Lake Superior Area Research Articles

Neoproterozoic sedimentation and tectonics of the Laurentian midcontinent: Detrital zircon provenance of the Jacobsville Sandstone, Lake Superior Basin, USA and Canada

AbstractThe Neoproterozoic Jacobsville Sandstone outcrops along the south and east shores of Lake Superior, USA. It records intraplate deformation, some during its deposition and some afterwards, after the c. 1,100 Ma Midcontinent Rift (MCR) failed. Here we analyse 549 new detrital zircon ages from five sites, combined with prior data. Initially, local palaeo‐topography controlled the source material, including the MCR‐adjacent Penokean and Archaean rocks. With time the percentage of distal sources increased, including the c. 1,300–980 Ma Grenville orogeny and 1,480–1,360 Ma Granite‐Rhyolite Province. Sites near the Keweenaw fault contain a significant number of MCR‐age zircons, presumably uplifted to the surface, indicating fault motion during deposition. Only a relatively small percentage of 1,090–980 Ma Grenville‐age zircons from collisions to the east are present, suggesting that they were not efficiently transported to the Lake Superior area. This work is innovative in that it is the first to use detrital zircon geochronology to understand the internal stratigraphy of the Jacobsville Sandstone, whose provenance provides new information about the Neoproterozoic tectonic and sedimentary history of Laurentia.

Growth, overprinting, and stabilization of Proterozoic provinces in the southern Lake Superior region

New geochronologic data in the southern Lake Superior region provide key information on the timing and nature of tectonic activity that pre-and post-date initial Paleoproterozoic growth of Laurentia during the geon 18 Penokean orogeny. The obducted Pembine ophiolite formed along the edge of a Paleoproterozoic ocean basin at least 30 m.y. prior to Penokean island arc/microcontinent accretion beginning at 1860 Ma. Following Penokean orogenesis, intrusion of mafic dikes at 1817 ± 2 Ma indicate a period of extension that coincided with a 30 m.y. gap in orogenic felsic magmatism at 1835–1805 Ma (between the Penokean and Yavapai orogenies) and likely represents relaxation of Penokean compression and a tectonic switch to intra-arc extension related to initiation of Yavapai subduction. Subsequent Yavapai arc accretion (1750–1720 Ma) resulted in pervasive ductile deformation of the dikes and host rocks at temperatures of ~700 °C, previously attributed to Penokean deformation. Geon 16 Mazatzal overprinting of the accreted Penokean and Yavapai provinces was widespread but of overall lower metamorphic grade (greenschist facies), and the thermal effects of the 1476–1470 Ma shallow level Wolf River batholith was limited to a 10–15 km wide contact zone surrounding the intrusion.In contrast to the Archean Superior Province to the north, Paleoproterozoic terranes in the southern Lake Superior area experienced widespread low-temperature reheating and cooling of shallow crustal levels at ca. 1.1–1.0 Ga attributed primarily to magmatic underplating with little subsequent Neoproterozoic exhumation. In the southern Lake Superior region widespread magmatic underplating likely thickened, strengthened, and stabilized Proterozoic Penokean-Mazatzal lithosphere but destabilized Archean cratonized Superior Province lithosphere to the north.

Chemically-oscillating reactions during the diagenetic oxidation of organic matter and in the formation of granules in late Palaeoproterozoic chert from Lake Superior

