Revised Chinle Stratigraphy of the Chama Basin

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. DOI: 10.2113/gsrocky.43.2.121

This is a recent paper with some interesting and possibly key implications for stratigraphic correlations in the Chinle Formation. Historically, stratigraphic correlation for Late Triassic non-marine rocks in western North America has been accomplished mainly through the use of biostratigraphy (e.g., Camp, 1930 [phytosaurs]; Colbert and Gregory, 1957 [vertebrates]; Litwin et al., 1991 [palynomorphs]; Good, 1993 [invertebrates], Lucas, 1993 [vertebrates]); however, these biostratigraphic-based correlations have not been unambiguous and are often contested and revised (e.g., Long and Murry, 1995; Langer, 2005; Rayfield et al., 2005; Hunt et al. 2005; Parker, 2006). More recent studies (e.g., Riggs et al. 1996; Zeigler, 2008; Irmis and Mundil, 2008; Dickinson and Gehrels, 2008) generating isotopic dates as well as magnetostratigraphic data may be more reliable to provide correlations and reconstruct Late Triassic paleogeography. However, much of this work is in preliminary stages and/or unpublished outside of dissertations and/or abstracts. Nonetheless, some important resolutions have been made regarding the Late Triassic of the southwest U. S. such as confirming that the lower portion of the Glen Canyon Group is Triassic in age and not Jurassic (Molina-Garza et al. 2003), providing isotopic ages for the Chinle Formation (Riggs et al., 2003; Irmis and Mundil, 2008); and reconstructing the Chinle-Dockum River systems (Riggs et al. 1996; Dickinson and Gehrels, 2008).

Zeigler et al. (2008) examine the sedimentology and magnetostratigraphy of the Chinle Formation in the Chama Basin of north central New Mexico. This area is well known for its paleontology containing no less than four major vertebrate quarries (Canjilon, Snyder, Coelophysis, and Hayden) and the areas have produced the type materials of several taxa including Coelophysis bauri, Eucoelophysis baldwini, Typothorax coccinarum, and Dromomeron romeri (e.g., Colbert, 1989; Hunt and Lucas, 1993; Long and Murry, 1995; Sullivan and Lucas, 1999; Zeigler et al., 2003; Irmis et al., 2007). Furthermore, this area has produced the vast majority of known dinosauromorph (including dinosauriforms and dinosaurs) material from the Late Triassic (e.g., Colbert, 1989; Irmis et al., 2007). Thus, correlating these deposits to the rest of the Chinle Basin is of extreme importance.



In the Chama Basin, the members of the Chinle from oldest to youngest are the “Mottled Strata”, Agua Zarca Sandstone, Salitral Shale tongue, Poleo Sandstone Lentil, Petrified Forest Member, and the Siltstone Member (Stewart et al. 1972). Lucas (1993) raised the Chinle Formation to group status and correlated the Agua Zarca “Formation” to the Shinarump “Formation” and the siltstone member (erroneous termed the Red Siltstone Member by Zeigler et al. 2008) to the Rock Point “Formation”. This last correlation has been highly contested with other workers (e.g., Dubiel, 1989) suggesting correlation with the Owl Rock Member instead. In later papers, Lucas and colleagues (e. g., Lucas et al., 2003) separated out various members of the Salitral “Formation” and the Petrified Forest “Formation”. It is important to note that the Coelophysis Quarry is situated in the siltstone member; whereas the Snyder, Canjillon, and Hayden Quarries are in the Petrified Forest Member. Based on stratigraphic position and vertebrates Lucas (1993) correlated the siltstone member (his Rock Point) with the Redonda Formation (Dockum Group) in eastern New Mexico and the Poleo Sandstone to the Sonsela Sandstone of northeastern Arizona. Finally, Heckert and Lucas (2003) formally named the “Mottled Strata” the Zuni Mountains Formation.

