Regional-scale Geologic Interpretation
of Seismic Reflection, Gravity, and Magnetic Profiles
Collected along the Western Arm of the Midcontinent Rift System,
Upper Peninsula of Michigan, Wisconsin, Minnesota and Iowa

Wisconsin Geological and Natural History Survey Open-file Report 2002-01, 18 p. +

Albert B. Dickas

Department of Geology
University of Wisconsin-Superior
Superior, Wisconsin 54880 -2898

Now at:

1355 Jefferson Forest Lane
Blacksburg, Virginia 24060

Michael G. Mudrey, Jr.

Wisconsin Geological and Natural History Survey
University of Wisconsin-Extension
3817 Mineral Point Road
Madison, Wisconsin 53705-5100
email mgmudrey@wisc.edu

Abstract
Introduction
Description and Interpretation of Seismic Profiles

Plate 1 Profile MCR-1 Ontonagon Co. to Gogebic Co., MI shot points 105-525
Plate 2 Profile MCR-2 St. Louis Co., MN shot points 1510-2105
Plate 3 Profile MCR-3 Douglas Co. to Washburn Co., WI shot points 245-950
Plate 4 Profile MCR-4 Barron Co., WI to Benton Co., MN shot points 1590 to 2505
Plate 5 Profile MCR-5 Goodhue Co., MN to Scott Co., MN shot points 1685-2300
Plate 6 Profile MCR-7 Winnebago Co. to Mitchell Co., IW shot points 600-1125
Plate 7 Profile MCR-8 Hamilton Co. to Story Co, IW shot points 105-440
Plate 8 Profile MCR-9 Hardin Co. to Webster Co, IW shot points 485-885
Plate 9 Profile MCR-10 Guthrie Co. to Taylor Co., IW shot points 1785-2625
Plate 10 Profile MCR-ll Carroll Co. to Story Co, IW shot points 1785-2575
Plate 11 Profile MCR-12 Butler Co. to Webster Co., IW shot points 920-1550
Plate 12 Profile MCR-13 Harrison Co. to Mills Co., IW shot points 857-1325

Summary and Conclusions
Acknowledgments
References

Figure 1
Location seismic lines along the Midcontinent Rift
Figure 2
Bouguer gravity anomaly map of Midcontinent showing locationof seismic lines
Figure 3
Shaded relief of total field aeromagnetic anomaly map of Midcontinet showing location of seismic lines
Table 1
Profile number, location, station designations, and length (miles and kilometers) for the Petty-Ray Geophysical Midcontinent Rift System Speculative Seismic Program, 1984-1985

ABSTRACT

In 1984 and early 1985 the western arm of the Midcontinent Rift System (MRS) was geophysically sampled by a series of reflection seismology, gravity, and magnetic profiles. This program, totaling approximately 1340 km. (833 mi), was undertaken in response to the developing interest being shown in the Midcontinent Rift System as an hydrocarbon frontier by the American oil and gas industry. Overall 34 sub-sets of data were collected in the form of 12 profiles, oriented both across and sub-parallel to the central axis of the Midcontinent Rift System. The structural and stratigraphic interpretation of these profiles, indicates that the consensus Lake Superior rift model, configured as a central horst enveloped by half-graben basins, is valid along the trend of the western arm of the Midcontinent Rift System, from the upper peninsula of Michigan south and west to southern Iowa.

This report is limited to the description and discussion of regional-scale structure and seismic stratigraphy.  It is prepared to encourage and stimulate additional analyses at detailed scale level. Such detailed evaluation is necessary to the better understanding of the Midcontinent Rift System, its past role in middle Proterozoic development of the Midwestern U.S., and its future significance in the determination of the economic geology potential of the Lake Superior basin.

INTRODUCTION

From 1984 to 1986 the western arm of the Midcontinent Rift System was geophysically evaluated by a series of reflection seismology programs financed by the federal government and private industry. Federal sponsorship by the U. S. Geological Survey and the Geological Survey of Canada supported the Great Lakes Interdisciplinary Multidisciplinary Program on Crustal Evaluation (GLIMPCE) program (Behrendt and others, 1989; Cannon and others, 1989).  U.S. Department of Energy funding permitted purchase of seismic profiles shot by Grant Norpac, Inc. in Lake Superior (McGinnis and Mudrey, in press). Private industry interests focussed on two orthogonal grid programs along the southwest shore of Lake Superior, in Douglas, Bayfield, Ashland, and Iron counties, Wisconsin, by Grant Norpac, and Seismograph Service Corporation, under contract respectively to Texaco USA and Amoco Production Company (USA) (Dickas, 1999).   These proprietary onshore programs has not been released to the public. A third sequence of profiles, collected on a speculative basis, was shot by Petty-Ray Geophysical Division of Geosource Inc. from June 1984 to February 1985. This set of seismic reflection profiles is the subject of this report.

