The Wall Mountain Tuff was deposited about 36 m.y. ago in paleovalleys extending from west of Salida to beyond Castle Rock, Colorado, a distance of at least 140 km. The Gribbles Run paleovalley, 16 km northeast of Salida, is exceptionally well exposed by modern dissection and reveals a complex interplay of primary and secondary structures formed during deposition of the Wall Mountain Tuff. Deposition occurred in laminar boundary layers between the bottom and sides of the channel and ash flows passing above. The overlapping sequence of events during deposition was (1) agglutination and incipient collapse of glassy particles, (2) laminar shearing of the compacting and welding mass to form a primary foliation analogous to flow banding in lavas, (3) expulsion of gases from the collapsing spongy mass and concentration of these gases along shear planes, (4) formation of gas pockets in places where the volume of gases expelled exceeded that which could be accommodated on shear planes, (5) elongation of gas pockets and pumice to form a primary lineation in the plane of the foliation, (6) statistical alignment of the long axes of solid particles parallel to the direction of flow and imbrication of the long axes so that they dip sourceward relative to the foliation planes, (7) development of primary flow folds with axes perpendicular to the lineation, and (8) end of forward motion. By the time forward motion ceased within a given layer, the tuff had the rheological properties of a rhyolite lava. The high viscosity of the welded tuff preserved open cavities and prevented differential compaction over lithic fragments or primary folds.
More rapid deposition along the sides of the paleovalley than along its axis caused inward accretion of welded tuff with steep primary flow foliation to form a U-shaped cross-channel profile. Secondary folds, whose axes parallel the lineation, and concurrent growth faults formed locally by creep toward the valley axis. Spectacular internal unconformities developed where undeformed tuff was deposited over primary or secondary folds, yet all the tuff welded together to form a remarkably uniform simple cooling unit. Episodic downstream movements of a few metres, as the Wall Mountain Tuff adjusted to its bed and to its rising center of gravity, opened swarms of tension cracks along certain horizons. These tension cracks dip steeply in the downstream direction and provide a useful indicator of flow direction. Other structures useful in determining flow direction are imbricated crystals, streamlined ridges and grooves in lineated gas cavities, and upstream dip of axial planes on asymmetric primary folds.
Emplacement temperature is the dominant factor in determining whether a tuff undergoes primary or secondary welding. If the temperature is well above the softening point, the glassy particles will agglutinate and collapse during deposition in the laminar boundary layer, and the tuff will show megascopic laminar flow structures (primary welding). If the temperature is below the softening point, deposition occurs as loose ash, and welding is a postemplacement process (secondary welding). Most tuffs undergo secondary welding and only the preferred orientation of solid particles and the grain-size distribution remain as evidence of laminar flow during deposition. The type of welding may vary both laterally and vertically within some ash-flow sheets and impose a significant facies variation.
