A Genetic Facies Tract for the Analysis of Sustained Hyperpycnal Flow Deposits

Facies analysis performed for more than 10 yr in several lacustrine and marine basins allowed the distillation of a genetic and predictive facies tract of general application to the recognition of sustained turbulent (noninertial) hyperpycnal flow deposits. The facies tract is composed of three main genetic facies groups related to bed load (B), suspended load (S), and lofting (L) transport processes. Facies B (bed load) is the coarsest grained and relates to shear and frictional drag forces provided by the overpassing long-lived turbulent (hyperpycnal) flow. Three main subcategories are recognized, termed B1 (massive or crudely bedded conglomerates), B2 (pebbly sandstones with low-angle asymptotic cross-stratification), and B3 (pebbly sandstones with diffuse planar lamination and aligned clasts). Facies S is finer grained and relates to the gravitational collapse of suspended load transported by the hyperpycnal flow. Four subfacies types are recognized, termed S1 (massive sandstones), S2 (parallel laminated sandstones), S3 (sandstones with climbing ripples), and S4 (massive siltstones and mudstones). Facies L (lofting) relates to the buoyancy reversal of the hyperpycnal flow provoked by lift of a less dense fluid (in this case, fresh water) typically found in marine and other saline-receiving basins. The finest materials suspended in the flow (very fine–grained sand, silt, plant debris, and mica) are lifted from the substrate and settle down forming silt/sand couplets of great lateral extent. Facies L develops only in marine/saline environments, whereas facies S3 and S4 are more common in lacustrine environments. Hyperpycnites are often very complex, showing internal erosional surfaces and gradual facies recurrences related to deposition from long-lived and highly dynamic (fluctuating) flows. This complex behavior results in the accumulation of composite beds having an internal facies arrangement that strongly departs from conventional facies models developed for surgelike flows. Facies B characterizes transfer zones, and its occurrence allows the prediction of sandstone deposits (facies S) basinward. Facies L is mostly developed in flow-margin areas. In marine settings, the reversing buoyancy effect (lofting) at the hyperpycnal flow margin will result in less lateral continuity of sandstone bodies compared with those related to resedimentation processes, with important consequences for hydrocarbon reservoirs.