A series of events and changes in conceptual hydrocarbon genesis models occurred over the past seven decades, and by 1980 had targeted the sedimentary rock sequence, deposited within horst and half-graben structures of Middle Proterozoic age and located throughout the upper Midwestern states, as a Precambrian hydrocarbon exploration frontier ( Dickas, 1988).   The most significant of these events include:

The Midcontinent Rift system is a geophysically identified tectonic structure that can be traced from northeastern Kansas into southern Ohio by way of the Lake Superior basin district (Dickas and Mudrey, 1997).   This rift is of Precambrian (Keweenawan) age and began developing approximately 1.1 Ga. For most of its 2,500 km. extent, this continental-class system is buried below Phanerozoic strata (Fig. 1). Outcrops are found only in east-central Minnesota, northwestern Wisconsin, and the Upper Peninsula of Michigan.   The Midcontient Rift is defined principally from regional gravity (Fig. 2) and magnetic (Fig. 3) anomaly maps.The most accepted tectonic model of the western arm of the Midcontinent Rift System (the target for the Petty-Ray speculative seismic shoot which is the subject of this report) is that of a central horst partly covered by clastic rock, bounded by high-angle faulting, and flanked by wedge-shaped basins filled with clastic rock. This model was developed in conjunction with land and offshore field studies in the southwestern Lake Superior area, and has been adopted for Minnesota, Iowa, Nebraska, and Kansas. In the Lake Superior outcrop belt, the oldest recognized rifted rocks are layered basalt of Middle Keweenawan age. Overlying this basalt is an Upper Keweenawan is a thick sedimentary sequence composed of the Oronto (older) and Bayfield (younger) groups. These sedimentary strata are entirely clastic with textures ranging from conglomerate to shale (mudstone). The Oronto Group is more immature mineralogically and texturally, and more complex structurally, than the Bayfield Group. The Oronto Group is also differentiated by one of its units, the Nonesuch Formation, containing organic shale that yields liquid hydrocarbon which bleeds from subsurface fractures and vugs in a Michigan copper mine. Conditions of deposition for both groups were closely allied with an aquatic, continental environment. Specific paleogeography ranged from alluvial fan and plain, to deltaic, flood plain, fluvial, and lacustrine environments. Prior to seismic profiling, lithostratigraphic relationships southwest of the Lake Superior outcrop belt were known from fewer that a dozen wells drilled into the Precambrian, including two record-depth industrial tests drilled by Texaco USA in Kansas (1984) (Kansas Geological Survey and Texaco USA, 1988), and by Amoco Production Company (USA) in Iowa (1987) ( Anderson, 1990). Analyses of the Upper Keweenawan sedimentary and basalt rock columns penetrated by these boreholes suggest Lake Superior stratigraphy can be correlated throughout the Midcontinent Rift System trend. Extensive reviews of the geology and geophysics of the Midcontinent Rift System are given in Van Schmus and Hinze (1985), Dickas (1986) , Behrendt and others (1989), and Cannon and others (1989, 2001).

Leasing activities along the entirety of the western arm of the Midcontinent Rift System began in late 1983. By mid-1984 numerous geophysical programs (reflection seismology, gravity, and magnetics) had been placed under contract by interested parties. In July, 1984 Petty-Ray Geophysical (Geosource), Denver, Colorado (today in business as Halliburton Geophysical Services, Inc. of Houston, Texas), announced in trade journals a 1,150 km (700 mi) speculative Vibroseis reflection survey along the Midcontinent Rift System trend in Iowa, Minnesota, Wisconsin, and the Upper Peninsula of Michigan. As offered for sale, this geophysical package is composed of twelve (12) profiles (numbers 1 through 13 with number 6 canceled) totaling a measured 1,340 km (833 mi.). Specifics of location and length are given in Table 1 . Each profile also contains gravity and magnetic data collected concurrently with and parallel to the seismic reflection profiles.

DESCRIPTION AND INTERPRETATION OF SEISMIC PROFILES

In this section the individual seismic profiles are described and broadly interpreted as to lithostratigraphy and regional structure. Discussion will involve individual, final stack processed profile segments, as listed on Table 1 . The reproduction of seismic profiles in this report is limited to no more than 30 percent of the overall length of each numbered profile, according to a proprietary agreement with Halliburton Geophysical Services, Inc., the present owner of the data. This agreement further stipulates that the location of individual shot points will not be identified, vertical scale representation will be one inch equals one second two-way-travel time, horizontal scale will be keyed to a 10 km. basis, and gravity and magnetic profiles will be displayed to the same horizontal scale as that used for seismic representation. Individual profile locations are shown on Figure 1 .

