ML20049J365

Review of Weston Geophysical Rept on Bedrock Structure in Vicinity of Midland Nuclear Plant

ML20049J365
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Site:	Midland
Issue date:	03/01/1982
From:	Jennifer Fisher MICHIGAN STATE UNIV., EAST LANSING, MI
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EICLOSURE 2 REVIEW 0F WESTON GEOPHYSICAL'S REPORT (FEBRUARY 1982) ON THE BEDROCK STRUCTURE IN THE VICINITY OF THE MIDLAND NUCLEAR PLANT by James H. Fisher flarch 1,1982 Dohgo!$oojj, 4

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LIST OF FIGURES Page 1.

Normal faul t g rad i ng i nto a monocl ine...........................

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Basement structure cross-section......................

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Pal eogeol ogic ma p o f Il l i noi s...................................

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Traverse structure map in the Midland vicinity..................

7 5.

Traverse structure in the Howell and Albinn-Scipio area........

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JAMES H. FISHER

[

GEotooy - MICH STATE UNIV-East LANSING, MICHIGAN 48824 AREA CODE 517 353 4366 REVIEW OF WESTON GEOPHYSICAL'S REPORT (FEBRUARY,1982)ON THE BEDROCK STRUCTURE IN THE VICINITY OF THE MIDLAND NUCLEAR PLANT Weston Geophysical Corporation is to be commended for this study of the bedrock in I

the vicinity of the Midland Plant of Consumers Power Company. It is apparent that they have made a thorough review of the literature on the Michigan Basin. The acquisition of 1

l well data is likewise very complete, including not only wells drilled for petroleum but also brine wells, injection wells, and coal core holes. Their interpretation of the data is valid as one possible explanation of the structural events in this area. However, it is axiomatic that no two geologists using the same set of data points will contour an area in exactly the same f ashion. The determination of f aults in a subsurface study is highly subjective. If the feature sketched in Figure I were buried under 3000 feet of sediment in an area where well control was limited, one geologist might place a f ault along the entire iength of the feature; another might fault part of it and leave the remainder as l

l monoclinal dip; still a third could consider the entire feature as a monocline with no l

l f aulting involved. In contouring, just how much of a kink must you have in the contour l

l pattern in order to draw in a f ault? It varies with the individual.

In the last ten years, many of the geologists working in the Michigan area have come to the conclusion the Precambrian basement is faulted in a series of horst and graben features (Figure 2) with a general NW-SE trend. The faults aopear to be mostly high angle normal f aults, although a few proven reverse f aults have been found. Vertical movements of these blocks have produced the fold structures utserved in the overlying sedimentary layers. Movement of these blocks has occurred intermittently throughout time; some shif ts occurred as early as Late Precambrian time; some changes took place during Ordovician or Silurian time but the major movement occurred in Late Mississippian time.

Af ter Late Mississippian time the Michigan Basin was for all practical purposes, tectonically dead.

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'Ihis is also true of the other major basins of the stable interior, the Williston and the Illinois Basins. The deformation of the Late Mississippian Chester beds of Illinois is clearly shown in the paleogeologic map (Figure 3) drawn at the major unconformity at the base of the Pennsylvanian sequence. The overlying Pennsylvanian beds dip gently toward the center of the basin where they attain a maximum thickness of 2500 feet. The same situation exists in the Williston Basin with Late Mississippian deformation of the Big Snowy group, followed by a major unconformity overlain by about 500 feet of gently dipping Pennsylvanian beds. The Michigan Basin, as a part of this arc of stable interior basins, has evolved in a similar fashion.

This brings us to the matter of the " lineaments" observed on LANDSAT imagery.

First let me state that I am not an expert on LANDSAT imagery but I have had discussions with experts in that field and they tell me that the use of LANDSAT imagery in areas of heavy vegetative cover or thick glacial drif t is highly speculative with regard to bedrock structure.

No one questions the use of LANDSAT to delineate glacial features in Michigan - the difficulties arise with the remaining " lineaments", which have no apparent relationship to glacial features, when the suggestion is made that they conform to bedrock f eatures, possibly f aults.

In my opinion there are two situations under which LANDSAT imagery could show iaults in Michigan:

-1.

Solutions moving upward along a bedrock fault cot!d work their way through thin glacial drift and alter the soil moisture regime in such a way as to produce an anomalous band of vegetation. Considering the thickness of the glacial drif t (200'+) in.the Midland area and the erratic nature of glacial deposits, this explanation seems unlikely.

2.

Bedrock faults sufficiently active at the present time to propagate upward through not only the Pennsylvanian beds but through the glacial sequence as well. Either the f aulting would have to be repetitive, or the lateral or vertical

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t displacement of a single movement would have to be of a magnitude sufficient i

to offset the effects of erosion and deposition in concealing the structure.

The Weston report (I-l and 1-2) cites the GeoSpectra map (Weston's Figure 2) which

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shows four northeast trending linear zones, one of which is marked " definite f ault", the

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others are identified as " anomalous dip". These lines are superimposed on a structure contour map drawn on the Traverse and are at an angle to the structure contours. In comparing GeoSpectra's map with my map on the Traverse (Figure 4)I find their contours t

generally comparable to mine, placing the Midland plant on the south flank of a broad syncline which trends roughly east-west. However I find nothing anomalous about the dips on either my map or GeoSpectra's. Anomalous dips, to me, are abrupt increases or i

decreases in dip shown by a bunching of contour lines or a widening of the regular spacing.

I do not see either of these phenomena on the GeoSpectra map and therefore,1 fail to understand these heavy lines cutting across contours and identified as " anomalous dips".

