Djupadal Formation

The Djupadal Formation is a geologic formation in Skåne County, southern Sweden. It is Early Jurassic (probably Pliensbachian-Toarcian, or Late Toarcian) in age. It is part of the Central Skåne Volcanic Province, know by the discovery of basalt tuff layers, including Sandåkra, Korsaröd and Djupadal.
An original analysis of the location of Korsaröd led to a Toarcian-Aalenian age,Sivhed (1984) p. 26 but was dismissed in 2016, when a series of Palynogical samples recovered a Late Pliensbachian and probably Lower Toarcian age for the Korsaröd Outcrop.Vajda, Linderson & McLoughlin (2016) p. 127 The same year this result was also challenged by an in-depth study of the Lilla Hagstad neck that yield a Late Toarcian Age. The formation was deposited in the Central Skane region, linked to the late early Jurassic volcanism. The Korsaröd member includes a volcanic-derived Lagerstatten with exceptional fern finds. The data provided by fossilized wood rings showed that the location of Korsaröd hosted a middle-latitude Mediterranean-type biome in the late Early Jurassic, with low rainfall ratio, delayed to seasonal events. Superimposed on this climate were the effects of a local active Strombolian Volcanism and hydrothermal activity.

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Description

Djupadalsmölla is a geological site in southern Sweden, notable for its volcanic tuff and related rock types formed from ancient volcanic activity.Augustsson (2001) p. 24 First documented in 1826, the site contains basalts from early volcanic eruptions. The Anneklev exposure near Höör revealed the first volcanic neck, with additional volcanic remnants at Jällabjär, Rallate, and Djupadalsmölla itself. The rocks here are primarily tuff, containing basaltic bombs with pyroxene, olivine pseudomorphs, and occasional conifer wood fragments ranging from small pieces to large logs.Eichstädt (1883) p. 412 The Rönne River has carved a 20-meter-deep valley through the Precambrian basement, exposing Lower Jurassic strata, including volcanic tuff, on the southern valley side. These strata extend northwest, suggesting the valley follows a partially exhumed sub-Jurassic depression.Lidmar-Bergström, Olsson, & Olvmo (1997) p. 99 The valley floor includes modern sediments from a Late Weichselian meltwater channel and eroded fluvioglacial deposits. A nearby roadcut shows kaolinized basement beneath Toarcian sediments, and a borehole penetrated 44 meters of kaolinized gneiss. Small ridges, cupolas in gneiss, and tors in amphibolite are also visible at the valley bottom.

Geology

The Djupadalsmölla site lies within the Sorgenfrei-Tornquist Zone (STZ), a 20–60 km wide fracture zone in southern Sweden and the Baltic Sea, part of the larger Tornquist Lineament stretching over 1000 km from the North Sea to the Black Sea. This zone marks the boundary between the East European Craton and Early Paleozoic terranes of central Europe. The STZ formed through continental rifting from the Permian to Cretaceous, similar to the North Sea system, with complex horst and graben structures from Late Cretaceous–Paleogene tectonics. Rifting from the Carboniferous to Mesozoic, linked to the breakup of Pangea, led to mafic alkaline magmatism, including a 70 km wide Permo-Carboniferous tholeiitic dyke swarm across Scania and Bornholm. The Central Skåne Volcanic Province, a 30 by 40 km Mesozoic volcanic field, features around 100 volcanic plugs and necks forming steep hills in central Scania. This volcanism likely originated from a continental lithospheric mantle reservoir or edge-driven mantle convection.

Stratigraphy

The Djupadalsmölla pyroclastic deposit is up to 10 meters high and 20 meters wide, extending over 100 meters westward along the Rönne River valley.Eichstädt (1883) p. 413 It consists of a 3-meter-thick Jurassic sequence overlying kaolinized Paleozoic gneiss, starting with 2 meters of sandstone and claystone, topped by 1 meter of green-brownish tuffaceous rocks.Norling, Ahlberg, Erlström & Sivhed (1993) p. 50 The tuff contains mostly lapilli (30–50%) and ash, with some red patches and concretions of coarse ash, calcite, and wood fragments.Augustsson (2001) p. 25 The deposit suggests moderate explosivity, with short transport paths indicated by minimal mechanical weathering, thick layering, and few basaltic bombs. The presence of wood and lapilli points to terrestrial deposition, likely from Strombolian volcanism tied to a regional rift, as evidenced by over 100 volcanic necks in central Scania.Augustsson (2001) p. 27Augustsson (2001) p. 28

