Early Jurassic

In the Early Jurassic, a series of spreading centers formed between North and South America, marking the beginning of Pangea break-up (Mpodozis and Ramos, 2008).

From: Andean Tectonics , 2019

Overview of the Jurassic Period☆

K.N. Page , in Reference Module in Earth Systems and Environmental Sciences, 2014

Continents

The Early Jurassic inherited the pole-to-pole massive supercontinent of Pangaea. The opening of the North Atlantic began the process of dismantling this huge landmass, initially with South America moving away from North America to form the marine 'Hispanic Corridor.' The latter linked the previously separated East Pacific regions and the west Tethyan regions of Europe. There is circumstantial evidence from faunal migrations that this passage may have been intermittently open during the Lower Jurassic, providing a migration passage for shallow-marine faunas, but it was not until the later Middle Jurassic that it became a more open passage.

During the Middle Jurassic, oceanic crust began to form in the Atlantic, as rifting began to separate Europe from Greenland and North America. Despite this gradual opening up, however, the general layout of the continents maintained strong east–west and north–south physical barriers to marine faunal migrations throughout the period, leading to well developed bioprovincialism ( Figure 1 ) that did not finally break down until the mid-Cretaceous.

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Late Permian-Early Jurassic Paleogeography of Western Tethys and the World

C.R. Scotese , A. Schettino , in Permo-Triassic Salt Provinces of Europe, North Africa and the Atlantic Margins, 2017

8 Earliest Jurassic Paleogeography (Hettangian and Sinemurian, 201.3–190.8   Ma), Fig. 15

During the earliest Jurassic, 200 million years ago, the world changed. A global plate tectonic reorganization took place that signaled the end of Pangaea and the beginnings of the world's modern ocean basins and continents. Though the cause of this plate reorganization is still unknown, the event is marked by a major mass extinction (Blackburn et al., 2013; Palfy & Kocsis, 2014) and a massive volcanic eruption (Central Atlantic Magmatic Province, or CAMP, related to a massive mantle plume or hotspot located in the Central Atlantic, see Fig. 9) covering more than 10   million   km2 (McHone, 2000; Hames, McHone, Renne, & Ruppel, 2003; McHone & Puffer, 2003).

The earliest Jurassic paleogeography shown in Fig. 15 tells only part of the tale. Based primarily on the published paleogeographies of Ziegler (1982, 1989, 1990), Cope et al. (1992), and Dercourt et al. (1993, 2000) , the early Jurassic paleogeography, when compared to the largely terrestrial environments of the late Permian and Middle Triassic, is predominantly marine. This was due to both rising eustatic sea levels ( Fig. 12) and the extension and subsidence associated with the initial phases of the opening of the Atlantic Ocean. Table 4 provides more detailed attribution of the paleogeographic sources for each region. Key geographical, geologic and tectonic features are shown on the index map, Fig. 10.