PROFILE MCR-1 (Plate 1).   MCR-1 was shot in three segments along a north to south heading from northern Ontonagon County (near the shore with Lake Superior) to southern Gogebic County (across the Keweenaw Fault) in the Upper Peninsula of Michigan (detail map). Part 1 was shot across terrane principally associated with the Keweenawan Supergroup, composed in Michigan of the syn-rift Powder Mill and Portage Lake volcanic groups, plus the overlying syn-rift Oronto (sedimentary) Group and the post-rift Jacobsville Sandstone. Adjacent to the south shore of Lake Superior all reflection horizons, igneous and sedimentary, display a north dip, compatible with outcrop measurements. Volcanic horizons can be traced to depths of 4.2 sec twt (two-way travel time) and correlated laterally over a distance of approximately 25 km. Reflections from layers of porphyritic volcanic rock and/or interflow clastic units, are identified by correlation to surface outcrop and by a corresponding increase in total magnetic intensity. The reflection units increase in separation and decrease in degree of plunge in the down-dip direction, suggesting surface emplacement of the volcanic layers were concurrent with deepening of the basin during tectonic evolution. The individual formations in the overlying Oronto Group cannot be differentiated as this part of the rift column is normally seismically transparent throughout the western arm of the Midcontinent Rift System. Lateral correlation of basement volcanic rock is terminated at the surface at the two-thirds point along Part 1. These terminations are associated with the Keweenaw Fault, a listric fault with a northerly dip. The surface position of this fault plane is constrained by a rapid decrease in both magnetic and observed gravity anomalies and by surface mapping. Distinctive reflections within the upper 1.5 sec twt of the southern third, Part 1, correlate with the northern half of the Jacobsville basin, that half-graben identifying the southern extent of the Midcontinent Rift System in the south-central Lake Superior region. Surface mapping suggests these reflections are associated with the Jacobsville Sandstone (Upper Peninsula equivalent to the Bayfield Group of Wisconsin), which conformably overlies the Powder Mill volcanic rock sequence. Both volcanic and sedimentary dips are to the north at low angles, mapped at the surface as less than 10 degrees, away from the drag effect caused by Keweenaw Fault movement.

At midpoint of Part 2 the Jacobsville basin terminates at the most southerly extent of Keweenawan age rock. The remainder of Part 2 and Part 1A will not be discussed as the infrequent reflection packages seen here are all correlated to pre-Keweenawan basement rock not associated with the Midcontinent Rift System. This profile is discussed in more detail in Hinze and Braile (1990) .

PROFILE MCR-2 (Plate 2).   MCR-2 consists of two parts beginning in the west several kilometers inside eastern St. Louis County, Minnesota and terminates near the shoreline of Lake Superior in Cook County, Minnesota (detail map). This profile is representative of that small percentage of speculative seismic surveys conducted in frontier areas that would never have been collected had the operators been familiar with the regional geology. While ninety percent of this profile traverses territory clearly identified in the literature as Midcontinent Rift system terrane, a review of geology indicates the northwestern shoreline of Lake Superior consists of intrusive and extrusive igneous rock. The Keweenawan basalt of the North Shore Volcanic Group overlies troctolite, gabbro, and anorthosite of the Duluth Intrusive Suite. There is no refIective character apparent anywhere throughout the profile's 6.0 sec twt depth (Plate 2) as this profile traverses bedrock that is 80 percent plutonic. One might expect some degree of reflection within the minor volume volcanic rock sequence; however such is not seen, suggesting these units are lacking either the interlayered clastic beds or the porphyritic zones common to lithostratigraphically equivalent volcanic rock found along the south shore of Lake Superior. The last 10-12 km. of Part 2 (not shown) displays several reflection packages below a depth of 1.8 sec twt. This area lies beyond the northwestern edge of the Duluth Complex and there bedrock is consists of granitic intrusives of the 2.6 to 2.75 Ga Lac La Croix and Giants range granitoid sequence of northern Minnesota.

The gravity profile associated with this seismic line shows flat values throughout Part 1, and steadily declining values to the northwest along Part 2. The declining values are indicative of regional northwesterly thinning of the Duluth Complex. The magnetic profile is composed of numerous short wave length anomalies, suggestive of a series of small displacement faults. One departure from this trend occurs midpoint in Part 1, at the northwesterly edge of the North Shore Volcanic Group.

PROFILE MCR-3 (Plate 3).   MCR-3 was shot in two sections. The combined profile extends from northwestern Douglas County, Wisconsin south to a termination in southern Washburn County, Wisconsin (detail map). Part 1, the northern half of this profile, is easily broken into thirds from a seismic reflection perspective. The principle feature along this part of the seismic line is the St. Croix horst, forming the central structural sector of the Midcontinent Rift System. The northern third of Part 1 was shot over the north-flanking Bayfield basin, a wedge-shaped, thick sequence of sedimentary strata made up of the Bayfield Group and underlying sequences of Oronto Group rock. This sequence is seismically transparent with rare, weak reflectors suggesting near-horizontal structure. The northern faulted edge of the St. Croix horst is identified by correlation to at least three exposures of the Douglas Fault within Amnicon Falls and Patterson Falls state parks, a gabbroic stock exposed immediately south of the Douglas Fault in Amnicon falls State Park (Dickas and Mudrey, 1991; Cannon and others, 2001), and a sharp change in the signature of the magnetic and gravity profiles. The The listric Douglas Fault can be traced to a depth of 4.0 sec twt, form the north flank of the Ashland syncline,  to the central east-west trending structure of the St. Croix horst in northern Wisconsin. This strong package of reflectors, representing the Chengwatana Group of basalt, is caused by a combination of interflow velocity contrasts and sedimentary sequences. The axis of the Ashland syncline is poorly defined due to a return to general transparent reflection conditions within the southern third of Part 1. This syncline is a simple, unfaulted, symmetrical fold in which a thin (1.0 sec twt) column of Oronto Group rock overlies a thick basaltic column, the latter calculated to be as great as 7,000 m.

