THE MANSON IMPACT STRUCTURE: A LATE CRETACEOUS METEOR CRATER IN THE IOWA SUBSURFACE
compiled by Raymond R. Anderson
Introduction
The Manson Impact Structure, named for the town of Manson near the center of the structure in north-central Iowa, has a diameter of 37 km (23 miles) making it the largest intact, on-shore meteorite crater in the United States. The structure is present at the bedrock surface, but is buried beneath 20 to 90 m (70 to 300 feet) of glacial till, and there is no apparent surface expression of the structure. The crater was probably produced in the Late Cretaceous (about 74 million years ago) by the impact of a stoney meteorite about 1.5 miles (2 km) in diameter.
History of Manson Investigations
The area of the Manson Structure has been known as a region of anomalous geology since 1912, when samples collected during the drilling of a town water well at Manson proved to be unlike other rocks in the area. The nature of the anomalous geology remained a mystery until 1953, when cooperative drilling of two research cores by the Iowa Geological Survey-U.S. Geological Survey led to its interpretation as a cryptovolcanic feature (a crater created by a blast of volcanic gas). In 1966, evidence was discovered proving the Manson Structure to be of meteor impact origin.
Recent Investigations of the Manson Structure
In 1991 and 1992 a second joint Iowa Geological Survey Bureau-U.S. Geological Survey investigation of the Manson Impact Structure included the drilling of 12 research cores totalling over 4000 feet (1200 m) of impact rocks. These drill holes were primarily located along an east-west radius of the structure (the line of a seismic profile). Study of the cores and other data showed the Manson Impact Structure to be a very well-preserved complex impact structure, with a large central peak, an outer ring of down-dropped strata known as the terrace terrane, and an intermediate crater moat region ( cross-section). Investigation of the drill cores disclosed the presence of six primary types of impact rocks. These impact rocks include:
- Brecciated crystalline rocks (core photo 1 & 2)--large blocks of brecciated basement rocks (granites and gneisses) that form the core of the central uplift (photomicrographs 1--2--3--4).
- Crystalline-clast breccia with sandy matrix (core photo)--smaller brecciated blocks of crystalline basement rocks in a matrix of sand- to silt-size fragments of these rocks (photomicrograph 1 ).
- Crystalline-clast breccia with melt-rock matrix (core photo)--smaller fragments of crystalline basement rocks in a matrix dominated by finely powdered crystalline rocks and impact melt-rocks (photomicrographs 1--- 2-- 3).
- Keweenawan shale-clast breccia (core photo)--a breccia dominated by large to small blocks of black Precambrian (Keweenawan) age shale and melt-rock in a matrix of black shale and melt-rock grains.
- Overturned ejecta flap--an overturned sequence of clastic sedimentary rocks ranging in age from Precambrian (Keweenawan) through Devonian.
- Phanerozoic-clast breccia (core
photo)--a breccia dominated by Cretaceous and
Paleozoic clasts in a shale matrix (photomicrographs
1--2--3).
The first four of these impact rocks types were found only on the central peak, and the overturned ejecta only on the terrace terrane, but the Phanerozoic-clast breccia was the uppermost unit in all terranes of the crater.
The Geological Society of America will be publishing a compilation of recent Manson research papers in the Spring of 1996. This 484 page compilation, GSA Special Paper 302 , The Manson Impact Structure, Iowa: Anatomy of an Impact Crater, edited by C. Koeberl and R.R. Anderson, will include 22 research papers and will be available from the Geological Society of America, Boulder, Colorado.
For more information contact Ray Anderson at (319)335-1575
or (e-mail: Raymond.Anderson@dnr.iowa.gov)