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 © 2013 Geological Society of America The Geological Society of America 18888 201320 Field Guide 33 2013 CELEBRATING ADVANCES IN GEOSCIENCE 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 rep- resents 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 Uni t 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 sedi- mentation in carbonate rocks. Cyclic patterns were recognized in deposits of carbonate materials, and described in terms of dis- tance 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 sedi- mentation (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 depos- ited. 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).