The turbiditic sand-lobes hosted within confined basins at the active margins provide interesting analogues of some deep-sea clastic reservoirs. Their depositional architecture derives from the complex interplay of external forcing vs. intrabasinal and autocyclic controls on sedimentation. Therefore, knowing to which factors responds any measurable depositional variable should help to quantify and to forecast facies changes and stacking pattern styles throughout a turbiditic reservoir; vice versa the statistical knowledge of sedimentary trends could help to infer the most influent controls on deposition. At this purpose, we applied a simple statistical analysis to the study of the well-known and well-exposed Cengio Turbidite System (CTS; Late Oligocene, episutural Tertiary Piedmont Basin, Northern Italy). It consists of several stacked sandstone bodies (“lobes”) which were deposited within a narrow and confined structural depression. At present the CTS succession is exposed over a 14 km2 wide area, where it can be as thick as 170 m. The outstanding features of the CTS are: 1) stacking of several almost tabular sandstone-mudstone bodies, thin to medium bedded and metres to decametres thick, that pinch out onlapping the marginal slope mudstones. Progressive filling of the basin determined widening of the depositional area through time, as it is shown by shifting of onlap terminations; 2) coupling of thick-bedded and amalgamated sandstone units, that are flat-shaped and metres to decametres thick, with the sandstone-mudstone units, mostly in the lower part of the CTS; 3) occurrence of different facies trends from the depocentres towards the distal sectors and towards two different types of terminations (i.e. “mild onlaps”, almost parallel to palaeocurrent trends and “abrupt onlaps”, almost normal to the same). A simple uni- bivariate statistical analysis was performed to quantify facies associations and describe the behaviour through space and time of several sedimentary variables: grain-size, bed thickness, sand/mud ratio, frequency of facies, percentage of amalgamated bed vs. total number of beds, graded/massive facies ratio, laminated/massive+graded facies ratio. The relationship between basin confinement and size and efficiency of sediment flows was explored by the analysis of bed thickness distribution (thickness of beds vs. facies, frequency distribution of bed thickness). This analysis helped to separate the influence of basin morphology and of the changing degree of confinement (that is the most influent local factor, linked to intrabasinal tectonics) from the external controls on volume and type of sediment load of the turbiditic flows entering the basin. The outstanding feature is the different behaviour of the considered variables throughout the sandstone-mudstone units and the amalgamated sandstone bed-sets, moving away from the onlap terminations. In fact, the statistics of facies variation of the sandstone-mudstone units provided evidence of flow transitions occurring downcurrent with respect to the depocentre and onlap areas. By contrast the thick-bedded and amalgamated sandstone bed-sets showed some changes only close to their terminations. The cumulative frequency distribution of bed thickness follows a segmented power-law with two steps, corresponding to deviations from the power-law trend, that occur for bed thickness respectively larger than 32 cm and 134 cm. The correspondence between thickness of beds and facies suggests that the first step can be interpreted as a consequence of the down-current transition from high-density flows to low-density turbulent flows. It occurs within the population of beds forming the sandstone-mudstone units. The thick beds of the amalgamated sandstone bed-sets contribute almost exclusively to the second step. Therefore it presumably indicates the critical thickness of the largest beds that could spread throughout the entire basin. These facts can be interpreted considering the differences of flow volume and efficiency that characterize the deposition of the two types of sandstone bodies, with respect to basin morphology. It seems reasonable that, even if the basin was widening due to progressive filling, the large-volume and poorly efficient, sand-laden sediment flows of the amalgamated sandstone bed-sets were fully contained within an area of enclosed bathymetry, that was also too small to permit the development of flow transitions. Differently, the turbidity flows that formed the mudstone-sandstone bodies were small and efficient enough, with respect to the basin width, to be able to segregate downcurrent textures and facies. The striking difference between the two types of turbiditic units may be attributed to the repetitive increase of volume of the sediment gravity flows entering the basin that forced the depositional system to fill the available room and then to aggrade. The progressive widening of the depositional area made this mechanism less efficient through time, as it is witnessed by the disappearance of the massive and amalgamated sandstone units in the upper part of the CTS and through the overlaying turbiditic units. Yet, whether the external control capable to force the repetitive changes of flow volume and density was climate cyclicity or something else, like a sequence of tectonic pulses, remains unclear.
|Titolo:||Statistical analysis of stratal patterns and facies changes: deconvolving controls on the depositional architecture of deep-sea confined clastic units|
|Parole Chiave:||Statistical analysis, turbidites, Cengio Unit, Tertiary Piedmont basin|
|Settore Scientifico Disciplinare:||Settore GEO/02 - Geologia Stratigrafica e Sedimentologica|
|Data di pubblicazione:||mar-2006|
|Tipologia:||Book Part (author)|
|Appare nelle tipologie:||03 - Contributo in volume|