One of the goals of Zeigler et al. (2008) was to test some of these correlations using geological mapping magnetostratigraphy and provide the following results:

1) The Zuni Mountain Formation is not mappable at the 1:24000 scale and thus is not a valid formation.

2) The various proposed members of the Salitral and Petrified Forest “Formations” cannot be distinguished by mapping and therefore are rejected.

3) The Poleo Sandstone is of almost entirely reverse polarity whereas the Sonsela Sandstone bed is of normal polarity and therefore cannot be correlative. [Note that this is the same bed which Jeff Martz and I now call the Jasper Forest bed of the Sonsela Member].

4) The siltstone member is predominantly of normal polarity whereas the type Rock Point (in Utah) and the Redonda Formation are of reverse polarity. Thus the siltstone member is not assignable to the Rock Point, nor is it correlative with the Redonda.

5) The siltstone member (and thus the Coelophysis Quarry) shares a paleo pole position with the lower part of the Moenave Formation of the Glen Canyon Group, which has alternatively been considered to be Triassic or Jurassic.

These conclusions reveal limitations of relying solely on biostratigraphy to correlate non-marine stratigraphic units. Late Triassic terrestrial fossils do have biostratigraphic utility, however, more work needs to be done (e.g., larger sample sizes) to determine accurate stratigraphic ranges of taxa, and this work needs to be done in concert with detailed palynology, magnetostratigraphic, stratigraphy, and anchored with robust isotopic dates.

One aspect of the Zeigler et al. (2008) paper that confuses me is the rank of the Chinle (and Kate and I have discussed this numerous times). In the beginning of the paper they suggest that because Spencer Lucas did not propose his change in rank for the Chinle in peer-reviewed literature, usage of Chinle Group for all non-marine Upper Triassic strata in the western U. S. is not valid. However, without any real justification or explanation they then ‘formally’ raise the Chinle to group rank only in the Chama Basin. When I mentioned this to a Chinle Formation veteran colleague of mine he exclaimed that this would only make things more confusing and Chinle nomenclature was already confusing enough. I have to agree. I really do not see the utility in raising the Chinle to group rank at all, nevermind in such a small portion of the formation. Now if one uses Chinle Group they will have to state whether this is sensu Lucas (1993) or sensu Zeigler et al. (2008). I am really not sure how any of this will be resolved.

Overall the new paper by Zeigler et al. (2008) provides intriguing hypotheses regarding the Chinle Formation and emphasizes how much work is left to do on that unit.

The photo above is of the Hayden Quarry in the Petrified Forest Member and is from here.

REFERENCES

Camp, C. L. 1930. A study of the phytosaurs with description of new material from western North America. Memoirs of the University of California, 10:1-174.

Colbert, E. H. 1989. The Triassic dinosaur Coelophysis. Museum of Northern Arizona Bulletin 57:1-160.

Colbert, E. H., and J. T. Gregory. 1957. Correlation of continental Triassic sediments by vertebrate fossils. Geological Society of America Bulletin 68:1458-1467.

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. Depositional environments of the Upper Triassic Chinle Formation in the eastern San Juan Basin and vicinity, New Mexico. U.S. Geological Survey Bulletin 1801B:1-22.

Good, S. 1993. Molluscan paleobiology of the Upper Triassic Chinle Formation, Arizona and Utah. Unpublished PhD Dissertation, University of Colorado, Boulder.

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.

Hunt, A. P. and S. G. Lucas, S.G. 1993. Stratigraphy and vertebrate paleontology of the Chinle Group (Upper Triassic), Chama Basin, north-central New Mexico. New Mexico Museum of Natural History Bulletin 2:61-69.

Hunt, A. P., Lucas, S. G., and A. B. Heckert. 2005. Definition and correlation of the Lamyan: A new biochronological unit for the non-marine late Carnian (Late Triassic). New Mexico Geological Society Guidebook, 56:357-366.

Irmis, R. B., and R. Mundil. 2008. New age constraints from the Chinle Formation resolve global comparisons of Late Triassic vertebrate assemblages. Journal of Vertebrate Paleontology 28:95A.

