material balance in manganese nodule formation north central pacific abstract all data necessary to calculate the metal balance for mn nodules in a well investigated area of the north central pacific are now available the nodules lie on porous siliceous ooze and receive more than 96 of their metal content from the underlying ooze by diagenetic mobilisation only a very small portion of the metal contained in the sediment has to be mobilised to form the nodules which explains why the expected depletion of metals from the sediments has never been demonstrated

rubidium strontium dating of ore deposits hosted by rb rich rocks using calcite and other common sr bearing minerals the authors have tested a rb sr technique that permits ore deposits to be dated using common gangue minerals such as calcite and fluorite the only conditions the deposit must meet are that 1 it have minerals with a low rb sr ratio and 2 it be enclosed by wall rock with a high rb sr ratio because hydrothermal minerals acquire a strontium isotope composition that is usually similar to that of the wall rock minerals with low rb sr ratio should record and retain the isotopic composition that the wall rock had at the time of mineralization the difference between that ratio and that of the wall rock at present is a function of time and the rb sr composition of the wall rock the technique was tested fusing fluorite and calcite from three deposits ranging in age from tertiary to precambrian in all cases the age determined here closely resembles that obtained by conventional k ar and rb sr dating methods the precision however can be poor and depends chiefly on the strontium isotope heterogeneity of the wall rock and its rb sr enrichment 36 references 1 figure 1 table

comparative precambrian stratigraphy and structure along the mid continent rift the mid continent rift is a geophysically identified tectonic structure that has been traced from kansas northeastward into the lake superior district a related arm has been identified by gravity as extending from eastern lake superior southeastward into the lower peninsula of michigan this rift is of precambrian keweenawan age and began developing approximately 1 1 b y b p for most of its extent this continental class rift system is buried below phanerozoic strata outcrops are found only in east central minnesota northwestern wisconsin and the upper peninsula of michigan the most accepted tectonic model of the mid continent rift is that of a central horst partly covered by clastic rocks bounded by high angle faulting and flanked by basins filled with clastic rocks this model was developed in conjunction with field studies in the southwestern lake superior area and generally has been adopted for minnesota iowa nebraska and kansas a comparable model has yet to be presented for the related arm extending into southern michigan in the lake superior outcrop belt the oldest recognized rifted rocks are layered basalts of middle keweenawan age overlying these basalts is an upper keweenawan thick sedimentary sequence composed of the oronto older and bayfield younger groups these sedimentary rocks are almost entirely clastic with texture ranging from conglomerate to shale mudstone rock colors are principally reds and browns with secondary shades of green and white the oronto group is more immature mineralogically and texturally and more complex structurally than the bayfield group the oronto group also is differentiated by one of its units the nonesuch formation which contains organic shales that bleed liquid hydrocarbons from subsurface fractures and vugs in a michigan copper mine conditions of eposition for both groups are closely allied with an aquatic continental environment specific paleogeography ranged from alluvial fan and plain to deltaic flood plain fluvial and lacustrine southwest and southeast of the lake superior outcrop belt stratigraphic relationships are known principally from fewer than a dozen significant wells drilled into the precambrian within the mid continent rift trend analysis of the proposed upper keweenawan red clastic sedimentary rock column within these wells suggests that the lake superior stratigraphy can be provisionally correlated throughout the mid continent rift trend

geophysical investigations and crustal structure of the north american midcontinent rift system abstract geophysical investigations have had a prominent role in studying the 1100 ma midcontinent rift system because of limited surface exposures and deep drilling of the rift rocks gravity and magnetic anomaly data delineate the rift system its central volcanic filled graben and overlying and adjacent post rift sedimentary basins and the major faults of the system over its 2000 km length in central north america crustal seismic studies indicate a high velocity crust that is thickened to as much as 55 km along the rift deeply penetrating seismic reflection data obtained across the rift show that the crust was thinned to less than one half of its original thickness during rifting and the present crust has been thickened by addition of up to a total of 30 km of mantle derived volcanic rocks within grabens and clastic sediments deposited in overlying sag basins as well as by intrusions into the crust and crustal underplating these data also confirm the occurrence of a post rift regional compressional event that locally caused broad folding of the rift rocks and reactivation of the graben normal faults as high angle reverse faults with vertical throws of several kilometers geophysical data indicate that the rifting event had a profound effect on the entire vertical range of the crust and that structures and thus the tectonic evolution are similar along the entire length of the rift

