Chocolay Group Research Articles

New U-Pb zircon tuff ages and revised stratigraphic correlations in the Superior craton during the Great Oxidation Episode

Major environmental changes during the Great Oxidation Episode are recorded in 2.5–2.0 Ga sedimentary successions across the world. In North America, the Huronian Supergroup is the most complete succession, preserving three glacial intervals with possible equivalents in the Lake Superior region. The third and youngest glacial unit (Gowganda Formation, Cobalt Group) of the Huronian Supergroup has been correlated with glacial deposits of the Chocolay Group in the Lake Superior region. Here we present the results of in situ SHRIMP U-Pb geochronology of zircon in thin tuff layers from the basal Wewe Slate, Chocolay Group, from drill-core in the Marquette Range area, upper Michigan. Zircon U-Pb data from two intervals ∼16 m apart yielded weighted mean 207Pb/206Pb ages of 2174 ± 9 Ma and 2172 ± 6 Ma, which define the depositional ages of the Wewe Slate and the conformably underlying Kona Dolomite. In the Huronian Supergroup, SHRIMP U-Pb dating of zircon in a tuff bed from the Gordon Lake Formation, Cobalt Group, yielded a weighted mean 207Pb/206Pb age of 2318 ± 8 Ma, interpreted to be the age of deposition. Re-examination of previously published U-Pb zircon data from the Gordon Lake Formation (Rasmussen et al., 2013) yields a slightly older weighted mean 207Pb/206Pb age of 2310 ± 5 Ma. Our results show that the Wewe Slate, and probably the Kona Dolomite, are ∼150 million years younger than the ∼2.32 Ga Gordon Lake Formation, with which they were previously correlated. Our results suggest that the glacial deposits in the Lake Superior region represent a fourth Paleoproterozoic glaciation in North America, whose age range corresponds to that of the fourth and final early Paleoproterozoic glaciation in southern Africa. We propose that the Chocolay Group was deposited after the Huronian Supergroup and is broadly coeval with the rift-passive margin succession (Cycle 1 of the Kaniapiskau Supergroup) in the New Québec Orogen, Canada, whose deposition has been linked to rifting and breakup along the eastern margin of the Superior craton at ∼2.22 Ga.

Did prolonged two-stage fragmentation of the supercontinent Kenorland lead to arrested orogenesis on the southern margin of the Superior province?

Recent geochronological investigations reinforce the early suggestion that the upper part of the Paleoproterozoic Huronian Supergroup of Ontario, Canada is present in the Animikie Basin on the south shore of Lake Superior. These rocks, beginning with the glaciogenic Gowganda Formation, are interpreted as passive margin deposits. The absence of the lower Huronian (rift succession) from the Animikie Basin may be explained by attributing the oldest Paleoroterozoic rocks in the Animikie Basin (Chocolay Group) to deposition on the upper plate of a north-dipping detachment fault, which lacks sediments of the rift phase. Following thermal uplift that led to opening of the Huronian Ocean on the south side of what is now the Superior province, renewed uplift (plume activity) caused large-scale gravitational folding of the Huronian Supergroup accompanied by intrusion of the Nipissing diabase suite and Senneterre dikes at about 2.2 Ga. Termination of passive margin sedimentation is normally followed by ocean closure but in the Huronian and Animikie basins there was a long hiatus -- the Great Stratigraphic Gap -- which lasted for about 350 Ma. This hiatus is attributed to a second prolonged thermal uplift of part of Kenorland that culminated in complete dismemberment of the supercontinent shortly before 2.0 Ga by opening of the Circum-Superior Ocean. These events caused regional uplift (the Great Stratigraphic Gap) and delayed completion of the Huronian Wilson Cycle until a regional compressional tectonic episode, including the Penokean orogeny, belatedly flooded the southern margin of the Superior province with foreland basin deposits, established the limits of the Superior structural province and played an important role in constructing Laurentia.

