High-resolution sequence stratigraphy of Upper Triassic succession (Carnian - Rhaetian) of the Zarzaitine outcrops (Algeria): a model of fluvio-lacustrine deposits
Résumé
The detailed facies analysis of the Zarzaitine outcrops (Illizi Basin in the In Amenas area of Algeria) allows the depositional environment of the Upper Triassic succession to be defined: braided rivers within an arid and humid alluvial plain, low sinuosity rivers within a humid alluvial plain, lake deposits and marginal sabkha. The description of the outcrops helps to define three types of genetic units from a proximal to a distal depositional environment: fluvial, fluvio-lacustrine and lacustrine. The spatial and temporal evolutions of the genetic unit were characterised by five specific stages from dry to wet climate conditions, inducing sediment supply and lake-level variations. During the first two stages (stage 1 and 2) under a dry climate, the lake level was low and sediments mostly by-passed and poorly preserved. During stages 3 and 4, an increase in humidity and rainfall induced a rise in the lake-level and the development of vegetation, as well as a decrease in the sediment supply, although the sediment preservation were then at its maximum. The last stage (stage 5) marked the beginning of a decrease in the humidity, the minimum of sediment supply and the maximum of the lake level. Therefore, the recognition and the description of genetic units within this fluvio-lacustrine environment help to demonstrate the interaction between climate, sediment supply and lake-level variation, at the scale of these units. Six stratigraphic cycles have been recognised in the vertically stacked genetic units and can be grouped in three megacycles (denoted as IIII). The first megacycle, attributed to Carnian to Norian, corresponds to the base of braided river systems with some ephemeral channels developed in an arid environment where some aeolian deposits were preserved. It evolved trough time to more humid conditions favouring the development of extensive floodplain, associated with hydromorphic soils, and perennial lake environments. During the Carnian times and Norian, the Zarzaitine area was not connected to the Berkine basin northward, a Hercynian unconformity palaeorelief forming a drainage divide. According to the directions of the palaeocurrents, the sediment provenances were mostly from the southwest and the north. The connection with the Berkine basin only occurred during the upper part of the megacycle II deposition, characterised by fluvio-lacustrine environments. The maximum flooding of the megacycle I, Norian in age, could be correlated with the early Norian maximum flooding observed on the Saharan platform. The late NorianRhaetian second megacycle was mostly a lacustrine environment associated with extensive floodplain, with the development of a hydromorphic palaeosol with root imprints attributed to a warm and wet climate. The maximum flooding episode of this second megacycle could be attributed to the major marine trangression recorded in Algeria, during the Rhaetian, and could be correlated with a relative sea-level rise. A dolomitic level, attributed to sabkha environments, marks the beginning of the retrogradational trend of the third cycle, attributed to the RhaetianLiassic. The palaeocurrent of the upper part of megacycle II and of the megacycle III were always oriented toward the northeast, attesting that the relief was located to the southwest.