A NEW PRINCIPAL REFERENCE SECTION FOR THE MANCOS SHALE (LATE CRETACEOUS) AT MESA VERDE NATIONAL PARK

 

James I. Kirkland

Dinamation International Society

P. O. Box 307

Fruita, CO 81521

R. Mark Leckie

Dept. of Geology & Geography

University of Massachusetts

Amherst, MA 01003

William P. Elder

Branch of Paleontology and Stratigraphy

U.S. Geological Survey

Mail Stop 915, 345 Middlefield Rd.

Menlo Park, CA 94025

 

ABSTRACT

The Mancos Shale crops out extensively across nearly the entire Colorado Plateau, where it ranges from 200 to 1500 m in thickness and spans up to 20 myr of the Late Cretaceous. The formation is of interest because it records dynamic changes in relative sealevel that resulted in complex intertonguing relationships with terrestrial strata and, consequently, the development of important organic fuel resources. Although the Mancos Shale has received much study, no detailed investigation previously had been undertaken at its type area in the Mancos Valley of southwestern Colorado, where the formation was named for poorly exposed slopes of marine strata that lie between benches formed by the Dakota Sandstone and Mesaverde Formation.

Because of poor exposure in the Mancos Valley, we selected a principal reference section just to its west, below Point Lookout at the north end of Mesa Verde National Park and extending out of the park to the north. This site was chosen because it presented a steep but accessible exposure of the entire Mancos Shale, and the National Park status of much of the section insured its protection and accessibility to researchers in perpetuity.

High-resolution event stratigraphic principles (HIRES) were employed in measuring and sampling the section. This method entails trenching the entire sequence to fresh rock and sampling extensively for sediments, geochemistry, microfossils, and macrofossils. Although some short-term disturbance of the land resulted from this work, an extraordinary data base was generated for the Mancos Shale that can benefit both science and Mesa Verde National Park.

 

INTRODUCTION

The Mancos Shale was named by Cross (in Cross and Purington, 1899) for "typical exposures" in the Mancos River Valley between the La Plata Mountains and the Mesa Verde near Mancos, Colorado (Fig. 1A). Cross estimated a thickness of 1200 feet (366 m) for the Mancos in its type area and provided a brief description of the formation, but no specific type section was designated. While acknowledging that fossils of the Mancos Shale were characteristic of several distinct formations east of the Front Range, Cross concluded that subdivision west of the Front Range in the Mancos was not practical. Cross and others (1899) first noted that "the whole formation was well exposed along the north face of Mesa Verde near Mancos". Subsequent workers have investigated these nearly continuous exposures below Point Lookout on the north side of Mesa Verde National Park. In this area, Pike (1947) measured 2191 feet (668 m) of section in the Mancos and recognized five faunal zones, and Wanek (1959) measured a total thickness of 1997.5 feet (609 m), not including the transition interval with the overlying Point Lookout Sandstone.

This previous work on the Mancos Shale in its type area was superficial in comparison to recent detailed research conducted on contemporaneous marine strata of the Western Interior in other areas (e.g., Pratt, et al., 1985, Nations and Eaton, 1991). As such, we initiated a study utilizing modern high-resolution event stratigraphic methods (HIRES; Kauffman, 1988) in order to establish a principal reference section for Mancos Shale in the type area. In the Cretaceous of the Western Interior, this study represents one of the greatest stratigraphic thicknesses and time intervals to ever be explored utilizing HIRES techniques. The copious data gleaned will be under analysis for years to come.

 

HIGH-RESOLUTION EVENT STRATIGRAPHIC METHODS AND PRINCIPLES

Kauffman (1988) introduced the term HIRES to describe high-resolution correlation methods that employ the detailed collection of all types of physical, geochemical, and bioevent data in order to maximize chronostratigraphic correlation. This technique was first pioneered in the Western Interior by Hattin (1962), in a study of the Carlile Shale of Kansas, and was subsequently employed by him in numerous other detailed studies of the Cretaceous in Kansas. Kauffman and others (1991) noted many studies that have used this methodology and provided examples of useful types of event stratigraphic information that can be collected. In brief, this method requires a freshly exposed outcrop section on which lithological features to cm-scale can be observed (i.e., ash beds, limestone or concretion beds, shell beds, storm beds, sediment bypass or disconformity surfaces). After the physical data are recorded, the section is excavated for macrofossil collection, and closely-spaced bulk samples are taken for microfossil and geochemical analyses. This collecting methodology results in the definition of diverse event data that are integrated into a composite event stratigraphy for that section and compared to event stratigraphies of other sections to yield highly refined correlations. In addition, these diverse data provide detailed information that can be used to interpret depositional history.