Thermal history of low metamorphic grade Paleoproterozoic sedimentary rocks of the Penokean orogen, Lake Superior region: Evidence for a widespread 1786 Ma overprint based on xenotime geochronology

Paleoproterozoic strata in northern Michigan, Wisconsin, and Minnesota were deposited between 2.3 and 1.75 Ga within the rifted margin and subsequent foreland basin of the Penokean orogen. These strata show evidence for multiple regional metamorphic events previously attributed entirely to the Penokean orogeny (1875–1835 Ma). Metasandstones from the Marquette Range Supergroup and the Animikie, Mille Lacs, and North Range Groups were sampled at multiple localities across Minnesota, Wisconsin and Michigan for metamorphic xenotime suitable for in situ SHRIMP U–Pb geochronology. All samples are from the northern Penokean foreland basin where the metamorphic grade is greenschist to sub-greenschist and the strata are virtually undeformed. Xenotime U–Pb ages in these samples have a bimodal population with means of 1786 ± 4 Ma ( n = 32) and 1861 ± 10 Ma ( n = 9). Xenotime of both ages are contained in metasandstones from the basal Chocolay Group in Michigan and Wisconsin and the Mille Lacs Group and North Range Groups in Minnesota. The older age records a regional low-temperature thermal event that is slightly older than the overlying Menominee Group in Michigan and the Animikie Group in Minnesota and Ontario. This 1861 Ma event coincides with regional uplift that led to the formation of the unconformity between the Menominee Group and the overlying Baraga Group in Michigan; hence xenotime growth must have occurred at shallow burial depths. Younger units from the Menominee and Baraga Groups in Michigan and the Animikie Group in Minnesota, record only the 1786 Ma event. A dominant 1800–1790 Ma metamorphic monazite population that overprints Penokean-interval monazite has been documented within amphibolite- to granulite-facies rocks immediately north of the Niagara Fault Zone within the vicinity of gneiss domes and granitic plutons. In contrast, the 1786 Ma xenotime ages are from low-grade, virtually undeformed rocks 50–150 km from the high-grade zones and thus do not appear to reflect a local thermal imprint. Rather, the geographic extent of the 1786 Ma xenotime growth event suggests that it reflects a basin-wide, subtle thermal pulse. It is proposed that the xenotime ages record widespread subtle heating triggered by renewed subduction along the orogen due to Yavapai-interval convergence. The 1800–1700 Ma Yavapai terrane forms an accretionary belt throughout the central and southwestern U.S. and truncates the southern part of the Penokean orogen in central Wisconsin and southeastern Minnesota, about 200 km south of the sample sites. Alternatively, an 1800–1765 Ma interval of gravitational collapse of overthickened crust of the Penokean orogen immediately north of the Niagara Fault Zone may have driven a northward flow of hydrothermal fluids which subtly but pervasively altered the northern parts of the Penokean foreland and resulted in xenotime growth.

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.

Age constraints for Paleoproterozoic glaciation in the Lake Superior Region: detrital zircon and hydrothermal xenotime ages for the Chocolay Group, Marquette Range Supergroup

A geochronological study of the Chocolay Group at the base of the Paleoproterozoic Marquette Range Supergroup in Michigan, Lake Superior Region, is attempted for the first time. Age data from detrital zircon grains and hydrothermal xenotime from the basal glaciogenic formation, the Enchantment Lake Formation, and the stratigraphically higher Sturgeon Quartzite and its equivalent, the Sunday Quartzite, provide maximum and minimum age constraints for the Chocolay Group. The youngest detrital zircon population in the Enchantment Lake Formation is 2317 ± 6 Ma; in the Sturgeon Quartzite, it is 2306 ± 9 Ma, and in the Sunday Quartzite, it is 2647 ± 5 Ma. The oldest hydrothermal xenotime age in the Enchantment Lake Formation is 2133 ± 11 Ma; in the Sturgeon Quartzite, it is 2115 ± 5 Ma, and in the Sunday Quartzite, it is 2207 ± 5 Ma. The radiometric age data in this study implies the depositional age of the Chocolay Group is constrained to ~2.3–2.2 Ga, which proves its correlation with part of the Huronian Supergroup in the Lake Huron Region, Ontario, and reveals the unconformity that separates the Chocolay Group from the overlying Menominee Group is up to 325 million years in duration. The source(s) of the ~ 2.3 Ga detrital zircon populations in the Enchantment Lake Formation and Sturgeon Quartzite remains an enigma because no known rock units of this age are known in the Michigan area. It is speculated that once widespread volcano-sedimentary cover sequences in Michigan were removed or concealed prior to Chocolay Group deposition. The hydrothermal xenotime ages probably reflect basinal hydrothermal fluid flow associated with the period of extension, involving rifting and major dyke formation, that affected the North American provinces between 2.2 and 2.1 Ga.

