The Calvin 28 cryptoexplosive disturbance is an isolated, nearly circular subsurface structure of Late Ordovician age in southwestern Michigan. The structure is defined by 107 wells, is about 7.24 km in diameter and consists of a central dome, an annular depression and an encircling anticlinal rim. Seismic and geophysical well log data confirm that an intricate system of faults and structural derangement exists within the structure. Deformation decreases with depth and distance from the structure. U.S.G.S. topographic maps and aerial imagery show the structure is reflected as a subtle surface topographic rise controlling local drainage. Igneous or diapiric intrusion and solution collapse are rejected as possible origins for Calvin 28 on the basis of stratigraphic, structural and geophysical evidence. A volcanic origin is inconsistent with calculated energy requirements and an absence of igneous material. Although shock-metamorphic features are unidentified, microbreccias occur in deep wells that penetrate the structure. Morphology and structural parameters support an impact origin

Milstein, Randall L.

English

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N09 - 2 1  3 5  8 
THE CALVIN 28 CRYPTOEXPLOSIVE DISTURBANCE, CASS COUNTY, 
MICHIGAN: EVIDENCE FOR IMPACT ORIGIN. Randall L. Milstein, Michigan Geological Sumy,  Michigan Depart- 
ment of Natural Resources, Lansing, MI. 48909. 
. The Calvin 28 cryptoexplosive disturbance is an isplated, near1 circular subsurface structure of Late Ordovician age in 
southwestern Michigan. The structure is defined by 107 wells, is about 7.24;m in diameter and consists of a central dome, an annular 
depression and an encirclin anticlinal rim. Seismic and geophysical well log data confirm that an intricate system of faults and structural 
derangement exists wthin tfe structure. Deformation decreases wth  depth and distance from the structure. U.S.G.S. topographic maps 
and aerial ima e y  show the structure is reflected as a subtle surface to 
trusion and soktion collapsf are rejected as possible origins for Calv ing  on the basis of stratigraphic, structural and eophysical 
evidence. A volcanic origin is inconsistent w th  calculated ene requirements and an absence of igneous material. *!though shock- 
metamorphic features are unidentified, microbreccias occur in%ep wells that penetrate the structure. Morphology and structural 
parameters support an impact ongin. 
I".Geoph ical data, geologic mappin and drilling have delineated a subsurface structure in Calvin Township Cass 
County, Michigan, centere21.4km southwest of the vilkge of Calvin Center. The structure consists of a central domal uplift bohnded by 
an annular depression and an encircling anticline. The structure has a diameter of 7.24km. Surface topogra hy is gently rollin 
rain with 30 to 133m of drift. The underlying Paleozoic strata are 1333m in thickness and di northeashvanfat 5 to llm/km. %ree 
Devonian oil fields are associated with this structure. Two are located in conjunction with t i e  anticlinal rim and one in conjunction with 
the central uplift. 
their equivalents in the annular depression. The rim has a maxlmum width of 15km. 
In the Smith XI-20 test well rock units below the Middle Silurian Clinton have anomalous thickness. For example the local thickness of 
the St. Peter Sandstone is about 7m: the St. Peter in the Smith #l-20 ispver 172m thick. The Ordovician Praine du Chien Group is ab- 
sent and the Cambnan Trempealeau Formation is greatly reduced in thickness. 
ANNULQR: An inner annular depression about lkm wide se arates the outer rim zone from the central uplift. Devonian 
strata lie 28m bel? their regional level and 41m below equivalent,strata in t%e central uplift. Within the annular de ression, no test 
wells have been dnlled to targets older than Devonian age. Seismic data confirm the presence of the depression at &pth. 
CENTRALUPLIFT: The extent of structural uplift exhibited by the central dome is 415.5m. Geophysical well logs, and well cuttings 
show the absence or anomalous thickness of many regionally distinct strati 
ample, in the Hawkes-Adams X1-28 test well a thinned section of the Latef%dovician Cincinnatian Senes rests directly on the Late 
Cambrian Trempealeau. The Lawson X1 test wel1,while shoulng a full complement of regional strata, exhibits extreme variations in their 
thicknesses, especially the Middle and Late Ordovician sequences. 
Geophysical well logs show both well bores to be intersected by %vera1 faults. Di meter readin 
dips throughout the disrupted sectign,,with readings as,high as 78 . In the lower &Om of the wepbore, the dip decreases, from top to 
bottom, from near 70 to roughly 5 wth  a persistent dip to the northeast, suggesting a waning of deformation at depth. 
Calvin 
=urate descriptive logs and welysamples, correlation between subsurface points is quite reliazle. 
Data indicate rocks of Late Cincinnatian age are present in all control points both on and off structure. Control points on structure dis- 
Correlatin away from the structure, Early and Middle Cincinnatian lithology 
