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PROVINCE OF ONTARIO
DEPARTMENT OF MINES
HON. PHILIP T. KELLY, Minister of Mines H. C. RICKABY, Deputy Minister M. E. HURST, Provincial Geologist
Geological Circular No. 6
OF THE ONTARIO DEPARTMENT OF MINES
ISSUED June, 1957
The Sedimentary Petrology of the Mississagi Quartzite in the Blind River Area
By J. P. McPOWELL
PRINTED BY ORDER OF THE LEGISLATIVE ASSEMBLY OF ONTARIO
TORONTO Printed and Published by Baptist Johnston, Printer to the Queen©s Most Excellent Majesty 1957 TABLE OF CONTENTS Geological Circular No. 6
PAGE PAGE Abstract...... l Sedimentary Sructures Continued Introduction...... l Cross-Stratification...... 13 Acknowledgments...... 2 Graded Bedding...... 19 General Geology...... 2 Measurements...... 19 Stratigraphy...... 2 Results...... 22 Table of Formations...... 3 Interpretation...... 24 Structure...... 3 Source Direction of Mississagi Lithology and Stratigraphic Features of Quartzite...... 24 the Mississagi Quartzite...... 3 Difference in Current Direction Lower Mississagi...... 4 between Lower Mississagi and Middle Mississagi...... 6 Upper Mississagi...... 24 Upper Mississagi...... 6 Significance of Pebble Data...... 27 Sedimentary Structures...... 7. Origin and Sedimentary History...... 28 Ripple Marking...... 8 Economic Aspects...... 31
LIST OF ILLUSTRATIONS PAGE Megaripples, Locality 59, Striker township...... 11 Closeup of megaripples snowing fine material in troughs...... 11 Cross-stratification in ripple marks...... 12 "Decapitated" cross-stratified ripple marks...... 12 Intersection of cross-stratification planes and bedding planes...... 15 Typical pebble layer at base of cross-strata...... 17 Angular relationship of cross-strata and basal plane...... 17 Typical trough cross-bedding...... 18
FIGURES PAGE 1 Index map showing area studied ...... facing l 2 Nomenclature used in the Blind River Area...... 5 3 Stratification and migration of ripple marks...... 9 4 Cross-stratification from ripple migration...... 10 5 Current direction data ...... back pocket 6—Types of cross-stratification...... ©. . ., ...... 14 7 Block diagram illustrating intersection of cross-bedding and bedding...... 15 8 Types of graded bedding in the Mississagi quartzite...... 21 9 Procedure for correction for tilt of the beds...... 23 10 Comparison of current directions in Upper and Lower Mississagi quartzite...... back pocket 11 Thickness of cross-bedded units...... back pocket 12 Data on distribution of maximum pebble size...... 25 13 Percentage of black chert in chert and quartz pebble conglomerates...... 26 14 Source of Mississagi quartzite...... facing 26 15 Comparison of fluvial, tidal, and Mississagi rose diagrams...... facing 28 16 Comparison of current azimuths and ore trends...... , ...... 30
(ii) Insert to face page l
DISTRICT OF ALGOMA
/f
o rt n e l
EXPLANATION
1. Endikoi Lake 2. Ten Mile Lake 3. Quirke Lake 4. Whiskey Lake 5. Elliot Lake 6. Motinenda Lake 7. Chiblow Lake 8. Lake of the Mountains 9. Lauzon Lake 10. Rock Lake 10
Figure l—Index Map showing Area within which Mississagi Quartzite was Studied. The Sedimentary Petrology of the Mississagi Quartzite in the Blind River Area BY J. P. McDowell
ABSTRACT The attitudes of 1,230 cross-beds were measured at 124 localities in 29 townships between Bruce Mines and Massey, Ontario; 49 asymmetrical ripple marks were also measured and included in the analysis of cross-bedding. The mean cross-bedding dip azimuth for the Upper Mississagi was 109 degrees, and for the Lower Mississagi 158 degrees, indicating that the flow of sand deposit ing currents was from northwest to southeast in both units, but from a slightly different direction. The maximum pebble size was measured at 15 localities and showed an increase in size towards the west. The percentage of black chert in the conglomerates also increased in the same direction, giving further support to a westerly source for the sediments. Assuming that the pebble size increased exponentially towards the source as is the normal case, the data show that the source area was from 130 to 250 miles west-northwest of Thessalon. The Mississagi quartzite was also studied and interpreted with respect to environment of deposition. The planar type of cross-bedding, large-scale ripple marks, regional uniformity of current directions, and the variation in size and composition of the pebbles in the conglomerate beds favour a fluvial origin for the Mississagi sands. The finely laminated silts, graded bedding, and abundant ripple marking in the upper portion suggest a marine submergence or a large lake for the deposition of the Middle Mississagi silts and argillites. The ore-bearing conglomerates are believed to represent gravels deposited in ancient streams beds. Semi-continuous gravel beds were formed by migration of stream channels by lateral corrasion. Similarity in trend of current directions in the Mississagi, as measured in cross-bedding and ripple marking, and the trends of orebodies as outlined by drilling, suggest that the two trends are related. If these trends are genetically related, then this is strong evidence of a syngenetic origin of the ore.
INTRODUCTION1 The observation of numerous outcrops displaying excellent bedding, cross- bedding, ripple marking, and other sedimentary structures in the Mississagi quartzite during previous field investigations in the district of Algoma, led the Ontario Department of Mines to employ the author, during the field season of 1956, to make a special sedimentological study of the Mississagi. The major work was done in the Blind River area covered by map No. 1970 of the Geological Survey of Canada. Supporting data were also obtained from a much wider area comprising about 2,000 square miles, as shown in the key map (frontispiece).
1This report is based on observations made during the field season of 1956 and will be subject to revision as a result of further field work during 1957. The purpose of the project was to study, measure, and map the various sedimentary structures and other features of the Mississagi quartzite in order to interpret its origin and sedimentary history. From studies on the Colorado Plateau and from mining experience in the Rand deposits in South Africa, it is evident that cross-bedding may be of economic importance. In the Colorado Plateau district, cross-bedding has been used to predict trends of channels containing ore and also to aid in prospecting for new ore.1 © 2 The validity of cross-bedding as an indication of channel trend and the relation of cross-bedding to known channels have been discussed at length by Potter3 and Edwards.4 In the Rand, cross-bedding has been used in conjunction with the long axes of pebbles, as an indication of which direction to extend the face when ore has run out in a particular play. Reinecke5 noted that the cross- bedding direction in the Witwatersrand was parallel to the axes of nearby pay- streaks. The implications of the above statements are obvious. If the major ore trends are channel controlled, then because of the demonstrated close relation between cross-bedding and channel directions, the cross-bedding may be used to predict such trends and thus serve as a guide to both exploration and development.
ACKNOWLEDGMENTS The author acknowledges the help of E. M. Abraham, who first recognized the possibilities and importance of such a study, and who directed the author to the best localities in the area. Able assistance was provided by Jerry Barbeau, Michael Moes, and Andrew Rickaby, who worked with the author. F. J. Petti- john of The Johns Hopkins University, Baltimore, Maryland, U.S.A., has aided materially by his many suggestions. Finally, the author must acknowledge the aid of his wife, Lorilee, who served as camp cook for the entire season and as a willing assistant after the regular assistants returned to the university.
GENERAL GEOLOGY Stratigraphy The rocks in the area, all Precambrian in age, fall into three age groups: (1) pre-Huronian basement, (2) Huronian, and (3) post-Huronian intrusives. The pre-Huronian basement rocks consist chiefly of granite and granite gneiss batholiths and a schist complex composed chiefly of basic metavolcanics. Metasediments of the Sudbury series which may or may not be pre-Huronian,6© 7 are also present.
