Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85
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Palaeogeography, Palaeoclimatology, Palaeoecology
journal homepage: www.elsevier.com/locate/palaeo
Early Jurassic microbial mats—A potential response to reduced biotic
activity in the aftermath of the end-Triassic mass extinction event
Olof Peterffy a, Mikael Calner a, Vivi Vajda a,b,⁎
a
b
Department of Geology, Lund University, SE-22362 Lund, Sweden
Swedish Museum of Natural History, P.O. Box 50007, SE-104 05 Stockholm, Sweden
a r t i c l e
i n f o
Article history:
Received 6 June 2015
Received in revised form 4 December 2015
Accepted 22 December 2015
Available online 30 December 2015
Keywords:
Hettangian
Mass extinction
Microbial mat
Cyanobacteria
Wrinkle structures
Sweden
a b s t r a c t
Wrinkle structures are microbially induced sedimentary structures (MISS) formed by cyanobacteria and are common in pre-Cambrian and Cambrian siltstones and sandstones but are otherwise rare in the Phanerozoic geological record. This paper reports the first discovery of Mesozoic wrinkle structures from Sweden. These are
preserved in fine-grained and organic-rich heterolithic strata of the Lower Jurassic (Hettangian) Höganäs Formation in Skåne, southern Sweden. The strata formed in a low-energy, shallow subtidal setting in the marginal parts
of the Danish rift-basin. Palynological analyses of fine-grained sandstones hosting the wrinkle structures show
that the local terrestrial environment probably consisted of a wetland hosting ferns, cypress and the extinct conifer family Cheirolepidaceae. Palynostratigraphy indicates a Hettangian age, still within the floral recovery phase
following the end-Triassic mass extinction event. The finding of wrinkle structures is significant as the presence
of microbial mats in the shallow subtidal zone, (in a deeper setting compared to where modern epibenthic microbial mats grow) suggests decreased benthic biodiversity and suppressed grazing in shallow marine environments in the early aftermath of the end-Triassic mass extinction event.
© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Microbes and eukaryotic algae dominate the Precambrian fossil record, but with the evolution of grazing metazoans and increasing rates
of bioturbation in the early Phanerozoic, cyanobacteria became restricted to more marginal marine environments (Lan, 2015). However,
during time intervals of severe environmental stress with extreme
conditions, microbes could expand over a wider area of the shelf and
contribute to the development of anachronistic facies (Sepkoski et al.,
1991; Mata and Bottjer, 2012; Forel et al., 2013), atypical of
metazoan-dominated Phanerozoic sea floors. This has been documented in association with the Late Devonian (Wood, 2000) and endTriassic mass extinction events (Pruss et al., 2004; Kershaw et al.,
2012; Forel et al., 2009), and the late Silurian Lau Event, a taxonomically
small but ecologically important crisis (Calner, 2005, 2008). Younger
examples of this phenomenon in association with mass extinctions
have not been known until recently, when Ibarra et al. (2014) documented unusually extensive microbialites from the Triassic–Jurassic
boundary interval in southwestern United Kingdom, and suggested a
linkage to the end-Triassic mass extinction event.
⁎ Corresponding author at: Swedish Museum of Natural History, P.O. Box 50007, SE-104
05 Stockholm, Sweden.
E-mail address: [email protected] (V. Vajda).
Modern microbial mats are restricted to marginal marine environments, such as tidal flats and salt marshes where burrowing and grazing
is minimal. Rapid fluctuations in salinity, water depth, currents and
temperature form unfavourable conditions for benthic organisms, leaving the extremophile organisms to flourish. Fenchel (1998) showed that
a four month old mat can be completely ingested in a time span of
weeks by gastropods and other grazers. Post-burial burrowing may further obliterate mat laminae (e.g., Hagadorn and Bottjer, 1997; Mata and
Bottjer, 2009).
Microbial mats are formed by cyanobacteria, generally by multilayer
microbial communities composed of several cyanobacterium species
(Ramsing et al., 2000). The morphology of cyanobacterial mats is a
result of a combination of factors, such as: environment, sediment characteristics and dominant cyanobacterial species (Ramsing et al., 2000).
An advanced biofilm of extracellular polysaccharide forming a laterally
extensive, organic layer may be called a microbial mat (Noffke, 2010).
