Field Guides

Home , Purgatoire River, South Valley Park

Downloaded from fieldguides.gsapubs.org on November 7, 2013

Field Guides

Fossils and geology of the Greenhorn Cyclothem in the Comanche National Grassland, Colorado

Steve Miller

Field Guides 2013;33;269-278 doi: 10.1130/2013.0033(10)

Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Field Guides Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society.

Notes

Fossils and geology of the Greenhorn Cyclothem in the Comanche National Grassland, Colorado

Steve Miller Western Interior Paleontological Society, P.O. Box 200011, Denver, Colorado 80220, USA

ABSTRACT

The Cretaceous Western Interior Seaway experienced several transgressive/ regressive cycles during its existence. The Greenhorn Cyclothem, the sixth such cycle, is signifi cant because of the symmetry of deposition, and because of the expression of cyclical climatically infl uenced deposits within. This fi eld trip will illustrate evidence of both of these cycles.

INTRODUCTION

The Comanche National Grassland in southeastern Colorado offers exposures of sediments deposited in the Western Interior Sea- way during the Greenhorn Cyclothem. This ﬁ eld trip offers a look at the geology and fossils that characterize the record of this marine environment. The human history of the area and early geological study also play a role in understanding this region of Colorado. The Comanche National Grassland occupies two areas of signiﬁ cant size in southeastern Colorado: the Timpas and Carrizo Units (Fig. 1). The Timpas Unit is situated in Otero County ~200 miles south of Denver. This area offers access to geology that represents a greater expanse of time than does the geology in the Car- rizo Unit. While the Carrizo unit largely includes exposures of the Dakota Sandstone through the lower Greenhorn Formation, the Timpas Unit (not including the Purgatoire River Canyon) exposes the Dakota Sandstone (Cenomanian age, ~110 Ma) through the Figure 1. Generalized map of Colorado showing locations of the Tim- Smoky Hills Chalk Formation (Coniacian age, ~87 Ma). pas and Carizzo Units of the Comanche National Grassland.

Miller, S., 2013, Fossils and geology of the Greenhorn Cyclothem in the Comanche National Grassland, Colorado, in Abbott, L.D., and Hancock, G.S., eds., Classic Concepts and New Directions: Exploring 125 Years of GSA Discoveries in the Rocky Mountain Region:: Geological Society of America Field Guide 33, p. 269–278, doi:10.1130/2013.0033(10). For permission to copy, contact [email protected]. ©2013 The Geological Society of America. All rights reserved.

269 270 Miller

One of the largest features in the Timpas Unit is an area situated between the north rim of the Purgatoire River Canyon and limestone escarpment parallel to the river. The Fort Hays Limestone caps the bluffs and forms slightly dipping cuestas where drainages to the north serve as tributaries to the Arkansas River. The prairie, bluffs, and stream channels are readily vis- ible in satellite imagery (Fig. 2). Tributaries of the Purgatoire River cut across this area in places with fl ows trending gener- ally north-to-south, contributing to extensive exposures of the canyon rim rock. A stage coach trail took prospectors and settlers over the prairies in the area between the bluffs and the Purgatoire River Canyon. After 1873, this trail became the New Santa Fe Stage Road (Scott, 1972), although it is shown as the New Santa Fe Trail (Thompson et al., 1894) as shown in Figure 3. To this day, a few homesteads can still be seen along this route. All three fi eld trip stops are in the area shown in Figure 3, including Minnie Canyon and Davis Canyon. Davis Canyon shown in this map is today known as Vogel Canyon. Figure 4, illustrated in early geologic work (Stose, 1912) describes the prevailing geologic profi le of the hard rock plain adjacent to the canyon, the prairies, and the bluffs. Timpas Lime- stone, the original name of the uppermost bed in this illustration, has been replaced by the name Niobrara. Timpas is the name of a small town along the railroad and Santa Fe Trail in Otero County. As old as it is, this profi le perfectly encapsulates the geologic focus of this fi eld trip. Some of the earliest work regarding relative age dating and sedimentation rates of the Bridge Creek Member of the Greenhorn

Figure 3. Area of the U.S. Geological Survey Timpas quadrangle from 1912 showing the Santa Fe Trail crossing the prairies between the Da- kota canyon rim rock and bluffs capped by the Niobrara Limestone. Note Minnie Canyon and Davis Canyon (now Vogel Canyon); two of the named locations on this ﬁ eld trip.