Filamentous and coccoidal microfossils have been reported since the 1950's from a range of granular cherts from the late Palaeoproterozoic southwestern Superior Craton, Canada-United States. However, the chemical and mineral compositions of granules, the presence of microfossils in granules, and the common presence of granules in intercolumnar space of stromatolitic chert are poorly documented and explained. Furthermore, the depositional model for the origin of granules in wave-agitated waters does not entirely explain their mineral diversity nor their characteristic morphologies and patterns. We report on the crystallinity of organic matter, mineral diversity, and compositions of microfossils in granules from three different kinds of late Palaeoproterozoic cherts, namely phosphatic, organic, and haematitic. Stromatolitic organic-rich chert from the Gunflint Fm contains granules with euhedral carbonate and equidistant concentric laminations of organic matter, akin to fractal patterns from the Belousov-Zhabotinsky (B-Z) chemically-oscillating reaction. These granules also contain authigenic anatase, ferric-ferrous silicates, and Fe-oxides. Filamentous and coccoidal microfossils similar to those of the Gunflint occur in chert from the Biwabik Formation and share morphology, and co-occur with Mn-siderite and apatite. Granules in phosphatic chert in the Michigamme Formation often contain filamentous and coccoidal microfossils composed of organic matter, sericite, and apatite. Bulk carbonate associated with these Michigamme granular phosphatic chert beds has systematically negative δ13Ccarb values around −3.1±0.9‰ (1σ) and δ18Ocarb-SMOW between +20.8 and +30.7‰, which suggest some contribution from the diagenetic oxidation of organic matter. Notably, residual carboxylic acid is detectable in C-XANES spectra of organic matter from granular phosphatic chert, which is a residual reactant of B-Z type reactions. Along with previously reported observations of pyritised microfossils from the Gunflint Formation, these distinct mineralogies indicate variable modes of preservation for the products of chemically-oscillating reactions that likely relate to the availability of different oxidants in the diagenetic environment. We conclude that the late Palaeoproterozoic shallow-marine environments of the Lake Superior area were populated by morphologically similar micro-organisms, and that the diagenetic oxidation of organic matter through chemically-oscillating reactions contributed to the formation of spheroidal rosettes, granules, and concretions during a late Palaeoproterozoic Great Putrefaction Event. Granules in chert that contain organic matter or microfossils thus provide a reliable petrographic context to search for a record of putrefaction of microbial life on the early Earth and on other ancient planetary surfaces.

Emplacement age and isotopic composition of the Prairie Lake carbonatite complex, Northwestern Ontario, Canada

AbstractAlkaline rock and carbonatite complexes, including the Prairie Lake complex (NW Ontario), are widely distributed in the Canadian region of the Midcontinent Rift in North America. It has been suggested that these complexes were emplaced during the main stage of rifting magmatism and are related to a mantle plume. The Prairie Lake complex is composed of carbonatite, ijolite and potassic nepheline syenite. Two samples of baddeleyite from the carbonatite yield U–Pb ages of 1157.2±2.3 and 1158.2±3.8 Ma, identical to the age of 1163.6±3.6 Ma obtained for baddeleyite from the ijolite. Apatite from the carbonatite yields the same U–Pb age of ~1160 Ma using TIMS, SIMS and laser ablation techniques. These ages indicate that the various rocks within the complex were synchronously emplaced at about 1160 Ma. The carbonatite, ijolite and syenite have identical Sr, Nd and Hf isotopic compositions with a 87Sr/86Sr ratio of ~0.70254, and positive εNd(t)1160 and εHf(t)1160 values of ~+3.5 and ~+4.6, respectively, indicating that the silicate and carbonatitic rocks are co-genetic and related by simple fractional crystallization from a magma derived from a weakly depleted mantle. These age determinations extend the period of magmatism in the Midcontinent Rift in the Lake Superior area to 1160 Ma, but do not indicate whether the magmatism is associated with passive continental rifting or the initial stages of plume-induced rifting.

Geochronology of Proterozoic metamorphism in the deformed Southern Province, northern Lake Huron region, Canada