geconies evident. kocks of the Middle Ordovician age, &renton and%lack River Groups) are involved in the deformation and appear 
faulted and abnormally thick on the flank of the central uplift, and in the per! heral anticline. These rocks were deposi,ted,brior to the 
structure's formation. The aF,of, the event responsible for the formation of &Ivin 28 has been placed prior to Ear y Silunan time, but 
after deposition of the Early incinnatian age rocks of the Utica Shale. 
v e v o n i a n  were not cored in these wells, the search for shock-related deformation features was 
limited to thin sections prepared from well cuttings. 
The examinations showed no evidence of high pressure, high strain-rate or hi h temperature shock effects, however the limited 
availability and small size of samples for insFction was a ma'or restriction. ff thorough microscopic search for shock metamorphism 
would have to involve cores and the less restncted use of wed cuttings. 
-. A microbreccia was identified in thin sections of rocks of Late Ordovician through Late Cambrian a e from deep 
test wells drilled into the structure. The breccia is composed of both fractured and unfractured, subrounded to rounded ioating quartz 
grains imbedded in a carbonate matrix. 
The occurrence of the microbreccia at different stratigraphic intervals and at multiple locations about the, structure, suggests that it is not 
the result of uphole cavings. In addition, des ite the small pofion of well cuttings used to make each thin section, the microbreccia is 
apparent in each thin section. The lithology o? the microbreccia is the same regardless of the unit in which it occurs and contrasts 
markedly with the normal litholo of the units. The variation of quartz grain mo,rphology, in combination with the carbonate matrix, 
suggests the need for distinctly dfirerent depositional envlronments if the breccia IS to be attributed to normal sedimentary processes. 
The characteristics of the microbreccia are consistent with those noted in macro and micro breccias associated with cryptoexplosive struc- 
tures and impact craters (1). 
. The energy re uired to form the Calvin 28 structure can be estimated by usin the empirical scalin law, 
D IcfKnE"1 3.4, derived from the effects 3 nuclear e losions on sedimentary targets (2). The calculate%ener required to k r m  the 
S t r u c t u F  is 1x lO"26 ergs. %bile this value exceeds enerqestimates for known s i n g g r  explosive en- 
dogenetic events, it would be considered a conservative value for energy released by a ypervelocity impact (2). 
f%%%%%attributable to hydrothermal or known volcanic processes has not been recognized in well samples. Microbreccia as- 
sociated with the structure contains no volcanic material. If igneous material had been resent at the site, even in small amounts, it 
would be difficult to explain it's absence by weathering processes. The absence of signitcant carbonate and evaporite deposits in the 
Cambrian and Ordovician rocks underlying the structure, effectivelyeliminates a solution subsidence-collapse origin. Seismic profiles in- 
dicate no intrusive body beneath the structure. A geophysical investigation by Ghatge (3) showed no grawty or magnetic anomalies to 
one milligal or gamma associated wth  the structure. 
Structures exhibiting similar characteristics to Calvin 28 are limited to impact craters and c toexplosion structures. The Calvin 28 Struc- 
ture exhibits many of the megascopic features of the c toexplosion structure of Dietz (4)%ost significant are the structure's size, sub- 
circular nature, as well as its intense, locallzed s t r a t ig rq i c  deformation. Lastly, this feature has no obvious relation to known volcanic 
or tectonic activity. 
Throu htout the region surrounding the Michigan Basin a number of cryptoe losion structures have been identified in the surface and 
subsurface. Cptoexplosives located in Kentland, Indiana, Glasford, Illinois,%ynn Creek, Tennessee and Rock Elm, Wisconsin have 
been corn are to Ca mn 28 based on specific characteristics (5  and 6). Results of the comparison indicate that similarities in structure 
and morpRology exist between the features. 
graphic rise controlling local drainage. Igneous or diapiric in- 
lacial ter- 
. Middle and Late Devonian formations in the outer rim are 15 to 9m higher than 
phic units in wells drilled into the central uplift. For ex- 
taken in the Lawson #1 show random 
. Cincinnatian rocks of the Late Ordovician a e (Richmond Group) are involved in the deformation, so the  
structure is clearly older than Earl Silurian. Lithologic units witkin the Cincinnatian are readil correlatable and when com- 
lay a lack of Earl and Middle Cincinnatian stratigraph 
. A microscopic investi ation for shock-metamorphic features was done on samples from three 
OF P Q S S I B L E .  No volcanic material has been identified in association with the Calvin 28 structure. 
ORIG4NAL PAGE IS 
OF POOR QUALITY 
https://ntrs.nasa.gov/search.jsp?R=19890011987 2020-03-20T02:43:27+00:00Z
OR!UNAL b;:GE 5s 
THE CALVIN 28 CRYPTOEXPLOSIVE DISLZTRBANCE 
Randall L. Milstein 
OF POOR QUALrn 
123 