Irmis, R. B, Nesbitt, S. J., Padian, K., Smith, N. D., Turner, A. H, Woody, D., and A. Downs. 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317:358–361.

Langer, M. C. 2005. Studies on continental Late Triassic tetrapod biochronology. II. The Ischigualastian and a Carnian global correlation. Journal of South American Earth Sciences, 19:219-239.

Litwin, R. J., Traverse, A., and S. R. Ash. 1991. Preliminary palynological zonation of the Chinle Formation, southwestern U.S.A. and its correlation to the Newark Supergroup (eastern U.S.A.). Review of Palaeobotany and Palynology 68:269-287.

Long, R. A., and P. A. Murry. 1995. Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. New Mexico Museum of Natural History and Science Bulletin, 4:1-254.

Lucas, S. G. 1993. The Chinle Group: Revised stratigraphy and chronology of Upper Triassic non-marine strata in the Western United States. Museum of Northern Arizona Bulletin 59:27-50.

Lucas, S. G., Zeigler, K. E., Heckert, A. B. and A. P. Hunt. 2003. Upper Triassic stratigraphy and biostratigraphy, Chama Basin, north-central New Mexico: New Mexico Museum of Natural History and Science Bulletin 24:15-39.

Molina-Garza, R. S., Geissman, J. W., and S. G. Lucas. 2003. Paleomagnetism and magnetostratigraphy of the lower Glen Canyon and upper Chinle Groups, Jurassic-Triassic of northern Arizona and northeast Utah. Journal of Geophysical Research 108, B4, 2181: 1-24.

Parker, W. G. 2006. The stratigraphic distribution of major fossil localities in Petrified Forest National Park, Arizona. Museum of Northern Arizona Bulletin 62:46-61.

Riggs, N. R., Lehman, T. M., Gehrels, G. E., and W. R. Dickinson. 1996. Detrital zircon link between headwaters and terminus of the Upper Triassic Chinle-Dockum paleoriver system. Science 273:97-100.

Riggs, N. R., Ash, S. R., Barth, A. P., Gehrels, G. E., and J. L. Wooden. 2003. Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: An example of dating a continental sandstone. Geological Society of America Bulletin 115:1315-1323.

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

Sullivan, R. M. and S. G. Lucas. 1999. Eucoelophysis baldwini, a new theropod dinosaur from the Upper Triassic of New Mexico, and the status of the original types of Coelophysis. Journal of Vertebrate Paleontology 19:81-90.

Zeigler, K.E. 2008. Stratigraphy, paleomagnetism, and magnetostratigraphy of the Upper Triassic Chinle Group, North-central New Mexico and preliminary magnetostratigraphy of the Lower Cretaceous Cedar Mountain Formation, Eastern Utah. Unpublished PhD dissertation, University of New Mexico, 224p.

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

K. E. Zeigler, S. Kelley, 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 (2), 121-141 DOI: 10.2113/gsrocky.43.2.121

Zeigler, K. E., Heckert, A. B. and S. G. Lucas. 2003. Paleontology and Geology of the Upper Triassic Snyder Quarry (Revueltian), North-Central New Mexico: New Mexico Museum of Natural History and Science Bulletin 24, 132 p.

No Carnian aged deposits in the Chinle Formation?

For the last couple of years one of the more intriguing hypotheses regarding the Chinle Formation is the possibility, based on proposed revisions of the Late Triassic time scale (Muttoni et al., 2004; Furin et al., 2006), that the entire unit was Norian-Rhaetian in age, rather than Carnian-Norian. The Carnian-Norian age was based on palynology (e.g., Litwin et al., 1991) as well as vertebrate biostratigraphy (e.g., Lucas and Hunt, 1993). In fact, a faunal and floral turnover near the middle of the Chinle Formation was hypothesized by many workers to possibly represent the Carnian-Norian boundary and was evidence for an end-Carnian terrestrial extinction. First noticed by Camp (1930) and Gregory (1957) this turnover was more fully documented by Long and Ballew (1985) who noted the presence of two distinct faunas differentiated by the phytosaurs “Rutiodon A” and “Rutiodon B”, as well as the aetosaurs Calyptosuchus and Typothorax. Lucas and Hunt (1993) subsequently named these the Adamanian and Revueltian land-vertebrate faunachrons.