Microbial mats form through the cooperation of individual microbes,
which secrete a biofilm of strongly adhesive extracellular polymeric
substance (EPS; Stoodley et al., 2002). Physically, they behave like viscoelastic fluids, in that they may both undergo elastic, reversible deformation as well as deform irreversibly if subject to sufficiently high shear
stress for a certain amount of time (for most mats around 18 min;
Thomas et al., 2013). These films can develop on almost any surface
on Earth and their complexity even resembles that of eukaryotic tissue,
facilitating optimal temperature, salinity and nutrition levels for the
http://dx.doi.org/10.1016/j.palaeo.2015.12.024
0031-0182/© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85
suggested that wrinkle structures reflect the actual crinkled upper
surface of a living mat, whereas Noffke et al. (2002) and Noffke
(2010) argued that they form due to post-burial deformation of the
mat by the pressure exerted by the overlying sediments. The
overload causes dewatering as the liquid bound in the mat escapes
laterally, thus crinkling the mat. Porada et al. (2008) on the other
hand, proposed a model with tidally induced oscillations in ground
water, which liquefy and reorganize sediments trapped under the
sealing layer of a microbial mat. This effect would be strongest in
the shallow subtidal zone. Laboratory experiments with viscoelastic
films carried out by Thomas et al. (2013) developed this model
further. As the water and the biofilm both have different viscosities
and behave like two immiscible fluids, they respond differently to
shear stress. A harmonic so called Kelvin–Helmholtz instability is induced at the interface, giving rise to the characteristic Kinneyia ripple
pattern in the biofilm.
This study aims to describe Early Jurassic wrinkle structures from
shallow marine, siliciclastic strata at the Kulla Gunnarstorp coastal cliff
section (Fig. 1), in the southernmost province of Sweden. We further interpret the depositional environment and, through palynological analyses, investigate the temporal relationship between these wrinkle
structures and the end-Triassic mass extinction event. The global record
of Mesozoic MISS is very sparse, and Swedish MISS have thus far been
documented only from Palaeozoic strata (e.g., Martinsson, 1965;
Calner, 2005; Calner and Eriksson, 2011). Thus the Early Jurassic structures of this study are the youngest yet described from Sweden, and
constituent microbes (Costerton et al., 1995; Noffke, 2010). Biofilms and
EPS not only hamper erosion by binding sediment grains together; the
smooth upper surface also has the effect of reducing the shear stress
exerted by eroding currents (Paterson, 1994).
The preservation-potential of microbes differs depending on the
depositional context. In carbonate depositional environments, stromatolites and oncoids may be formed. Owing to early cementation, these
are rigid, generally easily distinguished structures with a long history
of research (e.g., Walter, 1976; Tewari and Seckbach, 2011). Their
counterparts in siliciclastic sediments are less well known, since the
preservation potential of microbes on those substrates is lower and
identification is more challenging leading to the under-representation
of fossil cyanobacteria mats from siliciclastic settings in the literature.
Nevertheless, over the past two decades, more research has focused
on traces of microbial activity in siliciclastic settings as these are
especially interesting for astrobiologists studying extra-terrestrial
analogues.
Wrinkle structures have been attributed to the sediment-stabilizing
ability of these microbial mats and is an umbrella term used for so called
runzelmarken, Kinneyia ripples and “elephant skin” (Hagadorn and
Bottjer, 1997, 1999). They are microbially induced, commonly oversteepened surface irregularities developed on sand- and siltstones
(Hagadorn and Bottjer, 1997, 1999). Kinneyia-type wrinkle structures
(or just Kinneyia structures) feature minute flat-topped, winding crests
separated by troughs or pits, resembling small scale interference ripples
(Martinsson, 1965; Porada et al., 2008). Hagadorn and Bottjer (1997)
0
100
200
77
300 m
N
Road 111
P
Norway
Sweden
nn
Fe
Denmark
ia
nd
ca
os
Kulla
Gunnarstorp
n
Helsingborg
er
rd
Bo
eå
se
n
Malmö
ne
el
om
R
Lund
e
Zo
on
tZ
ul
Fa ugh
n
o
le
Tr
da
b
le
om
Fy
V
ne
Zo
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Kulla Gunnarstorp
coastal cliff section
u
Fa
N
Jurassic strata
20 km
Fig. 1. Geological map of southern Sweden (with Sweden inset), Jurassic deposits highlighted in blue. The beach exposure at Kulla Gunnarstorp is marked with an arrow.
78
O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85
among the first in relatively close temporal connection with the endTriassic mass extinction event.
2. Material and methods
The wrinkle structures described herein derive from the lower part
of the Helsingborg Member (topmost Höganäs Formation) at Kulla
Gunnarstorp northwest of Helsingborg (Fig. 1; Pieńkowski, 1991a, b).