1 W. L. Stokes, Primary Sedimentary Trend Indicators as Applied to Ore-Finding in the Carrizo Mountains, Arizona and New Mexico, U. S. Atomic Energy Comm., R. M. E.-3043, pt. l, 1953. 2J. D. Lowell, "Application of Cross-Stratification Studies to Problems of Uranium Explora tion, Chuska Mountains, Arizona." Economic Geology, Vol. 50, 1955, pp. 177-85. 3 P. E. Potter, "Petrology and Origin of the Lafayette Gravel, Pt. l Mineralogy and Pe trology," Journal of Geology, Vol. 63, No. l, 1955, pp. 1-38. 4 John D. Edwards, Studies of Some Early Tertiary Red Conglomerates of Central Mexico, U. S. Geol. Surv., Professional Paper 264-H, 1955, pp. 152-83. 5L. Reinecke, Origin of Witwatersrand System, Trans. Geol. Soc. South Africa, Vol. 33, 1930, p. 111-32. 6W. H. Collins, North Shore of Lake Huron, Geol. Surv. Can., Mem. 143, 1925, pp. 67-69. 7Jas. E. Thomson, Geology of Baldwin Township, Ont. Dept. Mines, Vol. LXI, 1952, pt. 4. 2 Overlying these plutonic and metamorphic rocks are two groups of sediments of Huronian age: the Bruce series and the Cobalt series. The following table shows the relations between these series and the formations within each:
Table of Formations PRECAMBRIAN: HURONIAN: Cobalt series \Gowganda/Lorrain quartzite formation
Unconformity [Serpent quartzite l [Espanola limestone R - J Espanola formation \ Espanola greywacke nruce series ^ (Bruce limestone l Bruce conglomerate 1. Mississagi quartzite Unconformity PRE-HURONIAN:
These sediments have not been involved in strong folding and metamorphism, and, therefore, show only slight recrystallization and development of cleavage. All the sedimentary structures and textures are perfectly preserved. Intruding the pre-Huronian and Huronian groups are numerous basic dikes and sills. These intrusives have been mapped as Keweenawan by most workers, but this correlation has been questioned by Thomson.1 The intrusives have caused local deformation and recrystallization of the invaded rocks.
Structure In the Blind River area, the Huronian strata have been gently folded into a large syncline and anticline plunging gently towards the west. The dips in the Mississagi range from 12 to 70 degrees on the flanks but approach O degrees on the nose of the anticline. The average dip is 22 degrees. On the south side of the anticline, along highway No. 17, the Mississagi has been steepened and overturned near a large fault that placed the Mississagi against the Sudbury series(?). Recrystallization is much greater close to this fault. This folding, coupled with the resistance of the Mississagi formation to erosion, makes the unit an ideal one to study. The Pleistocene glaciation has polished the quartzite so that all the detailed textures and structures stand out. Many glacial lakes, controlled by the structure and Mississagi stratigraphy, make the formation easily accessible from the few roads that penetrate the area.
LITHOLOGY AND STRATIGRAPHIC FEATURES OF THE MISSISSAGI QUARTZITE The Mississagi quartzite ranges from 650 to 3,500 feet in thickness in the Blind River area. A complete section is not present everywhere. The most complete section available was measured in drill core from a hole on the Stanleigh Uranium Mining Corporation, Limited, property and will be used as a standard for comparison with other sections and for a discussion of lateral modifications. For mapping and descriptive purposes, the Mississagi quartzite is best divided into three parts. These parts have been called Lower, Middle, and Upper by those
1Jas. E. Thomson, op. cit. working in the area in recent years. Roscoe1 has proposed formation names for different units in the Mississagi, but as yet these names have not been generally adopted. (See Figure 2.) Wherever the contact between the Mississagi and the basement is visible, it is marked by residual weathering products and soil layer formed before deposition of the Mississagi. 2© 3 This paleosol varies markedly in thickness and also with the character of the basement rocks. Thicknesses noted by the author ranged from 2 to 10 feet, but Roscoe4 has noted from O to over 50 feet on a proposed correlation chart. Where this paleosol has developed on a granite terrain, it is very difficult to pick out the contact since the granite grades upward through weathered granite to arkose, to feldspathic quartzite, with hardly a break. The paleosol overlying basic rocks is very dark in colour. It is easily weathered and forms a prominent trench.
Lower Mississagi The Lower Mississagi is a coarse-grained-to-conglomeratic feldspathic quart zite. It varies from O to 700 feet in thickness from north to south. In an east-west direction, it remains fairly uniform in thickness. The Mississagi immediately above the paleosol consists, in most instances, of quartz and chert conglomerate composed of well-rounded pebbles %-S inches in diameter. The pebble bands and layers are discontinuous along strike, but in general occur in a zone 10-50 feet thick. It is this pebble zone that is the uranium- bearing horizon in the district. The uranium is present in very small amounts of pitchblende, brannerite, and thucholite in the matrix of the conglomerate, with considerable amounts of pyrite and minor chalcopyrite, pyrrhotite, and galena.5 The conglomerate zone, however, is not always exactly at the base of the forma tion but may be underlain by as much as 50 feet of coarse feldspathic sandstone. Above the conglomerate zone is a coarse-to-conglomeratic quartzite. Sporadic quartz pebble layers are present, and large angular feldspar grains as large as l centimetre occur in some horizons. The rock contains about 80-85 percent quartz and 15-20 percent feldspar. The sorting is only fair as the average sorting index of samples studied (the number of times the largest grain exceeds the smallest) is 39. The degree of sorting is based on the following divisions: Largest grain exceeds the smallest by a factor of: 8 times or less...... good sorting 8 to 128 times...... fair sorting greater than 128 times...... poor sorting A comparison of the sorting between the Lower and Upper Mississagi is given in the table on page 7. This part of the Mississagi is characterized by the trough or festoon type of cross-bedding, which is discussed more fully in the section on sedimentary structures. Above this coarse, feldspathic portion, the Lower Mississagi becomes a finer-grained feldspathic quartzite showing both better sorting and rounding of grains. This portion forms only the upper 100 feet or less of the Lower Mississagi.
XS. M. Roscoe, Geol. Surv. Can., Topical Report No. 4, 1956. 2 R. Pumpelly, and C. R. Van Hise, "Observations upon the Structural Relations of the Upper Huronian, Lower Huronian, and Basement Complex on the North Shore of Lake Huron," American Journal of Science, 3rd. ser., Vol. 43, 1892, p. 230. 3W. H. Collins, op. cit., pp. 38, 39. 4 Roscoe, op. cit. 6E. M. Abraham, Geology of Parts of Long and Spragge Townships, Blind River Uranium Area, District of Algoma, Ont. Dept. Mines, P.R. 1953-2, 1953. 0 c o j o O (A c O JC o* tt: 0 Q. o ^. IA 0) ^ o s* c O (A O (A UJ TJ Jj (O Q. i • w s - JC 2. a: IA
e -a O tt .5 '•^ y\ "3 W GO \V\ '-o 0 u. w. C •D 3 v. o •~w ^ \ \ w *- Q 0) \\ 0 C 0) Jomencla Regional o i c 11956)f( E (A c JC (A i o o 0 u. V V 4K. i \ ^ S CA •o O Z 1n .-. n c c o* V .0.5 o M Q. 4 tt) ^c ( 5 -S — k. /i 1 Q) •o JO /l O * f? S 1 E o f O T o f o l 9 1 9) 1 3 — **"* tmenclatui W /.i O CL |2| O *L O. 1 CO \ < -o W to M J- 0 C JC O — I l JA * - ?^r ~— w S k. c ^ 4A ' •O o 3 •o Q. 5 w c •o 1 T) - c .* Q. o j o GC W i D ? (A i ^ E ii|? u. ^ g 85 o O* S ffi u. o 0 ^-* X-* c i-s. If) 00 CM E 00 O) c o o O* 1- O — 1IO CO ^ tA E ^ C l tt o in o .Z u. E o i 5 o It contains conspicuously less feldspar (10-15 percent) and lacks the coarse angular feldspar grains so characteristic of the section below.
