Guidebook—Geology of South-central Kansas
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Spillway at Eureka City Lake and the different rocks units that are exposed there.
Stop 1—Eureka City Lake Spillway
The spillway at Eureka City Lake is a good place to learn about the basic bedrock geology in south-central Kansas. Eureka City Lake was built in the 1930's as a Works Progress Administration (WPA) project and is a water supply lake for Eureka, about 5 miles to the south. The spillway is a means by which water can move out of the lake during times of extremely high water levels.
Several distinctive rock layers are visible at the spillway; in fact, this site has exposures of limestone, shale, and sandstone, the three common rock types in Kansas. These rocks were deposited during the Pennsylvanian Period of geologic history (also known as the Coal Age), from about 320 to 290 million years ago. During that time, Kansas was near the equator, the climate was warmer, and a shallow sea advanced and retreated repeatedly across eastern Kansas.
Geography of North America during the Pennsylvanian Period, about 300 million years ago. Present-day Kansas was near the shore of the shallow sea (from Wicander and Monroe, 1989).
Position of the plates during the Mississippian and Permian periods, the two periods bracketting the Pennsylvanian Period. During the Pennsylvanian, Kansas lay near the equator (from Tarbuck and Lutgens, 2000).
At times the sea left behind minerals that eventually became limestone, the brown and tan rocks in the spillway. Limestone is made up of calcium carbonate, debris composed of sea shells and other marine life, and minerals that were deposited onto the shallow ocean floor. Some of those limestones contain fossils of the invertebrate animals that were common during the Pennsylvanian--corals, sponges, and crinoids. These creatures secreted calcium carbonate to form shells and other hard body parts that make up these limestones.
At other times, rivers deposited mud into the oceans; these muds eventually formed shales, the softer, thinly layered rocks in between the limestones. Occasionally, this area was at or slightly above sea level and sandstone was deposited by rivers flowing into estuaries and deltas. Limestone, shale, and sandstone are sedimentary rocks--that is, they are made up of fragments of rock or shells that were deposited by wind or water. Except for rare exceptions (one of which we will see at Stop 2), rocks at the surface in Kansas are exclusively sedimentary in origin.
In order to identify different rock layers, geologists have given each of them a name, based on the location where it was first described (see How Rocks are Named). Here at the spillway, overflow from Eureka City Lake has formed a waterfall over two layers in the formation called the Bern Limestone. The uppermost layer is the Wakarusa Limestone Member; the lower one is the Burlingame Limestone Member (see photo at top of page). Both members are named for outcrops near towns south of Topeka. These limestones are separated by the Soldier Creek Shale Member.
Below the Burlingame Member is a thick sequence of gray shale and sandstone in the upper part of the Scranton Shale, the Silver Lake Shale Member. Below the Silver Lake Member is a sandstone layer, which is also part of the Scranton Shale. The sandstone contains mica flakes and woody fragments and also displays various sedimentary structures, including ripple marks. A thin coal layer can also be found in the Scranton.
Ripple marks in sandstone at Eureka City Lake.
The different sedimentary rocks in this sequence indicate changing environments of deposition. The coal and ripple-marked sandstone were formed slightly above sea-level, perhaps in a swampy delta. The shale most likely was deposited as sea level rose slightly. As water got deeper and the shoreline was farther away, the influx of sediment decreases. The water cleared up enough for shell-secreting invertebrates to flourish. The calcium carbonate shells they formed are the raw materials making up the limestones we see at the top of the sequence. This sequence recurred hundreds of times here in Kansas during the Pennsylvanian and early Permian (from about 320 to 275 million years ago), creating the repetitive sequence of limestones and shales that make up the Flint Hills and Osage Cuestas.
Stop 1 to Stop 2
From Stop 1, we backtrack south on North State Street to Eureka and turn left (east) on 7th Street. We take 7th Street for 0.4 miles to Jefferson Street and turn right (south) for one-half mile to U.S. Highway 54 or River Street. We turn left (east) on Highway 54 and continue east for 31 miles to Yates Center and the junction with U.S. Highway 75. We take Highway 75 to the south for about 13 miles until we reach the small town of Buffalo.
As we travel east through Greenwood and Woodson counties to Yates Center, we remain in the Osage Cuestas physiographic region. On the way to Buffalo, the highway makes a jog and passes over Rose Dome--a broad, gentle uplift formed by igneous intrusion below the surface. This has brought limestone in the Stanton Formation to the surface, and it is quarried near the highway. The Stanton can be traced northward to the Kansas City area. In this quarry, gentle dips can be seen in the limestone beds, reflecting the arching of the bedrock.
When we reach Buffalo, we turn right (west) at the Micro-Lite sign and proceed through town on Micro-Lite Street. The Micro-Lite plant is located on the west edge of town, along the railroad tracks.
From the Micro-Lite plant, we head west, passing clay pits in the Weston Shale Member of the Stranger Formation, which provided raw material for a brick plant that once operated in Buffalo. After about 4 miles, we turn right (north), passing the Wildcat Ranch. Proceed 1.25 miles and then turn left (west) and go 0.5 mile and turn right (north) at the entrance to the mine and proceed about 0.75 miles to the loading area, where we will park the bus and then walk into the quarry area (Stop 2).
This guidebook is also available in print form as Kansas Geological Survey, Open-file Report 2001-41, from KGS Publications Sales office, 785-864-3965.
Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey. Text by Jim McCauley, Liz Brosius, Rex Buchanan, and Bob Sawin, Kansas Geological Survey.
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