Stratigraphic and tectonic settings of Proterozoic glaciogenic rocks and banded iron-formations: relevance to the snowball Earth debate

Among Palaeoproterozoic glacial deposits on four continents, the best preserved and documented are in the Huronian on the north shore of Lake Huron, Ontario, where three glaciogenic formations have been recognized. The youngest is the Gowganda Formation. The glacial deposits of the Gowganda Formation were deposited on a newly formed passive margin. To the west, on the south side of Lake Superior, the oldest Palaeoproterozoic succession (Chocolay Group) begins with glaciogenic diamictites that have been correlated with the Gowganda Formation. The >2.2 Ga passive margin succession (Chocolay Group=upper Huronian) is overlain, with profound unconformity, by a >1.88 Ga succession that includes the superior-type banded iron-formations (BIFs). The iron-formations are therefore not genetically associated with Palaeoproterozoic glaciation but were deposited ∼300 Ma later in a basin that formed as a result of closure of the “Huronian” ocean. In Western Australia, Palaeoproterozoic glaciogenic deposits of the Meteorite Bore Member appear to have formed part of a similar basin fill. The glaciogenic rocks are, however, separated from underlying BIF by a thick siliciclastic succession. In both North America and Western Australia, BIF-deposition took place in compressional (possibly foreland basin) settings but the iron-formations are of greatly different age, suggesting that the most significant control on their formation was not oxygenation of the Earth’s atmosphere but rather, emplacement of Fe-rich waters (uplifted as a result of ocean floor destruction?) in a siliciclastic-starved environment where oxidation (biogenic?) could take place. Some of the Australian BIFs appear to predate the appearance of red beds in North American Palaeoproterozoic successions and are therefore unlikely to be related to oxygenation of the atmosphere. Neoproterozoic glaciogenic deposits are widespread on the world’s continents. Some are associated with iron-formations. Two theories have emerged to explain these enigmatic BIFs. According to the snowball Earth hypothesis (SEH), ice-covered oceans would have permitted buildup of dissolved Fe. Precipitation of Fe-rich sediments would have taken place following reoxygenation of the hydrosphere as the ice cover disappeared. A second theory involves glaciation of Red Sea rift-type basins. Fe-charged brines in such basins would have precipitated on being mixed with “normal” seawater as a result of glacially driven thermal overturn. Both theories provide an explanation of the hydrothermal imprint on the geochemistry of Neoproterozoic BIF but the restricted development of BIF (relative to glacial deposits), evidence of rift activity such as significant facies and thickness changes, and association with volcanic rocks, all favour deposition in a rift environment. Cap carbonates are one of the cornerstones of the SEH. Escape from the snowball condition is said to have resulted from buildup of atmospheric CO 2 while the weathering cycle was stopped. Under such conditions, the first siliciclastic deposits following glaciation, should be extremely weathered, and should be overlain by sedimentary rocks that show a gradual return to more “normal” compositions. Using a chemical index of alteration (CIA) it can be shown that, in the case of the Gowganda Formation, the CIA shows a gradual upward increase, opposite to that predicted by the SEH. The Earth underwent severe climatic perturbations both near the beginning and end of the Proterozoic Eon but whether it attained a totally frozen surface condition (as postulated under the SEH) remains speculative.

Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region

A rhyolite in the Hemlock Formation, a mostly bimodal submarine volcanic deposit that is laterally correlative with the Negaunee Iron-formation, yields a sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon age of 1874 ± 9 Ma, but also contains inherited Archean zircons as old as 3.8 Ga. This precise age determination for the classic Paleoproterozoic stratigraphic sequence of northern Michigan, the Marquette Range Supergroup (MRS), necessitates modification of previous depositional and tectonic models. Our new data indicate that the Menominee Group, previously ascribed to continental rifting during early, pre-collision phases of the Penokean orogenic cycle, is coeval with arc-related volcanic rocks now preserved as accreted terranes immediately to the south and is more aptly interpreted as a foredeep deposit. We interpret these to be second-order basins created by oblique subduction of the continental margin rather than basins formed on a rifting margin. Along with a recently reported age for the Gunflint Formation in Ontario of 1878 ± 2 Ma, our data suggest that an extensive foredeep in the western Lake Superior region was the locus of iron-formation deposition during arc accretion from the south. Further, we interpret the lower MRS (Chocolay Group), a glaciogenic and shallow-marine succession that lies atop Archean basement, to be equivalent to the upper part of the Huronian Supergroup of Ontario and to represent the original continental rifting and passive-margin phase of the Penokean cycle. The upper MRS (Baraga Group) represents deeper marine basins, dominated by turbidites and lesser volcanic rocks, resulting from increased subsidence and continued collision. A stitching pluton, which cuts correlatives of the Hemlock Formation in a thrust sheet, yielded a U–Pb zircon age of 1833 ± 6 Ma, consistent with other post-tectonic plutons in Wisconsin and northern Michigan, indicating that Penokean convergence lasted no longer than ~40 million years.