The northern quarter of Part 2 identifies the southern, northerly dipping flank of the Ashland syncline. The south border of the St. Croix horst,  Lake Owen Fault, which is regionally identified from potential field data,  is poorly constrained to that point where a north-dipping package of strong reflectors grade gradually into a 6.0 sec twt vertical transparent sequence that constitutes the remaining four-fifths of Part 2. This transparent section has been identified by shallow drillholes as the little studied River Falls syncline (Dickas, 1986).

Profile MCR-3 is discussed in more detail under the section labeled "Interpretation of Profile AA" in Chandler and others (1989) .

PROFILE MCR-4 (Plate 4).   MCR-4 was shot southwest of MCR-3 and crosses the axial trend of the Midcontinent Rift System as it changes to a more north-south orientation (detail map). Profile MCR-4 was shot from east to west beginning in Barron County, Wisconsin and ending in Benton County, Minnesota. The St. Croix horst is constrained by the Douglas Fault to the west and the Lake Owen Fault to the east, both buried under a thin wedge of Phanerozoic (Lower Paleozoic and Pleistocene) strata. The edge of the Bayfield basin to the west and the River Falls syncline to the east determine the extent of the Midcontinent Rift System. Subsurface mapping with the aid of borehole data suggests that the principal difference in the comparison of this line with that of MCR-3 is the lack of Oronto Group strata overlying the synclinal basalt pile forming the center of the St. Croix horst and the presence of Phanerozoic strata.

Processing of  MCR-4 was split in four section, designated from east to west 4, 4A, 4B, and 4C. At the mid-point of Line 4 an isolated package of east dipping reflectors, centered at 1.0 sec twt, identifies the pre-Phanerozoic subcrop of the Lake Owen Fault.

To the east the geometry of the River Falls syncline is poorly identified by discontinuous weak basement reflections off the sedimentary-igneous rock interface and by discontinuous, moderate reflections off strata comprising the Oronto-Bayfield sequence filling in this basin. Similar reflection packages in Line 4C and the western half of Line 4B are associated with the Bayfield basin, indicating that both Midcontinent Rift System flank basins have similar, mirror-image structural configurations. Undulating basin-basement reflections on Line 4C suggest a series of small-displacement faults at this horizon.

The western half of Line 4 and Line 4A traverse the St. Croix horst. The 3 to 4 km thick basalt pile forming the basement of this structure is clearly indicated by strong, laterally continuous reflectors (Plate 4). Within the central part of Line 4A and overlying the basalt reflectors, a 0.8 sec twt thick transparent zone is seen. Ordinarily, this sequence would be interpreted as Oronto Group sedimentary strata, as identified to the northeast within the Ashland syncline. Associated outcrops indicate that here, however, the volcanic pile is topped by a massive basalt sequence lacking either reflectivity alterations or interflow strata. On the western end of Line 4A this near surface transparent zone abruptly increases in thickness to 1.6 sec twt. While published maps do not agree, this zone may be interpreted as a northern extension of the Twin Cities (Minneapolis) basin, a Minnesota counterpart to the Wisconsin based Ashland syncline.

The magnetic, and especially the gravity profile, collected concurrently with seismic profile MCR-4, are most helpful in constraining the bounding faults identifying the St. Croix horst in this part of the Midcontinent Rift System.

MCR-4 is discussed in more detail in the section labeled "Interpretation of Profile BB " in Chandler and others (1989) and the regional aeromagnetics is discussed extensively by Cannon and others (2001) .

PROFILE MCR-5 (Plate 5).   MCR-5 was collected in three sections along an east-west line beginning in northeast Goodhue County, Minnesota and ending approximately 119 km. to the west along the La Seur-Scott County line, Minnesota (detail map). MCR-5, constituting the eastern part of this profile, gives excellent resolution regarding basement depth and overall width of the River Falls syncline. At this latitude this half-graben bounding basin is over twice the width as seen at the latitude of MCR-4. Two sets of laterally persistent reflectors dominate this line, one off the basalt basement and the other situated mid-section within the overlying Oronto-Bayfield Group sedimentary sequence. The latter may represent an angular unconformity forming the contact of these groups, as seen on seismic reflection profiles collected offshore Lake Superior by the GLIMPCE group (Cannon and others, 1989).  Both groups of reflectors appear to thin toward the basin edge the east of the beginning of MCR-5. Interflow reflections within the basalt column are not common.

MCR-5B poorly identifies the western flank Bayfield basin. A mid-line reflection package, centered at 2.0 sec twt, probably emanates from basement basalt. This package displays a prominent synformal geometry formed by the intersection of the southernmost extent of the northeast-southwest striking Douglas Fault and the Belle Plaine Fault system, a series of northwest trending, left-lateral displacements which structurally separate the Midcontinent Rift System into en-echelon first-order rift segments (zones) (Dickas and Mudrey, 1989). The eastern third of MCR-5A identifies the termination of the River Falls syncline against the Cottage Grove-Hastings Fault system (the southerly extension of the Lake Owen Fault) forming the eastern flank of the St. Croix horst in this locale (Morey, 1977). This horst is structurally defined in the remainder of MCR-5A as an unfaulted sag of thickness in excess of 2.3 sec twt of highly reflective basalt overlain by a thin (maximum 0.8 sec twt) seismically-transparent column of Solor Church sedimentary rock (Plate 5).