Table 4. Sources of Paleogeographic Information for Western Tethys

Source Age Region
Late Permian Middle Triassic Early Jurassic
Baud, Marcoux, Guiraud, Ricou, and Gaetani (1993) X – – Tethys
Biju-Duval, Dercourt, and LePichon (1977) – X X W. Tethys
Boucot, Chen, and Scotese (2013) X X X Global
Bradshaw et al. (1992) – – X Great Britain
Cope et al. (1992) X X X Great Britain
Dercourt et al. (1985) – – X Tethys
Dercourt et al. (1993) X X X Tethys
Dercourt et al. (2000) X X X Tethys
Evans et al. (2003) X X X Northern Europe
Fensome and Williams (2001) – – X Maritime Canada
Gaetani and Chuvachov (2000, chap. 3) X – – Tethys
Gaetani et al. (2000a, chap. 4) – X – Tethys
Gaetani et al. (2000b, chap. 5) – X – Tethys
Gaetani et al. (2000c, chap. 6) – X – Tethys
Glennie, Higham, and Stemmerik (2003, chap. 8) X – – Northern Europe
Goldsmith, Hudson, and Van Veen (2003, chap. 9) – X – Northern Europe
Golonka (2000) X X X Global
Golonka (2007) – X X Global
Husmo et al. (2003, chap. 10) – – X Northern Europe
Kazmin and Natapov (1998) X X X N. Eurasia
Kiessling et al. (2002) X X X Global
Le Tourneau and Olsen (2003) – – X NE USA
Marcoux et al. (1993a) – X – W. Tethys
Marcoux et al. (1993b) – X – W. Tethys
Olsen, Kent, Cornet, Witte, and Schlische (1996) – – X NE USA
Rees, Ziegler, and Valdes (2000) – – X Global
Ronov et al. (1984) X – – Global
Ronov et al. (1989) – X X Global
Schandelmeier and Reynolds (1997) X X X NE Africa
Schettino and Turco (2011) – – – W. Tethys
Scotese and Langford (1995) X – – Global
Scotese (2014a) – X X Global
Scotese (2014b) X – – Global
Scotese (2014c) – – X Global
Scotese (2014d) X X – Global
Smith, Taylor, Arthurton, Brookfield, and Glennie (1992) X – – Great Britain
Smith et al. (1994) – X X Global
Stampfli, Mosar, Favre, Pillevuit, and Vannay (2001) – – X Tethys
Thierry (2000, chap. 7) – – X Tethys
Turco et al. (2007) – X X W. Tethys
Warrington and Ivimey-Cook (1992) – X – Great Britain
Yilmaz, Norton, Leary, and Chuchla (1996) X X X W. Tethys
Ziegler et al. (1983) – X X Global
Ziegler et al. (1997) X – – Global
Ziegler (1982) X X X Northern Europe
Ziegler (1988) X X X Northern Europe
Ziegler (1989) X – – Northern Europe
Ziegler (1990) X X X Northern Europe

In Western Europe, the once-towering Central Pangean Mountains were reduced to a few upland areas with modest relief (Scotland, Ireland, Armorica, the Massif Central, Iberia, the Corsica-Sardinia Highlands, Bohemia, and Morocco), separated by moderately deep marine basins. To the south, in the west-central Mediterranean region, active sea floor spreading had subdivided the once broad, shallow, carbonate platform of the Triassic, into a few shallow carbonate banks separated by deep troughs. As stated in the introduction, we believe that Adria did not remain fixed to Laurasia during the late Triassic-early Jurassic time interval as envisaged by Schettino and Turco (2011), but moved independently with respect to both of the large surrounding plates.

The regional climate also had changed. During the Triassic, Pangaea moved nearly 25 degrees to the north (Fig. 11). Consequently, Western Europe entered the wetter, monsoonal climate of the warm temperate belt. Iberia, Morocco, Corsica, Sardinia, Apulia and Adria, though located in the northern part of the dry northern subtropics, were less arid. Only Tunisia, Libya, and the western shelf of Morocco were subjected to the intense arid conditions that characterized much of the Triassic and late Permian. It is in these regions that we find the largest accumulations of salt and evaporite deposits in the earliest Jurassic (the Oued Mya, d'Dahar, and Tataouine basins of Tunisia and Libya; Thierry, 2000; see also Chapters 25 and 17) as well as the deepwater salt accumulations of the early rift basins of the Atlantic Ocean (Essaouira and Tarafaya; Salvan, 1972).

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A Review of Mesozoic-Cenozoic Salt Tectonics Along the Scotian Margin, Eastern Canada

M.E. Deptuck , K.L. Kendell , in Permo-Triassic Salt Provinces of Europe, North Africa and the Atlantic Margins, 2017

Abstract

Widespread Late Triassic and earliest Jurassic salt deposition took place during the latter stages of rifting between Nova Scotia and Morocco. Extensive seismic data coverage along the Scotian margin provides a clearer picture of the margin structure, the distribution of synrift strata, and the extent of the original salt basin. The primary salt layer lies principally beneath the present-day slope in the southwest and the present-day shelf in the northeast. Postrift mobilization of salt played an important role in the structural and stratigraphic development of the margin. In the west, most of the expelled salt lies immediately above the primary salt basin; in the east, expelled salt largely escaped and now lies seaward of the primary salt basin. Recent mapping efforts have substantially improved our understanding of the timing and distribution of different postrift salt tectonic styles. This chapter introduces the reader to the wide diversity of salt tectonic elements found along the Scotian margin, linking the different salt tectonic styles to regional patterns in sedimentation.

