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Transcript
A Brief Geologic History of the Kansas City Area
C.G. Spencer
The Precambrian Era (prior to 500 million years ago [mya])
The oldest rocks found around Kansas City are “only” about 296 to 300 million years old.
That is less than 10% of the total age of the earth, so most of geologic time is not recorded in the
rocks we find here. However, we can use information generated from studies of rock layers
exposed in other places in the mid-continent of North America to piece together a general idea of
how geologic environments changed in the time prior to 300 million years ago.
The oldest rocks thus far discovered in Missouri were encountered in deep wells drilled
in the northern part of the state. Those rocks are about 1.8 billion years old, and are mostly
igneous and metamorphic rocks formed when a small continental crust plate, driven by the
mechanisms of plate tectonics, crashed into the southern (?) margin of the ancient North
American plate. Plate collisions seem to have continued in this general vicinity until about 1
billion years ago, adding a wide variety of igneous rocks that now form what we call the
“basement rocks” found several hundred feet beneath Kansas City. The rock record is a bit
sketchy over the next half billion years or so, but it is likely that the area remained near the coast
of the continent. For much of this time the region may have remained above sea level, and
subject to stream erosion and deposition. There may have been periods during which sea level
rose, inundating the area and leaving behind sediments of the sea floor.
Paleozoic Era (500-296 mya)
The oldest sedimentary rocks in Missouri are sandstones that date from the early
Paleozoic Era, about 500 million years ago. They represent beach sand deposited along the coast
by a rising sea. As sea level rose, it inundated an ancient landscape eroded into billion-year-old
igneous rocks. These earliest sediments lie atop the basement rocks below Kansas City. The
Paleozoic Era was dominated by repeated fluctuations of sea level, which resulted in rocks
formed from sediments deposited in alternating marine and delta environments. Most of the
rocks exposed in eastern Kansas and all of Missouri are made up of sediment deposited during
this Era.
Pennsylvanian Period (296-304 mya)
At just about this time, plate tectonics was joining all of the continents into one giant
continent. During this process the eastern and southern coasts of North America collided with
Africa and South America, respectively. North America was now positioned along the northern
part of this “super continent” and was situated near the Equator. Most of the continent was in the
Southern Hemisphere. The Kansas City area was part of the continental shelf somewhere along
the northwestern margin.
As Africa and South America collided with North America, the Appalachian and
Ouachita Mountain ranges were uplifted. The tectonic forces caused small fluctuations in the
elevation of the crust all across North America. Numerous rivers transported large quantities of
eroded sediment from these mountains, and river deltas and broad alluvial plains built out from
the shoreline toward the west and north.
At about the same time, extensive episodes of glaciation occurred on the Southern
Hemisphere continent. When glacial ice formed sea level fell, exposing the continental shelf.
When melting occurred, sea level rose and the shelf was inundated once again.
These processes combined to create a complex pattern of sea level elevation, shifting of
the coastline, and changes in depositional environments along the equatorial northern continental
shelf, where our area was then located. The region alternated between marine and non-marine
environments as the coastline advanced and retreated and deltas came and went. The sequence of
rock layers exposed around Kansas City reflects the changing position of the ancient shoreline.
Shallow sea, tidal flat, coal swamp, river channel and alluvial plain environments have all been
interpreted in the geologic evidence (rock types and fossils).
When sea level fell, the position of the coast retreated to the west and north (called a
regression). The depositional environments became more and more “terrestrial” in nature,
consisting of deltas, braided streams, swamps and estuaries. Shale, sandstone and thin coal
seams formed in these environments.
When sea level rose, the shoreline swept across the area to the east and south (called a
transgression). Warm, shallow seas dominated (no more than a few 100s of feet deep); marine
life was abundant, often living on reefs. Fragments of shell material of animals and calcareous
algae collected as sediment on the seafloor, and later hardened into the limestone beds we see
today.
Periodically during these inundation stages, mud was transported into the area from
deltas building out from the distant shoreline. Sometimes this influx was dramatic, leaving
behind thick deposits that became marine gray shale (marine fossils are found within these beds).
At other times the mud transport was sporadic or limited, producing thin shale seams between
two layers of limestone.
There is one mystery regarding the beds deposited in the area: no one knows exactly
where in this sequence the hard, organic-rich, black shale formed. Some geologist hypothesize
that these mud sediments were deposited when sea level was high (and are therefore indicators of
deep water); others hypothesize that the sediment formed in shallow, near-shore environments of
stagnant water. Interestingly, these shale beds occupy the same relative position in the rock
sequence as coal beds do in slightly older rocks.
