You can't change who you are, but you can change what you have in your head, you can refresh what you're thinking about, you can put some fresh air in your brain. Ernesto Bertarelli
The sun was peering over the horizon, and through the windshield, as I scurried across the High Plains at sunrise.
I was able to take a small break from my self-quarantine on Saturday with a quick one-day field excursion to the Cretaceous chalk beds of western Kansas. I found out that the Wichita Gem and Mineral Society (WG&MS) was taking a field trip to western Kansas and that a former student of mine was leading the trip. In fact, Jerry was a one of my early students graduating in 1972 from Fort Hays State two years after I had arrived. I had not seen the lad since that date but had been in recent email contact. I decided that Colorado Springs was closer to the chalk beds than Wichita so decided to tag along and get some fresh air. As a former teacher I could not help but add a few comments at the outcrops.
A group of 15 or so fossil pickers met in Oakley, Kansas, (north of the chalk beds) at 10:00 am. Unfortunately, my brain was a tad slow on that day and I sort of forgot my Mountain Daylight Time was an hour behind the Oakley Central Daylight time; therefore, no roadside stops for breakfast and coffee! But, off we went at 10:15 heading south to outcrops along the Smoky Hill River. Mostly we were driving on wind-blown loess (essentially dust) of Pleistocene age; however, ~250 feet of the Neogene Ogallala Formation/Group (sands, silts, etc.) forming the famous Ogallala Aquifer was not far below the surface.
The High Plains are in a severe drought and dust (much reworked loess) is on every country road.
The chalk beds are the eroded remnants of the Smoky Hill Chalk Member of the Niobrara Formation of Late Cretaceous age, about 82-87 Ma. The chalk that forms the bulk of the Member formed in a vast inland epeiric sea that stretched from the Arctic Ocean to the Gulf of Mexico. In the mid to late Cretaceous this sea, known by geologists as the Western Interior Seaway (WIS), split North America into three major subcontinents. In the United States the western boundary was the tectonic zone associated with various mountain building events generated by the Farallon Oceanic Plate being overridden and subducting under the North American Plate. Large amounts of coarse- to fine- sediment was eroded into the inland sea, To the east lay the remnants of former tectonic zones that created the Appalachian Mountains and their precursors. The land east of the seaway was non mountainous and did not contribute large amounts of sediment to the WIS although coastal and deltaic sediments are known.
The Western Interior Seaway during much of the Late Cretaceous. Map courtesy of the USGS and W. A. Cobban and R. C. McKinney.
Tectonic plate interactions in the western U.S during the time of the WIS. Sketch courtesy of the National Science Foundation and Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Cooler waters from the Arctic ocean were the first to invade the U.S. and came to a standstill in the earliest part of the late Cretaceous (perhaps the latest part of the early Cretaceous) and is known as the Mowry sea. These waters carried a cool water fauna rich in Inoceramid bivalves. The warmer water from the Gulf of Mexico carried a much different fauna (a tropical fauna with different bivalves) but also transgressed across the Continent and finally met the cooler arctic water in the early part of the late Cretaceous. Central and western Kansas was situated in the deepest part of the WIS (~2500 feet) and has a nice fossiliferous section of lime rocks in the Greenhorn, Carlile, and Niobrara formations.
The Paleogeographic Map of the Early Cretaceous ~125 Ma, compiled by Ron Blakey at Colorado Plateau Geosystems, shows the Arctic and tropical waters still struggling to meet up and complete the Western Interior Seaway.