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Influence of Salt Diapirism on the Basin Architecture and Hydrocarbon Prospects of the Western Iberian Margin

R. Pena dos Reis , ... B. Rasmussen , in Permo-Triassic Salt Provinces of Europe, North Africa and the Atlantic Margins, 2017

3.1 Layered Salt Stratigraphy

The Late Triassic to Early Jurassic evolution of the Lusitanian Basin corresponds to the opening of intracontinental basins to marine conditions at the western edge of the Iberian Margin. This evolution is related to the earliest phases of the Pangea break-up and to the opening of the Western Tethys along a complex network of major fractures leading to marine incursions ( Wilson, 1988; Watkinson, 1989; Pena dos Reis et al., 2011).

In the Lusitanian Basin, the basal sequence evaporite stratigraphy occurs from Late Triassic to Early Jurassic with salt interspersed with siliciclastic deposits known as the Silves Group (Palain, 1976; Azerêdo et al., 2003), red-beds composed mainly of coarse sands and clays, locally containing halite and gypsum (Fig. 2). This group includes the Conraria, Castelo Viegas and Pereiros Formations (Rocha et al., 1996), equivalent to Units A, B and B2   +   C of Palain (1976), respectively (Fig. 3). Salt occurrences are present in all of these lithostratigraphic units, showing prevalence of climatic evaporitic conditions throughout the Late Triassic and Early Jurassic where evaporitic lakes were complemented by seawater evaporation and salt precipitation. Salt occurrences are related to the basal alluvial fan infill, the Conraria Formation (50–150   m thick). This unit shows a finning-upward trend, with coarse conglomerates and sandstones at the base (A1 of Palain, 1976), grading into fine-grained siliciclastics, with abundant claystones (A2 of Palain, 1976). These claystones show some salt evidence, appearing as halite pseudomorphs and some gypsum. A second finning-upward cycle, corresponding to the Castelo Viegas formation (around 100   m thick), presents strong lateral variations and it includes coarse- to fine-grained siliciclastics in more proximal areas, whereas in distal areas silty clays are predominant. A third cycle is represented by the Pereiros Formation (50–100   m thick), a sandy unit with dolomitic intercalations, dolomitic crusts and laminated stromatolites. The Silves Group is followed by a carbonated unit, the Coimbra Formation (around 100   m thick), containing dolomites, marls and limestones with Sinemurian restricted to open marine fauna (Fig. 2). These carbonates onlap the basin's infill and represent the opening of the basin to marine conditions, which would widen and deepen along the Pliensbachian, defining a clear transgressive pattern for the Lower Jurassic.

This lithostratigraphic sequence is well known and studied at the eastern part of the basin, between Coimbra and Tomar, where it outcrops due to the Alpine uplift along the basin margin (Fig. 1). In central and western parts of the basin, this sequence lies beneath a 3–5   km thick Jurassic and Cretaceous cover and cannot therefore be observed within its normal stratigraphic position. However, diapiric activity brought it up to the surface and the Dagorda Formation might be considered as a lateral equivalent of the Silves Group (Fig. 3). Towards the deeper domains of the basin (i.e., towards the West), the salt in the Dagorda Formation is more abundant and thick, resulting from evaporation in semiarid climatic conditions. The same plastic and less dense units are also recognizable in seismic lines, either piercing the entire Mesozoic cover, either occupying a normal stratigraphic position between the Silves siliciclastics and the first thick carbonate layers of the Coimbra Formation.

The Dagorda formation is most probably a diachronous unit, particularly at its base where the coarse-grained red-beds retrograded and the more distal facies gradually expanded. This unit represents part of the Norian and the Rhaetian deposits, while in some areas favorable conditions for salt precipitation may have endured even during most of the Hettangian (Fig. 3).

