CLEMENTE, PILAR, Instituto de Geologia Economica, Madrid, Spain

ABSTRACT: Depositional Sequences and Fluvial Architecture in the Cameros Extensional Basin, North-Central Spain, Upper Jurassic-Lower Cretaceous

In the Tithonian-Berriasian and Aptian, basin fill of the Cameros basin is formed by a depositional megasequence of fluvial and lacustrine sediments. Basin evolution is related to the second stage of rifting in the North Atlantic. In the first stages of extension, the basin is compartmentalized due to differential subsidence. As the extension continues, the subbasins merge to form one large basin.

The megasequence is subdivided into five unconformity-bounded depositional sequences (SD1-SD5). Each sequence has a duration of 2.5-10 m.y. and a thickness of 400-1200 m. The internal sequence architecture is formed by a thick fluvial depositional system, which toward the top is overlapped by an expanding lacustrine facies.

The architecture of the fluvial systems in depositional sequences SD1-SD3 consists of small, isolated sandstone bodies in a mudstone matrix, and results from the evolution of distal, high-sinuosity fluvio-lacustrine coastal plains. Depositional sequences SD4-SD5 comprise larger and thicker multistory mixed (conglomeratic sandstone) and sandstone bodies. They originate from wider and nonconfined fluvial systems (conglomeratic and sandy braid plains). In SD5 this facies interfingers with a second fluvial system dominated by ephemeral streams.

The evolution of fluvial architecture is controlled by the balance between subsidence, sediment supply, and relative sea level change. In a reservoir-equivalent setting, the understanding of this evolutionary process and its resultant architecture provides a better insight in reservoir distribution and interconnectedness.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.