Navajo Volcanic Field: Ship Rock

Location: Chuska Volcanic Field, approx. 35.3 to 37° NS and 108-109° EW; Ship Rock, 36°, 41' N latitude, 108°, 50' W longitude, San Juan County
Type: Group of intrusions and associated extrusive rocks. (Chuska Mountains, northwestern New Mexico)
Age: Age: Middle Oligocene; fission track ages at Ship Rock (on southern dike) 27 ?3 and 32?3 million years; K-Ar, 30.6 million years

Famous icon of the volcanic Southwest; classic example of a large volcanic neck.





Shiprock Dike. Photo L. Crumpler


Geological Overview

Ship Rock is a volcanic neck located within the Colorado Plateaus province in the northwestern (“Four Corners”) region of New Mexico and in the middle of Navajo country. It is part of what is known as the Chuska volcanic field, a diffuse group of small intrusions, dikes, and some extrusive rocks scattered between Gallup, New Mexico and Farmington, New Mexico. Intrusive rocks in the field, like most Colorado Plateaus province intrusives, include some unusual petrologies: minette, vogesite, basaltic tuff, and tuff breccia. The Buell Park diatreme, which consists of kimberlite, is also part of the field.

Ship rock is thought to represent the near-surface interior of a maar-type eruption. The intrusion is 500 m in diameter at its greatest width, and rises 600 m above the surrounding plains. It consists of tuff breccia and fractured and comminuted host rocks with thin, sheet-like intrusions of minette locally cuttting through the entire mass. The intrusion is estimated to have been emplaced at a minimum of 750 m and a maximum depth of 1000 m below the original surface. Host rocks at the current level of erosion are late Cretaceous, marine origin Mancos Shale.

Six dikes radiate from a point just west of the central intrusion, the three largest are 9, 4, and 3 km in length and trend S12 degrees E, N80 degrees W, and N55 degrees E respectively. The other three dikes are approximately parallel to the northern dike, trend N55 degrees E, and are less than 1 km in length. Several small plugs occur along the length of some of these dikes.

The northeast dike is actually segmented into 35 short dikes with intervening Mancos Shale. The dikes range from 0.6 to 4.6 m in width and 8 to 395 m long. Each dike segment is oriented somewhat differently from adjacent dike segments, and the pattern as a whole is not strictly en échelon. However, the trend of all the segments together is remarkably linear. This suggests that they are segmented from a larger dike at depth.

The central intrusion appears not to have strongly disturbed local sedimentary strata, which are flat-lying adjacent to the intrusion. Considerable amounts of the host rock have, however, slumped into the throat of the intrusion along the margins. Locally the breccias consist in large parts of comminuted sedimentary materials with rare fragmens of minette and biotite mixed within the breccia. As is common with breccias within many volcanic necks, the mixture of comminuted sandy host material and juvenile material is so extreme in the breccias that it is often difficult to distinguish sedimentary and volcanic materials on a hand specimen basis.

Near the top and along some of the margins there is distinct inward-slumping and inward-dipping stratification of the tuff breccia. This configuration is typical of the structure seen for the smaller Rio Puerco volcanc necks in the much younger Mount Taylor volcanic field farther east. It is largely on the basis of the inward-dipping stratification, that the current level of exposure is interpreted to be within several hundred meters below the original crater. Additional evidence is the observed maximum thickness of the Tertiary Chuska Sandstone
In addition to sedimentary materials, some rounded cobbles of crystalline basement rocks occur within the tuff breccia. And some breccia bodies consist almost entirely of comminuted host rocks. Calcite and calcite veins are common throughout many of the breccia masses.
Minette consists of alkali feldspar, biotite or phlogopite, and diopside. Diopside (pyroxene), phlogopitic biotite and olivine occur as phenocrysts in many hand samples.

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Text and photos, Dr. Larry Crumpler, New Mexico Museum of Natural History and Science

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