Geology and Paleontology of Northwestern Kansas: Public Field Trip
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Geology and Paleontology of Northwestern Kansas: Public Field Trip May 3, 2003 Liz Brosius Jim McCauley Bob Sawin Rex Buchanan Kansas Geological Survey Open-file Report 2003-25 1 2 Geology and Paleontology of Northwestern Kansas Liz Brosius, Jim McCauley, Bob Sawin, and Rex Buchanan Kansas Geological Survey Introduction Smoky Hills and the High Plains (see factsheets). Except for Stop 3, the field trip focuses on rocks As the title suggests, this field trip will focus on deposited late in the Cretaceous Period, about 80 the geology and paleontology—that is, the rocks and million years ago (see geologic timetable), specifi- fossils of northwestern Kansas. The five stops in cally, the Niobrara Chalk (fig. 2). The field trip is Logan, Gove, and Scott counties (fig. 1) will take us cosponsored by The Nature Conservancy, Kansas through two of the state’s physiographic regions: the Chapter. Stop 5 Stop 4 Stop 1 Stop 2 Stop 3 Fig. 1—Map of the May 3, 2003, field trip route. 3 The Legalities of Fossil Collecting in Western including I-70 and the Union Pacific Railroad, has Kansas allowed Oakley to survive, while Russell Springs has faded in importance. Commercial fossil collecting—collecting and Oakley is situated in the High Plains physio- restoring fossils for resale to private buyers and graphic region (see factsheet); the elevation of our museums—became a contentious issue in the 1980’s starting point is 3,062 feet. Beneath Oakley is up to when a market for fossils, particularly large verte- 226 feet of Ogallala Formation (fig. 2), the primary brate fossils, began to develop. Landowners in the component of the High Plains aquifer. As we will chalk beds became concerned that fossil hunters discuss in more detail at Stop 3, the Ogallala con- were trespassing on property to collect fossils and sists of sediments that eroded off the Rocky Moun- selling fossils without making landowners aware of tains during the Tertiary Period (see geologic time- their market value (McCauley et al., 1997). table). In response, the 1990 legislature passed a law The High Plains aquifer supplies the numerous (Kansas Statutes Annotated 21-3759) requiring center-pivots that irrigate the fields around Oakley. commercial fossil hunters to “obtain written authori- Nearly all the irrigation in Logan County occurs zation of the landowner to go upon such along the northern fringe of the county. As we pro- land for such purpose and when requesting such ceed south along U.S. 83 toward the Smoky Hill written authorization has identified oneself to the River, we will see the landscape and agriculture landowner as a commercial fossil hunter.” Com- change. The irrigated fields will be replaced by mercial fossil hunters must also provide landowners dryland farms and shortgrass pastures and range- with “a description of the fossil” they intend to col- land. Once we enter the Smoky Hills physiographic lect and receive owner authorization, in writing, to region (see factsheet), the topography will become remove it. more dissected and outcrops more common. This legislation applies only to commercial fossil Much of the High Plains is covered by a fine, collecting. It does not apply to casual collectors loose silt called loess. Much of this loess was de- who are searching for fossils for their own use or for posited as windblown dust during the ice ages of the use in a classroom, or to groups such as school chil- last few hundred thousand years. Although glaciers dren, 4-H’ers, or boy and girl scouts. Of course, never reached this part of Kansas, glaciers in the such casual collectors must obey existing laws re- Rockies to the west and in the upper Midwest and lated to trespass and must secure owner permission northern Plains produced large amounts of finely before entering private property to search for fossils. ground rock material that was carried by strong This permission is particularly important because winds and deposited across large areas, including some landowners in the area have signed lease the High Plains of Kansas. agreements with fossil collectors, in essence giving This loess has buried the Ogallala and older rocks those collectors exclusive right to take fossils from that would ordinarily crop out across the High their property. These agreements are somewhat akin Plains. Outcrops are restricted to those areas where to leases for mineral rights, and allow the landowner stream erosion has removed the overlying loess. and the fossil hunter to share in the proceeds from Our first glimpse