Detached forced-regressive shoreface wedges at the Southern Iberian continental palaeomargin (Early Cretaceous, Betic Cordillera, S Spain)

Abstract

The Lower Cretaceous (Barremian to Early Aptian) Cerrajón Formation on the Southern Iberian Continental Palaeomargin is represented in the study area by five sharp-based shoreface sandstone wedges (5–15 m thick). The sandstone packages abruptly pinch out landwards onto outer-ramp limestones and marls (Los Villares Formation). Sandstone tongues are interpreted as prograding wave-dominated shorefaces distally evolving to thin micro-hummocky cross-stratified sandstone beds alternating with marls deposited in a storm-wave siliciclastic-dominated outer ramp. In the context of sequence stratigraphy, most of the characteristics in outcrops for detached forced-regressive deposits have been tested at the sandstone wedges. Sandstone bases are characterized by a regressive surface of marine erosion (RSME) underlying a calcarenite containing reworked inner-ramp fossils and glauconite from the preceding highstand carbonate-dominated outer ramp (Los Villares Formation). The shoreface sandstones are in turn cut by incised channels (e.g. 2nd sandstone wedge) and filled with Lithophaga-bored sandstone concretions. Lithophaga-bored sandstone concretions are interpreted as deriving from (1) concretion formation in the forced-regressive to lowstand shoreface sandstone during early diagenesis, (2) exhumation of sandstones hosting concretions along incised channels, and (3) coastal winnowing of poorly cemented sandstone host leaving the sandstone concretions which were bored and colonized by coastal fauna in situ. Incised channels truncating shoreface sandstones and sandstone concretions colonized by coastal fauna are interpreted as recognition criteria for a second sea-level fall that occurred after a lowstand sea-level stage. The term paired-forced-regressive is used for a systems tract resulting from a double sea-level dip (two sea-level falls occurring during a single sea-level change cycle). Repeated fourth-order fall-to-rise cycles of relative sea level controlled the architecture and facies distribution of system tracts on the South Iberian Continental Palaeomargin during the Barremian to Early Aptian (Early Cretaceous).