MINNESOTA GEOLOGICAL SURVEY

MATT WALTON, Director

STRATIGRAPHY AND SEDIMENTOLOGY OF THE LOWER PROTEROZOIC VIRGINIA FORMATION, NORTHERN MINNESOTA

M.E. Lucente and G.B. Morey

Report of Investigations 28 ISSN 0076-9177

UNIVERSITY OF MINNESOTA

Saint Paul - 1983

STRATIGRAPHY AND SEDIMENTOLOGY OF THE LOWER PROTEROZOIC VIRGINIA FORMATION, NORTHERN MINNESOTA

CONTENTS

Abstract ••• • 1 Introduction.

Stratigraphic nomenclature. • • • • • • • • • • • • • • • • • • • • • 4

Stratigraphy. • 4 Lower argillaceous lithosome • • • 5 Upper silty and sandy lithosome. 5

Petrology • • • • • • • • • • • • 5 Argillite and silty argillite. • 5 Sandstone and siltstone. 7 Feldspathic graywacke 7 Arkose. • • • • • •• 9 Limestone. • • • • • • 9 Carbonate concretions. • • 9 Cherty sideritic iron-formation. • .12 Chert •• • .12 Ash-fall tuff •• .12

Sedimentologic attributes • • • • • • .12 Pelagic and hemipelagic deposits • • .12 Turbidite deposits • • • • • .14 Thick-bedded turbidites • .14 Thin-bedded turbidites. • .15 Bottom-current deposits. • .15 Chemically precipitated deposits .15

Sedimentation and provenance. .15

Regional sedimentologic relationships •••••••••••••••• 18

Conclusions • • • .19

References cited. .19

Appendix - Lithic logs of the Virginia Formation. .21

ILLUSTRATIONS

Figure 1--Geologic map of northern and east-central Minnesota • • • • 2

2--Correlation chart • • • • • • • • • • • • • • 2

3--Geologic map of the Mesabi range. 3

4--Cross section of the Virginia Formation • • • • • • • • • • 4

5--Photographs of laminated argillite. • •••••• 6

6--Diagrams summarizing mineralogic composition. • • • • •• 7

7--Photomicrograph of feldspathic graywacke. .10

8--Photomicrograph of unaltered plagioclase in feldspathic graywacke • • •• • •••••• 10

9--Photomicrograph of highly altered plagioclase in feldspathic graywacke •••••••••••••••••• 10

iii Figure 10--Photomicrograph of altered plagioclase with twinning in feldspathic graywacke. . • • • • • • • • • • • • 10

11--Photomicrograph of quartz in feldspathic graywacke. 10

12--Photomicrograph of intergrown plagioclase and quartz in feldspathic graywacke. •• •••• • •••.• 10

13--Photomicrograph of phyllitic rock fragment in feldspathic graywacke ••••• . • • • • • 11

14--Photomicrograph of recrystallized clay matrix in feldspathic graywacke. • •• ••••• • ••••• 11

15--Photomicrograph of pseudomatrix material in f eldspathic graywacke • • •• ••••• • • 11

16--Photomicrograph of authigenic sericite as rim cement in feldspathic graywacke.. •••••• • • 11

17--Photomicrograph of detrital K-feldspar surrounded by authigenic cement in arkose. . • • • • • • • . • 11

18--Photomicrograph of quartz partly replaced by poikilitic sparry calcite cement in arkose • • • • • • • • • • 11

19--Photograph and radiograph of allochemical limestone 13

20--Diagram showing abundance and types of bedding features in graywacke and siltstone 14

21--Photograph of a typical thick-bedded turbidite. 16

22--Photograph of a typical thin-bedded turbidite • 16

23--Photograph of typical bottom-current deposits 16

24--Submarine fan environmental model • • 18

25--Diagrams comparing Virginia, Rove, and Thomson Formations with a hypothetical submarine fan sequence • • 18

TABLE

Table 1 - Mineralogic composition of selected Virginia Formation sandstones • • . • • • • • • • • • • • • • • • • .8

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iv STRATIGRAPHY AND SEDIMENTOLOGY OF THE LOWER PROTEROZOIC

VIRGINIA FORMATION, NORTHERN MINNESOTA

By

M.E. Lucente and G.B. Morey

ABSTRACT

The lower Proterozoic Virginia Formation, a thick sequence of argil­ lite, siltstone, and graywacke, forms the top of the Animikie Group on the Mesabi range in northern Minnesota. It is overlain by thick Quaternary deposits, but is exposed locally where the underlying Biwabik Iron Formation has been mined. Drill cores available from four sites provide stratigraphic and sedimentologic data, and the 450 m of core from a site south of Biwabik and 342 m of core from south of Calumet are proposed as alternate reference sections to the original type locality. The Virginia Formation can be divided into two informal members--a lower argillaceous lithosome and an upper silty and sandy lithosome. The argillaceous lithosome thickens toward the west end of the Mesabi range where it also contains appreciable quan­ tities of chert and a discrete unit of cherty, sideri tic iron-formation. The upper silty and sandy lithosome also consists predominantly of argil­ lite, but is characterized by intercalated beds of siltstone and fine­ grained graywacke. Mineralogic and sedimentologic data indicate that the graywackes were derived largely from granitic and low-grade metasedimentary and metavolcanic rocks like those now exposed in the Archean greenstone­ granite north of the Mesabi range, and that they were deposited by turbidity currents on a southward-dipping paleoslope into a basin south of the range. In detail, the sedimentologic attributes of the formation resemble "thicken­ ing- and coarsening-upward" turbidite sequences associated with the lower and mid-fan parts of a prograding submarine fan complex.

INTRODUCTION

The Virginia Formation of early Protero­ an unknown distance; broadly correlative rocks zoic age forms the uppermost lithostratigraph­ reappear again in east-central Minnesota where ic unit in the Animikie Group on the Mesabi they are assigned to either the Rabbit Lake range in northern Minnesota (Figs. 1 and 2). Formation of the Cuyuna range or the Thomson Along most of the Mesabi range, the Animikie Formation of Carlton and Pine Counties (Fig. Group forms a gently dipping homocline that 2) • strikes east-northeast and dips 5°-15° to the south. The homoclinal structure is inter­ Because of the nearly ubiquitous mantle rupted locally by faults (Fig. 3) and south­ of Pleistocene and Holocene materials, the west-plunging folds. An unknown amount of Virginia Formation is exposed only locally in lower Proterozoic strata was eroded after several iron-ore mines along the Mesabi range tilting and prior to emplacement of the Duluth where a few meters of strata can be seen just Complex at the eastern end of the range in above the Biwabik Iron Formation. The absence middle Proterozoic time. During Cretaceous of natural exposures and the lack of subsur­ time the lower Proterozoic strata were exten­ face information have precluded any detailed sively weathered, and the resulting saprolite stratigraphic and sedimentologic studies of was subsequently covered by a thin veneer of the Virginia Formation. However approximately nonmarine to marine strata. Much of the 1,060 m of diamond drill core of essentially bedrock is now covered by glacial and fluvial unmetamorphosed Virginia Formation at four deposits formed during Pleistocene and sites (Fig. 3) were obtained during an eval­ Holocene times. uation of the possibility of underground mining of taconite in the underlying Biwabik The Virginia Formation is a thick Iron Formation (Pf leider and others, 1968). sequence that consists dominantly of argil­ Al though these cores represent only a small lite, siltstone, and fine-grained graywacke. sample, they are the most complete and con­ It gradationally overlies the Biwabik Iron tinuous record of the Virginia Formation Formation along the south side of the Mesabi available in the public domain and provide the range, and is inferred to extend southward for materials that form the basis for this study...... EXPlANATION

Rocks yomqet thoo lower Prot ..Ql:ok:. undivided ~ ~ Known or inferred IOWl ProterOlOic plutonic: rocks ~ Lower Proterozoic stratified rocks; Iron -formofion shown solid block flZllllJ

o ~O 20 Mila t--r-'-r-' o ro 20Killilmllm

Figure 1. Generalized geologic map of the Lake Superior region showing the distribution of lower Proterozoic rocks and the major iron ranges.

Correlation Chart of Lower Proterozoic Rocks, Northern Minnesota Carlton and Pine Counties Mesabi Range Gunflint Range

Thomson Format ion

? ~ ! Trommold Formation '" Bi wobik Iron ~ ~ E E ~ ~ Mahnomen Formation

~ lillie Falls ~ I------V,f/ ~ Randall and Glen Township ~ Formations

Figure 2. Correlation chart of lower Proterozoic rocks, northern and east-central Minnesota.

2 94° 930 92° 4~~~~ __~ --~~q------~~-L __~ __~ ~ __~ __~ ______-+-48°

v

v •Cotton de V I

47D'-r~~~ __-______A_IT_K_JN_~~C- ____-~~_-J-~ ____------~----~~470 9ZO EXPLANATION

gr sm ms mv

K, e lastie sedimen tary rocks, undivided. dc, Duluth Complex, undivided. v, Virginia Formation. bi, Biwabik Iron Formation and Pakegama Quartzite . gr, granitic rocks including the Giants Range batholith in the central port of the sheet. ms, metamorphosed sed imentary and felsic volcanic rocks, undivided. mv, metamorphosed maf ic-intermediate volcanic rocks. sm, paragneiss and schist -rich migmatite gm, granite -rich migmatite. contact-- fault $ Drill hole discussed in text

Figure 3. Geologic map of the Mesabi r ange and environs (modified from Morey and others , 19B2) showing the loca­ tions (C , Calumet; K, Keewatin; Bu, Buh l; B, Biwabik; 17 , 700 , Drill Hole 17 , 700) of drill cores discussed in the text.