regional shortening fabrics in eastern north america far field stress transmission from the appalachian ouachita orogenic belt paleozoic carbonates of cratonic eastern north america comprise the footwall of the appalachian ouachita fold and thrust belt and contain a layer parallel shortening lps fabric that is preserved by mechanically twinned calcite shortening directions are generally parallel to the appalachian ouachita thrust transport direction in carbonates of the thrust belt proper restored width 400 km and within carbonates up to 1700 km into the foreland giving a pre thrusting sedimentary prism 2100 km wide through which compressive orogenic stresses were transmitted the shortening strain magnitudes 6 and the inferred calcite twinning differential stress magnitudes 90 mpa decrease exponentially away from the orogenic front calcite twinning strain patterns in other adjacent tectonic provinces such as the grenville laramide keweenawan rift and newark basin are distinct from the twinning strains preserved in the cratonic paleozoic carbonates appalachian orogen far field stresses calcite microstructures

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

amplitude blanking in seismic profiles from lake baikal abstract imaging of the deepest sedimentary section in lake baikal using multichannel seismic profiling was hampered by amplitude blanking that is regionally extensive is associated with water depths greater than about 900 m and occurs at sub bottom depths of 1 2 km in association with the first water bottom multiple application of a powerful multiple suppression technique improved the quality of occasional discontinuous dipping primary reflections but failed to substantially alter the non reflective character of the blanking zone detailed analysis of amplitudes from original data and synthetic models show that the threshold for detecting primary energy in deep water of lake baikal occurs when the primary is about 14 20 db less than the multiple energy the blanking occurs because of anomalously low reflectivities of the deep sediments coupled with this 20 db limitation in real data processing the blanking cuts across seismic stratal boundaries and is therefore probably unrelated to depositional lithologies the deepest early rift deposits inferred to come from a mixed fluvial and lacustrine setting do not easily explain the widespread and uniform character of the blanked deposits more likely blanking occurs because of processes or phenomena that physically alter the deposits causing them to be non reflective and or highly attenuating no single process explains all the observations but a combination of diagenesis overpressure and the presence of dispersed free gas at sub bottom depths of 1 2 km offer plausible and possible conditions that contribute to blanking

origin and stratigraphic assessment of pre mt simon clastics precambrian of michigan basin previous workers have synthesized the results of recent deep drilling in the michigan basin with the concept of keweenawan rifting their model correlates the pre mt simon clastics in the sparks et al 1 8 well with those in the state beaver island 1 well identifies them both as turbidites and places the pre mt simon sequence stratigraphically higher in the section than the jacobsville formation and freda sandstone reexamination of the cores and samples from the sparks et al 1 8 suggests a lacustrine origin for the pre mt simon clastics whereas paleomagnetic age data suggest a time equivalence with the freda sandstone the pre mt simon clastics at beaver island are possibly of marine origin and are considered to be time equivalents of the jacobsville formation

