The Isle of Portland shows exposure of uppermost Jurassic oolitic carbonate all along its coast and within several inland quarries. The exposure quality is very high with a potential 3D control. The site has a potential to understand the 3D architecture and the sedimentary dynamic of an oolitic system. However, only punctual observations of logs (1D), sometimes correlated have been published. The papers place a shore line between the Isle and the continent striking NEE-SWW and facing towards the Channel. Facies changes are attributed to rapid sea-level variations and Walter’s Law.
Following some preliminary field observations, we have decided to shoot an extensive GPR survey of the same stratigraphic interval (The Portland Freestone). With a total of 85 GPR profiles, we have produced grids on top of most of the coastal cliffs and quarry faces. We have encountered 3 main architectures, 2-m-high bars with steep clinoforms, 10s of metres-wide channels plugged with a variety of organism and stacked aggrading bundles of multidirectional dunesets. The whole dataset does not illustrate any major unconformity which could be attributed to a sharp sea-level drop. We have interpreted our sedimentary architecture to be the result of mixed hydrodynamic conditions associated with wave activity and tidal currents. The Isle shows a island barrier complex which migrates basinwards but also expands laterally, filling up the available space and cannibalising itself. More proximal facies are effectively observed in the north of the Isle towards the former continent or lagoon. However, the survey clearly shows that the shore zone progrades broadly towards the SW, partially away from the Channel, with an obtuse angle compared to former reconstructions. This result suggests that the oolitic shore zone system forms a large spit platform, pointing south, illustrating a contorted and complex shoreline/barrier.
The combination of sedimentology and geophysics allowed to reevaluate the depositional system of the Isle of Portland oolitic deposits. It also challenges the sea-level reconstruction of this interval and the palaeogeographic reconstructions in the basin by showing shoreline trajectories. The multidisciplinary approach permitted to view and analyse a Jurassic depositional system almost alike what can be done on active oolitic systems. The approach has a tremendous potential to better understand cliff exposures and in particular to produce reservoir analogues.