The studied specimens derive from strata immediately below and
above a distinct marker bed at the southernmost end of the Kulla
Gunnarstorp coastal cliff section. Additional rock samples from the
stratigraphic interval containing the wrinkle structures were analysed
for grain size and mineralogical content by light microscopy. The rock
slabs hosting the wrinkle structures illustrated herein are stored in the
reference collections at the Department of Geology, Lund University,
Sweden.
A single sandstone sample hosting wrinkle structures on the upper
bedding plane, and with the cyanobacterial film preserved, was
analysed palynologically with the aim to date the bed and to aid
palaeoenvironmental interpretation. The sample was processed according to standard palynological procedures at Global Geolab Ltd., Canada.
Approximately 10 g of sediment was first treated with 40% hydrochloric
acid (HCl) to remove potential calcium carbonate, and further macerated in 45% hydrofluoric acid (HF). The organic residue was sieved using a
10 μm mesh and mounted in epoxy resin on two microscopic slides.
Three hundred pollen and spores were counted and the percentage of
each palynomorph taxon was calculated (Table 1). The slides and
residues are deposited at the Swedish Museum of Natural History,
Stockholm, Sweden and illustrated specimens are identified by
S-numbers.
Table 1
Distribution of palynomorphs in the examined Kulla Gunnarstorp wrinkle structure sample with relative abundance in % for each taxa.
Palynomorphs
Number of specimens
%
Bryophytes
Foraminisporis jurassicus
Stereisporites antiquasporites
Stereisporites psilatus
Total bryophyte spores
2
2
2
6
0.7
0.7
0.7
2.0
Lycophytes
Acanthotriletes varius
Retitriletes austroclavatidites
Retitriletes semimuris
Uvaesporites spp.
Total lycophyte spores
2
7
6
2
17
0.7
2.3
2.0
0.7
5.7
Ferns
Deltoidospora spp.
Osmundacidites wellmanii
Conbaculatisporites mesozoicus
Cyathidites australis + C. minor
Laevigatosporites ovatus
Striatella scanica
Trachysporites fuscus
Total fern spores
66
8
3
51
2
1
6
137
22.0
2.7
1.0
17.0
0.7
0.3
2.0
45.7
Gymnosperms
Alisporites spp.
Araucariacites australis
Classopollis chateaunovi
Perinopollenites elatoides
Podocarpidites spp.
Ricciisporites tuberculatus
Vitreisporites pallidus
Total gymnosperm pollen
Total %
8
4
24
91
7
5
1
140
300
2.7
1.3
8.0
30.3
2.3
1.7
0.3
46.7
100.0
3. Geological setting and stratigraphy
Upper Triassic–Lower Jurassic strata of Sweden are restricted to the
southernmost province, Skåne and to adjacent offshore areas where
they form the marginal parts of the Danish Basin (Norling et al., 1993;
Ahlberg et al., 2003; Vajda and Wigforss-Lange, 2009; Pott and
McLoughlin, 2011; Vajda et al., 2013; Fig. 1). The succession includes
three distinct formations together representing a gradual transition
from continental to fully marine depositional conditions (Fig. 2). The
lowermost of these formations, the Kågeröd Formation (Norian), is composed of immature sandstones, arkoses and abundant extraformational
conglomerates formed through the accumulation of debris flows and
stream processes in proximal alluvial environments under low and
episodic precipitation regime. The overlying Höganäs Formation
(Rhaetian–Hettangian) is dominated by mudstone and fine-grained
sandstone with subordinate palaeosols and coal seams, reflecting an
unprecedented change in climate and run-off in the latest Triassic
(Pieńkowski, 2002). This environmental change, evident in the lithology,
has been attributed to the break-up of the supercontinent Pangea
(Ahlberg et al., 2003).
The Kulla Gunnarstorp coastal cliff section is exposed along the
shore c. 10 km north of the city of Helsingborg and belongs to the
uppermost part of the Höganäs Formation. The Höganäs Formation is
sub-divided into three members, namely the Triassic Vallåkra and
Bjuv members and the Jurassic Helsingborg Member (Fig. 2).The
Vallåkra Member is composed mainly of claystone, and reflects the
transition from the arid Pangaean Kågeröd redbeds to more humid
conditions. The Bjuv Member comprises floodplain deposits and hosts
a prominent coal seam at its base (known as the B-seam). The youngest
member of the Höganäs Formation is the Helsingborg Member, which
in its lowermost part comprises coarse-grained and poorly sorted
sandstones, the Boserup beds (Larsson, 2009). Palynological
assemblages from the Boserup beds show a transition from typical
Rhaetian to typical Hettangian palynomorphs, and were thus inferred
by Lindström and Erlström (2006) to include the T–J boundary. Overlying deposits reflect a transgressive phase and are predominately
composed of deltaic sediments, with some tidal marine influences
(Norling et al., 1993). These are the sediments investigated in this
study, at the Kulla Gunnarstorp section. The section has previously
been described by Troedsson (1951), Pieńkowski (1991a), Norling
et al. (1993) and Ahlberg (1994). Palynostratigraphical work has
been carried out by Larsson (2009). Troedsson (1951) and Ahlberg
(1994) attributed the section to the Hettangian, whereas both
Pieńkowski (1991a; using sequence stratigraphy) and Larsson (2009;
palynostratigraphy) concluded that the outcrop is of Sinemurian age.
