50m
Figure 4. Local stratigraphie variations between Proudfoot Lake and Roberts Lake, Roberts Township.
Figure 5. Plane bedded pebble conglomerate, Roberts Township. Staff is 1.5m long.
conglomerates are all of local origin. While oligomictic quartz pebble conglomerate was recorded at one location, most of the conglomerates are composed of clasts of schistose metasediments, acid or felsic metavolcanics and hornblende gneiss, with local concentrations of granite (Fig. 6) . Quartz content of the conglomerates normally ranges from 2% to 50% at the base of the sequence, to as much as 80% in higher parts of the sequence. Conglomerates are rare in the upper member. Ifriere present they are typically oligomict quartz granule or small pebble conglomerate, containing abundant pyrite and having a high background radioactivity. The relative abundance of quartz may indicate the extent of reworking of the conglomerates (cf Fig. 4) .
Figure 6. Polymict cobble conglomerate, Roberts Township.
Sandstones in the basal member in Roberts Township are of very coarse to very fine grade, with medium and coarse varieties predominating in all areas except southern Roberts rlbwnship/ where fine and very fine grained laminated varieties predominate. In the northern exposures, where the sandstones are intimately associated with thick cobble and pebble conglo-merates, sedimentary structures include ripple planar and trough cross
lamination, plane bedding and massive bedding. Sandstones in the upper member are typically medium to coarse sand grade, appear massive or are characterized by trough and ripple cross-stratification or plane bedding.
Argillites are locally important in the basal member, they are typically plane bedded to laminated and have a high background radio-activity.
Hutton, Parkin, Aylmer and Rathburn Townships
South of Roberts Township the "Mississagi Formation" is poorly exposed;
the sequence in this area (Fig. 3D-F) is dominated by planar and trough cross stratified mediun and coarse feldspathic and lithic feldspathic sandstones (Long 1976) . This sequence appears to have been deposited on an irregular topographic surface, as indicated by inferred position of the contact in Norman and Tathburn townships mapped by Dressier (1982) and central Hutton Township as mapped by Meyn (1970) . Long (1976) suggests development of local relief in excess of 131m in the latter area.
The lower conglomeratic member of the formation is poorly exposed.
Scattered quartz pebbles were observed in coarse sandstones in the "paleo-valley" in central Hutton Township. Thomson (1960) indicates that most of the conglomerates encountered in the subsurface in this area are dominated by subangular pebbles of quartz and chert. Minor boulders of granite,
argillite and rhyolite are also recorded, but are not as predominant as in Roberts Township. Discontinuous quartz pebble conglomerates are exposed along the east bank of the Vermilion River in Hutton Township (Fig. 3D) and near Fletcher Lake in Parkin Township (Fig. 3E, F) . Conglomerates at these localities are predominantly massive with well rounded to subrounded
quartz pebbles. Minor chert, rhyolite and granite are present in conglomerates in the Hutton Township locality, while only quartz clasts were seen in the pyritiferous pebble and cobble conglomerates in the Parkin Township locality.
Sandstones in the basal member are of very fine to coarse sand grade and include massive, plane bedded and planar cross stratified varieties.
Argillites and argillaceous sandstones are exposed in association with
conglomerates along the Vermilion River (Fig. 3D) . As in Roberts Township these are plane laminated or massive, with high background radioactivity.
The upper member is doninated by planar and trough cross stratified sandstones, but includes 80 to 120 m of argillaceous fine and very fine sandstone in Parkin and Rathburn townships. This unit is typically plane laminated with only minor ripple cross lamination, and resanbles the Pecors Formation in the Sudbury and Elliot Lake areas. It may be correlative with a 35 m thick mudstone unit in the subsurface in Telfer Township (Fig. 3G) .
Telfer, Fraleck and Grigg Townships
The "Mississagi Formation" is exposed in a belt, parallel to the Wanapitei River which extends from central Fraleck Township to central Grigg Township, and is present in the subsurface to the east. The basal mamber is best developed in central Grigg Township (Fig. 3K) . It consists of up to 75 m of interbedded cong loner ate, sandstone and mudstone with
some high uranium values concentrated in the more argillaceous parts of the sequence. The unit thins rapidly to the south (Fig. 31, H) and is apparently absent in Fraleck Township. Less than two metres of conglomerate is recorded at the base of the sequence in Telfer Township (Fig. 3G).
