Upper Cretaceous marine-continental transition (Leonese Area, NW Spain) defined from integrated outcrop and seismic stratigraphy

摘要

The Upper Cretaceous succession of the Leonese Area (NW Spain) comprises mixed clastic and carbonate sediments. This succession is divided into two lithostratigraphic units, the Voznuevo Member and the Bonar Formation, which represent fluvial, shoreface, intertidal, subtidal and open-shelf sedimentary environments. Regional seismic interpretation and sequence stratigraphic analysis have allowed the study of lateral and vertical changes in the sedimentary record and the definition of third-order levels of stratigraphic cyclicity. On the basis of these data, the succession can be divided into two second-order depositional sequences (DS-1 and DS-2), incorporating three system tracts in a lowstand to transgressive to highstand system tract succession (LST-TST-HST). These sequences are composed of fluvial systems at the base with palaeocurrents that flowed westward and south-westward. The upper part of DS-1 (Late Albian-Middle Turonian) shows evidence of intertidal to subtidal and offshore deposits. DS-2 (Late Turonian-Campanian) comprises intertidal to subtidal, tidal flat, shallow marine and lacustrine deposits and interbedded fluvial deposits. Two regressive-transgressive cycles occurred in the area related to eustatic controls. The evolution of the basin can be explained by base-level changes and associated shifts in depositional trends of successive retrogradational episodes. By using isobath and isopach maps, the main palaeogeographic features of DS-1 and DS-2 were constrained, namely coastline positions, the existence and orientation of corridors through which fluvial networks were channelled and the location of the main depocentres of the basin. Sedimentation on the Upper Cretaceous marine platform was mainly controlled by (i) oscillations of sea level and (ii) the orientation of Mesozoic faults, which induced sedimentation along depocentres. Copyright (c) 2013 John Wiley & Sons, Ltd.