Our field excursion was to hunt for fossils in the upper part of the Niobrara carbonate rocks in the Smoky Hill Chalk. These rocks are often covered by the Late Cretaceous Pierre Shale, a black shale that represents a shallower water deposition and can be observed to the west of Oakley at McAllister Buttes near old Fort Wallace. Although the Pierre has a spectacular fauna of coiled and straight cephalopods in many states, outcrops in Kansas are tough to locate and prospect. The late Cenozoic Ogallala Formation or Group, overlying the Pierre and/or Niobrara, crops out around the area and represents sediments eroded off the rising Rocky Mountains to the west and which originally covered Kansas east to the Flint Hills. Today the eastern edge of the Ogallala has eroded westward (and is still eroding) so that it now covers perhaps 25% of the State and defines the High Plains Physiographic Province, ~ Hays west. However, eastward flowing streams such as the Arkansas, Smoky Hill, Saline, Solomon, and Republican Rivers have carved through the Ogallala and exposed rocks of the Niobrara Formation and technically these exposures along the rivers are park of the Smoky Hills. This is where the crew was heading—off the High Plains at Oakley south to the exposures in the chalk beds along the Smoky Hill River (big finger heading west).
Map courtesy of Kansas Geological Survey.
The yellow and gray colors of the chalk are due to weathering profiles. Photos courtesy of Aaron and his drone.
Kansas is almost devoid of public lands where bone diggers can prospect; however, the Wichita club has a member who had permission from a landowner to collect fossils. The Smoky Hill Chalk does not have a bountiful number of invertebrate fossils available for collecting. There are only two common inoceramid clams and sparse coiled and straight shelled cephalopods and they do not have the shiny nacre on the outside of their shells that would make them attractive. It seems as if acid water dissolved the aragonite (a carbonate minerals) that formed the nacre. In addition, the most common fossils are: 1) a large (up to 3-5 feet), very thin shelled (quarter of a inch) inoceramid clam named Platyceramus; and 2) a dime size oyster named Pseudoperna. The clam needs lots of careful attention and plaster to extract from the rocks while the oyster is one of those fossils where your collection only needs a half dozen or fewer. It seems as if the big Platyceramus sort of floated on the oozy muddy bottom while the only attachment for the oysters was on the clam shell. Outcrops have thousands of broken pieces of Platyceramus covered with Pseudoperna weathering out of the rocks.
A ”breaking up” Platyceramus covered with attached Pseudoperna (by head of hammer).
Pseudoperna congesta attached to an
inoceramid shell. Photo courtesy of the
National Science Foundation and Creative Commons
Attribution-NonCommercial-ShareAlike 4.0 International License.
But fossil prospectors do not come looking in the chalk beds for clams and oysters—they are after the big boys and girls, the marine vertebrates such as mosasaurs, bony fish, flying reptiles, sharks, and plesiosaurs. The most common large vertebrate fossil in the Chalk is the bony fish known as Xiphactinus (formally known as Portheus when I was in school). These massive fish reached a length of ~17 feet and the most famous specimen (~13 feet long) was collected by George Sternberg in Gove County and contained a “recently swallowed” 6-foot-long fish named Gillicus. The field trip crew found several portions of fish vertebrae, probably of Xiphactinus, and those certainly caused some excitement. I found several large scales in splitting chalk layers that I can only presume came from this model.
This skull of Xiphactinus in my collection is a cast. The original was found a few decades ago and now resides in a museum.
Sharks of various sizes are also represented in the Chalk Beds but almost always by teeth. The cartilaginous skeletons of sharks are rarely preserved. The most common sharks in the Chalk are Cretoxyrhina, Cretalamna, and Squalicorax. However, shell crushing teeth of the shark Ptychodus are occasionally found. During the collecting trip on Saturday I only saw a couple of Cretoxyrhina teeth that were picked up from the surface by the group.
Squalicorax, Cretoxyrhina, Cretalamna: photos courtesy of Mike Everhart & Oceans of Kansas. A tooth from Ptychotus, a shell crushing shark.
Although hundreds of the extinct reptilian fossils known as mosasaurs (often thought of, although wrongly, as marine lizards) have been collected from the chalk, their remains still seem relatively rare for amateur collectors. Over the years while at Fort Hays I and our students found only a few scattered remains of the beasts, mostly isolated teeth, and vertebrae. None were collected on Saturday.
