Shear-generated stratification: A diagnostic indicator of laminar sandy debris flow processes in apparently massive submarine fan sandstones

Subtle stratification is locally observed within otherwise massive sandstone beds in submarine fan deposits. The genetic mechanism and process significance of this stratification, however, remain unclear. This study investigates the origin of this stratification and its implications through a detailed analysis of outcrops of the Middle Triassic Baifeng Formation in the Baise area, southern China. We identify three principal lithofacies based on their distinct sedimentary fabrics and structures: (1) laminated siltstone (LSl), (2) graded sandstone (GS), and (3) massive sandstone (MS). These lithofacies are organized into vertical successions or are individually present—namely, MS → GS → LSl, and more commonly, MS, MS → LSl, MS → GS, and GS → LSl. These successions are partly analogous to the Bouma Sequence, which is characterized by MS → GS → LSl. Massive sandstones are characterized by sharp, flat bases and uneven tops and contain floating gravel clasts and mudclasts. Basal dish and water-escape structures are present, and locally, subtle, often deformed stratification with convolute structures occurs within otherwise massive intervals. These sedimentary characteristics of the massive sandstones, combined with the prevalence of incomplete vertical successions, indicate deposition from cohesive, laminar, plastic gravity flows that underwent en masse freezing, consistent with the mechanics of sandy debris flows. Therefore, we interpret the subtle stratification as a primary syn-sedimentary structure generated by internal shearing within sandy debris flows, and propose it as a key diagnostic indicator for such processes. This study refines the understanding of gravity flow processes in submarine fan systems and offers new criteria for identifying sandy debris flow deposits in the sedimentary record.