 

THE MESA VERDE STUDY

The principal reference section at Mesa Verde was measured, described, and collected during the summers of 1988 and 1989 (48 days on the outcrop). The entire Mancos Shale thickness (700 m) was exposed in 95 trenches dug in 23 different areas from sections 19 and 20, T36N, R14W, north of Mesa Verde National Park, to section 5, T35N, R14W, at the base of Point Lookout within the park (Fig. 1A). Numerous event stratigraphic units (particularly ash beds) present throughout the Mancos allowed precise correlation between trenches and the establishment of a composite section for the formation (Fig. 1B). The section was measured using a refined Jacob's staff (Elder, 1989), and lithologic units and meter levels were marked by flagged nails prior to detailed description and collection. In order to allow correlation to the subsurface data, gamma-ray measurements were taken through the entire section using a portable gamma-ray scintillometer.

After section description, two-kilogram bulk sediment samples were taken at one-meter intervals or less through the lower 494 m of section and at two-meter intervals or less above, resulting in a total of 664 bulk samples. These samples are being analyzed for micropaleontology, clay mineralogy, and geochemistry (e.g., Leckie and others, 1991). Finally, the trenches were extensively sampled for macrofossils, which were locally abundant. Many macrofossils are not recoverable unless fresh unweathered rock is examined. After all sampling and analyses, the trenches were backfilled. No evidence of the trenches was apparent after a few rains.

Although the methods used in this study required some land disturbance, long-term impact is minimal and is greatly out-weighed by benefits to both science and the Park Service. These benefits include the documentation of the paleontologic resources at Mesa Verde National Park and their preservation for future scientific study. The Park Service plays a role in this preservation not only by protecting the land on which the fossils are found, but also by the housing of the many molluscan specimens collected in this study at the Mesa Verde Research Center, where they will be available to scientists for further study.

 

RESULTS

Among the initial results of the project are the recognition of 686 lithological units and numerous bioevent levels in the 700 m thick Mancos Shale (Fig. 1B). Fossiliferous strata in the lower 647 m of section provided age control through that interval, and fossils in the overlying Point Lookout Sandstone allowed a lowermost Campanian age to be placed on the top of the section. Several thousand macrofossil specimens representing over 110 species, several of which are new (Fig. 2), were recovered through the 20 late Cenomanian to early Campanian molluscan biozones recognized. Refined placement of molluscan biozone boundaries was facilitated by both compacted specimens from the shale and uncrushed fossils from concretions and calcarenite beds (Fig. 2). The recovery of a specimen of Uintacrinus in the upper Santonian Desmoscaphites erdmanni Zone indicates a significant range extension for this rare taxon. Study of the foraminifers also is under way (Fig. 2), and an integrated molluscan-microfossil biostratigraphy is being developed.

In addition, we have greatly refined the lithostratigraphy in the Mancos Shale type area. We now recognize most of the Colorado Front Range lithofacies units in the thick shale and mudrock sequence at Mesa Verde reference section, where we can differentiate eight members in the Mancos; these members include the Fairport, Blue Hill, and Smoky Hill lithofacies equivalents, and two new members with regional extent, an unnamed calcareous member and an unnamed upper member (Fig. 1B). Also, several stratigraphic breaks were noted in the sequence. The most significant break occurs at the basal contact of the Smoky Hill member, where the entire lower Coniacian and much of the middle Coniacian are omitted, reflecting the "Carlile-Niobrara" disconformity. Finally, the outcrop gamma-ray profile has permitted correlation into the subsurface of the northern San Juan Basin, including the recognition of prominent bentonites.

 

CONCLUSIONS

This study demonstrates that paleontologic research in national parks can be beneficial to both science and the Park Service. Science benefits by having research sites that are protected and accessible to researchers in perpetuity, and the Park Service benefits by having increased knowledge of their paleontological resources and by being able to distribute this knowledge to the public through resultant papers and public brochures. Our results are being prepared for an illustrated scientific publication that also will be suitable for sale to the public at Mesa Verde National Park. Although short-term land disturbance is necessary to bring sedimentological and paleontological research up to modern standards, this can be done without undo impact or long-lasting effects. These types of detailed research studies are compatible with the mission of our national parks.

 

ACKNOWLEDGEMENTS

We express our gratitude to Mesa Verde National Park for their support. We appreciate the helpful people of Mesa Verde National Park and ARA Leisure Services Inc. for making our stay at Mesa Verde enjoyable and productive. In particular we thank Liz Bauer, Mary Griffitts, Nancy Harris, and Jack Smith of the Mesa Verde Research Center and park rangers Jim Lynch and Dave Dyson. A special thanks to Stanley Hindmarsh for access into the section north of the park. The extensive field data collected would not have been possible without the able assistance and good humor of David Finkelstein, "Sooz" Kirkland, Richard Cashman, Christein Farnham, Constance Hayden-Scott, Huaibao Liu, George "Skip" Price, John Roy, Peter Vaz, Oona West, Dave Wolny, and Richard Yuretich. Invaluable assistance through numerous discussions about Colorado Plateau stratigraphy, as well as access to electric logs, was provided by C. M. Molenaar (USGS-Denver). We also acknowledge helpful discussions with Donald Hattin (Indiana Univ.), Erle Kauffman (Univ. of Colorado), William A. Cobban (USGS-Denver), Robyn Wright-Dunbar (Rice Univ.), Elana Leithold (North Carolina State Univ.), and Robert Zech (USGS-Denver). We thank Marie Litterer for drafting assistance and Maxine Schmidt and Oona West for the foraminifer photos. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for field and laboratory research support to R.M.L. Additional support for field work was provided by the University of Massachusetts (Faculty Research Grant to R.M.L.), Amoco Production Company (Denver), and NSF Grant No. EAR-8904608 to R.M.L. and R. Yuretich.