of the Ogallala occurs as we cross the sale of any fossils. Because these leases may the Middle Branch of Hackberry Creek, about 8 give the person holding the lease exclusive right to miles south of Oakley. As we continue south, collect on a piece of property, landowners cannot deeper into the Smoky Hill River valley, we enter an allow other people, including casual collectors, to area where the Ogallala has been removed by ero- collect on their land. sion and the Niobrara Chalk from the Cretaceous Period pokes out from its loess covering. From Oakley to STOP 1 Our first stop will be at Monument Rocks, where we’ll get a close-up look at the Niobrara Chalk. We begin our field trip in Oakley, at the cross- roads of two federal highways, U.S. 40 and U.S. 83. STOP 1—Monument Rocks Located in the extreme northeastern corner of Logan County, Oakley is the county seat, replacing the Monument Rocks is a series of chalk monoliths more centrally located former county seat of Russell in western Gove County (fig. 3). Like Castle Rock Springs. Its location on major transportation routes, in the eastern part of the county, Monument Rocks 4 Holocene Pleistocene QUATERNARY Fig. 3—Monument Rocks, Gove County. Ogallala Fm. Miocene/Pliocene Pierre Shale served as a landmark for early travelers and pio- neers. It remains a popular tourist site in the Smoky Hill River valley. Monument Rocks was carved by wind and water in the thick chalk of the Smoky Hill Chalk Member of the Niobrara Chalk (fig. 2). These rocks were deposited during the later part of the Cretaceous Period, about 80 million years ago, when Kansas (and the western United States) was covered by a vast inland sea, several hundred feet deep (figs. 4, 5). Hundreds of feet of shale, limestone, and chalk were deposited on the floor of this relatively shallow sea. Chalk is a soft, fine-textured form of limestone, composed primarily (90–99%) of calcite. In its pure form, it is white, but it may be colored by iron oxide or other impurities. At Monument Rocks, harder layers within the chalk protect the relatively soft chalk below from erosion, creating the distinctive CRETACEOUS Niobrara Chalk Upper Cretaceous buttes or monuments. Nonetheless, erosion contin- ues to wear away pieces of these monuments, as was Smoky Hill Chalk Member demonstrated by the dramatic toppling of Cobra Rock in 1998 and the collapse of Castle Rock’s tall- est spire following a thunderstorm in July 2001 (fig. 6). Chalk is made up of tiny marine organisms that lived near the surface and piled up on the seafloor after death. As the tiny shells accumulated, a soft limy ooze formed, perfect for engulfing and preserv- Fig. 2—Stratigraphic column of rocks encountered ing the remains of other animals that fell to the bot- on this field trip. tom of the sea (some of which we’ll see at the Key- stone Gallery’s museum). 5 Alluvial Barrier islands lowlands S Subduction zone e v i Land e Cretaceous Int r a in Mountains n a C t d n ordi C u L o o Shallow Sea a M a r l a n l le a m i f o h ra erior Seaway c i r d e la n e a s p Mobile Belt t m p s A o a u n n d t a l o i n w l b a u n i d l d s i n g Limestone and reefs Fig. 4—Geography of North America during the Cretaceous Period, about 100 million years ago. Present- day Kansas is outlined in red (from Wicander and Monroe, 1989). As we will discuss in more detail at our last stop, river is 480 feet lower than our starting point in the Niobrara Chalk in western Kansas is world fa- Oakley. We then travel west back to U.S. 83, pass- mous for its fossils. Beginning in 1868, with the ing numerous Niobrara outcrops before we arrive at discovery of a large swimming reptile called a ple- the Keystone Gallery on the west side of the high- siosaur, the Smoky Hill Chalk Member produced a way. variety of large vertebrate fossils that attracted pale- ontologists from around the world. STOP 2—Keystone Gallery Monument Rocks has been designated a National Natural Landmark. Although Monument Rocks is Located on U.S. Highway 83, about 7 miles open to the public, visitors should bear in mind that southwest of Monument Rocks, the Keystone Gal- it is located on private property. lery is a museum and gift shop operated by Chuck Bonner and Barbara Shelton. This unique gallery STOP 1 to STOP 2 features a variety of fossils collected from the Niobrara Chalk, as well as original artwork by From Monument Rocks we travel south, again Chuck Bonner. crossing the Smoky Hill River. Here the bed of the 6 Asia Europe North America Africa India South America Fig. 5—Position of the continents during the later part of the Cretaceous Period (Blakey, 2001).