Submarine Canyons and Fans in a Rift-Climax Event: Analysis of Two Contrasting Late Jurassic Systems in the Lusitanian Basin (Portugal)

Pimentel, Nuno ¹; Pena dos Reis, Rui ²; Group, Margins Exploration ³
(1)Geology Dep., Lisbon University, Lisboa, Portugal. (2) Earth Sciences Dep., Coimbra University, Coimbra, Portugal. (3) Margins Exploration Group, Lisbon & Coimbra Universities, Portugal.

The Lusitanian Basin in Western Iberia is related with the Cretaceous North Atlantic opening. A previous late Jurassic rifting event, related with the seafloor spreading in the Central Atlantic between Morocco and Nova Scotia, is recorded by transitional to coastal sedimentation, with high TOC in restricted areas (Oxfordian source-rock). A late Oxfordian increase of subsidence and accommodation space promoted deep marine carbonated sedimentation.

An abundant and wide detrital input marks the latest Oxfordian rift-climax, corresponding to an intense structuration of the basin, driven by transtensional faults, together with extensive salt motion. Fluvio-deltaic inputs from NE, W and E, were carried by a longitudinal drainage towards SW, into the depocentric areas, where shallow to deep-marine turbiditic deposits accumulated over 2000 m thick, with rates of 1500 m/my. In these areas, coarse deposits coming from inland and platform scarps, were also delivered into the marine basin submarine fans, by feeding canyons and talus debris-flows.

On the western border, a Paleozoic basement rift-shoulder delivered coarse granitic and quartzitic clasts, as seen in Santa Cruz outcrops very close to a piercing diapir. Here, as a result of the coeval salt motion, the rift-climax sequence is only 100 m thick, with broad and thin sandy channels and shallow turbidites, followed by a deeply incised conglomeratic feeding canyon, showing good reservoir characteristics. This sequence is overlaid by progading deltaic lobes.

On the eastern border, late Jurassic carbonates have been salt domed and uplifted in rift-shoulders, as seen at Montejunto area, delivering extremely coarse calciclastic sediments with olistholits, followed by later siliciclastic turbidites coming from the basement. The rift-climax sequences attained around 600 m thick, and present multiple broad coarse-grained feeding channels, showing poor reservoir characteristics.

These two contrasting and contemporaneous submarine fans systems and the resulting characteristics of the incision and infill, show the influence of source-area composition and of coeval diapiric activity, in the reservoir properties. The proximity between the late Oxfordian source-rocks and the early Kimmeridgian submarine fans reservoirs, as proved by several oil-shows and oil-seeps, promoted the development of an efficient upper Jurassic rift related petroleum system, in the Lusitanian Basin.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.