Middle Mississagi The Middle Mississagi is a finely laminated siltstone and argillite with inter bedded fine-grained quartzite in the lower part. One of the most striking features is an unusual boulder conglomerate. This conglomerate, easily seen in drill core, is widespread according to Roscoe.1 It ranges in thickness from 10 to 200 feet. Lakes and swamps tend to form on the Middle Mississagi so that surface outcrops of the conglomerate are not prominent except along the northern belt of Mississagi outcrops between Quirke Lake and Ten Mile Lake where the conglomerate rests directly on the basement. The conglomerate is characterized by granite, greenstone, and quartzite pebbles and boulders up to several feet in diameter randomly dispersed in a greywacke matrix. The boulders seem to be those found in basement rock north of the outcrop although greenstone pebbles are also prominent. The conglomerate is probably a time-rock unit, or one that was deposited at the same time over the entire area, since it transgresses the lithology of the Mississagi at a very low angle. For example, the conglomerate is at the base of the Middle Mississagi and rests directly on the basement at Quirke Lake and Ten Mile Lake, whereas at North Nordic Lake over 200 feet of argillite, interbedded siltstone, and very fine quart zite, are present below the conglomerate. Above the conglomerate the unit contains no interbedded coarse material, but consists of fine silt and argillite. The transgression of this conglomerate across the lithology of the Mississagi is the basis for the revision in stratigraphic nomenclature proposed by Roscoe.2 (See Figure 2.) The recognition of this transgression was an important observa tion, as it may prove to be the key to correlation in places where the stratigraphy is in doubt. However, the areal extent of this conglomerate is as yet unknown so that its value as a reference plane on which to base stratigraphic nomenclature is uncertain. Perhaps it would be better to retain the present division of the Mississagi into Lower, Middle, and Upper units until further careful geological mapping has been completed. In the meantime, recognized sub-units of these major divisions could be given letter designations to aid in mapping and strati graphic studies In the argillite and siltstone beds are laminae that show micro-graded bedding. In the upper portions siltstone predominates over argillite, and ripple marking is common. These ripples show a cross-bedded cross-section. In a few cases the ripple was observed to have migrated, thus forming a small lens-shaped cross-bedded unit.
Upper Mississagi The contact of the Middle with the Upper Mississagi is not sharp but shows a fairly rapid change from siltstone to medium-to-coarse-grained feldspathic sand stone. The ripple marking, common near the top of the Middle Mississagi, is also found in the lower part of the Upper Mississagi. The Upper Mississagi is a fairly uniform feldspathic quartzite containing 85-90 percent quartz and 10-15 percent feldspar. It is noticeably finer-grained,
aS. M. Roscoe, personal communication. 2S. M. Roscoe, op. cit. better sorted, and less feldspathic in most places.The table below compares the sorting of the Lower and Upper Missassagi in several thin sections studied. COMPARISON OF SORTING INDICES IN LOWER AND UPPER MISSISSAGI QUARTZITE Lower Upper Mississagi Mississagi 53 24 50 7 13 5 20 116-f^ 17 5 8 20 30 136-^-9 = Average sorting index...... 39 15 In the Ten Mile and Quirke Lake areas the Upper Mississagi is course-grained in some parts. In this area the Lower Mississagi is present for only a short distance along the contact north of the Algom-Quirke mine. The Upper Mississagi is characteristically coarse-grained for 350-500 feet above the contact with the Middle. Pebble beds are present in this zone, which are commonly only one pebble thick. The pebbles are chiefly quartz with minor black chert. The black chert becomes increasingly more abundant to the west. (See Figure 13, page 27.) Above this zone, sporadic pebble zones are common. The pebbles increase in size (see Figure 12, page 25), and the zones become more abun dant towards the west. Planar-type cross-bedding is characteristic of the Upper Mississagi and is especially abundant in the lower portion near the contact with the Middle. In the Quirke Lake-Ten Mile Lake area trough cross-bedding is also present in the Upper Mississagi and is apparently related to the coarser grain size and poorer sorting of the upper unit in this area. The Bruce conglomerate overlies the Mississagi. No great time break between the two was noted. At Ten Mile Lake there is a complete gradation from Missis sagi into Bruce, and the exact contact cannot be determined. At Quirke Lake, however, the contact is irregular, the Mississagi bedding has a swirly appearance, and irregular pieces of Mississagi are included in the Bruce. The Mississagi does not appear to have been consolidated at the time the Bruce was deposited. It is evident, from the generalized discussion of the Mississagi lithology above, that the sedimentation was not everywhere uniform throughout Mississagi time. This is strikingly portrayed by the thinning and complete pinching out of the Lower Mississagi from Elliot Lake to Quirke Lake, except for a thin remnant left to the north of the Algom-Quirke mine. The abrupt change in lithology from Lower to Middle to Upper Mississagi is another example.
SEDIMENTARY STRUCTURES Several sedimentary structures were found in the Mississagi formation in addition to normal bedding. Of these structures, cross-bedding1 is the most
1 Variously called current bedding, false bedding, cross-stratification, cross-lamination, foreset bedding, torrential bedding, inclined bedding, and oblique bedding. All are grouped under cross- stratification by McKee and Weir. (Edwin D. McKee and Gordon W. Weir, "Terminology for Stratification and Cross-Stratification in Sedimentary Rocks," Bull. Geol. Soc. Amer., Vol. 64, 1953, pp. 381-90.) important since it is the most abundant. Ripple marking is present and locally important. Graded bedding is also present, but is the least important of the structures observed. Ripple Marking Ripple marking is one of the most commonly recognized primary structures occurring in arenaceous sedimentary rocks. In general, ripple marks consist of alternating subparallel ridges and troughs. These ridges and troughs have three important properties related to their genesis that are useful for descriptive pur poses: amplitude, from bottom of trough to crest of ridge; wave length, or distance from one crest to the next (or from trough to trough); and shape, or whether they are symmetrical or asymmetrical. The first two properties are easily measured and range from fractions of an inch to tens of feet. The third property is a subjec tive one, as it is not always easy to determine the asymmetry. In the Blind River area, however, the distinction was easily made. Two major types of ripple marks are defined on the basis of whether they are symmetrical or asymmetrical. The symmetrical ripples are formed on loose, granular material by the oscillation of water in wave action. They possess a direc tion, but show no "sense." Because oscillatory forces form them, they remain stationary. The asymmetrical ripples form in the same type of material in response to a current of water (or air). These ripples are not stationary, but move down current by the erosion of grains from the gentle upstream slope of the ripple and redeposition on the steeper downstream slope. The size of wave ripples is influenced by the size of the material in which they form, the size of the waves, and the depth of the water.1 © 2© 3 They are oriented parallel with the shore line, but since the forces forming them are oscillatory, they show no direction of deposition. Current ripples, on the other hand, give evidence of the direction of the depositing currents, the strength of these currents,4 the depth of water,5 the amount of sediment load,6 and may eventually be useful in postulating the type of flow or kind of water body (marine or fluvial)7 when they are more fully under stood. In this study, the directional qualities and internal structure are most important. Ripple marking is present throughout the Mississagi formation. However, it is common only in the Middle Mississagi and the lower part of the Upper Missis sagi. In almost all cases, the ripples are asymmetrical or current ripples. Only a few wave ripples were observed. The internal structure of current ripples was observed and studied in a number of places. In the uppermost part of the Middle Mississagi, where siltstone is the dominant rock type, the ripples commonly show cross-stratification as a result of migration. (See Figure 3.) In many cases, these cross-laminated ripples are overlain by normal laminated silts and hence preserved. In other cases, how ever, the ripples have migrated without destroying the gentle current side of the ripple in order to construct the down-current cross-laminae, thus forming a small
1 G. K. Gilbert, "Ripple Marks and Cross-Bedding," Bull. Geol. Soc. Amer., Vol. 10, 1899, p. 138. 2 E. M. Kindle, Recent and Fossil Ripple Marks, Geol. Surv. Can., Museum Bull. No. 25,1917, p. 29. 3E. M. Kindle, and W. H. Bucher, Treatise on Sedimentation, The Williams and Wilkins Co., Baltimore, Md., 1932, p. 662. 4Ibid.,p. 666. 5A. Hider, Mississipi River Commission reports, 1882, pp. 83-88. *E. M. Kindle, op. cit., p. 23. 7J. Hulsemann, Grossrippeln und Schragschichtungs-Gefuge im Nordxee-Watt und in der Molasse, Senkenbergiana, Lethata, Band 36, 1955, p. 359-88. 8
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Megaripples, Locality 59, Striker township; wave length, 2 Vi feet; amplitude, 6 inches.