Local thickness variations of the manber reflect development of paleo-valley systans in the Archean surface. In Grigg Township, on the flanks of the paleovalley system (3J), conglomérâtes include both weakly stratified, clast supported small pebble conglomerates, and structureless matrix supported conglomeratic sandstones with local inverse coarse tail grading at the base.
Clasts in these conglomerates are angular to subangular, and consist pre-dominantly of vein quartz with some rare granitic rock fragments and no greenstones. Conglomerates in the core of the paleovalley system (Fig. 3K) are coarser than those on the flanks. Maximum recorded grain size at the base of the sequence is very large pebble grade. Conglomerates exposed near the top of the sequence in the paleovalley have a more cyclic appearance
(Fig. 7) and show a marked lateral variation in grain size and thickness.
Mudrocks associated with these quartz-pebble conglomerates have high back-ground radioactivity.
The upper member is dominated by planar and trough cross stratified medium and coarse sandstones. A locally developed sequence of laminated fine sandstones and siltstones was reported in the upper member by Peters
(1969) in Fraleck Township and is present in the subsurface in Te If er Township (Fig. 3G) .
Figure 7. Intact framework conglomérâtes, Grigg Township.
Stobie Township
The basal Huronian sequence in Stobie Township rests directly on an irregular surface of Archean metavolcanics, metasediments and grano-diorite. The sequence consists of a basal member of conglomerate sandstone and argillite up to 23 m thick, overlain by a thicker sequence sandstone
(Fig. 3, M, N).
West of Stobie Lake the sequence consists of massive fine to very-coarse sandstones with minor granite conglomerate units. South of Stobie Lake the basal 13 m of the formation contains disrupt framework conglo-merates with minor subrounded to very angular cobbles and boulders of
local origin (up to 80 cm) set in a matrix of slightly muddy small pebbly very coarse sandstone. This mixtite rests dis conformably on an irregular surface of Archean quartz monazite which shows no sign of paleosoil develop-ment (Fig. 8) . Mixtites higher in the sequence are interbedded with massive medium and fine sandstones which are in turn overlain by up to 10 m of plane
laminated silty fine and very fine sandstone which grades locally into mudstone with high background radioactivity. Granules and small pebbles are present in this sequence both as distinct conglomerate beds and as a dispersed function: pyrite is present locally in concentrations of 1 to 8%.
Figure 8. Mixtite at base of section in Stobie Township. Unconformity, at base of staff is irregular.
The upper member of the "Mississagi Formation" in this area is dominated by medium and coarse grained sandstone, whose thickness is
controlled by erosion at the base of the Gcwganda Formation. The sandstones include massive, plane bedded, ripple laminated and planar and trough cross
stratified varieties, and are affected locally by Sudbury type brecciation (cf Speers 1957) involving abrasion of breccia fragments by gas streaming processes.
Turner, Seagram, Danorest and Clary
The basal member of the Mississagi Formation in this area consists of a 6 to 68 m thick package of sandstone conglomerate and argillite
(Fig. 3). Intact framework conglomerates in the area of granule to
cobble grade, are commonly polymict. Thin beds of oligomict quartz pebble conglomerate intimately associated with medium grained sandstones with high background radioactivity were found in the vicinity of "Discovery Pond"
in Turner Township. Average thickness of polymict conglomerates ranges from a few centimetres to 20 m.
Where exposed the conglomerates appear massive to weakly planar stratified. Examples in the northwest of Turner Township fined upwards from cobbly very large pebble conglomerate at the base, to large pebble conglomerate at the bop in one instance and small pebble conglomerate in another. Both conglomerates were overlain by massive and plane bedded medium, coarse and very coarse sandstone, and pebbly granule conglomerate, and had a matrix and medium to coarse sand to granule grade, with abundant pyrite. Clasts in these conglomerates are predominantly rounded to sub-rounded and consist of up to 40% quartz, 10-15% locally derived meta-sandstone and 45-50% felsic metavolcanics.