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Evolution and Biostratigraphy

Coordinated byF.M. Gradstein , ... S. Esmeray-Senlet , in Geologic Time Scale 2020, 2020

3G.2 Jurassic

Planktonic foraminifera originated in late Early Jurassic (e.g., Gradstein, 1976; Hart, 1980; Caron, 1983; Wernli, 1988; BouDagher-Fadel et al., 1997; Wernli and Görög, 2007) and, for reasons poorly understood, only underwent proliferation of species and geographic spreading from mid-Cretaceous onward (Fig. 3G.1). Early planktonic foraminifera from the Jurassic are known to be usually of low frequency and only seldom reach sporadic mass occurrences. One issue is that the aragonitic tests of these Jurassic microfossils hampered fossilization. Indeed, a majority of samples in wells or outcrop sections may lack specimens of these taxa. In this context it is not so much surprising that Jurassic foraminifera were only rarely encountered in samples and are limited to low- and mid-latitude marine basins of Pangaea, adjacent to the true oceans (e.g., Görög and Wernli, 2003; Hudson et al., 2009; Gradstein et al., 2018). The Jurassic oceans themselves were empty of planktonic foraminifera.

Figure 3G.1. Stratigraphic ranges of Jurassic planktonic foraminiferal species according to Gradstein et al. (2017a,b, 2018) and references herein.

The early planktonic foraminiferal record from Toarcian through Tithonian is represented by only two trochospiral genera possessing a microperforate wall texture and a cancellate (Conoglobigerina) or smooth/pustulose (Globuligerina) surface pattern and fewer than 10 species have been identified so far (see review by Gradstein et al., 2017a,b and references herein). Therefore the evolution of planktonic foraminifera does not follow the evolutionary diversity pattern of nannofossils and dinoflagellates, and the Tithonian appears to be a bottleneck for planktonic foraminifera, with a sparse record and virtual extinction of taxa. Jurassic Globuligerina oxfordiana likely evolved into the Cretaceous Favusella hoterivica.

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The Cretaceous Period

A.S. Gale , ... M.R. Petrizzo , in Geologic Time Scale 2020, 2020

27.2.1.2.3 Planktonic foraminifera

Planktonic foraminifera originated in late Early Jurassic (e.g., Hart, 1980; Caron, 1983; Gradstein et al., 2017) and only underwent radiation and geographic spreading from mid-Cretaceous onward. In general, planktonic foraminifera from Berriasian to Barremian have a scattered geographic and stratigraphic record. A progressive increase in abundance and diversification of genera and species is observed from the mid-Barremian (Aguado et al., 2014) to the Aptian (Premoli Silva and Sliter, 1999). The major turnover observed across the Aptian–Albian boundary interval represents the most dramatic event in the Cretaceous evolutionary history of planktonic foraminifera after the mass extinction at the Cretaceous/Paleogene (K/Pg) Boundary (e.g., Leckie et al., 2002; Huber and Leckie, 2011; Petrizzo et al., 2012, 2013; Kennedy et al., 2014). During the early to middle Albian planktonic foraminifera diversified rapidly with the continuous increase in morphological complexity and the appearance of newly evolving lineages all characterized by novel morphological and wall texture features.

The Early Cretaceous biozonal scheme currently adopted is based on the work by Moullade (1966) implemented and/or modified according to subsequent biostratigraphical studies or syntheses by Longoria (1977), Sigal (1977), Salaj (1984), Caron (1985), Gorbatchik (1986), Banner et al. (1993), Coccioni and Premoli Silva (1994), Robaszynski and Caron (1995), BouDagher-Fadel et al. (1997), Moullade et al. (2005), and Premoli Silva et al. (2018).

The subsequent pattern of evolutionary changes in the planktonic foraminifera from the Cenomanian to the Turonian corresponds to increased speciation and enlargement in test size of trochospiral keeled and unkeeled taxa and mirror the overall trend of rising sea level and global warming and increase of the density gradient within the surface water. After the maximum Late Cretaceous global Warmth registered in the early Turonian, and a relatively stasis from the late Turonian to the early–middle Coniacian, planktonic foraminifera underwent a major compositional changes in the late Coniacian–Santonian marked by high rates of species diversification and the appearance of newly evolved keeled, biserial, and multiserial taxa (e.g., Wonders, 1980; Caron and Homewood, 1983; Hart, 1999; Premoli Silva and Sliter, 1999). The radiation in the late Coniacian–Santonian time interval is followed by extinctions of some keeled taxa in the latest Santonian–earliest Campanian. In general, the Coniacian–Santonian time interval represents the transition from the mid-Cretaceous extreme greenhouse to more temperate climatic conditions in the Campanian–Maastrichtian that determined the onset of climatic bioprovinces. As a consequence, distinct biozonation schemes for the Tethyan, Boreal, Austral, and Transitional Provinces have been developed (e.g., Caron, 1985; Nederbragt, 1990; Huber, 1992; Premoli Silva and Sliter, 1999; Robaszynski and Caron, 1995; Li et al., 1999; Petrizzo, 2003; Campbell et al., 2004).

