Fine-grained distal deposits of a mixed siliciclastic-carbonate marine system: Origin of mud and implications on mixing processes

Abstract

Mixed siliciclastic-carbonate sediments in the distal settings of marine systems (i.e., offshore and basin areas) are the result of the complex interaction between production of benthic and pelagic biogenic carbonates and the exportation of terrigenous, and/or carbonate mud from shallow-marine settings. Thus, the origin of mud (particles , 63 mm) of different compositions and the processes responsible for mixing them play a key role in the final distal deposits of mixed systems. Although the understanding of these topics in shallow settings has made significant progress, one of the critical issues is to recognize them in the offshore and basinal settings for understanding the link between shallow-marine and deep-marine areas. In this study, a thick Lower Cretaceous succession (330-650 m) of the Neuquén Basin, composed mostly of mixed (siliciclastic-carbonate) fine-grained marine deposits, was studied through a high-resolution analysis of petrography, SEM analysis, and mineralogy, to define and characterize the origin of carbonate and terrigenous mud as well as to characterize the processes involved in the mixing of terrigenous and carbonate mud on a storm-dominated ramp. Contrary to previous suggestions, most of the carbonate recognized in the basinal setting is composed of silt- to sand-size skeletal remains and micrite (micarbs) and is likely derived from a pelagic biogenic factory combined with carbonate derived from the maceration of benthic organism. Furthermore, micarbs are possibly diagenetic in origin but with a distally produced carbonate precursor. In contrast, the siliciclastic fraction is entirely detrital, being silt-size quartz particles and clay minerals (mainly illite) the most important constituents. Most of the terrigenous mud delivered from shallow areas acts to dilute the distally produced carbonate mud. However, a bloom of carbonate production able to dilute terrigenous supply is recorded, and its origin is likely triggered by the input of waters charged with nutrients from the paleo-Pacific Ocean in addition to nutrients derived from proto-Andes volcanic activity. This demonstrates that the relation between the terrestrially derived siliciclastic components and the biogenically produced carbonates in mixed systems is not linear, but a complex interaction of climate, sea-level, origin of the components, and hydrography of the basin, among others.