Periplatform ooze in a mixed siliciclastic-carbonate system - Vaca Muerta Formation, Argentina

摘要

Thick carbonate mud successions and marl/limestone alternations are often found in periplatform environments. The sources of these fine-grained sediments and the processes that govern their accumulation and distribution in the basin are frequently a matter of debate. This case study from a mixed silicidastic-carbonate succession in the Neuquen Basin, Argentina, relates the origin of the carbonate-dominated intervals to shelfal carbonate production. The Tithonian-early Valanginian basinal strata (Vaca Muerta Formation) contains several carbonate-rich intervals at different stratigraphic levels. The thickest of these intervals is the mid-late Tithonian Los Catutos Member in Sierra de la Vaca Muerta. This wedge-shaped package of micritic limestones and intercalated marls thins basinwards. Proximal-distal variations and cyclic partitioning in fades and geochemical proxies indicate highstand shedding and a periplatform origin for the Los Catutos Member. The proposed source is a coeval 20 m-thick pure carbonate interval located 50 km south of the Sierra de la Vaca Muerta within the mixed siliciclastic-carbonate shelf fades of the Picun Leufu Formation. Younger carbonate-rich intervals in the center of the basin (Puerta Curaco area) show similar characteristics to the distal portion of the Los Catutos Member and are also interpreted as periplatform deposits but sourced from carbonate shelves farther east. The high sedimentation rates of the Los Catutos Member require an efficient mechanism, other than tides and winds, to deliver significant amounts of fine-grained carbonate sediment to the basin. This mechanism is envisioned to be sediment-charged hyperpycnal flows cascading from the platform to the basin, analogous to what is observed in modern carbonate systems. The occurrence of periplatform ooze in different stratigraphic levels of the siliciclastic-dominated Vaca Muerta Formation demonstrates that off-shelf transport and distribution of carbonate mud in the basin are also common processes in mixed clastic-carbonate environments. (C) 2019 Elsevier B.V. All rights reserved.