Eustatic implications of late Miocene depositional sequences in the Melilla Basin, northeastern Morocco

The age (similar to 5.78 Ma or lower chron C3r) of the major drawdown of the Paleo-Mediterranean Sea during the Messinian Salinity Crisis has been established by combining results from stratigraphy, paleontology, magnetostratigraphy, and argon dating for a late Miocene sedimentary succession in the Melilla Basin, NE Morocco. This event is inferred from a marine-to-continental series of carbonate and siliciclastic rocks that record the end of Messinian marine deposition in the Melilla Basin and presumably marks the final isolation of the Paleo-Mediterranean Sea. The evidence from the Melilla Basin is approximately coeval with an increase in benthic foraminiferal delta(18)O values from a deep-marine section in the Bou Regreg valley, NW Morocco (Hodell et al., 1994). This increase suggests that a glacio-eustatic lowering of sea level, at least, contributed to the final closure of the Mediterranean during the Messinian Salinity Crisis. The marine-to-continental succession onlaps a carbonate complex that contains evidence for multiple relative sea-level changes leading up to the main drawdown. From bottom to top, the carbonate complex is composed of: (1) an onlapping ramp; (2) a prograding bioclastic platform; (3) a prograding and, locally, downstepping Porites-reef complex; and (4) a topography-draping sequence composed of grainstones, Porites reefs, and stromatolites (terminal carbonate complex of Esteban, 1979). The transgressive ramp correlates to relatively low values of benthic foraminiferal delta(18)O values from a Tortonian-to-lower Messinian section at Bou Regreg (Hodell et al., 1994). This correlation indicates, at least in part, a link between rising sea level and a reduction in global ice volume during deposition of the ramp. A major fall in relative sea level (similar to 60 m) occurred near the demise of the reef complex during chron C3n.1n at 5.95 +/- 0.10 Ma. This signals the initiation of drawdown and changing environmental conditions in the Melilla Basin (a marginal basin), and perhaps the entire Paleo-Mediterranean Sea. A megabreccia interpreted as forming by solution collapse of evaporites on the basin margin of the reef complex occurs at the base of the terminal carbonate complex. Updip, a major subaerial unconformity separates the reef complex and terminal carbonate complex. Evaporite deposition likely occurred during this exposure event and has been dated at 5.82 +/- 0.02 Ma near the base of chron C3r. We contend that these evaporites, restricted to the shallow Melilla Basin, are related to the continuation of the initial stage of the major drawdown of the Paleo-Mediterranean Sea.