Geologic History. Expansion in this an element of the Rio Grande rift began about 36 million years back.

Geologic History. Expansion in this an element of the Rio Grande rift began about 36 million years back.

Expansion in this an element of the Rio Grande rift started about 36 million years back. Rock debris that eroded through the developing highlands that are rift-flank along with wind-blown and playa pond deposits, accumulated within the subsiding Mesilla Basin. These basin fill deposits, referred to as Santa Fe Group, are 1500 to 2000 foot dense beneath Kilbourne Hole (Hawley, 1984; Hawley and Lozinsky, 1993). The uppermost sand, silt, and clay of this Pliocene to very very early Pleistocene Camp Rice development, the youngest device of this Santa Fe Group in this the main basin, are exposed into the base of Kilbourne Hole. The Camp Rice development had been deposited with 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 area, a flat surface that developed along with the Camp Rice development, represents the utmost basin fill for the Mesilla Basin at the conclusion of Santa Fe Group deposition about 700,000 years back (Mack et al., 1994). This surface is mostly about 300 ft over the Rio Grande that is modern floodplain. The outer lining created during a time period of landscape security. Basalt flows from the Portillo volcanic field are intercalated with all the top Camp Rice development and lie in the Los Angeles Mesa area.

The Rio Grande started initially to decrease through the older Santa Fe Group deposits after 700,000 years back in reaction to both changes that are climatic integration associated with the river system using the gulf coast of florida. This downcutting had not been a constant process; there have been a few episodes of downcutting, back-filling, and renewed incision. This development that is episodic of river system resulted in the forming of several 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 edges for visit this page the Afton basalt moves, showing that the crater is younger than 70,000 to 81,000 years of age. Pyroclastic rise beds and vent breccia blown through the crater overlie the Afton basalt movement. The crater formed druing the ultimate phases associated with eruption (Seager, 1987).

Volcanic Features

Bombs and bomb sags

Volcanic bombs are blobs of molten lava ejected from the volcanic vent. Bombs are in minimum 2.5 inches in diameter and generally are usually elongated, with spiral surface markings acquired since the bomb cools because it flies although the fresh air(Figure 5).

Bomb sags are typical features into the pyroclastic beds that are suge. The sags form whenever ejected volcanic bombs effect in 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 shows a volcanic bomb that has deformed the root deposits. Photograph by Richard Kelley.


Lots of the bombs that are volcanic Kilbourne Hole contain xenoliths. Granulite, charnokite, and anorthosite are normal xenoliths in bombs at Kilbourne Hole; these xenoliths are interpreted to express items of the reduced to center crust (Figure 7; Hamblock et al., 2007). The granulite may include garnet and sillimantite, indicative of a metasedimentary origin, 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 basaltic andesite, 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 provided data that are important the structure and heat regarding the mantle at depths of 40 kilometers under the planet’s area ( ag e.g., Parovani and Reid, 1989; Hamblock et al., 2007). Some olivine within the mantle xenoliths is of enough 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 stone 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 kinds by unsteady and pulsating turbulence in the cloud.

Hunt’s Hole and Potrillo Maar

Lots of the features described above may also be current at Hunt’s Hole and Potrillo maar (Figure 9), that are found towards the south of Kilbourne Hole. Xenoliths are uncommon 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.

de Jager MargrietGeologic History. Expansion in this an element of the Rio Grande rift began about 36 million years back.