3 STRATIGRAPHIC NOMENCLATURE and south of Keewatin intersect only 92 m and 175 m of strata, respectively, and therefore The term "Virginia Slate" was first pro­ have limited stratigraphic value. The cores posed by Van Hise and Leith (190 1, p . 360) for are described in detail in the appendix, and exposures of " •.• dense, fine-grained, gray to may be examined at the U.S. Bureau of Mines black, sometimes graphitic slate and argilla­ Core Depository, Fort Snelling, Minnesota. In ceous siltstone .•• in numerous test pits and addition to the unmetamorphosed material, drill holes west of the town of Virginia in approximately 158 m of metamorphosed Virginia the central part of the Mesabi range." strata from a fifth site--Drill Hole 17,700 However because the strata are dominantly (Morey and others, 1972)--also was examined argillite, rather than true slate, and because for stratigraphic data, but a strong meta­ beds of siltstone and sandstone make up an morphic overprint precludes any detailed sedi­ appreciable part of the sequence, the name was mentologic study of this material. changed to "Virginia Formation" by White (1954, p. 18) and that terminology is retained here. STRATIGRAPHY

The original type locality is not easily The Virginia Formation consists of inter­ accessible because the area in and around the calated beds of argillite, carbonaceous city of Virginia contains numerous iron-ore argillite, argillaceous siltstone, siltstone, mines. Furthermore Van Rise and Leith never very fine to fine-grained graywacke, and described a specific type section. However lesser amounts of arkose, limestone, chert, the 450 m of Virginia core obtained from south cherty sideritic iron-formation, and ash-fall of Biwabik and the 342 m of core obtained from tuff. In general the minor rock types are south of Calumet (Fig. 3) are representative confined to the lower part of the formation, of the Virginia Formation at the eastern and where they are intercalated with beds of western ends of the Mesabi range, respective­ argillite and carbonaceous argillite. In ly. In lieu of easily accessible exposures at contrast, beds of siltstone and graywacke a well-defined type locality, it is therefore occur predominantly in the upper part of the proposed here that the core material obtained formation. These differences serve to divide from these localities be established as alter­ the Virginia Formation into two informal native reference sections to the original type members--a lower argillaceous lithosome and an locality. Additional cores from south of Buhl upper silty and sandy litho some (Fig. 4).

SW N.E. Calumet Keewatin Buhl Biwabik 17,700 o ..,... ..,... ..,... 0

50 50

wo ~oo

~50 Upper silty ond sandy li thosome ?-?-1-1_1 t __ !--t====- ~ ...... :::::::l--· ·--?--?-1 200 . -r--?__ ,:==:-- "*E , ____ 1----!----1----1---- · ", 250 --=?===:; . 250 '" Q) Q) '"c: .-?-?-?-?-?-?_, '"c: ~ 300 .----?----?-? ~~--- 300 ~ ~ :E I- Interbedded Lower arg illaceous li thosome ~ t_ I- 350 350

400 - ~ 400

450 "-"'- 450 Cherty -Sideritic 500 Iron Formation 500 Biwab ik Iron Formation Figure 4. Diagrammatic northeast-southwest cross section of the Virginia Formation showing inferred relations between various rock types described in the text. Stratigraphic data from Drill Hole 17,700 (Morey and others, 1972) are included for comparative purposes. Datum is the inferred top of the Biwabik Iron Formation; vertical exaggeration is approxi­ mately x 500.

4 Lower Argillaceous Lithosome range is characterized by appreciable quan­ tities of bedded chert. At the site south of The lower argillaceous lithosome is Keewatin the Biwabik-Virginia contact is 23 m approximately 125 m thick in Drill Hole 17,700 beneath the uppermost bedded chert unit, at the eastern end of the Mesabi range and whereas south of Calumet it is 88 m beneath approximately 140 m thick at the drill site the uppermost chert unit (Fig. 4). south of Biwabik. At the Biwabik site it con­ sists predominantly of dark-colored, fissile, The lower argillaceous lithosome south of carbonaceous argillite (Fig. 5a). Very thin Calumet is also characterized by scattered, beds and laminae of dark-grayish-black argil­ centimeter-thick beds of arkose and by laceous siltstone, light-grayish-black silty approximately 27 m of intercalated iron-poor argillite, and bluish-gray to white chert also argillite and cherty sideritic iron-formation occur locally at both sites. This lithosome (Fig. 4). The cherty sideritic rocks are thickens westward; it makes up the entire 175 lithologically similar to those that occur in m of Virginia strata intersected south of the upper part of the Biwabik Iron Formation Keewatin, and 300 m of the formation inter­ at this locality (Pfleider and others, 1968; sected south of Calumet. However these White, 1954). reported thicknesses are somewhat arbitrary in that both lower and upper contacts are gradational. Upper Silty and Sandy Lithosome

At Biwabik and Buhl, the contact with the Approximately 300 m of the upper silty Biwabik Iron Formation is placed arbitrarily and sandy lithosome were recovered from the at the top of the uppermost occurrence of drill site south of Biwabik (Fig. 4). bedded chert (Pfleider and others, 1968). Although the contact with the underlying Al though the chert beds, and hence the in­ argillaceous lithosome is gradational, the ferred contact, need not be precisely correla­ unit can be distinguished mainly by its appre­ tive either spatially or temporally, the strat­ ciable quantities of siltstone (30% of the igraphic interval immediately beneath the section) and fine-grained sandstone (5% of the uppermost chert beds at both localities is section) • The intercalated siltstone and similar, and therefore is broadly correlative sandstone beds, which generally occur in from place to place. Consequently this inter­ packages of strata 1 m to 5 m thick, are light val, which has been intersected at many places colored and range in thickness from several along the eastern and main parts of the Mesabi millimeters to approximately a meter. More or range (White, 1954), has been used traditional­ less monotonous intervals of argillite 6 m to ly to define the approximate top of the 30 m thick separate the siltstone-sandstone Biwabik Iron Formation. Intercalated beds of packages. These argillite intervals are light limestone, argillite, chert, slaty silicate­ colored, silt rich, and nonfissile; they con­ carbonate-facies iron-formation, and ash-fall tain discrete laminae of light-gray siltstone tuff in the interval indicate that clastic (Fig. 5b). sedimentation, chemical sedimentation, and volcanism were all more or less contempora­ neous processes in the transition from Biwabik PETROLOGY to Virginia time. For this study 70 thin sections represen­ From near Keewatin westward along the tative of all rock types encountered in the western part of the Mesabi range, the lime­ diamond drill cores were cut perpendicular to stone-bearing interval and its associated rock bedding and studied using standard flat-stage types were not deposited, and the Biwabik­ petrographic procedures. The percentage of Virginia transition is gradational over potassium feldspar in the coarser grained several tens of meters of dominantly argilla­ samples was estimated by staining thin sec­ ceous strata and lesser amounts of interbedded tions etched by hydrofluoric acid with a chert. Consequently the Biwabik-Virginia con­ potassium cobaltinitrite solution. X-ray tact is arbitrarily placed where iron-bearing diffraction was used to supplement petrograph­ minerals characteristic of the Biwabik Iron ic observations in rocks too fine grained for Formation no longer make up an appreciable optical studies. Lastly, sedimentary textures proportion of the argillite, as inferred from and structures too large to be studied in specific gravity values (pfleider and others, standard-size thin sections were examined 1968). As Gruner (1946) first noted, argilla­ using polished slabs and X-ray photography ceous material containing about 20 percent according to the techniques of Hamblin (1971). iron in the Biwabik Iron Formation has a spe­ cific gravity of about 3.0 to 3.2, whereas typical iron-poor argillite of the Virginia Argillite and Silty Argillite Formation has a specific gravity that rarely exceeds 2.85 and generally is about 2.75. The term argillite is used here for both Thus defined, the lower part of the Virginia massive mudstone and fissile shale in which Formation in the western part of the Mesabi the original clay minerals have been recrystal-

5 Figure 5. Polished slabs of core showing typical laminated argillite from (a) the lower argillaceous lithosome, and (b) the upper silty and sandy lithosome.

lized to various phyllosilicate minerals, Quartz always exceeds plagioclase in modal either during diagenesis or during a very low abundance, and potassium feldspar is typically grade metamorphic event. As defined, argil­ a trace to minor component. Both the quartz lite is the dominant rock type in the Virginia and the feldspar have irregularly embayed and Formation. It comprises 70 to 90 percent of indistinct grain boundaries indicative of the lower argillaceous litho some and approxi­ recrystallization. Patchy areas of fine­ mately 65 to 70 percent of the upper sandy and grained calcite replace and enclose the silty lithosome . phyllosilicate minerals, and euhedral grains of pyrite commonly occur in argillaceous units Argillite varies in color from black to containing abundant carbonaceous material. light gray; darker hues dominate in the lower lithosome where carbonaceous material is an The modal mineralogy of the clay- size abundant constituent, whereas lighter hues matrix, as measured by relative peak heights dominate in the upper litho some where silt­ on X-ray diffractograms, is remarkably uniform size detritus occurs either randomly inter­ throughout the Virginia Formation. Further­ spersed within discrete argillite beds or as more there is no evidence of in situ albitiza­ laminae and very thin beds between argillite tion or authigenesis of potassium feldspar in layers. Many of the silt-rich layers are the argillite samples studied. Thus even either size or content graded. Typically they though the original clay-size detritus has are poorly sorted and have between 5 and 30 been thoroughly recrystallized, it appears percent framework grains . Principal constit­ that all the argillaceous rocks were of simi­ uents include, in approximate order of abun­ lar original composition and subjected to dance, muscovite, chlorite , quartz, plagio­ similar diagenetic or low-grade metamorphic clase , and potassium feldspar. Muscovite and processes . chlorite occur in subequal proportions.

