Eocene foreland basin carbonatae facies, the external Sierras, Spanish Pyrenees.
This thesis explores the controls on carbonate platform formation in foreland
basins through a study of the facies, and depositional architecture, of the Middle
Eocene Guara Limestone Formation, from the External Sierras, Northern Spain. The
Guara Limestone Formation formed in a ramp environment on the Iberian foreland
margin of the South Pyrenean Foreland Basin. The facies are foraminifera and algal
limestones, with minor shallow marine siliciclastics. A facies model has been erected
indicating 19 facies, grouped into 6 facies associations.
Using these facies and associations, the evolution of the platform has been
studied. A progradational lime-mud and clastic rich lowstand systems tract marks the
initiation of deposition, the lowstand systems tract being deposited during a period of
low relative sea level rise. This is overlain by an aggradational and retrogradational,
carbonate grain rich, transgressive systems tract. This was deposited as the rate of
relative sea level rise increased. Parasequences have been redefined herein to allow
successions of a similar stratigraphic hierarchy to be encompassed in the same name.
The aggradational section of the platform containing both shallowing and deepening
upward parasequences. The deepening upwards parasequences were created by base
level rise driven by tectonic subsidence and eustatic sea level rise. The aggradational
platform margin indicates that inner-ramp production, even with the absence of coral
reefs, was able to keep pace with relative sea level rise. Relative sea level rise was
sufficiently rapid to preclude the development of peritidal facies and evaporites, despite
suitable arid climatic conditions.
Platform retrogradation, in the late transgressive systems tract, and eventual
drowning, was caused by a further increase in the rate of relative sea level rise. This
was created by an increase in the rate of foreland subsidence due to the formation of
antiformal stacks in the Pyrenean Axial Zone to the north. Following drowning, a
progradational, clastic and lime-mud rich highstand systems tract developed. Initially
the rate of relative sea level rise was rapid during the highstand systems tract, this rate
probably decreasing as the sequence boundary is approached. The observed increase
through time of the rate of tectonic subsidence is typical of foreland basins, and is in
contrast to the exponential decay of subsidence seen in passive margins.
A number of other controls can be seen to have affected the Guara Limestone
Formation ramp. These may affect any carbonate system; though some may be
favoured specifically in foreland basin settings. Tidal action formed a series of
grainstones shoals at the shelf margin, tidal effects may be favoured in narrow foreland
basins due to tidal amplification, and also the limitation of wave and storm effects due
to a restricted fetch. The basin was well circulated, with effective exchange between
basin and platform, and salinity was normal to possibly slightly lower than normal. The
biota displays a chlorozoan assemblage, but is depleted in corals due to their global
decline at this time. Sediment and nutrient input onto the platform was low, leading to a
resource limited environment favouring the development of large benthic foraminifera.
Localised tectonics, in the form of small scale folding, produced a series of marked
effects on the platform, these include: the generation of angular local unconformities,
and a variation and narrowing of biofacies belts.
In summary, foreland basins may display a complicated interaction between
eustatic sea level variation and tectonic subsidence. In contrast to other basin types, this
tectonic subsidence increases through time until eventual uplift. This provides a
dominant control on the stratal architectures observed. This thesis illustrates, therefore,
the potential of the use of such detailed facies and platform models to elucidate both the
local, and the regional scale, controls on platform development and basin evolution.