The Cretaceous of Lebanon in the Middle East (Levant) context
Résumé
The Cretaceous succession of Lebanon was first described and dated mainly by Dubertret & Vautrin (1937), Dubertret (1955), and Saint-Marc (1974), supplemented by the later contributions of Smirnova & Mroueh (1980) for the Chouf Sandstones, Noujaim-Clark & Boudagher-Fadel (2001, 2002) for the Salima Formation. After two years of field work using facies sedimentology and sequence stratigraphy, and as part of the Middle East Basins Evolution (MEBE) Programme, we propose to divide the Cretaceous of Lebanon into three parts (Fig. 1): (a) Valanginian to upper Aptian, (b) upper Albian to Turonian, and (c) Post-Turonian to Eocene. The first period is represented by several depositional sequences each bounded by emersion surfaces (D1 to D3, Fig. 1). The top of the Jezzine Fm. is unique because it is an emersion (karstified) surface in northern Lebanon but appears to be only a simple transgression surface in the South. During these emergences the coastal sedimentary prism was shifted into the present-day offshore. The transgressions that initiate all three sequences, including that of the upper Albian, which is coincident with the beginning of period 2, starts with the deposition of volcaniclastics that may continue into the overlying, deepening-up, shallow-water marine deposits. This relationship suggests that these depositional sequences were tectonically-controlled, that is the transgressions were initiated by the renewal of an extensional regime accompanied by volcanism. As a result of the strong lowering of base level, most emersions are associated with incised valleys, particularly spectacular in Northern Lebanon. The strongest is at the base of the upper Albian sequence (Fig. 2). It corresponds to an emergence that lasted throughout the whole lower to middle Albian interval. The second period corresponds to the emplacement of a large system of carbonate platforms that covered much more of the Arabian craton. Changes of relative sea-level created a number of depositional sequences (not shown on Fig. 1) but drops in base level were not as great as those that occurred during the first period, except perhaps for the sequence ending near the Albian-Cenomanian boundary where an emersion surface of this age has been seen in the Anti-Lebanon and even in sections west of the Levant fault. These mild oscillations in relative sea-level are responsible of the spectacular sandwich of shallow-water (often rudist-bearing) carbonate facies alternating with finely-bedded or even laminated mudstones of Mount Lebanon. The Cenomanian-Turonian boundary, situated at the very base of the Ghazir Fm., is here coincident with the beginning of a deepening trend that peaks in the lower Turonian. The third period corresponds to an acceleration of the drowning of the Arabian craton, probably as a response to the beginning of the collisional trend responsible for the closing of the Tethys. On the Levant platform, deposition of micritic limestones and chalks began and continued into Eocene times. Several paleogeographic maps have been drawn that take into account our results in Lebanon and also include, tentatively, published data from adjacent countries. The task is not easy mainly because of the many remaining uncertainties about the age of Lower Cretaceous sandstones in Israel, Syria and Jordan. Two of them are presented (Fig. 3). The (?) Berriasian – Valanginian sequence is preserved only in some half-grabens in northern Lebanon under the "transgressive" Barremian Chouf sandstones. The term "transgressive" should be considered in terms of accomodation space because the facies is fluvial at its base, and becomes deltaic to marginal marine only at its summit. These sandstones were deposited most abundantly in a W-E oriented saddle west of the Levant fault. East of the fault their thickness abruptly decreases. The lower portion of the poorly dated sandstones of the Kurnub Group in Syria and Jordan, as well as their equivalent in Israel have been tentatively attributed to the lower and upper Aptian rather than to a lateral equivalent of the Chouf sandstones. This choice is based on a better coherence in the Aptian depositional systems from a regional standpoint. This tentative correlation remains to be fully supported by new biostratigraphic analyses. The whole of the lower Aptian is represented by a transgressive half-cycle (Jeita Fm.) made of a number of smaller carbonate sequences (megarippled calcarenites, rudist-bearing wackestones, etc.), often with emergent features at the top, and by a regressive half-cycle represented by the lagoonal mudstone facies of the Jezzine Fm. The barrier facies of the Jezzine Fm. is not known in Lebanon. In the coastal plain of Israel, thick calcarenites have been found at this stratigraphic level in exploration wells. These calcarenites could be part of the facies belt that separated the lagoonal mudstones from those of the open sea. If this view is correct, this barrier facies should continue to the north in offshore Lebanon (Fig. 3A). In west central Lebanon the upper Aptian (Dar El Beidar Fm.) is made up of a number of small shallow-water carbonate sequences. To the North, the thickness of this formation decreases markedly, mainly because it was eroded during the Albian emergence. Paleogeographic maps of the Aptian are hard to make due to stratigraphic uncertainties about the Kurnub sandstones in adjacent countries, as for example in Jordan where they onlap Paleozoic sandstones. The maps drawn for the upper Albian (Fig. 3B) to Turonian interval indicate a system of prograding and retreating carbonate platforms both in the coastal range of Syria and in the Rutbah high in Syria. Central and southern Lebanon remained subsident enough that only deeper-water mudstones were deposited there, at least during the Late Albian and Early Cenomanian. The Bekaa valley also remained a low. This suggests a synsedimentary vertical movement of the Yamouneh Fault.
Domaines
Stratigraphie
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