The following graduate term paper was written and submitted by me "in partial completion of the requirements" for a Geology For Teachers course I took at Northern Arizona University in the summer of 2004, during what is probably best characterized as the second-phase (of four, I think) of my post-baccalaureate academic career.
I must admit: I was sincerely intrigued by the content of this course. Most likely this is due to the fact that I was fortunate to have had Dr. Larry Middleton as an instructor. His depth of knowledge and passion for understanding complex geologic processes and structures was truly inspiring to me. I have continued to be attentive to and interested in geology ever since. I credit Dr. Middleton (and also my good friend and professional geologist, Joe "Rockman" Hazel) with instilling within me this abiding interest in rocks and stuff.
I got a good grade on this paper, which pleased me immensely, as I put a lot of effort into it. It's about some of the really interesting non-volcanic, sedimentary geology found on the eastern end of Mount Elden (of which I have written on several other occasions) that's conveniently bisected by the Heart Trail, a trail I ride perhaps once (never more than twice) in any given riding-season.
After I completed this paper, I uploaded it to Wikipedia where it became the initial seed for the site's current Mount Elden entry.
An Overview of the Geology of the Eastern Sedimentary Block of Mount Elden
A paper submitted in partial completion of the requirements for
30 September 2004
Mount Elden, a.k.a. Elden or Eldon Mountain, located in central Coconino County northeast of Flagstaff, Arizona, takes its name from one of the region’s earliest Anglo settlers, John Elden, who, along with his family, established a homestead on the mountain’s lower slopes and grazed sheep on the open grasslands below during the late 1800s (Cline, 1976).
Despite its rugged appearance, steep relief and massive size, covering nearly 15 square miles of surface area (Kluth 1974), Mount Elden is easily accessible via an extensive, well-developed road and non-motorized trail system. This paper will concern itself primarily with specific geologic features on the eastern slope of Mount Elden that are accessible via the Mount Elden Lookout Road, the Sunset Trail, and the Heart Trail.
AN OVERVIEW OF THE GEOLOGY OF MOUNT ELDEN
Mount Elden is one of five large peripheral silicic features within the greater San Francisco Mountain volcanic system which include the nearby Dry Lake Hills, the Hochderffer and White Horse Hills to the northwest, and O’Leary Peak to the northeast (Nations, et al, 1986). Because these features developed within close proximity of the San Francisco Mountain strato-volcano there is a strong likelihood that each is a geologic subsidiary of the larger mountain (Kluth, 1974).
Geologically, Mount Elden is a lava dome comprised almost entirely of dacitic lava flows which emerged from several vents. According to Kluth these features emerged as intrusive emplacements within sedimentary blocks or as viscous extrusive flows in which the younger flows partially covered the older, lower flows. Geologic evidence suggests that the eruption of the mountain, which occurred at linear vents along regional faults, was a non-explosive event that took place during several flow sequences. Likewise, because of the high viscosity of the dacite, it is likely that the mountain formed in the short period of a few month’s time (Kluth, 1974). The mountain’s overlapping flows commonly take lobe-like shapes which display a variety of flow characteristics and features including concentric benches, spires, ramping shear fractures, longitudinal tension fractures, and conjugate shear fractures. (Nations, et al, 1986).
Occurring within the central part of Mount Elden, on the mountain’s eastern and northwestern flanks are two sedimentary blocks. Both of these blocks display characteristics of uplift that most likely occurred as dacitic magma intruded into the sedimentary layers at a shallow depth and uplifted the layers causing them to dip away from the mountain (Kluth, 1974). These blocks of sedimentary material are unique to the mountain, which otherwise is uniformly composed of silicic dacite. Kluth notes that volcanic material predating the Elden orogeny can be identified among the alluvial material at the base of the eastern sedimentary block. He also notes that the greater uplift and deformity on the eastern block suggests that there was a greater volume of intrusion emplaced beneath this region.
THE EASTERN SEDIMENTARY BLOCK OF MOUNT ELDEN
The eastern flank of Mount Elden contains the oldest rocks on the mountain. Due to the intrusion of igneous material, the sedimentary beds of the late Devonian to Permian periods have been thrust upward, overturned, and subsequently have been tipped off the mountain’s side. This orographic activity, which began during the Tappan Volcanic Period, 0.5-0.03 million years ago (Kluth, 1974), created a large, steeply inclined area of disturbed sedimentary deposits that are predominantly inverted to their standard order of geologic deposition (Figure 1c).
In his dissertation Kluth includes an oblique aerial photograph of the eastern sedimentary block of Mount Elden which includes hand-drawn outlines identifying, generally, the location of the various sedimentary and igneous strata. Johnson provides a substantially more accurate depiction of this block, adapting an earlier geologic map of Mount Elden originally drawn by Holm (1988). I have relied extensively on both of these sources in attempting to interpret the geology of the Heart Trail, which generally bisects the eastern sedimentary block. I have combined both of these illustrations to create a generalized cross-section of the eastern sedimentary block (Figure 1c); where these sources do not concur I have attempted to make note of their disparity.
According to Johnson, the Jerome member of the Devonian Martin formation corresponds in geologic character and lithography to the late Devonian Temple Butte formation identified in the Grand Canyon strata (Figure 1c), within which is an exposed horizon of fossilized fish remains.
