| Popis: |
Lava-fed deltas are extraordinarily useful indicators of fossil water (and ice) levels in glacial, marine, and lacustrine environments. Deltas fed by ‘a‘ā lava should be at least as common as those sourced in pāhoehoe, yet they have been rarely described. Although facies models for pāhoehoe lava-fed deltas are well established, the architecture and lithofacies of ‘a‘ā-fed equivalents are substantially different and have thus far largely been unrecognized. This can have profound consequences for paleoenvironmental investigations, particularly those attempting to reconstruct past ice sheets. Essential features of ‘a‘ā lava-fed deltas include (1) a subaerial ‘a‘ā lava capping unit comprising massive internal sheet lava overlain by clinkers; (2) a crudely developed subaerial to subaqueous transition (passage zone); (3) a chaotic subaqueous association of abundant lava lobes and hyaloclastite with admixed vesicular, often reddened (oxidized) lava clinkers; and (4) rare subaqueous stratification with predominantly lower dips (∼10°–20°) than in deltas fed by pāhoehoe lava (∼25°–40°). We develop a generic facies model and investigate the emplacement conditions of ‘a‘ā lava-fed deltas in order to facilitate the recognition and environmental interpretation of these important sequence types in ancient successions. |