Larsson's (2009) age assessment was based upon the occurrence
of the key taxon Cerebropollenites macroverrucosus and the spore
Retitriletes semimuris.
The basal part of the overlying Rya Formation (Sinemurian –
Aalenian) is composed of poorly consolidated and cross-bedded quartz
arenites (Döshult Member), including an extraformational conglomerate with crystalline basement clasts, overlain by mudrocks with abundant ammonites, the latter implying deposition in a fully marine,
although near-shore environment.
4. Results
4.1. Sedimentology
The Kulla Gunnarstorp locality exposes an approximately 250 m
long and 2–3 m high coastal cliff section (Fig. 1) with interbedded,
organic-rich, dark grey to black shale, mudstone and very fine grained
quartz arenite lenses. The strata dip slightly to the south so that at
least 7–8 m of stratigraphy is accessible. The sedimentary facies
comprise rhythmically bedded heteroliths displaying primarily wavy
to lenticular bedding (Fig. 3a), and subordinate flaser bedding.
O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85
System
Stage
Formation
Member
Early
Jurassic
Toarcian
Triassic
201.6
Pliensbachian
Rya Formation
Rhaetian
Norian
Spheripollenites-Leptolepidites
Katslösa Member
Chasmatosporites
Döshult Member
Hettangian
Helsingborg Mb
Höganäs Formation
Miospore Zone
Rydebäck Member
Pankarp Member
Sinemurian
79
Bjuv Member
Vallåkra Member
Kågeröd Formation
C. macroverrucosus
Pinuspollenites-Trachysporites
Transitional Spore-spike Interval
Corollina - Ricciisporites
Undefined
Fig. 2. Stratigraphic column of the Triassic–Jurassic units of Skåne correlated with local pollen zones (from Koppelhus and Nielsen, 1994; Larsson, 2009).
Subaquatic synaeresis cracks (Fig. 3b) occur in the lenticular facies, but
true, subaerial desiccation cracks have not been found anywhere in the
section.
Wrinkle structures were found in situ in what Pieńkowski (1991a)
referred to as Unit 5 (Fig. 3c–d). This unit consists of wavy bedded
heteroliths hosting trace fossils, such as Rhizocorallium (Fig. 3e), together with prominent drainage channels with intraformational basal conglomerates. The sand beds are generally fine-grained, but one 3–4 cm
thick, grey quartz-wacke forms a laterally continuous layer in this unit
and can serve as a marker bed (Fig. 3d). It consists of well-sorted,
coarse-grained sandstone with a white, silty matrix that weathers to a
rusty yellowish colour. It is weakly cemented and has a high effective
porosity. Mineralogically, the bed is dominated by quartz (c. 95%), but
feldspars and smaller grains of heavy minerals also occur. Thin coatings
of pyrite are evident on some grains.
4.2. Wrinkle structures
The wrinkle structures are preserved exclusively on the upper bedding planes of 1–3 cm thick beds of current-rippled quartz arenites
(Fig. 4a–f). The arenites are very well sorted and fine grained, with relatively high contents of mica. Epichnial, endichnial and hypichnial trace
fossils (Diplocraterion and Scolithos) are relatively common with bioturbation indices ranging from 2 to 4 (sensu Droser and Bottjer, 1986),
which means that the primary sedimentary structures are disturbed
but the sediment is not homogenized and individual burrows are clearly
discernible. The lower bedding plane also hosts horizontal furrows together with slightly irregular hypichnial protrusions, interpreted as
resting traces and scour marks.
The wrinkle structures cover flat surfaces of an area ranging from 30
to 60 cm2 and are all less than 1 mm high. Smooth current ripple
foresets occur adjacent to the wrinkles, sloping down into topographically lower, wrinkle-free areas (Fig. 4b–c).
