Henri Pauc

Very high resolution seismic investigation of the sedimentary infill of the Leucate lagoon (Aude and Pyrenees-Orientales - SE France)

A very high resolution seismic investigation has for the first time allowed the imaging of the sedimentary infill of a Mediterranean lagoon. The Leucate lagoon is part of a lagoon system located along the shore of the western Gulf of Lions, from the Rhone delta to the Spanish border. These lagoons are separated and protected from the sea by sandy barriers, also called lidos, which are the result of a process of shore regularization by waves. The seismic data, obtained by using a boomer-Seistec, coupled with lithological and radiocarbon data [Martin, 1978] previously collected from cores, have revealed three main sedimentary formations : The basal formation represents the substratum of the lagoon and the middle and upper formations its infill. The basal formation displays a very uniform seismic facies with reflectors almost constantly dipping towards the sea. It comprises conglomeratic sediments and is interpreted as a progradating fluvio-deltaic formation. Its upper surface is erosional and is locally deeply incised. In the shallowest parts of the lagoon, where the basal formation almost emerges, its upper part is reworked by modern processes into multiple cut-and-fill structures. The middle formation overlaps the basal formation, and constitutes the main depositional unit of the lagoon fill (up to 20 ms twtt thick). The seismic facies are variable and correspond to sand and clay sediments deposited under fluvio-lagoonal to lagoonal and marine conditions. The upper formation represents the upper part of the infill. It rests above the middle formation through a conformable surface, locally slightly erosional, and overlaps the basal formation along the western rim and in the shallowest parts of the basin. The thickness of this upper formation does not exceed 3mst wtt. It mainly consists of clay sediments of lagoonal origin. The main characteristic of this upper formation is a thin sole of very dense sand at the base. This bed is also a remarkable seismic reflector, and is interpreted as resulting from the maximum marine flooding of the system. This occurs before the beginning of the barrier construction, and the progressive closure of the lagoon. Another remarkable aspect of the upper formation is the simultaneity of its basal part with the lido construction. In this upper unit, the seismic data allow the imaging of the lateral passing between the planar bedded sediments of the infill, with the sigmoidal beds representing washover fans that construct the lagoonal side of the lido. The uppermost part of the formation represents the final and present-day stage of the lagoonal infill since the final closure by the barrier. Dating, performed on cored sediments, allow the sediments of the basal formation to be assigned to the Middle Pleistocene (with no more precision). The erosion of the top of the basal formation is interpreted as fluvial incision during the last sea level fall. The lagoon infilling is of Holocene age and comprises two stages : the first and main stage corresponds to fluvio- to marino-lagoonal sedimentation, and occurred before 6,000 B.P. The second corresponds to the recent to modern infilling that began around 4,000 B.P. with the construction of the lido and the closure of the lagoon. The two stages are clearly separated by a period of maximum marine flooding. The thickness of the lagoon fill is relatively limited, probably no more than 20 m.

Matière organique et pollution par les hydrocarbures dans les sédiments superficiels du golfe d'Arzew (mer Méditerranée, Algérie)

The distribution of global organic parameters (organic carbon, nitrogen, hydrolyzable carbon) was established in surficial sediments of the Gulf of Arzew. Moreover, humic substances, humin and hydrocarbons have provided information on the organic matter origin and evolution in the identified sedimentary areas: prodeltas at the mouth of the dry rivers Cheliff and Macta; muddy mid-deposits and carbonated shallow environments. The higher organic matter concentrations characterize the muddy mid-deposits and especially the western part located near the Arzew and Bethioua harbours (TOC > 0.9 %; Nt = 0.35 % d.w.). The marine origin of the organic matter is supported by low CN ratios (5–9) in the total sediments and the position in the Van Krevelen diagram of the humic acid HC, NC and OC atomic ratios. The predominant marine character results from an important phytoplanktonic production. Only organic matter of the Cheliff prodelta is characterized by a double marine and continental signal. The degradation state of organic matter shows that the compounds are more degraded in the western part of the gulf, towards the east to west transfer of matter. The Arzew and Bethioua harbours areas are enriched in hydrocarbons (90–180 mg kg−1) according to the N.S.A. norm (1975) and represent significant petrogenic pollution levels.

L'intrusion saline et la dynamique des materiaux en suspension au contact fluvio-marin: Regime de crue et regime d'etiage dans l'oued Mazafran (Ouest Algerois)

Abstract In the lowest part of the Oued Mazafran Valley (30 km west of Algiers, Algeria), the entry of saltwater creates a salt wedge. This structure can be found upstream as far away as 5 km from the river mouth during the low water stage when the river flow is zero. The river-seawater interface is indicated by the water reverse ionic composition. The river suspensions react to this salt wedge in two ways: First, from a dynamic point of view, the flow becomes equal to zero at the upstream limit of the salt wedge, inducing the deposition of part of the suspension due to excess load. The second is an electrochemical reaction which causes the finest and reactive part of the suspended material to flocculate through the interface.