6 sandstone and Siltstone chert or possibly iron-formation (Fig_ 11) and quartzite. The plutonic igneous rock frag­ Beds of sandstone and siltstone range in ments are composed dominantly of intergrown grain size from fine sand to medium silt, are grains of plagioclase and quartz (Fig. 12). A poorly sorted, and are very well indurated by few dark-colored rock fragments have a very a recrystallized clay-size matrix of phyllosil­ fine grained texture and may be of volcanic icate minerals similar to those observed in origin. Large grains of chlorite, biotite, argillite. Modal analyses of 20 selected and muscovite also are present locally in samples are summarized in Table 1. Most of minor quantities; these detrital grains also the sandstones contain somewhat more feldspar than quartz and from 21 to 35 percent matrix material. Siltstone samples are the fine­ grained equivalents of the sandstones and both QUARTZ, CHERT AND QUARTZITE can be classified as feldspathic graywackes (Fig. 6). A few sandstone beds contain less than 15 percent matrix material, and accord­ ingly are classified as arkoses (Fig. 6).

Feldspathic Graywacke

Detrital quartz makes up 16 to 35 percent of the sandstone samples examined and 36 to 52 o • percent of the framework grains. Apparent • o.• long dimensions of the quartz grains are M • markedly variable from sample to sample, • 0 ranging from 0.03 mm to 0.3 mm, but rarely • ... exceed 0.2 mm. Most of the sand-size grains ...... are angular to subangular in shape (Fig. 7), but the original shapes of many of the silt­ size grains are difficult to resolve because FELDSPATHIC LITHIC of incipient recrystallization with enclosing GRAYWACKE GRAYWACKE matrix material at grain boundaries. There is no evidence of in situ precipitation of authi­ FELDSPAR ROCK FRAGMENTS genic quartz, but a few detrital grains have rounded or abraded quartz overgrowths. MATRIX Feldspar forms 23 to 36 percent of the sandstone samples and 42 to 61 percent of the framework grains. Plagioclase dominates over potassium feldspar; both occur as angular to subrounded grains with average long dimensions about equal to those of the quartz grains. Although fresh grains occur (Fig. 8), much of the plagioclase is highly sericitized and nearly opaque (Fig. 9). Where measurable, the plagioclase grains have an apparent index of • refraction less than quartz, and hence prob­ ably a sodic composition. • •..... %. Potassium feldspar occurs in trace amounts • :. ~ to as much as 5 percent at the eastern end of . ~ the Mesabi range, but it makes up as much as ... • FELDSPATH IC GRAYWACKE ~ 16 percent of the graywacke samples from south j5~------... __--~~ of Keewatin. Microcline with tartan-plaid ...... • twinning dominates among the fresher grains, ARKOSE but twinning is difficult to resolve in many grains because of extensive alteration (Fig. FELDSPAR FELDSPATHIC QUARTZ, 10). Rare overgrowths on some feldspar grains QUARTZITE CHERT AND always show evidence of rounding or abrasion. QUARTZITE Figure 6. Summary of the mineralogic com­ Rock fragments of sedimentary, plutonic position of the Virginia Formation using igneous, and metamorphic derivation average the classification scheme of Pettijohn from trace amounts to 11 percent of the ( 1957) • Circles, hole south of Biwabik; graywacke samples studied. Sedimentary rock squares, hole south of Keewatin; triangles, fragments include grains of recrystallized. hole south of Calumet.

7 Table 1. Mineralogic composition of selected Virginia Formation sandstones (estimated from 400 points per thin section)

Framework constituents Matrix constituents Cement Access7 Values recalculated to 100%

Sample K- Chert & Mus & Qtz+ Dark6 Cal- K- Number 1 Qtz2 Plag3 spar Qtzite IRF4 MRF 5 Chlor Feld color cite spar Qtz8 Feld RF9 Qtz Feld Matrix 1 0

B-173-8 21 24 2 Tr 11 Tr 3 18 4 2 19 - 5 42 52 6 30 37 33 B-198-3 26 25 2 Tr Tr 4 22 4 Tr 13 - 4 45 47 7 28 36 35 B-215-10 17 26 2 2 3 6 29 4 1 4 - 4 34 50 16 23 35 42 B-630-10 20 26 Tr 1 Tr 7 23 2 2 14 - 5 39 48 13 28 35 36 B-1024-2 25 22 5 2 Tr 4 18 3 2 10 - 9 47 47 6 35 3 30 B-1051-6 23 31 2 2 2 4 14 3 6 6 - 3 39 51 9 31 1 28 B-1082 27 23 Tr Tr - 2 11 6 12 9 - 7 52 44 4 34 29 37 B-1099-1 20 31 5 Tr Tr 3 25 4 15 3 - 5 34 61 5 23 41 36 B-1111-4 27 31 4 Tr Tr 2 6 2 - 23 - 2 42 55 3 39 50 11 B-1112 24 28 3 Tr - 2 20 2 3 17 - 2 42 54 4 30 39 31 K-520-7 23 20 8 4 2 4 10 6 17 Tr - 6 44 46 10 31 32 37 K-530 28 17 8 2 3 2 12 4 18 2 - 4 50 42 8 34 28 37 (XI K-570-6 23 22 12 2 Tr 1 1 3 31 2 - 2 42 57 1 27 36 36 K-586-6 16 12 16 Tr - Tr Tr 2 2 48 - 2 36 64 - 17 30 52 K-589-11 33 18 8 2 3 3 4 2 18 6 - 2 52 39 9 52 39 9 C-823-7 16 11 31 Tr - 1 2 2 4 24 4 4 27 71 2 25 63 12 C-855-8 18 11 34 _12 Tr 4 4 1 2 14 12 2 27 67 6 26 64 10 C-906 15 14 30 Tr 1 2 14 2 4 9 4 4 26 71 3 19 56 25 C-924 18 10 28 Tr - Tr Tr Tr 5 16 15 4 29 61 10 30 62 18 16 4 48 Tr 1 1 2 15 10 4 31 69 22 71 5 C-942-3 _. __ .------

1. B, Biwabik; K, Keewatin; C, Calumet; numbers refer to depth in feet and inches below collar 2. Includes quartz grains with abraded overgrowths in trace amounts 3. Includes feldspar grains with abraded overgrowths in trace amounts 4. Igneous rock fragments. Dominantly of plutonic origin; includes grains of possibly volcanic origin in trace amounts 5. Metamorphic rock fragments 6. Dark-colored constituents. Includes carbonaceous material and very fine grained, dark-colored material of indeterminate but possibly volcanic origin 7. Includes heavy minerals and detrital grains of muscovite, chlorite, and biotite 8. Includes chert and quartzite 9. Rock fragments exclusive of chert and quartzite 10. May include digested rock fragments 11. Trace amounts of less than 1 percent 12. Not observed may be of plutonic igneous origin. Metamor­ framework grains. On the average, somewhat phic rock fragments include phyllitic and less sodic plagioclase (~12%) occurs than quartz schistose rocks (Fig. 13) composed of various (~16%). Grains of perthitic or microperthitic proportions of sericite and chlorite; quartz, potassium feldspar dominate the feldspar popu­ sericite, or chlorite; and quartz, plagio­ lation, and most are highly altered, vacu­ clase, and sericite. olized, and surrounded by authigenic over­ growths (Fig. 17). Both the host grains (Fig. The matrix material in the graywackes is 18) and their overgrowths are extensively similar to that in the argillites. It con­ replaced by sparry calcite cement. Because of sists mostly of sericite and chlorite with extensive alteration and replacement, it is some very fine silt-size quartz and feldspar difficult to determine if the original detri­ and randomly distributed carbonaceous material. tal grains were microcline or orthoclase. The phyllosilicates occur as irregularly shaped patches having no preferred orienta­ As in the feldspathic graywackes, most tion. The fibrous or lacy habit of individual matrix material is intergrown chlorite and minerals within the patches (Fig. 14) implies sericite, but the arkose contains less car­ that the phyllosilicates were recrystallized bonaceous material which occurs as discrete from some original clay-size protolith lenses or laminae rather than as irregular (protomatrix to orthomatrix according to the patches. The origin of the phyllosilicates is nomenclature of Dickinson, 1970). There also uncertain, but because well-sorted sediments is evidence that some of the matrix formed by typically lack much original detrital clay­ the "squeezing" of the low-grade metamorphic size material, it seems likely that at least rock fragments (Fig. 15) around resistant some of the matrix formed from recrystallized framework grains so as to make the former phyllitic and schistose rock fragments (pseudo­ appear as matrix (pseudomatrix of Dickinson, matrix of Dickinson, 1970). 1970). Furthermore, because some of the seri­ cite occurs as thin coatings on a few of the framework grains (Fig. 16), some of the matrix Limestone may have formed by authigenic processes (epimatrix of Dickinson, 1970). Thus not all Beds of light-gray limestone with trace of the clay-size material in the feldspathic to minor amounts of silt-size quartz and graywackes is of detrital origin. feldspar occur throughout the Virginia Forma­ tion, but are most abundant in the upper silty Calcite cement also occurs in many and sandy lithosome. Individual beds range in graywacke samples, mainly as isolated patches thickness from 4 cm to 8 cm, and have sharply of micrite enclosed within the phyllosilicate defined bottom contacts and gradational tops. matrix. Some of the calcite, however, has a For the most part they have a very fine coarse-grained sparry habit and poikilitically grained micritic texture that locally has been encloses both framework grains and matrix either recrystallized to a sparite or replaced material. Thus the calcite appears to have by rhomb-shaped grains of dolomite. Many beds had a complex paragenetic history. also contain scattered grains of euhedral pyrite. The micritic texture is similar in thin section to that of the Biwabik-Virginia Arkose transi tion zone, but in hand specimen these limestone beds have a parallel-laminated, Centimeter-thick beds of arkose inter­ cross-laminated, or convoluted fabric (Fig. sected south of Calumet differ petrographi­ 19) that the transition zone does not. Thus cally from the feldspathic graywackes by most of the limestone units in the Virginia containing appreciably less matrix material, Formation proper are allochemical, formed by and in several other ways: (1) The framework the mechanical deposition of fine-grained grains are better sorted and consist predomi­ calcareous detritus. nantly of potassium feldspar with lesser amounts of quartz and plagioclase. (2) Authigenic potassium feldspar occurs as an Carbonate Concretions early-formed cementing agent, whereas sparry calcite occurs exclusively as a parageneti­ Carbonate concretions occur throughout cally late, intergranular mineral. (3 ) the Virginia Formation. Most lie within Although metamorphic rock fragments and the argillaceous intervals and are lenticular or accessory minerals, including detrital grains elliptical in shape. Although most concre­ of muscovite, biotite, and chlorite, have the tions are even grained and structureless, some same general abundances as in the feldspathic have an internal fabric defined by silt-size graywackes, sedimentary and igneous rock detritus that is crudely graded, laminated, or fragments are lacking. cross-bedded, and many are rimmed by cone-in­ cone structures. The concretions themselves Feldspar makes up 26 to 52 percent of the typically consist of sparry calcite that arkose samples and 61 to 71 percent of the poikilitically encloses various proportions of

