Relationships Between Internal Structures and Petrographic Textures of Basalt Flows: Example from a Continental Flood Tholeiite Province

摘要

Excellent exposures of numerous, well-preserved flows of continental flood tholeiite in south-central Washington provide opportunity to study primary internal structures of subaerial basalt flows. Typical intraflow structures (top to bottom) of a flow include: ropy to brecciated, vesicular flow top; coarsely columnar basalt (upper colonnade); hackly to columnar basalt with relatively small columns (entablature); relatively large, regular columns (lower colonnade); and a glassy basal zone that may be pillowed, vesicular, or highly fractured. One or more of these intraflow structures may be absent or present in multiples in any given flow. Predominance of certain combinations of intraflow structures define three basic types of flows. Type I flows consist principally of relatively large (> 0.9 m), irregular columns with little or no entablature and a poorly developed flow top. Type II flows exhibit alternating tiers of entablature and colonnade 1 to 5 m thick, which give way upward to a more uniform entablature that commonly contains vesicular layers. Type III flows have sharp entablature-colonnade contacts and typically exhibit an upper colonnade. Petrography of samples from different intraflow structures and flow types shows that entablature invariably has abundant black, tachylitic glass; whereas, colonnade typically has much less abundant, brown, transparent glass. Simple conductive cooling models do not explain sharp contacts between the distinctive textures of entablature and colonnade or textural differences between flow types. These textural differences require either different cooling rates or perhaps different oxygen fugacities across a relatively sharp contact within a flow. (ERA citation 81:026429)