Late Paleozoic to Quaternary (300 to 2 mya)
The 300 million years between the end of the Pennsylvanian and the beginning of the
Quaternary is also missing from the local rock record. It is likely that some deposits were laid
down, at least through the Permian, and were subsequently eroded. During the Mesozoic (the
Age of Dinosaurs) seas covered much of what is now called the High Plains, but there is no
evidence that they extended this far to the east (there is some evidence that the Missouri
Bootheel was inundated during this time). Our best guess is that the region has remained above
sea level since the end of the Paleozoic.
Quaternary Period - Pleistocene Epoch (2-0.01 mya)
Pleistocene glacial and stream deposits overlie the Pennsylvanian rocks at Kansas City.
The contact between them is called an unconformity. Thick layers of glacier sediments (till) were
deposited sometime prior to about 600,000 years ago (and perhaps as long ago as 800,000 years).
This was during the maximum advance of the ice sheets on the North American continent.
The present-day positions of the Kansas and Missouri Rivers were established along the
front of the ice sheet. Two lobes of glacial ice pushed across the river valleys: one near
downtown Kansas City and another in eastern Jackson County near Buckner. Lakes were
formed upstream of these "ice dams". In addition, meltwater flowing off the ice sheets scoured
deep channels in the Turkey Creek and Little Blue valleys (over 100 feet deep in places).
Small patches of glacier meltwater deposits are found up to 15 miles south of the farthest
advance of the ice sheet. These are probably the remnants of what was once a vast outwash plain
created along the margin of the melting ice sheet.
As the ice sheets retreated, the Kansas and Missouri River valleys carried tremendous
meltwater discharges that caused massive downcutting. Today, alluvial deposits over 100 feet
thick are present in these valleys, and are sources of drinking water.
No other ice sheets advanced as far south as Kansas City. However, meltwater from ice
sheets to the north contained large quantities of silt and clay, which were deposited on the
floodplains of the local streams. When this material dried, it was easily picked up by winds and
deposited on surrounding uplands. These deposits are called loess and there are several episodes
of loess deposition in Kansas City. The most recent loess deposits in Kansas City date from
perhaps 15,000 years ago.
Quaternary Period - Holocene Epoch (10,000 years ago – today)
The Holocene Epoch in Kansas City is dominated by modification of the land surface,
primarily by stream and mass wasting processes. Channel and floodplain deposits of the Kansas
and Missouri Rivers, and their tributaries, are the predominant sediment deposits in the area.
KC Rock Identification Guide
C.G. Spencer
LIMESTONE
Limestone is usually some shade of gray, although it sometimes weathers to a tan or
brown color. It is a hard rock, and may shatter into sharp flakes when struck by a hammer. Fresh
surfaces may exhibit a sparkly appearance due to the tiny crystal grains of the mineral calcite,
which make up most of the rock. Calcite also reacts with hydrochloric acid, producing vigorous
bubbling due to release of carbon dioxide. Nearly all of the local limestones contain fossils.
Some contain layers of shell fragments that are interpreted to indicate wave action in a surf or
shoal environment.
Limestone beds are resistant to erosion and cap most of the local hilltops. They also form
ledges in road cuts. Sometimes there are weathered fractures in limestone layers, particularly at
the top of a hill. These are places where water can enter readily and dissolve the limestone. Often
these “joints” are filled with reddish-brown soil.
SHALE
Non-marine shale beds are usually some shade of gray or tan. They are rarely
fossiliferous due to the poor preservation environment, but a few of the clay-rich ones have
leaves and other terrestrial plants preserved in them. They sometimes contain sandy layers, with
flakes of mica, and these often contain hard, reddish-brown nodules of iron oxide that formed as
a result of periodic saturation. A few sandy shale layers apparently were deposited in either tidal
flats or shallow, calm water because they contain ichnofossils, the trails, tracks or burrows of
bottom-dwelling clams, worms or arthropods.
Marine shales are generally soft and clay-rich. Often the shale weathers into small chips
or chunks. They may be any color, from dark gray to reddish or greenish. Because they were
deposited offshore in muddy water, they can contain abundant fossils (whether the animals lived
in these muds, or the shells washed in is not always apparent). These shales will react with
hydrochloric acid if adequate calcite is present (they are then referred to as being “calcareous”).
Black shale does not fit into either of those two descriptions. There are several of these
layers in the local rock sequence. The distinguishing characteristic is that these shales are very
hard, black, and split into thin sheets (which are sometimes mistaken for slate). They have few
fossils, although they often contain hard, ovoid nodules (rich in phosphate) that formed around a
fossil fragment.
SANDSTONE
Sandstone or very sandy shale is typically light gray to tan and is usually micaceous. The
rocks around KC are interpreted to indicate a delta or stream channel depositional environment.