Geological Society of America | GEOLOGY | Volume 47 | Number 2 | www.gsapubs.org 171 Discovery of vast fluvial deposits provides evidence for drawdown during the late Miocene Messinian salinity crisis Andrew S. Madof 1 , Claudia Bertoni 2 , and Johanna Lofi 3 1 Chevron Energy Technology Company, Houston, Texas 77002-7308, USA 2 Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK 3 Géosciences Montpellier, CNRS, Université de Montpellier, Montpellier 34095, France 1 GSA Data Repository item 2019062, methods involving 2-D and 3-D seismic interpretation as well as generating isochrons and spectral decompositions, is avail- able online at http://www.geosociety.org/datarepository/2019/, or on request from editing@geosociety.org. CITATION: Madof, A.S., Bertoni, C., and Lof, J., 2019, Discovery of vast fuvial deposits provides evidence for drawdown during the late Miocene Messinian salinity crisis: Geology, v. 47, p. 171–174, https://doi.org/10.1130/G45873.1. ABSTRACT The late Miocene Messinian salinity crisis (MSC) was a signifcant oceanographic event that caused widespread evaporitic accumulation throughout the Mediterranean Basin. Although multiple hypotheses exist regarding the origin of evaporitic and post-evaporitic deposits, researchers remain divided on the magnitude of base-level fall, and on whether these accumulations record deep-water or non-marine conditions. Here, we introduce a previously unknown, upper Mes- sinian fuvial deposit comparable in size to the late Miocene Nile River fuvial valley fll and show that near-complete desiccation of the eastern Mediterranean was responsible for its development. The basin-wide accumulation, which is located offshore Cyprus, Syria, Lebanon, and Israel, lies directly atop deep-basin evaporites and related erosional surfaces, and is one of the largest known riverine deposits associated with the terminal MSC. From marked onshore incision and basinward thinning trends, the source of the accumu- lation is presumed to be a formerly unidentifed drainage basin in southern Turkey and western Syria; the deposit extends >500 km into the western Levant Basin, where its depositional sink is marked by six well-developed backstepping lobes. Based on the deposit’s seismic stratigraphy and morphology, which provide clear evidence of sub- aerial exposure, we question current hypotheses proposing a deep- water origin for late Messinian accumulations. We also draw specifc attention to the development of extensive circum-Mediterranean non- marine conditions prior to Zanclean marine transgression, and to the previously overlooked role of fuvial systems in diluting hypersaline lakes in evaporitic basins. INTRODUCTION The Messinian salinity crisis (MSC) was a major late Miocene oceano- graphic event that led to the emplacement of >1 × 10 6 km 3 of evaporites in the Mediterranean Basin (Ryan, 1973) in <640 k.y. (i.e., between 5.97 and 5.33 Ma; Manzi et al., 2013). Although multiple hypotheses were initially proposed to explain the origin of the evaporitic and post-evaporitic deposits, a shallow-water deep-basin model was generally accepted (Hsü et al., 1973). While some workers have refned this model, others have re- adopted an earlier and alternative hypothesis for the MSC: a deep-water deep-basin model (i.e., small-magnitude base-level fall; see Roveri et al., 2014). Proponents of this idea envisage these evaporites as deep marine, suggest that late Messinian post-evaporitic accumulations are subaqueous in origin (Gvirtzman et al., 2017), and conclude that subaerial exposure had little to no bearing on the MSC. Although proprietary data (i.e., those acquired during offshore hydro- carbon exploration) can be used to test hypotheses related to the MSC, accessibility issues regularly preclude further investigation. This is par- ticularly true in the eastern Mediterranean, where questions regarding the crisis largely remain unanswered. To address this issue, we present previ- ously unpublished two- and three-dimensional (2-D and 3-D) seismic data from the Levant Basin, test hypotheses related to origin of latest Messinian deposits, and evaluate the claim that during the MSC, “the eastern Mediter- ranean became evaporated to near dryness” (Wallmann et al., 1997, p. 31). NAHR MENASHE DEPOSIT Interpretation of 2-D and 3-D seismic data (see Methods in the GSA Data Repository 1 ) from offshore Cyprus, Syria, Lebanon, and Israel has led to the recognition of a formerly unidentifed basin-scale accumulation. This deposit, herein termed Nahr Menashe (Fig. 1), has an areal extent approxi- mately equal to that of the Messinian Nile River (Eonile) fuvial valley fll (Abu Madi Formation) and a volume of >4150 km 3 (calculated from 2-D seismic data in two-way traveltime [TWTT] and using an interval velocity of 2925 m/s). From its position and morphology, as well as interpreted age and depositional environment, we show that the Nahr Menashe is one of the largest riverine accumulations associated with the terminal MSC, and that it deposited in a subaerially exposed, actively deforming Levant Basin. Position and Morphology The Nahr Menashe is situated directly atop deep-basin Messinian evapo- rites (Fig. 2A), with its lower and upper boundaries (intermediate erosional surface [IES] and top erosional surface [TES]; see Lof, 2018) forming conformable to unconformable contacts with surrounding units. When traced toward the southwest, the top of the Nahr Menashe is coincident with the upper boundary of the Abu Madi Formation, offshore Egypt (Fig. 2B); to the northeast, the surface shallows (Fig. 2C) and deepens (Fig. 2D). The Nahr Menashe reaches a maximum thickness of 300 ms (TWTT) in areas offshore of northwestern Lebanon and western Syria and thins to the southwest (Figs. DR1A–DR1F and DR2A–DR2F in the Data Repository). In the Levant Basin, the Nahr Menashe consists of a major axial accu- mulation fanked by smaller transverse deposits (Fig. 3A). The trunk-like axial accumulation extends >500 km in a northeast-southwest to east- west direction and is >20–50 km in width; the deposit terminates at six Manuscript received 6 August 2018 Revised manuscript received 5 December 2018 Manuscript accepted 11 December 2018 https://doi.org/10.1130/G45873.1 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. 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