The wrinkles occur either as elongate or pitted forms. The former
features up 1–2 mm wide, semi-continuous, commonly bifurcated and
subparallel ridges, which are separated by slightly wider (1–2 mm)
round-based troughs (Fig. 4d). The crests are flattened and slopes
over-steepened. The pitted forms on the other hand, have round bases
and are generally oval, with their short axes ≤1 mm and the long axes
1–3 mm (Fig. 4e). In many cases they are slightly interconnected with
each other, showing a transition to more elongate or kidney-shaped
forms. Slopes are steep or overhanging, and the crests form an irregular
network. Their morphology is concordant with the description of
Kinneyia-type wrinkle structures of Porada et al. (2008). Since the underlying bedforms are indiscernible below the wrinkles (non-transparent wrinkle structure of Noffke, 2000), these were probably formed by a
thick epibenthic mat (cf. Noffke, 2010).
Trace fossils are notably abundant in the upper bedding plane
(Fig. 4f) and preserved as up to 16 mm thick and several centimetres
long epichnial furrows, filled either with clay or medium- to even
coarse-grained sand. Backfill structures are common and occur in several of the furrows. All trace fossils in the upper bedding plane post-date
the formation of the wrinkle structures.
4.3. Palynology—stratigraphical and palaeoenvironmental implications
The sandstone sample containing the MISS studied for palynology
hosts a well-preserved terrestrial palynoflora of relatively low diversity,
and 15 spore and seven pollen taxa were identified (Table 1). The spores
attain c. 53% of the assemblage and gymnosperm pollen grains c. 47%. No
algae or dinoflagellates were encountered. Fern spores dominate the
spore component, which in turn are strongly dominated by the trilete
spores Cyathidites spp. (18%) and Deltoidospora spp. Key taxa, such as
Trachysporites fuscus and Striatella scanica occur in low numbers.
Lycophyte spores reach a relative abundance of 5% including the Early Jurassic key taxa Retitriletes semimuris and Retitriletes austroclavatidites.
Bryophyte spores reach 2%, represented by Stereisporites spp. and
Foraminisporites jurassicus (Fig. 5). The gymnosperm component is
strongly dominated by Perinopollenites elatoides, followed by Classopollis
chateaunovi (Fig. 5). Based on the composition of the flora (Table 1),
such as the abundance of specific taxa together with key-species, a
Hettangian age is inferred for the beds hosting the wrinkle structures.
The assemblage is similar to that identified by Larsson (2009) although less diverse, possibly due to the fact that Larsson sampled the
more productive mudstones that occur between the microbial matbearing sandstones within the sampled succession. Larsson (2009)
interpreted the Kulla Gunnarstorp strata as Sinemurian based chiefly
on the presence of the miospore taxa C. macroverrucosus and R.
semimuris, taxa that were also recovered in this study. However, ranges
of Cerebropollenites macroverucosus and Retitrilites semimuris extend at
least from the base of the Hettangian (Koppelhus, 1991; Vajda, 2001;
Cirilli, 2010; Lund, 1977; Morbey, 1975; Pedersen and Lund, 1980).
Therefore, none of these taxa serves as a marker for the Sinemurian. A
Hettangian age for the studied assemblage is instead supported by comparison with assemblages of similar composition, e.g., with the
Munkerup assemblage from Bornholm, Denmark of Koppelhus
(1991), characterized by high relative abundances of P. elatoides,
Pinuspollenites minimus, Deltoidospora toralis, Calamospora,
Chasmatosporites spp. and Alisporites spp. together with low numbers
of C. macroverrucosus. Koppelhus (1991) referred the Munkerup assemblages to Lund's (1977) Hettangian Pinuspollenites–Trachysporites
Zone. Cirilli's (2010) comprehensive work on northern European
Triassic–Jurassic palynological revealed that C. macroverrucosus ranges
from the topmost Triassic (where it is sparse), through the Hettangian
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O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85
Fig. 3. Illustrations of outcrop features at Kulla Gunnarstorp locality. (A) Tidally influenced heterolithic bedding with bi-directional ripples. (B) Trace fossils of the Skolithos ichnofacies and
numerous synaeresis cracks (subtidal desiccation cracks) on the upper bedding plane of current-rippled, fine-grained sandstone (some of the synaeresis marks marked with white
arrows). (C) Overview of strata in the southern end of the studied section. Note variation in heterolithic facies and erosional down-cutting between the wavy- and flaser-bedded facies
units. (D) Close up of in situ wrinkle structures. The rust-coloured bed in the top right is the marker bed. (E) Rhizocorallium ichnofossil immediately above marker bed.
(where it reaches low but consistent relative abundances), and up into
the Sinemurian. The presence of the pollen Ricciisporites tuberculatus is
further indicative of an assemblage older than Sinemurian and rather
indicates an early to mid-Hettangian age.