Metamorphic geochronology and thermochronology of the deformed Southern Province north of Lake Huron in Ontario, Canada reveals a complex and protracted tectonothermal history for the Proterozoic southern Laurentide margin that is distinct from that of the Lake Superior region to the west. While Penokean-interval deformation is well-constrained in the Lake Superior area, a well-defined geon 18 geochronometric signature is lacking in the Huronian vicinity. The oldest metamorphic ages revealed by in situ monazite U-Th-total Pb electron microprobe analyses are ca. 1800 Ma, with a strong geon 17 (Yavapai-interval) and local geon 14/15 magmatic influences. Furthermore, conventional 40Ar/ 39Ar incremental step-heating of hornblende indicates Paleoproterozoic heating through 500 °C occurred only adjacent to geon 17 intrusions; regional analysis of mica cooling ages suggests that Mesoproterozoic thermal overprinting of primary geon 17 magmatism and metamorphism is limited to south of the Murray fault. The Penokean orogeny, long considered the dominant Paleoproterozoic event in the Great Lakes region, is now recognized as only the first of several accretionary events that impinged on the southern Laurentide margin. However, the Penokean orogen is restricted to the 2100 Ma embayment of the Superior Province rifted margin. Moreover, subsequent tectonothermal pulses (Yavapai, Mazatzal, and ca. 1450 Ma) recorded in the rocks of the Southern Province in Ontario mimic that documented across much of the central and western United States. Based on the distinct lack of Penokean-interval ages and the strong geon 17 and geon 14 total-Pb and 40Ar/ 39Ar geochronometric signature, the tectonothermal history of Southern Province more closely resembles that of the southwest United States than the Lake Superior region.

Determining the Provenance of native copper artifacts from Northeastern North America: evidence from instrumental neutron activation analysis

The emergence of archaeological interest in native copper in the mid-1800s developed in concert with explanations that privileged the Lake Superior area over other potential sources of copper. Most scholars have thus assumed that when copper artifacts first appeared in Northeastern North America, they arrived as finished implements or were locally made from Lake Superior raw materials. Procurement models that point to Lake Superior as the sole source of native copper have been widely accepted in the absence of systematic large-scale testing. This article evaluates the dominant model for native copper procurement and presents trace element data derived from instrumental neutron activation analysis (INAA) to determine whether hunter-gatherers in the Northeast utilized one dominant source of copper or in fact exploited a number of geological deposits. I specifically report on the chemical characterization of copper from 13 discrete geological deposits and 18 archaeological sites dating to the Late Archaic (ca. 5000–3000 B.P.) and Early Woodland (ca. 3000–2000 B.P.) periods to suggest that the dominant model for native copper procurement is oversimplified.

Carbon isotope record for the onset of the Lomagundi carbon isotope excursion in the Great Lakes area, North America

Carbonate units of the Chocolay Group in the Lake Superior area, USA and the Gordon Lake Formation of the correlative upper Huronian Supergroup, Ontario, Canada were deposited after the last Paleoproterozoic glacial event and an episode of intense chemical weathering. The Huronian Supergroup contains at the base ∼2.45 Ga volcanics and is intruded by the 2.22 Ga Nipissing diabase dykes and sills while the Chocolay Group is bracketed in age between ∼2.29 and 2.20 Ga. The Lomagundi (∼2.22–2.1 Ga) carbon isotope excursion started after the Paleoproterozoic glacial epoch and before a plume breakout event at 2.22 Ga. Therefore, the Chocolay and Upper Huronian carbonates were deposited either before or during the onset of the Lomagundi event. Notably, thin carbonates of the basal Gordon Lake Formation and thick carbonate succession of the Kona Dolomite in the northeastern exposures of the Chocolay Group record δ 13C values as high as +9.5‰ versus V-PDB. Similar to other successions deposited during the Lomagundi event, both units contain pseudomorphs and molds after sulfates. This observation suggests that seawater sulfate contents rose dramatically in association with the onset of the Lomagundi event and the rise of atmospheric oxygen. Carbonates in the western and southern exposures of the Chocolay Group (Randville and Bad River dolomites, and Saunders Formation) previously assumed to be equivalent to the Gordon Lake Formation and Kona Dolomite have carbon isotope values close to 0‰. Based on basin analysis, we infer that these carbonate units were deposited during a negative carbon isotope excursion after the Lomagundi event started and are slightly younger than the Kona Dolomite and Gordon Lake Formation. This interpretation implies that the carbonate platform in the Great Lakes area transgressed to the west over shallow-marine and fluvial deposits. The negative carbon isotope excursion in the Lake Superior area might correspond to similar δ 13C values of the Mooidraai Dolomite in the Griqualand West Basin, South Africa supporting correlation between Paleoproterozoic successions of North America and South Africa and the notion of three global glaciations in the Paleoproterozoic Era. Carbonates of the Mille Lacs Group (Trout Lake, Glen Township, and Denham formations) in Minnesota have δ 13C values ranging from −1.2 to +2.5‰. Combined with geochronologic constraints, these data suggest that these units were deposited after the Lomagundi excursion and are related to the rifting event that led to development of the so far unrecognized ∼2.0 Ga passive margin in the Lake Superior area.

