Geodinamica Acta 18/6 (2005) 431–446
Paleoenvironment reconstructions and climate simulations
of the Early Triassic: Impact of the water and sediment supply
on the preservation of fluvial systems
Samuel Péron a, Sylvie Bourquin a*, Frédéric Fluteau b and François Guillocheau a
a Géosciences
Rennes, UMR 6118 du CNRS, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France;
de Paléomagnétisme, IPGP, Université Paris VII, 4 place Jussieu, 75252 Paris cedex 05, France
b Laboratoire
Received: 14/12/2004, accepted: 07/11/2005
Abstract
Paleoenvironmental reconstructions and climatic modelling allow us to investigate the influence of water and sediment supply on the
preservation of fluvial systems within a given geodynamic context. To simulate climate, we need global-scale paleoenvironmental and
paleotopographic reconstructions. However, the present study only covers the West-Tethys domain, where sedimentological and stratigraphic data allow us to check climate simulation results against geological data. We focus our modelling on the Olenekian, with the aim
of characterizing the impact of climate on fluvial sedimentation in the West-Tethys domain. The climatic simulations show that paleoclimates differ between Western Europe and North Africa. A more humid climate is simulated over North Africa, whereas a rather arid
climate prevails over Western Europe. In Western Europe, the sediments are preserved for the most part in endoreic basins and the presence of rivers in an arid environment suggests that these rivers are mainly fed by precipitation falling on the North Africa Variscan
Mountains. In North Africa, sedimentation is exclusively preserved in exoreic basins (coastal plain sediments). Consequently, the lack
of preserved fluvial systems in endoreic basins in North Africa either could be due to a shortage of accommodation space in this area, or
is linked to the climatic conditions that controlled the water and sediment supply.
© 2005 Lavoisier SAS. All rights reserved.
Keywords: Fluvial sedimentation; Early Triassic; Paleogeographic maps; West-Tethys domain; Climate simulations
1. Introduction
The deposits of fluvial environments are preserved either
in exoreic basins, where the fluvial systems are connected to
the sea, or in endoreic basins, without any marine connexion.
Fluvial systems in endoreic settings are mostly preserved
during warm arid or during humid climatic phases, which are
associated with aeolian or lacustrine deposits, respectively.
This can be observed at the present day [1], [2], [3], [4]… as
well as through geological time [5], [6], [7], [8], [9]…
* Corresponding author.
E-mail address: [email protected]
© 2005 Lavoisier SAS. All rights reserved.
In the continental realm, the main difficulty is quantifying
the impact of allocyclic factors (climate, eustatic effects,
deformation and sediment supply, Fig. 1) on the preservation
of sedimentary systems [10], [11], [12], [13], [14], [15]. In
fact, the preservation of stratigraphic cycles is controlled by
two parameters: the accommodation (A) and the sediment
supply (SS) [16], [17], [18], [19], [20], [21], [22]. The
accommodation characterizing the space available for sedimentation is controlled by the deformation of the basin
(subsidence/uplift of the basin, denoted ASU, fig. 1), eustatic
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Fig. 1 Influence of allocyclic parameters along longitudinal profiles with multiple local baselevel.
(noted E, Fig. 1) and/or lake level variations (noted LL,
Fig. 1, controlled either by basement deformation, noted
ASU, or eustatic, noted E, and/or climate, noted ALL). The
sediment supply (noted SS, Fig. 1) is controlled by the climate (noted Sc, Fig. 1) and/or by the deformation of the basin
basement (noted SD, Fig. 1) or the source area of the siliciclastic sedimentation [23], [24], [25], [26], [27], [28]…
Consequently, sedimentation is controlled only by two
external factors, climate (noted C, Fig. 1) and deformation
(noted D, Fig. 1), while eustatic variations (E) are tectonically (noted ATE, Fig. 1) or climatically induced (noted AGE,
Fig. 1)).
To investigate the relationship between the sedimentation, climate and deformation of basins, we use both
sedimentological analyses and climate modelling to study
the well-preserved Triassic fluvial successions of Peri-Tethyan basins. The Triassic is a transitional period associated
with the onset of the break-up of the Pangean supercontinent
and the formation of the Mesozoic basins in a globally warm
and dry climate. Detailed sedimentological and stratigraphic
analyses of Triassic fluvial deposits within the European
geodynamic context allow us to characterize the evolution of
the fluvial style through time.
Numerical modelling is a powerful tool for simulating climates at both global and regional scales. Climate represents
an equilibrium state between the different components of the
global climatic system (atmosphere, ocean, biosphere, cryosphere and lithosphere surface). Because of the complexity
of this system, and despite the development of powerful
computers, the entire climatic system cannot be simultaneously simulated. Nevertheless, present-day or past climates
can be modelled by specifying certain components of the climatic system as boundary conditions, such as
paleogeography, sea-surface temperature and cryosphere.
