Petrographic Analysis of the Loftarstone Resurge Deposit in the Lockne Impact Structure, Sweden

Abstract

Impact cratering has played a fundamental role as a surface modifying process on Earth and other celes- tial bodies. It has significantly influenced crustal reshaping, basin formation and the development of magmatic ac- tivity and hydrothermal systems. Of the roughly 200 impact structures that have been identified on Earth, the ma- jority occurred on land, with significantly fewer known examples preserved from marine environments. Although impact cratering has become a well-established field of research, the formation and development of marine impact craters is less well understood and requires further investigation to understand the unique processes involved.
This work investigates the Lockne impact structure, formed in a marine environment 455-458 Ma. The crater, measuring 7.5 km across, is found in the Jämtland province of Sweden and has been well preserved in the Caledonian nappe system. It provides a unique opportunity to study the material and depositional processes related to the backflow of water (resurge) that occurred after the formation of the initial crater. The material found in these resurging waterfronts formed the Lockne resurge deposit, a unit which consists of reworked crater and bedrock ma- terial. The upper arenitic section of this deposit is know as the Loftarstone, which is the focus of this thesis.
The Loftarstone is a fine-gravel- to mud- sized deposit, exhibiting cross-lamination and graded structure. Similarly to the underlying section (Lockne breccia), the Loftarstone contains clasts of orthoceratite limestone, granitoid fragments- and minerals and melt fragments. Unique to this deposit is the presence of shocked quartz, a diagnostic product of the impact event.
While an overview of the sedimentary structures and general composition of the Loftarstone has been inves- tigated, there has been no detailed studies into the petrography, chemistry and spatial distribution of constituents across the impact site. This study therefore provides information on the petrography across 17 sample locations, showing evidence for compositional variations across the impact site. It demonstrates the presence of a clay-rich matrix with variable levels of FeO and MgO and provides evidence for a heterogenous composition of melt frag- ments, including potential intermingled textures between calcite and silicate melts. Shock features in quartz and feldspar were studied, including planar fractures (PFs) and planar deformation features (PDFs). High twin densities in calcite (350 twins/mm) were also observed, leading to a discussion about their formation and origin. Deposition- al processes across the impact site were deemed complex, influenced by the access to source rock material and characteristics of the material.

Abstract (Swedish)

Meteoritnedslag har spelat en grundläggande roll som ytmodifierande processer på jorden och andra himlakroppar. De har haft stor betydelse för omstruktureringen av jordskorpan, bassängbildning samt bildan- det av magmatisk och hydrotermal aktivitet. Av de cirka 200 nedslagsstruktur som identifierats på jorden har de flesta bildats på land, medan betydligt färre exemplar finns bevarade från marina miljöer. Trots att processer relate- rade till kraterbildning har blivit väletablerade forskningsfält är marina nedslag fortsatt mindre väl förstådda. Fort- satta studier krävs därför för att klarlägga unika processer relaterade till dessa nedslagsmiljöer.
I denna studie undersöks Lockne nedslagskrater, en struktur som bildades i en marin miljö för 455–458 Ma sedan. Denna krater, som återfinns i Jämtland, Sverige, har en diameter på 7.5 km och har bevarats väl under den Kaledoniska orogenesen. Kratern erbjuder en unik möjlighet att studera material och depositionsprocesser kopplade till återflödet av vatten in i nedslagsområdet (återsvallet), efter det inledande nedslaget. Återsvallssedimenten i Lockne består av omarbetat krater- och berggrundsmaterial som transporterats av vattenmassorna. Den övre are- nitiska delen av denna avsättning är känd som Loftarstenen. Detta är lagret som har varit i fokus för denna uppsatts.
Loftarstenen är en avsättning med storleksfraktioner mellan fint grus och lera och uppvisar korsskiktning, lami- nering och en graderad struktur. Den innehåller, likt den underliggande avsättningen (Lockne breccia), klaster av ortoceratitkalksten, granitfragment och smältprodukter. Unikt för Loftarstenen är även förekomsten av chockad kvarts, en produkt från nedslaget.
Trots att övergripande undersökningar kring sedimentära strukturer och Loftarstenens innehåll har genom- förts, finns det inga detaljerade studier kring avsättningens petrografi, kemiska sammansättning och den rumsliga fördelningen av beståndsdelar i kraterområdet. I denna studie beskrivs därför petrografin av Loftarstenen, baserat på 17 olika provtagningsområden. Detta påvisade en mineralogisk variation mellan proverna. Resultaten visar även på förekomsten av smältfragment med heterogen komposition samt interaktioner mellan kalcit och silikatsmäta. Matrix visades vara ler-rik med varierande nivåer av järn och magnesiumoxider. Chockstrukturer undersöktes även, där strukturer identifierade som planära sprickor (PFs) och planära deformationsstrukturer (PDFs) återfanns i kvarts och fältspat. Individuella fall av höga tvillingdensiteter (350 tvillingar/mm) i kalcit noterades också, vilket leder till en diskussion kring dess bildande och ursprung. Depositionsprocesserna i kraterområdet kunde fastställas som komplexa och under inflytandet av de omarbetade materialets egenskaper samt tillgängligheten av material från ursprungsbergarten.

Popular Abstract

Some 455 million years ago, a meteorite slammed into the shallow sea in what is now central Sweden, forming the Lockne impact crater. The impact unleashed colossal forces, sending seawater rushing back into the crater in a violent resurge. In the aftermath, a distinct layer of rock known as the Loftarstone was deposited. Its composition preserves traces of the extraordinary forces at work when a meteorite strikes the sea.
The Lockne impact crater, spanning 7.5 km across, is one of the few well-preserved marine impact structures on Earth, offering a rare opportunity to explore the turbulent processes of crater formation beneath the sea. Unlike terrestrial craters, marine impacts interact with water and bedrock in complex ways that are still not fully understood. The Loftarstone, a distinct rock layer formed by the resurge of seawater after the impact, holds valuable clues to these interactions. Despite its significance, the detailed composition and formation of this layer has remained relatively unexplored. Studying it deepens our understanding of impact dynamics in marine settings and broadens our view of Earth’s geological past.
Close examination of the rock, under both light and electron microscopes, uncovered a varied blend of fragments from the crater and its surroundings, woven together with fine material and forming intricate textures. Some mineral grains showed signs of being affected by intense pressures, hinting at the extreme conditions involved. Melted fragments were also observed further indicating the high-energy nature of the impact. The rock’s makeup and structure varied across different areas, reflecting changes in the depositional environment and the movement of materials during the resurge. Notably, high-pressure features in calcite were identified, offering additional clues to the intense forces at play. These findings not only shed light on the chaotic aftermath of the impact but also provide key insights into the processes that shaped the formation of the Loftarstone. The creation of the Loftarstone provides a rare window into the dynamic processes that occurred during and after the Lockne impact, highlighting the powerful forces at work when a meteorite strikes a marine environment. This study contributes to our understanding of impact cratering in marine settings, revealing the complexity of how water, bedrock, and extreme pressures interact during such events. The insights gained from the Loftarstone not only enhance our knowledge of this specific impact but also pave the way for future research into similar events on Earth, offering a deeper understanding of the processes that have shaped our planet's history.

• author: Barreby, Linn
• supervisor: Carl Alwmark; William Hyde
• organization: Department of Geology
• course: GEOR02 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Impact cratering, resurge deposit, shock metamorphism, petrographic analysis
• publication/series: Dissertations in Geology at Lund University
• report number: 702
• language: English
• id: 9207119
• date added to LUP: 2025-07-31 11:27:35
• date last changed: 2025-07-31 11:27:35