1 Secondly, I question the validity of the " definite f ault" shown in the northeast corner of 3

the map. I cannot justify it with the well control (petroleum tests)in that area and unless t

GeoSpectra has test hole or seismic information not generally available, with which they i.

l can support a fault in that location,I question its existence.

i j

Assuming that the " anomalous dips" shown on the GeoSpectra map are based on I

LANDSAT imagery, the following quotes from a 1975_ publication by Ben Drake i

(Environmental Research Institute of Michigan) and Robert K. Vincent (President of GeoSpectra) are of interest:

Page 945 i

"Many of the recambrian and Paleozoic structures known from drilling o

or geophysical studies are not indicated on the LANDSAT-1 mosaic.

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I Drake, Ben and Vincent, Robert K. (1975), Geologic Interpretation of LANDSAT-1 imagery of the Greater Part of the Michigan Basin; Proceedings of the Tenth International Symposium on Remote Sensing of Environment.

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8 Considering the thickness of the Phanerozoic rocks and the extensive glacial drif t,it is not surprising that indications of the Precambrian and Phanerozoic, lithologies, bedrock patterns, and structures in the Southern Peninsula generally are lacking on the mosaic. The f act that structures in the Paleozoic and Precambrian rocks cannot be seen or recognized on the mosaic does not cast any doubt on the presence of the structures. It only means that the structures either have no surf ace expression or are indicated at the surface in such a manner or scale that they connot be idenitified on the mosaic.

Among the Precambrian and Paleozoic structures that are not indicated on the mosaic are the Firidlay Arch, Algonquin Arch, and Chatham Sag (Brigham,1971); the Electric Fault in southwestern Ontario and trending E-W across the St. Clair River delta (Brigham,1971); the Peck Fault in St. Clair l

and Sanilac Counties (Brigham, 1971); and the Bowling Green Fault (Ells, 1

1969)."

Page 946 "We do not know the mechanism (s) by which the indication of a buried structure, possibly several thousand meters deep, can be propagated to the surface, especially through sedimentary rocks that have not been tectonically deformed. Investigation of the origin (s) of the features is continuing."

It is apparent from the above that GeoSpectra has reservations concerning the positive identification of these linear features as faults. I too would be concerned with LANDSAT imagery in the Michigan area that would f ail to define major faults such as the Electric and the Bowling Green f aults where there is a vertical throw of several hundred feet. According to Weston (I-14) vertical displacement on one f ault in the Midland area is about 150 feet, however, " vertical displacement generally is no more then a few tens of feet".

9

9 It should be noted that Weston, in their LANDSAT study of Michigan, considered the lineaments that were not related to glacial features (and therefore classed as unknown) to be only coincidentally related to bedrock structure (III-2).

Weston's LANDSAT study does not show (III-4) the Howell, Lucas-Monroe, Albion-Scipio structures.

Figure 5 shows the Traverse structures in the Howell and Albion-Scipio areas.

Most Michigan geologists, given a map of Michigan and asked to sketch in the major faults, would indicate faults (among other places) at Howell and Albion-Scipio. They would, however, be hard pressed to produce solid evidence to support faulting.

Consider the case of the Albion-Scipio field with approximately 900 wells on twenty acre spacing. This certainly gives a wealth of control, but contouring done on a five foot interval does not demonstrate faulting.

Cores taken in the field occasionally contain a fracture, but offsets are in the order of a few inches.

Every form of geophysics known to the industry has been run across this field without result.

Despite all of this, most geologists would argue that Albion-Scipio has to be developed along a fault.

A second exampie is the Howell anticline.

The differences in eleva-tion on the Traverse on the southwest limb of the anticline between closely spaced wells is in the order of 900 feet.

I ahve indicated a fault there.

The U. S. Geological Survey's geologic map of the United States shows a fault there.

The Michigan Survey's geologic map of Michigan does not show a fault along the Howell.

Ells (1969, p.75)I argues that the southwest flank of the Howell anticline is a steep monoclinal dip and that evidence for faulting-is lacking.

Thus it is apparent that the determination of fault locations in Michigan is a very subjective affair.

What is one man's fault is another man's monocline.

With regard to the ability of LANDSAT imagery to locate bedrock faults through a 200 foot cover of glacial drift, it is interesting I Ells, Garland D. (1969),. Architecture of the Michigan _ Basin:

Mich. Basin

- Geol. Soc., Ann. Field Guidebook.

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11 to note that in the locations previously cited, areas where most Michigan geologists would place faults--LANDSAT shows nothing.

This includes the Howell area where the vertical offset may be 900 feet and the glacial drift thickness 100 to 150 feet.

Weston in II-l states that the pre-Pennsylvanian folds and faults are not reflected in the overlying Pennsylvanian strata and that the structures found in the Pennsylvanian sequence are non-tectonic structures.

I am in agreement with this.

CONCLUSIONS 1.

I would discount the identification of bedrock features in the Midland arca by LANDSAT imagery.

2.

The Michigan Basin was tectonically inactive after Late Mississippian time.

Fold and fault structures of pre-Pennsylvanian strata are not seen in the Pennsylvanian sequence.

(Strutz,T.A.,

1978).I 3.

Weston's subsurface maps indicate numerous faults in the Midland area.

Considering the theory that the Precambrian basement in Mich-igan is faulted and in view of the change in structural grain of the Traverse in the Midland area from E-W to NW, it is possible to assume faulting of the magnitude shown by Weston.

However, I would stress that this is only one solution.

Perhaps other geologists, not informed of any LANDSAT determinations, would contour the area as the unfaulted margin of a syncline.

4.

In my opinion, the original trends in the Midland area identified by GeoSpectra as " anomalous dips" and by inference, as faults are not supported by sound geological evidence.

I Strutz, T. A. (1978), A Pre-Pennsylvanian Paleogeologic Study of Michigan unpublished M.S. thesis, Mich. St. Univ.