The lower Proterozoic Fern Creek Formation, northern Michigan: mineral and bulk geochemical evidence for its glaciogenic origin

Bulk chemical data indicate the Fern Creek Formation (Early Proterozoic, Marquette Range Supergroup, Lower Chocolay Group) originated as a glacial till, resolving an old argument about the correct interpretation for diamictite-containing units within the formation. There are poor correlations among SiO2, Al2O3, and K2O, and the chemical index of alteration averages 55. These are qualities characteristic of tills and other sediments accumulated in the absence of chemical weathering and transport-induced sorting and inconsistent with an origin as a fluvial deposit (the principal alternate hypothesis). The Archean Carney Lake Gneiss might be the source for at least the lower portions of the Fern Creek Formation. Compositions are similar, and rare earth element trends in the Fern Creek Formation are consistent with derivation from an Archean high-grade terrane. Pegmatites in the gneiss could be the source for monazite, huttonite, and an unnamed fluor-hydroxy-rare earth mineral that occurs in the midsection of the Fern Creek Formation. In addition, mineral and bulk chemical trends are consistent with models involving deep erosion of a complex Archean source, similar to models previously developed for Gowganda tills in the Huronian Supergroup. Samples of the Sturgeon River Quartzite occurring above the Fern Creek Formation were also analyzed. The bulk chemistry is consistent with a well-sorted, supermature quartz arenite.

Two early Proterozoic successions in central Wisconsin and their tectonic significance

Geological mapping of 7,500 km 2 of previously virtually unmapped Precambrian rocks in central Wisconsin suggests the presence of two early Proterozoic sedimentary-volcanic-plutonic successions. Evidence for the two successions includes structural, stratigraphic, metamorphic, and lithologic relations that are consistent throughout the area. The older succession unconformably overlies Archean rocks. It is characterized by amphibolite-facies metamorphism, folding about west- to northwest-plunging isoclinal (F-1) fold axes with more open, coaxial F-2 folding. Basal quartzite and pelitic sediments of the older succession are overlain by volcanic rocks and intruded by syntectonic tonalite, granodiorite, and trondhjemite plutons. The younger succession consists primarily of a greenschist-facies calc-alkaline basalt-rhyolite sequence that unconformably overlies the older amphibolite-facies rocks. Structures in the greenschist sequence trend northeasterly with fold axes that plunge easterly. The plutonic rocks are dominantly syntectonic to late tectonic granites and quartz monzonites with only minor tonalite. In most areas, the two successions have been juxtaposed along major faults. The older amphibolitic sequence may correlate with the Chocolay Group farther north in the Lake Superior region, and the younger green-schist rocks with the Menominee and/or Baraga Groups. If this correlation is correct, the deformation, metamorphism, and intrusion of the green-schist succession would have occurred during the Penokean Orogeny. Metamorphism to amphibole rank, isoclinal folding, and tonalite intrusion of the older succession would be a major pre-Penokean erogenic event within the early Proterozoic that may correlate with the post-Chocolay-pre-Menominee break in northern Michigan, Wisconsin, and Minnesota. We suggest that this earlier event has been unrecognized and that it may be a significant part of the early Proterozoic history of the Lake Superior region.

The Chocolay Group, Lake Superior region, U.S.A.: Sedimentologic evidence for deposition in basinal and platform settings on an early Proterozoic craton

The early Proterozoic Chocolay Group in the Lake Superior region is a cratonal sequence consisting of a basal unit overlain successively by quartzite, dolomite, and slate formations. Two regional depositional environments are described: (1) present-day structural troughs (Marquette, Menominee, and Felch, Michigan) show evidence of having sedimentary-basin precursors; and (2) other areas (Gogebic Range, Michigan) show evidence of sedimentation in platform settings. The basal unit in the Marquette and Menominee troughs is as much as 160 m thick and was probably deposited by alluvial processes. In the Gogebic Range, the basal unit is less than 10 m thick and resembles lag deposits at the base of transgressive sequences. The overlying quartzite contains cross-beds that show two regional sedimentary-vector (paleocurrent) distributions: (1) unimodal low-variance distributions with sedimentary vectors oriented subparallel to local structural trough axes (Marquette, Menominee, and Felch troughs); and (2) polymodal distributions with sedimentary vectors showing greater dispersion about the mean (Gogebic Range). As indicated by many varieties of sedimentary structures (silicified evaporites, cryptalgal structures, intraclast breccias, and so on), the carbonates and interstratified siliciclastics of the Chocolay Group were deposited in subtidal to supratidal conditions. Taylor noted, in 1972, that certain members of the Chocolay Group carbonates thicken toward the center of the Marquette trough, with concomitant facies changes. The uppermost pelite unit of the Chocolay Group was probably deposited below normal wave base. It is concluded that the basins and platforms were formed on a craton undergoing rift-type tectonism.

