Sedimentological analysis of marine Pleistocene deposits of southeastern Tunisia: evidence of two positive eustatic pulsations during the marine isotopic substage 5e (Eemian, Tyrrhenian)

International audienceAbstract Detailed sedimentological and petrographic analysis of marine Pleistocene deposits along the coastal area of southeastern Tunisia allow to identify two distinct lithostratigraphic units separated by an erosion surface (fig. 2 et 5 A and B). These two commonly superposed units form a ridge parallel to the coast. The palaeocoastal morphology was more irregular than the present-day coastline with areas either more protected or more exposed than now (fig. 3). The lower unit overlies an erosion surface cutting into Mio-Pliocene and Villafranchian deposits. It consists of fine-grained bioclastic quartz-rich sands (fig. 5 C), locally overlain by thin marl layers containing benthic foraminifera and ostracods. The very fine facies (silts and clays) represent relatively protected areas while the coarser facies developed in the more exposed zones. These deposits locally display a well-developed aeolian facies that terminates the sedimentary sequence. This unit, well developed in Jerba Island and Jorf peninsula, strongly resembles the stratigraphic unit of “Khnis” as defined by Mahmoudi [1988] on the coast of Central Tunisia. The upper unit is the better developed in the studied area. It consists of carbonate deposits composed mainly by ooids and peloids (fig. 5 D) and contains also a warm Senegalese fauna, especially Strombus. Locally, in the exposed areas, it shows a coarse facies which resulted mainly from the erosion of the calcareous Villafranchian deposits. This unit displays a remarkable shallowing-upward sequence from shoreface to aeolian dunes (fig. 5 E and F). It constitutes the lateral equivalent of the stratigraphic unit of “Réjiche” as defined by Mahmoudi [1988] in Central Tunisia. These two units, called in this study “quartz-rich unit” (the lower sequence) and “carbonate unit” (the upper one), developed during two distinctive sedimentation phases associated with two sea-level highstands separated by a marine regression. During the first highstand sea-level was about 3 m higher than today whereas it was at about +5 m during the second highstand [Jedoui, 2000]. Along the Mediterranean coasts the Strombus paleobeaches, which are contemporary with the carbonate unit, are well developed and traditionally called Tyrrhenian deposits. Their radiochemical dating, using corals, gives ages of about 125 ka [Hearty et al., 1986 ; Miller et al., 1986 ; Dumas et al., 1991 ; Vai et Pasini, 1996]. We obtained the most reliable uranium/thorium dates in southeastern Tunisia on oyster shells. Results show that the two units developed during the marine isotopic substage 5e [last interglacial ; Jedoui, 2000]. This evidence suggests that substage 5e was characterised by at least two eustatic maxima separated by a lowering of sea level during a marine regression. Our results are therefore in agreement with recent palaeoclimatic reconstructions and in particular with sea level reconstructions and marine oxygen isotope records that indicate the distinct possibility of two positive eustatic pulsations during the last interglacial [Hillaire-Marcel et al., 1996 ; Kindler et al., 1997 ; Plaziat et al., 1998]. Variations in the petrographic content of the two outlined units reflect drastic palaeoclimatic fluctuations in southeastern Tunisia during the last interglacial. The establishment of wetter climatic conditions at the beginning of marine isotopic substage 5e than today was responsible for an enhanced terrigenous materiel supply from the continent as showed by siliciclastic sedimentation along the coast (lower unit). Our data are in agreement with the strong sea surface salinity lowering observed in Mediterranean basins at the beginning of the last interglacial period [Kallel et al., 2000]. The regression of these conditions during the second half of the last interglacial favoured a carbonate sedimentation (upper unit) in southeastern Tunisia

Shallow-marine microporous carbonate reservoir rocks in the Middle East: relationship with seawater Mg/Ca ratio and eustatic sea level

The formation of shallow-marine microporous carbonate reservoir rocks remains poorly understood in spite of their economic importance, particularly in the Middle East. In this paper, we investigate relationships between the stratigraphic occurrence of these carbonates in the Middle East and (i) the evolution of the Mg/Ca ratio in seawater; and (ii) cyclic variations in relative sea-level. An inventory of carbonate formations in the Middle East was compiled for three geological time intervals characterised by different seawater chemistries: the Late Carboniferous to Triassic (aragonite seas); the Cretaceous (calcite seas); and the Cenozoic (transitional from calcite to aragonite seas). For each time interval, carbonate formations described as microporous have been listed. During the Cretaceous calcite sea, eleven microporous carbonate formations were deposited in the Middle East. However, no microporous carbonates were formed during the Late Carboniferous to Triassic, a time of aragonite seas. During the Cenozoic, four of the five microporous carbonate formations recorded were deposited before the transition from calcite to aragonite seas. Thus, these shallow-marine microporous carbonates appear to have developed from precursor muds which were mainly composed of low-Mg calcite crystals. Moreover, during the Cretaceous and the Cenozoic, microporous carbonate formations in the Middle East were generally associated with major transgressions and highstands of relative sea level. The relatively high stability of low-Mg calcite muds may explain why shallow-marine microporous carbonates formed during time intervals with calcite seas. In contrast to muds composed of aragonite or high-Mg calcite crystals, the original microfabric (including intercrystalline microporosity) of low-Mg calcite muds can partly survive moderate diagenesis.status: publishe

Lacustrine microporous micrites of the Madrid Basin (Late Miocene, Spain) as analogues for shallow-marine carbonates of the Mishrif reservoir Formation (Cenomanian to Early Turonian, Middle East)

Shallow-marine microporous limestones account for many carbonate reservoirs. Their formation, however, remains poorly understood. Due to the lack of recent appropriate marine analogues, this study uses a lacustrine counterpart to examine the diagenetic processes controlling the development of intercrystalline microporosity. Late Miocene lacustrine microporous micrites of the Madrid Basin (Spain) have a similar matrix microfabric as Cenomanian to Early Turonian shallow-marine carbonates of the Mishrif reservoir Formation (Middle East). The primary mineralogy of the precursor mud partly explains this resemblance: low-Mg calcites were the main carbonate precipitates in the Cretaceous seawater and in Late Miocene freshwater lakes of the Madrid Basin. Based on hardness and petrophysical properties, two main facies were identified in the lacustrine limestones: a tight facies and a microporous facies. The tight facies evidences strong compaction, whereas the microporous facies does not. The petrotexture, the sedimentological content, and the mineralogical and chemical compositions are identical in both facies. The only difference lies in the presence of calcite overgrowths: they are pervasive in microporous limestones, but almost absent in tight carbonates. Early diagenetic transformations of the sediment inside a fluctuating meteoric phreatic lens are the best explanation for calcite overgrowths precipitation. Inside the lens, the dissolution of the smallest crystals in favor of overgrowths on the largest ones rigidifies the sediment and prevents compaction, while partly preserving the primary microporous network. Two factors appear essential in the genesis of microporous micrites: a precursor mud mostly composed of low-Mg calcite crystals and an early diagenesis rigidifying the microcrystalline framework prior to burial

Hydrodynamic behaviour of Nummulites: implications for depositional models

Climate change and earth-ocean atmosphere systems