The masthead photo across the top of my blog was taken a few years ago from Buffalo Pass in the Chuska Mountains on the Arizona-New Mexico border when my good friend, geologist Wayne Ranney and I looked down on Ship Rock to the east. That trip also resulted in my a post on Ship Rock which I invite you to visit at: http://written-in-stone-seen-through-my-lens.blogspot.com/2011/01/ship-rock.html.
Taken with a long lens in the steamy haze of late day, this is how we saw the stone monolith from ten miles to the southeast in New Mexico. This time the Chuska’s were on the horizon with Ship Rock in the foreground, the reverse of my masthead. Note the long dike to the south (left), and a smaller dike and three volcanic plugs off to the north.
Ship Rock projects upward 1,583 feet from its base at an elevation of 5,494 feet. My fanciful juxtaposition provides a helpful perspective into its height which, as a solitary structure, somehow seems deceptively smaller. The Empire State Building’s spire tops out only at 1,545 feet.
THE NAVAJO VOLCANIC FIELD
Ship Rock erupted on the Navajo Volcanic Field of over 30,000 square kilometers in the Four Corners region of extreme northwestern New Mexico. Emplaced during the Late Oligocene to Early Miocene (28 to 19 Ma), the field contains over 100 diatremes with related dikes and plugs, and also preserves various flows and sills.
Diatremes are not unique to the Navajo Volcanic Field. Although diatremes on the Navajo field aren't visibly situated along exposed faults, as elsewhere on the Colorado Plateau, they are likely associated with NE-SW Laramide-age faults at depth that represent re-activated Precambrian fractures. These rents in the crust probably facilitated the ascent of magma in a tectonic regime that converted compression to extension fueled by changes in oceanic Farallon Plate subduction-geometries beneath the continental North American Plate. The result is potassium-rich mafic dikes and explosive diatreme volcanoes that erupted along the ancient faults and dot the Navajo Volcanic Field today.
On the map above and the cross-section below, Ship Rock and the Navajo Volcanic Field are situated on the physiographic province called the Four Corners Platform of the Colorado Plateau. The platform is bounded by the Laramide-age San Juan Basin to the east and the Defiance Uplift to the west. The northeast-trending Hogback monocline bounds the Four Corners Platform 25 km east of Ship Rock, while the north-south trending Mitten Rock monocline lies 30 km to the west. Created by Laramide compression, these uplifts and their monoclines are of Late Cretaceous and Early Tertiary age.
|A geologic cross-section through northwestern New Mexico. |
(Modified from wrri.nmsu.edu/publish/watcon/proc41/Semken.pdf)
|Illustration of a diatreme such as Ship Rock with vertical-cone geometry|
(Modified from cas.muohio.edu/~rakovajf/WTTW%20Diatreme.pdf)
Ship Rock is composed of two rock types: a minette and a tuff breccia (specifically a "serpentized ultramafic microbreccia"). Minette, an older term but still used, is a lamprophyre, a dark igneous rock lacking plagioclase that's common in dikes, laccoliths, stocks and small volcanic intrusions. They are greenish-gray to black in color with a low silica-content. Minettes erupted at high temperatures from a source deep in the mantle accounting for their high potassium content (ultrapotassic). In contrast, the olive-green to brown breccia, being a solid-gas mixture, is thought to have been generated during intrusion of minette magma into cooler, hydrated mantle at lower temperatures. Both rock types are xenolith-bearing as we shall see.
The geometry of the volcanic neck-radial dike system on the landscape is exquisitely revealed from the air. Ship Rock has three major dikes of mafic minette known as the South, West and Northeast Dikes, and four minor dikes. They are similar in composition being minette, are of varying length, demonstrate varying degrees of exhumation and project outward from the neck roughly at 120°. The overall divergence pattern suggests that the dikes merge at depth, and the consensus is that the root zone remains deeply buried.
|This dramatic southeast aerial view captures Ship Rock and its dike system at sunset.|
(Photo courtesy of Alex S. MacLean, Landslides Aerial Photography www.Landslides.com and www.AlexMacLean.com)
Many aspects of diatremes remain unclear particularly timing and emplacement sequencing. Debated in particular is whether radial dikes form after diatreme emplacement or whether ascending magma first propagates through dikes, the central pipes of which become the diatreme. The orientation and spatial association of the dikes and plugs represent discrete events during their formation. Although the south dike radiates from Ship Rock’s base, notice that a linear projection does not directly point to it. You can also see this on the aerial photo above.
