Bed-thickness and grain-size trends in a small-scale proglacial channel–levée system; the Carboniferous Jejenes Formation, Western Argentina: implications for turbidity current flow processes



Preserved in Quebrada de las Lajas, near San Juan, Argentina, is an ancient subaqueous proglacial sedimentary succession that includes a small-scale (ca 50 m thick and ca 200 m wide) channel–levée system with excellent exposure of the channel axis and levée sediments. Coeval deposition of both the channel axis and the levées can be demonstrated clearly by lateral correlation of individual beds. The channel axis consists predominantly of a disorganized, pebble to boulder conglomerate with a poorly sorted matrix. The channel axis varies from 10 to 20 m wide and has a total amalgamated thickness of around 50 m. Beds fine gradationally away from the cobble–boulder conglomerates of the channel axis within a few metres, transitioning to well-organized pebble to cobble conglomerates and sandstones of the channel margin. Within 60 m outboard of the channel axis in both directions, perpendicular to the trend of the channel axis, the mean grain size of the beds in the levées is silt to fine-grained sand. Deposits in this channel–levée system are the product of both debris flows (channel axis) and co-genetic turbidity currents (channel margins and levées). Bed thicknesses in the levées increase for up to 10 to 25 m away from the channel axis, beyond which bed thicknesses decrease with increasing distance. The positions of the bed thickness maxima define the levée crests, and the thinning beds constitute the outer levée slopes. From these relationships it is clear that the levée crest migrated both away from and toward the channel axis, and varied in height above the channel axis from 4 to 5 m (undecompacted), whereas the height of the levée crest relative to the distal levée varied from 4·5 to 10 m, indicating that the channel was at times super-elevated relative to the distal levée. Bed thickness decay on the outside of the levée crest can be described quite well with a power-law function (R2 = 0·85), whereas the thickness decay from the levée crest toward the channel axis follows a linear function (R2 = 0·78). Grain-size changes are quite predictable from the channel margin outward, and follow logarithmic (R2 = 0·77) or power-law (R2 = 0·72) decay curves, either of which fit the data quite well. This study demonstrates that, in at least this case: (i) levée thickness trends can be directly related to channel-flow processes; (ii) individual bed thickness changes may control overall levée geometry; and (iii) levée and channel deposits can be coeval.