North American Midcontinent Rift Research Articles

Confirmation of progressive plate motion during the Midcontinent Rift's early magmatic stage from the Osler Volcanic Group, Ontario, Canada

AbstractAs the supercontinent Rodinia was assembling ca. 1.1 billion years ago, there was extensive magmatism on at least five Proterozoic continents including the development of the North American Midcontinent Rift. New paleomagnetic data from 84 lava flows of the Osler Volcanic Group of the Midcontinent Rift reveal that there was a significant and progressive decrease in inclination between the initiation of extrusive volcanism in the region (ca. 1110 Ma) and ca. 1105 ± 2 Ma (during the “early stage” of rift development). Paleomagnetic poles can be calculated for the lower portion of the reversed Osler Volcanic Group (40.9°N, 218.6°E, A95 = 4.8°, N = 30) and the upper portion of the reversed Osler Volcanic Group (42.5°N, 201.6°E, A95 = 3.7°, N = 59; this pole can be assigned the age of ca. 1105 ± 2 Ma). This result is a positive test of the hypothesis that there was significant plate motion during the early stage of rift development. In addition to being a time of widespread volcanism on Laurentia and other continents, this interval of the late Mesoproterozoic was characterized by rapid paleogeographic change.

Primary curvature in the Mid-Continent Rift: Paleomagnetism of the Portage Lake Volcanics (northern Michigan, USA)

Rocks of the North American Mid-Continent Rift (MCR) in the Keweenaw Peninsula of northern Michigan display a change in structural trend from east to west, varying in strike from 100° in the east to 35° farther west, before returning to a more east–west trend of 80° near Wisconsin. In general, curvature can be described either as of primary origin, meaning that the arc developed in its present curved state, or as of secondary origin, when the arc formed from an initially straighter geometry. A powerful tool in evaluating the origin and degree of curvature in any deformed belt is paleomagnetism, where coincident change in declination and strike is evidence for secondary curvature. Several paleomagnetic studies have been completed on rift-related sedimentary rocks, as well as lava flows, from the MCR in the Lake Superior region. Paleomagnetic directions for the sedimentary rocks vary much more in declination than in inclination, possibly reflecting a vertical axis rotation. If secondary rotation has affected the sedimentary rocks, then underlying volcanic rocks should also demonstrate a similar rotation. Thirty-one sites were collected from the Portage Lake Volcanics in the most highly curved part of the Keweenaw Peninsula in the Upper Peninsula of Michigan. Sites were chosen to maximize the variation in structural trend. Thermal demagnetization results showed two components in samples from most sites, a lower temperature A component (< 580 °C) as well as a higher temperature B component (> 580 °C). Both components were tested for primary remanence using a conglomerate test. The A component, carried by magnetite, passes the conglomerate test at a 95% confidence level, and is therefore considered a primary remanence. The B component, carried by hematite, fails at the 95% level and is considered a secondary magnetization. Most importantly, declination of the primary (A) magnetization between sites shows no correlation with strike, demonstrating that vertical axis rotations cannot explain the curvature of the Mid-Continent Rift. We conclude that curvature in the Lake Superior Region is a primary feature, likely reflecting a pre-existing zone of weakness that was exploited during rifting.

Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution

Volcanism in the Midcontinent rift system lasted between 1108 and 1086 Ma. Rates of flood-basalt eruption and subsidence in the western Lake Superior region appear to have been greatest at the beginning of recorded activity (estimated 5 km/Ma subsidence rate at 1108 Ma) and rapidly waned over a period of 1–3 Ma during a magnetically reversed period. The age of the paleomagnetic polarity reversal is now constrained to be between 1105 ± 2 and 1102 ± 2 Ma. A resurgence of intense volcanism began at 1100 ± 2 Ma in the North Shore Volcanic Group and lasted until 1097 ± 2 Ma. This group contains a ca. 7 Ma time gap between magnetically reversed and normal volcanic sequences. A similar disconformity appears to exist in the upper part of the Powder Mill Group. The average subsidence rate during this period was approximately 3.7 km/Ma. Latitude variations measured from paleomagnetism on dated sequences indicate that the North American plate was drifting at a minimum rate of 22 cm/year during the early history of the Midcontinent rift. An abrupt slowdown to approximately 8 cm/year occurred at ca. 1095 Ma. These data support a mantle-plume origin for Midcontinent rift volcanism, with the plume head attached to and drifting with the continental lithosphere. Resurgence of flood-basalt magmatism at 1100 Ma may have been caused by extension of the superheated lithosphere following continental collision within the Grenville Orogen to the east.

