a detectable resistivity anomaly. However, resistivity measurements proved useful in investigating the tectonic fracture zones crossing the area in the vicinity of the structure. Following the impact model, a reconstruction of the pre-erosional structure resulted in an approximately 50% wider original crater than seen today.

Impact origin for the Hummeln structure (Sweden) and its link to the Ordovician disruption of the L chondrite parent body

Several studies of meteorites show that a large disruption of an asteroid occurred ca. 470 Ma in our solar systems asteroid belt. As a consequence, a large number of meteorite impacts occurred on Earth during the following few million years. The finding and characterization, for the first time, of planar deformation features in quartz grains from rocks collected at the Middle Ordovician Hummeln structure (Sweden) prove the hypervelocity impact origin of the structure. The unambiguous shock features allow us to close an similar to 200-yr-old discussion about its origin, and further the hypothesis of enhanced asteroid bombardment during the Middle Ordovician, adding an impact crater to the increasing number confirmed and properly dated from this period. Despite its relatively small size (similar to 1.2 km in diameter), similar to the young Meteor Crater (Arizona, USA), and its old age, the Hummeln structure is remarkably well preserved, contradicting the general assumption that small craters are not preserved on Earth for more than a few tens of thousands to a couple of million years. (Less)

SEISMIC IMAGING OF THE MAIN CRATER OF THE PROPOSED SIRENTE METEORITE CRATER FIELD (CENTRAL ITALY)

We present seismic imaging of the subsurface structure of the main crater of the proposed Sirente meteorite crater field (Abruzzo, Central Italy). The crater field has been suggested to have formed by a small meteorite impact and consists of a main, dominant crater (~120 m in diameter) and a group of much smaller craters (~10 m in diameter on average). The main crater has a prominent elevated rim. The crater field is found within lacustrine sediments overlying limestone. Two shallow reflection profiles with P waves were acquired across the structure when the small lake, which occupies the main crater, was ice-covered. Profile RFL 2 is 130 m long and crosses the main structure rim to rim. Profile RFL 1 is 78 m long and roughly transversal to profile RFL 2. Two CMP processing sequences were applied on raw data. A short processing sequence allowed recognition of the main features of the subsurface structure of the crater: a deep (53 m on average), rootless, bowl-shaped geometry, a deepseated central uplift structure and three different seismic facies representing the infilling of the bowlshaped basin. These include lateral onlap reflectors may be interpreted as an analogue to the “breccia lens” in craters formed on rocky targets, indicating the occurrence of collapse events (slumping) during the crater modification stage. A long processing sequence allowed a more detailed imaging of the bowlshaped basin and the structures underlying and surrounding the crater, such as compaction of strata below the rim. The structural features interpreted from our survey are consistent with the impact hypothesis. Apparently, they do not support other proposed mechanisms of formation as the structure seems both rootless and deep.

Pleistocene Lake Bonneville as an analog for extraterrestrial lakes and oceans: Chapter 21

Abstract Geomorphic confirmation for a putative ancient Mars ocean relies on analog comparisons of coastal-like features such as shoreline feature attributes and temporal scales of process formation. Pleistocene Lake Bonneville is one of the few large, geologically young, terrestrial lake systems that exemplify well-preserved shoreline characteristics that formed quickly, on the order of a thousand years or less. Studies of Lake Bonneville provide two essential analog considerations for interpreting shorelines on Mars: (1) morphological variations in expression depend on constructional vs erosional processes, and (2) shorelines are not always correlative at an equipotential elevation across a basin due to isostasy, heat flow, wave setup, fetch, and other factors. Although other large terrestrial lake systems display supporting evidence for geomorphic comparisons, Lake Bonneville encompasses the most integrated examples of preserved coastal features related to basin history, sediment supply, climate, and fetch, all within the context of a detailed hydrograph. These collective terrestrial lessons provide a framework to evaluate possible boundary conditions for ancient Mars hydrology and large water body environmental feedbacks. This knowledge of shoreline characteristics, processes, and environments can support explorations of habitable environments and guide future mission explorations.

Diagenetic analogs to hematite regions on Mars: Examples from Jurassic sandstones of southern Utah, USA

Diagenetic hematite concretions are common in the eolian Jurassic Navajo Sandstone in southern Utah (and some correlative units in Arizona and Nevada). The zones of alteration formed by structurally and stratigraphically influenced subsurface groundwater flow and localized iron oxide precipitation within porous sedimentary rocks. In many geologic systems on Earth, iron is a sensitive fluid flow indicator1. Mobilization and precipitation of iron oxides and sulfides requires specific variations in fluid chemistry. Precipitation of iron oxides in discrete concretionary zones further requires specific host rock characteristics. These characteristic color variations and zones of mineralization in the Jurassic Navajo Sandstone occur in a variety of cementation patterns with structural and stratigraphic relationships that have been well documented. Iron for the concretions is likely sourced internally from hematite grain coatings. Near surface, meteoric waters and processes of weathering commonly distribute disseminated iron films that impart a pink to orange-red color to the sandstone early in the depositional or burial history. The disseminated iron oxides are commonly mobilized and removed by reducing fluids, leaving the sandstone white. When these fluids mix with oxidizing groundwater in the Utah example, concentrated hematite precipitates, typically in the form of spherical balls. Many other concretion geometries commonly occur where anisotropy and preferential fluid flow pathways exist. Some of these shapes include pipes, sheets, bulbs, angular bricks, and repetitive bands. The differing geometries appear to be primarily a function of permeability barriers and pathways. Both sandstone coloration and the presence of hematite concretions (+/- other iron oxide minerals) record evidence of past fluid flow and reactions in subsurface sedimentary rocks. These are products of low-temperature, near-surface, hydrologic, chemical diagenetic reactions. Biomediation can also enhance the diagenetic precipitation of cements. In addition to elucidating a complex history of fluid flow in Utah subsurface, analysis of these concretions can help us to better understand the recently discovered hematite concretions on Mars. The NASA Mars Exploration Rover (MER), Opportunity has discovered spherical nodules in Meridiani Planum, that have been identified to be predominately hematite in composition5,6. These Mars concretions bear a remarkable resemblance to hematite-cemented concretions in sandstones of southern Utah. Hematite is one of few minerals currently found on Mars that can be genetically linked directly to water-related processes7. Although the general process of chemical precipitation has been proposed, diagenetic concretionary precipitation, or ferruginization, has been previously overlooked as a potential formation mechanism. This terrestrial analog in Utah has important implications for biomediated precipitation and for subsurface and potentially atmospheric chemical conditions on Mars.