Closeup of megaripples showing fine material in troughs.
Sorby1 noted the formation of small-scale cross-bedding by the migration of ripples in this manner and drew essentially the same figure as Figure 4. An unusual ripple marking was discovered by Abraham in Striker township
1 H. C. Sorby, "On the Structures Produced by the Currents Present During the Deposition of Stratified Rocks," The Geologist, Vol. 2,1859, pp. 137-47. 11 and restudied by the author. (See Locality 59 on Figure 5, in back pocket.) Here the ripples might best be termed megaripples, since they reach an amplitude of 6 inches and have a wave length of 2^ feet. These ripples also show cross-stratifica-
Cross-stratification in ripple marks.
"Decapitated," cross-stratified ripple marks. tion and are in the process of forming a cross-bedded layer through migration and decapitation. A second area of such ripples was also noted, and the same cross- stratification was present. Ripples, then, may be an important mechanism in the formation of cross-bedding.1
J H. lilies, Die Schragschichtung influviatilen und litoralen Sedimenten, ihre Ursachen, Messung und Auswertung, Mitt. Geol. Staatsinstitut Hamburg, Heft 19, 1949, pp. 89-109. 12 Cross-Stratification Cross-stratification,1 here used to include both cross-bedding and cross- lamination, is probably the most common primary structure preserved in sedi mentary rocks exclusive of normal stratification. McKee and Weir2 define cross- stratification as "the arrangement of layers at one or more angles to the dip of the formation. A cross-stratified unit is one with layers deposited at an angle to the original dip of the formation." Cross-stratification is formed by the passage of a current of water (or air) over granular sediment, causing the deposit to be extended downstream by the addition of inclined strata. These strata and the cross-stratified unit as a whole have several properties, which are of interest and importance in classification, description, and mode of origin. Of these properties, the shape and thickness of the cross-stratified unit, and the shape, thickness, and attitude of the cross- stratum are the most important parameters. Before these parameters can be discussed intelligently, however, the major types of cross-bedding and their relationship to currents in which they were formed must be considered. McKee3 and Kiersch4 found that cross-stratification can best be divided into three groups, depending on the nature of the lower bounding surface and the shape of that surface. The first division is based on whether the lower surface is one of erosion or non-erosion. If the surface is one of non-erosion, the cross- stratification is called simple. When the surface is one of erosion, a further sub division is made on the basis of whether that surface is planar or curved. If the surface is erosional and planar, the cross-stratification is called planar; if curved, it is called trough cross-stratification. Figure 6 illustrates the three types and the shapes of the cross-strata that make up these types. The distinction between simple and planar cross-stratification is a difficult one to make, since it is not always easy to ascertain whether or not the lower bound ing surface was erosional. Both types certainly grade into each other. On the other hand, the distinction between planar and trough cross-stratification is a sharp one, and there is little chance of confusion. The two types appear to be unrelated. All three types of cross-stratification were observed in the Mississagi, but the planar and trough types were the most important. The only clear examples of simple cross-stratification were observed in the cross-laminated silts of the Middle Mississagi. These units were all less than two centimetres. In order to form this type of cross-stratification, the depositing medium must have been carrying its maximum load, or some degree of erosion would have resulted. Planar cross-bedding was the most important type present in the Mississagi. This is the type often called torrential cross-bedding, but as this implies a genesis that may be strongly questioned, the word has not been used in his report. Planar cross-bedded units in the Mississagi range in thickness from less than 3^ inch to 72 inches. They are characteristic of the Upper Mississagi and the
1 Terminology according to McKee and Weir in which cross-stratification is used to include all strata inclined to the major stratification, regardless of size or shape. Using this classification, cross-stratification is divided into the two subgroups, cross-bedding and cross-lamination, based on whether the cross-strata are greater or less than l centimetre in thickness. 2E. D. McKee and G. W. Weir, op. cit. 3 E. D. McKee, "Classification and Interpretation of Cross-Lamination." Bull. Geol. Soc. Amer., Vol. 59, 1948, p. 1378. 4G. A. Kiersch, Small-scale Structures and Other Features of Navajo Sandstone, Northern Part of San Raphael Swell, Utah, Bull. Amer. Assoc. Petroleum Geologists, Vol. 34, 1950, pp. 923-42. 13 I o Z) H o e S (T h- -S 5
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14 Intersection of cross-stratification planes and bedding plane showing intersection is a straight line.
Intersections of bedding and cross-bedding surfaces Bedding surface
Cross-bedding surface Figure 7 — Block diagram illustrating structure shown in above photo.