Figure 4. Graphic from the U.S. Geological Survey Apishapa Folio (Stose, 1912) that illustrates the general geographic pro- ﬁ le of the Dakota, Greenhorn, and Timpas (Niobrara) Lime- stones. Key features illustrated are the sandstones that indicate the shallower levels of the seaway at the start and end of the Figure 2. Satellite image showing Purgatoire River and the limestone es- cycle, and the limestones that formed during the maximum ex- carpment formed by the Fort Hays Member of the Niobrara Formation. tent of the seaway in between. Greenhorn Cyclothem, Comanche National Grassland 271

Formation based upon astronomic cycles was attempted in this the Greenhorn Cyclothem. The geological evidence is described area (Gilbert, 1895). These historical observations later became in seven phases and characterized as “asymmetrical stratigraphi- validated in the context of Milankovitch Cycles and their effect cally but nearly symmetrical lithologically” (Hattin, 1964). The on climate. Greenhorn Cyclothem is described in much more detail—13 The oldest rocks of the Timpas Unit are Permian mudstones phases—by Kauffman (1977b). and siltstones in Purgatoire River Canyon (Tweto, 1979). The Indeed, the striking symmetry was recognized over 100 Purgatoire River Canyon also exposes rocks of Triassic, Jurassic, years ago (Stose, 1912). Further, within this symmetrical sedi- and early Cretaceous ages. mentation sequence, alternating couplets of shale and limestone The Purgatoire River Canyon also contains one of the larg- were studied to determine the age of the Cretaceous deposits est dinosaur track sites in North America, expressing more than (Gilbert, 1895). 1,300 tracks over a distance of more than 400 km (Lockley and Hunt, 1995). Stop 1. Dakota Sandstone

THE GREENHORN CYCLOTHEM Our fi rst stop (Fig. 3) will be at Minnie Canyon where we will examine the Dakota Sandstone. The goal of this stop is to The goal of this fi eld trip is to provide a look at sediments observe the beginning of the Greenhorn Cyclothem, the deposits deposited during the Greenhorn Cyclothem, and observe the of clastic material delivered by fl uvial systems from the west. paleofl ora, paleofauna, and ichnofossils these coastal and marine deposits yield today. The Greenhorn Cyclothem was the sixth transgressive-regressive cycle of the Western Interior Seaway, sometimes referred to in other literature as T6 (Kauffman, 1977b). As a concept, the cyclothem was developed in 1932 as a way to describe cyclic deposits in Pennsylvanian strata that included coal deposits. Economic considerations motivated geologists to make these observations, but their observations were also largely confi ned to coal seams, ignoring additional observations that were at the time deemed insignifi cant (Weller, 1964). Studies in cyclic sedimentation followed this work by a few decades as rocks of entirely marine origin were recognized in the mid-1950s and early 1960s. A study of rock layers in south- ern Arizona in the early 1950s identifi es patterns of cyclic sedimentation in carbonate rocks. Cyclic patterns were recognized in deposits of carbonate materials, and described in terms of distance above and below wave base confi rmed that cycles can be recognized in marine environments (McKee, 1960). The infl uences of cyclic sedimentation are described in the areas of three general theories: diastrophic, climatic, and sedimentation (Weller, 1964). During this fi eld trip, we will look at a cycle of deposits that have been related to each of these infl u- ences. Diastrophic changes include both tectonic and eustatic infl uences. Climatic changes today are considered in terms of Milankovitch Cycles. The sequence of beds deposited during the Greenhorn Cyclothem provides one of the best opportunities to test models of sequence stratigraphy in a large, tectonically complex epicon- tinental basin. Within that study, Kauffman and others identifi ed tectonism, eustasy, and climate-infl uences on the strata deposited. Hence, each of the forcing mechanisms suggested by Weller (1964) appears to be infl uential in the deposition of the Dakota Sandstone through the Carlile Formation (Kauffman et al., 1990). Figure 5 shows the portion of the Comanche National Figure 5. Segment of Comanche National Grassland stratigraphic Grassland stratigraphic column that describes the sequences of section showing Greenhorn Cyclothem deposits from the Dakota strata observed. The canyon rim rock, Dakota Sandstone, and up- Sandstone through the Carlile Formation (image courtesy of USDA section through the Carlile Formation comprise the sediments of Forest Service). 272 Miller