The Late Cretaceous biozonal scheme currently adopted is based on Caron (1966), Pessagno (1967), Robaszynski et al. (1984, 1990, 2000), Robaszynski and Caron (1979), Masters (1977), Robaszynski and Caron (1995), Premoli Silva and Sliter (1995) and implemented and/or modified according to studies by Tur et al. (2001), Petrizzo (2000, 2001, 2003, 2019), Bellier and Moullade (2002), Lamolda et al. (2007), Petrizzo and Huber (2006), González-Donoso et al. (2007), Huber et al. (2008), Gale et al. (2011), Petrizzo et al. (2011, 2015, 2017), Pérez-Rodríguez et al. (2012), Elamri and Zaghbib-Turki (2014), Coccioni and Premoli Silva (2015), Haynes et al. (2015), Huber et al. (2017).

The taxonomy of Early and Late Cretaceous planktonic foraminifera is currently under revision using a more systematic and evolutionary framework by the Mesozoic Planktonic Foraminiferal Working Group that has produced a taxonomic database ([email protected]) available online at http://www.mikrotax.org (see Huber et al., 2016 for further details).

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Fluvial-Tidal Sedimentology

R.W. Dalrymple , ... D.A. Mackay , in Developments in Sedimentology, 2015

1.3.4 Tilje Formation, Offshore Norway

The Tilje Formation (Pliensbachian, Early Jurassic) occurs in the subsurface of the Halten Terrace, which lies 100–200  km northwest (seaward) of Trondheim, Norway, beneath the Norwegian continental shelf (Fig. 1.13A and B ). Deposition occurred in an overall transgressive setting in which accommodation was generated by crustal stretching and thermal relaxation associated with extension during the early stages of the opening of the North Atlantic Ocean (Doré, 1991). The Tilje is underlain by the alluvial-plain deposits of the Åre Formation and is overlain by the shelf mudstones of the Ror Formation (Fig. 1.13C). The Tilje Formation itself is 150–200   m thick, and, like the Lajas Formation (see above), consists of tabular units that are composed of alternating prodeltaic mudstones and deltaic sandstones and tidal–fluvial-channel deposits (Ichaso and Dalrymple, 2014; Ichaso Demianiuk, 2012; Martinius et al., 2001). The previous sedimentological interpretation indicated that the Tilje accumulated in a succession of tide-dominated estuaries and deltas (Martinius et al., 2001), but more recent work (Ichaso and Dalrymple, 2014; Ichaso Demianiuk, 2012) proposes a more mixed-energy coastal setting, intermediate between the river- and tide-dominated end members. Sediment input was generally from the north and northeast, and the succession accumulated in a southward-opening embayment, in which tidal action was accentuated, whereas the intensity of wave action was reduced by the sheltered setting. Fluid-mud deposits are abundant in the fluvial–tidal mouth bars and tidally influenced distributary channels (Ichaso and Dalrymple, 2009).

Figure 1.13. General (A) and local (B) location maps of the study area in the Tilje Formation, central Norwegian continental shelf. The core from which the illustrated deposit (Fig. 1.20) comes was taken in the Smørbukk Field (B). (C) Generalized stratigraphic section for the Tilje Formation in the Smørbukk Field, showing the progression of depositional environments. "P.Z." refers to the "production zones" used internally by the field operator (Statoil). The red (black in the print version) star in (C) shows the stratigraphic level from which the studied example comes.

 (C) From Ichaso Demianiuk (2012).

