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The Jurassic-Cretaceous boundary in Lebanon
Sibelle Maksoud, Bruno Granier, Raymond Gèze, Christopher Toland, Dany Azar

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-06 03:30 PM – 03:50 PM
Last modified: 2018-11-30

Abstract


Louis Dubertret is considered as the father of the modern stratigraphy in the Levantine area. His work remains the basis of any geological or paleontological contribution in Lebanon. However, the stratigraphical units he introduced when mapping the region, i.e., J6 = Kimmeridgian, J7 = uppermost Jurassic, C1 = Neocomian [The "Néocomien" sensu Dubertret comprised all the Cretaceous strata below the Aptian, i.e., including the Barremian], C2a = lower Aptian, and C2b = upper Aptian" (Dubertret, 1955), and later renamed by Walley (1997, 2001) respectively as "Bikfaya, Salima, Chouf, Abeih, and Mdairej formations") are all facies-driven. Because this concept is nowadays considered outdated, the Lebanese stratigraphy was in need for a full revision.

In 2013, our team initiated a full revision of the lower Cretaceous in Lebanon based on a holistic approach of stratigraphy (micropaleontological, biostratigraphical, and lithostratigraphical studies). We were able to: identify regional stratigraphical discontinuities and related bounded units; document their typical paleontological assemblages with both micro- and macrofossils; ascribe them more accurate ages; and better understand both vertical and, consequently (Walther's law), lateral facies changes.

Dubertret considered the Bikfaya Cliff, symbolized as J6, to be Kimmeridgian in age. However, the occurrence of Campbeliella striata (Carozzi, 1954) gave better resolution, permitting a late Kimmeridgian – Tithonian date.

The Salima Formation auct., above the J6, was considered Jurassic in age. Its microfossil contents (Granier et al., 2016) suggested it should be divided into two different units: 1) an unnamed unit with Longicollaria insueta (Řehánek, 1986) as a marker for the middle Tithonian, and 2) the oolitic limestones of the Salima that could be early Valanginian in age. According to S. Ferry (personal communication), the macrofossil contents of this oolitic unit (e.g., brachiopods, identified by Y. Alméras) validates the age ascription.

The "Grès du Liban" of Heybroek (1942), "Grès de base" or "C1" of Dubertret (1955), or "Chouf Formation" of Walley (2001) was considered an interval comprising continental strata only. However, Granier et al. (2016) reported marine occurrences at its base that are dated as early Barremian. The "Grès du Liban" sensu Granier et al. (2015) comprises the "C1" and the base of the "C2a" of Dubertret (1955). In its upper part, there are locally lacustrine and palustrine facies corresponding to a series of Lagerstätten (including fishes, turtles, ostracods, etc.), as well as amber sites with arthropod inclusions (Azar, 1997; 2007; Azar et al., 2003).

The Salima Formation emend., which corresponds to an overall transgressive oolitic unit, is an unconformity-bounded unit. Based on our investigations: 1) the lower unconformity corresponds to a hiatus that encompasses at least the duration of the Berriasian; 2) the upper discontinuity corresponds to a hiatus that encompasses at least the upper Valanginian and the Hauterivian. Regardless of the selected option for the Jurassic-Cretaceous boundary (Tithonian/Berriasian or Berriasian/Valanginian), it unambiguously fits into the discontinuity at the base of the Salima Formation in Lebanon.