The Cretaceous in the Alentejo Basin (Portugal): From the Delta Wedge to the Basin Floor Fan

Abstract

With recent 3D seismic data acquired in the exploration concessions in the Alentejo basin, it was possible to recognize the entire spectrum of the Cretaceous clastic depositional system, from coastal to deepwater. The basin configuration has been quite stable, with proximal coastal environments to the NE passing to deepwater systems in the SW, with a relatively continuous transgression taking place through Cretaceous and Cenozoic times.

The prograding delta top has been dated Maastrichtian with biostratigraphic analysis of a drop core performed in the area. This has helped to constraint the stratigraphy of the Upper Cretaceous in the proximal area, although due to high compartmentalization of the basin the correlation with the distal area is more difficult to achieve. It can be observed that this delta wedge started to develop possibly even before Cretaceous times and had several episodes of growth and erosion; at least two major sequences can be observed directly in seismic.

This delta will feed the basin floor fans interpreted as being present in the distal SW area of the basin, flowing along a NE-SW regional graben, with several channelized features supporting the model. Slope deposits can be inferred through forward modeling, with meandering channels in the toe of slope and more straight NE-SW oriented channels in the distal part of the slope, forming widespread basin floor fan deposits in the SW corner of the basin.

The Lower Cretaceous in the Alentejo basin is characterized by a mixed carbonate/siliciclastic setting seen in the shallow water wells on the shelf, as opposed to the deep water environment, with the deposition of coarser sands in the slope and finer-grained sands in the basinal area. This Early Cretaceous turbidite system is capped by a much shalier Upper Cretaceous probably corresponding to the Cenomanian -Turonian transgression observed in the onshore Portugal and associated with the Oceanic Anoxic Event. The Late Cretaceous will terminate with the initiation of the tectonic inversion of the whole basin, conditioning the deposition of the overlaying Tertiary strata to local depocentres confined by the inverted, anticlinal structures.

With the studies performed in the characterization of this siliciclastic system, we propose a new depositional model for the Lower Cretaceous, to support the unique seismic features observed in the Alentejo basin.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015