the origin distribution and hydrocarbon generation potential of organic rich facies in the nonesuch formation central north american rift system a regional study abstract precambrian unmetamorphosed organic rich clastic rocks whilst uncommon in the geologic record are of interest with respect to their potential for hydrocarbon generation the 1 1 billion year old nonesuch formation is primarily a lacustrine deposit which is part of a thick sequence of volcanic and clastic sedimentary rocks that filled an aborted rift in central north america previous organic geochemical investigations of the nonesuch formation have been limited to isolated samples from the basal portion of the nonesuch formation at a copper mine white pine in northern michigan in the present study 183 outcrop and core samples of the nonesuch formation from northern wisconsin and michigan upper peninsula were collected for sedimentologic and organic geochemical analyses total organic carbon toc values for the samples ranged from 0 0 to 2 5 and showed a strong correlation between organic richness and depositional environment detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two major organic petrographies 1 and 2 which could be further distinguished using combined pyrolysis gas chromatography mass spectrometry py gc ms whereas the composition of organic petrography 1 kerogens is primarily aliphatic the composition of organic petrography 2 kerogens is dominated by aromatic and or phenolic constituents the geographic and stratigraphic distribution of rocks bearing the two organic petrographies indicate that variable source input or preservation superimposed over maturity differences is responsible for both organic petrographic and kerogen pyrolysate compositions

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

precambrian oil inclusions in late veins and the role of hydrocarbons in copper mineralization at white pine michigan liquid oil was trapped as primary fluid inclusions in calcite crystals in late cu fe sulfide bearing veins that crosscut and offset the cupriferous shale deposit at white pine the age of the calcite 1047 35 ma is an entrapment age and thus a minimum age for the oil close temporal and spatial associations of oil and metallic sulfides in the late veins suggest that liquid and solid hydrocarbons may have been dominant controls of the main stage white pine copper mineralization

early non marine life evaluating the biogenicity of mesoproterozoic fluvial lacustrine stromatolites abstract the record of life on land or in non marine environments during the precambrian is sparse limiting our ability to understand life outside of marine settings before the advent of animals stromatolites from such environments are known but demonstrating stromatolite biogenicity remains difficult as stromatolite growth can be controlled by a spectrum of biologic chemical and biologically mediated processes stromatolites from the mesoproterozoic 1 09 ga copper harbor conglomerate an alluvial fan fluvial and lacustrine deposit offer an interesting test for the presence and nature of microbial life in shallow mesoproterozoic non marine settings stromatolites from a siltstone facies are interpreted as biogenic as they contain detrital rich laminae likely indicative of trapping and binding by microbes and fenestral fabrics suggestive of desiccation or lift off structures in mats via the presence of gas perhaps o2 from photosynthesis or other gases from mat decay the stromatolites formed as microbial mats grew over a mudflat or sandflat with carbonate filled desiccation cracks on an erosive topography and thus provide evidence for life in a very shallow predominantly desiccated environment stromatolites from a conglomerate facies are microdigitate and record both isopachous laminae with radial fibrous calcite fans and botryoids typically considered abiogenic in origin as well as wavy conical laminations likely indicative of the former presence of microbial mats the conglomerate facies stromatolites are interpreted to have formed in a flooded braidplain setting with restricted circulation this study supports the suggestion that microbial communities were abundant in non marine environments in the midcontinent rift during the mesoproterozoic it also highlights how variable environmental factors can influence stromatolite growth even in similar depositional settings and with a consistent microbial presence

a possible southeastern extension of the midcontinent rift system located in ohio in 1988 a stratigraphic test core drilled in southwestern ohio penetrated a previously unknown structure and sedimentologic unit core analyses disclosed a sequence of lithic arenite and siltstone of probable pre phanerozoic age deposited within an alluvial fluvial environment a seismic reflection profile across the core site shows a sequence of strong horizontal paleozoic reflectors unconformably overlying eastward dipping layered units of poor reflectivity beyond the core hole total depth a regime of excellent reflectors is seen the pre mount simon upper cambrian sedimentary sequence encountered has been defined as the type section of the middle run formation point counter analyses of typical middle run formation samples indicate that this sequence is similar in composition to middle proterozoic units associated with the midcontinent rift system in the lake superior region the newly discovered structure in ohio is also similar to half graben structures composing the lake superior basin portion of the midcontinent rift on the basis of similarities in lithology stratigraphy structure and proximity to regional gravity anomalies the middle run formation and its apparent half graben basin is proposed as evidence for the probable extension of the midcontinent rift system southward from its normally accepted termination in southeastern michigan into southwestern ohio