5. Discussion
5.1. Palaeoenvironment
The Early Jurassic was characterized by a hot climate with an atmospheric carbon dioxide concentration (pCO2) around 900–1000 ppm
(Beerling and Royer, 2002; Bonis et al., 2010; Steinthorsdottir et al.,
2011; Steinthorsdottir and Vajda, 2015; Sha et al., 2015); more than
double that of modern post-industrial levels.
The palaeoenvironmental proxies at the Kulla Gunnarstorp locality
indicate the presence of coastal wetland communities with ferns and
Taxodium (Perinopollenites) representing the major part of the flora.
Taxodium is included in the Cupressaceae family (cypress) and grows
presently in a broad range of warm moist habitats. Their co-existence
with ferns (spores making up over 50% of the assemblages) and the
lack of the drought-resistant Pinaceae indicate that the Taxodium pollen
grains identified within the studied sample were probably produced by
forms with wetland adaptations similar to the modern bald cypress. The
extinct Cheirolepidiaceae represent a group of gymnosperms that during the Jurassic were widespread globally. They were possibly related
O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85
81
Fig. 4. Illustrations of wrinkle structures found at Kulla Gunnarstorp. All scale bars = 1 cm. (A) Elongate wrinkle structures cut by epichnial tracefossils. Smooth area in the lower, middle
part represents a ripple foreset. (B) Close-up of elongate, subparallel ridges. Note the steep slopes of wrinkles, far exceeding the normal angle of repose for sand. (C) Pitted and reniform
morphology. (D) The wrinkle structure fills out the area between the ripple foresets, forming a leveling structure. (E) The one specimen found in situ, also forming a leveling surface. The
degree of bioturbation is not evident from the photo, but endichnial and hypichnial traces are abundant (bioturbation index 3). (F) Close-up of epichnial trace fossil shown in Fig. 4A,
formed by a vermiform invertebrate, cutting the ridges of the wrinkle structure.
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O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85
Fig. 5. Light micrographs of representative pollen and spores from the investigated samples, scale bar = 10 μm, same magnification for all illustrated specimens. Taxa, sample number and
England Finder Reference (EFR) and S- numbers. (A) Stereisporites antiquasporites (Wilson and Webster, 1946) Dettmann, 1963, WRS-1B, EFR-P40/3, (S085450). (B) Retitriletes semimuris
(Danze-Corsin and Laveine, 1963) McKellar, 1974, WRS-1B, EFR-Q31/1, (S085451). (C) Retitriletes austroclavatidites (Cookson, 1953) Döring et al., in Krutzsch 1963, WRS-1A, EFR-L23/2,
(S085452). (D) Deltoidospora spp. WRS-1A, EFR-S38/2, (S085453). (E) Deltoidospora spp., WRS-1A, EFR-D22/3, (S085454). (F) Conbaculatisporites mesozoicus Klaus, 1960, WRS-1A, EFRE20/1, (S085455). (G) Foraminisporis jurassicus Schultz, 1967, WRS-1B, EFR-O20/3, (S085456). (H) Cyathidites australis Couper, 1953, WRS-1A, EFR-O27/2, (S085457). (I) Acanthotriletes
varius Nilsson, 1958, WRS-1B, EFR-Q16/1, (S085458). (J) Striatella scanica (Nilsson) Filatoff and Price, 1988, WRS-1B, EFR-U16/2, (S085459). (K) Classopollis chateaunovi Reyre, 1970, WRS1B, EFR-K11, (S085460). (L) Perinopollenites elatoides Couper, 1958, WRS-1B, EFR-Q15/3, (S085461). (M) Vitreisporites pallidus (Reissinger) Nilsson, 1958, WRS-1B, EFR-D43/3, (S085462).
(N) Ricciisporites tuberculatus Lundblad, 1945, WRS-1A, EFR-L41/1, (S085463).
to the Araucariaceae or Cupressaceae (Watson, 1988; Mehlqvist et al.,
2009) and were represented by a range of growth forms from to large
trees to small shrubs (Kürschner et al., 2013). Although they seem to
have been adapted to a wide range of ecological settings, from arid to
wet environments including mangroves (Watson, 1988; Kürschner
et al., 2013), the xeromorphic morphological characters of their leaves,
have generally been interpreted as an adaptation to an arid, coastal environment. We envisage the Classopollis-producing conifers to have
inhabited a coastal setting, closer to the shore compared to the other
plants, growing on a drier, sandy substrate enriched in salts, a habitat
not suitable for the otherwise arid-tolerant pines. Importantly, we
interpret the presence of the disaster taxon R. tuberculatus to indicate
that the vegetation was still within the recovery phase after the Tr–J extinction event and close to the system boundary.
