Sédimentologie de faciès de la plate-forme carbonatée du Pliensbachien inférieur des Monts des Traras orientaux (Algérie Nord-occidentale)

Abstract

In the present study, we investigate six sections, which belong to the lower Pliensbachian outcrops in the Traras Mountains, north-west of Algeria (Western Tethys). The detailed sedimentological study allowed to distinguish 23 facies, regrouped into five facies associations. The deposits are suggested to be formed in inter-supratidal to shallow subtidal environments, above the fair-weather on a tropical ramp. The diagenetic history of the "Zailou Formation" has been investigated for the first time under cathodoluminescence and from carbon and oxygen isotope analysis (bulk rock and cements). In the phase of early diagenesis, we distinguished ten generations of early cements, forming four types of discontinuities, (i) marine, (ii) emerged surfaces, and (iii) composite surface. Non luminescent cements indicate precipitation or recrystallization within oxidized conditions, while the cements with various luminescence suggest recrystallization within semi-restricted (orange), to restricted conditions (dull), during shallow burial conditions, resulting in negative oxygen values. The positives values of δ13 C indicate marine or meteoric conditions, whereas the negative values characterizing some levels reflect incorporation of organic matter within the intertidal and supratidal zone. Among the eogenetic cements, asymmetric cements, resembling pendants cements, have been recorded in thin sections from the Ras El Manara section. Geopetal fabrics indicate that these seemingly “pendant cements” are, in some places, oriented upwards, i.e. grown in the opposite direction than expected, or they grew from grains towards the pore centres. These observations disprove their origin as gravitational cements precipitated from pendant water droplets on the bottom sides of grains as in the vadose zone. In contrast, a formation in the marine phreatic zone, under strong tidally-driven horizontal pore-water flow, seems more probable. The detailed sedimentological and diagenetic study allowed highlighting meter-scale shallowing-upward cycles. The cycles as mainly produced by autocyclic processes under steady relative sea-level rise; subtidal cycles are interpreted to have been controlled by lateral migration of shoals, while peritidal cycles are thought to be generated by progradation of inter- and supratidal flats into lagoonal sediments. In this case, sediment transport would control the thickness and duration instead of rate at which accommodation was created. However, the superposition of supratidal facies upon subtidal facies, as well as the superposition of marine and subaerial cements on the same surface (composite surface), would not reflect an autogenic formation, but rather reflect absolutely an allocyclic origin (eustatic sea level fall, or tectonic uplift), resulting in the emergence of the surface, after an initial flooding.