Transformation, partitioning and flow–deposit interactions during the run-out of megaflows

Abstract

Four megabeds (I to IV) were recognized throughout the Cerro Bola inlier, a glacially influenced depositional area of the Carboniferous Paganzo Basin, south-western La Rioja Province, Argentina. Such anomalous thick beds are associated with the collapse of an unstable basin margin after periods of large meltwater discharge and sediment accumulation. Failure of these previously deposited sediments triggered mass flows and associated turbidity currents into the basin. Megabed I is up to 188 m thick and was deposited during a transgressive stage by re-sedimentation of ice-rafted debris. Also part of the transgressive stage, Megabeds II, III and IV are up to 9 m thick and are associated with a dropstone-free period of flooding. Megabeds were subdivided into three divisions (1 to 3) that represent a spectrum of flow properties and rheologies, indicative of a wide range of grain support mechanisms. These divisions are proposed as an idealized deposit that may or may not be completely present; the Cerro Bola megabeds thus display bipartite or tripartite organization, each division inferred to reflect a rheologically distinct phase of flow. Division 1 is a basal layer that consists of clast-supported and matrix-supported, pebble conglomerate, rarely followed by weak normally graded to ungraded, very coarse- to coarse-grained sandstone. This lower interval is interpreted to be the deposit of a concentrated density flow and is absent in bipartite megabeds. Division 2 is represented by a mud-rich sandstone matrix with dispersed granule to pebble-size crystalline and mudstone clasts. It also includes fragments of sandstone up to boulder size, as well as rafts of cohesive muddy material and wood fragments. Division 2 is interpreted to be a result of debris-flow deposition. A debrite-related topography, resulting from the freezing of high yield strength material, captures and partially confines the succeeding upper division 3, which fills the topographic lows and pinches out against topographic highs. Division 3 is rich in mudstone chips and consists of very coarse-grained, dirty sandstones grading upward to siltstones and mudstones. It is interpreted to be a deposit of a co-genetic turbidity current. Spectral gamma ray and petrographic analyses indicate that both debrite and co-genetic turbidite have high depositional mud content and are of similar composition. One of the megabeds is correlated with an initial slump-derived debris flow, which suggests that the mass flow becomes partitioned both at the top, generating a co-genetic turbidity current and, at the base, segregating into a concentrated density flow that seems to behave as a gravelly traction carpet.