These boundary conditions should be reconstructed using
the best available data. Although this is an easy task for the
present day, it is much more problematic for the distant past.
To overcome this difficulty, the sensitivity of climate is
investigated with different boundary conditions that take
account of these uncertainties. In this study, we test the sensitivity of climate to the paleotopography and, consequently,
its influence on sediment supply and the preservation of fluvial systems.
We focus our modelling on the Late Early Triassic
(Olenekian), and the impact of climate on fluvial sedimentation in the western Tethys domain. Indeed, we need globalscale reconstructions to simulate climate. In this study, however, we focus on Western Europe, where sedimentological
and stratigraphic data allow us to check the results of climate
simulation against geological data.
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2. Paleogeographic reconstructions
2.1. Plate tectonic reconstructions
In the framework of this study, the plate tectonic reconstruction for the Early Triassic makes use of paleomagnetic
data while assuming a geocentric axial magnetic field. The
paleolocations of Laurussia (North America and Baltica)
and Siberia have been calculated using apparent polar wander paths (APWP) defined by well determined poles [29].
Because the Gondwanan poles are relatively sparse, the
position of this continent with respect to Laurussia has been
extensively discussed in the paleomagnetic literature (two
alternative Pangean plate reconstructions, respectively
called Pangea A and Pangea B, have been proposed [30],
[31], [32]. However, this problem is beyond the scope this
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paper (for further discussion see [33], [34]. We here consider
the best and most recent paleomagnetic poles to compute an
APWP for Gondwana [33], [35], [Besse, pers. comm.].
Southern Asia is made up of a mosaic of accreted blocks that
rifted away from Gondwana during the Late Paleozoic and
Mesozoic [36], [37]. The locations of these blocks during the
Early Triassic have been calculated using the paleomagnetic
poles from Enkin et al. [38].
2.2. Land-sea distribution
Two paleogeographic reconstructions are attempted here:
we firstly build up as accurate a map as possible on a global
scale ([39] to [43], Fig. 2A), and, secondly, a map at higher
resolution focused on West Tethys ([44] to [67], Fig. 2B). To
establish this second paleogeographic map, we use pub-
Fig. 2 Palaeogeographic maps for Scythian (245 Ma). (A) Global paleogeographic map representing a synthesis of different reconstructions: (i) Plate
tectonic kinematic reconstruction (after [30]), ([35]), Besse, pers. comm., (ii) shoreline and palaeoenvironment reconstructions (after [39], [40], [41],
[42], [43]), (iii) topography reconstructions (after [34], [40]) (B) detailed palaeogeographic map of the West-Tethys domain: (i) kinematic
reconstruction (see above), (ii) reconstructions of palaeoenvironments and fluvial systems: in Germany (after [44], [45], [46], [47]), in England (after
([48], [49]), in the Netherlands (after [50], [51], [52]), in the North Sea (after [53]), in the Maghreb (after [54], [55]), in Spain ([56], [57], [58]), in the
Eifel basin (after [59], [60]), in southern Scandinavia (after [61]), in Sardinia (after [62], [63] [64]) and in France (after [65], [66]), and Gaetani ([67]),
(iii) topographic reconstructions (after [34], [40]).
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lished paleogeographic maps [42], [43], [67], [68] as well as
sedimentological and sequence stratigraphy analyses of the
continental environments. During the Early Triassic, the
oceanic areas are dominated by the Panthalassa and the
Tethys Oceans. On the world scale, the land-sea distribution,
the continental sedimentation areas and the orogenic belts
are estimated from published data. On the scale of Western
Europe, the sedimentological studies and sequence stratigraphy analysis of the continental environments allow us to
constrain the river basin size, the areas in erosion, transit and
sedimentation, as well as the fluvial style (alluvial fan,
braided, meandering or anastomosing) and the associated
paleoenvironments (aeolian, flood-plain, lacustrine or sabkha). We consider the paleogeographic maps for the Late
Scythian (Olenekian).
2.3. Paleotopography
It remains very difficult to estimate the elevation of
mountain ranges in the distant past. There is still no absolute
method for quantifying paleoelevations at the global scale
and through geological time. Nevertheless, indirect methods
can provide a range of paleoelevations. The estimates of
mountain height are firstly based on the geodynamic context. The Early Triassic represents a transition between a
period of mountain building (Variscan and Urals orogenic
belts) and the initial stages of the break-up of Pangea. However, since there are still many uncertainties about
paleoelevations, we propose two reconstructions here: the
first one corresponds to a low-relief scenario (Fig. 3A), and
the second to a high-relief scenario (Fig. 3B).