Considered controversial, the hypothesis that most if not all of the Chinle is actually Norian received strong support by the recent announcement of a new 206Pb/238U age of 219.2 ± 0.7 Ma from the base of the Blue Mesa Member in New Mexico (Mundil and Irmis, 2008). This is important for several reasons, firstly, the Chinle Formation has very well documented vertebrate, invertebrate, plant and trace fossil assemblages and is arguably a keystone unit for study of the Late Triassic terrestrial record. Second, it is the basis for the Adamanian land-vertebrate faunachron which has been used to correlate the first appearance of dinosaurs globally (e.g., Heckert and Lucas 1999, 2000). The implications of this will be the focus of Randy Irmis’ presentation at the Society of Vertebrate Paleontology annual meeting next month and therefore will not be discussed further here. What I would like to discuss is the slight possibility that despite these new findings maybe not all of the lower Chinle is now Norian.

In the four corners are of the western United States (Arizona, Utah, Colorado, New Mexico), the Chinle Formation uncomformably overlies the Middle Triassic (Anisian) Moenkopi Formation and consists of seven members which from oldest to youngest are the Shinarump, Bluewater Creek, Blue Mesa, Sonsela, Petrified Forest, Owl Rock, and Rock Point (Note: another unit, the Mesa Redondo Member, is locally situated between the Shinarump and Blue Mesa members but is lithologically distinct from the Bluewater Creek and the relationships between these units are not completely understood). The date provided by Mundil and Irmis (2008) is near the Bluewater Creek/Blue Mesa contact. As the new hypothesized date for the Carnian-Norian boundary is now around 230 ma, this would still leave approximately 10 million years of Norian time below this contact and would presumably include the Shinarump and Bluewater Creek. This is important because instead of just having an uncomformity between the Moenkopi and Chinle that encompasses the Ladinian, this unconformity would now encompass the Ladinian-Carnian as well as a portion of the early Norian. Whereas this might possibly explain the lack of rhynchosaur material (believed to have died out at the end of the Carnian or in the early Norian) from the Chinle Formation, it would also suggest that the Chinle is not temporally equivalent to all of the Dockum Group (eastern New Mexico and Texas) or most of the Newark Supergroup (eastern U.S.) as previously supposed. I’m getting dangerously close to the contents of Randy’s SVP abstract here which is embargoed for the next month, so I will stop this line of thought, but what I want to look at in more detail is the possibility that the Shinarump may not be Norian.

The Shinarump member consists mainly of extrabasinal conglomerates and sandstones that fill paleovalleys carved into the underlying Moenkopi Formation. Once considered its own formation, the Shinarump is now considered to represent the basal member of the Chinle Formation. Unfortunately, the high energy environment that deposited the conglomerates and sands is not conducive to preserving fossils; however in some places mudstone facies do preserve material, most notably near Cameron Arizona. Ash (2005, 2006) has documented this flora and found that it is distinct from the rest of the Chinle Formation in possessing several forms, most notably a seed fern, that more closely resembles archaic forms from the Paleozoic. This suggests that the Shinarump, although still Late Triassic, may be much older than the rest of the Chinle. Unfortunately the vertebrates are not any help. Heckert et al. (2003) documented material that is purportedly from the Shinarump near Cameron, and found that the fauna contains metoposaur and phytosaur material typical of the rest of the Chinle. Furthermore, the flora does also contain forms found in the younger Chinle units including the pollen (Litwin et al., 1991; Ash, 2005). Finally in his excellent dissertation, Jeff Martz discusses in detail that Riggs et al. (1996), in an important but often overlooked paper, used detrital zircons to correlate the Shinarump (and the Santa Rosa Formation of the Dockum) with the marine Auld Lang Syne Group, which is early Norian in age (Martz, 2008). Thus, these authors (Riggs et al., 1996) had suggested a Norian age for the entire Chinle over a decade ago. Despite this, the idea that the Shinarump may still be Carnian is intriguing because it would suggest the presence of a sizeable unconformity (TR-4?) between that unit and the rest of the Chinle Formation.

