Architecture of intereruptive and syneruptive facies in an Andean Quaternary palaeovalley: the Huarenchenque Formation, western Argentina

Luis Spalletti,Ferrán Colombo

doi:10.5027/andgeov46n3-3170

Abstract

The Huarenchenque Formation is a volcano sedimentary unit deposited to the east of the Plio-Quaternary Andean Magmatic Arc. In order to define depositional settings, two lithofacies associations (fluvial and pyroclastic) were defined. The fluvial facies association is composed of polymictic conglomerates with the predominance of basalt- dominated clasts, coarse- medium-grained conglomeratic sandstones and medium- to coarse-grained sandstones. These deposits occur as stacked or single bodies, display both sheet and channelized geometries, and contain a range of internal sedimentary structures, such as planar, low angle stratification and cross-bedding. This facies association is interpreted as the deposit of a multichannel fluvial system characterized by high bed load, steep gradient and non-cohesive bank materials. Facies and architecture of the fluvial deposits are the result of high bank full discharge related to rapid deglaciation of the Andean Last Glacial Maximum. The pyroclastic facies association is characterized by lapilli and ash tuffs deposited from air fall, pyroclastic density current, and density stratified surge mechanisms. In the Huarenchenque Formation the fluvial and the pyroclastic facies associations show a clear physical separation, suggesting that sedimentation occurred in two distinct (intereruptive and syneruptive) phases. During the long-lived intereruptive phases the sedimentary record corresponds mainly to the deposits of the gravelly braided fluvial system, whereas during syneruptive phases the fluvial valley was almost entirely occupied by primary pyroclastic deposits related to high-explosive episodes of the neighbor Andean strato-volcanoes. Although most of the cross-bedded sandstones and conglomerate sandstones are rich in basaltic fragments, some strata are composed almost entirely of pumiceous fragments, while in others there is a marked alternation between “basalt” and “pumiceous” foresets. These attributes reflect the preservation of intrabasinal pyroclastic fragments and allow suggest that: i. explosive volcanic events could be more frequent than reflected by the pyroclastic deposits themselves; ii. syneruptive pyroclastic materials could be eroded (even eliminated) by the fluvial system; iii. contributions of primary pyroclastic material persisted during intereruptive (fluvial-dominated) phases.

Highlights

The episodic nature of volcanic eruptions may profoundly impinge on sedimentary environments, and the influence of volcanism on the sedimentary record has been addressed in several classic studies (Smith, 1987, 1991; Waresback and Turbeville, 1990)
Based on the concept of syneruptive and intereruptive lithofacies, numerous investigations have been conducted in order to reveal how volcanic eruptions affect depositional environments and how volcanic impacts change with time (Kuenzi et al, 1979; Palmer et al, 1993; Bahk and Chough, 1996; Major et al, 1996; Pierson et al, 1996; Valentine et al, 1998; Kataoka et al, 2009; Németh et al, 2009; Pierson et al, 2011)
Sedimentation occurred in two distinct phases: intereruptive and syneruptive

Summary

Introduction

The episodic nature of volcanic eruptions may profoundly impinge on sedimentary environments, and the influence of volcanism on the sedimentary record has been addressed in several classic studies (Smith, 1987, 1991; Waresback and Turbeville, 1990). The lithofacies around a subaerial volcanic edifice can be classified as syneruptive and intereruptive lithofacies based on the differences in constituent sediments and depositional features (Smith, 1991). Based on the concept of syneruptive and intereruptive lithofacies, numerous investigations have been conducted in order to reveal how volcanic eruptions affect depositional environments and how volcanic impacts change with time (Kuenzi et al, 1979; Palmer et al, 1993; Bahk and Chough, 1996; Major et al, 1996; Pierson et al, 1996; Valentine et al, 1998; Kataoka et al, 2009; Németh et al, 2009; Pierson et al, 2011). During an intereruptive period, normal fluvial processes are dominant due to gradual decrease in available coeval volcaniclastic contribution, and intereruptive lithofacies are commonly confined in and around the channels, accompanied with incision of the syneruptive lithofacies (Gihm and Hwang, 2014)

Methods

Discussion

Conclusion