tectonic setting of the midcontinent rift system abstract the continental lithosphere which hosts the midcontinent rift mcr system comprises a crustal assemblage ranging in age from 3 6 to 1 5 ga the most voluminous igneous activity along the mcr occurred within late archean 2 7 ga crust presumably due to magmatism associated with a rising mantle plume two major arms of the rift extend to the southeast and the southwest where rifting magmatism and sedimentation were confined to a 2000 km long rift that cuts through the various crustal provinces ranging in age from 3 6 to 1 5 ga tensional reactivation of pre existing crustal discontinuities may have influenced the location of parts of the mcr however direct cause and effect are difficult to distinguish for want of sufficient exposures the most active phases of magmatism and rifting occurred between 1109 and 1087 ma a u pb age of 1097 ma for the southern end of the mcr shows that magmatic activity was synchronous throughout its length there is evidence of rifting and magmatism prior to 1109 ma but there is little evidence of magmatism after 1087 ma rifting may have effectively ceased by that time rifting may have ceased as a result of compressional forces being exerted on the foreland during the grenville collisional event spanning 1030 970 ma and compressional reactivation of normal faults may have resulted in partial inversion of the mcr chemical data and u pb and sm nd isotopic data for igneous rocks along the rift are similar to those for mcr rocks in the lake superior area which are best explained in terms of a mantle plume model however since the distal arms of the mcr were developed in proterozoic lithosphere and did not involve nearly as much magmatism as in the lake superior area mantle plume sources may not have been as important in formation of the distal magmas data from the buried arms are also consistent with the mafic magmas being derived by decompression melting of proterozoic lithosphere during rifting

atlantic core top calibration of the u37k index as a sea surface palaeotemperature indicator abstract a field calibration of the uk index with sea surface temperature is discussed through analysis of an extensive suite of surface sediments n 109 from the northeastern atlantic 2 s 75 n values of u 37 k are compared with sea surface temperatures for overlying waters measured at different depths and seasons to obtain a correlation suitable for palaeotemperature reconstructions the best fit is obtained using surface 0 m temperatures corresponding to caloric winter and autumn months however the annual average spring and summer surface temperature equations also have high correlation coefficients and are also appropriate for climatic studies the results further validate the general applicability of the u 37 k as a climatic proxy because the calibration equations are valid over a wide range of surface water temperatures 0 28 c for different algal populations and are representative of the average contribution of alkenones to sediments as found in sediment cores

new model of the midcontinent rift in eastern minnesota and western wisconsin the midcontinent rift contains a central zone of mostly volcanic and associated intrusive rocks flanking and in places overlying these mafic igneous rocks are clastic sedimentary basins which range in thickness from less than 1 5 km southwest of the west end of lake superior to more than 3 0 km near the minnesota iowa border new gravity and magnetic modeling of the rift has shown that most of the volcanic and intrusive rocks are contained within a central block which in minnesota and wisconsin extends to a depth of at least 10 to 20 km the geometry of this central block which ranges from fairly symmetric to very asymmetric appears to reflect a change from predominantly a graben to only a half graben development the modeling has also suggested the presence of a major prerift intrusive body along the minnesota iowa border which could have locally controlled the rift s development

serpentinization and the origin of hydrogen gas in kansas hydrogen gas occurs in ten kansas wells near the mid continent rift system since 1982 two wells have yielded small amounts of gases containing an average of 29 37 mole h2 the remainder being chiefly n2 with only traces of hydrocarbons isotopic compositions for hydrogen dgrd 740 to 836 pmil imply near ambient about 10 c equilibration temperatures for the gases which are among the most deuterium depleted in nature and resemble the h2 rich gases described from ophiolites in oman isotopic values for the kansas n2 differ slightly from those of the atmosphere but not enough to rule out an atmospheric origin because they are low in ch4 and co2 expected byproducts of biogenic activity the gases are probably abiogenic in origin the existence of such gases near a major rift system containing mafic rocks and not far from known kimberlites is consistent with an origin from reactions involving fe 2 oxidation for example during serpentinization because the gases may be associated with kimberlites and deep seated rifting mantle outgassing is possible but such an origin would be difficult to reconcile with the low isotopic temperatures the h2 gases from kansas and elsewhere seem to be too low in pressure to have commercial value however neither the kansas gases nor those from other h2 occurrences have been adequately examined to assess their importance as potential resources