The Chengwatana Volcanics, Wisconsin and Minnesota: petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent rift

The southernmost exposed rocks of the North American Midcontinent rift system (1100 Ma) consist of 3000 m of mafic volcanic flows and minor interflow sediment exposed along the St. Croix River in Minnesota and Wisconsin. The flows are mostly high-Fe tholeiitic basalt with plagioclase phenocrysts and ophitic to subophitic clinopyroxene. Abundant secondary chlorite, epidote, and actinolite indicate the group was metamorphosed to greenschist facies (~350 °C). Low sodium (M4 site) and tetrahedral aluminum (AlIV) contents of actinolite indicate low-pressure metamorphism (0.25 GPa) and imply a geothermal gradient of 45 – 50 °C/km. Low magnesium (Mg# = 0.37–0.58) and Ni contents (36–185 ppm) indicate the basalts have undergone significant fractionation and are not primary mantle melts. Incompatible element abundances are inversely correlated with Mg#, and most samples plot within either high or low trace element groups (e.g., Ti, P, Zr). The basalts are enriched in the light rare earth elements and Th, and are variably depleted in Ta and Nb relative to La and Th. Initial 143Nd/144Nd compositions of the group range from 0.51099 to 0.51122 (initial εNd = −4.5 to +0.1). Most flows have isotopic compositions within a relatively limited range (initial εNd = −2.5 to −1.6), but exhibit variable trace element abundances. Flows with the highest and lowest initial 143Nd/144Nd ratios have isotopic compositions that are inversely correlated with trace element abundances and ratios (e.g., La/Yb, Th/La, Th/Ta). The combined geochemical data suggest that the Chengwatana basalts originated from plume-derived melts and underwent variable fractional crystallization and crustal contamination. These melts may have interacted with lithospheric mantle enriched during Penokean subduction.

Thermal History of the 1.1-Ga Nonesuch Formation, North American Mid-Continent Rift, White Pine, Michigan

The Nonesuch Formation, a black siltstone, was deposited in the Lake Superior portion of the mid-continent rift system during middle Proterozoic rifting. Despite its age of nearly 1.1 Ga, the Nonesuch Formation contains liquid petroleum and solid pyrobitumen. Numerical modeling techniques were used in this study to constrain the thermal history of the Nonesuch Formation at White Pine, Michigan. Thermal modeling addresses two issues: (1) the utility of illite-smectite expandability as a limiting parameter for describing the thermal history of ancient (Proterozoic) sedimentary rocks, and (2) the potential for hydrocarbon maturation within the Nonesuch Formation at White Pine. A range of potential burial histories for the Nonesuch Formation was constructed based on geologica evidence. Time-temperature histories were calculated for each burial history model assuming one-dimensional, transient, conductive heat flow. Results of numerical time-temperature models were then evaluated on the basis of organic and inorganic thermal maturity indicators including Rock-Eval® pyrolysis data and biomarker indices (correlated to equivalent vitrinite reflectance values), in addition to illite-smectite expandability. Illite-smectite expandability values appear generally consistent with measured organic thermal maturity values. The preferred set of cases also produces calculated illite-smectite expandability and vitrinite reflectance values consistent with thermal maturity indicators (that do not include vitrinite) collected in the field. Modeling results demonstrate tha a combination of elevated basal heat flow and rapid burial during the Proterozoic is required to produce the observed thermal maturities at White Pine, Michigan. More specifically, modeling indicates that maximum temperatures for the Nonesuch Formation, 110 to 125°C, were reached at about 1075 Ma, coincident with a maximum burial depth of about 6 km, although 4 km represents a more plausible value. The results support a thermal history consistent with in-situ oil generation at White Pine; however, thermal modeling alone cannot rule out the possibility that oil was generated and expelled elsewhere and migrated into the area with fluids expelled during an episode of postrift compression.