15 upper 100 feet of the Lower Mississagi. The planar erosional surfaces between sets of cross-strata are commonly marked by a pebble layer. This layer may be only one pebble thick, composed of quartz and black chert pebbles of remarkably uniform size. (See upper photo, page 17.) Planar cross-bedding in the Mississagi appears to have been formed only if the sands showed a fair degree of sorting. It is not the common type in the more poorly sorted Lower Mississagi and portions of the Upper Mississagi in the Quirke Lake area, which are not so well sorted as the normal Upper Mississagi. The planar type occurs in strata ranging from very fine sand to very coarse and con glomeratic sands, so that the grain size is not an important restriction. From the definition of planar cross-bedding, it is evident that the upper and lower surfaces are planes. The shape of the cross-strata within the set are not included in this definition, but all those observed in the Mississagi were essentially planar with at most a few degrees of curvature. Thus the intersections of the bedding plane and the cross-bedding planes are essentially straight. (See photo, page 15.) A few cross-strata were observed to be tangent to the basal plane of the cross-bed. In almost all cases, however, the cross-strata meet the basal plane at an angle. (See photos, page 17.) Because of the planar nature of both the cross-stratified unit and the inclined cross-strata, the angular relationship between the two planes may be computed after measuring the attitude of the bedding and the attitude of the cross-strata, and correcting for tilt of the beds. This angular relationship is called the inclination. The inclination, assuming that the original bedding was essentially horizontal, represents the angle of repose for the conditions under which the cross-strata were deposited. For dry sand, this angle has an approximate upper limit of 34 degrees.1 McKee2 has shown in experiments that the same sand has a lower angle of repose in water than in air. Thus the maximum inclination for cross-bedding should be somewhat less than 34 degrees. The actual range of inclinations measured in the Mississagi was 3-53 degrees. The mean inclinations ranged only between 10.5 and 29.9 degrees, however, with 81 percent beween 16 and 25 degrees. The number of individual readings greater than 34 degrees, the maximum angle of repose, was less than }^ of l percent. These discrepancies could be accounted for by errors in measurement of the attitude of the cross-strata, the attitude of the true bedding, and the factors related to rock deformation. Any more thorough explanation of these high inclinations is beyond the scope of this report. It is important to note, however, that the majority of the inclinations are less than the maximum angle of repose as should be expected from McKee©s experiments3 on the angle of repose of wet sand. The assumed direction of current flow that formed the. cross-bedding will be normal to the intersection of the cross-stratum and the bedding and in the direction of dip of the cross-stratum. The thickness of the cross-stratified unit is also easily measured. The formation of planar cross-bedding may be intimately associated with ripple marking as discussed earlier. Kindle4 states that "cross-bedding represents in many instances one phase of a phenomenon called sand waves, which are noth ing more than current made ripple-marks . . . when the current is overloaded with sediment. The crests are often 15 to 35 feet apart and rise 2 to 3 feet above
X R. A. Bagnold, The Physics of Blown Sand and Desert Dunes, Methuen, London, 1941, p. 201. 2E. D. McKee, Report on Studies of Stratification in Modern Sediments and in Laboratory Experiments, Office of Naval Research, Project Nonr 164 (00) NR081 123, 1955. 3 E. D. McKee, op. cit. 4 E. M. Kindle, op, cit., p. 55 16 the troughs." lilies1 noted that cross-bedding formed on the bottom of streams by deposition of sediment on the lee side of ripples, sand ridges, and deltas. These cross-bedded units were 10-50 centimetres thick, contained cross-strata with an average inclination of 27 to 30 degrees, and were continuous for as much as 30 metres along the stream.
Cross-bedded layer with quartz and chert pebble layer marking erosion plane at base.
Cross-strata meeting basal plane at an angle. (See photo above.) Cross-bedding may also form in some other manner, possibly by the deposi tion of foreset beds off an abrupt steepening in slope. In this case, one would expect the cross-strata to be sinuous along the strike, not straight as observed. (See Figure 7, page 15.) One must also posutulate some means of forming a steepening in slope in a regular manner. McKee2 has formed cross-bedding in this
1 H. lilies, op. cit. 2 E. D. McKee, op. cit. 17 manner in the laboratory by deposition in a standing body of water. This is no more than deltaic deposition. Trough or "festoon" cross-bedding is also common in the Mississagi and is the characteristic form found in the more poorly sorted Lower, and similar por-
Typicol trough cross-bedding. tions of the Upper Mississagi in the Quirke Lake area. This type of cross-bedding was first studied in detail by Knight1 who called it "festoon" bedding from its ap pearance in certain orientations. This terminology has been in common usage,
1 S. H. Knight, The Fountain and the Casper Formations of the Laramic Basin, Univ. Wyoming Pub. in Sci., Geol., Vol. l, 1929. 18 but trough cross-bedding is used here as a more general, descriptive term. This follows the terminology suggested by McKee and Weir.1 Trough cross-bedding, as shown in the photos on page 18 and in Figure 6, consists of wide, shallow, concave upward channels, which are U-shaped in plan view, and wedge- or lens-shaped in longitudinal section. These structures in the Mississagi ranged from 4 inches wide, l inch deep, and 40 inches long, to 48 inches wide, 6 inches deep, and 120 inches long. In other areas, trough cross-bedding has been observed, but on a much larger scale. Stokes2 recorded the average dimen sions as 5 feet wide, l ^ feet deep, and 20 feet long. Knight3 measured a maximum size of 1,000 feet wide, 100 feet deep, and several thousands of feet long. All of these examples show about the same ratio of width to depth to length. Knight4 describes the formation of trough cross-bedding as: the result of: (1) the erosion of plunging troughs, having the shape of a quadrant of an elongate ellipsoid; (2) the filling of the troughs by sets of thin laminae conforming in general to the shape of the trough floors; (3) the partial destruction of the filling laminae by subsequent erosion, producing younger troughs. . . . Although the mechanics of the process by which the troughs were eroded and filled is not thoroughly understood, it is concluded that the same force operating through varying degrees of intensity, eroded and filled the troughs. In other words, the channel was cut and strata deposited in it almost im mediately by the same stream during a single continuous phase of activity. No one has observed the formation of trough cross-bedding nor been able to reproduce it in the laboratory. Stokes5 believes, however, that it is formed under turbulent conditions and, therefore, turbid water during a period of high water. At this time the velocity, and thus the transportation of sediment, would be high. The reason we have not observed it in modern sedimentation is because of the manner of its formation. Although trough cross-bedding is abundant in the Mississagi, it is difficult to make meaningful measurements on it because of its small scale and curved shape. The only reliable measurement is the azimuth of the axis of the trough. As this measurement can only be made on the bedding-plane surface, it requires special conditions for this type of cross-stratification to be of use. These conditions were provided in the Blind River area only along lake shores and on a few ridge tops. Here the azimuth of the trough and the attitude of the bedding could be easily measured and correction made for the tilt of the beds. Graded Bedding Graded bedding is not characteristic of the Mississagi formation, but does occur in three characteristic ways. These three types are illustrated in Figure 8. Graded bedding was not of importance in determining the directional properties of the Mississagi. MEASUREMENTS It was desired to measure those sedimentary structures which have direc tional properties in order to ascertain the direction of the depositing currents, and thus interpret the source direction of the Mississagi sediments. These structures consisted of planar cross-bedding, trough cross-bedding, and ripple marking as described earlier in the report.
1 E. D. McKee and G. W. Weir, op. cit. 2 W. L. Stokes, Primary Sedimentary Trend Indicators as Applied to Ore-Finding in the Carrizo Mountains, Arizona and New Mexico, U. S. Atomic Energy Comm., R.M.E.-3043, pt. l, 1953. 3S. H. Knight, op. cit. 4S. H. Knight, "Festoon Cross-Lamination (Abstract)," Bull. Geol. Soc. Amer., Vol. 41, 1930. p. 86. 5W. L. Stokes, op. cit. 19 It is obvious that all outcrops in the area could not be visited and the direc tional structures measured. Therefore, some system had to be established whereby the objectives of the study could be fulfilled in the time available. This system should be delimited by the following factors: (1) greatest possible areal coverage; (2) over-all abundance and distribution of directed structures; (3) local and areal variation in current direction; (4) ease of measurement; and (5) convenience in analysis of data. Others have analysed the problems of sampling design in connection with similar directional studies.1 © 2 The approximate number of measurements taken in the Mississagi per unit area is based on these studies, although modified to fit the restricting factors of this particular study. No limiting sample design should be devised and rigorously followed without prior knowledge of the field conditions. It was found from the size and distribution of lakes, excellence of exposure, and other topographic conditions, that the most satisfactory and uniform cover age of the area was obtained by spacing the measured localities at intervals of 1-1^ miles wherever possible. This density of coverage gave about five localities per township. At each locality, an attempt was made to measure five directional structures. No fewer than two were measured. However, if the exposures were good, and the structures abundant, more than five were often measured. If the structure measured was planar cross-bedding, which was the most abundant directional structure, two measurements were made of the attitude of the planes of cross-stratification as far apart as possible in the same cross-stratified unit. This was done to compensate for minor irregularities in the plane of cross- stratification and thus give a better average reading for each bed. In order that the measurements on trough cross-bedding and ripple marking could be compared and evaluated on an equal basis with planar cross-bedding, two measurements of these structures were also taken on each bedding plane. The fact that two measurements were taken on each of five horizons at the average locality, gave ten measurements in all. The number ten simplifies the analysis of data to such a degree that it is worthy of consideration. The number ten is also sufficient and very useful for showing the raw data graphically as percentages in the form of histograms. (See Figure 5, back pocket.) The correction for tilt of the beds for both planar and trough cross-bedding was made by use of the equal-area Schmidt net in the manner illustrated by Figure 9. Whitaker3 has challenged this method as involving some error in areas of arcuate trends, such as the Appalachians,4 because arcuation implies some degree of rotation about a vertical axis. The method has considerable justification, however, as shown by Kopstein5 in the Harlech Dome, Scotland. Here, the directional properties of the Cambrian sediments vary little more than a few degrees over the entire area in a region that has undergone a high degree of folding.