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Sedimentary and tectonic development of the Ordos Basin and its hydrocarbon potential

Renchao Yang , A.J. (Tom) van Loon , in The Ordos Basin, 2022

3.2.3 Early Jurassic

Probably no sedimentation took place during the Early Jurassic, apart from the late Early Jurassic. It cannot be excluded that some sedimentation occurred locally, but no traces are left due to the prevailing erosional conditions.

Although the erosion surface between the Late Triassic top part of the Yanchang Formation and the overlying late Early Jurassic Fuxian (or the latest Early Jurassic Yan'an Formation) is variable, there is hardly an angular unconformity between the Triassic and Jurassic. This indicates that the basin underwent a fairly similar uplift over large distances. It is therefore understandable that no clear traces of compression deformation are present; distinct signs of contemporaneous extension-related faulting are present, however, in the surrounding coal basins. It must consequently be deduced (Liu et al., 2006) that the setting of the basin was somewhat extensional in its center but strongly extensional in its periphery.

In the beginning of the early Early Jurassic, sedimentation started again in the basin, but regionally denudation continued. The lower parts of the basin became filled by the Fuxian Formation (and contemporaneous equivalents), which represents mainly alluvial fans and braided rivers. The predominantly sandy and gravelly material was deposited, particularly in the central and southern parts of the basin (Fig. 1.9).

Fig. 1.9

Fig. 1.9. Paleogeography of the Ordos Basin when the Fuxian Fm. was deposited (early Early Jurassic).

 Modified from Guo, Y., Li, W., Guo, B., Zhang, Q., Chen, Q., Wang, R., Liu, X., Ma, Y., Li, Z., Zhang, M., Li, B., 2019. Sedimentary systems and palaeogeographical evolution of the Ordos Basin. J. Palaeogeogr. 21, 293-320 (in Chinese, with English abstract)..

The situation did not change much initially when the Yan'an Formation accumulated (Fig. 1.10), but later a transgression occurred, so that—in addition to the fluvial and delta deposits—more shallow-lacustrine sediments could form in the expanding lake. The deposits of meandering rivers occur mainly in the northern part of the basin, whereas delta-front deposits occur near the cities of Wuqi, Zhidan, and Yan'an and southeast of Huachi. The delta-front deposits, in contrast, prograded eastward near Huachi and amalgamated with the delta-front sand bodies in the south. Eventually, tectonic uplift took place, and the entire basin shrunk (Guo et al., 2019).

Fig. 1.10

Fig. 1.10. Paleogeography of the Ordos Basin when the Yan'an Fm. was deposited (late Early Jurassic).

 Modified from Guo, Y., Li, W., Guo, B., Zhang, Q., Chen, Q., Wang, R., Liu, X., Ma, Y., Li, Z., Zhang, M., Li, B., 2019. Sedimentary systems and palaeogeographical evolution of the Ordos Basin. J. Palaeogeogr. 21, 293-320 (in Chinese, with English abstract).

The Yan'an Formation houses large quantities of coal and the fluvial sandstones contain oil reservoirs. These developed in an environment that initially evolved from a braided to a meandering river. Subsequently, a lacustrine delta developed, eventually followed by a stage in which a network of streams and a residual lake dominated.

The depocenter then was situated east of the cities of Mizhi, Jingbian, Wuqi, and Yijun, adjacent to a small quiet basin in Shanxi Province, east of the present-day Ordos Basin. During the accumulation of the Yan'an Fm., the depocenter expanded to the north, while some lakes of limited size still existed in the southern part of the basin. The lake sediments consist mainly of mudstone, oil shale, sandstone, and deep-water gravity-flow deposits, but along the lake margins peat swamps were well-developed all the time, and the resulting coals are widely distributed. The distribution of the coal seams was, of course, controlled by the sedimentary environment. The coal seams thin with increasing depth of the lacustrine water body; they are consequently absent near Yan'an, where the depocenter was situated. The thickness of most sediments formed during this time span is 200–300   m, but reaches more than 300   m near the cities of Tianchi, Ansai, and Northern Etoke Banner, where an accumulation center existed (Liu et al., 2006).

At the end of the Early Jurassic, the basin was uplifted again, resulting in an erosional unconformity between the Yan'an Formation and the overlying Middle-Jurassic Zhiluo Formation. Due to differential denudation, which was strongest in the south-west, the uppermost preserved part of the Yan'an Formation is relatively young in the north and northwest, but older in the south and southeast.