hydrocarbon source rock evaluation of middle proterozoic solor church formation north american mid continent rift system rice county minnesota hydrocarbon source rock evaluation of the middle proterozoic solor church formation keweenawan supergroup as sampled in the lonsdale 65 1 well rice county minnesota shows that 1 the rocks are organic matter lean 24 of 25 samples have less than 0 8 organic carbon 2 the organic matter is thermally post mature probably near the transition between the wet gas phase of catagenesis and metagenesis dry gas zone and 3 the rocks have minimal potential for producing additional hydrocarbons genetic potential 0 30 mg hc g rock although no direct evidence exists from which to determine maximum burial depths the observed thermal maturity of the organic matter requires significantly greater burial depths a higher geothermal gradient or both it is likely t least on the saint croix horst that thermal maturation of the organic matter in the solor church took place relatively early and that any hydrocarbons generated during this early phase were probably lost prior to deposition of the overlying middle proterozoic fond du lac formation keweenawan supergroup

interpretation of seismic reflection gravity and magnetic data across middle proterozoic mid continent rift system northwestern wisconsin eastern minnesota and central iowa seismic reflection gravity and magnetic data were used along four profiles to study the mid continent rift system between northwestern wisconsin and central iowa mafic lavas in medial horsts are characterized by strong laterally continuous reflections that indicate a stratigraphic package as much as 10 km thick mafic roots extending beneath the reflective sequence to mid crustal depths were defined from gravity modeling in northwestern wisconsin southeastern minnesota and central iowa a strong normally directed remanent magnetization was modeled for most lavas although some lavas deep in the section of northwestern wisconsin and central iowa were modeled as magnetically reversed the horsts are apparently bounded by faults that are vertical or dip moderately inwar several of these faults originated as growth faults in fault bounded grabens but these faults were transposed into reverse faults by a late stage compressional event the medial horsts are flanked by several half graben basins that contain as much as 5 km of sedimentary material inferred to be lithologically akin to the oronto and bayfield groups of michigan and wisconsin several synclinal basins characterized by 2 4 km of sedimentary rocks are developed atop the medial horsts when viewed regionally the four interpretive profiles show a marked asymmetry with regard to the off axis placement of the deep mafic roots and the positions of postulated growth faults this asymmetry is consistent with the rift propagating as a series of northeast striking en echelon half grabens separated by northwest striking scissor faults or accommodation zones

structure of the southern keweenawan rift from cocorp surveys across the midcontinent geophysical anomaly in northeastern kansas this is the published version copyright 1984 american geophysical union all rights reserved

flexural modeling of the midcontinent rift a basement profile obtained from seismic reflection data has been used to constrain a two dimensional flexural model of basin formation for the midcontinent rift at a latitude of 45 sup 0 25 n model parameters included the thickness of the elastic plate the basin width and the maximum basin thickness modeling suggests that flexure produced a deep narrow basin along the rift axis and that the crust was thinned at the time of basin formation to an elastic thickness of 9 6 km for a plate ruptured by rifting or 5 7 km for an unbroken plate with corresponding maximum basin thicknesses of 14 km and 16 km respectively the plate thickness depends most strongly on the basin width and is well constrained by the seismic data although erosion may have narrowed the basin the maximum basin thickness is poorly constrained because of the lack of seismic data for depths greater than about 10 km and because the strata at the center of the rift have been disturbed by a postrift compressional event which produced the st croix horst despite uncertainty about the basin thickness the load required to flex the crust to produce the midcontinent rift basin is too large to be attributed tomore the weight of the central flood basalts unless the basin subsided into a fluid less dense than the solidified basalts on the basis of seismic refraction data and by analogy with other rifts we hypothesize that a magnetic rift pillow intruded in the lower crust the basaltic pillow subsequently solidified to produce a large high velocity region in the lower crust centered under the rift axis as determined from deep seismic refraction this crystallization and cooling may be responsible for the sag phase of rift evolution as evidenced by laterally widespread occurrence of postvolcanic sediments copyright american geophysical union 1988 less