Geochemistry of glacial sediments in the area of the Bend massive sulfide deposit, north-central Wisconsin

Geochemical exploration in northern Wisconsin has been problematic because of thick glacial overburden and complex stratigraphic record of glacial history. To assess till geochemical exploration in an area of thick glacial cover and complex stratigraphy samples of glacial materials were collected from cores from five rotasonic boreholes near a known massive sulfide deposit, the Bend deposit in north-central Wisconsin. Diamond drilling in the Bend area has defined a long, thin zone of mineralization at least partly intersected at the bedrock surface beneath 30–40 m of unconsolidated glacial sediments. The bedrock surface has remnant regolith and saprolite resulting from pre-Pleistocene weathering. Massive sulfide and mineralized rock collected from diamond drill core from the deposit contain high (10s to 10,000s ppm) concentrations of Ag, As, Au, Bi, Cu, Hg, Se, Te, and Tl. Geochemical properties of the glacial stratigraphic units helped clarify the sequence and source areas of several glacial ice advances preserved in the section. At least two till sheets are recognized. Over the zone of mineralization, saprolite and preglacial alluvial and lacustrine samples are preserved on the bedrock surface in a paleoriver valley. The overlying till sheet is a gray, silty carbonate till with a source hundreds of kilometers to the northwest of the study area. This gray till is overlain by red, sandy till with a source to the north in Proterozoic rocks of the Lake Superior area. The complex glacial stratigraphy confounds down-ice geochemical till exploration. The presence of remnant saprolite, preglacial sediment, and far-traveled carbonate till minimized glacial erosion of mineralized material. As a result, little evidence of down-ice glacial dispersion of lithologic or mineralogic indicators of Bend massive sulfide mineralization was found in the samples from the rotasonic cores. This study points out the importance of determining glacial stratigraphy and history, and identifying favorable lithologies required for geochemical exploration. Drift prospecting in Wisconsin and other areas near the outer limits of the Pleistocene ice sheets may not be unsuccessful, in part, because of complex stratigraphic sequences of multiple glaciations where deposition dominates over erosion.

Tectonic setting of the Midcontinent Rift system

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.

Crustal Contamination Identified in Keweenawan Osler Group Tholeiites, Ontario: A Trace Element Perspective

In the Lake Superior area, voluminous Middle Proterozoic tholeiitic basalt with minor rhyolite and interflow sediments occupy the Midcontinent Rift and overlie Archean and older Proterozoic basement. The 3000 m-thick section of gently dipping (~10°SSE) basalts on the Black Bay Peninsula is divided into three suites based on the relationship between stratigraphy, petrography, and geochemical variations (e.g., Mg-number, $$A1_{2}O_{3}$$, La/Sm, and Gd/Yb). The Lower suite consists of augite-olivine porphyritic lavas with high Mg-number, $$SiO_{2}, TiO_{2}$$ and Gd/Yb, but low La/Sm. The overlying Central suite flows are either aphyric or plagioclase porphyritic and have lower Mg-number. $$SiO_{2}$$, La/Sm and Gd/Yb decline upward through the Central suite. The Upper suite lavas are either aphyric or plagioclase porphyritic, and element ratios plot on slightly different trends compared to the Central suite. The Lower suite has a narrow compositional range; the average of the four most primitive lavas is used...