The potential field profiles that accompany seismic profile MCR-5 correlate with the gross structure as stated above, but not to the extent as seen along MCR-3 and MCR-4. MCR-5 is discussed in detail under the heading "Interpretation of Profile CC " in Chandler and others (1989) and aeromagnetic data are analyzed by Cannon and others (2001) .

PROFILE MCR-6. Although advertised in promotional advertisements (Oil and Gas Journal, July 30, 1984, p. 36), profile MCR-6 was never shot. This is probably due to the fact that at the announced southern Minnesota latitude it would have crossed the heart of the Belle Plaine Fault system, thus being of minimal value in the interpretation of Midcontinent Rift System structural and seismic stratigraphy.

PROFILE MCR-7 (Plate 6).   MCR-7 consists of two principle lines (7A and 7C), offset and tied together by the very short (2.82 km.) line 7B. This composite profile begins on the Howard-Mitchell County, Iowa border and terminates to the west in eastern Winnebago County, Iowa (detail map).  This profile reveals little information regarding the northern Iowa part of the Midcontinent Rift System. The only reflection package of moderate intensity was limited to the upper 0.6 sec twt  and originates from the relatively thin covering of Phanerozoic strata that cover the Midcontinent Rift System. This reflection package is laterally persistent, horizontal in dip, and extends across all three lines. Seismic detail is lacking beneath cover. A one-hundred milligal increase in the accompanying gravity profile in the western half of Line 7C helps regionally locate the Thurman-Redfield and Northern Boundary structural zones, the central Midcontinent Rift System horst-defining faults as mapped in Iowa by previous studies (Anderson and Black, 1981).

PROFILE MCR-8 (Plate 7).   MCR-8 is a short (56.12 km.) Midcontinent Rift System strike-oriented profile shot in a single segment along a north-south direction from northern Hamilton County to southern Story County, Iowa (detail map). Surface Phanerozoic sedimentary strata are seen as a series of almost continuous, horizontal reflectors which bottom out at approximately 0.6 sec twt.  In the northern half of MCR-8, beneath the Phanerozoic section and overlying a thick (3.0 sec twt) laterally continuous, southerly-dipping package of strong reflectors, a relatively thin (maximum of 1.0 sec twt ) seismically transparent zone is present (Plate 7). The strong reflectors emanate off the Thor volcanic rock sequence, the Iowa eqivalent to the Chengwatana and Powder Mill Portage Lake volcanic rock sequence of Wisconsin and Michigan. While Thor volcanic reflections become much weaker in the southern half of MCR-8, the reflections that are present there display an apparent north dip, indicating the Thor volcanic rock sequence form a gentle symmetrical synclinal sag. Such a structure would suggest this longitudinal profile was shot over the central Midcontinent Rift System horst, known in Iowa as the Iowa horst. The overlying transparent section in this interpretation represents Solor Church equivalent sedimentary rock filling in the Stratford basin, one of a series of isolated basins incompletely covering the Iowa horst.

Stratford basin strata thin to the north terminus of this profile to a seismic thickness of 0.1 sec twt, indicating the basin boundary lies immediately beyond this profile. The synclinal axis of the Iowa horst appears to lie mid-point in this profile in a sector of poor seismic resolution. Interpretation of the magnetic and gravity profiles accompanying MCR-8 might allow the placement of at least one principal fault offsetting  basement volcanic rock sequence in the southern profile half.  If such were correct, this fault(s) would represent the most southeastern expression of the Thurman-Redfield structural zone. A subdued expression of the Ankeny basin, one of several which combine to form the southeastern flank of the Midcontinent Rift System in Iowa, is seen, beyond the point of possible faulting in the form of weak, volcanic-basement reflectors which intercept the terminous MCR-8 at an approximate depth of 2.2 sec twt. This interpretation is supported by analyses of line 11-3 MCR-11.

PROFILE MCR-9 (Plate 8).   MCR-9 was shot in five sections, only four of which will be discussed here as Part 9A is a short (4.43 km.) north-south line tieing Line 9 to Line 9B. MCR-9 begins in eastern Hardin County, Iowa and terminates in western Webster County, Iowa (detail map). This entire profile is seismically capped by Phanerozoic reflectors displaying near horizontal attitude and extending to a depth of 0.55 sec twt. Beneath these surface units an east to west composite of lines 9D, 9, 9B and 9C presents a high resolution first-order, portrayal of the Midcontinent Rift System in central Iowa, that being a central horst topped by a thin sedimentary sequence and separated by fault systems from wedge-shaped, sedimentary rock-filled, flank basins.

The majority of Line 9D was shot over the wedge-shaped Wellburg basin which forms the eastern flank of the Midcontinent Rift System at this latitude (Sims, 1990)  and thus is a structural counterpart to the River Falls syncline to the north. A moderately intense, westerly-dipping reflection package at depths of 2.3 to 2.6 sec twt identifies the volcanic basement of the Wellburg basin within the eastern third of Line 9D. The overlying 1.75 sec twt thick seismically transparent sequence represents infilling Solor Church and younger Fond du Lac Formation strata. The latter unit is the lithostratigraphic equivalent to the basal Bayfield Group of Wisconsin. In the western half of 9D poorly defined westerly dipping reflections, possibly derived from basement volcanic rock sequence, phase through a zone of no reflections into a thick (minimum of 3.0 sec twt) package of fair to moderate, easterly dipping reflectors associated with the volcanic core of the Iowa horst. The termination of these opposing dip sequences help to seismically constrain the Thurman-Redfield structural zone, which at this latitude is composed of three to four normal displacements distributed over a 12 to 15 km. wide path.

