IN SITU EXPERIMENTS ON THE GROWTH TEXTURAL DEVELOPMENT OF SUBAERIAL MICROSTROMATOLITES, CHAMPAGNE POOL, WAIOTAPU, NZ

摘要

Potential origins for a range of common silica sinter fabrics are evident from a continuing study of experimentally grown microstromatolites at Champagne Pool. Periodic collection of these actively accreting subaerial deposits provides incremental windows into the key stages of textural development. The results have particular significance for the formation of analogous spicular and finely laminated subaerial sinters formed by hydrodynamic mechanisms of surge, splash or wave action. At Champagne Pool, vitreous, granular, and filamentous-network sinter fabrics result from the silicification of bacterial filaments and, to a certain extent, vitreous and granular fabrics also form abiotically. External silicification of filaments is typically accompanied by rapid intracellular infill, resulting in loss of evidence of a biotic origin for porous and vitreous sinter fabrics. Transitions between alternating vitreous-porous laminae may be induced by variations in the supply of silica-charged waters, i.e. limited by periods of low wave action, yielding a filament-based porous, granular fabric. Vitreous laminae form where subaerial silica supply matches or exceeds growth rate of filament networks. Consequently, a decrease or increase in wave action may result in vitreous-porous or porous-vitreous transitions, respectively. Alternatively the formation of thin silica crusts derived from progressive silicification of films of mucilaginous exopolymeric substances, which drape and bridge topographies, create a barrier against further silica input. Hence silica crusts cause transitions in fabric types or promote the formation of fenestrae.