Textual heterogeneity in massive sandstones from the Grès d'Annot and Numidian Flysch: Implications for depositional processes

Abstract

ABSTRACT Vertical bed scale heterogeneity in six massive sandstone beds is investigated using digital image analysis to determine flow processes. Images parallel to the bedding plane and perpendicular to the apparent grain long-axis orientation were acquired to minimize the uncertainty in the grain-size and fabric, and increase the statistical significance of the data. Hypothesis testing was used to reduce the subjectivity in assigning vertical trends within each bed. Results show that a significant part of the deposit contains statistically significant vertical variation in grain-size and fabric (ρ-values < 0.01). At the bed scale, grain-size can be normally graded or non-graded. However, at shorter length scales, complex grading patterns emerge in the different percentiles of the grain-size distribution, including “oscillating,” “diverging,” and “converging” trends involving the fine- and coarse-tail percentile. Grain fabrics in the bedding-parallel sections show a consistently flow-oblique trend with an average azimuthal deviation of 48°, but becoming increasingly flow aligned at the tops of most beds. The majority of the bedding-perpendicular sections show imbrication angles > 15°, with an average imbrication angle of 78°. Both up- and down-current imbrication polarities are observed in equal abundance. These textural trends are interpreted as a product of active near-bed sedimentation processes that operate in high concentration near-bed layers, which develop at the base of concentrated, turbulent flows, and active fluidization processes that operate during sediment deposition. The short length-scale grain-size trends are likely a reflection of fluctuations in the sediment concentration in these near-bed layers and elutriation, which led to differential grain segregation processes influencing the different grain-size percentiles. The high shear stresses and frequent grain interaction in the near-bed layers, with concomitant fluidization, also generate the oblique bedding-parallel fabric and very high imbrication angles.