Early Proterozoic Sedimentary Basins of the Lake Superior Region

Studies of the complexly folded and metamorphosed early Proterozoic strata (1.8 to 2.1 b.y. B.P.; Van Schmus, 1976) of the Lake Superior region (Fig. 1) have revealed features indicating a complex sedimentary-tectonic evolution. These rocks, termed “the Marquette Range Supergroup,” and which include stratigraphic equivalents in Minnesota and Ontario (Cannon and Gair, 1970), have been divided into four groups (James, 1958), which, in ascending order, are the Chocolay, Menominee, Baraga, and Paint River Groups. In Minnesota and Ontario, the stratigraphic equivalents of the Chocolay, Menominee, and Baraga Groups are the Mille Lacs (Chocolay Group equivalent in the Cuyuna area), and lower (Menominee Group equivalent) and upper (Baraga Group equivalent) Animikie Group (Morey, 1978a) (Table 1). The stratified rocks of the Marquette Range Supergroup and stratigraphic equivalents occur in four structural settings (Fig. 1A): (1) elongate, generally synclinal, structural troughs bounded by Archean rocks; (2) extensive homoclines; (3) outliers on granitic basement; and (4) mantling rims around early Proterozoic gneiss domes (Sims, 1976). Some rims also form steep narrow troughs (for example, the Michigan River trough; Klasner and Cannon, 1974), which are here grouped with gneiss-dome rims as opposed to the other structural troughs (type 1 above) which appear to show no genetic relation to gneiss domes. Most of the data discussed herein concern the first two types of structural settings. It is the purpose of this paper to suggest a relationship between the four types of structural settings and the mode of original sedimentation. Specifically, we propose that elongate early Proterozoic sedimentary basins existed in eastern Upper Michigan at the sites of what became structural troughs later in the early Proterozoic; that is, the eastern Upper Michigan structural troughs are simply appressed sedimentary basins. As will be discussed, the Animikie basin (as reported by Morey, 1967; Morey and Ojakangas, 1970; and Alwin, 1979) represents a more “stable” type of sediment receptacle (types 1, 2 structural exposure) with a sedimentary history broadly comparable to the eastern Upper Michigan basins. Finally, it is noted that individual basin history was complex, with some more areas showing evidence of both basinal and platformal sedimentation.

Chocolay Group--Early Proterozoic Cratonic Sequence: ABSTRACT

The Chocolay Group of the southern Lake Superior region (upper Michigan, Wisconsin, and Minnesota), a stable-shelf assemblage bounded by unconformities, resembles Phanerozoic cratonic sequences. The Chocolay Group consists of a lenticular basal conglomerate overlain successively by quartzite (0 to 700 m), dolomite (30 to 800 m), and locally slate; it is capped by a regional unconformity. This sequence, although generally thicker, is similar to cratonic sequences (e.g., Sauk) in that both become finer upward, contain texturally mature sediments, are areally extensive, and are contained by unconformities. New data on the Chocolay Group show interesting divergences from a quiescent stable-shelf model. Paleocurrents from cross-beds in quartzites show strong unimodality (^sfgr = 20 to 60°) atypical of stable platforms. Current directions from ripple marks and cross-beds crudely parallel later Precambrian troughs. Locally there are apparent reversals in the fining-upward trend of the Chocolay Group. Granite and basalt pebbles and abundant feldspars are present locally in dolomitic formations, but are absent in the underlying quartzites. The presence of irregular topography and the occurrence of uplifting locally are suggested by data which include thinning of quartzite members, variations in modal percent of detrital feldspar, and intraformational unconformities in the dolomites. Based on large stratigraphic thickness, variations in sedimentary texture and mineralogy, and unimodality of paleocurrents, analogy to a simplistic stable craton interior is rejected. Rather, the influence of pericratonic tectonic conditions, including fault-bounded troughs, is suggested. End_of_Article - Last_Page 486------------