Figure 6. The Minnie Canyon locale with approximate boundaries of the Mesa Rica, Pajarito, and Romeroville Members of the Dakota Sandstone.

This formation rims the Purgatoire River Canyon and its sils from the Pajarito include tracks (Fig. 10) similar to those tributaries. The Dakota Sandstone is of late Albian and early found in nearby Bent County (Schumacher, 2003). In addition Cenomanian age (Kauffman, 1977a). to fossil tracks, fossil plants are abundant in the Pajarito Member Here, the Dakota Sandstone is informally subdivided into (Fig. 11). three subunits and shown in Figure 6: the Mesa Rica Sand- The Romeroville crops out on the upper surfaces around the stone (Dakota Group), Pajarito Formation (Dakota Group), and canyon where it has resisted weathering. In the Timpas Unit, the Romeroville Sandstone (Dakota Group) (Kues and Lucas, 1987). Romeroville shows characteristics used to describe the “upper The basal Mesa Rica is bioturbated and shows many nearshore Dakota” in Pueblo County: “avalanche cross beds” (Kauffman, environment features such as ripple marks, as shown in Figure 7 1977b) and horizontal beds in frequent, alternating sequences and abundant ichnofossils including Diplocraterion and Rhizo- characterize the Romeroville in the Timpas Unit. Figure 12 corralium. The uppermost surface of this subunit contains abun- shows a typical Romeroville bedding sequence. dant ripple marks and dinosaur tracks such as Magnoavipes (Fig. The paleoenvironments preserved in the Dakota signal the 8) and Caririchnium (Fig. 9). transgression of the Western Interior Seaway. The ﬂ uvial sands The Pajarito is characterized by alternating beds of shale show a dynamic, but deepening surface of clastic material as the and sandstone. Some of the sandstone shows crossbedding. Fos- seaway expanded to the west.

Figure 7. Ripples preserved in the lower portion of the Mesa Rica Member of the Dakota Sandstone. These ripples with somewhat pointed crests and smooth troughs are roughly symmetrical. The arrow indicates bidirectional ﬂ ow. The scale closer to the ripples is in inches (image courtesy of Steve Wagner). Greenhorn Cyclothem, Comanche National Grassland 273

Figure 10. Ornithopod Charirichnium track from a lower sandstone bed in the Pajarito (Dakota Group).

Figure 8. Theropod Magnoavipes track, in positive relief, near the top of the Mesa Rica (Dakota Group).

Figure 11. Fossil leaves in the Pajarito (Dakota Group). The scale closer to the impression is in centimeters.

Figure 9. Ornithopod Charirichnium track on the top of the Mesa Rica (Dakota Group).

Figure 12. Typical bedding characteristics of the Romeroville (Da- kota Group). These cross beds indicate the sand was deposited in deltaic environment. The scale is six inches long (image courtesy of Shellie Luallin). 274 Miller

Figure 13. An old homestead sits on a bench of the Bridge Creek Member of the Greenhorn Limestone northeast of Vogel Canyon. Likely this homestead was very close to the New Santa Fe Trail as shown on the U.S. Geological Survey topographic map in Figure 3.