Lines 9 and 9B were shot over the unfaulted central sector of the Iowa horst, seen here as a broad sag of thick (at least 3.0 sec twt) Thor volcanic reflections overlain by a prism-shaped zone of non-reflectors that reaches a maximum thickness of 0.6 sec twt. The latter zone represents Solor Church sedimentary rock contained within the Stratford basin. The many reflectors identifying the Thor volcanic rock sequence suggest this igneous pile is differentiated by flows of contrasting character and/or by interflow sedimentary units such as is seen in outcrop in the Lake Superior basin.

Line 9C, forming the westernmost part of profile MCR-9, seismically identifies, from east to west, the western flank of the Iowa horst, listric faulting associated with the Northern Boundary structural zone, and the eastern portion of the wedge-shaped Duncan basin (Sims, 1990) (Plate 8). The volcanic basement of the Duncan basin is clearly defined by laterally persistent, easterly-dipping reflectors found at depths ranging from 1.9 to 2.3 sec twt. A structural rollover within these basement volcanic rock sequence at the western terminus of Line 9C is probably associated with structural complexities caused by the Manson Impact Structure, a 35 km. diameter astroblem that impacted the southern flank of the Duncan basin approximately 66 Ma and is centered in southeastern Pocahontas County, immediately west of Webster County (Hartung and Anderson, 1988). Solor Church and Fond du Lac sedimentary rock fills in this basin to the maximum thickness of 1.7 sec twt over the volcanic sequence. The sharp seismic definition of faulting identifying the western boundary of the Iowa horst on this profile suggests that at this latitude the Northern Boundary structural zone is not as wide nor bifurcated as indicated on published maps (Sims, l990).

The potential field surveys conducted concurrently with MCR-9 in general support the above seismic interpretation. Additional discussion is available under the heading "Interpretation of Profile DD"  in Chandler and others (l989).

PROFILE MCR-10 (Plate 9).   MCR-10 was shot, in two lines, along a north-south oriented traverse beginning in northwest Guthrie County, Iowa and terminating in south-central Taylor County, Iowa (detail map). Near-surface Phanerozoic strata can be traced across both lines from a depth of about 0.4 sec twt in the north, gradually increasing to about 0.6 sec twt in the south. Line 10-1, comprising the north half of this profile, was initiated on the northwestern flank of the Iowa horst. This northern terminus can be projected onto the western half of Line 11-1, MCR-ll, which lies approximately 11 km (7 mi) to the north. Employing this projection the pre-Phanerozoic portion of the initiation of Line 10-1 is seen as a seismic transparent zone, bottoming at 2.1 sec twt and interpreted to be the same "massive Thor sequence'' identified on profile MCR-11. This massive sequence appears to unconformably overlie weak, discontinuous reflectors representing the older Thor basalt rock sequence. The base of the massive sequence plunges to the south until it is disrupted, close to the Adair-Guthrie county line, by a structural displacement which forms the northwesterly fault of the Thurman-Redfield structural zone. The placement of this fault is supported by accompanying magnetic data. Overlying this fault, a lens shaped transparent zone, having maximum development between depths of 0.5 and 1.4 sec twt in northern Adair County, unconformably underlies the near horizontal Phanerozoic sequence. It is suggested this lens represents one of several isolated and un-named Solor Church filled basins that cover the Iowa horst.

In the southern half of Line 10-1, displacements of isolated packages of Thor basalt reflectors and associated potential field data, identify the main fault zone of the Thurman-Redfield structural zone in southern Adair County. In the northern half of Line 10-2, underlying the Phanerozoic sequence and overlying the basaltic basement, a wedge-shaped semi-transparent zone identifies the eastern flank of the Shenandoah basin, forming the southeastern flank of the Midcontinent Rift System in southern Iowa. This basin, lying between depths of approximately 0.6 to 1.7 sec. twt at the northern terminus of Line 10-2, thins to a feather edge at midpoint of this line (Plate 9). In the southern half of Line 10-2 the only reflectors are associated with Phanerozoic strata. The deeper rocks lie outside the boundaries of the Midcontinent Rift System and are identified by Sims (1990) as granitoid units of Early Preoterozoic age.

PROFILE MCR-11 (Plate 10).   MCR-11 was shot east to west from southeastern Story County to central Carroll county, Iowa (detail map). While no Precambrian outcrops occur in the vicinity, seismic interpretation of this profile is aided by the presence, approximately 14 km. west of the western terminus, of a hydrocarbon exploration borehole. The Amoco M.G. Eischeid #1 well was spudded 16 March, 1987 in Sec. 6, T 83 N, R 35 W, Carroll County, and plugged and abandoned at a total depth of 5,441 m (17,851 feet) after penetrating approximately 1,100 m (3,600 feet) of Phanerozoic (Pleistocene, Cretaceous, and Pennsylvania to Cambrian) strata, overlying about 4,300 m (14,100 ft) of unmetamorphosed Precambrian clastic rocks, and bottoming in 30 m (100 ft) of gabbro and associated mafic rock. Precambrian rock was divided into a 2,200 m (7,200 ft) thick "Upper 'Red Clastic"' series, and an underlying 2,100 m (6,900 ft) thick "Lower Red Clastic" series. These two sequences have been proposed to be lithostratigraphically similar, respectively, to the Keweenawan-age Oronto and Bayfield groups of the Lake Superior basin (Anderson, 1990).

