Mixed carbonate-siliciclastic contourite drift deposits associated with the entrance of an Atlantic-Mediterranean corridor (late Miocene, southwest Spain)

Carbonate contourite drifts are poorly documented in the onshore record because of the difficulty of implementing diagnostic criteria for their recognition. Accordingly, little is known about the relative position of carbonate drifts with respect to ancient carbonate platforms, seaways and shallow passages within the context of palaeoceanography. This study presents a fossil example of mixed carbonate-siliciclastic drift cropping out in a quarry in Osuna (Sevilla province, southern Spain) at the northern end of the Guadalhorce Corridor, a Miocene strait connecting the Mediterranean Sea and the Atlantic Ocean in the Betic Cordillera. Based on the facies and sedimentary structures, the studied succession is divided into three units: 1) the lower unit, Unit 1, is a 33-m thick succession of large carbonate bodies with mega cross-stratification pointing to the southeast and secondarily to the northwest interpreted as a contourite drift; 2) the intermediate Unit 2 is a 0.5–2-m thick terrigenous conglomerate body eroding the top of Unit 1; and 3) the uppermost Unit 3 consists of a 6-m thick siliciclastic-dominated succession with herringbone cross-stratification and a dominant direction of the structures to the northwest interpreted as tidal deposits. The large-scale sediment bodies with mega cross-beds, the presence of reactivation surfaces with grain-size changes, and the unidirectionality of the structures were diagnostic for the recognition of Unit 1 as drift deposits. The dominant sedimentary structures pointing to the southeast in the drift were generated by Atlantic inflow into the Mediterranean. This challenges the classical “siphon” model for the Atlantic-Mediterranean water-mass circulation pattern for this age. The conglomerates of Unit 2 evidence regional uplift of the southern margin of the Guadalquivir Basin that promoted a change in the depositional mode from a bottom-current dominated (Unit 1) to a tide-dominated environment (Unit 3) after the closure of the Guadalhorce Corridor in the Messinian.