Basaltic pahoehoe lava-fed deltas: large-scale characteristics, clast generation, emplacement processes and environmental discrimination

Abstract

AbstractBasaltic pahoehoe lava-fed deltas are important coastal constructions of many oceanic islands and continental flood basalt provinces. Whilst littoral processes associated with their formation have been described, little is known about subaqueous processes and products. This study is primarily focused on field studies of lava-fed deltas from the James Ross Island Volcanic Group (JRIVG), Antarctica, but also on other published studies of lava-fed deltas, and on information from studies of coarse-grained alluvial deltas. Seven coherent lava facies and eight subaqueously deposited clastic facies from the JRIVG are described and interpreted. Clastic facies are dominated by cobble-sized angular lithic and fluidal lithic-vitric breccias. The fluidal lithic-vitric breccias are derived from various slope failure processes acting on large-volume ponded lavas in the frontal crest area, or from gravity-driven ductile detachment of the margins of active pillow and sheet lavas on the steep subaqueous slope. Angular lithic breccias are generated mostly by similar brittle processes operating on cooled and jointed lavas ponded upslope. Subaqueous emplacement is mostly by density-modified grain flows with associated small buoyant plumes of finer sediment, and by high-density turbidity currents, many of which infill debris chutes. Basaltic lava-fed deltas have large-scale characteristics and processes that are similar to those of Gilbert-type and gravitationally-modified Gilbert-type alluvial deltas. Important contrasts that influence processes and facies in lava-fed deltas include the absence of any effluent force and the presence of hot clasts. Study of lava-fed delta deposits is important for palaeoenvironmental analysis because marine examples record relative sea level changes, and englacial examples provide evidence of minimum ice sheet thicknesses and meltwater levels. Characteristics that may be used to distinguish lava-fed deltas in marine and englacial lake environments are discussed, and littoral zone facies analysis is emphasized. Suggestions for future research include correlation of the physical parameters of subaerial and subaqueous lavas and the nature of cogenetic clastic products, submersible dives on active deltas, detailed facies analysis of individual tangential foreset beds, and comparative studies with steep coarse-grained alluvial deltas.