Bedrock Geology of Mackinac County, Michigan.

ABSTRACT

This study produced a series of maps showing the bedrock topography and geology of Mackinac County, Michigan, to assist in the analysis and development of groundwater supplies. It is based on data from 1,305 drill holes penetrating bedrock and is stored in the Statewide Groundwater Data Base. Shown are bedrock topography and a revision of bedrock geology that include structure and stratigraphic relationships of units from the Stonington Formation of Ordovician age to the Mackinac Breccia of Devonian age. Revisions include changes in formation locations. Potential applications of these maps include analyses of groundwater conditions, development of groundwater supplies, and identification of areas subject to contamination.

INTRODUCTION

Mackinac County, located in the southeastern part of the Upper Peninsula of Michigan (Figure 1), has 1,895 water wells registered in the groundwater database housed in the Chippewa County Environmental Health offices, a regional center for the eastern Upper Peninsula of Michigan. Of these wells, 1,470 intersect bedrock at an average depth of 44 feet. The greatest density of data is in the southeastern and northwestern parts of the county. Few wells were drilled into bedrock in the central portion of the county because of sparse human population and extensive glacial overburden. Water well log descriptions were interpreted based on lithologic descriptions given by the drillers. The data from 165 wells were discarded due to unclear lithologic descriptions. Therefore, information from 1,305 of the 1,470 holes that intersected bedrock (Figure 1) was used to generate the maps of this study using C-Map, Surfer, and Geobase. Our maps update the Mackinac County information on the 1987 northern Michigan bedrock geolo gic map (Reed and Daniels 1987).

We produced (1) two three-dimensional maps showing bedrock topography, surface topography, and approximate placement of the Niagara Escarpment (Figures 2 and 3); (2) a new geologic map (Figure 4) showing bedrock structure and stratigraphy for comparison with Reed and Daniels's 1987 bedrock geologic map (Figure 5); and (3) a set of three cross sections showing bedrock stratigraphy, structure, and topography (Figures 6-8).

BEDROCK TOPOGRAPHY

A three-dimensional map of the bedrock topography was created using the computer software programs C-Map and Surfer (Figure 2). Bedrock topography is dominated by a narrow zone of high relief extending generally east-west across the county. The abrupt change of elevation is consistent with the location of the Niagara Escarpment that forms a ridge with about 200 feet of relief. The Escarpment outcrops in many places, one of which is in Marquette Township, Sec. 25, T43N, R1W. Another area of high relief, which may be structural or erosional in origin, occurs in the northeastern part of the county.

Bedrock topography controlled the deposition of glacial material in Mackinac County, and, as a result, surface topography tends to be a subdued replica of bedrock topography (Figure 3). Overburden of glacial origin varies from 10 feet to 150 feet in thickness and is composed of varying amounts of clay, sand, and gravel.

STRATIGRAPHY AND BEDROCK GEOLOGY

Bedrock units mapped in Mackinac County are represented by rocks from the Ordovician to the Devonian System. The units are discussed in ascending order. The Ordovician System includes the Stonington Formation, consisting of interbedded fossiliferous carbonate and shale; the Big Hill Dolomite consisting of argillaceous dolomite and limestone; and the Queenston Shale, consisting of alternating layers of gypsum, limestone, and gray, green, and red shale. The Stonington Formation and Queenston Shale are present in the northeast corner of Mackinac County. The Big Hill Dolomite is not found in Mackinac County on our revised map. The formational contacts are farther north than previously mapped.

The Lower Silurian includes the Manitoulin Formation, which consists of fossiliferous dolomite and limestone, and the Cabot Head Shale, which is a shale with thin interbeds of limestone containing the evaporites anhydrite and gypsum. The Middle Silurian is represented by the Burnt Bluff Group, which is composed of light gray, calcitic dolomite and limestone, and the Manistique Group, which consists of brown or gray, cherty dolomite that contains beds of lime mudstone. The Upper Silurian includes the Engadine Group, consisting of a massive, hard, bluish-white dolomite, and the Point aux Chenes Shale, consisting of green and red shales, along with thin beds of dolomite and thin beds and small irregular masses of gypsum.

The Devonian System is represented by the Mackinac Breccia, which is composed of fragmented shale, limestone, and dolomite from formations of the Upper Silurian and Lower Devonian. This unit represents a collapse of overlying dolomites into older, Late Silurian shales due to solution of salt and gypsum from the underlying shales. This probably occurred during a period of marine withdrawal and emergence of land in Mid-Devonian time.

