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Title: Eocene foreland basin carbonatae facies, the external Sierras, Spanish Pyrenees.
Author: Pedley, Antony.
ISNI:       0000 0001 3482 118X
Awarding Body: Royal Holloway, University of London
Current Institution: Royal Holloway, University of London
Date of Award: 1994
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Geology