Across MCR-ll continuous, near surface reflectors recorded to a rather uniform depth of 0.6 to 0.7 sec twt identify near horizontal Phanerozoic sedimentary units. Unconformably underlying these units, the three lines that compose profile MCR11 characterize a typical Midcontinent Rift System cross-section of the central Iowa horst enveloped by flank basins. Line 1-3 begins over the northern slope of the Ankeny basin and ends over the west flank of a wide (12 km) structural block, bounded by two faults and forming the Thurman-Redfield structural zone. The eastern-most of these reverse faults displaces the original continuity of the sediment filling in the Ankeny basin. These strata, the combined thickness of which thins to the east, can be seismically subdivided into two units. Within the fault block the lower (Oronto Group equivalent) unit is characterized by a 1.1 sec twt thick package of fairly strong reflectors, while the upper (Bayfield Group equivalent) is seen as an overlying, somewhat seismically transparent unit about 1.0 sec twt thick (Plate 10). The basement volcanic rock sequence along this line does not display discernable reflection characteristics.

According to published information, the seismic interpretation of the western two-thirds of Line 11-2 is subject to debate. Here a latterly persistent, slightly synformal package of strong reflectors, with a structural axis identified at a depth of 2.7 sec twt, is clearly associated with Thor volcanic rock sequence forming the basement of the Iowa horst. The question is whether the overlying, pre-Phanerozoic, semi-transparent package can be interpreted as younger, more massive units of the Thor sequence (as suggested by Anderson (1990, p. 33), or are Solor church equivalent strata (as mapped by Sims, 1990).  While either interpretation allows for displacement of the Iowa horst along reversely faulted borders, potential field surveys taken concurrently with this profile favor the "Thor sequence" interpretation.

Within the western half of Line 11-1 the western fault boundary of the Iowa horst is defined by a reverse fault separating footwall Oronto and Bayfield equivalent strata from overlying, synformal units (upper Thor or Solor Church?) forming the wedge-shaped hanging wall.  Anderson (1990)  interprets the basement units of the western strata of MCR-1, below 2.7 sec twt, are composed of pre-Keweenawan crystalline rock.

PROFILE MCR-12 (Plate 11).   MCR-12 was shot in three parts along the latitude beginning in southwest Butler County, Iowa and terminating in north-central Webster County, Iowa (detail map). As this traverse is parallel to, and lies approximately 15 km north of profile MCR-9, its interpretation should closely correlate to that presented above for MCR-9.

Line 12-1, the easternmost segment of this profile, is capped by Phanerozoic reflectors down to a depth of between 0.5 and 0.6 sec twt. This line was initiated over the Wellburg basin and displays a basement Thor volcanic rock sequence plunging gently in a westerly direction beginning at an approximate depth of 2.1 sec twt. A comparison of this depth to a correlatible depth of 2.3 sec twt, as identified on MCR-9, would suggest the volcanic basement of the Wellburg basin is dipping in a southerly direction along the line connecting the eastern terminations of MCR-9 and MCR-12. The wide, multi-step fault Thurman-Redford structural zone is seen as a vertical transparent zone followed to the west by a antiform structure, both contained in the western half of Line 12-1. This antiform, composed of moderately strong volcanic reflectors, is repeated in the eastern portion of Line 12-2 and is followed immediately to the west by yet another vertical zone composed of several traces of laterally persistent, weak reflector horizons. This latter zone probably identifies the western-most fault composing the Thurman-Redford structural zone. The remainder of Line 12-2 is composed of shallow, laterally persistent, sub-horizontal, weak reflecting horizons which grade with depth to a strong package of sub-horizontal reflectors. The latter package bottoms out at an approximate depth of 2.4 sec twt. Below 2.4 sec twt, several reflecting sequences indicate a broken, possibly faulted, hummocky structure. The relationship of this deeper terrane to the overlying terrane might be that of a separate angular unconformity, suggesting the Iowa horst was formed during two stages of tectonic development. The shallow, western portions of Line 12-2, representing the uppermost portion of the Iowa horst, does not allow for seismic interpretation indicating a Precambrian sedimentary sequence, such as is seen on MCR-9. Rather, sub-horizontal volcanic reflectors appear to be non-conformably overlain by flat-lying Phanerozoic strata (Fig. 13). This interpretation permits the regional identification of l the northern edge of the Stratford basin as lying between lines MCR-9 and MCR-12. This geographic relationship is verified by interpretations presented here in the discussion of profile MCR-8.

