Environmental implication of subaqueous lava flows from a continental Large Igneous Province: Examples from the Moroccan Central Atlantic Magmatic Province (CAMP)

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Highlights

  • The best exposed pillow-lavas and volcano-sedimentary successions in Morocco, which contribute to characterize the environmental prevalent at this area of Pangea.

  • The field analysis of these volcanic sequences in the High-Atlas, including the morphology and the internal structure of the basaltic flows, and the presence of reddened surfaces marking the separation of lava flows from individual eruptions allowed describing the composition of each of the formations.

  • The subaquatic lava flows present variable aspects. They locally correspond to densely packed pillows, to pillows dispersed into more or less abundant hyaloclastite matrix, and more or less extended sheet flows.

  • A morphometry of the pillow-lavas was performed by measuring the horizontal (H) and vertical (V) diameters of individual pillows.

  • Mineralogy of the analysed sedimentary intervals is very diverse.

Abstract

The Late Triassic-Early Jurassic volcanic sequence of the Central Atlantic Magmatic Province (CAMP) of Morocco is classically subdivided into four stratigraphic units: the Lower, Middle, Upper and Recurrent Formations separated by intercalated sediments deposited during short hiatuses in volcanic activity. Although corresponding to a Large Igneous Province formed in continental environment, it contains subaqueous lava flows, including dominant pillowed flows but also occasional sheet flows. We present a study of the morphology, structure and morphometry of subaqueous lava flows from three sections located at the Marrakech High-Atlas (regions of Aït-Ourir, Jbel Imzar and Oued Lhar-Herissane), as well as an analysis of the sediments, in order to characterize them and to understand their environmental meaning.

The analysis of clays by the diffraction method X-ray revealed the presence of illite, mica, phengite, céladonite, talc and small amounts of quartz, hematite, calcite and feldspar, as well as two pairs of interbedded irregular (chlorite Smectite/chlorite-Mica). Fibrous minerals such as sepiolite and palygorskite were not detected.

The peperite of Herissane region (Central High Atlas) provided an excellent overview on the factors favoring the magma-sediment interaction. These are the products of a mixture of unconsolidated or poorly consolidated sediments, low permeability with a low viscosity magma. The attempt of dating palynology proved unfortunately without results.

Introduction

Earth history is punctuated with occasional emissions of large volumes of basaltic magma, termed to Large Igneous Provinces (LIP's), (Sheth Hetu, December 2007, Coffin and Eldholm, 1994, Ernst and Buchan, 2001, Bryan and Ernst, December 2007), They have induced drastic climate changes and mass extinctions. One of the largest LIP is the Central Atlantic Magmatic Province (CAMP), which was emplaced around 201 My ago, during the initial stages of Pangea supercontinent break-up that gave rise to the Central Atlantic opening (Olsen et al., 2004, Marzoli et al., 2004).

Although corresponding to a continental volcanic event, CAMP volcanism presents evidence for interaction of the eruptive activity with water (e.g., Manspeizer, 1980, Olsen et al., 2003). In southern Portugal this interaction is mostly the result of subterranean water leading to an abundance of phreato-magmatic pyroclastic deposits (Martins et al., 2008). These are absent in Morocco, but in the other hand there are frequent examples of subaqueous lava morphologies that include pillow lavas (Foye, 1924, Olsen et al., 2005, Greenberger et al., 2015) and subaquatic sheet flows indicating the presence of sufficiently deep surface water bodies into which the lavas flowed (Youbi et al., 2003). The present work aims at contributing to the physical and environmental characterization of the CAMP subaqueous basaltic successions of the Marrakech High-Atlas regions of Aït-Ourir (N31 32 59.2 & W7 40 2.26), Jbel Imzar (N31 35 46.9 & W7 25 49.3), and Oued Lhar-Herissane (N31 36 44.7 & W7 22 48.8). These areas exhibit some of the best exposed pillow-lavas and volcano-sedimentary successions in Morocco, which contribute to characterize the paleo-environmental conditions prevalent at this area of Pangea during the volcanic activity.

Section snippets

Geodynamic and geological setting

Intracontinental rifting, leading to Pangea break-up and Central Atlantic opening, initiated during latest Permian-earliest Triassic times and propagated northward along the trend of the Variscan orogen (Ruellan, 1985, Ruellan et al., 1985, Medina, 1995, Medina, 2000, El Arabi et al., 2006). Pangea rifting was accompanied by CAMP extrusion (Marzoli et al., 1999) a LIP mostly composed of low-titanium tholeitic basalts. CAMP magmatism is represented by the remains of intrusive rocks (crustal

The structure of subaqueous flows

The subaquatic lava flows in the High Atlas region present variable aspects. They may locally correspond to densely packed pillows, to pillows dispersed into more or less abundant hyaloclastite matrix, and more or less extended sheet flows presenting well-developed undulated and thin columnar jointing. Pillow lavas and hyaloclastite containing pillows may pass laterally into sheet flows. At the Middle Fm. of Aït-Ourir and Oued Lhar, the subaquatic sequence starts with densely packed pillows,

Pillow lavas morphometry

A morphometry of the pillow lavas was performed by measuring the horizontal (H) and vertical (V) diameters of individual pillows. Whenever possible the measures were obtained on sections as perpendicular as possible to the axis of the pillow. This task was extremely difficult in outcrops where the pillows were intensely weathered and observed in a 2D surface; at Oued Lhar, however, the outcrop allows a 3D observation of the subaquatic sequence. At all locations (Aït-Ourir, Jbel Imzar and Oued

Mineralogy of sedimentary beds and peperites

Sediments were sampled along the Jbel Imzar (Fig. 7) and Sidi Rahal (Fig. 8) sections and analysed by X-Ray diffraction at Instituto Andaluz de Ciencias de la Tierra (Granada, Spain).

The lithostratigraphic analysis of the volcanic succession at Jbel Imzar revealed 14 superimposed flows, organized into four formations and separated by sediments. Three sedimentary intervals were sampled: E1–red and black-shales beneath the lavas from the Lower Fm.; E2 –peperitic sediment (White et al., 2000,

Discussions and conclusions

The succession of lava flow is subdivided into the Lower, Middle, Upper and Recurrent Formations. The emplacement of basalts may induce an important thermal effect onto the adjacent sediments; therefore train an effect on the clay and mineral assemblages.

The pillow lavas and some silty and sandy sediment indicate subaquatic environment.

Some sediment between basalt flows have been affected by hydrothermal fluids circulation, resulting into interactions at the basalt-sediment interface. This is

Acknowledgments

Many thanks to:

-Dr. Michel Seranne, CNRS-GEOCSIENCES, University of Montpellier for his remarks and his suggestions.

-Prof. Anne de Vernal (Geotop-UQAM, Canada) for palynoflore analysis.

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