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Development of an Upper Triassic-Lower Jurassic Evaporite Basin on the Saharan Platform, North Africa

P. Turner , K. Pelz , in Permo-Triassic Salt Provinces of Europe, North Africa and the Atlantic Margins, 2017

1 Introduction

This contribution concerns the late Triassic and early Jurassic salt basins of the Saharan Platform of northwest Africa. These salt deposits are an integral part of the key petroleum systems in North Africa and form seals to the giant gas and oil fields of Hassi R'Mel and Hassi Messaoud ( MacGregor, 1998). They were deposited during the opening of the central Atlantic and spanned the Triassic-Jurassic boundary when major climatic changes took place.

These pericratonic salt basins that developed along the northern margin of the Saharan Craton include the Oued Mya and Berkine basins of Algeria as well as the Ghadames Basin of Tunisia and western Libya (Figs. 1 and 2). These basins are largely undeformed and lie south of the Atlas-Tellian thrust fronts. They sit unconformably on Paleozoic and are bounded to the north by the Telamzane-Jifarah High, the Allal and Idjarene ridges to the west, the Gargaf High to the southeast and the Hoggar Massif to the south. The Amguid-El-Biod Ridge and Allal-Idjarene form major north-south spurs, which protrude into the basin and reflect the basement control of Pan-African lineaments (Coward & Ries, 2003).

Fig. 1. Google earth image of North Africa showing main structural features and sedimentary basins of the Saharan Platform. The outlined areas show the main study area (Fig. 2) and the map of evaporite thickness variations (Fig. 10).

Fig. 2. Regional location of wells and cross sections used in the study. Fig. 4A and B are key seismic lines. The distribution of Triassic (Sequence 1) and Jurassic (Sequence 3) salt is shown in relation to areas of Paleozoic uplift.

The northern margin of this basin is not seen because of the overthrust Tellian and southern Atlas system of Tunisia and northern Algeria. However, significant evaporite basins were formed in this area and have been subject to halokinesis such that a diapiric system crops out in the central part of the Tunisian Atlas (Rusk, 2001; Jallouli et al., 2005; Masrouhi, Bellier, & Koyi, 2014).

The Triassic-Jurassic basin-filling sequence contains about 1.5   km of fluvio-lacustrine clastics, shallow-marine carbonates and shales, evaporites and marine mudrocks with thin carbonates and siltstones (Fig. 3). The fluvial clastics include the late Triassic Argilo-Grèseux Inférieur (TAG-I), which extends from Algeria to southern Tunisia, and its equivalents, the Kirchaou and Ra's Hamia (Kurrush) formations of southern Tunisia and western Libya. The TAG-I is an important hydrocarbon reservoir and hosts many oil fields in eastern Algeria and southern Tunisia, including the giant El Borma, Ourhoud, and Hassi Berkine South (HBNS) fields (Boote, Clark-Lowes, & Traut, 1998; Acheche, M'Rabet, Ghariani, Ouahchi, & Montgomery, 2001; Turner et al., 2001; Galeazzi, Point, Haddadi, Mather, & Druesne, 2010). In the study area, the TAG-I rests unconformably on the subcrop of Devonian-Carboniferous aged sediments. The oldest Triassic deposits occur further eastwards in Tunisia and Libya. These include the Ouled Chebbi Formation, which includes marine shales and interbedded marine carbonates of pre-Ladinian age, which can be linked to the Tethyan domain (Ben Ismail, 1991a; Guiraud, 1998; Soussi, Abbes, & Belayouni, 1998; Soussi, 2000). During this period a number of subbasins existed, isolated by basement relief, as in the Zarzaitine area (Bourquin, Eschard, & Hamouche, 2010). The clastic sequence is overlain by the Triassic Argilo-Carbonaté (or Triassic Carbonate) and its equivalents, which represent a basin-wide marine incursion of Carnian age. Further west and south, around the basin margins younger Triassic clastics are present, such as the Triassic Argilo-Grèseux Supérieure (TAG-S). Thickness variations in these clastic pulses were controlled by movements on the basement lineaments moderated by sea level and climatic fluctuations (Busson & Cornée, 1989; Ait Salem et al., 1998; Guiraud, Bosworth, Thierry, & Delplanque, 2005; Bourquin et al., 2010).

