AUTHORS: Riva, Di Cuia
YEAR: 2011
Batrust Carbonate Conference (Bristol, UK)
The existence of intra-platform basins (depressions) and associated deposits is poorly understood in the subsurface due to the quality and type of available data and the characteristics of these kind of deposits. The presence of these depositional environments can have a large impact on decisions related to exploration activity targeting carbonate sequences because they can represent source rocks located laterally or within a carbonate reservoir, permitting a direct connection from the source to the reservoir.
Many of these depressions are tectonically controlled basins bounded by active fault scarps, with irregular shapes, often difficult to predict in the subsurface because related to sub-seismic structural features and they may represent surprises in new exploration wells, because of the difficult to predict their relatively “random” distribution.
The shape is difficult to recognize in seismic, because of the thickness of these basins, the contrast in seismic velocities and often because of the seismic data quality. Moreover, these intraplatform “black shale ponds” may also lead to confusion when trying to interpret the stratigraphy – as they by nature could not be expected in this setting.
Despite the difficulties in clearly identifying Intra-platform (or intra-shelf) basins (depressions) using subsurface data they represent common feature within many extensive Mesozoic carbonate platforms of the Tethys realm.
We have studied and collected information on the deposits associated to this depositional environment both from outcrop and subsurface data. The studied sequences span from the Triassic Moena and Forni formations in the Southern Alps to the Cretaceous of the Southern Apennines thrust belt (Southern Italy) and of the NW Zagros thrust belt (Northern Iraq).
The sedimentation inside these basins is complex, due to the irregular topography generated by different normal/wrench fault movements. The “classic” black shales of this sort of euxinic basins are often remobilized and involved in slumpings, associated with debris flow deposits with clasts coming both from the anoxic seafloor and the surrounding carbonate platform scarps. The walls bounding these basins can be actively eroded and channelized, representing a source for clastic sediment production (carbonate breccia and megabreccias) organized often in lobes or eventually remobilized again in other slumping events.
The oxic/anoxic interface on the water column can be located some meters below sea-level, allowing active carbonate production in the surrounding carbonate platforms.
The micropaleontological content is normally very poor, with diffused ostracods and rare planktonic foraminifera. The scarce planktonic content is related to the stressed environment that reflects also the preservation of calcitic tests.
The micropaleontological content can be interpreted as related to a very stressed anoxic environment in partial communication with the open water. These connections could have been temporarily closed during lowstands or due to tectonic movements. In extreme situations, the restricted anoxic circulation can evolve into hypersaline conditions: a salina develops permitting the precipitation of gypsum and anhydrites.