Geologic History. Extension in this the main Rio Grande rift started about 36 million years ago.

Geologic History. Extension in this the main Rio Grande rift started about 36 million years ago.

Expansion in this right area of the Rio Grande rift started about 36 million years back. Rock debris that eroded through the developing rift-flank highlands, along with wind-blown and playa pond deposits, accumulated when you look at the subsiding Mesilla Basin. These fill that is basin, referred to as Santa Fe Group, are 1500 to 2000 foot thick beneath Kilbourne Hole (Hawley, 1984; Hawley and Lozinsky, 1993). The uppermost sand, silt, and clay of this Pliocene to very early Pleistocene Camp Rice development, the unit that is youngest for the Santa Fe Group in this an element of the basin, are exposed within the base of Kilbourne Hole. The Camp Rice development had been deposited by way of a south-flowing braided river that emptied in to a playa pond when you look at the vicinity of El Paso.

The Los Angeles Mesa surface, a surface that is flat developed together with the Camp Rice development, represents the utmost basin fill associated with the Mesilla Basin at the conclusion of Santa Fe Group deposition about 700,000 years back (Mack et al., 1994). This area is approximately 300 ft over the contemporary Rio Grande floodplain. The top created during a time period of landscape security. Basalt moves through the Portillo volcanic field are intercalated with all the top Camp Rice development and lie in the Los Angeles Mesa area.

The Rio Grande began to reduce through the older Santa Fe Group deposits after 700,000 years back as a result to both changes that are climatic integration associated with river system utilizing the gulf coast of florida. This downcutting wasn’t a constant procedure; there have been a few episodes of downcutting, back-filling, and renewed incision. This episodic growth of the river system resulted in the synthesis of a few terrace amounts over the Rio Grande between Las Cruces and El Paso.

Basalt that erupted about 70,000 to 81,000 years back from a collection of vents called the Afton cones found north-northeast of Kilbourne Hole flowed southward. The explosion that created Kilbourne Hole erupted through the distal sides regarding the Afton basalt moves, indicating that the crater is more youthful than 70,000 to 81,000 yrs . old. Pyroclastic rise beds and breccia that is vent from the crater overlie the Afton basalt movement. The crater formed druing the final phases of this eruption (Seager, 1987).

Volcanic Features

Bombs and bomb sags

Volcanic bombs are blobs of molten lava ejected from the volcanic vent. Bombs have reached minimum 2.5 inches in diameter consequently they are frequently elongated, with spiral surface markings acquired whilst the bomb cools since it flies although the fresh air(Figure 5).

Bomb sags are typical features when you look at the pyroclastic beds that are suge. The sags form whenever ejected volcanic bombs effect to the finely stratified rise beds (Figure 6).

Figure 5 – Volcanic bomb from Kilbourne Hole. Figure 6 – Hydromagmatic deposits exposed in cliffs of Kilbourne Hole. The arrow features a bomb that is volcanic has deformed the root deposits. Photograph by Richard Kelley.


A number of the volcanic bombs at Kilbourne Hole have xenoliths. Granulite, charnokite, and anorthosite are typical xenoliths in bombs at Kilbourne Hole; these xenoliths are interpreted to express items of the low to middle crust (Figure 7; Hamblock et al., 2007). The granulite may include garnet and sillimantite, indicative of the origin that is metasedimentary or the granulite may include pyroxene, suggestive of a igneous beginning (Padovani and Reid, 1989; Hamblock et al., 2007). Other upper crustal xenoliths include intermediate and silicic-composition volcanic stones, clastic sedimentary stones, basalt and andesite that is basaltic and limestone (Padovani and Reid, 1989; French and McMillan, 1996).

Mantle xenoliths (Figure 8) consist of spinel lherzolite, harzburgite, dunite, and clinopyroxenite. Research of these xenoliths has supplied data that are important the structure and temperature for the mantle at depths of 40 kilometers underneath the planet’s area ( ag e.g., Parovani and Reid, 1989; Hamblock et al., 2007). Some olivine into the mantle xenoliths is of adequate size and quality to be looked at gem-quality peridot, the August birthstone.

Figure 7 – Crustal xenoliths from Kilbourne Hole. Figure 8 – Mantle xenolith from Kilbourne Hole.

Surge beds

A pyroclastic rise is hot cloud which contains more gasoline or vapor than ash or stone fragments. The cloud that is turbulent close towards the ground area, frequently leaving a delicately layered and cross-stratified deposit (Figures 3 and 6). The layering kinds by unsteady and turbulence that is pulsating the cloud.

Hunt’s Hole and Potrillo Maar

Lots of the features described above will also be current at Hunt’s Hole and Potrillo maar (Figure 9), that are positioned towards the south of Kilbourne Hole. Xenoliths are unusual to absent at Hunt’s Hole (Padovani and Reid, 1989), but otherwise the maars are comparable. contrary to Kilbourne Hole, Potrillo maar just isn’t rimmed by way of a basalt movement, and cinder cones and a more youthful basalt movement occupy a floor of Potrillo maar (Hoffer, 1976b).

Figure 9 – View into the western from Potrillo maar looking toward Mt. Riley and Mt. Cox, two Cenocoic that is middle dacite . Photograph by Richard Kelley.