One final note is that the faunal turnover mentioned at the beginning of this post is close to the base of the Sonsela Member based on detailed mapping and revised biostratigraphic work done by Jeff Martz and myself in Petrified Forest National Park. Thus this turnover (which may also correspond with a floral turnover) is in the early-middle Norian and does not represent an end-Carnian event. More on this later.

REFERENCES

Ash, S.R. 2005. A new Upper Triassic flora and associated invertebrate fossils from
the basal beds of the Chinle Formation, near Cameron, Arizona. PaleoBios 25:17–34.

Ash, S.R. 2006. Chilbinia gen. nov., an archaic seed fern in the Late Triassic Chinle Formation of Arizona, USA. Palaeontology 49:237–245.

Camp, C. L. 1930. A study of the phytosaurs with description of new material from western North America. Memoirs of the University of California 10:1-174.

Furin, S., Preto, N., Rigo, M., Roghi., G., Gianolla, P., Crowley, J.L., and S. A. Bowring. 2006. High-precision U-Pb zircon age from the Triassic of Italy: Implications for the Triassic time scale and the Carnian origin of calcareous nannoplankton and dinosaurs. Geology 34:1009–1012.

Gregory, J.T. 1957. Significance of fossil vertebrates for correlation of Late Triassic continental deposits of North America. Report of the 20th Session of the International Geological Congress 1956, Section II:7-25.

Heckert, A.B., and S.G. Lucas. 1999. Global correlation and chronology of Triassic theropods (Archosauria: Dinosauria). Albertiana 23:22-35.

Heckert, A.B., and S.G. Lucas. 2000 [imprint 1998]. Global correlation and chronology of Triassic theropods. Gaia 15:63-74.

Heckert, A.B., Lucas, S.G., and J. W. Estep. 2003 [imprint 2002]. Lower Chinle Group (Upper Triassic: Upper Carnian) tetrapods from the vicinity of Cameron, Arizona. New Mexico Museum of Natural History and Science Bulletin 21:73-76.

Litwin, R.J., Traverse, A., and S.R. Ash. 1991. Preliminary palynological zonation of the Chinle Formation, southwestern U.S.A., and its correlation to the Newark Supergroup (eastern U.S.A.). Review of Palaeobotany and Palynology 68: 269-287.

Long, R. A. and K. L. Ballew. 1985. Aetosaur dermal armor from the Late Triassic of southwestern North America, with special reference to material from the Chinle Formation of Petrified Forest National Park. Museum of Northern Arizona Bulletin 54:45-68.

Lucas, S.G., and A.P. Hunt. 1993. Tetrapod biochronology of the Chinle Group (Upper Triassic), western United States. New Mexico Museum of Natural History and Science Bulletin 3:327-329.

Martz, J.W. 2008. Lithostratigraphy, chemostratigraphy, and vertebrate biostratigraphy of the Dockum Group (Upper Triassic), of southern Garza County, West Texas. Unpublished PhD dissertation. Texas Tech University, Lubbock, 504p.

Mundil, R., and R. Irmis. 2008. New U-Pb age constraints for terrestrial sediments in the Late Triassic: Implications for faunal evolution and correlations with marine environments. International Union of Geological Sciences (IUGS) meeting abstracts Oslo 2008 (online at: http://www.cprm.gov.br/33IGC/1342538.html).