1 Paul Edwin Potter, and J. S. Olson, "Variance Components of Cross-Bedding Direction in some Basal Pennsylvanian Sandstones of the Eastern Interior Basin: Geological Application." Journal of Geology, Vol. 62, No. l, 1954, pp. 50-73. 2 Paul Edwin Potter, and Raymond Siever, "Sources of Basal Pennsylvanian Sediments in the Eastern Interior Basin, 1. Cross-bedding," Journal of Geology. Vol. 64, No. 3, 1956, pp. 225-44. 3J. C. Whitaker, "Direction of Current Flow in some Lower Cambrian Clastics in Maryland," Bull. Geol. Soc. Amer., Vol. 66, 1955, p. 765. ] 4Arcuation in the Appalachians implies that crustal shortening has been more intense in areas of greatest degree of arcuation. Therefore, Appalachian sediments have not only been transported westward about subhorizontal rotation axes, but arcuation has produced some rotation of the sediments about vertical axes. 5F. P. H. W. Kopstein, Graded Bedding of the Harlech Dome, Doctor of Philosophy thesis, University of Groningen, 1954. 20 c o E E o o c O Z Q Q LU GO N O ©utw Q o Ld 'a8! Q i < o: CD O) •DTJ
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21 In addition to the directional properties of the sedimentary structures, the thickness of each planar cross-bedded unit was measured, as the average thickness of the sets for each locality might show a relationship to the direction of sediment transport. This relationship should be true if the factors controlling the formation of planar cross-bedding are as Schwarzacher1 showed in some experimental studies. His studies showed that the thickness of cross-bedding units depended on the interaction of three factors: (1) relative height of water level; (2) configuration of the bottom; (3) amount of sediment supplied. The last factor would be the most important in controlling a thickening of cross-bedded units in the source direction. The maximum pebble size was also measured at a number of localities on the assumption that the pebble size would increase in the source direction. The percentage of black chert with respect to the total pebble composition was also determined to see if the relative percentages varied systematically. In most cases, this was done by an actual pebble count, but in a few cases where the distribution was spotty and the black chert scarce, a visual estimate was made.
RESULTS The results of the directional measurements at each locality are shown for the whole area by rose diagram in Figure 5 (in back pocket). The rose diagram is based on the percentage of readings in 30-degree classes for each locality. Thus the length of each sector of the diagram represents the percentage of current azimuths measured in that 30-degree direction interval. The mean current azimuth2 has been represented by a small arrow outside the rose diagram to aid the reader in interpreting the diagram. The rose diagrams have been included to illustrate the degree of "spread" in the distribution of directional properties at each locality. From Figure 5, it is readily seen that the current direction during Lower Mississagi time was more southerly than during Upper Mississagi time. Figure 10 (in back pocket) was constructed to illustrate this difference in current direction in a more striking fashion. In this illustration, the Upper and Lower Mississagi have been presented separately with all readings for each township summed and plotted at the centre of the township. The summations of all measurements in the area for each unit have also been included in the form of rose diagrams. The difference between the mean current azimuths of the Upper and Lower Mississagi is 49 degrees.3 The current direction, or the direction of sediment transport, established by the above measurements, is further supported by an increase in the average thickness of the cross-bedded units from east-southeast to west-northwest. (See Figure 11, in back pocket.) This interpretation is based on the hypothesis pro posed by Schwarzacher,4 that the "cross-bedding units will thicken towards the source of supply of the sediment." The direction of increase in thickness cor responds more closely with the mean current azimuth of the Upper Mississagi as the majority of the planar cross-bedded units of which the thickness was measured are Upper Mississagi. A moving average was made of the thickness data by averaging the values of four neighbouring townships and plotting the mean at
*W. Schwarzacher, "Cross-Bedding and Grain Size in the Lower Cretaceous Sands of East Anglia," Geological Magazine, Vol. 90, 1953, pp. 322-30. 2For all localities at which the individual azimuth readings lie within 180 degrees of one another and do not show a strong asymmetrical distribution, the mean current azimuth is the arithmetic mean. For all other localities, the mean current azimuth was determined by graphical vector analysis. 3 Based on 274 measurements in the Lower Mississagi and 987 measurements in the Upper Mississagi. 4 W. Schwarzacher, op. cit. 22 J!
0) •o O u 2 l i O) their common corner. This form of representation smooths out many of the irregularities in the raw data and emphasizes the major trends that may be present. The paucity of data in the Bruce Mines area did not permit the extension of the moving average as far west as might be desired. Anomalous figures occur around the edge of the area due to irregular distribution of the raw data. These anomalies are called "edge effects" and should be disregarded. The data on maximum pebble size are shown in Figure 12. These data show a coarsening from east to west, which is also indicative of a westerly source. The average pebble size increases from about 16 millimetres in the eastern third of the area to 34 millimetres in the western third. The pebble beds also become more abundant towards the west. The proportion of black chert also increases in abundance towards the west, as illustrated by Figure 13. This map is based on the relative percentage of black chert to total chert and quartz pebbles in a particular horizon.
INTERPRETATION Source Direction of Mississagi Quartzite The directional measurements, thickness of planar cross-bedded units maximum pebble size, abundance of pebble horizons, and proportion of black chert, all show that the current directions during Mississagi time were from the northwest quadrant to the southeast quadrant. Hence it is safe to assume that the source of the Missassagi sediments was from the west and northwest. One of the most common arguments against the use of cross-bedding as an indication of the direction of the source area is the possibility of confusing cross- beds built by longshore currents with those formed by streams. Here one is seemingly confronted with a great dilemma of determining in just which direction the source area lay. For the area covered by Huronian sediments, however, this problem has been essentially solved. Pettijohn1 has shown that, over all the area of Huronian quartzites that have been measured in the United States and Canada, the average current direction for each is from the northwest to the southeast. These measurements are based on cross-bedding and include the Sioux, Baraboo, Waterloo, Barron, Sturgeon, Mesnard, Mississagi, and Lorrain quartzites. The data presented in this report agree with these findings. As these measurements represent about 20,000 feet of sedimentation, and the area covered was several thousand square miles, the stability in current direction is of a very high order, such as one would associate with the regional slope. Hence it is more likely that the cross-bedding was related to the regional slope than to longshore currents. The other data on thickness of cross-bedding, maximum pebble size, and proportion of black chert lend support to this conclusion.
Difference in Current Direction between Lower Mississagi and Upper Mississagi The difference in average current direction between the Lower Mississagi and Upper Mississagi is not easily explained. Although this difference is a real one in the area in which the measurements were made, it may be only of local importance, or it may be a regional feature. Because of the difficulty in finding good exposures to measure in the Lower Mississagi, and the lack of outcrop in the Quirke Lake-Ten Mile part of the area, most of the measurements were con centrated in the Lake Matinenda and Lake Lauzon area north of Blind River.