In the high-relief scenario (Fig. 3B), the Appalachians
underwent their final orogenic event during the Early Permian. The estimates of elevation of the Appalachians vary
from 3 [69] to 6 km [70]. According Fluteau et al. [34], a
moderately elevated mountain range about 4 km high still
existed during the Late Permian. Thus, an elevation of about
3,000 m (slightly less at the model resolution) is attributed to
the highest peaks of the mountain range in the Olenekian.
The Hercynian (Variscan) mountain range reached its maximum height during the Late Carboniferous. Fluteau et al.
[34] suggests that the height of the range could have been
around 2000-3000 m in the Late Permian. We can thus estimate that the elevation of the Hercynian (Variscan)
mountain range was about 1,500 m during the Olenekian.
This result is confirmed by sedimentological analyses of fluvial successions in France, which allow us to constrain the
paleoelevation of these series using the topographic slope
gradients of each depositional environment [15]. Western
Europe is also characterized by numerous areas of moderately high relief that are generally elongated N-S, uplifted
during the onset of the Pangean breakup (Fig. 2B). These
topographic areas correspond to the uplifted parts of
grabens. An elevation of about 500 m is proposed for such
areas. The collision between Laurussia and the Siberia-
Fig. 3 Paleoelevations reconstruction of the Early Triassic (Olenekian)
for climate simulations: (A) low-relief scenario and (B) high-relief
scenario for the Hercynian (Variscan) and Appalachians.
Kazakstan block led to the uplift of the Urals during the Late
Permian [39], [40], [43]. Large amounts of siliciclastic sediment were deposited farther westward in the Urals seaway,
suggesting that this mountain range was strongly eroded by
mid-latitude precipitation during the Late Paleozoic and
Early Mesozoic. This sedimentation also suggests that the
relief was high enough to trigger precipitation over its flank.
Based on these considerations, we estimate that the elevation
was about 3 km during the Early Triassic. The southern
landmasses are characterized by some remnants of old
mountain ranges and by the Gondwanalides along the Panthalassa coast [41], [42], [43].
In a lower elevation scenario, we reduce the elevation of
the Hercynian (Variscan) —Appalachians mountain range
by a factor of 2. This lowering allows us to test the sensitivity
of the climate to the height of the mountain range in the
West-Tethys domain.
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3. Climate modelling
We used the Laboratoire de Métérologie Dynamique
(LMD, France) Atmospheric General Circulation Model
(LMDZ) [71]. It involves a 3D climatic model based on the
equations of atmospheric dynamics, thermodynamics and
the physics of the atmosphere. This 3-D atmospheric model
has a standard resolution of 96 points regularly spaced in
longitude, 71 points regularly spaced in latitude and 19 vertical layers unevenly spaced from the surface to 40 km high.
The equations are solved using a discretized atmosphere.
The purpose of each numerical experiment is to calculate the
equilibrium of the atmosphere with boundary conditions
representing the other components of the climatic system
(paleogeography, surface temperature of the oceans, albedo
and roughness of the ground, extension of the ice caps,
orbital parameters, chemical composition of the atmosphere,
CO2 in particular, and the solar constant). The experiments
are run for 20 years. Then, to remove the effects of the internal variability of the model and obtain the average climate,
the mean values are calculated over the final 10 years. These
experiments are qualified as “snap-shot experiments”. This
grid point model includes a full seasonal cycle as well as a
diurnal cycle. This 3D atmospheric model has been
employed to investigate present and past climates [72, 73,
74, 75, 76]. Moreover, the LMDZ model has been validated
by checking its capacity to simulate the current climate.
The two paleogeographic reconstructions discussed here
are used to force the LMDZ Atmospheric General Circulation Model. Other boundary conditions are fixed in the set of
experiments. There is no global dataset of sea-surface temperatures (SST) for the Early Triassic. Thus, we have to
define SST on a daily basis for each oceanic grid point. We
applied the same SST values as in Fluteau et al. [34], which
have been used to simulate the Late Permian climate.
The CO2 atmospheric content is fixed at 1,200 ppm
according to the geochemical modelling of the carbon cycle
due to Berner [77]. Nevertheless, we know that large uncertainties remain. In the present study, we lower the solar
constant by 1% (1,343 Wm–2 instead of 1,370 Wm–2), and
use the present-day orbital parameters. We also assign the
values of albedo and roughness of continental points according to vegetation. Because of the lack of any global
biospheric reconstruction, we use a “transfer” function calibrated on the present day to assign an albedo and a roughness
at each grid point according to the elevation and the latitude.