Stop 2. Greenhorn Formation—Bridge Creek Member 1987). Based on the study of the fossil oyster Pseudoperna from the Jetmore and Pfeifer (Bridge Creek equivalents in Kansas), The second stop on this fi eld trip (Fig. 3) will be on a prairie Hattin (1975) concludes the depth of the Western Interior Sea- between the Purgatoire River Canyon and the bluffs capped by way during the deposition of the Greenhorn Formation—at its the Fort Hays Limestone Member of the Niobrara Formation. An maximum—was ~200 m (500 ft). arroyo transects the prairie and offers a glimpse into the Green- Beds at this particular locality are rich with the inoceramid horn formation, specifi cally the upper Hartland Shale Member Mytiloides and have yielded several specimens of the heteromor- and the majority of the Bridge Creek Member. The goal of this phic ammonite Puebloites greenhornensis as shown in Figure 16. stop is to observe carbonate and shale depositions during the Greenhorn Cyclothem when the Western Interior Seaway was at Stop 3. Carlile–Niobrara Contact its greatest eustatic highstand (Kauffman et al., 1990). Here, the Hartland Shale is largely devoid of fossils except The third stop (Fig. 3) will be on the bluffs from which for taxonomically indeterminate inoceramids (Nagrodski et al., the Greenhorn benches and canyon rim Dakota features can be 2012) that occur in somewhat resistant blocks of fi ssile, blue- observed as depicted in Figure 4. Under the “Timpas” Limestone weathered shale. (Stose, 1912) that caps the bluffs, there is a thin sandstone layer. The resistant basal beds of the Bridge Creek Member can The goal of this stop is to observe the regression of the Western be traced from the Front Range of Colorado southeast to New Interior Seaway from the deposit of fi ne sediments in the Blue Mexico and eastward to Kansas (Cobban and Scott, 1972; Hat- Hill shale to the coarser and somewhat sandier, wave and beach tin 1975). Historically, these beds also were noted as forming benches between the Dakota and Niobrara as described in Figure 4, above (Stose, 1912). Figure 13 shows an old homestead built on a bench of the Bridge Creek Member near the second stop of the fi eld trip. Alternating beds of shale and limestone occur in the Bridge Creek Member (Fig. 14). The rhythmic nature of deposition has been a topic studied for over 100 years, and is thought to be infl uenced by astronomic cycles. Among the earliest study of astronomical cycles infl uencing deposition Gilbert described the deposition of limestone during intervals of wetter climate and shale during intervals of dryer climate (Gilbert, 1895). More recently, however, the concept of orbital forcing on the climate through Milankovitch Cycles is thought to have caused shale deposition during intervals of wetter climate and limestone deposition during more arid intervals (Keller and Pardo, 2004). These beds have been well-documented as illustrated in Figure 15 (Schumacher, 2012). The rhythmic nature of the hard limestones and soft, marly shales indicates the probable climate-driven cyclic infl uences of deposition within the Green- Figure 14. A Western Interior Paleontological Society member takes horn Cyclothem. The Bridge Creek Member represents deposi- notes of a fossil fi nd in a limestone bed within the Bridge Creek Mem- tion during alternating arid and humid climatic episodes (Hattin, ber of the Greenhorn Limestone. Greenhorn Cyclothem, Comanche National Grassland 275

Figure 15. Detailed stratigraphic column showing alternating limestone and shale sequences in the Bridge Creek Member of the Greenhorn Formation. This stratigraphic illustration is the re- sult of measurements made at multiple sites, including the site at Stop 2. (Image courtesy of Bruce Schumacher.) 276 Miller

inﬂ uenced Juana Lopez Member (Fig. 5). At this site, the sandstone is the Juana Lopez Member, the uppermost member of the Carlile. The limestone unconformably overlying the Juana Lopez Member is the Fort Hays Member of the Niobrara Formation. The Juana Lopez Member has been determined to be of late Turonian age through the study of its paleofauna (Dane et al., 1966, and Hook and Cobban, 1980). Calcarenites, some with abundant bioclastic debris, as shown in Figure 17, constitute the bulk of the Juana Lopez lithology (Dane et al., 1966). Other invertebrate fossils found in abundance in this Colorado exposure include the oysters Camleolopha bellaplicata and C. lugubris, and the scaphite Scaphites whitﬁ eldi. Here, as at the Juana Lopez Reference Section in Sandoval County, New Mexico, the Carlile–Niobrara Contact is uncon- formable (Dane et al., 1966). Figure 18 shows a view of the top of the Carlile Formation and base of the Niobrara formation. In the Comanche National Grassland, the Juana Lopez Member ranges from ~80 cm to 165 cm in thickness. At the Type Section, near Figure 16. The fossil with the coolest name from Colorado: Pueb- loites greenhornensis comes from the basal beds of the Bridge Creek Formation.