1 F. J. Pettijohn, "Paleocurrents of Lake Superior Precambrian Quartzites," Geol. Soc. Amer.. Bull., Vol. 68, 1957, p. 477. 24
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'tl
0) D O) The difference could be related to a number of factors, such as a different source direction or local shift in current direction due to pre-Huronian topography. The stability of the current direction throughout the Huronian as noted above, sug gests that these variations in current direction are more likely of local than regional importance.
Significance of Pebble Data The increase in pebble size, in the number of pebble horizons, and in the proportion of black chert towards the west all combine to give a unified picture of currents diminishing in strength from west to east. Thus these currents would leave behind the coarsest material, and the less common types, such as the black cherts, would become less plentiful. Sternberg1 observed the decline in size of pebbles in the downcurrent direc tion and noted that the decrease in pebble size from the source was proportional to the weight of the pebble in water and the distance traveled. Barrell2 noted that Sternberg's "law" may be expressed mathematically as: where Wo is the weight of the pebble at the source, W is the weight of the pebble at distance, x from the source, and a is the coefficient of size reduction. The formula may also be expressed in terms of the diameters of pebbles rather than the weights. This form is more useful from the standpoint of measurement. This function is only valid if the current in which the pebbles were transported also decreased in velocity exponentially, as would be the case in a stream with a normal gradient. Assuming that these factors did decrease exponentially, then one can compute x, the distance to the source, at least accurately enough to give an order of magnitude. Comparing the decrease in size of the pebbles in the Mississagi per unit distance with a number of other conglomerates and gravels plotted by Pettijohn3 one finds that a ranges between 0.006 and 0.010; 128 millimetres can be taken as a good estimate of yo, since chert beds do not often exceed this figure in thickness. It is also a good estimate of the maximum-size boulder a stream can carry as it leaves the mountain front, (y) can be given a maximum limit of 34 millimetres, which is the average pebble size for the western third of the area, and a minimum limit of 29 millimetres, which is the average pebble size for the entire area. With these estimates, the approximate limits of the source area for the Mississagi sedi ments can be computed by substituting in Sternberg's law as follows: MINIMUM MAXIMUM v 34 29 ___ = e —O.OIOX ___ = e —0.006X 128 128 l l reducing numerator to l.... —— = e -o.olOx —— se -o.ooex 3.76 8.83 taking negative logs...... 1.324 x = 132 miles x - 247 miles
J H. Sternberg, Untersuchungen uber langen—und querprofil gesckiebefuhrende Flusse, Z., Bauwesen, Vol. 25, 1875, pp. 483-506. 2J. Barrell, "Marine and Terrestial Conglomerates," Bull. Geol. Soc. Amer., Vol. 36, 1925, pp. 279-342. 3F. J. Pettijohn, "Sedimentary Rocks," (second edition) Harper and Brothers, New York, N.Y., 1957, p. 529. 27 Thus a good estimate of the distance to the source of the Mississagi sediments would be from 130 to 250 miles west-northwest of Thessalon. If the direction of source is taken as the average of that found for the Upper Mississagi and that for the Lower Mississagi, the source would be in the middle of what is now Lake Superior. (See Figure 14.) Origin and Sedimentary Rocks Of primary concern in the interpretation of the Mississagi sedimentation is the environment of deposition in which it was formed. Unfortunately, no fossils are present to facilitate this part of the interpretation, and no associated lithologies can be considered as diagnostic of a particular environment. The origin of the Mississagi has been variously attributed to the action of beach or river processes.1 Collins2 attributed the whole Bruce series to intermittent shallow submergence. The observations presented in this report tend to favor a fluvial origin for the sands, coupled with a shallow submergence for the argillite and siltstone. The type of cross-bedding, inclination of cross-strata, and thickness of cross- bedded units have been used by some to interpret environments. No one has systematically recorded data in recent or fossil deposits of known origin, however, which could be used to distinguish between the cross-bedding formed in aeolian, fluvial, and marine environments. Therefore, cross-bedding cannot be used as an absolute indication of environment of deposition of the Mississagi. Pettijohn,3 however, has shown that the distribution of cross-bedding azimuths in the Huronian quartzites, including the Mississagi, as expressed in rose diagrams is similar to younger deposits of known fluvial origin. The distribu tion is not similar to that found on tidal belts4'5 since in this environment cross- bedding forms in two opposing directions resulting from currents associated with ebb and flood tides. (See Figure 15.) The uniformity of the Huronian cross- bedding through time, as discussed earlier, supports a fluvial origin. The poor sorting and coarseness of the Lower Mississagi and the sporadic pebble horizons of the Upper Mississagi rule out an aeolian origin for the forma tion. The sorting of the sands in the Lower Mississagi is not as good as one would expect in a beach deposit, but is more characteristic of fluvial sands. The sorting of the Upper Mississagi is inconclusive. The pebble horizons in the Upper Missis sagi are well sorted, a characteristic especially typical of beach gravels. Most of the evidence discussed above favors a fluvial environment of deposi tion for the Missassagi sands. Reinecke6 also concluded that the Witwatersrand system was fluvial in origin. Based on this hypothesis, a sedimentary history of the Mississagi follows. The paleosol and residual deposits present at the base of the formation suggest that the pre-Huronian surface had undergone considerable weathering prior to the deposition of the Mississagi. Since the contact of the Mississagi with the basement is very straight and uniform, this surface must have been one of fairly low relief.7 With uplift to the northwest, the coarse, feldspathic, poorly sorted
1F. R. Joubin, and D. H. James, "The Algoma District," Canadian Mining Journal, Vol. 77, 1956, p. 156. 2 W. H. Collins, North Shore of Lake Huron, Geol. Surv. Can., Mem. 143, 1925, p. 60. 3F. J. Pettijohn, op. cit. 4R. Brinkmann, Gerichtete Gefuge in klastischen Sedimenten, Geologischen Rundschau, Vol. 45, 1955, No. 2, pp. 562-68. 6J. Hulsemann, Grossrippeln und Schragschichtungs-Gefuge im Nordsee-Watt und in der Molasse, Senkenbergiana, Lethata, Band 36, 1955, p. 359-88. 6L. Reinecke, Origin of Witwatersrand System, Trans. Geol. Soc. South Africa, Vol. 33, 1930, p. 111-32. 7W. H. Collins, op. cit., p. 57. 28 Insert to face page 28
A- LAFAYETTE(fluviol) (175) B- UPPER MISSISSAGI C- BRANDYWINE (fluvial) (987) (86)
O- MEERESMOLASSE(tidal) F- WEVERTON (fluvial ) (59) (136) E- LOWER MISSISSAGI (274)
A- POTTER (1955) B- THIS REPORT C- SCHLEE (1956) O- HULSEMANN (1955) /O 10 20 30 40 PERCENT E- THIS REPORT F- WHITAKER (1955) EXPLANATION