Muttoni, G., Kent, D.V., Olsen, P.E., Di Stefano, P., Lowrie, W., Bernasconi, S.M., and F. M. Hernandez. 2004 Tethyan magnetostratigraphy from Pizzo Mondello (Sicily) and correlation to the Late Triassic Newark astrochronological polarity time scale: Geological Society of America Bulletin 116:1043–1058.

Riggs, N. R., T. M. Lehman, G. E. Gehrels, and W. R. Dickinson. 1996. Detrital zircon
link between headwaters and terminus of the Upper Triassic Chinle-Dockum
paleoriver system. Science 273:97-100.

The TR-J Terrestrial Extinction Actually Early Jurassic?

Adam Yates most recent post over at Dracovenator and a new abstract by Zeigler and Geissman has got me thinking more about faunal transitions between the Late Triassic and Middle Jurassic. As I stated in an earlier post, Chinle Formation faunal composition remains relatively consistent from the oldest to youngest localities and it is not until you get into the uppermost units of the formation and higher that you start to see some changes. Lucas and Tanner (2007) provides a good documentation of faunal change in the western U.S.A. through this interval and demonstrates that the lowermost Dinosaur Canyon Member (Moenave Formation) and the basal portion of the Wingate Sandstone (both units previously argued to be Jurassic in age and in the Glen Canyon Group) are most likely latest Triassic in age. This is based several lines of evidence including magnetostratigraphy, lithostratigraphic correlation, and biostratigraphy (the presence of phytosaur body fossils and pseudosuchian trace fossils). The upper Moenave, upper Wingate, and the Kayenta Formation lack these fossils. In addition, Lucas and Tanner (2007) place the youngest known Chinle Formation fossil assemblage (the Ghost Ranch Coelophysis Quarry) in the Rock Point Member, which they consider to be laterally equivalent to the base of the Wingate and the lower Dinosaur Canyon Member. They also consider this assemblage to be latest Norian in age based on palynology and the presence of the aetosaur Aetosaurus.

Zeigler and Geissman (2008) argue that based on magnetostratigraphy that the Ghost Ranch Coelophysis Quarry is not in the Rock Point and that it may be even younger than previously supposed. As I have noted previously, Zeigler (2008) correlates the site (using magnetostratigraphy) with the lower Moenave and now Zeigler and Geissman (2008) suggest that the uppermost Chinle Formation is at least Rhaetian and may even be Hettangian in age! This would extend the range of phytosaurs and other non-crocodylomorph pseudosuchians into the Early Jurassic. Thus there would be no terrestrial Triassic/Jurassic extinction, at least not in western North America.

Furthermore, Adam Yates recent post suggests that there may have been an end Early Jurassic extinction that spelled the end of coelophysoids and basal sauropodomorphs, followed by the rise of tetanurans and eusauropods in the Middle Jurassic. If Zeigler and Geissman and Yates are correct there would have been two major faunal turnovers in the very short period of time (approx. 30 million years) encompassing the Early Jurassic. In the earliest Jurassic we would see the disappearance of non-crocodylomorph pseudosuchians and the rise of a dinosaur dominated fauna, including the first basal sauropodomorphs in North America (which are not found in the Late Triassic of that continent*). Approximately 24 million years later we get the Early-Middle Jurassic turnover discussed by Yates and an explosion in dinosaurian diversity. Very interesting and the reason why research on the vertebrate fossil record of the lower Glen Canyon Group in becoming very important and needs to be expanded.

*Note: the only purported evidence of Late Triassic sauropodomorphs in North America are the ichnotaxa Tetrasauropus and Pseudotetrasauropus (e.g., Lucas and Tanner, 2007); however, Rainforth (2003) has determined that these taxa probably represent tracks made by pseudosuchians.

REFERENCES

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.

Rainforth, E.C. 2003. Revision and re-evaluation of the Early Jurassic dinosaurian ichnogenus Otozoum. Palaeontology 46, 803–838.

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).