Figure 18. Section showing the dark ﬁ ssile Blue Hill Shale grading into the Juana Lopez Member atop the Carlile. The Juana Lopez is repre- Figure 17. A small example of dense, bioclastic calcarenite from the sented here as brown, sandy, and brittle. Large blocks of the basal Fort Juana Lopez Member showing a specimen of Cameleolopha lugubris Hays Member of the Niobrara Formation sit unconformably—~80 cm and Ptychodus whipplei. The scale shown is in centimeters (image above the end of the staff—on the Juana Lopez Member. The staff is courtesy of Steve Wagner). 1.5 m in length. Greenhorn Cyclothem, Comanche National Grassland 277

Figure 20. An example of a commonly found external mold of the ammonite Prionocyclus wyomingensis from the Juana Lopez Member. The scale is six inches long.

Field Opportunity in the Comanche National Grassland. I thank Figure 19. A large Prionocyclus macombi is exposed on the underside Malcolm Bedell who came on many of the early scouting trips. of a slump block of the Juana Lopez. This specimen was found in the Most importantly, I thank my wife Jo-Anne for her support of lower one-third of the member. The scale is six inches long. my avocation in paleontology.

REFERENCES CITED

Santa Fe, New Mexico, the Juana Lopez Member is ~32 m thick Cobban, W.A., and Scott, G.R., 1972, Stratigraphy and Ammonite Fauna of the (Hook and Cobban, 1980). Graneros Shale and Greenhorn Limestone near Pueblo, Colorado: U.S. Geological Survey Professional Paper 645, 108 p. Ammonites Prionocyclus macombi and P. wyomingensis Dane, C.H., Cobban, W.A., and Kauffman, E.G., 1966, Stratigraphy and have been observed and collected in this area, and indicate the Regional Relationships of a Reference Section for the Juana Lopez Mem- relatively thin deposit of Juana Lopez in Otero County encom- ber, Mancos Shale, in the San Juan Basin, New Mexico: U.S. Geological Survey Bulletin 1224-H, 15p. passes the same time interval as the much thicker Juana Lopez Gilbert, G.K., 1895, Sedimentary Measurement of Cretaceous Time: The Jour- Type Section. The two ammonite species are used to biostrati- nal of Geology, v. 3, no. 2, p. 121–127, doi:10.1086/607150. graphically date ~31 m of the Type Section total thickness (Hook Hattin, D.E., 1964, Cyclic Sedimentation in the Colorado Group of West- Central Kansas: Kansas Geological Survey Bulletin 169, p. 205–217. and Cobban, 1980). Figure 19 shows P. macombi and Figure 20 Hattin, D.E., 1975, Stratigraphy and depositional environment of Greenhorn shows P. wyomingensis. Limestone (Upper Cretaceous) of Kansas: Kansas Geological Survey The Juana Lopez formation marks the lowest point of regres- Bulletin 209, 128 p. Hattin, D.E., 1987, Pelagic/Hemipelagic Rhythmites of the Greenhorn Lime- sion and therefore marks the end of the Greenhorn Cyclothem. stone (Upper Cretaceous) of Northeastern New Mexico and Southeastern The Niobrara Cyclothem followed abruptly leaving the Fort Colorado, 1987, New Mexico Geological Survey Guidebook, 38th Field Hays Member of the Niobrara lying unconformably above. Conference, Northern New Mexico. p. 237–247. Hook, S.C., and Cobban, W.A., 1980, Reinterpretation of type section of Juana Lopez Member of Mancos Shale: New Mexico Geology, v. 2, p. 17–22. ACKNOWLEDGMENTS Kauffman, E.G., 1977a, Geological and biological overview: Western Interior Basin, in Kauffman, E.G., ed., Cretaceous Facies, Faunas, and Paleoenvi- ronments across the Western Interior Basin: Field Guide, North American I offer thanks to Bruce Schumacher, of the USDA Forest Service Paleontological Convention, II: Rocky Mountain Association of Geolo- for his mentorship, guidance and support for Western Interior gists, Rocky Mountain Geologist, v. 14, p. 75–99. Paleontological Society (WIPS) trips to the Comanche National Kauffman, E.G., 1977b, Second Day, Upper Cretaceous Cyclothems, Biotas, and Environments, Rock Canyon Anticline, Pueblo, Colorado, in Kauff- Grassland. Louis Taylor also earns my greatest thanks for his man, E.G., ed., Cretaceous Facies, Faunas, and Paleoenvironments across mentorship. We value him not only for giving us answers, but the Western Interior Basin: Field Guide, North American Paleontological for causing us to ask more and more questions as we continue Convention, II: Rocky Mountain Association of Geologists, Rocky Moun- tain Geologist, v. 14, p. 129–152. to visit and document this region. I thank Angela Matthias who, Kauffman, E.G., Harries, P.J., Kirkland, J.I., Sageman, B.B., and Wolfe, D.G., along with Lou, supported my proposal to develop the WIPS 1990, Sequence Stratigraphy and Facies Analysis of the Greenhorn 278 Miller