After the Permo-Triassic ecological crisis, the Early Triassic
vegetation could have been relatively sparse or different
from that mimicked by the albedo and roughness values proposed in this study. Nevertheless, our primary goal is to
investigate the sensitivity of climate to paleogeography.
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4. Sedimentation of the Triassic successions
in the western Tethys domain
A sedimentological study was first focused on the Paris
Basin, where a complete set of subsurface data (cores and
well-logs) is available [7], [78], [79]. Comparisons and correlations with the Germanic Basin [44], [45], [46], [80], [81]
allow us to determine the evolution of the river basins from
the Early to Late Triassic in Western Europe (Fig. 4). At the
beginning of the Early Triassic (Induan), the sedimentation
area was restricted to the central part of the Germanic Basin:
the Lower Buntsandstein Formations, which are characterized by a playa system, are only found in the central basin
and do not have any equivalents on the western margin [82].
These deposits characterize the regressive tendency of the
uppermost Zechstein (“Bröckelschiefer”), which continues
into the Lower Buntsandstein [46], [83]. The angular unconformity at the base of the Upper “Bröckelschiefer” was
caused by non-deposition or erosion of the uppermost Zechstein at the basin margin [45], [84]. During the Olenekian
(Figs. 2B; 4A), the sedimentation is characterized in the
western Germanic basin by braided fluvial systems evolving
laterally, toward the central part of the basin, into more or
less ephemeral lakes [44], [45], [46], [75], [80], [82] and
aeolian deposits [50], [85]. Moreover, in the northern Germanic Basin, alluvial fan deposits are described that come
from the Fennoscandian Massif (Fig. 2B). During the Olenekian, in the north-western Tethys domain, fluvial deposits
are preserved in a large endoreic basin.
Above the major Hardegsen unconformity, dated as
Olenekian [45], [51], [86], [87], the Triassic exhibits a
change in fluvial system style. This discontinuity is marked
by an erosional surface, characterized by the first development of paleosols (“Zone limite violette”). During the
Hardegsen Phase, an important structural event occurs in
NW Europe [51], leading to the formation of rift systems in
NW Germany [45], [88]. Above this surface, the Triassic
series show an onlap relationship, which implies some tectonic activity just before the renewal of fluvial
sedimentation. The fluvial systems are the lateral equivalents of playa and, eventually, sabkha and marine deposits.
In Anisian and Ladinian times, the Germanic Basin is connected with the Tethys Ocean [89], and the marginal fluvial
systems are developed in an exoreic basin setting (Fig. 4B).
In the Paris Basin, the Upper Triassic sedimentation (Fig.
4C) represents braided rivers (Carnian and Norian) associated with lacustrine environments [7], [15]. In the central
basin, the climate is arid and characterized by evaporitic sebkhas [79], [90]. These fluvial systems are located in more
numerous small drainage catchments (around 50 km long)
compared with the Early Triassic systems (> 1,000 km
long). In fact, there is no a single drainage catchment, but a
succession of small catchments that reflect the independence
of the present-day Paris Basin from the Germanic Basin
since the Upper Carnian [78], [79].
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Fig. 4 Evolution of fluvial systems from the Early to Late Triassic in Western Europe. (A) During the Olenekian (after [66]), at the scale of the
Germanic Basin, the rivers sources are mainly located in the present-day Armorican Massif, the paleocurrents indicate a general direction towards the
NNE. (B) During the Anisian, the fluvial systems are connected to the Tethys sea. (C) During the Lower Carnian and Norian, fluvio-lacustrine
sedimentation takes place (after [7]).
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This Triassic fluvial evolution (Fig. 4), from large endoreic river basins to a coastal system and small river basins,
could result from the geodynamic evolution of Pangea. In
fact, the Early Triassic represents the beginning of the Pangea break up, and the formation of large endoreic basins
where sediments could be preserved. Consequently, the
preservation of fluvial systems and their spatial distribution
can be attributed to a tectonic setting controlling the accommodation space. On the other hand, the stratigraphic
architecture observed in the fluvial environments could be
the result of other allocyclic factors.