The South dike has the appearance of a long, fortress-wall (below) and is not perfectly linear but has slight curves and offsets, and in places is discontinuous. Notice the undulation of the solidified magma stream of minette. At the time of emplacement, the dike was confined by sedimentary host rocks, here, the flat-lying Late Cretaceous Mancos Shale. The Mancos (pronounced MAN-cuss "like a cowboy") is a marine mudstone deposited some 60 million years ago during the first transgression of the Western Interior Seaway. Below it is the seaway's nearshore Dakota Sandstone and above it, the seaway’s Mesaverde Group, which in the region of Ship Rock has been completely unroofed by erosion.
Dike emplacement is governed by many factors such as regional host rock density, stress levels and orientations, magma pressurization, temperature, gas content and exosolution patterns. Although dike-emplacement can intersect the surface if shallow enough and result in extrusion of magma from a vent, this is not the case at Ship Rock where emplacement occurred at depth (estimated at over 1,000 m) with solidification well-below the subsurface. Exposure only occurred after erosional exhumation. Studies of Ship Rock's dike system have proven invaluable in the emerging science of planetary geology such as on the giant dike systems of Mars.
The stagnant, muddy environment during the deposition of the Mancos Shale contained a surprisingly rich marine fauna of the Western Interior Seaway. I stumbled on this tightly-coiled, fossil marine gastropod (sea snail) of the Genus Turritella while walking along the south dike near the base of Ship Rock, 500 miles from the nearest present-day ocean!
Some segments of the Mancos that are in contact with the dike have the original stratification preserved and appear to encase the dike. Proximity to the dike has sheltered these sections from erosion. From a distance, I suspected that the dike might have regionally metamorphosed these sections of Mancos thereby conferring a resistance to erosion. This theory was proposed for resistant sections of Navajo Sandstone in proximity to the Mule Ear diatreme at Comb Ridge, but I saw no metamorphic alteration in the Mancos on close inspection.
This close-up of the South dike exhibits its heavily-eroded superior surface and its fenestrated breadth. One can sense the direction of flow lineation, tremendous horizontal injection-energy and turbulence of magma flow where minette appears in elongated, parallel vesicles as if tubular. Portions of the dike also exhibited bulging at the expense of the host rock in order to accept a greater volume of magma. It's dynamic geology frozen in time!
With afternoon shadows beginning to grow, the south face displays its eroded and fractured surface with large patches of dark minette cutting through lighter-colored tuff breccia. Notice the uplifted bedrock at its base and the scattered talus of boulders. The minette does not contain vesicles indicative of gas content, implying that the magma was gas-poor and likely cooled underground. Ship Rock is composed of tuff breccia at mid-diatreme and shot through at the base with small branching dikes. I'm standing just west of the radial South dike's crest. The dirt road on which we travelled passes through the visible break in the dike. Seen previously, the strike of the dike is offset from the direction of the neck. The beginning of the West dike is seen at the far left.
As previously mentioned, both the minette of Ship Rock's neck and dikes contain xenoliths of diverse geochemical populations. They provide clues to the diatreme's depth of origin and information about the otherwise inaccessible mantle and crust such as the location of major lithospheric boundaries and their tectonic histories. These magmas were the only ones that penetrated the Colorado Plateau at the end of the Laramide Orogeny around 30 million years ago when border magmatism was voluminous. The subject of ultramafic magma generation on the Colorado Plateau is complex and likely related to the angle of Farallon Plate subduction beneath North America (amongst other theories) and fractional crystallization.
Inclusions also are good indicators of magma flow direction when they become synonymously-oriented and imbricated. The xenolith to the right was embedded in the South dike and appears to be Precambrian granite derived from the mantle which itself formed over a billion years ago! Ship Rock's magma source emplaced through an amazing thickness of basement rocks…well over 20 miles.
A few brecciated lithic-fragments of Mancos Shale were incorporated into the molten matrix of the dike. I spotted these nearest to the South dike’s connection with the base of Ship Rock, undoubtedly related to the high injection force and velocity of the magma when it hit the Mancos.
The setting sun begins to cast a magical glow. In the foreground, the Mancos Shale slopes upward to meet the south dike with blocks of car-sized minette talus scattered about. Dikes are found in association with every diatreme within the Navajo Volcanic field. A small diatreme known as The Thumb (seen on my masthead above) has no dikes visible surficially, but they have been mapped via imaging of gravity anomalies within the subsurface volcanic system. Undoubtedly, they someday will be exhumed. An inspection of Ship Rock (here at its south face), indicates patches of dark minette rock cutting through the lighter tuff-breccia suggesting that the dikes here were emplaced after the major eruption of the diatreme.
Nightfall. Ship Rock seems illuminated by its own aura.
We returned to Ship Rock the following morning to experience sunrise. Please visit my next post to see what geological surprises we found on the north side! Also, check out Wayne Ranney's blog for his posts on our Colorado Rockies excursion at http://earthly-musings.blogspot.com.