2-D Tomographic Imaging Across the North American Mid-Continent Rift System

Summary 2-D traveltime tomography was used to model the seismic-velocity structure of the Mid-Continent Rift System underneath Lake Superior. the data set came from Line A of the 1986 Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) experiment. In-line data analysed in this study were recorded on four land stations and on five ocean-bottom seismometers. Shot spacing on the 240 km long line was approximately 333m. Both direct and refraction arrival times were used to image the upper crust before expanding the process to cover greater distances and depths. Crustal thickness was determined using reflection (PmP) data from the Moho. Several known near-surface structures were resolved. an extensive high-velocity zone (7.0-7.2kms-1) under the central basin of the lake was found to be composed of a plume-like structure rising to a depth of approximately 10km. This plume originates from a broader-based (approximately 50km) mantle bulge, and probably indicates an area of crustal weakness along the rift. A smaller, less complex intrusion appears under the northern basin of the lake. Moho depths increase from about 36 km under the southern flank to about 55 km under the central basin, decreasing again to about 45 km in the north. There is some evidence in the PmP seismic record that the Moho under the central basin is a disrupted transition zone. the velocity model presented here corroborates this. A stability test applied to the tomography velocity model-along with comparisons to other Line A models-indicate that the result is a reliable and a reasonable representation of the Mid-continent Rift System under Lake Superior. These findings support the theory that this complex structure is part of a failed Keweenawan (1100 Ma) tectonic rifting event.

Thermal Maturation and Organic Richness of Potential Petroleum Source Rocks in Proterozoic Rice Formation, North American Mid-Continent Rift System, Northeastern Kansas

A recent well in northeastern Kansas penetrated 296 ft (90.2 m) of dark gray siltstone in the Precambrian Mid-Continent rift (Proterozoic Rice Formation). Correlations indicate this unit may be as thick as 600 ft (183 m) and is possibly time-equivalent to the Nonesuch Shale (Middle Proterozoic) in the Lake Superior region. The upper half of this unit qualifies as a lean source rock (averaging 0.66 wt. % TOC), and organic matter in it is in the transition stage between oil and wet gas generation. The presence of the gray siltstone in this well and similar lithologies in other wells is encouraging because it indicates that source rock deposition may be common along the Mid-Continent rift, and that parts of the rift may remain thermally within the oil and gas window. Microscopic examination of calcite veins penetrating the dark gray siltstone reveals numerous oil-filled and subordinate aqueous fluid inclusions. Homogenization temperatures indicate these rocks have been subjected to temperature of at least 110-115°C (230-239°F). Burial during the Phanerozoic is inadequate to account for the homogenization temperatures and thermal maturity of the Precambrian rocks. With the present geothermal gradient, at least 8250 ft (2.5 km) of burial is necessary, but lesser burial may be likely with probably higher geothermal gradients during rifting. Fluorescence colors and gas chromatograms indicate compositions of oils in the fluid inclusions vary. However, oils in the fluid inclusions are markedly dissimilar to the nearest oils produced from Paleozoic rocks.