direct measurement of timing underpinning a reliable petroleum system model for the mid continent rift system the amoco 1 eischeid well sec 6 t83n r35w carroll county iowa was the first significant well drilled along the flank of the iowa horst providing an opportunity to evaluate the stratigraphic and structural history of the enigmatic mid continent rift system of the united states new thermal history data obtained from cutting and core samples show that three paleothermal events affected the drilled section at the amoco 1 eischeid well results show that three significant cooling events occurred sometime between 300 and 200 70 and 50 and 35 and 10 ma the first of these events was associated with a high paleogeothermal gradient 35c km demonstrably greater than the present day value 16c km these new results provide direct evidence of a significantly higher paleogeothermal gradient and by inference higher heat flow during the paleozoic in addition results show that maximum maturity of the rich precambrian source the nonesuch shale equivalent was reached during the 300 200 ma event and not 600 700 m y earlier as previously proposed previous work has been significantly influenced by the view that the craton was relatively stable or behaved uniformly throughout the phanerozoic previous models impose a proterozoic time of generation and entrapment for any precambrian source intervals with recognition of significant phanerozoic heating events the opportunity for later generation emerges thus reducing the preservation time and increasing the probability of entrapment and preservation needed for exploration success in the region furthermore these events mapped at the mid continent rift system may be of great regional extent and correlative in time to other well known events along the north american plate margins e g alleghanian laramide ancestral rockies orogenies thus offering insights to new possible exploration targets within the rift perhaps indicative of deep seated processes in the mantle beneath the north american craton during the phanerozoic

rift structure beneath the michigan basin from cocorp profiling cocorp consortium for continental reflection profiling surveys across the mid michigan gravity anomaly near the mcclure sparks no 1 deep well delineate a thick layered sequence of relatively strong reflectors that define a structural trough within the precambrian beneath the michigan basin these deep reflections which correlate spatially with the mid michigan gravity and magnetic high are interpreted to be a pile of basaltic extrusive rocks intercalated with clastic rocks and correlative with keweenawan volcanics exposed in the lake superior region to the northwest a reflection poor zone between the base of the paleozoic as found in the deep well and the top of the deep layered sequence probably corresponds to the upper keweenawan clastic assemblage direct evidence of faulting within the inferred volcanic sequence is lacking although sharp flexures structural benches and dips of as much as 10 suggest subsidence controlled by fault block motions reflections from beneath the inferred volcanic pile are rare the deepest clear event being at about 18 km there is no unequivocal evidence on these sections to indicate whether precambrian structures were reactivated to affect the overlying paleozoic strata of the michigan basin proper although data quality may be insufficient to detect such effects

a geophysical profile of the southern margin of the midcontinent rift system in western lake superior a 70 km long seismic reflection profile in western michigan provides new insight into the nature distribution and structure of the keweenawan supergroup volcanic and overlying sedimentary rocks and the controversial keweenaw fault along the southern boundary of the midcontinent rift system in the lake superior basin interpretation of the 5 s reflection data constrained by surface geology magnetics and gravity modeling shows that volcanic rocks which cropout north of the keweenaw fault dip northerly to depths of the order of 17 km south of the fault volcanic rocks overlain by 2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the south range the volcanic pile within the basin thickens rapidly to the north of the keweenaw fault suggesting that the volcanics were deposted in an extensional fault bounded basin clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event the thickness of the volcanic filled basin implies that the upper crust was almost completely broken during the rifting event the available evidence is interpreted to show the keweenaw fault as a moderate to high angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence there is no evidence from the seismic profiling for major faulting except for the keweenaw fault intrusions or folding of the keweenawan supergroup in this region

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting