Depositional architecture and transgressive–regressive cycles within Liassic backstepping carbonate ramps in the Basque–Cantabrian basin, northern Spain

Abstract

An integrated stratigraphical, sedimentological, geochemical and sequential analysis has been carried out on the Liassic rocks of the Basque–Cantabrian basin in northern Spain using outcrop and subsurface data. The study documents the progressive drowning and backstepping of an epeiric carbonate ramp that originated during the worldwide late Triassic–early Jurassic transgression. Two major carbonate depositional systems have been recognized within the drowning succession: (1) a late Triassic–early Sinemurian shallow-marine ramp, which reflects a progressive marine transgression over Triassic continental deposits; (2) a late Sinemurian–Toarcian set of hemipelagic ramps, which recorded the progressive drowning and backstepping of the ramp system during a second major event of marine flooding. Sequence stratigraphic analysis of the Liassic succession has revealed that transgressive–regressive trends constitute the most evident cyclicity. A major transgressive–regressive cycle and six subsidiary transgressive–regressive facies cycles have been defined in the basin and compared with those defined in other European basins. Although a good correlation exists for the major transgressive–regressive cycle, significant discrepancies have been found for the number, limits and extent of the transgressive–regressive facies cycles reflecting the effects of local tectonics. The overall backstepping within the Basque–Cantabrian basin resulted in a dominance of transgressive episodes across a range of different scales. Accordingly, regressive intervals are less well developed except during limited periods of active tectonics as occurred in the mid-Sinemurian. These progressively longer and deeper marine flooding intervals, followed by relative stillstand or minor sea-level falls, are interpreted to reflect high thermal subsidence rates coupled with a Liassic eustatic transgression.