9 Figure 7. Photomicrograph of a typical Figure 8. Typical framework constituents in feldspathic graywacke (plane light). Note feldspathic graywacke. Unaltered pla­ that most grains are angular to subangular. gioclase with well-developed polysynthetic Scale approximately 650 microns. twinning. Note detrital chlorite in the lower right of the photomicrograph. Scale approximately 100 microns.

Figure 9. Typical framework constituents in feldspathic graywacke. Left center, highly altered plagioclase; top center, composite Figure 10. Typical framework constituents in quartz; right center, semicomposite quartz. feldspathic graywacke. Bottom left, al­ Note that the framework grains are dis­ tered plagioclase with vaguely developed persed in a poikilitic sparry calcite polysynthetic twinning; top right, altered cement. Scale 100 microns. microcline with tartan-plaid twinning. Note irregular grain boundaries on both quartz and feldspar grains. Scale 100 microns.

Figure 11. Typical framework constituents in feldspathic graywacke. Top right, highly altered untwinned plagioclase; left center, Figure 12. Typical framework constituents L~ stretched composite quartz; bottom right, feldspathic graywacke. Right center, rock fragment of recrystallized chert or igneous rock fragment consisting of inter­ very fine grained quartzite. Scale 100 grown plagioclase and quartz. Scale 100 microns. microns.

10

quartz, feldspar, and phyllosilicates. In SEDIMENTOLOGIC ATTRIBUTES contrast, the cone-in-cone structures have a fine-grained, microsparic texture and bladed Four discrete sedimentologic processes or radial extinction patterns which imply that were operative in the Virginia Formation at the cones formed from bladed aragonite that one time or another. These are: (1) The depo­ was subsequently transformed to calcite. sition of mudstone by pelagic processes in­ volving the slow accumulation of fine-grained sediments from dilute suspension, or by hemi­ Cherty Sideritic Iron-Formation pelagic processes involving the action of slowly flowing diffuse turbidity currents. Beds of cherty sideritic iron-formation (2) The deposition of poorly sorted sandy and intersected south of Calumet (Fig. 3) are silty beds, including allochemical limestone irregularly laminated and grayish brown in units, by the action of sediment-laden tur­ color. Lenses and angular granule- to pebble­ bidity currents. (3) The deposition of well­ size fragments of light-bluish-gray and white sorted sandy beds by the action of nontur­ cryptocrystalline chert commonly occur in the bidity currents that reworked previously iron-rich layers. deposi ted sediments. (4) The deposition of chert and iron-rich beds by chemical processes The cherty sideritic material consists of during periods of reduced clastic influx. finely intergrown siderite and chert in pro­ portions of about 3:1. Chert grains are rarely more than 0.01 mm in diameter, and much pelagic and Hemipelagic Deposits of the siderite is microcrystalline although some grains are as large as 0.05 mm in The Virginia Formation is dominated diameter. A few beds have secondary rhombs of throughout by mudstone, now recrystallized to ankerite or dolomite that poikilitically argillite, but the character of the argillite enclose grains of chert and siderite, par­ changes with stratigraphic position. Argil­ ticularly near the edges of chert fragments. lite in the lower argillaceous lithosome typi­ Although dominantly chemical in origin, the cally occurs in thick and uninterrupted iron-rich units contain scattered, well­ sequences. Because of their more or less rounded, sand-size grains of quartz, which are homogeneous texture, these argillite units clearly detrital in origin, and large blades of were probably formed by the gradual sedimen­ chlorite, which may be authigenic in origin. tation of suspended clay. In modern environ­ ments, similar fine-grained, black, carbona­ ceous and pyritic sediments form in a variety Chert of sedimentologic regimes ranging from re­ stricted shallow lagoons to deep basins with Beds or nodules of light-bluish-gray to anoxygenic bottom conditions. However, the white and dark-gray to black chert generally extensi ve development of the lower argilla­ less than 2 em thick occur commonly in the ceous lithosome, the apparent lack of asso­ lower part of the Virginia Formation, par­ ciated shallow-water strata, and the presence ticularly in the western part of the Mesabi of primitive amino acids of organic origin in range. Although not studied in detail, these some of the argillite beds (Niehaus and Swain, units, regardless of color, appear to consist 1972), imply that sedimentation occurred by entirely of an interlocking mosaic of individ­ pelagic processes in a large basin having ual crystallites and therefore appear to be anoxygenic bottom conditions. typical sedimentary cherts. Much of the argillite in the upper silty and sandy lithosome also may have formed by Ash-Fall Tuff pelagic processes. However many of the argillaceous beds in this li thosome contain Olive-green, structure less , waxy-appear­ appreciable quantities of randomly dispersed ing tuff generally occurs in beds less than 7 silt. Silt-size detritus also occurs con­ cm thick which have sharp upper and lower con­ centrated in very thin laminae that have well­ tacts. In thin section, the tuff is typically defined lower contacts. Therefore it appears very fine grained and consists dominantly of likely that some of the argillaceous material sericite; however ghosts of shardlike features in the upper lithosome formed by hemipelagic or of euhedral feldspar grains having oscil­ processes involving the action of either weak latory zones are preserved in a few samples. turbidi ty currents or bottom currents. Al­ However because of extensive crystallization, though the pelagic and hemipelagic deposits the protoliths of these ash-fall tuff varieties typically exhibit parallel bedding types, could not be determined. scattered stratigraphic intervals generally less than a meter thick are characterized by

12 Figure 19. Typical allochemical limestone. Polished core slab on the left and radiograph on the right are of the same specimen.

13

of the siltstone beds 20 em or more thick Chemically Precipitated Deposits start with the Bouma A division. These beds also typically contain other Bouma divisions. The iron-bearing strata intersected south Consequently thick-bedded turbidites are not a of Calumet are sedimentologically similar to major sedimentological attribute of the upper the upper part of the Upper Slaty member of silty and sandy lithosome. the Biwabik Iron Formation in the westernmost part of the Mesabi range (White, 1954). Inasmuch as the Biwabik-Virginia contact is Thin-bedded Turbidites gradational, it seems that iron-formation pre­ cipitation and clastic sedimentation were con­ Most sandstone and siltstone beds in the temporaneous, and that the iron-bearing strata Virginia Formation are thin-bedded turbidites. mark episodes of diminished clastic input into However thin-bedded turbidites starting with the basin rather than major changes in basin the Bouma B division are relatively rare in geometry in early Virginia time. the upper silty and sandy lithosome south of Biwabik. Most of those which are 20 em or The presence of bedded chert in the lower more thick also contain Bouma C and D divi­ part of the Virginia Formation also implies sions. Typically however, the B division is that clastic sedimentation was episodic in poorly developed and the C and D divisions early Virginia time. The fact that the chert make up hal f to two thirds of the sedimen­ units are free of iron-bearing minerals tation unit. A few sedimentation units be­ implies that the chemical precipitation of tween 5 em and 20 em thick also contain the B silica and siderite were not contemporaneous division, but most of these begin with the processes. Therefore the reappearance of Bouma C division (Fig. 22). The C division is siderite-bearing strata in the Virginia Forma­ rarely present in sedimentation units less tion must mark a period of increased influx of than 5 em thick and most of these very thin dissolved iron or carbon dioxide into the sedimentation units consist entirely of the D chemical environment of deposition. division. The D division, wherever it occurs, appears to pass uninterrupted into very finely laminated, hemipelagic sediments of the Bouma SEDIMENTATION AND PROVENANCE E division. A simple depositional model can account for the sedimentologic attributes described Bottom-Current Deposits above. Early Virginia time was characterized by the slow deposition of black mud--now The upper part of the lower argillaceous argillite--in quiet water below wave base. lithosome intersected south of Calumet con­ The presence of locally abundant quantities of tains scattered sandstone lenses 1 cm to 3 cm carbonaceous material implies deposition under thick (Fig. 23) that have many sedimentary anoxygenic conditions, possibly in deep water. attributes in common with sediments deposited Beds of iron-formation, limestone, dolomite, by bottom-hugging currents in modern environ­ and chert, interlayered with the argillites in ments (Hubert, 1964). The sandstone lenses the Biwabik-Virginia transition zone indicate have sharply defined upper and lower contacts, short periods of diminished clastic supply to and in places appear to fill bottom irregular­ the depositional basin and an episodic return i ties. They are better sorted and contain to a sedimentologic regime dominated by chemi­ less matrix material than either the thick­ cal processes like that of late Biwabik time. bedded or thin-bedded turbidites described However, the relatively abrupt change from a above. Furthermore they commonly are either chemical to a clastic sedimentologic regime laminated or cross-bedded, but they are not implies that the depositional system underwent graded and they lack typical Bouma sequences. an abrupt transformation, most likely involv­ ing subsidence of the basin and uplift of the Although the term "contourite" is used bordering craton. The added presence of minor for deposits of bottom-hugging currents in amounts of volcanic material in the chemical modern environments, it applies particularly to clastic transition implies that the trans­ to sediments transported and deposited by formation was accompanied by volcanism. currents flowing parallel to bathymetric con­ However because the volcanic material is very tours and driven by tides, internal waves, or fine grained, pyroclastic in origin, and sub­ thermohaline gradients (Heezen and Hollister, ordinate in quantity, it seems likely that its 1963) • Circulation patterns in the Virginia source area was located elsewhere in the Lake Formation, however, cannot be reconstructed Superior region. from available data, and therefore these sandstone lenses are termed bottom-current The sedimentologic significance of bottom­ deposits to avoid implication as to origin or current deposits in the lower argillaceous orientation of the bottom currents. lithosome is not well understood. Heezen and

