Outcrop-Scale Identification of Forced and Normal Regressive Deposits in Carbonate Systems: From Aptian Example from the Western Maestrat Basin (Iberian Chain, Spain)

Bover-Arnal, Telm¹
 Salas, Ramon²
 Moreno-Bedmar, Josep A.²
 Bitzer, Klaus¹

Discriminating between forced and normal regressive deposits is of importance to oil exploration. The processes governing their formation are different as well as their resulting architecture and lithofacies, which determine important features of petroleum plays. The key to distinguishing these two types of deposit lies in the identification and correlation of the different bounding surfaces with a sequence stratigraphic significance. In the Aptian platform-to-basin carbonate succession preserved in the Galve sub-basin (western Maestrat Basin), these key surfaces are easily recognizable and correlatable at outcrop-scale, facilitating the identification of two distinct highstand systems tracts (normal regression), one forced regressive wedge systems tract (forced regression), one lowstand prograding wedge systems tract (normal regression) and one transgressive systems tract (transgression). A large prograding carbonate platform dominated by rudist bivalves and corals makes up the first highstand systems tract. This systems tract is topped by the subaerial unconformity (sequence boundary), which is characterized by extensive palaeokarst development towards the land. The forced regressive wedge systems tract corresponds to a basin-floor calcarenitic wedge displaying low-angle cross-bedding, which is bounded below by the basal surface of forced regression and topped by the correlative conformity (sequence boundary). The basal surface of forced regression is marked by a bioturbated smooth surface separating highstand basinal marls from the aforementioned basin-floor component. The correlative conformity is characterized by a sharp surface that limits the detached forced regressive wedge (below) from the lowstand prograding wedge systems tract (above). The lowstand prograding wedge systems tract corresponds to a small carbonate platform located in a basinal position that downlaps onto the forced regressive deposits and onlaps landwards. At the top of the lowstand platform, a hardground marks the transgressive surface. Landwards, this surface is superposed onto the subaerial unconformity. Above the transgressive surface, the transgressive systems tract displays a retrograding pattern of the lowstand platform and marly sedimentation. The maximum flooding surface surmounts the transgressive deposits. The subsequent establishment of new carbonate platforms with rudists, nerineid gastropods and corals characterizes the second highstand systems tract.