[The nomenclature and classification of the geologic units described in this report follow the usage of the State Geological Survey of Kansas. They differ somewhat from usage adopted by the U.S. Geological Survey.]
Rocks which occur in the subsurface, but do not crop out in Brown County, range from Precambrian to Pennsylvanian (Virgilian) in age. Much of the following discussion of these buried rock units is from a report on the Forest City Basin by Lee (1943).
The Precambrian rocks which underlie Brown County are believed to be principally granite. The depth to the upper surface of the Precambrian ranges from 2,500 to 3,000 feet below sea level. The Precambrian rocks occupy an asymmetrical synclinal trough, the west flank of which rises sharply in the direction of the Nemaha Anticline. East of the synclinal axis the beds are less steeply dipping. Only one well in Brown County (sec. 8, T 1 S, R 15 E) has reached the Precambrian, at a depth of 4,016 feet.
The Precambrian rocks in Brown County are overlain by a succession of Paleozoic rocks. The Lamotte Sandstone of Late Cambrian age is the oldest of these rocks and is probably present in the part of the synclinal basin extending through the central part of the area. Rocks of the Arbuckle Group of Late Cambrian and Early Ordovician age overlie the Lamotte, where present, and the Precambrian in other areas. Few wells have penetrated these rocks and little is known of their thickness and lithology in the county; however, they probably underlie all of the area. The St. Peter Sandstone of Middle Ordovician age unconformably overlies Arbuckle rocks and in turn is unconformably overlain by the Middle Ordovician Viola (Kimmswick) Limestone and the Upper Ordovician Maquoketa (Sylvan) Shale. In a well in sec. 24, T 4 S, R 16 E, 265 feet of Viola and Maquoketa were penetrated.
Rocks of Silurian age rest unconformably on older rocks in the county. The Silurian rocks consist principally of dolomite in this area, but locally some limestone is interbedded with the dolomite. A thickness of 263 feet of Silurian rocks was present in the well in sec. 24, T 4 S, R 16 E, but these rocks thin toward the southeast as indicated by a log of a well in sec. 17, T 6 S, R 20 E, where the Silurian is absent. Rocks of Devonian age unconformably overlie Silurian rocks in Brown County. The Devonian rocks are predominantly dolomites. In some areas hundreds of feet of older rocks were removed by erosion prior to the deposition of Devonian beds. In the well in sec. 24, T 4 S, R 16 E, 163 feet of Devonian rocks were penetrated; however, in the well in sec. 17, T 6 S, R 20 E, where Silurian rocks were absent, more than 300 feet of Devonian rocks were present.
The Chattanooga Shale of Late Devonian and Early Mississippian age is about 231 feet thick in Brown County. This shale thins toward the southeast and is overlain by Mississippian rocks. The well in sec. 24, T 4 S, R 16 E, penetrated 282 feet of Mississippian; however, the Mississippian rocks do not exceed 100 feet in thickness in the northwestern part of the county and are locally absent over the Nemaha Anticline a few miles west of Brown County.
Rocks of the Cherokee Group of the Pennsylvanian System are about 650 feet thick in Brown County. In this area the Cherokee is composed principally of gray shale, although some black shale is present. The sandstone commonly present in the Cherokee in eastern and southeastern Kansas occurs sparsely in the county. The Cherokee Group thins toward the south and west and is absent in local areas in adjacent Nemaha County where it was never deposited over local highs on the Nemaha Anticline. In Brown County this unit is conformably overlain by rocks of the Marmaton and Pleasanton groups which have a combined thickness of about 200 to 250 feet. In this area the contact between the Marmaton and the Pleasanton is not distinct.
Rocks of the Kansas City Group overlie the Pleasanton rocks in the county. These, together with the overlying Lansing Group rocks, are composed of alternating beds of limestone and shale and some sandstone, and although the individual beds range considerably in thickness, the aggregate thickness of the rocks is 325 to 350 feet.
The Stanton Limestone of the Lansing Group is overlain by 130 to 195 feet of shale, sandstone, sandy shale, and minor amounts of limestone of the Douglas Group. In local areas in northeastern Kansas, the Weston Shale and the Iatan Limestone members of the Stranger Formation of the Douglas Group are present. These units, which formerly comprised the Pedee Group, have been entirely removed in parts of the area, and the Tonganoxie Sandstone Member of the Stranger Formation rests unconformably on the Stanton Limestone. West of the outcrop area along the Missouri River, the Weston Shale Member thins rapidly. In Brown County the Weston is probably no more than a few feet thick and may be absent locally. The Iatan Limestone is also absent locally. The thick sandstone beds commonly found in the Douglas Group in the area south of the Kansas River are not present in Brown County; however, sandy shale beds are common in the upper part.
The Lawrence Formation of the Douglas Group is overlain by a sequence of four limestone formations and three shale formations that comprise the Shawnee Group. In Brown County this group has a nearly constant thickness of about 300 feet. The individual beds of the Shawnee Group are comparatively uniform in thickness and lithology and are traceable over considerable distances. The upper and lower contacts of the Shawnee Group appear to be conformable with beds above and below.
Kansas Geological Survey, Geology
Placed on web May 29, 2009; originally published May 1967.
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