15 Figure 23. Polished core slabs showing bottom­ current deposits from the lower argillaceous litho­ some south of Calumet. The top slab shows vaguely defined climbing-ripple sets, whereas the sandstone Figure 22. Polished slab of lenses in the bottom slab core showing typical thin­ appear to be structureless. bedded turbidites from the upper silty and sandy lithosome south of Biwabik. Note that most siltstone uni ts are CO or 0 turbid­ ites.

Figure 21. Polished slab of core showing a typical thick-bedded turbidite from the lower argillaceous lithosome south of Keewatin. Note that the upper part is finely laminated and the remainder is massive to vaguely laminated (AB turbidite).

16 Hollister (1963) have suggested that detrital cut off at the source or is diverted else­ material affected by bottom currents in modern where, the fan will be abandoned and covered environments is moved by turbidity currents to by rather uniform layers of pelagic and hemi­ deeper water environments where it is sub­ pelagic mud. Thus the presence of thick-bed­ sequently reworked and retransported. However ded turbidites intercalated with hemipelagic bottom currents in modern environments are or pelagic muds in the Virginia Formation most active around the edges of a depositional south of Keewatin records the birth and death basin--not in its deepest parts (Bouma and of the feeder system for one submarine fan Hollister, 1973). If these observations can complex. be extrapolated to the Virginia Formation, it appears that the area of the Mesabi range was Most sedimentologic models developed to located toward the edge of the depositional account for the graywacke sequences in north­ basin in an area where both turbidity and bot­ ern Minnesota have suggested that the edge of tom currents were active. the depositional basin was located to the north of the Mesabi range in an area where The basin of deposition must have con­ strata subsequently were eroded in post-Vir­ tinued to subside in Virginia time at a rate ginia time (e.g., Morey, 1973). This conten­ such that filling did not keep pace with tion cannot be corroborated by direct paleo­ downwarping. As the water gradually deepened, current observations from the unoriented drill a bottom slope developed, and silt- and sand­ cores, but many of the mineralogic attributes size detritus was introduced into the basin by in the Virginia Formation imply a source area means of turbidity currents. Most sandstone to the north of the Mesabi range. and siltstone beds in the Virginia Formation are thin-bedded turbidites that were deposited Quartz and· feldspar having types that by slow-flowing currents. In modern environ­ indicate a source area of dominantly plutonic ments, analogous thin-bedded turbidites form or high-rank metamorphic rocks are relatively along the lower parts of submarine fan com­ abundant in the Virginia Formation. Although plexes (Walker, 1978, 1979) in environments not as abundant, stretched composi te quartz considerably distant from the source of the types in the formation imply that the source turbidity currents (Fig. 24). However, simi­ area also contained a variety of low-rank lar thin-bedded turbidites can be deposited by metamorphic rocks. The sparse presence of slowly flowing currents in environments near "granitic" rock fragments and sand-size grains the source of the currents under conditions of of chert, iron-formation, phyllite, schist, diminished sediment supply or in environments and fine-grained volcanic rocks of uncertain characterized by a very gently sloping bottom affinity also indicates a source area of mixed topography. Thus the presence of thin-bedded composition. Because many of the phyllitic turbidites in the Virginia Formation need not and schistose rock fragments have been at indicate deposition in an environment well least partially recrystallized to form pseudo­ removed from the source of the turbidity matrix material, it is difficult to estimate currents. However the widespread occurrence the abundance of low-grade metamorphic rocks of thin-bedded turbidites in the Virginia in the source area. Nonetheless the presence Formation does indicate simultaneous deposi­ of pseudomatrix material in the graywackes of tion from a number of partially overlapping the Virginia Formation is consistent with the submarine fan complexes, so that the sedimen­ conclusions of Morey and Schulz (1977) who tologic attributes of any single fan complex suggested on chemical grounds that other lower would be modified by deposits from adjacent Proterozoic graywacke sequences in Minnesota fan complexes. were derived from a mixed source of quartz monzonitic, metasedimentary, and metavolcanic In contrast to the general situation rocks like those which now crop out in the ·outlined above, the amalgamated sandstone Archean greenstone-granite terrane to the sequence intersected south of Keewatin appears north of the Mesabi range. to consist of thick-bedded turbidites which form, by analogy with modern turbidite environ­ The above conclusions notwithstanding, ments (Fig. 24), in areas near or within chan­ the occurrence in some of the graywackes of nels that feed submarine fan complexes (Nor­ sand-size quartz and feldspar grains with mark, 1978). In modern environments feeder abraded overgrowths and the presence of detri­ channels commonly originate near the edge of a tal limestone beds having sedimentary struc­ deposi tiona I basin, and thus the presence of tures indicative of turbidite deposition imply thick-bedded turbidites could be considered an that the source area was not entirely confined indication of proximity to a basin margin. to igneous and metamorphic rocks of Archean However, feeder channels extending far into age. Although speculative, it seems very the depositional basin also have been observed likely that both the terrigenous and calcare­ in modern environments. They can form any ous detritus were derived from lower Protero­ place where the sediment supply increases con­ zoic units deposited either shortly before the siderably or where the paleoslope into the time of Virginia deposition or from contempo­ basin is increased by tectonism. Conversely raneous shelf deposits formed near the strand­ if the sediment supply for a fan complex is line during Virginia time.

17 Regardless of the precise composition of bidites and other sedimentary attributes indic­ the source area, all of the mineralogic evi­ ative of deposition from moderately fast dence implies that a craton existed to the flowing turbidity currents (Morey, 1969); it north of the Mesabi range. It follows that also contains thick to medium-thick beds of the northern edge of the Virginia depositional quartzose sandstone that have sedimentologic basin also was located to the north of, and attributes indicative of bottom-current activ­ possibly subparallel to, the east-northeast­ ity (Morey, 1969, p. 37). trending strike of the Mesabi range. Unfor­ tunately however, there is nothing in the The Rabbit Lake Formation is not exposed geologic evidence to indicate how far north to any great extent, but scattered drill holes of the Mesabi range the edge of the deposi­ have penetrated a stratigraphic succession tional basin was. very much like that in the Virginia Formation at the western end of the Mesabi range (Marsden, 1972). Although the Rabbit Lake REGIONAL SEDIMENTOLOGIC RELATIONSHIPS Formation has not been studied in detail, it can be subdivided into a lower black carbona­ The Rove Formation of the Gunflint range ceous argillite or slate member some 60 m in northeastern Minnesota and adjoining parts thick; an intermediate cherty sideritic iron­ of Ontario, the Rabbit Lake Formation of the formation member as much as 300 m thick; and Cuyuna range, and the Thomson Formation of an upper member of gray, green, or black argil­ east-central Minnesota are broadly correlative lite or slate intercalated with thin to thick (Fig. 2) with the Virginia Formation. They beds of graywacke, chert, and iron-formation. all occupy similar stratigraphic positions in the Animikie Group and are characterized by The stratigraphic position of the Thomson broadly similar stratigraphic successions Formation is somewhat problematic because (e.g., Morey, 1973, 1978). neither its upper nor lower contact is exposed. Goldich and others (1961, p. 5) , There are marked stratigraphic and sedi­ however, have suggested on the basis of mentologic similarities between the Virginia radiometric dating that the Thomson Formation and Rove Formations (Fig. 25). At least 970 m is lower Proterozoic in age, and Morey and thick, the Rove Formation contains a lower Ojakangas (1970) have demonstrated that it is argillaceous unit 120 m to 130 m thick, which lithologically and sedimentologically akin to is overlain by 600 m of argillite and inter­ the upper part of the Rove Formation. The calated thin-bedded turbidites. The uppermost Thomson Formation differs in detail from the 240 m of the exposed section contains tur- upper part of the Rove Formation in that tur-

Hypothetical pro-grading submarine fan sequence Virginia Formation Rove Formation Thomson Formation

".':::',··:· S';'.':'

."a Q'5 ~~I:::~=~""

0.8.~ C . :::l g

Figure 24. Submarine fan environmental model (modified from Walker, 1979) showing major turbidite types recog­ Figure 25. Diagrammatic stratigraphic and sedimentologic nized in the Virginia sequences in the Virginia, Rove, and Thomson Formations Formation and other compared to a hypothetical prograding submarine fan se­ lower Proterozoic gray­ quence as proposed by Walker (1979). No vertical scale wacke sequences in is implied; the stratigraphically higher Thomson Forma­ Minnesota. No relative tion lacks evidence of mid-fan channels because of its scale is implied. location in the axial part of the depositional basin.