In fact, the sedimentological studies of Triassic fluvial
deposits located in the western part of the Germanic Basin
[7], [66], [91], [92], reveal stratigraphic cycles at different
scales (ranging from genetic units, with a duration of 20 to
100 ka, to minor cycles with a duration of 1 to 5 Ma). The
stratigraphic cycles reflect the variations in accommodation
space (A) and sediment supply (SS). At the scale of the
smallest stratigraphic cycle, the A and SS variations cannot
be tectonically induced. For the Middle Triassic (i.e. Ladinian to Anisian) and the Upper Triassic (i.e. Carnian to
Norian), the smallest stratigraphic cycles reflect sediment
supply and/or water table variations induced by changes in
sea level [66] or lake level [7], [15]. For the Early Triassic
(i.e. Olenekian), the sedimentological study of Péron [92]
leads to the following reconstruction of the depositional
environments (Fig. 4A): (1) braided rivers in an arid context,
with very few floodplain deposits, (2) marginal erg (braided
river and aeolian deposits and (3) playa lake environment
(fluvial and ephemeral-lake environment). The river catchments are mainly located in the present-day Armorican
Massif, and the paleocurrent directions are generally
towards the NNE (Fig. 4A). The facies associations are
essentially composed of stacked channel-fill facies with very
few flood plain deposits (< 3%) and without paleosols. In
more distal areas of the Germanic Basin, the sedimentation
corresponds to ephemeral lakes or aeolian deposits [46],
[50], [85], [93]. In the northern part of the basin, alluvial fan
sediments are present (Fig. 4A). In this arid context, very little influence is seen from sea-level variations, excepted
locally in the extreme eastern part of the Germanic Basin
[94] that corresponds to the maximum westward extent of
playa environments [66]. The smallest stratigraphic cycles,
which are observed within an environment without communication with the open sea, reflect sediment supply and lakelevel variations controlled by climate variations. Within the
Triassic series, this could arise from variations of precipitation in a warm climate. For the Olenekian, two hypotheses
could explain the preservation of fluvial systems in the European basins. According to one hypothesis, the siliciclastic
sediment supply from the Hercynian (Variscan) range was
constant, and thus the stratigraphic cycles result solely from
lake-level fluctuations. The lake levels could be controlled
by monsoonal activity, under the influence of sea-level fluctuations [80]. Alternatively, the basin was situated in an arid
context and the stratigraphic cycles result from sediment and
437
water supply variations controlled by precipitation over the
mountain ranges. Climate simulations are used to investigate
the relationships between sedimentation and climate during
the Early Triassic (Olenekian).
In North Africa, fluvial sedimentation during the
Scythian occurs only in the Tunisian and Libyan basins and
is characterized by coastal plain deposits [54], [55], [95]. In
the other Triassic basins (Morocco, Algeria), an angular
unconformity at the base of the Triassic represents the
boundary between pre-Triassic deposits and the Middle or
Upper Triassic sedimentary succession. Fluvial sedimentation without sea connection seems to be absent in this
western part of Tethys.
5. Climate simulation results
Two experiments (low topography, Fig. 3A, and high
topography, Fig. 3B) were run to simulate the Olenekian climate and test the impact of orography on climate at that time.
Firstly, we discuss a climate simulation in the case of low
topography (Fig. 3A). Then, we compare these results with
simulations in the case of high topography (Fig. 3B). Both
experiments are compared with proxy data. Finally, we consider the consequences of the simulated climate on
sedimentation over Western Europe.
The climate simulated in the low relief experiment is globally warm and mostly dry (Fig. 5). Indeed the global annual
mean surface temperature is 17.3°C, about 4°C above that of
the present-day. The difference in surface temperature
results from a change in land-sea distribution as well as the
absence of ice sheets and sea ice during the Scythian. The
global annual mean precipitation is about 3.2 mm/day
(1,150 mm/year). In fact, these first observations mask
important disparities in temperature and rainfall over the
Earth’s surface as well as large seasonal fluctuations.
The continents of Laurussia and Gondwana are separated
by a mountain range (mostly represented by the Appalachians) trending along a SW-NE axis. Because of the high relief
(up to 1,800 m, Fig. 3B), the mean annual surface temperature does not exceed 25°C (Fig. 5A). By contrast, the mean
annual surface temperature exceeds 30°C over two vast
mountainous areas located on either side of the equator, one
in Laurussia (including North America, southern Greenland,
and western Europe) and the other in northern Gondwana
(northern part of South America and North Africa).
When averaged annually, the simulation yields heavy
precipitation over the western part of the Appalachians,
whereas the easternmost part close to the Paleotethys Ocean
remains dry (Fig. 5B). In the subtropics, the two warmer
zones are characterized by weak precipitation, although Laurussia is much drier than northern Gondwana (Fig. 5B). A
vast area extending from the coast of Panthalassa to northern
Paleotethys receives less than 0.5 mm/day (180 mm/year) of
rainfall, which is close to the present-day threshold value for
a desert environment. In the southern hemisphere, the
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Fig. 5 Climate simulations for the Early Triassic (Olenekian), assuming low relief of the mountain range separating the continents of Laurussia and
Gondwana.
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warmer area (expressed as an annual mean) does not correspond to a drier climate. In fact, rainfall in the northern part
of Gondwana (North Africa and the Arabian Peninsula,
Fig. 5B) ranges from at least 1 mm/day (360 mm/year) up to
5 mm/day (1,800 mm/year).
