Field of Science

Mottled Horizons in the Basal Portion of the Chinle Formation

Strongly mottled sedimentary horizons are very common in the basal members of the Chinle Formation. The most prominent are the Temple Butte Member (Robeck, 1956) in the San Rafael Swell of Utah, and the "mottled strata" of Stewart et al. (1972). The "mottled strata" represent a zone of alteration, believed to have resulted from extreme pedogenesis (soil formation), found in the base of the Chinle Formation and sometimes in the top of the underlying Moenkopi Formation (Stewart et al., 1972; Dubiel, 1989).

According to Stewart et al. (1972) this mottled coloration is "striking and unmistakable" and consists of various rock types (mainly siltstone and sandstone, but also conglomerate and even some igneous rocks) that contain intricately mottled reddish purple, pale reddish brown, and light greenish gray. These strata are generally 5 to 25 feet thick, overly the Moenkopi Formation or the Shinarump Member of the Chinle Formation, and outcrops are discontinuous, occurring only locally. Despite stating that these strata are not really mappable at the 1:24000 scale, Heckert and Lucas (2003) provided a formal name, the Zuni Mountains Formation; however, this terminology has not been accepted by any other Chinle worker, and Zeigler et al. (2008) argued that it did not form a valid lithostratigraphic unit.

I still have yet to find this "distinctive" horizon in outcrop and through in many places the lateral muddy facies of the Shinarump (normally a conglomerate) is mottled. However, this unit contains many petrified logs, a charateristic of the Shinarump (Stewart et al., 1972). I have visited places where other authors (e.g., Heckert and Lucas, 2003) have stated the unit to be present (e.g., Six Mile Canyon, New Mexico; Placerias Quarry, Arizona) but the "mottled" rocks here are not the well developed horizon described by Stewart et al. (1972). Zeigler et al. (2008) found the same thing to be true for proposed outcrops of "mottled strata" in the Chama Basin of New Mexico.

Stewart et al. (1972) briefly mention a second mottled horizon near the top of the Mesa Redondo Member near St. Johns, Arizona. Jeff Martz and I have repeatedly found this horizon at roughly the same stratigraphic interval near St. Johns, near Concho (west of St. Johns), and even in the Petrified Forest National Park.

Earlier today, I did a brief reconnaissance of an outcrop of Chinle Formation rocks near the Tunnel Overlook (find map here) at Canyon De Chelly National Monument in northeastern Arizona. The majority of the monument consists of deep canyons carved into the Permian De Chelly Sandstone; however, several paleovalleys filled with Chinle sediments are scoured into the De Chelly Sandstone and thick outcrops of Chinle strata occur in the nearby city of Chinle (hence the name of the Formation).

This outcrop is of interest as it rests within one of these paleovalleys and thus represents basal Chinle strata. Would I find the "mottled strata" here? From the picture below it can be seen that the base of the outcrop consists of a chocolate brown siltstone horizon (unit 1) filling the paleovalley in the sandstone. Although hard to tell from this limited outcrop, this unit is similar to Moenkopi sediments. Above this is a layer of mottled yellow and gray siltstone and sandy siltstone (unit 2) capped by a resistant brown cross-stratified sandstone lens (unit 3). Above this is gray, non-mottled, siltstone (unit 4) and above this is a very hard strongly mottled unit (unit 5) very similar to the upper mottled unit found near St. Johns and Concho. The medial sandstone lens could possibly represent the Shinarump. See photos below.

View of Chinle Formation section exposed near Tunnel Overlook in Canyon De Chelly.

Units 2 and 3. Is Unit 2 the "Mottled Strata"?

Unit 5.

Close-up of mottling in Unit 5.

Unfortunately I only had a few minutes here, enough time to snap a few photos, but will definitely return in the near future for a more detailed investigation. I need to search laterally for the base of the section to find the Shinarump, and see if the basal brown unit is really the Moenkopi. The significance of these mottled units is unclear; however, if they truly are correlatable, they will be extremely important in sorting out the nomenclatural mess an correlations of the basal Chinle Formation.

REFERENCES

Dubiel, R. F. 1989. Depositional and climatic setting of the Upper Triassic Chinle Formation, Colorado Plateau; pp. 171-187 in Lucas, S.G., and A.P. Hunt (eds.), Dawn of the Dinosaurs in the American Southwest, University of New Mexico Press, Albuquerque.

Heckert, A. B., and S.G. Lucas. 2003. Triassic stratigraphy in the Zuni Mountains, west-central New Mexico. New Mexico Geological Society Guidebook 54:245-262.

Stewart, J.H., Poole, F.G. and Wilson, R.F. 1972. Stratigraphy and origin of the Chinle Formation and related Upper Triassic strata in the Colorado Plateau region: U.S. Geological Survey Professional Paper, 690, 336 p.

Zeigler, K. E., Kelley, S., and J. W. Geissman. 2008. Revisions to stratigraphic nomenclature of the Upper Triassic Chinle Group in New Mexico: New insights from geologic mapping, sedimentology, and magnetostratigraphic/paleomagnetic data. Rocky Mountain Geology 43:121-141.

1 comment:

  1. Hey, Bill. This post is very timely for me. I have just started reading my copy of Stewart, et. al., and was a little puzzled by the references to these "mottled strata" as the lower unit of the Chinle. I had always thought the Shinarump was the basal unit. Your comments put my confusion into perspective. I'm glad you included the pictures too. Not twelve hours before reading your post I had been sitting with my copy of Stewart thinking, "I wish I could see a picture of what this 'mottling' looks like, exactly".

    Sorry I missed you on your excursion to Canyon de Chelly. I hope we'll have a chance to get together later this summer.

    ReplyDelete

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