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.
OK bone pickers, here are some prime chalk beds!
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.
