The Coniacian sequence in the western Iberian Ranges (Spain): Stratigraphic architecture, systems tracts, biostratigraphy and high-frequency cyclicity

Abstract

The Coniacian 3rd-order sequence comprises nine 4th-order parasequences (PsCo1–PsCo9). PsCo1 and PsCo2 are considered a Shelf Margin Wedge as in a Lowstand Systems Tract (LST). Parasequences PsCo3 to the lower part of PsCo6 show an aggradational and retrogradational geometry with onlap on the basal boundary, representing the Transgressive Systems Tract (TST). The upper part of PsCo6 to PsCo9 represents the Highstand Systems Tract (HST) with a Highstand Normal Regression (HNR), upper part of PsCo6 and PsCo7. PsCo8 and PsCo9 show a significative retrogradation of the outer ramp facies and a Forced Regressive Systems Tract (FRST) with progradation of siliciclastic facies from emerged massif on the carbonate facies. The rapid northwards progradation during HST originates a change from a homoclinal ramp to a distally-steepened one. The Coniacian–basal Santonian age is based on the presence of Forresteria (Harleites) petrocoriensis (base of PsCo1) and of Cladoceramus undulatoplicatus (top of PsCo9). Metatissotia ewaldi, Tissotioides hispanicus, Gauthiericeras margae and Prionocycloceras iberiense zones characterize the middle Coniacian. Finally, the late Coniacian is characterised by the Protexanites burgeoisi and Hemitissotia spp. zones. The parasequences are considered to have originated from long eccentricity cycles (0.405 Myr) of Milankovitch's band. Correlation and the reconstruction of the depositional architecture allow quantifying the sedimentary processes and trends, systems tracts, and temporal amplitude of hiatuses. The sequence at the northern outcrops spans 3.65 Myr, and only 0.81 Myr at the southern ones, with most time contained in the hiatuses of the basal (2.43 Myr) and upper (0.405 Myr) sequence boundaries.