In the central sector of Line 12-3, the western flank of the Iowa horst is juxtaposed by Northern Boundary structural zone faulting against the southeastern flank of the Duncan basin. The angular unconformity suggested above to separate the two stage development of the Iowa horst, is even better defined in Line 123. Within the Duncan basin, the undulating volcanic basement lies at depths ranging from 1.9 to 2.3 sec twt. The overlying, seismically subdued sequence, capped by regional, flat-lying Phanerozoic strata, is composed of Solor church and Fond du Lac clastic rocks filling in the Duncan basin. A slight decrease in the apparent dip of the basement volcanic rock sequence at the western terminus of MCR-12 is probably caused by the nearby presence of the Manson astoblem (Hartung and Anderson, 1988).

The above interpretation is supported by the signatures of the regional gravity and magnetic lines which were collected concurrent with this profile.

PROFILE MCR-13 (Plate 12).   MCR-13 was shot in three sections along a north-south longitude line from southeastern Harrison County to south-central Mills County, Iowa (detail map). The entirety of this profile appears to be confined to the Iowa horst. Near horizontal reflectors off the surface Phanerozoic section cap all three segments, from a depth of about 0.7 sec twt in the north decreasing to about 0.6 sec twt in the south. Underlying this surface package on Line 13, the seismic stratigraphy is similar to that discussed above under MCR-10 and MCR-11, that being thick, massive basalt overlies deeper, basalt units of subdued reflectivity. These igneous units are separated by a zone of discontinuous, moderately strong reflectors that dip gently southward and appears to locally display unconformable geometry. A reverse seismic displacement, verified by accompanying magnetic and gravity data, is discernable on the southern portion of this line, and the overlapping portion of Line 13-1. While this displacement defines one of the faults comprising the Northern Boundary structural zone, other faults associated with this zone, as shown on published maps (Sims, 1990), are not apparent on either Line 13 or Line 13A (part 1). The latter segment displays a southerly dipping, uninterrupted series of moderate to strong reflectors, which unconformably underlie the Phanerozoic section.

This series of uninterrupted reflectors, associated with the Thor volcanic rock sequence composing the core of the Iowa horst, continues onto Line 13A (part 2) and south to the geographic position of the Pottawatomi-Mills county line. Here a thick (approximately 2.5 sec, twt) series of chevron-shaped reflectors identify the Mineola graben. The faulted edges of this structure, situated midpoint on the Iowa horst, are constrained by inflection points associated with negative magnetic and gravity anomalies collected concurrently with this seismic profile. The Mineola graben appears to be filled with Solor Church Clastic as identified by a characteristic transparent seismic pattern, and is approximately 10 km wide as measured along the line of MCR13. South of the Mineola graben the Thor basalt changes dip from southerly to northerly. Near the southern extremity of Line 13A (part 2) one of the reverse faults forming the Thurman-Redfield structural zone may be seen as a northerly dipping reflector package overlying a subhorizontal reflector package, the latter being seen at a depth of 2.3 sec. twt at the southern end of profile MCR-13.

SUMMARY AND CONCLUSIONS

A series of reflection seismic, magnetic, and gravity profiles collected in 1984 and early 1985 by the Petty-Ray Geophysical Division of Geosource, Inc. now doing business as Halliburton Geophysical Services, Inc. of Houston, Texas) constitute the principal source of-geophysical information available for interpretation of the geology of the western arm of the Midcontinent Rift System. This series, composed of 34 individual sub-sets of data, is presented in the form of 12 profiles geographically distributed from the upper peninsula of Michigan southwest to southern Iowa. The 34 sub-sets total a measured 1340.39 km (833.06 mi) and were recorded to 6.0 second, two-way travel time, equivalent to a depth of approximately 18 km (11 mi) assuming mean velocity to a 6.0 second reflector is 6 km (4 mi) per second. Each data-set is described and discussed in terms of interpreted regional geology and seismic stratigraphy. This discussion is presented in terms of the well-known stratigraphy and tectonic character of the Midcontinent Rift System exposed in west-central Minnesota, northwest Wisconsin, and the upper peninsula of Michigan.

The basic tectonic model of the Midcontinent Rift System, as configured by Van Schmus and Hinze (1985), Dickas (1986), and Cannon and others (1989) , is verified by the seismic and potential field data interpretations presented here. This consensus model depicts the western arm of the Midcontinent Rift System as a broad, synclinal sag interrupted by a central horst separating wedge-shaped, thick accumulations of low-density sedimentary rock overlying deeply buried basement volcanic rock sequence. The central horst, composed of a thick sequence of layered volcanic rock partly overlain by clastic rock, is bounded by a series of high-angle, listric faults. Initially developed as a symmetrical, extensional basin, the central horst was subsequently formed by late-phase compressional faulting.

The geologic and stratigraphic interpretations presented in this report verify throughout the geographic region of the Halliburton geophysical survey the gross characteristics of the consensus model described above. These interpretations should be considered as first order (regional) in scale and approximate in description. No attempt has been made to determine specific structure or to evaluate seismic stratigraphy beyond the formation level.

ACKNOWLEDGMENTS

We thank Jay Jones, Vice President and General Counsel, Geosource, Inc., Linda Heckman, Contract Paralegal, Halliburton Geophysical Services, Inc., John D. Anderson, Western Hemisphere Speculative Survey Manager, Halliburton Geophysical Services, Inc. for their interest in this study and their cooperation towards its publication.

This study was supported under grant #024069, funded by the 1991-1992 Applied Research Program of The University of Wisconsin

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