Micromorphs: Response of the Ammonite Fauna during the Toarcian Oceanic Anoxic Event (T-OAE) in the Es-Saffeh Mountains (Tiaret, Western Algeria) †

: The paleontological analysis of the fauna of ammonites collected in the marl–limestone series of the Pliensbachian and The Toarcian of the Es-Saffeh Mountains (Tiaret, western Algeria) brings new data to the Oceanic Anoxic Event of the lower Toarcian (T-OAE). During this time interval, hypoxia is characterized by a signiﬁcant disturbance of the global carbon cycle marked by a negative excursion of the isotope δ 13 C, δ 18 O and an increase in organic carbon content (TOC). Benthic life almost completely disappears, and microfauna (foraminifera) is absent. It should, however, be pointed out that the few specimens of a small size (swarf forms) collected in the marly levels and the well-identiﬁed specimens (ammonites) were attributed to the microshell forms.

The sedimentary record of the T-OAE is characterized by organic-rich sediments "black shales" associated with a distinctive negative excursion in the δ 13C recorded in organic matter, biomarkers, marine carbonates, and fossil wood from marine and continental sections (e.g., [6,8,).
The aim of this research is to analyze the ammonite's assemblages of the Pliensbachian-Toarcian limit of the Benia section (north western Algeria).The study of the ammonite's assemblages made it possible to analyze the morphological responses of adopted ammonites to paleoenvironmental changes.

Location and Geological Setting
The study region is located at the Es Seffah Mountain (Figure 1 The ammonites studied in this work come from the Benia section, which is located on the southern flank of the Es Saffeh Mountain (Figure 1).The studied outcrop is raised near the old "Lime kiln", which is located 2 km north western of the village of Bénia.The Nador Chain is organized into three topographic units arranged from north to south as follows: - The Nador Zérange: It corresponds to an anticline with a liasic core spilled towards the north west; the southern flank is clearly less disturbed and shows good development of the Jurassic series [50,51] The ammonites studied in this work come from the Benia section, which is located on the southern flank of the Es Saffeh Mountain (Figure 1).The studied outcrop is raised near the old "Lime kiln", which is located 2 km north western of the village of Bénia.

Materials and Methods
New bed-by-bed sampling in the Benia section located on the southern flank of the Es Seffah Mountain was exhaustively carried out to achieve a high-resolution record, resulting in 112 newly collected ammonites.Mean biometric parameters (length, width, thickness) were measured for complete specimens.Ammonites' associations allowed us to precisely characterize the upper Pliensbachian and lower Toarcian biozones.

Lithostratigraphic Framework
The section described in this work had already been the subject of several stratigraphic studies, which made it possible to subdivide the formation of the "Benia Marno-limestone" into several terms (a-f), ( [52][53][54][55]).In this work, we will retain the last subdivision of established by Sebane [56], which could be summarized in two lithological units (Figure 1):

Lithological Unit I: (Sublithographic Marly Limestones)
It was a close alternation of marl and limestone grouping together the "a & b" terms of Caratini (1970).The limestone beds were thick, more or less clayey, and sometimes compact or crumbling.In the upper part, the limestone banks were better defined, and their upper surfaces were highlighted by the accumulations of ammonites, belemnites, and trace fossils of benthic organisms.The marly levels were greenish gray in color, were not very thin, and contained abundant microfauna (foraminifera, ostacods).
The lithological nature of the sediments (marl and limestones), as well as the abundance of cephalopods and radiolarians, indicated an open marine environment.

Lithological Unit II (Marly Beds with Lumpy Levels)
It grouped together the terms (c and d) of Caratini (1970); the term c corresponded to a greenish marl base, revealing small gray-greenish limestone levels, which passed towards the top to small discontinuous and lumpy banks; the term d corresponded to clayey "ammonitico-rosso".It began with nodular to lumpy levels and was greenish in color, becoming reddish at the top.The limestone levels were separated by lumpy reddish levels.
The ammonites collected in this unit made it possible to distinguish two successive intervals (Figure 2):
The analysis of lithofacies and microfacies made it possible to define two types of environment: first, a deep environment, inhabited by small brachiopods (dwarf forms).The sedimentation showed significantly high levels of illite and pyrite.The combination of these characters indicated a narrow basin, having the shape of a deep gutter, poorly oxygenated on the bottom, where we noted the presence of a certain degree of confinement ( [56][57][58]).Then, a relatively deep environment where a clayey sedimentation rich in lumpy facies (Ammonitico-rosso facies) settled.The microfauna reappeared.These characters indicated a resumption of life due to the change in conditions that became more favorable.The transition between the first environment and the second was marked by the absence of benthic life (azoic episode) [56].

Lithological Unit II (Marly Beds with Lumpy Levels)
It grouped together the terms (c and d) of Caratini (1970); the term c corresponded to a greenish marl base, revealing small gray-greenish limestone levels, which passed towards the top to small discontinuous and lumpy banks; the term d corresponded to

Discussion
In the Nador Mountains during this time interval, hypoxia, favorable to the accumulation and preservation of organic matter, is characterized by a significant disturbance of the global carbon cycle marked by a negative excursion of the isotope δ 13 C, δ 18 O and an increase in organic carbon content (TOC).
Benthic life almost completely disappears, and microfauna (foraminifera) is absent ( [56][57][58][59]).Although the existence of a sexual dimorphism of the Dactylioceratidae is often discussed by certain authors [60], it should, however, be pointed out that the few specimens of a small size (swarf forms) collected in the marly levels and the well-identified specimens (ammonites) are attributed to the microshell forms and can be interpreted as a response to the conditions of the reducing environment during the Lower Toarcian, particularly at the end of the Polymorphum Zone and the beginning of the Levisoni Zone.