In comparison to Reed and Daniels's 1987 map (Figure 5), the updated geologic map (Figure 4) shows significant changes in locations of the contacts of the bedrock units. These changes coincide with the 1993 revised map of Chippewa County (McCann et al. 1994). On the whole, our data show that the formational contacts are more northerly than originally mapped. The most evident changes are apparent in the northeastern part of Mackinac County. The changes in the contact positions related to topography cause the Cabot Head Shale to appear thicker and the Burnt Bluff Group to appear thinner in map view. There is no evidence to support the irregular southeast-trending extension of the Manistique Group in the center of the eastern half of the county as shown on the 1987 map. The smoother contacts indicate that there is less structure and/or less erosional topography developed on the bedrock surface than was shown in the previous interpretation.

In a small area in northeastern Mackinac County, the Manitoulin Dolomite unconformably overlies the Stonington Formation, whereas the Manitoulin Dolomite overlies the Queenston Shale in the rest of the county. Due to insufficient data in this area of the county, this interpretation was made by extrapolation from Chippewa County. Contact relationships indicate that this unconformity is the result of nondeposition of the Upper Ordovician sediments associated with the Queenston Shale and the Big Hill Dolomite due to the presence of a topographic high. Subcrop patterns of the Point aux Chenes Shale and the Mackinac Breccia remain similar on each geologic map.

Three cross sections (Figures 6-8) represent the stratigraphy of the eastern, central, and western areas of Mackinac County. The average dip of the rocks in the county is less than 5 degrees to the south, about 50 feet per mile.

However, the dip of the beds varies slightly across the county.

Cross section 1 is located in the western part of Mackinac County (Figure 6). This cross section, vertical exaggeration of 268 to 1, represents approximately 11 miles in a northwest-southeast direction. In this area, the elevation of the bedrock surface on the Burnt Bluff Group and the Manistique Group increases and the thickness of the overburden decreases. This is evidence for the presence of the Niagara Escarpment that extends from Niagara Falls to the east through the southern part of the Upper Peninsula of Michigan, and west to the Door Peninsula which is located east of Green Bay, Wisconsin. The escarpment was formed because of the presence of Middle Silurian carbonate rocks, represented in Mackinac County by the Burnt Bluff Group, the Manistique Group, and the Engadine Group, which are more resistant to erosion than the underlying beds.

Cross section 2 (Figure 7), vertical exaggeration of 170 to 1, is oriented from northwest to southeast and extends approximately 12 miles. The increase in bedrock elevation with very little overburden shown above the Engadine Group is consistent with the placement of the Niagara Escarpment in the county. Cross section 3, vertical exaggeration of 28 to 1, extends from north to south for about 3 miles in the eastern part of Mackinac County (Figure 8). The elevation of the Burnt Bluff Group shows, once again, evidence for placement of the Niagara Escarpment.

CONCLUSIONS

The updated 1996 bedrock geologic map of Mackinac County, based on 1,305 drill holes, revises the 1987 northern Michigan bedrock geology map (Reed and Daniels 1987), The revisions are consistent with the revised geologic map of Chippewa County (McCann et al, 1993). Important features of the new map include an approximate placement of the Niagara Escarpment in the county; identification of an unconformity in the northeast part of the county; and a more refined understanding of the formation contacts in the subsurface. This study, which will assist in groundwater utilization, is one example of the many uses of the data stored in the Statewide Groundwater Data Base.

ACKNOWLEDGMENTS

We would like to thank Shirley Buzinski, the program coordinator for the Statewide Groundwater Data Base of the Michigan Department of Natural Resources, for providing assistance in computer applications, and Frank Chenier, Michigan Department of Natural Resources, for Insight regarding the local geology of Mackinac County. We also greatly appreciate the assistance of Joe Hyrman, the microcomputer support technician at the Chippewa County Correctional Facilities, who provided resources and suggestions regarding the visual presentation of the figures.

REFERENCES

MCCANN, SEAN, L. M. BROWN, M. RIBANT, S. LIEURANCE, F. J. CHENIER, M. A. GERE, JR., AND C. B. REXROAD. 1995. "Bedrock Geology of Chippewa County, Michigan." Michigan Academician XXVII(1): 81-93.

REED, ROBERT, AND JENNIFER DANIELS. 1987. Bedrock Geology of Northern Michigan, Michigan Department of Natural Resources/Geological Survey Division, Lansing, Michigan.