AN OVERVIEW OF THE HEART TRAIL
The Heart Trail bisects the eastern sedimentary block of Mount Elden generally in an east-west direction. Originally called the Elden Red Hills Trail, the trail was officially completed and rechristened the Heart Trail in the early 1990s. Trail construction efforts were spearheaded by Flagstaff residents many years prior (Mangum, 1992). However, it does not appear to have been in existence in any form at the time of Kluth’s research prior to the publication of his dissertation in 1974.
A PHOTOGRAPHIC TOUR OF THE GEOLOGY OF THE HEART TRAIL
The accompanying photos were taken on August 14, 2004, at various locations on the Heart Trail.
Looking eastward at the top of the ridgeline on Mount Elden, standing on the Sunset Trail on Quarternary dacite. From this vantage point, several features of the eastern sedimentary block can be observed. The Heart Trail begins at this point and descends nearly the entire uplifted block, terminating two miles later, approximately 1500 vertical feet below, at the base of the mountain.
Dacite spires above the Martin formation contact looking southeast. Kluth believes that these spires, which appear sporadically at a common elevation across this face of the mountain, were formed at the point of contact between the dacitic material and the sedimentary layers which are found immediately below them.
Sedimentary layers (foreground) below the dacite spires (background) looking westward. Two distinctly different rock colorations can be seen at this point. Kluth identifies the initial sedimentary unit in this vicinity as the Jerome member of the Devonian Martin formation. He ascribes to it an olive green to brown color, becoming bleached white where it contacts igneous material. The material on the left fits this description. He identifies the next unit as the Mississippian Redwall. In this vicinity he describes it as greyish in color, fitting the material on the right side of this photograph.
Standing in the light colored Devonian-Mississippian Martin-Redwall material looking eastward the contact between these units and the subsequent orange-red Pennsylvanian Supai group below is easy to identify. Kluth notes that the Supai group is identifiable only on this flank of Mount Elden. Due to fracturing it forms talus slopes instead of cliffs.
According to both Kluth and Johnson, this feature should be located within the Pennsylvanian Supai group. Here, looking southwest, the bedding planes can be seen to be steeply inclined. Kluth notes that the uplifted Supai material on the eastern block of Mount Elden dip between 50-70 degrees in a southeasterly direction; this feature would seem to comply.
Looking westward, immediately below the red sandstone of the Pennsylvanian Supai group, there are several small outcroppings of a lithologically different rock. Kluth and Johnson both indicate the presence of early Permian Hermit shale in this vicinity though Kluth indicates that the shales in this region have not been accurately mapped. Johnson indicates a large deposit of Hermit shale placing it between the Supai group and the Schnebly Hill formation.
A point of contact that appears to correspond with Johnson as the darker red Schnebly Hill formation (left) and buff-colored Coconino-Toroweap (right). Kluth identifies this vicinity as the boundary between the Supai and undifferentiated Coconino-Toroweap without mentioning the Schnebly Hill formation. Kluth also identifies the nearby sandstone knobs as the edge of the upturned eastern block.
Looking southwest, below the previous contact the sandstone material is now uniformly buff-colored. This loose fine-grained material seems to fit with Kluth’s description. Likewise, Kluth notes that within the Coco-weap exposures the material forms broad flat parks. Here, in its last mile, as the trail begins to traverse less steeply inclined terrain, a mixing of materials, chiefly sandstones, limestones and volcanics, also becomes evident.
Looking southeast at an outcropping of red sandstone immediately adjacent to the trail that appears to correspond to Johnson as an isolated emplacement of the Schnebly Hill formation within the alluvial surface material at the base of the mountain.
Two miles and 1500 vertical feet later, looking west, at the terminus of the Heart Trail. Here the geologic material is identified by Kluth and Johnson as a mixture of surficial deposits. The Heart Trail does not appear to cross any sizable, identifiable outcroppings of early Permian Kaibab limestone; however, the presence of such material would appear to be evident in the vicinity, particularly as a major component of the sandy park and small white knob immediately north and east of this position.
The eastern sedimentary block of Mount Elden is a unique geologic study area within the San Francisco Mountain volcanic area in Northern Arizona. However, to date, investigations that have concerned themselves with this region have not benefited from the presence of the Heart Trail, a relatively new route that generally bisects the block, which was constructed during the late 1980s and early 1990s.
Like the Grand Canyon many miles to the north and west, the eastern sedimentary block of Mount Elden contains geologic evidence of the Devonian, Mississippian, Pennsylvanian and Permian geologic periods in addition to ample evidence of both Quarternary and Tertiary volcanics. However, due to uplift from the intrusion of silicic magma within these sedimentary layers, radical fracturing, tipping and sluffing has occurred, leaving these layers uniformly exposed but inverted to their common order of deposition.
Ashworth, Donna, 1991, Biography of a Small Mountain, Small Mountain Books, Flagstaff, Arizona, p. 19.
Cline, Platt, 1976, They Came to the Mountain, Northern Arizona University with Small Town Press, Flagstaff, Arizona, p. 210.
Johnson, Heidemarie, 1991, A New Fish Fauna from the Upper Devonian Martin Formation, Mount Elden, Northern Arizona, Northern Arizona University, p. 9-15.
Kluth, Charles, 1974, Geology of the Elden Mountain Area, Coconino County, Arizona, Northern Arizona University, 89 p.
Mangum, Richard, et al, 1992, Flagstaff Hikes and Mountain Bike Rides, Hexagon Press, Flagstaff, Arizona, p. 66-67.
Nations, J. Dale, et al, 1986, Geology of Central and Northern Arizona: Field Trip Guidebook for Geological Society of America, Rocky Mountain Section Meeting, Flagstaff, Arizona, p. 27-30.