The mean annual precipitation in fact masks seasonal
changes both over the area of equatorial relief and in the subtropics. Precipitation occurs during the winter and spring all
along the mountain range, whereas the summer is dry for a
major part of this range (Fig. 6). A dissymmetry in precipitation on either side of the equatorial relief is controlled by
the geographic position of the relief and the axial trend of the
mountain range: moisture mainly arrives from Panthalassa
and the simulated precipitation over Laurussia remains weak
throughout the year, whereas a dry/wet season is simulated
over northern Gondwana. The summer overheating favours
the development of a thermal low pressure cell over Laurussia and northern Gondwana (Fig. 6A). Moist air masses
coming from Paleotethys cannot penetrate farther inland, so
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precipitation is very weak and does not exceed 0.5 mm/day
as observed during the autumn (Fig. 6B). During the winter
(austral summer, Fig. 6C), the paleogeographic configuration of Gondwana shifts the thermal low pressure cell
southwards, allowing Panthalassan moist air masses to penetrate far inland over the northern part of Gondwana, leading
to mixing with the Paleotethys moist air masses. Thus, the
austral summer is marked by a monsoon-type circulation
along the southern Paleotethys coast bringing moisture and
producing precipitation over north-eastern Gondwana. During the spring (austral autumn, Fig. 6D), a rainy season
(2-4 mm/day) is simulated over northern Gondwana, which
is due to the onset of the warming of the northern subtropics
and a weakening of the thermal low pressure zone above
northern Gondwana. We observe a maximum of radiative
solar flux located over the equatorial mountain range, which
amplifies precipitation over the relief, as well as a low-pressure cell, which remains in a southerly position
(corresponding more or less to the arid area). As for winter,
Fig. 6 Detail of West-Tethys climate simulations during the Early Triassic (Olenekian), assuming a low-relief scenario and for each season: (A)
summer, (B) autumn, (C) winter (austral summer), and (D) spring (austral autumn). See figure 5 for the location.
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rainfall from Panthalassan moist air masses can penetrate far
inland over the northern part of Gondwana.
At high latitudes (Siberia and southern Gondwana), the
mean annual temperature is close to freezing point, and may
even fall below this threshold value locally over elevated
areas (Fig. 5A). A strong latitudinal gradient is thus simulated in both hemispheres at mid-latitudes. This strong
thermal gradient drives the mid-latitude precipitation belts,
which could be locally enhanced over mountain ranges such
as the Urals (Fig. 5). A similar intensification is seasonally
observed over the Fennoscandian Mountain ranges. The
highest precipitation is generally simulated during the spring
of both hemispheres when the thermal gradient is strongest.
The Olenekian climate clearly depends on the prescribed
boundary conditions. However, the elevation of the mountain ranges remains a major source of uncertainty. Thus, we
performed a second experiment taking account of Hercynian
(Variscan) —Appalachians and Fennoscandian Mountain
ranges with higher relief (Figs. 3B, 7). The difference in elevation (Fig. 7A) between the two experiments reaches about
1,000 m for the highest areas of the Appalachians, and about
500 m for the eastern part of this range as well as the mountains located on both sides of the Arctic Seaway. Figures 7B
and 7C present the differences in mean annual temperature
and precipitation in response to the difference in elevation.
The change in elevation evidently leads to a cooling of more
than 6°C over the highest summits, along with a warming
over the adjacent lowlands in both hemispheres that exceeds
1°C over Gondwana. This warming could exceed 2°C over
northern Gondwana in winter. A decrease in precipitation is
simulated in the lowlands of Gondwana along the mountain
range (Fig. 7C). This drying is more pronounced in winter
and spring. No change is simulated during the summer and
autumn, which already correspond to the dry season. There
is no change in precipitation over Laurussia. A change in elevation modifies the atmospheric circulation, increasing the
advection of moisture over the relief, whereas the adjacent
lowlands become drier. This drying implies a decrease in
cloudiness and favours climatic warming.
We also compared the climate simulations with proxy
data.
6. Comparisons between climate simulations
and sedimentological analysis
While the climatic simulations were carried out at a global scale, our study is nevertheless focused on the western
Tethys basins. In this area, we can compare the simulations
with sedimentological data, allowing us to interpret the
results in terms of sediment supply and fluvial system preservation. We calculated monthly mean precipitation (i.e.
MMP, Fig. 8) and the monthly mean difference between
evaporation and precipitation (i.e. MMEvP, Fig. 9) and the
mean monthly temperature (i.e. MMT, not shown) in different regions selected according to their respective
Fig. 7 Detail of West-Tethys simulations during the Early Triassic
(Olenekian). (A) The difference in elevation between the two
experiments reaches about 1,000 m for the highest areas of the
Appalachians, and is about 500 m for the eastern part of this range and
for mountain ranges located on both sides of the Arctic Seaway.
