Patterns in the distribution of Aptian rudists and corals within a sequence-stratigraphic framework (Maestrat Basin, E Spain)

The ecological zonation of, and environmental controls on rudist and coral assemblages on carbonate platforms of the Old World have received more attention for Late Cretaceous examples than for their Early Cretaceous counterparts. This study accordingly investigates the vertical and lateral distribution of Aptian rudist bivalves and scleractinian corals on a carbonate platform succession from the western Maestrat Basin (E Iberian Peninsula). Here, colonial corals grew profusely on an isolated platform top environment during an earliest highstand stage of a long-term trend of relative sea level, as well as on marly slope settings during higher-frequency transgressive pulses. During the later highstand stage within a longer-term relative sea-level cycle, a facies belt dominated by autochthonous rudist bivalves overlaid the coral meadow that had developed on the isolated platform top. The internal part of this carbonate platform with rudists is dominated by slender elevator caprinids such as Caprina parvula, whereas requieniids and polyconitids predominate in the external zone. The abundance of caprinids in the internal platform is remarkable given that caprinid lithosomes of late Early Aptian age are usually rare in the northern margin of the Tethys. The proliferation of caprinids in this case was probably favoured by the apparently more isolated nature of the carbonate platform. On the slopes, the coral communities that flourished during higher-frequency transgressive pulses are overlain by carbonates with rudists, mainly requieniids, shed from the platform top during normal and forced regressive higher-frequency changes of relative sea level. Accordingly, the vertical change from coral-dominated to rudist-dominated facies in both platform top and slope settings records progradation. To decipher the long-term relative sea-level changes that controlled the deposition of this carbonate succession, a sequence stratigraphic analysis was performed. Two depositional sequences including a late Early Aptian (intra Dufrenoyia furcata Zone) forced regressive stage of relative sea level, which subaerially exposed and incised the Early Aptian succession to a depth of 21 m, were recognised. The incisions were back-filled with peritidal deposits during the subsequent marine onlap. The rudist- and coral-bearing carbonates were deposited along platform top to slope profiles lacking a barrier margin, and hence, lagoon environments.