The presence of several (up to 10 m wide) channel sandstones with
basal intraformational conglomerates and a mudstone facies strongly
enriched in organic material, along with bi-directional current ripples
in the heteroliths, suggests an extremely shallow, marginal marine
environment with a pronounced palynological influence from nearby
wetland vegetation communities. There is no convincing evidence for
repeated subaerial exposure of the succession, as neither palaeosols
nor rootlets have been identified. Subaquatic synaerisis cracks
O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85
(Fig. 3b) are abundant in certain beds but true desiccation cracks have
not been observed.
The ichnofauna at Kulla Gunnarstorp has been interpreted previously as representing mainly marine conditions, but authigenic minerals
(pyrite and siderite) precipitated during early diagenesis suggest brackish water influences (Pieńkowski, 1991a; Ahlberg, 1994). For the above
reasons, the exposed strata are herein interpreted as representing a very
shallow subtidal setting subjected to tidal flat progradation and
shallowing only rarely resulting in subaerial exposure.
6. Astrobiological implications
Due to their 3.4-billion-year fossil record, cyanobacterial mats are
important targets in astrobiological research (Noffke, 2015 and references therein). Studying fossil microbial mats provides information
about early Earth's ecosystems and might serve as an analogue for
early life on other planets and celestial bodies in our solar system.
Photosynthetic cyanobacteria (commonly preserved as stromatolites)
were a driving force in the evolution of oxygen- dependent life
(Kershaw et al., 2012) and these microbes influenced and changed the
composition of the atmosphere.
Phanerozoic microbially induced sedimentary structures (MISS) are
normally associated with tidal deposits, in which the fluctuating salinity
and water levels form extreme environments limiting burrowing and
grazing by invertebrates, thus facilitating the formation and preservation of microbial mats. In modern tidal flats, differing hydraulic conditions and moisture levels on the tidal flat will lead to the development
of diverse types of MISS depending on where in the tidal zone they form.
The upper intertidal zone is drained and flooded daily by tidal currents, whereas the lower intertidal belt is dry only during neap tides recurring every two weeks. The subtidal zone is always submerged. In
modern settings, microbial mats grow thicker between the beach and
the upper intertidal/lower supratidal zone, only to thin out substantially
towards the subtidal zone (Noffke and Krumbein, 1999; Bose and
Chafetz, 2009).
The lower supratidal conditions allow for favourable mat forming
conditions, owing to a delicate balance between inundation and desiccation. The fluctuation of water levels causes an environmental stress
for grazers, while still supplying the mats with moisture. However, if
the mats are perpetually wet, they become soft and more susceptible
to grazing (Bose and Chafetz, 2009). This balance is one explanation of
why modern microbial mats dramatically decrease in thickness in the
lower intertidal zone and seaward.
Even though there is broad agreement that Kinneyia-type wrinkle
structures form beneath thick, cohesive microbial mats, their exact
mode of formation is still debated. As discussed earlier, modern
epibenthic mats mainly grow in the supratidal to intertidal zones, but
according to the tidal hydrodynamic and ground water flow model advanced by Porada et al. (2008), Kinneyia-type wrinkle structures would
preferentially form in the subtidal zone.
It is notable that the wrinkle structures described herein occur in
strata lacking rootlets or other signs indicative of prolonged subaerial
exposure. It is striking that even desiccation cracks, which form in a
matter of hours in tidal settings, are absent. Therefore we interpret the
strata to have been deposited in a very shallow, low-energy subtidal,
i.e., below low-tide level, setting. This signifies that they were formed
in a deeper setting compared to where modern epibenthic microbial
mats grow, which all occur above low-tide level. Regardless of which
model is invoked for the genesis of wrinkle structures, the palaeosetting
in the subtidal zone requires an explanation.
6.1. MISS and their relation to the end-Triassic mass extinction event
Based on statistical analyses of the stratigraphic ranges of marine
taxa, Raup and Sepkoski (1982) delimitated five major mass extinctions
in Earth history. In the aftermath of both the late Devonian and the end-
83
Permian mass extinction events, microbial mats spread into environments not occupied prior to the event, and started to dominate in the
environments that they already inhabited (Forel et al., 2013; Mata and
Bottjer, 2012).