A borehole (KBH2) at Djupadalsmölla provides detailed stratigraphy. The crystalline basement is 6.32 meters of weathered red-whitish gneiss, transitioning to kaolinite at the top.Wahlquist (2023) p.11 The Jurassic strata include light to dark gray shale with a 30 cm silt/clay layer and coal, showing microfaults and volcanic fragments. The volcanic sequence, 19.50 meters thick, consists of lapilli tuff cemented by calcite or zeolite, with felsic xenoliths and accidental lithics. Four subunits include varying textures, cement types, and preserved wood, indicating a dynamic depositional environment.

Other nearby sites, like Karup, show 1–1.5-meter-thick pyroclastic layers with abundant charred and silicified wood. Koholma features 0.5 meters of green-brown tuff with large clasts and plant remains, likely from volcanic sliding flows.Norling, Ahlberg, Erlström & Sivhed (1993) p. 52 The Korsaröd Lagerstätte, a key outcrop, contains well-preserved fern fossils, linked to the Djupadal Formation and interpreted as a lahar deposit, comparable to modern Rotorua, New Zealand.Vajda, Linderson & McLoughlin (2016) p. 139

Lithology

The rocks at Djupadalsmölla are primarily volcanic tuff, composed of ash, sand, lapilli, and partially disintegrated basalt, with colors varying from blue-green to brown depending on weathering.Eichstädt (1883) p. 408 The tuff contains rounded grains, from pea to hazelnut size, cemented mainly by limestone.Eichstädt (1883) p. 409 Cavities are filled with secondary minerals like calcite, zeolites, and viridite, with cement consisting of limnic limestone and sandstone. The rocks are classified as either feldspar basalt or limburgite (glass basalt), containing augite, olivine, and minor magnetite and ferrite. The glass-like sections are porous, with olivine crystals and minor serpentine, cemented by limestone.Eichstädt (1883) p. 411 Volcanic bombs include red gneiss and amphibolite, alongside mica diorite, limestone, clay slate, and shale sandstone, often containing wood fragments.

At Lilla Hagstad, the lithology includes nepheline basalt with a dark glass matrix containing nepheline, augite, olivine, and magnetite crystals. Rounded lapilli, from hazelnut to pea size, indicate subaerial eruption products. The KBH2 borehole reveals volcaniclastic grains with swelling clay minerals from degraded ash and lapilli, cemented by calcite, chlorite, zeolites, and iron-rich siderite.Wahlquist (2023) p.20 Siliclastic interbeds, rare and initially linked to the Höör Sandstone, include arkose sandstone with ripple marks and sandstone/claystone layers with coal and plant remains.

Age and Correlations

The Djupadalsmölla deposits are dated to the Late Pliensbachian to Early Toarcian (approximately 184–176 Ma), based on palynology and radiometric data.Bomfleur, McLoughlin & Vajda (2014), Supplementary Material p. 2Vajda, Linderson & McLoughlin (2016) p. 134 A high-precision 40Ar/39Ar age of 176.7 ± 0.5 Ma from Lilla Hagstad confirms a Late Toarcian age. Earlier K–Ar dating yielded a range of 171–179 Ma, while 40Ar/39Ar analyses from volcanic plugs suggest 191–178 Ma. Palynological evidence from KBH2, including marine palynomorphs and specific pollen, supports a Latest Pliensbachian age.
The formation overlies the Höör Sandstone and correlates with the Rydebäck and Katslösa members of the Rya Formation, the Röddinge Formation, and the Sorthat Formation in Denmark, sharing abundant fern-derived miospores. It also aligns with the Fjerritslev and Gassum Formations in the Danish and Øresund Basins. Volcanic material was transported by fluvial channels to the Green Series of Grimmen and Dobbertin (Grimmen Formation), with clays likely derived from weathered volcanic deposits. Erosion of the underlying Hettangian–Sinemurian Höör Sandstone contributed sands to the North German Basin.