Difference in mean annual temperature (B) and precipitation (C) in
response to the difference in elevation for the western Tethys domain.
See figure 5 for the location.
paleoenvironments. Three areas belonging to Western
Europe are named “Germanic Basin” “Spain” and “Western
France” (Figs. 8A, 9A), while an area located in the northern
Gondwana continent is named “North Africa”, and another
area, named “North Africa Variscan Mountains” corresponds to the eastern part of the Hercynian (Variscan) —
Appalachians mountain range (Figs. 8A, 9A).
S. Péron et al. / Geodinamica Acta 18/6 (2005) 431–446
441
Fig. 8 Monthly mean precipitation for the low- and the high-relief scenario. (A) Location of the different regions selected, F: Fennoscandian
Mountains, GB: Germanic Basin, W: Western France, S: Spain, NAV: North Africa Variscan Mountains, NA: North Africa (See figure 2B for the
location). (B to G) Monthly mean precipitation for these different regions.
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S. Péron et al. / Geodinamica Acta 18/6 (2005) 431–446
Fig. 9 Monthly mean difference between evaporation and precipitation for the low- and the high-relief scenario for the sedimentation areas. (A)
Location of the different sedimentation areas, GB: Germanic Basin, S: Spain, NA: North Africa (See figure 2B for the location). (B to D) Monthly mean
difference between evaporation and precipitation for these three sedimentation areas.
Sedimentological studies of Western Europe show that
paleoenvironments during the Olenekian were made up of
large rivers, with their sources mainly located at the margins
of playa systems containing ephemeral lake and/or aeolian
deposits (Figs. 2B, 4A). The simulated climate in this area is
in agreement with these field observations. MMT (Fig. 5A)
does not fall below 26°C in both experiments, and MMP is
extremely weak, not exceeding 0.2 mm/day (Fig. 8B, C). In
terms of hydrological budget (MMEvP), we simulate over
western-Europe (Spain) a near equilibrium between evaporation and precipitation throughout the year, due to the
absence of available water in the soil (Fig. 9B), whereas
evaporation exceeds largely precipitation in the Germanic
basin (Fig. 9C). This paleoenvironment is compatible with a
simulated dry climate not controlled by the paleo-elevation
of the North Africa Variscan Mountains. The presence of
rivers in an arid environment suggests that these rivers were
fed by precipitation falling on adjacent areas.
The sedimentation in the Germanic basin is mainly represented by ephemeral lakes or aeolian deposits (Fig. 2B, 4A).
This is clearly consistent with the simulated climate, which
is warm and arid in this area. Monthly temperature (MMT,
not shown) ranges from about 20° in winter to almost 33°C
in summer. Precipitation is as weak as 0.4 mm/day. The high
evaporation rate (Fig. 9C) simulated in this area is due to the
fact that we assume a large lake in our paleogeographic
reconstructions. Thus, evaporation exceeds largely precipitation throughout the year. Without a large lake, the
evaporation rate would be in equilibrium with precipitation.
These results suggest that the lifetime of lake in this area
would have been short because of the high evaporation rate.
This is consistent with proxy data. The ephemeral lake
S. Péron et al. / Geodinamica Acta 18/6 (2005) 431–446
deposits occurring in the Germanic Basin during this period
can be attributed to precipitation flowing out of the Fennoscandian Mountain ranges, and/or are associated with water
supply imported by rivers from the North Africa Variscan
Mountains.
In the northern Germanic basin, the Triassic sediments
are mostly derived from the Fennoscandian highland as well
as from the remnants of the Bohemian massif (Fig. 2B). This
is consistent with our simulations, showing that seasonal
precipitation, mainly in winter, falls on the Fennoscandian
Massif (Fig. 8D). Indeed, precipitation simulated over the
Fennoscandian Mountain ranges does not vary significantly
in response to an increased paleoelevation of this range.
Precipitation simulated on the North Africa Variscan
Mountains (Fig. 8E) shows that the mountains produce a
water supply able to control the sediment supply (SS) of the
rivers. This scenario is consistent with the sedimentological
analysis (rivers in an arid climate context) and the paleo-current directions. The water supply evidently depends on the
elevation of the mountain ranges. The presence of large rivers suggests a voluminous water supply, suggesting that the
North Africa Variscan Mountains were still elevated during
the Olenekian. If we assume that the relief was already flattened at that time, the precipitation would have been too
weak to feed rivers. However, the simulation on Western
France shows a very weak precipitation (Fig. 8F). This scenario is inconsistent with sedimentological data: rivers come
from the Armorican Massif, which represent the fluvial transit area. Two hypothesis may arise: (1) either the
paleoelevation of the Armorican Massif has been under-estimated or (2) a continuous mountain range, located in the
western Iberian Peninsula, should be considered between
Armorican Massif and North Africa Variscan Mountains.
