Sedimentology of Jurassic syn-rift resedimented carbonate sandbodies.
This thesis discusses the sedimentology of three contrasting Jurassic carbonate sand
turbidite systems from Southern Europe: the Cutri Formation (Bathonian) of Mallorca;
the Vajont Limestone (Bajocian-Callovian) of northern Italy; and the Peniche sequence
(Toarcian-Aalenian) of the Brenha Formation of western Portugal.
These sandbodies all formed in syn-rift extensional settings which imposed a primary
morpho-tectonic control on both the source platform and depositional basin
morphology. The three sandbodies in question display varying geometries and
architectures and are discussed in terms of the palaeogeographic, tectonic and eustatic
controls that governed their individual development; as well as being used to test the
recently developed apron model against that of the submarine fan. In this context
oolitic carbonate aprons associated with palaeowindward and palaeoleeward platform
margins have been distinguished.
The Cutri Formation is interpreted as a oolitic base-of-slope apron, that displays a
minor single syn-rift thinning upward megacycle (retrogradational) trend indicative of
subsidence out-pacing sedimentation. The apron correlates with a eustatic sea-level
drawdown and was characterised by infrequently laterally correlatable, oolitic turbidite
units separated by hemipelagic interbeds. This sandbody is relatively sand-poor in
nature, and is interpreted as being sourced from a palaeowindward platform margin.
The Vajont Limestone is re-interpreted as an aggraded oolitic apron from its original
interpretation as a sub-marine fan. The apron is composed of stacked oolitic
grainstone turbidites and is locally up to 800m thick. It is interpreted as being sourced
from a stable `keep up' palaeoleeward platform margin, where dominant off-bank sand
transport led to development of line-sourced oolitic turbidites, which were actively
aggraded by on-going basin subsidence. Statistics were used to demonstrate a random
turbidite sequence which enhances the apron interpretation.
The Peniche sequence is reconfirmed as a carbonate-siliciclastic fan, its facies
development conforming to a siliciclastic sand-rich fan model. Statistical analysis
indicates a non-random (cyclic) turbidite sequence, thereby enhancing the fan
interpretation. The fan occurs as a localised development within fine-grained basinal
facies and correlates stratigraphically with a eustatic sea-level drawdown. Interpreted
as being sourced from a palaeoleeward margin, the sequence progradates from outer
fan lobes to a thick, multi-storey braided channel complex.
These syn-rift resedimented carbonate sandbodies have the potential to be
stratigraphically associated with basinal source rocks and therefore may be viewed as
prospective hydrocarbon reservoir facies.