Speculations on the origin of the North American Midcontinent rift

The Midcontinent rift is an example of lithospheric extension and flood basalt volcanism induced when a new mantle plume arrived near the base of the lithosphere. Very large volumes of basaltic magma were generated and partly erupted before substantial lithospheric extension began. Volcanism continued, along with extension and deep rift subsidence, for the ensuing 15 m.y. Much of the basaltic magma, including some of the earliest flows, was formed by partial melting of isotopically primitive asthenosphere contained in the plume head. The intense but relatively short duration of rifting and magmatism is a result of the dissipation of thermal and mechanical energy in the plume head. As the plume head spread beneath the lithosphere, it stretched the overlying lithosphere radially away from the Lake Superior region, the triple junction of the rift system, and partially melted to form the great volume of basalt and related intrusive rocks of the region. The plume arrived beneath a continent that was under compression as a result of the ongoing Grenville orogeny that affected a large region east of the rift. That compression prevented full continental separation and eventually returned the region to compressional tectonics as the energy of the plume head waned.

Geophysical investigations and crustal structure of the North American Midcontinent Rift system

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.

Organic matter and copper mineralization at White Pine, Michigan, U.S.A.

Indigenous and exogenous organic matter in sediments of the North American Midcontinent Rift were involved in precipitation of Cu-bearing minerals during main- and second-stage mineralization events at White Pine, Michigan. Samples of the Proterozoic Nonesuch Formation from the White Pine mine and adjacent areas were investigated petrographically and geochemically in order to determine the influence of organic matter on the precipitation of Cu and to determine whether mineralization influenced thermal maturity of organic matter. During main-stage mineralization, introduction of cupriferous fluids into the lower Nonesuch Fm. formed Cu-bearing sulfides by replacement of pyrite. Sulfur may have been added to the system via abiogenic sulfate reduction, with organic matter providing a direct source of reducing potential. In basal ore horizons where all sulfide had been incorporated into chalcocite during main-stage mineralization, native Cu also precipitated locally, with organic matter presumably acting as a direct reductant. Later compressional faulting at White Pine provided conduits for additional cupriferous fluids; these fluids deposited second-stage Cu mineralization. Inclusions of liquid petroleum and solid pyrobitumen in veins filling fractures associated with compressional faults record migration of petroleum into the White Pine district from the deeper, hotter, axis of the rift penecontemporaneous with second-stage fluids. Some of this petroleum may have acted as a reductant during precipitation of native Cu in basal ore horizons. Biomarker compositions of bitumens and isotopic compositions of individual n-alkanes from petroleums and bitumens suggest that these materials were all derived from organic matter deposited during sedimentation of the Nonesuch Fm.. Biomarker data demonstrate that bitumens from the White Pine area have significantly higher levels of thermal maturity than bitumens from unmineralized strata outside the mine area. Therefore, Cu mineralization at White Pine coincides with a thermal anomaly. Seep petroleums from the northeast and southwest domains of the mine contain different n-alkane distributions; we propose that this difference reflects elevated thermal maturity of seep petroleums spatially associated with second-stage mineralization in the southwestern domain of the mine.

Sulfur/carbon ratios and extractable organic matter of the middle proterozoic nonesuch formation, north american midcontinent rift

The Nonesuch Formation (ca. 1.1 Ga), host to the White Pine copper deposit, is a generally organic-rich unit of relatively wide extent. Selected strata, away from mineralization, were analyzed for their organic carbon and total sulfur contents and for the amount and composition of extractable organic matter. Sulfur to carbon ratios are covariant, but the ratios of total sulfur to organic carbon are generally higher than those found for marine Phanerozoic black shales deposited under oxic bottom water conditions. The sulfur to carbon ratios and the intimate association fo framboidal pyrite and organic matter are suggested to be the result of bacterial sulfate reduction in the Nonesuch sediments. Activity of sulfate reducing bacteria requires a supply of sulfate in the water column and implies that the depositional environment was not a low-sulfate aqueous environment. A marine embayment dominated more by sea water than riverine input is suggested as the environment of deposition based on the correlation of total sulfur and organic carbon, the relatively high total sulfur content, and on the absence of sedimentologic and geochemical criteria for saline lacustrine environments. Deposition of highly reactive organic matter in low concentrations coupled with high concentrations of reactive iron and the absence of bioturbation resulted in unusually efficient pyrite formation. Extractable organic matter in unmineralized Nonesuch strata is interpreted to be autochthonous in origin. Homologous series of n-alkanes and cyclohexyl alkanes, regular isoprenoids and monomethyl-branched alkanes were identified by gas chromatography. Ratios of pristane/phytane, pristane/n-C17 and phytane/n-C18 suggest relatively low thermal maturity for the overall unit with a slight increase in the level of thermal maturity from Michigan westward to Wisconsin. The aliphatic character and high extractability of organic matter indicate that the petroleum potential of the Nonesuch Formation will be good to excellent if the organic-rich facies thicken down-dip under Lake Superior or into the southern extension of the rift.