There are other rarer invertebrates and vertebrates in the Chalk that rockhounds usually do not observe when casually looking for specimens. But fossils of flying reptiles such as the famous pterosaurs, long necked plesiosaurs such as Elasmosaurs, belemnites (squid-like pens), straight- and coiled-ammonite cephalopods, sparse birds, a few floating crinoids, some barnacles, a few dinosaurs that probably floated into deeper water from the shoreline (bloat and float), and the really strange clams called Rudists that resemble horn corals. However, many rockhounds do not realize that the chalk is composed, almost entirely, of microscopic, compacted, carbonate plates derived from single celled algae. These plates are termed coccolithophores or coccoliths for short. The algae lived in near surface waters and upon perishing fell to the bottom by the gazillions and formed an oozy calcareous mud that ultimately hardened into chalk.
An interesting aspect of the Smoky Hill Chalk is that portions of it are silicified and known as Niobrarite or Smoky Hill Jasper. The silica percolated down from volcanic ash deposited in the overlying Ogallala Formation/Group. The silicified chalk was often used by Native Americans on the Great Plains to construct projectile points and scrapers.
Silicified chalk from the Smoky Hill Chalk Member found in a Trego County gravel terrace deposit.
The Niobrara Formation represents a fascinating part of Cretaceous history. As formations go it is not of tremendous length, perhaps around 5 million years from 87 Ma to 82 Ma, but it plays a critical part in our understanding of the Late Cretaceous rocks deposited in the WIS. In rockhound terms, the WIS rocks represent a series of transgressions and regressions of marine waters probably caused by rapid seafloor spreading (due to the Mid-Cretaceous Superplume of hot molten rock) far to the west. These fluctuations of marine waters in Kansas are somewhat minor compared to the major marine cycle (Zuni Sequence) that allowed water to occupy (transgression) and depart (regression) the WIS. The Zuni Sequence started in the Late Jurassic and lasted until the early Paleocene (Tertiary). Erle Kauffman, one of the most noted paleontologist/stratigraphers working in the Cretaceous, identified 10 worldwide Cretaceous cycles (T/R) in the Zuni Sequence of which the T/R 5-9 are present in Kansas. Kauffman estimated the earliest of the marine transgressions (T5) in Kansas started ~ 108 Ma with the warm water Kiowa Formation representing a marine transgression from the south; however, the Kiowa is restricted in its occurrence in Kansas. Better known is the overlying T/R 6 (~100 Ma to 88 Ma) composed of the Dakota Formation/Group representing shoreline and nearshore sedimentation followed by the further offshore Graneros Formation. Above the Graneros are the deep-water lime rocks of the Greenhorn and lower Carlile formations. The T/R7 (~88 Ma to 78 Ma) is composed of the upper Carlile, the Niobrara Formation (with maximum transgression in the Fort Hays Member) and regression in the lower part of the Pierre Shale. T/R cycles 8-10 are not exposed in Kansas as the WIS split into two bodies of water (north and south of central South Dakota) with the Kansas waters retreating south back into the Gulf of Mexico. In the latest Cretaceous the WIS was reaching the end of its “life” as the early uplift of the Rocky Mountains was playing havoc with the inland sea. The last remnants of the WIS persisted until the Paleocene in parts of South Dakota, North Dakota, and adjacent Saskatchewan. The seas of the Zuni Sequence represent the last marine waters to cover the craton of North America.
Jerry’s best find—a string of fish vertebrae.
I wish to thank Jerry and the WG&MS for leading and sponsoring this breath of fresh air and the chance to collect a few fossils. I was unable to stay past mid-afternoon; however, it appears the group was successful and located several partial fish skeletons. Sunday was a cold windy day and most Cretaceous rocks were seen from inside of the vehicle.
The best way to get a good group photo of a bunch of bone pickers is from Aaron’s drone.
I would encourage fossil pickers interested in Cretaceous rocks of the WIS to purchase (on-line book dealers) a copy of: Kauffman, Erle G., 1977, Cretaceous facies, faunas, and paleoenvironments across the Western Interior Basin: The Mountain Geologist, vol. 14, nos. 3 and 4. This journal is a publication of the Rocky Mountain Association of Geologists, Denver.