18 bidites starting with the Bouma A division are Walker's (1979) model involving the basinward very common regardless of stratigraphic posi­ progradation of a number of submarine fan tion (Morey and Ojakangas, 1970). The typi­ complexes provides a simple and elegant expla­ cally size- and content-graded nature of the nation for the various sedimentologic phenom­ Thomson turbidites implies that the Thomson ena observed in the lower Proterozoic gray­ Formation occupies a stratigraphic position wacke sequences in northern and east-central somewhat akin to the upper parts of the Minnesota. sequences described from the Mesabi and Gunflint ranges (Fig. 25). REFERENCES CITED

From a sedimentologic point of view, the Bouma, A.H., 1962, Sedimentology of some stratigraphic reconstruction summarized in flysch deposits: Amsterdam, Elsevier, Figure 25 resembles "thickening- and coarsen­ 168 p. ing-upward" sequences typically associated Bouma, A.H., and Hollister, C.D., 1973, Deep with prograding submarine fan complexes ocean basin sedimentation, in Middleton, (Walker and Mutti, 1973; Walker, 1979). Ac­ G.V., and Bouma, A.H., eds., Turbidites cording to Walker (1979), the progradation of and deep water sedimentation: Society of a submarine fan complex would result in a Economic Paleontologists and Mineralo­ stratigraphic sequence passing upward from gists (Pacific Section) Short Course lower fan, through mid-fan, into upper fan Notes (Los Angeles), p. 79-118. deposits. Thus progradation in the outer fan area would result in the deposition of a se­ Dickinson, W.R., 1970, Interpreting detrital quence of turbidites that would become thicker modes of graywacke and arkose: Journal bedded upwards. In this model, the upward of Sedimentary Petrology, v. 46, p. 695- transition from dominantly pelagic and hemi­ 707. pelagic muds to intercalated thin-bedded tur­ Goldich, S.S., Nier, A.O., Baadsgaard, H., bidites in the Virginia, Rove, and possibly Hoffman, J.H., and Krueger, H.W., 1961, Rabbit Lake Formations represents the tran­ The Precambrian geology and geochronology sition from pelagic sedimentation on a basin of Minnesota: Minnesota Geological Sur­ plain to sparse thin-bedded turbidite deposi­ vey Bulletin 41, 193 p. tion in the lower fan area to abundant thin­ Gruner, J.W., 1946, The mineralogy and geology bedded turbidite deposition in the outer part of the taconites and iron ores of the of the mid-fan area. The presence of thick­ Mesabi range, Minnesota: St. Paul, Office bedded turbidites in the Thomson Formation of Commissioner, Iron Range Resources and implies that sedimentation there occurred in Rehabilitation, 127 p. the inner part of a prograding submarine fan Hamblin, W.K., 1971, X-ray photography, in complex. Most likely this general thickening­ Carver, V.C., ed., Procedures in sedimen­ and coarsening-upward sequence reflects either tary petrology: New York, Wiley-Inter­ subsidence of the axial part of the deposi­ science, 653 p. tional basin or uplift of the source area, either of which would lead to steeper deposi­ Heezen, B.C., and Hollister, C.D., 1963, tional gradients in the basin and to increased Evidence of deep sea bottom currents from amounts of detritus being supplied to the abyssal sediments [abs.J: International basin. Union of Geodesy and Geophysics, v. 6, p. 111. CONCLUSIONS Hubert, J.F., 1964, Textural evidence for deposition of many western North Atlantic It can be concluded that sediments in the deep-sea sands by ocean-bottom rather Virginia Formation were derived in large part than turbidity currents: Journal of from the Archean greenstone-granite terrane Geology, v. 22, p. 757-785. located to the north of the Mesabi range, and Kuenen, P.H., 1964, Deep-sea sands and ancient to a lesser extent from lower Proterozoic turbidites, in Bouma, A.H., and Brouwer, strata slightly older or equivalent to the A., eds., Dev'elopments in sedimentology: Virginia Formation. The original muds in Amsterdam, Elsevier, 266 p. early Virginia time were deposited by a slow Marsden, R.W., 1972, Cuyuna district, in Sims, raining down of suspended clay particles in a P.K., and Morey, G.B., eds., Geology of basin whose axis was located to the south of Minnesota--a centennial volume: Minnesota the Mesabi range. It is inferred that con­ Geological Survey, p. 227-237. tinued subsidence led to a paleoslope dipping generally southward into the basin and to the Morey, G.B., 1969, The geology of the Middle transport and deposition of silt and sand by Precambrian Rove Formation in northeast­ turbidity currents that mostly flowed down the ern Minnesota: Minnesota Geological Sur­ paleoslope. In detail, the sedimentologic vey Special Publication SP-7, 62 p. attributes of the Virginia Formation resemble _____ , 1973, Stratigraphic framework of Middle those associated with deposition on the lower Precambrian rocks in Minnesota, in Young, and mid-fan parts of a submarine fan complex. G .M., ed. I Symposium on Huronian sedimenta-

19 tion: Geological Association of Canada 1967, Turbidite structures and their Special Paper 12, p. 211-249. relationship to proximal and distal depo­ , 1978, Lower and Middle Precambrian strat­ sitional environments: Journal of Sedi­ igraphic nomenclature for east-central mentary Petrology, v. 37, p. 25-43. Minnesota: Minnesota Geological Survey 1978, Deep-water sandstone facies and Report of Investigations 21, 52 p. ancient submarine fans: models for explo­ Morey, G.B., and Ojakangas, R.W., 1970, Sed­ ration for stratigraphic traps: American imentology of the Middle Precambrian Thom- Association of Petroleum Geologists Bulle­ son Formation, east-central Minnesota: tin, v. 62, p. 932-966. Minnesota Geological Survey Report of 1979, Turbidites and associated coarse Investigations 13, 32 p. clastic deposits, in Walker, R.G., ed., Morey, G.B., Papike, J.J., Smith, R.W., and Facies models: Geoscience Canada Reprint Weiblen, P.W., 1972, Observations on the Series 1, p. 91-103. contact metamorphism of the Biwabik Walker, R.G. and Mutti, E., 1973, Turbidite Iron-Formation, East Mesabi District, facies and facies associations, in Mid­ Minnesota, in Doe, B.R., and Smith, dleton, G.V., and Bouma, A.H., eds., D.K., eds., Studies in mineralogy and Turbidi tes and deep water sedimentation: Precambrian geology: Geological Society Society of Economic Paleontologists and of America Memoir 135, p. 225-264. Mineralogists (Pacific Section) Short Morey, G. B., and Schulz, K. J • , 1977, Petro­ Course Notes (Los Angeles), p. 119-157. graphic and chemical attributes of some White, D.A., 1954, Stratigraphy and structure Lower and Middle Precambrian graywacke­ of the Mesabi range, Minnesota: Minnesota shale sequences in Minnesota [abs.]: Geological Survey Bulletin 38, 92 p. Institute on Lake Superior Geology, 23rd Annual Meeting, Thunder Bay, Ontario [Proceedings], p. 34. Morey, G.B., Sims, P.K., Cannon, W.F., Mudrey, M.G., Jr., and Southwick, D.L., 1982, Geologic map of the Lake Superior region: Minnesota Geological Survey State Map Series S-13, scale 1:1,000,000. Niehaus, J. R., and Swain, F .M., 1972, Amino acids in some Middle Precambrian rocks of northern Minnesota and Southern Ontario, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota--a centennial volume: Minnesota Geological Survey, p. 272-277 • Normark, W.R., 1978, Fan valleys, channels and depositional lobes on modern submarine fans: characters for recognition of sandy turbidite environments: American Association of Petroleum Geologists Bulletin, v. 62, p. 912-931. Pettijohn, F.J. , 1957, Sedimentary rocks: Harper and Row, 718 p. Pfleider, E.P., Morey, G.B., and Bleifuss, R.L., 1968, Mesabi deep-drilling project, progress report no. 1: in Pfleider, E.P., and Berger, F.E •. , eds., Minnesota Mining Symposium, 29thf Annual: Minneapo­ lis, Uni versi ty of Minnesota Center for Continuing Education, p. 59-92. Van Hise, C.R. and Leith, C.K., 1901, The Mesabi district: u.S. Geological Survey 21st Annual Report, Part 3, p. 351-370. Walker, R.G., 1965, The origin and signifi­ cance of the internal sedimentary struc­ tures of turbidites: Yorkshire Geological Society Proceedings, v. 33, p. 1-29.

20 APPENDIX-- LITHIC LOGS OF THE VIRGINIA FORMATION Biwabik 1 of 3 !?rapnlc GrQphic Descrip tion Descr i ption Depth Column Description Depth Column ft. m.