Sterane and triterpane biomarkers in the Precambrian Nonesuch Formation, North American Midcontinent Rift

Molecular fossils and organic matter from microbial communities living around 1.1 Ga ago are well preserved in the Nonesuch Formation of the US Midcontinent Rift System. Although some compound classes of biological markers have been recognized repeatedly in previous studies of Nonesuch rocks and associated petroleums, steranes and triterpanes have not been detected. Failure to detect these compounds was due primarily to instrumental limitations and the relatively high thermal maturity of samples collected within zones of copper mineralization. Our analyses of Nonesuch samples from the White Pine Mine and nonmineralized localities in Michigan and Wisconsin reveal suites of C 26–C 30 steranes and C 27–C 35 pentacyclic triterpanes similar to those found in Phanerozoic oils and sedimentary rocks of marine origin. Stereoisomeric distributions in the Nonesuch samples are consistent with moderate thermal maturity except in the mineralized zone near White Pine.

Thermal Maturity of the 1.05 Ga Nonesuch Formation, North American Midcontinent Rift: Biomarker Ratios and Hopane and Sterane Stereoisomers

Differences in the apparent level of thermal maturity indicated by sterane and hopane stereoisomers arise from the relationship between time and temperature in the maturation process. As part of a continuing study of the approximately 1.05 Ga Nonesuch Formation, North American Midcontinent Rift. The authors have evaluated the thermal maturity of organic matter based on biomarker distributions and ratios of stereoisomers (17 samples). Steranes and hopanes, once thought to be absent from Nonesuch bitumen and petroleum, have been identified and quantified by multiple-reaction-monitoring gas chromatography mass spectrometry. Other indicators of thermal maturity including the methylphenanthrene index and the ratio of {alpha}{alpha}{alpha}/{alpha}{alpha}{alpha} + {alpha}{beta}{beta} steranes are covariant with the 20S/20S + 20R ratio of C{sub 29} sterane. The ratios of pristane/nC{sub 17}, phytane/nC{sub 18}, and trisnorhopane/trisnorneohopane (Ts/Tm) are not covariant with the 20S/20S + 20R ratio of C{sub 29} sterane. The ratios of pristane/nC{sub 17}, phytane/nC{sub 18}, and trisnorhopane/trisnorneohopane (Ts/Tm) are not covariant with the 20S/20S + 20R ratio of C{sub 29} sterane reflecting control by the source of organic matter. The long burial history of the Nonesuch has influenced the kinetic transformations of hopane and sterane stereoisomers to a greater extent than thermal influence. The 22S/22S + 22R ratio ofmore » C{sub 31} hopane equilibrates more rapidly than the C{sub 29} 20S/20S + 20R ratio even at relatively low burial temperatures given sufficient geologic time. The Nonesuch Formation is moderately mature with respect to petroleum formation and preservation despite a billion year long burial history.« less

Novel Isotopic Compositions of Total Organic Carbon and Bitumen in the Nonesuch Formation, North American Mid-Continent Rift: ABSTRACT

Novel Isotopic Compositions of Total Organic Carbon and Bitumen in the Nonesuch Formation, North American Mid-Continent Rift: ABSTRACT

