The Navajo volcanic field is a diffuse group of intrusions, dikes, and some extrusive rocks of early Neogene (mid-Cenozoic) age scattered between Gallup and Farmington, New Mexico and Window Rock, Arizona. The most famous intrusion is Ship Rock. Intrusive rocks in the Navajo volcanic field, like most of those elsewhere within Colorado Plateaus province, include some unusual ultramfic petrographies such as minette, vogesite, and kimberlite as well as altered and serpentinized basaltic tuff and tuff breccia.
As a rule these types of volcanic rocks are indicative of derivation of magmas from deep within the continental lithosphere and from a mantle source that is somewhat different in composition than "typical" mantle. Minette consists of alkali feldspar, biotite or phlogopite, and diopside. Diopside (pyroxene), phlogopitic biotite and olivine occur as phenocrysts in many hand samples.
The Buell Park diatreme, which consists of kimberlite, is also part of the field. Kimberlite is the host rock for diamonds in many places in the world, because kimberlite is erupted from great depths where diamonds are initially formed. Both Buell Park and the Green Knobs are ultramafic breccias and consist mostly of serpentinized olivine and pyroxene.
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 sur- rounding plains. It consists of tuff breccia and fractured and comminuted host rocks wtih 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 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 iconic Ship Rock as seen from the air. In the background are the Chuska Mountains. Photo, L. Crumpler
Views of Ship Rock and southern dike. Photos L. Crumpler
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 echélon. 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 consists in large part 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 some 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 in the smaller and much younger Rio Puerco volcanc necks of the 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.
The Beast, an isolated intrusion, is typical of the scattered smaller volcanic intrusions throughout the Navajo volcanic field.
Photo, L. Crumpler
Another type of exposure within the Navajo field are diatremes. The Green Knobs diatreme stands out markedly from the surrounding sedimentary rocks from the air. Photo, L. Crumpler
Outcrops on the Green Knobs are typical ultramafic granular materials that erode in pinnacles and towers. Photo L. Crumpler
Narbona Pass Volcano
The Narbona Pass (also “Washington Pass”) volcano is a little known example of the explosive vent structures that may have been common to many of the Chuska-Ship Rock volcanoes before they were eroded to their current deeper levels of exposure. The crater shape of Narbona is located on the upper surface of the broad mesa-like Chuska Mountains. This surface is some 300 to 400 m above the surface surrounding Ship Rock and similar intrusive structures of the Navajo field. Therefore, it is possible that Narbona is a preserved surface expression of the type of volcanism responsible for Ship Rock, and that Ship Rock is a deep exposure of the conduit for a Narbona type volcano. Narbona volcano itself must be considerably eroded given that it would be an surface remnant of early Neogene age. So the crater shape could be a function of erosion oif a relatively easily removed former volcanic cone. More information is needed to come to a conclusion.
Narbona volcano as seen from the air. The view is directed eastward. Photo by L. Crumpler
DEM and geologic map (re-drafted after Ehrenberg, 1978) of Narbona Pass crater. On the lower right is my original interpretation the possible relationship between the intrusive structure of Ship Rock and the surface volcano morphology before erosion using Narbona Pass volcano as a model.