Figure 15—Comparison of Distribution of Cross-bedding Directions in Mississagi, Known Fluvial and Known Tidal. Lower Mississagi was deposited by streams not yet fully adjusted to an increased gradient on an old-age topography. The weathered material on the basement rocks was eroded in some places and covered over in others. The lack of adjustment of the streams to the surface configuration may have accounted for the divergence in cross-bedding directions noted between the Upper Mississagi and Lower Mississagi. This lack of adjustment coupled with the poorer sorting may have controlled the formation of trough-type cross-bedding present in the Lower Mississagi through greater turbulence of the streams. The pebble horizons, which make up the ore zones, could easily have formed in the swifter currents of the shifting channels of these streams. As the streams must have been aggrading in order to deposit any thickness of sediments, the streams channels would tend to shift, thus laying down gravel bodies with areal dimensions equivalent to those of the meander belt. The dimensions of these gravel bodies would be thousands of feet wide and several miles long. Thus the gravel bodies would be of a continuous nature for considerable distances, but discontinuous on a regional basis. These gravel deposits formed in the meander belt could be considered as very wide channels with a very shallow depth. Marginal to the actual river channel itself, sand would be deposited, which would also form continuous bodies as the river shifted. As the streams became more adjusted to the uplifted surface, and the initial increase in stream gradient had been compensated for by erosion in the highlands and deposition of coarse, poorly sorted materials in the lowlands, the stream flow would become less turbulent. With a decrease in turbulence, the sands would become better sorted, thus enabling ripples to form. By migration, these ripples could lay down cross-bedded layers.1 A gradual change such as this would explain the better sorting characteristic of the upper portion of the Lower Mississagi. The change from the deposition of feldspathic sands in the Lower Mississagi to the deposition of finely laminated silts and clays in the Middle Mississagi, almost certainly necessitates a shift to tranquil waters. This could be accomplished by deposition in a deep-marine environment below wave base, or a very large lake. The finely laminated argillites may be related to annual deposition, in which case the depositing medium would most likely be fresh water.2 Near the top of the Middle Mississagi the fine laminations give way first to wavy bedding, then to an irregular interbedding of silt and argillite, and finally to ripple-marked and cross-bedded silts just below the Upper Mississagi. This type of relationship is typical of the littoral zone. A return to sand deposition in the Upper Mississagi was accomplished by a prograding deltaic coastal plain advanc ing over the Middle Mississagi silts and argillites. The lower part of the Upper Mississagi is ripple-marked and shows typically abundant cross-bedding. If cross-bedding can be formed by the migration of megaripples in streams, then the pebble layers commonly found on the erosional planes between cross- bedding units might be nothing more than a gravel pavement formed on the current slope of long wave length ripple marks. Thus these pavements would protect the upstream portion of the ripple from erosion and would be preserved through burial by the migration of ripples farther up current. This is essentially the conclusion arrived at by lilies.3 This hypothesis explains the difference in sorting, in cross-bedding, and in cross-bedding direction between the Upper Mississagi and the Lower Mississagi. It also explains the change in sedimentation from Lower to Middle to Upper
1 H. lilies, Die Schragschichtung influviatilen und litoralen Sedimenten, ihre Ursachen, Messung und Auswertung, Mitt. Geol. Staatsinstitut Hamburg, Heft 19, 1949, pp. 89-109. 2W. H. Collins, op. tit., p. 61. 3 H. lilies, op. tit. 29 Limits of cross-bedding directions
109"
UPPER MISSISSAGI
Mean II9 0
Median 133.5
158' LOWER MISSISSAGI
Direction of boundaries of Quirke ore (Abraham,1956)
Mean 118
Direction of boundaries of Nordic ore (Abraham, 1956)
Directions of maximum thickness of Lower Missis-
sagi in ore zones 123" (Roscoe, 1956)
Mean 135.5
Figure 16—Comparison of directional qualities showing similarities in the trends of cross-bedding, ore zones, and isopachs.
30 Mississagi and why this change in sedimentation does not occur at the same time over the entire area. The hypothesis also accounts for the fine lamination and graded bedding of the Middle Mississagi and the abundant small-scale ripple marking in its upper portion. The sporadic pebble zones often found on the erosional planes between cross-beds in the Upper Mississagi can also be logically explained. It does not explain the presence of the odd boulder conglomerate present in the Middle Mississagi. This conglomerate is most likely a time rock unit as dis cussed earlier, as it cuts across the lithology of the Mississagi. It must be due to some event which happened in a very short period of time relative to the deposi tion of the rest of the Mississagi. The distribution at one horizon, the great changes in thickness,1 and the textured nature of the conglomerate suggest that the conglomerate is a tillite. No further evidence of this origin was noted how ever. Other hypotheses to be considered are that it represents a mud flow or a volcanic agglomerate. The conglomerate must be carefully studied and mapped before any definite conclusion as to its origin can be made. This should be done before the conglomerate is used as an horizon on which to base correlation.
ECONOMIC ASPECTS The average trend of the cross-bedding azimuths, as measured in the Blind River area, approximate the trends of the ore bearing zones as shown on the geological map of Townships 149 and ISO.2 The cross-bedding direction also paral lels isopachs drawn on the Lower Mississagi3 in the ore bearing areas. Figure 16 is a comparison of the average cross-bedding direction, the ore trends as shown by Abraham, and the isopachs drawn by Roscoe. This correlation between sedimen tary trends and ore-bearing horizons suggests that the two factors are related. Assuming that the correlation is due to something other than chance, the relationship may be explained satisfactorily in two ways: first, that the ores are syngenetic, that is, brought in and deposited as detrital minerals at the time of deposition of the conglomerates; or second, that the mineralization was intro duced at a later ime in favorable zones, which in this case are conglomerates. As other conglomerates in the section and in neighboring formations are not miner alized, the first hypothesis of a syngenetic origin seems to be the most logical. Whether the ore be syngenetic or introduced at a later time along favorable sedimentary trends, the correlation remains important. As stated in the intro duction, cross-bedding and other sedimentary structures have been used ex tensively in other uranium districts to predict unknown ore trends and to extend known orebodies. The realization that the ore zones follow sedimentary trends may also prove to be a useful tool in the Blind River area, both for mining and further development work. The recognition that a correlation does exist between sedimentary trends and ore zones may also prove to be of importance in future mining districts yet to be discovered.
1 S. M. Roscoe, Geol. Sury. Can. Topical Report No. 4, 1956. 2 E. M. Abraham, Preliminary map of townships 149 and 150, Blind River area, district of Algoma, Ontario, Ont. Dept. Mines, 1956. 3 S. M. Roscoe, op. cit.
31
Figure 5
To accompany Ontario Department of Mines Geological Circular No. 6
TARBUTT TARBUTT ADD.
EXPLANATION
Rose diagram showing percentage distribution of current azimuths in Upper Mississagi
Rose diagram showing percentage distribution of current azimuths in Lower Mississagi
Approximate contact of Mississagi quartzite
112(8) Locality and number of measurements
X Mean current direction at locality 1957 jm FIGURE 5-CURRENT DIRECTION DATA Figure 11
To accompany Ontario Department of Mines Geological Circular No. 6
. - y
EXPLANATION
12.4 Average thickness of cross-bedded units at locality l —— 21.7— Mean thickness of cross-bedded units of four adjacent townships is./. Blind River — — ———rs. Scale Approximate contact of Mississagi quartzite
4 6 6 10 12 MILES 1957
FIGURE 11—THICKNESS OF CROSS-BEDDED UNITS Figure 10
To accompany Ontario Department of Mines Geological Circular No. 6
UPPER MISSISSAGI QUARTZITE
Rose diagram showing percentage distribution of current directions based on 986 measure ments in Upper Mississagi (Lake H u r O -Arrow indicates mean current direction
Rose diagram showing percentage distribution
of current directions based on 274 measure
ments in Lower Mississagi
-Arrow indicates mean current direction
EXPLANATION
Percentage distribution of current directions in Upper Mississagi by township
Percentage distribution of current directions in Lower Mississagi by township
North Channel (Lake Huron)
FIGURE 1O-COMPARISON OF CURRENT DIRECTIONS IN UPPER AND LOWER MISSISSAGI QUARTZITE