Spit-platform temperate carbonates: the origin of landward-downlapping beds along a basin margin

Lower Pliocene temperate carbonates exhibit landward-downlapping beds at the southern margin of the Carboneras Basin in south-eastern Spain. This rarely documented stratal geometry resulted from the accumulation of bedded bioclastic carbonate sand and gravel by longshore currents along a spit platform located a few hundred metres from the palaeoshoreline. The top of the spit platform was covered by shoals that extended over a gently dipping ramp inclined to the north. On the landward slope of the spit, sediments washed over from the shoal area were deposited in parallel-laminated beds with a southward dip of 8–11 . These beds aggraded and retrograded after an increase in accommodation space, probably related to an Early Pliocene eustatic sea-level rise. As a result, the beds downlap onto the underlying unconformity surface in a shoreward direction. Eventually, the depression between the shoreline and the spit platform was filled, and a gentle ramp became established. These Pliocene exposures in the Carboneras Basin and a similar Upper Miocene example in southern Spain suggest that landwarddownlapping stratal geometries can be expected in nearshore temperate carbonates along basin margins, and demonstrate a similarity in sedimentary dynamics to siliciclastic sands and gravels.