Fig. 3. Comparative lithostratigraphy of the Triassic-Jurassic in the Berkine-Ghadames basins eastern Algeria, southern Tunisia, and western Libya. The ICC chronostratigraphy is shown as a guide for the possible time framework of Saharan evaporite deposition.

The Triassic Argilo-Carbonaté passes upwards through a series of shales with thin dolomites into a thick carbonate-evaporite succession which includes shales, anhydrites, and halite spanning the uppermost Triassic of late Norian and Rhaetian age. This sequence is terminated by a regional dolomite marker referred to as the "D2 Marker." This marker separates the Rhaetian evaporites from a more widespread development of Lower-Middle Jurassic evaporites. These occur throughout the Berkine Basin and western Ghadames Basin and are contemporaneous with evaporite basins farther west in Morocco, indicating that in late Triassic-early Jurassic times the whole of North Africa consisted of a huge evaporitic basin complex (Leroy & Pique, 2001; Turner & Sherif, 2007). The Moroccan sections show remarkable similarity with those of the Berkine Basin. There are two evaporite successions spanning the Rhaetian-Sinemurian time interval which, like the Algerian sections, show increasing marine influence with time (Ouijdi, 2000; Tourani, Lund, Benaouiss, & Gaupp, 2000). Furthermore, geochemical studies of these evaporites show clear evidence of the mixing of marine and nonmarine brines as the Atlantic transgressed southward along the rifted continental margin (Clement & Holser, 1988; Horita, Zimmerman, & Holland, 2002).

In this paper we describe the stratigraphy and development of the late Triassic and early Jurassic evaporites of the Saharan Platform and discuss how they might relate to global changes at the Triassic-Jurassic boundary (Whiteside, Olsen, Kent, Fowell, & Et-Touhami, 2007; Steinthorsdottir, Jeram, & McElwain, 2011; Ikeda, Hori, Okada, & Nakada, 2015). The basin was silled, probably by the Paleozoic basement highs, and replenished by peri-Tethyan waters that periodically breached the sill. As the eastward opening of Tethys progressed and relative sea level rose, the basins were inundated and developed a system of enclosed evaporitic basins and narrow troughs which stretched across North Africa and linked the Central Atlantic Magmatic Province (CAMP) with the peri-Tethyan Ocean. Thus, the evaporites of the Saharan Platform are unique because they form a link between the Tethyan province and CAMP and contain a record of the changes that occurred at the Triassic-Jurassic boundary and the break-up of Pangaea.

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THE END OF THE PALEOZOIC AND THE EARLY MESOZOIC OF THE MIDDLE EAST

A.S. ALSHARHAN , A.E.M. NAIRN , in Sedimentary Basins and Petroleum Geology of the Middle East, 2003

Ghalailah Formation.

The type locality of the Late Triassic-Early Jurassic Ghalilah Formation is in Wadi Bih in the northern U.A.E. ( Figs. 6.31 and 6.33), where the beds have been described in some detail by Searle et al. (1983) and Alsharhan (1989). A three-fold division of the total thickness of 250 m (810 ft) follows. The lower 105 m (344 ft) of reddish quartz sandstone and marl, is followed by 80 m (262 ft) of flaggy grainstone and interbedded marl and capped by 65 m (213 ft) of ferruginous quartz sandstone alternating with buff and gray marl and calcareous shale is recognized. It has a fauna of lamellibranchs, gastropods, brachiopods and ostracods. In Wadi Ausaq in the northern U.A.E. (Fig. 6.31), the Ghalilah Formation is reduced to 5 m (16 ft) of buff, algal lime mudstone and, only 1 km to the north, by 20 m (66 ft) of limestone overlying a 1 m basal quartz sandstone. In these limestone developments, evidence of frequent emersion is provided by leached, bird's-eye textures, mud cracks and Neptunian dikes.

Fig. 6.33. Lithostratigraphy of the Norian to Liassic (Ghalilah Formation) in the northern Oman Mountains.

 (modified from Glennie et al., 1974) Copyright © 1974

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