Bed-scale vertical and lateral distribution of massive sandstone in a topographically confined basin (Peïra Cava, SE France): Implications for flow processes

In high-density turbidite (HDT) systems, proximal deposits typically show thick beds of massive and/or stratified sandstone capped by a thin low-density turbidite (LDT) interval of finer-grained laminated and rippled sandstone. Distally, the massive/stratified HDT interval thins and is replaced by the LDT interval as the dominant facies. Whereas this empirical observation is common to many ancient systems worldwide, a very different relationship is found in the topographically confined Peïra Cava sub-basin in SE France. Eight prominaent beds can be correlated across much of the exposed basin (3 x 11 km). Three massive sandstone facies are recognised in these beds: (1) ungraded (2) graded, and (3) massive with patchy texture. In proximal and medial sections, these facies are capped by a relatively thick LDT interval comprising up to 30% of the bed thickness. Moving to distal sections, the massive facies come to dominate the bed, in some cases forming 100% of the bed thickness. Along a downslope transect, three facies tracts are defined that all differ considerably from previously described trends. Facies tract I and II comprises ungraded massive sandstone and massive sandstone with patchy texture in distal sections, respectively. Facies tract III comprises repetitive sequence of ungraded and graded massive sandstones in distal sections. Statistical analysis of the vertical internal character in the eight beds show non-random, non-cyclic facies trends, with each massive facies exhibiting some level of dependency on other facies, suggesting sediment deposition depended on temporally evolving flow conditions. The ungraded and graded massive facies are interpreted as deposits of high-concentration near bed layers beneath surge-type turbidity currents. In contrast, the massive sandstone with patchy texture is interpreted to result from liquefied debris flows. Bed thickness trends and the presence of distinctive grain size breaks indicate depositing flows were bipartite (debris flow with an overlying concentrated turbulent layer), depletive, unsteady and pulsatory in proximal locations, but evolved distally into a turbulent flows with a simple surge structure. The distal dominance of these massive facies is here interpreted as a consequence of flow interaction with confining topography. This interaction induced gravity and body transformations in flows of facies tract I and II, whereas generation and passage of internal waves in distal sections created the necessary conditions for massive sandstone deposition in facies tract III.