Guidebook—Geology of Northeastern Kansas
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Stop1—Clinton Lake Spillway, continued
Although each rock layer here is only a few feet thick, these formations extend vast distances across Kansas and adjacent states. Geologists have given each of these rock layers a name, based on the location where it was first described (see stratigraphic column, below). The limestones and shales here are part of two formations, the Oread Limestone and the Lawrence Formation. The Oread Limestone is named after nearby Mount Oread, home of the University of Kansas, where the formation was first described by geologist Erasmus Haworth in 1894. In spite of its name, the Oread Limestone, like many other formations, contains rocks other than limestones. The Lawrence Formation, which lies below the Oread (and is thus older), is named for the city of Lawrence.
Stratigraphic classification of Upper Pennsylvanian rocks in Kansas (adapted from Zeller, 1968).
Geologists have divided the rock layers in the Oread and the Lawrence down into smaller units called members, much the way that biologists divide a genus into smaller groups called species. The rock layer at the bottom of the spillway is called the Amazonia Limestone Member, named for the small town of Amazonia, north of St. Joseph in Andrew County, Missouri. Above the Amazonia are shales that are also part of the Lawrence Formation. This part of the Lawrence Formation includes a thin coal layer called the Williamsburg coal bed, named for the small town of Williamsburg, Kansas. This coal layer is barely visible at the spillway, but it is exposed in a roadcut south of the dam. This part of the Lawrence Formation may have been deposited on the plain of an ancient delta, where local swamps had formed.
Limestone and shale units in the Lawrence and Oread Formations at the Clinton Lake Spillway.
Above the Lawrence Formation is the Toronto Limestone Member of the Oread Limestone. The Toronto is a thick, light-gray to brown limestone that contains a few fossils. Above the Toronto is the Snyderville Shale Member, which is gray, and above that is the Leavenworth Limestone. The Leavenworth is a thin layer of gray to brown limestone that was first described from exposures near the town of Leavenworth.
The next unit up is the Heebner Shale Member, one of the most recognizable rock layers in the spillway. The Heebner is very different from the Snyderville Shale and shales in the Lawrence Formation. The Heebner is very dark-black, in fact. It is fragile and thinly bedded, deposited in layers so thin that they resemble pages in a book. In places, the Heebner contains phosphate nodules, marble-sized rocks that are high in phosphate and are slightly radioactive. Geologists disagree about the conditions under which the Heebner was deposited. Some regard it as the kind of rock that was deposited where the Pennsylvanian sea was its deepest. Other geologists are less sure.
Closer view of the rock units exposed in the spillway at Clinton Lake.
Above the Heebner Shale is the Plattsmouth Limestone Member of the Oread Limestone. The Plattsmouth is a wavy-bedded, gray to brown limestone. In places it contains chert (or flint) and fossils such as crinoids, corals, or single-celled, wheat-grain-shaped animals called fusulinids.
One obvious characteristic of these rock layers, as they are exposed here at the spillway, is their cyclicity. That is, the rocks here are deposited in a regular vertical sequence of limestone, shale, limestone, shale, etc. This sequence of rocks, which geologists call a "cyclothem," is found not only in the Oread Limestone and Lawrence Formation, but is repeated in Pennsylvanian rocks above and below these formations. Geologists believe that this regular sequence of deposition is probably the result of fluctuations in sea level. As the sea levels deepened (in the range of tens of feet deep), limestone was deposited. As the seas shallowed, shale was deposited (except, perhaps, in the case of the Heebner, which some geologists believe represents the time when the ocean was at its deepest).
Geologists disagree, however, about the reasons for the change in those Pennsylvanian sea levels. The changing sea levels may represent times when polar ice caps were shrinking or growing. When the ice caps melted, sea levels rose, resulting in limestone deposition. At other times, when ice caps grew, sea levels went down and shale was deposited. There are other theories about the reasons behind the sea level change. Still, it is possible to look at the rock layers in the spillway as hard evidence of sea-level fluctuations in this long-ago ocean.
Directions to Stop 1
From the junction of Clinton Parkway (23rd Street) and Kansas Highway 10 on the west edge of Lawrence, drive west a short distance to Douglas County Road 458 (this is the road that crosses Clinton Lake Dam). Turn south (left). The spillway is about 1,000 feet from the intersection, but there is a small parking area on the east side of the road just 300 feet south. This parking area allows safer access to the bike path that leads to the spillway. Follow the bike path that leads into the spillway and provides access to the rocks exposed on the north side.
This guidebook is also available in print form as Kansas Geological Survey, Open-file Report 2000-55, 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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