100 :.0:6 ·.: 0 Glacial drift; cobbles 270 440 ;c,:·o.··:o· and boulders In sand _ c Argll lite, black, car­ .. - C---C bonaceous; scattered Silty arg II \I te, ~::-....::::""-.:. . VIRGINIA F~TION light-gray, and dark­ laminae of dark-gray, 135 ...... Arg Illite & arg I II a- -=~-~ silty argillite & mad .­ gray argillite; laminated ; scattered 110 ::...-::.:.:-:: .. ceous siltstone; lam­ 280 85 _l:_ gray, argillaceous 450 ...... I nated; I nter I ayers of --- s II tstone & rare II ght­ beds of laminated .. .::. .. ~ .. flne.... gralned, very thin c-__-C gray fine-grained, cross-bedded siltstone; 35 ::-_:::::-.::.-:: bedded, cross-bedded, calcareous graywacke scattered thick beds of .. .. I am I nated , graded, black, carbonaceous - c- arg Illite 120 .. .. ca I careous s II tstone 290 460 - C .... C .. - .. - - " - " - " ::-.. :.:-:- .. :.:c 130 -- ._ .. 300 470 -c C .... C .. - .. .. Argillite, laminated; ti::::::::ij Graywacke, I I ght-gray, ~_ .. _ calcite concretions Arg II I I te, silty arg II .... fine-grained, graded, _. . _ .. - cross-bedded .:.:...-::. - :.:...<:P.. - . lite, & siltstone; 140 Graywacke, fine-gr., 310 t-.. - .. - laminated to very thin 480 145 ...7~~ . Silty argillite, graded, laminated , bedded; Inter layered light-gray, and dark­ cross-bedded 95 t-..::::- ~ :: black, carbonaceous .§;?.:.:...-::-..: f..- •. _ •. - gray argillite; lami ­ ..:..::-....::::-.:. Argillite, laminated; argillite nated 45 .. -~ . calcite concretions S i I ty arg II II te and 150 320 ~ .. - .. - argillite; laminated; 490 - -_ .. - " - " - " Argll lite & argili. t.:..: ::-.:..:-:: ripple-scale laminae In siltstone, laminated; r ·· _··- s" ty beds; scattered 1.:.:;::. ,:;-::-",.:;::. _;:..;::; .'4. Interbeds of fine-gr ., 1- .. _ .. - beds of siltstone .... calc. siltstone 160 -.- .. _.. 330 500 Argillite, laminated, ,...... - .. - ...... calc. ; calcite ..- .. _ .. concretions; Interbeds _.. - ...... - .. _ Silty argillite, IIght­ of fine-gr . siltstone _.. - ...... ::...... :.: .. ..:.. gray, and siltstone; 170 340 510 laminated; some dark­ Graywacke, fine-gr ., 155 - "-"- gray argillite ~:t..~.~ . ~ graded, I am I nated, _.. _ .. - .. - .. - .. r::~.:~ .. cross-bedded 165 _ •. _ .. _ 180 350 520 .. - .. - .. Argillite, dk.-gray, Dominantly siltstone & & med .-gray, silty _.. _.. _. laminated, si Ity argillite; rare lami ­ argll lite; siltstone ~~~~ nae and very th I n beds units laminated, cross­ of It.-gray siltstone; 190 Arg I I I I te & arg I II • 360 530 - <53> ..- siltstone; laminated to bedded -.. '=:$ abundant concretions .. very thin bedded; - ._ _. Interbedded fine-gr . , .. - .. - .. .~- ...... laminated & cross­ - .. - .§...... bedded , locally 200 370 540 Graywacke, It.-gray,

- . ~ . - 230 400 570 - .- .. - ...... Argl" Ite & silty - Silty argillite, med .- .: ... argillite; laminated to "G:5_- ~~~~ gray; rare laminae of .. .. very th I n bedded; ...... ::. .. ..::. .. It ..... gray siltstone .. - .. - .. scattered thin beds of 175 240 .... laminated to rlpple- 410 580 ...... I aminated sll tstone & 125 - $ _ ._ .. - rare light- gray, flne­ - -- .::..~::.. grained graywacke Argillite, black, and --G:IG:L _ med .-gray, silty argil ­ 75 . ~..:.:.~ 250 420 590 lite; laminated ~.~. ~..~ . ~.~. ·i·~ Graywacke, med ..... gray, -- .. _ -_. ~ fine-gr., locally cal .... - .-_ ..- _. . _.. - 'i~~~ careous; I nter layered Argillite, black; rare j:;: black argillite laminae of dark-gray, 260 _ .. _ .. - Dominantly argillite, s II ty arg I I I I te -430 black, laminated 600 _ ...... , Graywacke, light- gray, 185 f.-gr.; Inter layered black argillite - 21

Biwabik 3 of 3

1280 c-c-c Argillite, black, 1110 345 J-..-.:....- . -.. _---_~ Arg I I I I te, s I I ty, -- - , - ,,_ .- I aminated I-~-~-I Siltstone, coarse-gr., c--~ f i ssile, carbonaceous .. calcareous, graded; ~_~_= cont'd. 1- -- - ~ thin beds of argilla­ ceous siltstone, laml­ C- -C f-i290 1460 445 - -- 1120 1- -- ~~~e nated argillite, & J silty argillite C-<:-C -C- 395 Argillite & silty 345 argillite; laminated - C- -C ~70 11 30 1- -- 1300 C-~C- I- - -C­ C-C-C 1480 11 40 --- 1310 ~-C-C- --- -C- - - Siltstone, coarse-gr . , calcareous c- -C ~320 1490 ~150 -c....-:-~~. Argi II ite & silty argll C-C-C 1-.. - .. - .. lite; carbonaceous; 455 - -- Argillite, black, .. - ..- .. - laminated C-C-C fissile; very rare I-·· C-·· laminae of silty C- -C 1160 arg I I I I te; Ioca I I Y 1330 405 .. _ ._ .. 1500 carbonaceous C-C-G- - -- ·· -c- 355 ,:...-:: :..:....::.:: Siltstone, fine-gr., C- -C graded 1- -- t:==..=I .. 15 10 - C-C 1170 1340 .--.. Argillite, blk., silty . - . - . ~ . Siltstone, coarse-gr., -Z-C-C 1- -- graded - C-C Argillite, black, 1- -­ _c=-=C- - f I ss I I e, carbonaceous, C- C-C 1520 I- -C­ 1180 - C- C- 1350 ___ and pyritic I-C-C-C --- -~~-~~Si Itstone, argillaceous, c-C-~ --- ~ t hin-bedded; Inter- 465 layered dark-gray, C- -C- 1360 C - s i I ty arg II II te 1530 11 90 - -- Argillite, black, c-c-C f-4 15 1-_- C_-_ fissile, carbonaceous, 1- --- I- -- pyritic C-C-C 3651- __ _ ~ S II tstone, f I ne-gr., 120C __ _ 1370 C _ graded, laminated 1540 C-C-C - -C- Argillite, black, I------fissile, carbonaceous; C- -C- -_-c_- C scattered thin laminae I- --- ___ of silty argillite; -C- - scattered calcite 1550 1210 - -C ~---c- ~~i.:. Si Itstone, f .-gr , cross) -l380 -C- - concretions bedded & laminated I- - -C- I--C- -- - t-c- -C I-G- -C Argillite, black, - -C- 1560 475 -C-C- 1220 -- - fissile; very rare 1390 laminae of silty ~ -..532 ---C-- --- arg Illite C- -C- Argillite, black, 425 -C-­ -.s; ~ - C f I ss I Ie, car bonaceous -- - - -C -- Tuff, green, massive 1570 1400 .. -= .. -.::-:. Argililte, silty, 1230 375 I- _ _ _ I- -C-C -C_-_- ~~~~~ massive C _ C Arg illite & sll ty arg. ; -C-- I- - -- - C-C thin bedded, carbo I- -C- C - C _ Tuff, green, massive 1410 1580 1240 I- --- I- - -C­ - C- Argillite, black, C - C f I ss II e, carbonaceous -:- . -:-: .-:-. S II tstone, very fl ne I--S--::Z­ ~c.~~ Tuff , grn, waxy, massive . . - - . ... grained, graded - -C­ I-C-C- - -I-I-I-~-Arglillte, bl k., fissile - -- Argillite, black, - - C- Graywacke , very fine 1590 BIWABIK IRON FORMATION 125C - - - fissile, laminated; and 1420 gr. , calcareous ; I Intorbedded chert, car- I- -- very thin bedded, silty C-C- - pyrite concretions 485 _ _ bonaceous arg I I I I te, 1----_-::. arg Illite I---C-­ ~C~ tuff, limestone, and -C- - ~----=-~.~~~~ do lomite Argillite, black, 1600 1260 1- -- 1430 1-435 -c-c­ fiSSile, carbonaceous; -~~ I------C­ 1610 -C -...c- 1270 1440 I-C-C- c- -c _ C=-~ -C-­ 1-- - tk?:c- C- -C 495 ~c:-=s :s Iron- formation, slaty

23 Buhl 1 of 1

..-...... _ .-...j Dr I ft- -sand, clay, 0- -0 scattered bou I dar 150 horizons 320 490 ~~~! Limestone, chert, & =.-~~ CRETACEOUS I- - C olive- green tUff; Inter- ...•. 0" ~ 45 : !> :-: ·o ~ Siltstone, fine- gr. , t::r:: ::::r: bedded 0- green i sh-gray, cgl. t--0--0 C- - 0 _ C - C - kg I" Ite, bl ack, car­ with chert fragments C-C-C bonaceous , fissile 160 °C_ E. Clay, It.-yellowlsh­ 330 500 gray, cgl . C- -0 =-C=C= ~- g-.f Clay, carb., pyritic; C- -C and arg II I aceous Chert & ollve-grn . tuff F-:":;oc=;;;-o~ s II tstone -C-C- Argillite, black, car­ 170 GO if Clay, carb., cgl . 340 r-c- -0 510 C- C - C bonaceous , f I ss II e Clay, black, plastic; -C-- -C-C- interlayered black, 155 C-C -C I- - - f I ss I Ie arg II II te - - - 105 ""C--C ~~~~ LI mestone, b I k., arg II I • 180 I- - - 350 520 "::..::..._ Argll lite, black, f ls­ f-c- -c- Argillite, bl k., R~~~ slle , cgl ., with chert 55 1----"-1 VIRGINIA FffiMATION - - - " fragments & Interbedded fissile, carbonaceous, _0....., black chert and dark- gray, silty, P.::::'c::::J-=1Arglillte, black, local ­ C- -C - 5 - 5 - Iron- formation , slaty, carbonaceous argi l lite; .::-.::.-:::. Iy calc., and dk.-gray, S - s- 5 s lileate facies 190 360 C- -C- laminated; carbonate 530 -- silty, argillite; laml­ -5-5- .. - .. - .. - . nated; beds and part- concretions with & wit hout cone- In - cone Ings of ollve-grn . tuff -C- -c structures common; - - - cOIMIOn I-C- -C scattered very thin 200 370 beds & laminae of 540 argi l laceous, ca lc ., graded, and laminated sll tstone 165