High resolution aeromagnetic survey of Lake Superior

A 57,000 line kilometer, high‐resolution aeromagnetic survey was flown in 1987 as a contribution to the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE). Existing aeromagnetic data from the United States and Canada were combined with the new data to produce a composite map and gridded data base of the Lake Superior region (Figure 1).Analysis of the new data permits more accurate definition of faults and contacts within the Midcontinent Rift system (MCR). The aeromagnetic map provides important information supplemental to the seismic profiles acquired under the GLIMPCE program in 1986, allowing lateral extension of the seismic interpretation. In particular, modeling of the data provides an independent assessment of a reflection seismic model derived along line A (Figure 2). The profile and gridded digital data are available to geoscientists through the Geophysical Data Centre of the Geological Survey of Canada (GSC), while the gridded data are available from the USGS‐EROS Data Center.GLIMPCE was established in 1985 to study the nature and genesis of the crust in the Great Lakes region. Program participants include the GSC, the U.S. Geological Survey (USGS), provincial and state surveys, and Canadian and American universities. In the Lake Superior area, a major objective of the program is to develop thermal, tectonic, and petrogenetic models for the evolution of the MCR and to evaluate these in the broader context of the tectonic evolution of the North American continent.Pre‐1982 geological and geophysical knowledge of the MCR in the Lake Superior region has been summarized by Wold and Hinze [1982]. The Lake Superior region provides a unique window on this Proterozoic rift system, exposing igneous rock of the Keweenawan Supergroup that disappears under Paleozoic cover to the southwest.

Toxic contaminants in rainfall in Canada: 1984

Replicate rain samples for most of the 1984 wetfall season were collected from four widely separated sites in Canada, and concentrations and loadings of a number of polychlorinated biphenyls and organochlorine pesticides were determined for each location. Comparisons were made among the sites and with results from other years. The major contaminants in 1984 were, in order of concentration, α-hexachlorocyclohexane (α-HCH or α-BHC), lindane and polychlorinated biphenyls (PCBs). Found at lower levels were dieldrin, methoxychlor (in the Lake Superior area), DDT residues, endrin (especially at Kouchibouguac), heptachlor epoxide and hexachlorobenzene. The loading of the α-HCH/lindane pair was higher at the Atlantic coastal site than at the other locations, possibly because of the greater precipitation there. No geographic trend for PCBs was apparent. For the other substances, no significant pattern to the differences between sites can be discerned. The qualitative similarities of concentrations and loadings at the widely separated locations, however, suggest long-range atmospheric transport. A comparison between results from two Lake Superior sites indicated no shoreline effect on the concentrations of over-lake versus shoreline samples. The differences observed were in accord with those observed in 1983 and were attributed to lack of replenishment from the lake surface.

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.

Comparative Precambrian Stratigraphy Along Mid-Continent Rift Trend: ABSTRACT

In 1983, leasing agents representing several major exploration corporations appeared in the Lake Superior district of northern Wisconsin, an area best known for its exposures of Duluth gabbro and Mellen black granite, and not for its hydrocarbon potential. The geologic column of interest is the 7,620 + m (25,000 + ft) thick sequence termed the Keweenawan Supergroup (Precambrian), a basal shale that has been Rb-Sr-age dated at 1,075 ± 50 billion years. Within the past decade, this sequence has been recognized as having been deposited in response to tensional processes resulting in the development of a rift system. This Mid-Continent rift is best outlined by gravity and can be traced from the Keweenawan Peninsula of Michigan across northern Wisconsin, Minneso a, Iowa, and Nebraska into northeastern Kansas, a distance of 1,290 km (800 mi). An organic-rich shale source unit is known in the Lake Superior area, where it creates intermittent subsurface oil seeps in the White Pine copper mine of Michigan. To the southwest along the trend of the rift, the extent of Keweenawan-equivalent sediments are masked by Paleozoic and younger sediments, increasing from a feather edge in central Minnesota to a reported 1,524 m (5,000 ft) in southwestern Iowa. In the mid-1960s, a natural-gas-storage drilling program centered on Dakota County, Minnesota, cored extensive thicknesses of Precambrian clastics, subsequently named the Solor Church Formation. This subsurface unit, together with surface exposures of the younger Hinckley Sandstone and Fond du Lac Formation, constitutes the Keweenawan section of Minnesota. Less is known of the Kewee awan-equivalent lithology of Iowa, where the Minnesota terminology is being used in geophysical studies of the Mid-Continent gravity high. Here, a late Precambrian red clastic sequence is known, apparently similar to that termed the Rice Formation in Kansas--a carbonate subordinate sequence of sandy shales and feldspathic sandstones. End_of_Article - Last_Page 1317------------