Eustatic Cycle (Late Albian–Middle Turonian), Western Interior Basin: Schumacher, B.A., 2003, An addition to the Dinosaur Freeway Megatracksite, Arizona to Minnesota: AAPG Rocky Mountain Section Meeting, Denver, Dakota Group (Upper Cretaceous), Bent County, Colorado: Ichnos, v. 10, Colorado, 16–19 September 1990, AAPG Search and Discovery Article p. 255–262, doi:10.1080/10420940390256302. #91002. Schumacher, B.A., 2012, Of Bombers and Bivalves: North American Ceno- Keller, G., and Pardo, A., 2004, Age and paleoenvironment of the Cenoma- manian occurrence of the rudist Durania sp. (Bivalvia: Radiolitidae) in nian–Turonian global stratotype section and point at Pueblo, Colo- the Greenhorn Limestone (Upper Cretaceous) of southeastern Colorado: rado: Marine Micropaleontology, v. 51, p. 95–128, doi:10.1016/j Kansas Academy of Science Transactions, v. 115, p. 117–134. .marmicro.2003.08.004. Scott, G.R., 1972, Historic Trail Map of the La Junta 2 Degree Quadrangle, Kues, B.S., and Lucas, S.G., 1987, Cretaceous Stratigraphy and Paleontology Colorado: U.S. Geological Survey, scale 1:250 000, 1 sheet. in the Dry Cimarron Valley, New Mexico, Colorado and Oklahoma, in Stose, G.W., 1912, Geologic Atlas of the United States: U.S. Geological Sur- Lucas, S.G., and Hunt, A.P., eds., New Mexico Geological Society Guide- vey, Apishapa Folio 186, 20 sheets. book, 38th Field Conference, northeastern New Mexico, p. 167–198. Tweto, O., 1979, Geologic Map of Colorado: U.S. Geological Survey, scale Lockley, M.G., and Hunt, A.P., 1995, Dinosaur Tracks and Other Fossil Foot- 1:500 000, 2 sheets. prints of the Western United States: New York, Columbia University Thompson, A.H., Johnson, W.D., and Barclay, A.C., 1894, Timpas quad- Press, 338 p. rangle: U.S. Geological Survey, scale 1:125,000, 1 sheet (Thompson— McKee, E.D., 1960, Cycles in Carbonate Rocks: American Journal of Science, geographer; Johnson—topographer in charge; Barclay—triangulation). Bradley Volume 258-A, p. 230–233. Weller, J.M., 1964, Development of the Concept and Interpretation of Cyclic Nagrodski, M., Shimada, K., and Schumacher, B.A., 2012, Marine verte- Sedimentation: Kansas Geological Survey: Bulletin 169, p. 607–621. brates from the Hartland Shale (Upper Cretaceous: Upper Cenomanian) in southeastern Colorado, USA: Cretaceous Research, v. 37, p. 76–88, doi:10.1016/j.cretres.2012.03.007. MANUSCRIPT ACCEPTED BY THE SOCIETY 19 JULY 2013

Printed in the USA