 

REFERENCES

Cross, W. and Purington, C.W., 1899. Geologic atlas of the United States, Telluride, Colorado: Folio 57: Washington, D.C., U.S. Geological Survey.

Cross, W., Spencer, A.C., and Purington, C.W., 1899. Geologic atlas of the United States, La Plata, Colorado: Folio 60: Washington, D.C., U.S. Geological Survey.

Elder, W.P., 1989. A simple high-precision Jacob's staff design for the high-resolution stratigrapher: Palaios, v. 4, p. 196-197.

Hattin, D.E., 1962. Stratigraphy of the Carlile Shale (Upper Cretaceous) in Kansas: Kansas State Geological Survey Bulletin 156, 155 p.

Kauffman, E.G., 1988. Concepts and methods of high-resolution event stratigraphy: Annual Reviews of Earth and Planetary Science Letters, v. 16, p. 605-654.

Kauffman, E.G., Elder, W.P. and Sageman, B.B., 1991. High-resolution correlation: a new tool in chronostratigraphy; in Einsele, G., Ricken, W. and Seilacher, A. (eds.), Cycles and Events in Stratigraphy: Springer Verlag, Berlin, p. 795-819.

Nations, J.D. and Eaton, J.G. (eds.), 1991. Stratigraphy, depositional environments, and sedimentary tectonics of the western margin, Cretaceous Western Interior Seaway: Geological Society of America, Special Paper 260, 216 p.

Leckie, R.M., Schmidt, M.G., Finkelstein, D., and Yuretich, R., 1991. Paleoceanographic and paleoclimatic interpretations of the Mancos Shale (Upper Cretaceous), Black Mesa Basin, Arizona; in Nations, J.D. and Eaton, J.G., (eds.) Stratigraphy, depositional environments, and sedimentary tectonics of the western margin, Cretaceous Western Interior Seaway: Geological Society of America, Special Paper 260, p. 139-152.

Pike, W.S., Jr. 1947. Intertonguing marine and nonmarine Upper Cretaceous deposits of New Mexico, Arizona, southwestern Colorado: Geological Society of America Memoir, v. 24, p. 1-103.

Pratt, L.M., Kauffman, E.G. and Zelt, F.B. (eds.) 1985. Fine-grained deposits and biofacies of the Cretaceous Western Interior seaway: Evidence of cyclic sedimentary processes. Society of Economic Paleontologists and Mineralogists Field Trip Guidebook No. 4, 1985 Midyear Meeting, Golden, Colorado, 245 p.

Wanek, A.A. 1959. Geology and fuel resources of the Mesa Verde area, Montezuma and La Plata Counties, Colorado: United States Geological Survey Bulletin 1072(M), p. 667-717.

 

Figure 1. (A) Generalized location map of the study area. (B) Lithostratigraphy of the new principal reference section of the Mancos Shale at Mesa Verde National Park, Colorado.

 

 

Figure 2. Selected fossils from the Mancos Shale. All specimens from the Mesa Verde section unless otherwise indicated. (A) Inoceramus (Magadiceramus) subquadratus Schlüter, x1, MEVE 65842, compacted juvenile from shale, Smoky Hill calcareous member. (B-C) Inoceramus n. sp., x1, MEVE 65843, (B) side and (C) dorsal views, uncrushed juvenile from calcarenite, Juana Lopez Member. (D) Plicatula n. sp., x2, MEVE 65844, latex peel of specimen from limestone, Bridge Creek Limestone Member. (E) Uintacrinus sp., x1, MEVE 65845, partial specimen from shale, unnamed upper member. (F) Collignoniceras woollgari (Mantell), x1, MEVE 65846, latex peel of crushed juvenile from shale, Fairport calcareous member. (G-H) Scaphites whitfieldi Cobban, x1, MEVE 65847, (G) side and (H) apertural views, uncrushed specimen from limestone concretion, Juana Lopez Member. (I) Modiolus n. sp., x1, MEVE 65848, compacted specimen from shale, Smoky Hill calcareous member. (J) Turritella n. sp., x2, MEVE 65849, compacted specimen from shale, Graneros Member. (K) Hedbergella delrioensis (Carsey), x110, umbilical view, planktonic foraminifer from Lohali Point, Arizona. (L) Heterohelix globosa (Ehrenberg), x180, side view, planktonic foraminifer from Lohali Point, Arizona. (M) Gavelinella dakotensis (Fox), x70, spiral view, benthic foraminifer, Graneros Member. (N) Neobulimina albertensis (Stelk and Wall), x120, side view, benthic foraminifer, Graneros Member.