The end-Triassic mass extinction is one of the less studied major mass
extinction events with regard to the precise timing and magnitude. Based
on U–Pb zircon dating of marine strata, the extinction has been dated at
201.6 ± 0.3 Ma by Schaltegger et al. (2008) and 201.31 ± 0.43 Ma by
Schoene et al. (2010). The event resulted in extinction of conodonts and
a severe decline of ammonoids and brachiopods among other groups,
but also led to dinosaurs taking over niches previously occupied by amphibians or mammal-like reptiles on land (Tanner et al., 2004; Akikuni
et al., 2010). Several extinction mechanisms have been proposed, including widespread volcanism (Central Atlantic Magmatic Province, CAMP)
associated with the break-up of Pangea (Marzoli et al., 2004; Tanner
et al., 2004).
Nonetheless, unlike the end-Permian mass extinction and despite a
decrease in ichnofauna, diversity and bioturbation rates during the
Rhaetian (Twitchett and Barras, 2004), a significant microbial response
associated with the end-Triassic mass extinction has so far been reported only from localities in the southwestern United Kingdom (Ibarra
et al., 2014). The general lack of stromatolites following the Tr–J event
has been attributed to a biocalcification crisis (van de Schootbrugge
et al., 2007; Mata and Bottjer, 2012; Vajda and Bercovici, 2014). However, such a scenario would not exclude microbial growths in siliciclastic
settings. We propose an explanation in which the expansion of
cyanobacterial mats in siliciclastic subtidal settings is a response to the
end-Triassic mass extinction, with lower faunal diversity and with a depleted invertebrate gene pool with inferior abilities to adapt to extreme
conditions. We propose that reduced grazing and bioturbation due to
the mass extinction event explain the presence and preservation of microbial mats in this Early Jurassic shallow subtidal zone. This is further
supported by the palynological results revealing that the enigmatic
“disaster” pollen taxon R. tuberculatus is present in the assemblage,
although in low relative abundance.
7. Conclusions
This study describes the first microbially mediated sedimentary
structures in the Mesozoic succession of Sweden and one of the first
global occurrences of earliest Jurassic microbial mats. The studied wrinkle structures are of Kinneyia-type and these form within the sediments
owing to the cohesiveness of microbial mats, although the exact mode
of formation remains unclear. Importantly, the studied Kinneyia structures derive from the shallow subtidal or lower intertidal zone, i.e., in
a deeper environment compared to where modern mats develop.
Biostratigraphy indicates a late Hettangian (earliest Jurassic) age of
the deposits, i.e., relatively soon after the Late Triassic mass extinction
event. Since perpetually wet, and thus soft, microbial mats are susceptible to predation. The presence of wrinkle structures may be explained
by lower bioturbation and grazing associated with the mass extinction.
List of taxa
Bryophyta (mosses)
Foraminisporis jurassicus Schultz 1967
Stereisporites antiquasporites (Wilson & Webster 1946) Dettmann
1963
Stereisporites psilatus (Ross 1943) Pflug in Thomson & Pflug 1953
Lycopsida (club mosses)
Acanthotriletes varius Nilsson 1958
Retitriletes austroclavatidites (Cookson 1953) Döring et al. in
Krutzsch 1963
Retitriletes semimuris (Danze-Corsin & Laveine 1963) McKellar 1974
Uvaesporites spp.
Filicopsida (ferns)
84
O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85
Conbaculatisporites mesozoicus Klaus 1960
Cyathidites australis Couper 1953
Cyathidites minor Couper 1953
Deltoidospora spp.
Laevigatosporites ovatus Wilson & Webster 1946
Osmundacidites wellmanii Couper 1953
Striatella scanica (Nilsson) Filatoff & Price, 1988
Trachysporites fuscus Nilsson 1958
Gymnospermopsida
Alisporites spp.
Araucariacites australis Cookson 1947
Classopollis chateaunovi Reyre 1970
Perinopollenites elatoides Couper 1958
Podocarpidites spp.
Ricciisporites tuberculatus Lundblad 1945
Vitreisporites pallidus (Reissinger) Nilsson 1958
Author disclosure statement
No competing financial interests exist.
Acknowledgements
This article is a contribution to United Nations Educational,Scientific
and Cultural Organization-International Union of Geological SciencesInternational Geoscience Programme (UNESCO-IUGS-IGCP) Project
632.
We thank the three anonymous referees for valuable comments and
suggestions. The laymen geologists of the Helsingborg Geology Club,
Sweden are acknowledged for sharing their collections from the Kulla
Gunnarstorp outcrop and wrinkle structures at this locality. Russ
Harms GeoLab Ltd. is thanked for palynological processing and Andrea
Santivanez for taking the miospore micrographs. This research was
jointly supported by the Swedish Research Council (VR) (201504264), Lund University Carbon Cycle Centre (LUCCI) funded through
the Swedish Research Council (grant 349-2007-8705) and the Royal
Swedish Academy of Sciences through the Swedish National Committee
for Geology to V.V.
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