The writer first encountered rocks from the Manson Impact Structure in 1965 when I stopped off in Iowa City, IA enroute to my field trip to the West Hawk Lake structure which I was about to study as part of an NSF grant that took me to Ottawa, Canada afterwards to collect samples from drill core into the structure's breccia deposits. While in Iowa City I met with Dr. Richard Hoppin who had first proposed Manson as an impact crater based on samples obtained from the structure as part of a deep well drilling program for groundwater recovery. Upon seeing thin sections of Manson breccias I immediately confirmed that those rocks had undergone high pressure shock wave damage. I published this information a year later in an article entitled "Shock Processes in Geology". Although Prof. Hoppin is credited with "discovering" Manson's apparent identity as an impact crater, I was (years later) credited at a Meteoritical Society meeting as the person who first described the shock effects.
Manson is still the second largest (31.5 km diameter) impact structure in the 48 United States. Interest in it was rekindled after publication of an age of formation near 65 million years. Thus it could have been the prime candidate for the impact event postulated to have killed the dinosaurs. (A more exact age of 74 million years, obtained later, disproved that hypothesis which was good in that the crater seemed too small to have had a worldwide effect.) Because of this interest, a consortium of investigators was formed in the 1990s to renew study of Manson. I was contacted to urge my participation. At that stage I had recently retired from my brief stint at Bloomsburg University, so I had abundant free time. I borrowed a petrograph microscope and Universal Stage from a colleague at NASA Goddard, set them up in my basement, acquired samples from Ray Anderson of the Iowa Geological Survey, had thin sections made (using a small grant from the Barringer Foundation), and spent many exciting hours doing a complete analysis of those rocks from a purely petrographic approach.
So, about 33 years after my last work on shocked rocks (the lunar samples in 1970), I conducted one more study of these fascinating phenomena. This was summarized in a paper, "Petrography of Shocked Rocks from the central peak at the Manson impact structure", published in 1996 with other Manson papers in Geological Society of America Special Paper 302 entitled "The Manson Impact Strucure: the Anatomy of an Impact Crater". Some of the main results I reported are restated below.
At the time of impact, Iowa was covered by Paleozoic rocks, as shown here:
Recall from page 18-2 that Manson has no surface expression since the sedimentary bedrock shown in the above map is entirely covered by masking glacial drift. As a reminder of the geology and morphology of Manson itself, here is a variation of the cross-section shown near the bottom of page 18-2:
The first (1953) drill hole, 2-A, went to a depth of 145 meters. A second drill hole (M-1), which in 1992 went down to 210 meters, penetrated into another part of the Manson structure; this also recovered core. Most of the core consists of breccias made up of clasts of Proterozoic arkoses, siltstones, and shales which filled a late Precambrian graben and of biotite granites, granodiorite, and gneiss that made up the crystalline basement rocks at impact time; in hole 2-A the lower samples are from blocks of basement rock carried up as the central peak. Occasional fragments of sandstone and shales from the thin supracrustal sedimentary cover were also noted.
Planar features are pervasive through most of the quartz-bearing rocks, attesting to the huge energy release from the impact (in millions of megatons). In the four photomicrographs shown next, these PDFs are evident. The quartz grains in the lower right are "toasted"; this refers to a condition that results in an optical appearance much like the color of white bread toast.
These planar features are mostly oriented along the so-called omega crystallographic plane which makes an angle of 23 degrees with the c-axis of quartz.
The next set of six photomicrographs show planar features with various orientations within the several feldspar minerals found in Manson samples.
Structural shock effects in the feldspars: A. Albite twins with alternate pairs showing brown alteration; B. Microcline, with twin units bent and offset by microfaults; C. Single set of PDFs in albite twins; poikolitic quartz (darker gray oval) also has PDFs; D. Bent twins containing PDFs; E. Diagonal (en echelon) PDFs in altered alternate twins; F. Deformation bands with single set of PDFs in a K-feldspar crystal.
The feldspars show other distinctive shock features, shown in these six photomicrographs:
For the above six: A. Flow bands in a plagioclase crystal that appears to have experienced intracrystalline melting; several quartz crystals have embayed boundaries; B. Recrystallized feldspar on left and quartz (right); C. Elongate feldspar microcrystals developed by recrystallization of partially isotropized single feldspar crystal; D. Clusters of feldspar with spherulitic texture; E. Spherules of recrystallized feldspar with internal radiate texture; F. Reaction rim "coronas" of clinopyroxene and plagioclase around quartz grains in a glassy (devitrified) matrix.
The next six photomicrographs show various shock effects in other minerals and in the matrix:
For the above six photomicrographs: A. Lenticular kink bands in muscovite; B. Brown hornblende with some type of lamellae (shock-induced); crystal has abnormally low birefringence; C. Titanite (dark) with thin PDF-like bands (shock-induced mechanical twinning?); D. Three sets of cleavage traces in elongate apatite crystals; E. Flow-banded glass selvage within fragmental matrix; F. The lower part consists of a clast composed on apparently shock-lithified quartz, feldspar, and other mineral fragments held in a dark matrix.
The writer attempted to produce another shock log (different from the West Hawk Lake log shown on page 18-2) for drill hole M-1 samples. It is shown here without its corresponding legend.
The Manson study, synopsized here, proved an exciting adventure for the writer after a 34 year hiatus from working directly on shock metamorphic signatures in impactites. Manson proved to be the equal of the Ries and Canadian impact structures in petrographic phenomena.
Primary Author: Nicholas M. Short, Sr.