Separating the Rock Point from the Chinle

William R. Dickinson, George E. Gehrels (2006). U-Pb ages of detrital zircons in Jurassic eolian and associated sandstones of the Colorado Plateau: Evidence for transcontinental dispersal and intraregional recycling of sediment Geological Society of America Bulletin, preprint (2008) DOI: 10.1130/B26406.1

Following on the heels of their earlier paper on Triassic paleodrainage pattern in North American (Dickinson & Gehrels, 2008) is this new paper in GSA Bulletin using detrital zircons to determine the provenance of Jurassic rocks (Glen Canyon and San Rafael Groups) of the Colorado Plateau. The exact location of the Triassic/Jurassic boundary in western North America has long been an issue of contention as has the exact correlations of many of the units. A redbed siltstone unit underlying the Wingate Sandstone was given two names, in Arizona it was named the Rock Point Member of the Wingate Sandstone (Harshbarger et al., 1957), whereas in Utah it was named the Church Rock Member of the Chinle Formation. The Triassic/Jurassic boundary was traditionally placed between the tops of these units and the overlying Wingate Sandstone (e.g., Stewart, 1957). Dubiel (1989) assigned the Rock Point as a member of the Chinle Formation and Lucas (1993) raised it to formational rank along with his rank increase of the Chinle to a group. Lucas and Hunt (1992) also assigned strata (the siltstone member of the Chinle Formation) above the Petrified Forest Member in the Chama Basin of New Mexico to the Rock Point Formation. Recently Zeigler et al. (2008) argued that based on differing paleomagnetic polarities, the siltstone member and the Rock Point could not be the same unit. Furthermore Zeigler and Geissman (2008) demonstrated that the siltstone member possesses a magnetic paleo pole similar to the Moenave Formation. Although alternatively considered Triassic (pre-1950s) or Jurassic (post-1950s) the strata in the lower part of the Moenave Formation, as well as the base of the Wingate Sandstone) are now believed to be Late Triassic in age based mained on vertebrate biostratigraphy (e.g., Lucas and Tanner, 2007).

The new paper by Dickinson and Gehrels also considers the basal Wingate and Moenave to be Late Triassic; however, based on detrital zircon provenance they consider the lower Moenave and the Rock Point/Church Rock to be lateral equivalents and remove the Rock Point/Church Rock from the Chinle Formation. This makes the top of the Owl Rock Member the top of the Chinle Formation, placing the Rock Point back into the Glen Canyon Group. In the Moab/Arches area the "black ledge" sandstone (at the base of the Church Rock Member) forms the base of the Glen Canyon Group, and the Church Rock is now part of the Glen Canyon Group as well.

Going back to the recent work by Zeigler et al. (2008) and Zeigler and Geissman (2008) regarding the stratigraphic position and nomenclature of the siltstone member. Based on the paleo pole the siltstone member as equivalent to the Moenave is supported by Dickinson and Gehrel's work; however, the issue of the differing magnetic polarities is circumstantial evidence that the Rock Point and siltstone member are not the same unit. Stratigraphic correlation is based on lithologic similarities and it is possible that the Rock Point between northern Arizona, Utah, and New Mexico is time transgressive as depostion is not always synchronous. It would be interesting to test the paleomagnetism of other Rock Point/Church Rock outcrops for comparison. Also unresolved is the lack of Owl Rock strata in Northern New Mexico. Dubiel (1989) argued that the siltstone member was a lateral equivalent of the Owl Rock and this also needs to be tested by sampling the Owl Rock.

All of this recent work by Kate Zeigler and colleagues as well as Dickinson and Gehrels appears to be raising as many questions as are being answered; however, it appears clear that we are getting closer to finally having a handle on these correlations and nomenclature problems, instead of relaying solely on a single line of evidence, vertebrate biostratigraphy.

REFERENCES

Dickinson, W. R., and G. E. Gehrels. 2008. U-Pb ages of detrital zircons in relation to paleogeography: Triassic paleodrainage networks and sediment dispersal across southwest Laurentia. Journal of Sedimentary Research 78:745-764.

Dubiel, R.F., 1989, Sedimentology and revised nomenclature for the upper part of the Upper Triassic Chinle Formation and the Lower Jurassic Wingate Sandstone, northwestern New Mexico and northeastern Arizona. New Mexico
Geological Society Guidebook 40:213–223.

Harshbarger, J.W., Repenning, C.A., and J.H. Irwin. 1957. Stratigraphy of the Uppermost Triassic and the Jurassic Rocks of the Navajo Country: U.S. Geological Survey Professional Paper 291, 74 p.

Lucas, S.G. 1993. The Chinle Group: revised stratigraphy and biochronology of Upper Triassic nonmarine strata in the western United States. Museum of Northern Arizona Bulletin.

Lucas, S.G., and A.P. Hunt. 1992. Triassic stratigraphy and paleontology, Chama Basin and adjacent areas, northcentral New Mexico. New Mexico Geological Society Guidebook 43:151–172.

Lucas, S.G., and L.H. Tanner. 2007. Tetrapod biostratigraphy and biochronology of the Triassic–Jurassic transition on the southern Colorado Plateau, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 244:242–256.

Stewart, J.H. 1957. Proposed nomenclature of part of Upper Triassic strata in southeastern Utah: The American Association of Petroleum Geologists Bulletin 41:441–465.

Zeigler, K.E., and J.W. Geissman. 2008. Magnetostratigraphy of the Upper Triassic Chinle Group and Implications for the Age and Correlation of Upper Triassic Strata in North America. Geological Society of America Abstracts with programs (online).http://a-c-s.confex.com/crops/2008am/webprogram/Paper47897.html

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.