The sedimentology and lithostratigraphy of the Upper Jurassic Lourinha formation, Lusitanian Basin, Portugal

Hill, Graham (1989). The sedimentology and lithostratigraphy of the Upper Jurassic Lourinha formation, Lusitanian Basin, Portugal. PhD thesis The Open University.



The Lusitanian Basin of West-Central Portugal is one of a family of formerly adjacent North Atlantic margin basins. Like others of its type (e.g. the Jeanne d'Arcand Western Approaches basins) early phases of rifting and subsidence occurred as far back as the Triassic.

During the Late Jurassic rifting phase in the development of the Lusitanian Basin marine shelf and slope systems were replaced by alluvial fan and fluvial environments which prograded from the nonh during the late Kimmeridgian. Continental environments, periodically punctuated by marine incursions, persisted over northern and central areas of the basin at least until the end of the Jurassic. The deposits of these continental and shallow marine systems are here defined as the Lourinha formation, which is sub-divided into five members.

Sedimentary structures, textures and sand body geometries suggest that the Praia da Amoreira member was deposited in a distal alluvial fan setting where sheet-flooding preceded the establishment of channelised flow. The presence of calcretes suggests a semi-arid/sub-tropical climate with markedly seasonal rainfall. The fan systems,confined to the western margin of the basin, passed laterally into meandering fluvial systems, the deposits of which form the Porto Novo member.

A marine transgression from the south resulted in the establishment of shoal-water delta systems, the deposits of which form the Praia Azul (lower delta plain) and Assenta (upper delta/fluvial plain) members. The largely continental Assenta member is punctuated by thin, laterally extensive marine horizons which suggest rapid relative rises in sea level. In northem and eastern parts of the basin the deposits of sandy fluvial systems are replaced by those of the gravelly Santa Rita member, which appear to have been sourced from the east.

In a number of the above units evidence for the operation of active extensional processes is to be found The deposition of the Lourinha formation preceded the final Aptian rifting event which resulted in the formation of the North Atlantic Ocean. However, it may have heralded and been contemporaneous with, an earlier phase of sea floor spreading, later abandoned, thought to have occurred In the area of the present day Tagus Abyssal Plain.

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