Oil and Gas Developments in North Mid-Continent in 1984

Common to the Committee on Statistics of Drilling District 12 area are the recent exploration activities associated with the Central North American rift system or Mid-Continent geophysical anomaly (MGA), a major feature that runs from the Lake Superior area south into Kansas. For the last several years, much preliminary geologic and geophysical work has been undertaken, which usually proceeds a major play. The primary purpose is to test the Cambrian and Precambrian sediments know to have oil seeps in Wisconsin and Michigan. In 1984, Texaco USA drilled the first deep test, which was in Kansas. Although the well was apparently a dry hole, Texaco's findings have not been released. Kansas had a very active year with 7,451 completions, 45 more than those reported in 1983. The success rate of all wells drilled for oil or gas (7,307) was 57.5%, down slightly from 59.3% in 1983. Drilling for oil continued to predominate with 3,783 oil wells and 419 gas wells completed. Total footage was 22,486,535, up 4% from 1983. The average depth of a test drilled for oil or gas was 3,026 ft. In Missouri, the number of wells drilled for oil or gas declined 17% from 1983 levels. Most drilling continued to be in the western part of the state. A deep test in Vernon County penetrated 2,080 ft of Precambrian rocks. In Nebraska, 12 new discoveries were made in the western part of the state. Seven found new oil reserves, and 5 were tight holes; all were classified as new-field wildcats. The average depth was 5,465 ft in the 7 discoveries where the operator reported the total depth. In Mills County, Iowa, 4 wildcats were drilled to the Cambrian with depths from 3,000 to 3,300 ft. All were located approximately 35 mi north of the Tarkio field in northwestern Missouri. It is estimated that 2,000,000 ac are leased in Iowa along the MGA. In Minnesota, 400,000 ac were leased during 1984. The leases were concentrated mainly along the MGA from Duluth to the Iowa border. About 1,000 mi of Vibroseis was run across this feature. In Wisconsin, regional geophysical surveys along the MGA have been run. Companies are now doing more detailed seismic work. Acreage leased from October 1983 to January 1985 was estimated at 214,000 ac. A dry hole was drilled 1,000 ft into quartzite in Barron County.

LAKE SUPERIOR AREA TEMPERATURE VARIATIONS∗

Variations in atmospheric circulation and spatial and temporal patterns of sea-level pressure anomalies over eastern North America are highly correlated in time with temperature trends in the Lake Superior area. Lake Superior region temperature trends have not been the same for all seasons. January and July temperatures were highest in the mid-1930s; those for April and October culminated later. Differences in the timing of temperature peaks within the Lake Superior region are related mainly to variations in pressure anomalies over western Canada and with associated anomalous air flow components across the Lake. Pressure anomalies over the southwestern United States and opposite anomalies to the east have been less important and they have affected mainly the temperatures over the eastern part of the region. About seventy to eightly percent of the temperature variance for the Lake Superior region is related to about twenty-five to thirty percent of the pressure variance for eastern North America.

Paleomagnetism of the Haliburton Intrusions: Superimposed magnetizations, metamorphism, and tectonics in the Late Precambrian

Three components of remanent magnetization have been isolated by af and thermal demagnetization of highly metamorphosed dioritic and gabbro-anorthositic rocks from Haliburton County, Ontario, in the Grenville structural province of the Canadian Precambrian Shield. The predominant A component, a normal magnetization, defines a paleoinagnetic pole at 142.5°E, 35.6°S (α95 = 6.3°, k = 37.6), near southeast Australia, in agreement with published poles from widely separated areas in the Grenville Province. Paleopoles for the normal B and reversed C magnetizations fall much farther north, at 172.3°E, 24.5°N (α95 = 15.7°, k = 9.4) and 167.4°E, 3.0°S (α95 = 6.6°, k = 53.5), respectively. The C pole is close to secondary poles of other studies, but the B pole has not been detected elsewhere within the Grenville Province. It lies among poles for late Keweenawan rocks of the Lake Superior area. The A and B components are frequently superimposed in individual samples. The three components have distinctive coercivity and blocking temperature spectra, which we interpret as recording mineralogy and physical conditions (temperature and duration of heating) at different stages of slow cooling following high-grade regional metamorphism of the Grenville Province. They constitute a record, from a single locality, of apparent polar wander between about 1200 and 950 m.y. The A magnetization was acquired at high temperatures, the B at much lower temperatures. Although both B and C magnetizations nay have been set during subsidiary thermal pulses, a single-stage heating-cooling model of Grenville metamorphism is not ruled out. The proposed age sequence is A-C-B, a sequence most naturally explained by convergence of separate Grenville and North American plates. However, one-plate models which would incorporate the A-C-B sequence into a ‘Grenville Loop’ in the North American polar wander path are also possible.

