Inferred transport conditions from clast fabrics of the Rosario Fm submarine channel gravels

MILANA, J. P. AND , KNELLER, B., DYKSTRA, M. AND THOMPSON, P.

Oslo

Congreso; . International Geological Congress; 2008

IUGS

            The clast fabrics of the Rosario Conglomerates suggest important differences in the transport conditions within channel complexes which are large multistoreyed conglomerate bodies, and single channels which are isolated gravel bodies within the muddy or heterolithic basinal deposits. A dominant a-parallel fabric combined with other factors observed in channel units within channel complexes suggest flows were selective but concentrated enough to develop a suspension-type fabric, and they cannot be ascribed to debris flows but to hyperconcentrated transport mechanism. Debris flow deposits are rare (<1%), usually occurring near channel complexes, and at least some of these originated as local transformations from dilute flows. Isolated single channels show deposits associated to flows of less energy but well stratified with modification from hyperconcentrated into more dilute flow, with typical bedload a-transverse fabrics. In this depositional system, we conclude that the flow stratification produces a clear separation between a gravel-transport carpet (bedload) and a suspension-dominated body, creating the suitable scenario for over-spilling the finer fractions and building up sandy, heterolithic levees. As there are no autocyclic mechanisms to explain the high-frequency cyclicity within the channel complexes, as it would be channel-filling processes in fluvial systems, a possible mechanism of gravel transport by sediment waves or bedforms is invoked to explain this cycles expressed as 3-4 m thick, laterally-restricted elements within channel complexes. Deposition of coarse and high-energy gravels, showing more reworking and therefore a low in the sediment concentration of the flow, would occur at the lows of these large bedforms; while finer-grained gravels, sandier and less sorted, but associated to almost similar flow conditions, would be deposited during moments of he flow having higher sediment concentration. On the other hand, gravel deposition in single channels is more difficult to explain due to the fact these bodies seems to be built by many distinct flow events. However, the general tendency of each distinct flow bed within the body is to be less energetic upwards, a process likely to occur in an channel-avulsion dominated scenario as this occurs as the conduit is being jammed with sediment. However, this does not fit with the convex upward shape of many of these late single-channel fills and thus, alternative hypothesis as a lobe-prograding or retrograding system are proposed to explain them .