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

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

Expansion in this right an element 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 within the subsiding Mesilla Basin. These fill that is basin, referred to as Santa Fe Group, are 1500 to 2000 legs dense beneath Kilbourne Hole (Hawley, 1984; Hawley and Lozinsky, 1993). The uppermost sand, silt, and clay associated with the Pliocene to very very early Pleistocene Camp Rice development, the unit that is youngest of this Santa Fe Group in this the main basin, are exposed within the base of Kilbourne Hole. The Camp Rice development ended up being deposited by a south-flowing river that is braided emptied into a playa pond into the vicinity of El Paso.

The La Mesa area, a surface that is flat developed in addition to the Camp Rice development, represents the utmost basin fill regarding the Mesilla Basin at the conclusion of Santa Fe Group deposition about 700,000 years back (Mack et al., 1994). This area is all about 300 ft over the Rio Grande that is modern floodplain. The top created during a time period of landscape security. Basalt moves through the Portillo volcanic field are intercalated using 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 climatic changes and integration for the river system because of the gulf coast of florida. This downcutting had not been a process that is continuous there have been a few episodes of downcutting, back-filling, and renewed incision. This episodic growth of the river system resulted in the forming 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 ports called the Afton cones positioned north-northeast of Kilbourne Hole flowed southward. The explosion that created Kilbourne Hole erupted through the distal sides regarding the Afton basalt flows, showing 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 last stages associated with the eruption (Seager, 1987).

Volcanic Features

Bombs and bomb sags

Volcanic bombs are blobs of molten lava ejected from a volcanic vent. Bombs have reached minimum 2.5 inches in diameter and are also usually elongated, with spiral surface markings acquired once the bomb cools because it flies although the atmosphere (Figure 5).

Bomb sags are typical features when you look at the pyroclastic suge beds. The sags form whenever ejected volcanic bombs effect to the finely surge that is stratified (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 normal xenoliths in bombs at Kilbourne Hole; these xenoliths are interpreted to express items of the low to center 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 an igneous beginning (Padovani and Reid, 1989; Hamblock et al., 2007). Other upper crustal xenoliths include intermediate and silicic-composition volcanic stones, clastic sedimentary rocks, basalt and basaltic andesite, and limestone (Padovani and Reid, 1989; French and McMillan, 1996).

Mantle xenoliths (Figure 8) consist of spinel lherzolite, harzburgite, dunite, and clinopyroxenite. Study of these xenoliths has supplied data that are important the structure and heat associated with mantle at depths of 40 kilometers underneath the earth’s area ( ag e.g., Parovani and Reid, 1989; Hamblock et al., 2007). Some olivine into the xenoliths that are mantle of sufficient 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 surge that is pyroclastic hot cloud which contains more gasoline or vapor than ash or rock fragments. The cloud that is turbulent close towards the ground area, usually leaving a delicately layered and cross-stratified deposit (Figures 3 and 6). The layering types by unsteady and pulsating turbulence in the cloud.

Hunt’s Hole and Potrillo Maar

Most features described above may also be current at Hunt’s Hole and Potrillo maar (Figure 9), that are found to the south of Kilbourne Hole. Xenoliths are uncommon to absent at Hunt’s Hole (Padovani and Reid, 1989), but otherwise the maars are comparable. As opposed to Kilbourne Hole, Potrillo maar is certainly not rimmed by a basalt movement, and cinder cones and a more youthful basalt movement occupy a floor of Potrillo maar (Hoffer, 1976b).

Figure 9 – View to your western from Potrillo maar looking toward Mt. Riley and Mt. Cox, two middle Cenocoic dacite domes . Photograph by Richard Kelley.

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