21 0 380 11 5 ~-_-.:­ 550 -G- - C- 65 c- -c 1- -- 220 390 t-C- -C 560

C- -C- ~~. ~. ~.~. ~ Ar gillit e , black, and _ -:. -::. dk.-gray, silty, thln­ ~--C- 230 bedded to I am I nated 400 510 -:.:.--- argillit e; Interlayered I-C- -0- .. It. gr ay ar gil l aceous 1=-<=-==-9 s I I tstone; scatter ed _~_ ~- very th I n beds of I- - _ - S i I tstone, coarse-gr. , 24 0 410 580 _ .. _ .. _ fine-gr. siltstone and .. .. calc., graded , & lam- _ .. __ .. arg II I. siltstone near 125 ,...... -'- " Inated; Inter l ayered _ .. __ top of unit. Partings ...... ~ .. with laminated bl k. __. . _ of ollve-grn . tuff 75 ~""';··~""'~· i· arg III Ite & dk . -gray, concentrated near base ~ .:. ...S. s II ty arg II I I te 250 - .. - - of unit ~20 590 I- _C-~ - Argillite, black, _.. _ .. - massive, carbonaceous; ~-~ -~ calcite concretions - C- C - COIMIOn C-=C =C 260 430 -C-C - 600 --- t-=r,..,.~Llmestone , argill ., blk. .. - . Argillite, blk. , car­ I- - .. -- rc ~c~ c bonaceous with laminae C-C - of silty argll lite 270 .. - .. -- 440 ~~~~ Limestone , argll I. , blk. 610 1--- - C:':' C::' Argillite, bl k., car­ C-C-C bonaceous with laminae 1-.. - - 135 r- C-~ - of silty arg II lite . . Siltstone, fine-gr . , 280 85 t-.. - - 450 '="'~=,,':::t graded , laminated, 620 -_-_-- cross- bedded; Inter­ ___ layered with black, t- - - .. - 1"== =""'1 fissile argillite 290 1- .. - - f"460 Argillite, black, car­ 630 - -- bonaceous, fissile; t- - .. -- M '""T" " ""T"'~ ' with thin laminae of - -- silty arg I I I I te & - - .. - argillaceous siltstone 470 . '"T" •• -,.... -, 640

- - .. -- .. __ .. - -.. - -- 145 310 .. i< __ 480 650 ·' ..,-.'''''T'· 95 - .. -- - - - BIWABIK IRON FffiMATION Argillite, blk., fis­ sile, & ollve-grn . tuff 24 Keewatin 1 of 2

210 I;";.: 0 .' 6 Glacial dr l ft, 380 115 ~~~ 550 1""0 0 0 undivided 65 ' - "- "- ' VIRGINIA FORMATION - -- Argillite, black, c-c- carbonaceous, and 220 _.. - - " 390 laminated argillite, 560 and si Ity argi Illte "'"T"-' ''''''T' c- c- c Tuff, mass I ve .. ------...... 230 Argillite, black, and 400 570 .. .. _ ...... dark-gray, silty argll- - - lite; laminated to _. _ _ th In- bedded ; scatter ed _ thin beds of laminated , Arg I I I I te & s I I ty Argillite, bl ack, and 580 195 240 cross- bedded , argilla­ 410 _.. _- argillite; laminated ,.... - ..,. ceous siltstone; abun ­ dark- gray, silty argil - lite; laminated -- dant soft sediment 125 Graywacke, med .-gr., _ " __ deformation structures 75 __ _ light- gray, cgl . ; graded , laminated 590 250 420 0000 '-'--'-"-T" • Argll lit e & silty - -"-- argillite; laminated

-'-- Graywacke, med.-gr., 260 Argillite, black, and 430 600 light- gray, structure­ arg II I aceous s I I t ­ less ' -r-' , " "'T . stone; thin bedded t------.. _ .. _~ . Argillite & silty Siltstone, fine- gr., argillite; laminated 270 --- 4 4C -_-_ -_ light- gray, graded 610 ' '"'T" '' , ' -.'' 185 r._~_~_ _.. _.. _.. Argillite, black, & 135 I- - - dark-gray, silty argil­ --- 450 lite; l aminated to 620 280 85 I- -- , " -'-" -Y- very th I n bedded .:.:..:... .:..:. Ar g II I I te, silty ar g 11 - .-__"T"..!.... •....j . lite, & f I ne- gra I ned --- - .. ---.. siltstone; laminated

290 I- - -- 460 630 . -:-...::.-::-.::.-::-~ Arg . & s i I ty arg II II te; laminated to very thin ' f " "'"T" ' "'T .- .. - .. - bedded ------...... 'T' .,.. Siltstone, very fine ...... ,: . :.,...... Argillite, silty argll ­ ~-__-- __--_ gr ., structureless 300 470 640 .. _ .. _ lite, & scatter ed beds Argillite, black, and ' , " "'T" " ""T" F==. :::::.~ Of fine- gr . laminated dark-gray, silty argil ­ 195 -- - to cross- bedded 1- -- lite; laminated .. _ ._ .. _ graywacke 650 310 480 145 1-.. - .. - "" .. _ ._-- Iiio=-iiii.. '=iii - i::.iit Argll I I te & s I I ty - .. ---- Ar gillite & silty 95 - -- - argillite; laminated " )"," "_ICI_IC.. =-..•~ =-I arglillte; laminae of ollve- grn . tuff 320 490 Siltstone, f . -gr. , 660 ~ - --::-- mass I ve; cg I. near 1- --- base Argillite & silty argillite; laminated I- - .. - 330 500 Tuff , ollve-grn., 670 massive ---- .. - --- - .. -- Arg . & silty argillite --- Argillite, dark- gray, Limestone, f .-gr., argillaceous; cross­ silty, structureless . -,.. • .. 205.L.. -'- bedded 340 510 f,680 155 ~= . = Argillite and silty 1-"- - - " --- Dominantly graywacke, argillite; laminated , .. .. dark- gray, flne- to 105 I- ....L... - locally calcareous; .. .,...-,-....,. medium- gr., In amalga­ locally abundant car­ 350 520 ------mated beds as much as 690 '0 . ;" ... '0 Graywacke, f .-gr . , bonate concretions 3 m thick; general I y _ -_- cgl . ; gr aded and structur e less to _ .. _ .. _ . I am I nated f-.-._.­ vag uel y laminated; •• ...1.....- •• -. scattered laminae of Argillite, black , and 530 360 dark- gray to black 700 - G=>- dark- gray, silty argil - -:...-:...-- 1---- _ .. - argillite -- lite

.. - .. _ .. - .,.... "'T' • ...,... •...., "'T' ''''''",. Siltstone & silty argll ­ 370 -- - 540 7 10 ; ::::::.::r"" lite with chert laminae 215 .-. --.. Ar gillite and sll ty , --­ ~...=-_-:-..:. arg i llite; laminated; r- -- -- c- - Arglll ite, black, car­ t- _ _ Interlayered 01 ive-grn. C-C bonaceous , pyritic tuff 165 25 Keewatin 2 of 2 Calumet 1 of 3

210 - - - Glacial drift, 720 undivided p::ci:::x:i~ Argl I lite & argilla­ 65 c-c- c ­ ___- ceous siltstone; thin VIRGINIA FORMATION __ _ bedded to laminated; -C-C­ @--C _ Interbedded thin beds 220 730 --- of chert & laml nae of -C-C­ a I I ve- grn. tuf f C-C-G: - C- C­ C- C- C ~-- 230 Argillite, black, car­ 740 - -- -C-C- bonaceous , fissile; ~- C scattered laminae and -C-C­ thin beds of silty, 225 C-C- C graded and cross­ 240 - C-C- bedded argillite 750 c-c-c­ Argll lite, black, car­ - C-C - bonaceous , pyritic; . C-:-.C-:-.$; c-c-C­ -G:-C-C- interbedded laminae & thin beds of chert 75 -C-C- -C-C- 250 760 f-c-C-C ~c~c£ -C-G­ ~-C-C­ ~--C-C -C-C­ f-C-C-C- -C-C­ C-C - C­ 260 C-C-C 770 - C-C­ -G - C­ C-C-C- C-C w: C-C - C­ -C-C- I-c~·:C<:.:c- 270 780 235 f-C-C- C­ C-C-C -C-C­ -C-C­ C-C-C C-C-C- -c ..6) c-C- c ­ C-C-C -c-c- 280 85 790 -C-C­ BI~IABIK IRON FORMATION 0-0- 0 Chert & argil lite, C-C-C -0- 0- Interbedded -C-C­ D-O-O Iron- formation , cherty, C-C-C- -0 290 silicate facies , con­ -ID"C­ 800 glomeratic, nonmagnetic C-C-G­ -C - C­ C-C-C 300 810 -C-GZ C-C-C­ 245 -C-{;­ C- C- C 310 -C-C- 820 k,;-C-{;- 95 I ~-{;­ C-C-C 320 C C C 830 - C-C­ C-C-C -C -C­ C- C - C 330 .. -,.-- Arg illite, sl I ty, 840 laminated , locally calcareous Argillite, black, 340 - - - pyritic, & dark-gr ay , 850 silty argillite; --- laminated. Scattered _....cs=::L .. pods & lenses of -l05 fine- gr., light-gray 350 arkose. Carbonate 860 concretions with & without cone- in-cone structures

360 870

370 _C:;; 880 @_ .. _ . C-C-C -C-C-

26