Emplacement of Pleistocene basalt flows near McCoy, Colorado

摘要

Intraflow structures are readily apparent in the Pleistocene basaltic lava flows that erupted from two cinder cones near McCoy, Colorado 0.66 Ma. Intraflow structures are important because each different structure develops by a different cooling mechanism, which are driven by environmental factors into which the lavas are emplaced. These flows are investigated with the modern interpretations of intraflow structures in basalts to describe their emplacement. Particular features that are associated with the McCoy basalts that indicate being at or near the paleosurface were used to map the pre-eruption topography. Upon mapping these features, these lavas are interpreted to have followed the paths of the old fluvial drainage systems of Rock Creek and Egeria Creek. Identifying the direction of the cooling front, which is preserved in plumose structures along columns, is the main tool used to delineate flow events. Results show complex arrangements of one-tiered and two-tiered flows that are interpreted as open channel lava flows that formed a series of stacked lava levees (two-tiered flows) and drained channel bottoms (one-tiered flows) as flow centers proceeded to drain. Subsequent flows spread across the flow tops and developed a series of stacked one-tiered flows after the lava valley completely filled. Entablatures are formed when water penetrates the solidified portions of the flows and increases cooling rates. The lava flows obstructed stream flow of paleo-Rock and Egeria Creeks. Water from these systems eventually breached the lava dams and flooded the flow tops, which is evidenced by the extensive entablatures in the outcrops. Water entered the flow interiors through brecciated flow tops and along fractures between open channel walls and its succeeding flows. A streamflow impoundment analysis of the time required for these streams to flood the basalt flow tops indicated the flood events possibly occurred within about one week to three months after basalt emplacement. Using Long and Wood's (1986) convective cooling model, the flows at McCoy completely solidified within three years after emplacement. Since emplacement, the fluvial systems have reestablished their flow paths along the western extent of the flows.