The latter hypothesis implies that water was supplied
directly from the North Africa Variscan Mountains. The climate of the Germanic basin is not sensitive to either an uplift
of the North Africa Variscan Mountain or the Fennoscandian
Mountain ranges.
During the Scythian in North Africa, the Triassic basins
mainly exhibit erosion or by-pass sedimentation. Olenekian
sedimentation is characterized exclusively by coastal plain
deposits located in the Tunisian and Lybian basins (Fig. 2B
and see chapter 4). Simulated temperature is very hot
throughout the year, about 30-33°C. In contrast to the other
areas discussed here, this region is subject to seasonal precipitation occurring mainly in winter and spring (Fig. 8G).
Clearly, the evaporation exceeds precipitation throughout
the year because of the heat (Fig. 9D). However, these
results are not inconsistent with the presence of rivers. This
local precipitation is added to the water supply coming from
rainfall over the North Africa Variscan Mountain. By means
of simulations, we show that the elevation of the North
Africa Variscan Mountains controls the aridity of the North
Africa basin as well as the water supply to rivers. During, the
Olenekian, the accommodation space available for the sedimentation (A) is located in Western Europe, and the
443
sediments are preserved in an endoreic basin where the
smallest stratigraphic cycles, (i.e. genetic units) seem to be
controlled by sediment and water supply variations induced
by precipitation over the mountain range. Consequently, in
this area, A and SS are in equilibrium (Fig. 10A and a). However, the sediment supply derived from areas of high relief is
not preserved in the endoreic setting of the North African
basins, but is preserved only as coastal deposits. Consequently, two hypotheses can be considered. In the first
hypothesis, we assume that no accommodation space (A < 0)
is created in this area, and that all the sediment supply transits to the sea (Fig. 10B and b). In the second hypothesis,
accommodation space is created (A > 0) but, because of the
climatic conditions in a basin subject to seasonal precipitation, the sediments are not preserved and they undergo bypass in continental areas. In fact, at the onset of the fluvial
sedimentation, the sediment supply fills in the basin, SS < A
and preservation is possible (Fig. 10B and c). When SS > A
(no subsidence, coupled with a constant sediment supply),
the sedimentation area could be in transit during an initial
stage and then become subject to erosion. As a result, all the
sediments transit to the sea and are preserved in coastal environments (Fig. 10b).
Fig. 10 Hypothetical longitudinal profiles of the European Triassic
endoreic basin (A) and of the North-Africa Triassic exoreic basins (B).
When the sediments of an endoreic basin (a) is connected to the sea all
the sediment supply will transit to the sea (b) or, alternatively, endoreic
basin can be preserved (b).
S. Péron et al. / Geodinamica Acta 18/6 (2005) 431–446
444
7. Conclusion and perspectives
By combining climate simulations with paleoenvironmental reconstructions based on sedimentological studies on
the scale of West Tethys, we propose a scenario for the preservation of fluvial systems in Triassic times. The
preservation of these systems is firstly controlled by accommodation space, which is itself tectonically induced in
continental environments. During the Early Triassic (Olenekian), fluvial sediments in Western Europe are preserved in
endoreic basins, while, in North Africa, they are preserved in
coastal environments without the development of endoreic
basins. The simulations demonstrate that climatic conditions
differed between these two areas, with North Africa being
more humid and subject to seasonal precipitation. In North
Africa, the absence of fluvial systems preserved in endoreic
basins could either be due to a lack of accommodation space
or could be linked to the specific climatic conditions of this
area. In the latter case, we might ask whether the water supply is able to control the preservation of fluvial systems.
To address this question, we propose (1) an analysis of the
Permo-Triassic transition in North Africa to show if sediments in this area were actually undergoing transit or being
eroded during the Scythian, (2) a comparison with other fluvial systems preserved in coastal environments under warm
climatic conditions during other geological periods (Cretaceous, for example).
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
Acknowledgements
[15]
We are grateful for the financial support of INSU/CNRS
programme ECLIPSE: Environnements et Climats du Passé,
entitled “Quantification des flux sédimentaires au Trias:
réponse des systèmes sédimentaires continentaux aux sollicitations climatiques et tectoniques”. We thank the LSCE/
DSM/CE at Saclay for providing computing time on supercomputers. We gratefully acknowledge Dr J. Totman
Parrish and Dr. G. Bachmann for their critical and constructive reviews of the paper. Dr. M.S.N. Carpenter post-edited
the English style.
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