Conclusions
The paleontological analysis of the fauna of ammonites collected in the marl-limestone series of the Pliensbachian and The Toarcian of the Es-Saffeh Mountains (Nador Mountais, western Algeria) brings new data to the Oceanic Anoxic Event of the lower Toarcian (T-OAE), well known over a significant part of the North West Europe and West Tethys shelves and basins [60].
The Dactylioceratidae, Harpoceratidae, and Arieticeratidae collected in the Es-Saffeh Mountains section made it possible to recognize all the passage terms from the Emaciatum Zone to the end of the Polymorphum Zone.The Levisoni Zone was recognized by the presence of Hildoceratidae [52].During this time interval, hypoxia, favorable to the The faunas studied were identical to those known in Western Europe and occupied the same stratigraphic positions.The vertical extension showed two important horizontals: the first, at the base of horizon XIII, corresponded to the extinction of the Hildoceratinae and the appearance of Hammatoceratidae and Grammoceratinae; the second, at the base of horizon XXII, marked the disappearance of Grammoceratinae and the emergence of Dumortiinae.
The Dactylioceratidae, Harpoceratidae, and Arieticeratidae collected in the Es-Saffeh Mountains section made it possible to recognize all the passage terms from the Emaciatum Zone to the end of the Polymorphum Zone.The Levisoni Zone was recognized by the presence of Hildoceratidae.

Discussion
In the Nador Mountains during this time interval, hypoxia, favorable to the accumulation and preservation of organic matter, is characterized by a significant disturbance of the global carbon cycle marked by a negative excursion of the isotope δ 13 C, δ 18 O and an increase in organic carbon content (TOC).
Benthic life almost completely disappears, and microfauna (foraminifera) is absent ( [56][57][58][59]).Although the existence of a sexual dimorphism of the Dactylioceratidae is often discussed by certain authors [60], it should, however, be pointed out that the few specimens of a small size (swarf forms) collected in the marly levels and the well-identified specimens (ammonites) are attributed to the microshell forms and can be interpreted as a response to the conditions of the reducing environment during the Lower Toarcian, particularly at the end of the Polymorphum Zone and the beginning of the Levisoni Zone.

Conclusions
The paleontological analysis of the fauna of ammonites collected in the marl-limestone series of the Pliensbachian and The Toarcian of the Es-Saffeh Mountains (Nador Mountais, western Algeria) brings new data to the Oceanic Anoxic Event of the lower Toarcian (T-OAE), well known over a significant part of the North West Europe and West Tethys shelves and basins [60].
The Dactylioceratidae, Harpoceratidae, and Arieticeratidae collected in the Es-Saffeh Mountains section made it possible to recognize all the passage terms from the Emaciatum Zone to the end of the Polymorphum Zone.The Levisoni Zone was recognized by the presence of Hildoceratidae [52].During this time interval, hypoxia, favorable to the accumulation and preservation of organic matter, was characterized by a significant disturbance of the global carbon cycle, marked by an increase in organic carbon content (TOC).Benthic life almost completely disappeared, and microfauna (foraminifera) were absent ( [56,59]).It should, however, be pointed out that the few specimens of a small size (swarf forms) collected in the marly levels and the well-identified specimens (ammonites) were attributed to the microshell forms and could be interpreted as a response to the conditions of the reducing environment during the Lower Toarcian, particularly at the end of the Polymorphum Zone and the beginning of the Levisoni Zone.
These levels were correlated to those described in the North West Tethyan basins, where this global anoxic event (Oceanic Anoxic Event: OAE) was recorded [50].During this period, the environmental conditions were also related to the Liasic tectonic event (eustatism), which played a key role in the paleogeographic evolution in North Africa and Europe.
), part of the Nador Mountains; it is located approximately 45 km SE of Tiaret city.The Nador Mountains are part of the pre-Atlas domain, which is bordered to the north by the external zone of the Tellian Domain, to the south east by the Atlas Domain, to the south by the Oran High Plains, and to the west by the Tlemcenian Domain.These deposits are sometimes covered by Miocene and Plio-Quaternary sediments.-The Taga Plateau: It occupies the southern flank of the anticlinal structure of Nador.It corresponds to the dolomitico-limestone formations of the Upper Jurassic period, which show a slight dip and a great extension towards the South.

Figure 1 . 1 .
Figure 1.Geological setting and stratigraphical succession studied: (A) Situation and geological map of Nador mountains; (B) Satellite image of the locality studied; (C) Synthetic lower Jurassic Figure 1.Geological setting and stratigraphical succession studied: (A) Situation and geological map of Nador mountains; (B) Satellite image of the locality studied; (C) Synthetic lower Jurassic lithostratigraphical column from Benia section; (D) Outcrop view of Benia section showing the distribution of the ammonites chronozones.

Figure 2 .
Figure 2. Stratigraphic distribution of ammonites from the Benia section.

Figure 2 .
Figure 2. Stratigraphic distribution of ammonites from the Benia section.