Ambient Pyrite in Precambrian Chert: New Evidence and a Theory

Ambient pyrites of two distinct types were described from middle Precambrian rocks of the Lake Superior area. A new class of this phenomenon is here described from middle Precambrian chert from western Australia. The newly found ambient pyrites are quite minute and characteristically occur in groups forming a "starburst" pattern. All three types of ambient pyrite may be explained in terms of pressure solution initiated by gas evolution from organic material attached to the pyrite. Thermal degradation of the kerogen produces the gases which, due to the impermeability of the encompassing chert, build up the pressures necessary to initiate solution. Pyrite appendages bear a striking resemblance to micro-organisms and, thus, constitute the smallest pseudofossils known.

Three Great Basins of Precambrian Banded Iron Formation Deposition: A Systematic Comparison

The origin of Precambrian banded iron formations is controversial. One type of evidence is a comparison of different occurrences. Three of the best-preserved basins of Precambrian iron formation deposition are here denned and compared systematically: the Hamersley Basin of Western Australia, the Animikie Basin of North America, and the Transvaal System Basin of South Africa. After a brief summary of the broad structural and stratigraphic evolution of each basin, their differences and similarities are reviewed. They differ mainly in: major stratigraphic sequence, thickness of iron formation, thickness variation, stratigraphic continuity, lithology of iron formation, nature of sedimentary structures, conspicuousness of varves (micro-banding), clastic association, and abundance of diagenetic riebeckite. With some reservations, they resemble each other in: age, basin size, chemistry and mineralogy, broad time-structural evolution of basin, precedence of chert, volcanic association, and in the restricted, medial and transitional stratigraphic status of the main iron formation. It is concluded that the Western Australian and South African iron formations are very closely similar, but that both differ markedly from the Animikie iron formations of the Lake Superior area. However, all three are more closely similar in chemistry than is required by the fact that their definition is chemical. Quantitative geochemical argument indicates a direct volcanic contribution of iron to the Hamersley Basin; it is possible that such vulcanicity is restricted to a limited medial part of basin development in a particular kind of basin-forming process.

Future of Lake Superior Iron Ore Supply

One hundred thirty-two tons of iron ore on the brigantine Columbia passed through St. Mary's canal in August, 1855, the first such cargo taken from upper to lower lake ports without breaking bulk. Lake Superior ore shipments mounted slowly till after 1880, and rapidly not till some time after the opening of Mesabi Range. Lake Superior district then began to supply an increasing share of national iron ore consumption; demand rose with population increase and with growth of per capita consumption of iron and steel, a consumption that by 1900 measured seven-fold that of a half century previous, and more than four times that of thirty years earlier. Average annual shipments for the decade 1921-1930 surpassed fifty million tons, reaching an all-time peak in 1929 when some sixty-three million tons moved from six upper lake to nearly three times as many lower lake ports. Some years after peak shipment the lake ore fleet yet consisted of some three hundred thirty vessels. Their combined carrying capacity, on the nineteen foot draft then current, totalled 2,780,500 tons per round trip. Since lake carriers can make as many as twenty-five round trips per season past records could fall without further additions to the ore carrying fleet. Following the first forty years of shipping ore from Lake Superior area, new discoveries seemed imminent enough to provide for all future demands; after a second four decades, with shipment proceeding at a much faster rate than during the first, a student of the iron ore industry finds the district confronted with a