Structures now identified as terrestrial impact Scars appear in two forms simple and complex craters (7). The board characteristics for 
complex crater forms compare favorably wth features identified in the Calvin 28 structure. 
Pike (8) and Grieve and others (7) state that impact craters exhibit specific characteristics and that these can be calculated based on 
relationships between the craters observable depth, diameter and StNCtUral uplift. Grieve and others (6) suggest a final complex crater 
form will exhibit an ap arent diameter (Da), which can be considered ap roximately e uivalent to the observed distance from rim to rim, 
a true depth (dt , whrct can be determined only by extensive drilling, anfan amount o? structural uplift (SU), calculated by the 
measurable up& of the deepest in place marker horizon. 
Only three deep test wells have been drilled into the disrupted sections of Calvin 28 and no reliable estimate of dt is available. Based on 
measurments made from structure contour maps, the Da of Calvin 28 is estimated a 7.24km. This estimate may be considered the maxi- 
mum, observable value for Da. Geophysical well logs gm a minimum estimate of S U  for Calvin 28 of 415Sm. This is based on compari- 
tive measurments of the lowest, observable in lace marker bed, the Cambnan Mt. Simon Sandstone, between the on structure 
Hawkes-Adams X1-28 and the off structure d d e n  Y1. 
B studyin the relationships between strati S 6  = 0.06ba**l.l. B solving for SU and ESt'h" the observed vayues, a calculated SU of &9m and a calculated Da of 6.93km are 
found. The author be6eves minor disagreements between calculated results and observed values are acceptable 'ven the maximum and 
minimum limitations placed on the observed values. Calvin 28 is considered to exhibit a recognized relationshiptetween stratigraphic 
displacement in the central uplift and its present form and dimension. 
While the identification of shock metamo hic features. a macrobreccia, or chemical anomalies would lend stron er support to an im act 
origin, seven characteristics of the Calvin % structure stron ly su port origin by impact: 1. The structure is circukr, with a central upfft, 
surrounding annular de ression and a peripheral anticline. 5. Cahn 28 is an isolated structure involving intense, large-scale deformation 
in otherwise flat-IyingsLta. 3. Deformation decreases with de th beneath and distance awa from the structure. 4. Calvin 28 exhib!ts a 
recognized relationship between strati 
neous material is associated with the sRcture. ?The event responsible for the structure's ongm is estimated to have released at least 1 
x 10**26 ergs of energy, wthout the development of magma. 
CONCLUSION. Comparison of the Calvin 28 cryptoexplosive disturbance with known endogenetic structures shows a notable lack of 
analogs. A yet unidentified endogenetic mechanism may be responsible for its origin, but the available evidence makes this unlike1 
Comparison of Calvin 28 to structures of known or suspected exogenetic origin suggest consistent structural and ph ical analogs. khile 
a considerable bod of interpretive data favors an impact ori 'n for the structure, ~ f i c  physical data indicative ocmpact cratering 
events is not availagle. Based on the available data it is conctded that the Calvin 
surface, high energy shock event, and that the event can best be attributed to hypervelocity impact. 
., and k F .  dolfe, 1982, in The satellites of Jupiter (Morrison, @$, Univ. of Arizona fk%,M277-339. 3 G?it e 
t 
uplift and the a parent diameter of acce ted terrestrial complex craters, (7) find, 
phic dis lacement in tl!e up!ift and crater form. 5. '&e ycurence of a microbreccia. 6. No ig- 
cryptoexplosive disturbance is the result of a near- 
ono Book Co p 1-17. 
L., 1984,,A geo hysical investigation of a possible astrobleme in southwestern Michigan, un ub. M.S. thesis, qestern Mickgan d i v . ,  
41p. 4) Dietz, Rg., 1959 Journal of Geolo toexplosive dis- 
turbance Cass Coun Gichi an, pub. Mpihdsis, 8niv. of Northern Colorad;, 87 . 6 Klstein, RL., 1988 Re . rt Xnvesti ation 28: 
Im act okigin of the &lvm 2#cy!oe$xive distueance Cass Count , Michigan, Rick Geo. Survey 38p. 'I> &eve, RA.E b.B, 
Rogertson and M.R Dence, 198 u ulti-ring Basins, L.P.S. Conf., d h ,  (Schultz and Merrill, &.), fiergamon Press, p.37-51.8) Pike, 
R.E., 1980, U.S.G.S.P.P.- 1046C, h p .  
1) French B.M. 1968 in Shock metamorphism of natural materials French and Short, 
v 67, 4F505. 5 )  Milsteig R L  1986, Im cporigin of the Calm 28 c 
GENERAL CHARACTERISIICS - CALVIN 28 CRYPTOEXPLOSIVE DISKJRBANCE 
Subsurface Structure Microbreccia. Yes Circular Shape 
Crater Diameter 7.24km Volcanic Material. None Identified Central Domal Uplift 
Structural Uplift. 415.5m Mineralization. None Identified Encircling Annular Depression 
Bedding Dips. 5' - 78' Microstructures. None Identified Anticlinal Rim 
Estimated Age. Late Ordovician Gravity Anomaly. None Intense Faulting 
Deformation Decreases with Depth Magnetic Anomaly. None Anomalous Bedding Thicknesses 


The Calvin 28 cryptoexplosive disturbance, Cass County, Michigan: Evidence for impact origin

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