The Precambrian nonesuch formation of the North American mid-continent rift, sedimentology and organic geochemical aspects of lacustrine deposition

Abstract The Precambrian Keweenawan Trough in the Lake Superior region of North America is part of a failed intractonic rift that is filled with a thick sequence of volcanic and sedimentary rocks. One unit in this sequence, the None-such Formation, is composed of black-to-green siltstones and shales and is conformable with the underlying alluvial Copper Harbor Conglomerate and the overlying fluvial Freda Sandstone. The formation is exposed as a lens in northern Michigan and Wisconsin and attains a maximum thickness of ∼ 200 m. Based on the integration of outcrop and core data, nine facies have been recognized in the formation; each has been assigned to one of three genetic facies assemblages. A marginal lacustrine assemblage, characterized by interbedded lithic sandstones, siltstones, mudstones and sandstone-shale couplets, is interpreted as a record of deposition on a sandflat-mudflat complex. A lacustrine assemblage is characterized by massive to well-laminated dark shaly siltstones, carbonate laminites, shales, siltstones and mudstones. These sediments were probably deposited in a progressively shallowing perennial lake. Bottom conditions were anoxic and became increasingly more oxic as lake regression progressed. A gradual transition from a lacustrine environment to a fluvial environment is represented by the red to brown, massive, horizontally laminated and rippled fine sandstones, siltstones and mudstones of the fluvial-lacustrine assemblage. Interactions among rates of subsidence, rates of sedimentation, fluctuations in lake level, possible changes in climate and differences in tectonic setting controlled sedimentation in the Nonesuch Lake and resulted in variable vertical facies sequences. Total organic carbon (TOC) analyses show a strong correlation between organic richness and the shale facies (lacustrine assemblage) and mudstone facies (marginal lacustrine assemblage). Detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two prevalent organic petrographies which could be distinguished using combined pyrolysis-gas chromatography-mass spectrometry (PY-GC-MS) and, to a limited extent, by stable carbon isotope mass spectrometry. The geographic and stratigraphic distribution of rocks containing the two organic petrographies suggests that differential preservation (e.g., microbial degradation) accounts for the difference rather than maturity or variable source input.

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).

Notes on the Geological Section of Michigan: Part I. The Pre-Ordovician

Previous articleNext article FreeNotes on the Geological Section of Michigan: Part I. The Pre-OrdovicianA. C. Lane and A. E. SeamanA. C. Lane Search for more articles by this author and A. E. Seaman Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 15, Number 7Oct. - Nov., 1907 Article DOIhttps://doi.org/10.1086/621460 Views: 67Total views on this site Citations: 5Citations are reported from Crossref PDF download Crossref reports the following articles citing this article:Nicholas L. Swanson-Hysell, Jahandar Ramezani, Luke M. Fairchild, Ian R. Rose Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis, GSA Bulletin 131, no.5-65-6 (Jan 2019): 913–940.https://doi.org/10.1130/B31944.1J. F. MILLER, R. L. ETHINGTON, R. ROSE Stratigraphic Implications of Lower Ordovician Conodonts from the Munising and Au Train Formations at Pictured Rocks National Lakeshore, Upper Peninsula of Michigan, PALAIOS 21, no.33 (Jun 2006): 227–237.https://doi.org/10.2110/palo.2004.p04-50R. Douglas Elmore, Gary J. Milavec, Scott W. Imbus, Michael H. Engel The Precambrian nonesuch formation of the North American mid-continent rift, sedimentology and organic geochemical aspects of lacustrine deposition, Precambrian Research 43, no.33 (May 1989): 191–213.https://doi.org/10.1016/0301-9268(89)90056-9 Anonymous Road log: International Geological Congress Field Trip T344, (Jan 1989): 25–62.https://doi.org/10.1029/FT344p0025Hans Becker Die präkambrische Geschichte des Lake Superior-Gebietes, Nordamerika, Geologische Rundschau 22, no.66 (Dec 1931): 385–411.https://doi.org/10.1007/BF01810145