Basin history

The Djupadalsmölla deposits formed within the Sorgenfrei-Tornquist Zone (STZ), part of the Trans-European Suture Zone separating Baltica from peri-Gondwanan terranes, active since the Late Ordovician (~445 Ma). At the Carboniferous–Permian boundary (~300 Ma), the Skagerrak-Centered Large Igneous Province influenced the region.Bryan & Ferrari, 2013, p.1058

The basins of southern Sweden and eastern Denmark formed in the Late Triassic to Early Jurassic, coinciding with the Central Atlantic Magmatic Province (CAMP), the largest igneous province in Earth’s history. The Central Skåne Volcanic Province was active during the Pliensbachian to Toarcian (184–176 Ma), with Strombolian-style eruptions near Early Jurassic oceans. No correlative pyroclastic beds have been identified in surrounding basins.Ahlberg et al., 2003, p.539
During the deposition of the Rya Formation’s Rydebäck Member, the Toarcian turnover caused widespread extinctions.Harries & Little, 1999

Environment

The Djupadalsmölla deposits, with moderately sorted lapilli tuff and abundant wood, indicate a terrestrial environment influenced by freshwater. Likely deposited in a fluvial setting with debris flows from nearby volcanoes like Äskekull, the site includes sandstone layers with ripple marks, suggesting coeval deposition with pyroclastics. Barium-enriched zeolites suggest hydrothermal seawater circulation, causing diagenetic changes.

The KBH2 borehole indicates a weathered basement, likely altered in the 25 Ma between the Rhaetian and Late Pliensbachian, followed by a dark shale layer suggesting a marine-influenced lagoon or bay. Palynology reveals a humid, moderately warm climate with conifer-dominated forests, ginkgoes, seed ferns, and fern understories, occasionally disrupted by forest fires. Volcanic clasts and charred wood confirm deposition through forested areas into a shallow, low-energy water body, followed by marine transgression.

At Korsaröd, freshwater algae and fossilized wood suggest a river-influenced, Mediterranean-type biome with low rainfall and active Strombolian volcanism, comparable to modern Rotorua, New Zealand. The vegetation, dominated by Cupressaceae and Erdtmanithecales, faced volcanic disruptions and hydrothermal activity, with rapid wood permineralization. Well-preserved Osmundastrum ferns indicate a moist gully engulfed by lahar deposits.

Sandåkra Lake System

North of the volcanic outcrops, the Sandåkra Sapropel, up to 150 meters thick, includes sandstones, clays, oil shales, and breccias, partly coeval with the Djupadal volcanism. Deposited in a 70-meter-deep limnic/lacustrine deposit, likely formed in a tectonic breach or topographic depression, it lacks marine palynomorphs and contains volcanic minerals in its upper sections.Nilsson (1958) p. 5 The lake, fed by ephemeral streams, developed anoxic conditions, similar to modern Lake Rotokakahi in New Zealand or the Toarcian Sichuan Lake ( Ziliujing Formation), with upper sections influenced by volcanic material akin to the Waimangu Volcanic Valley.Tralau (1973) p. 154

Fossils

Pseudofungi

Fungi

Acritarchs

Dinoflagellates

Chlorophyta

Bryophyta

Lycophyta

Equisetopsida

Filicopsida

Peltaspermales

Erdtmanithecales

Gnetales

Cycadophyta

Ginkgophyta

Coniferophyta

Arachnida

See also

* List of fossiliferous stratigraphic units in Sweden
* Kristianstad Basin
* Toarcian formations

* Mizur Formation, North Caucasus
* Sachrang Formation, Austria
* Saubach Formation, Austria
* Posidonia Shale, Lagerstätte in Germany
* Ciechocinek Formation, Germany and Poland
* Calcare di Sogno, Italy
* Marne di Monte Serrone, Italy
* Lava Formation, Lithuania
* Krempachy Marl Formation, Poland and Slovakia
* Rya Formation, Sweden

References

Bibliography

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{{Mesozoic Sweden, state=expanded

Geologic formations of Sweden
Jurassic System of Europe
Early Jurassic Europe
Jurassic Sweden
Pliensbachian Stage
Toarcian Stage
Sandstone formations
Coal formations
Coal in Sweden
Paleontology in Sweden