Mixtite was recorded in the subsurface, and at one locality within two metres of the base of the sequence in Turner Township.
Most sandstones in the basal member are massive-plane bedding, ripple lamination and trough cross stratification is present locally. Thin beds of mudstone are found interbedded with very fine and very coarse sandstone in outcrops of the basal member. These units are ccmnonly lenticular; one example contained dessication cracks. Thicker mudstone units were recorded in the subsurface.
Janes, fteNish Pardo and Clement Townships
Rocks of the "Mississagi Formation" crop out in a broad belt extending from the Grenville Front, in Janes Township to Clement Township. The basal Huronian sequence in this area consists of a conglomeratic sequence 40 to 80 m thick, overlain by about 270 m of sandstone (Fig. 3 U to X) .
The basal member of the formation is best exposed in Pardo and Clement Townships where it has a relatively uniform stratigraphy with two major conglomeratic horizons separated by 10 to 40 m of massive and trough cross-stratified medium grained sandstones. At the outlet of Tee Lake, in Pardo Township the lower conglomerate consists of 9 to 20 m of massive to weakly stratified cobbly large and very large pebble conglomerates, with interbeds of pebbly medium to coarse sandstone. Clasts in the conglomerate are predominantly subangular to moderately well rounded metasiltstone with lesser amounts of metavolcanic material. Clasts include only 10 to 15%
quartz résistâtes including vein quartz, and white and black chert. Grano-diorite pebbles are present above the 4 m level in trace amounts. The
lower conglomerates at this location are separated from an upper conglomerate unit by 24 m of lithic-arkosic sandstone. The upper conglomerate consists of 15 bo 20 m of large pebble, very large pebble and small cobble conglomerate, containing clasts up to 33 cms. The conglomerate is predominantly clasts
supported, though disrupt framework (matrix supported) conglomerates are present locally. Clasts in the conglomerate are typically rounded to sub-rounded, and include meta-argillite, metavolcanics, metaplutonics and minor granodiorite, garnetiferous schist, black chert and siliceous iron formation.
Vein quartz accounts for only one or two percent of the clasts present.
The upper member of the formation in this area consists predominantly of thick bedded planar and trough cross-stratified medium sandstones, with minor quartz pebble conglomerates.
Vogt Township
An isolated patch of the Mississagi Formation is preserved beneath the Gowganda Formation in south-central Vogt Ttwnship. In this area it consists of up to 36 m of medium to thickly bedded massive and planar cross stratified medium and coarse arkosic sandstone, with minor well sorted medium and large pebble quartz resistate conglomerates, and pebble cobble and boulder grade polymict conglomerates. Polymict conglomerates are confined to the base of sequence, and are thickest (up to 7 m) in the base of depressions interpreted as paleovalleys (Fig. 3 Y,Z ). Clasts in these basal conglomerates are typically angular to subangular and consist predominantly of locally derived iron formation, siliceous metavolcanics and metagreywacke. "Quartzite"
and vein quartz typically constitute less than 50% of the gravel fraction.
Resistate pebble conglomerate in the overlying sandstone sequence are medium to thickly bedded (20-55 cms) have low lateral continuity, and are usually devoid of internal structure. Planar bedding is visible in some units. Clasts in the conglomerates are predominantly subangular to rounded and consist of strain-recrystallized vein quartz ( > 50%) with lesser amounts of siliceous and sericitic metasediments and metavolcanics.
The matrix of these conglomerates is a moderately poorly sorted pyritic arkose in which all feldspar grains have been altered to sericite. Heavy minerals such as zircon and monazite are cormon.
SEDIMENTTOIOGY
Most of the "Mississagi Formation" in southern Cobalt Plain is of braided fluvial origin. Lake and pond deposits are recognized in several localities, in places forming sequences tens of metres thick. Long (1976, 1978) suggested that most of the Mississagi Formation was deposited in Platt type braided rivers (cf Miall 1978) with high width to depth ratios and highly unstable banks. More recent observations (Long, in press) indicate that while sandy Platte type river deposits predominate, especially in the upper manber, more proximal (conglomeratic) Platte, Donjeck and Scott type deposits (cf Miall 1978) are present in the lower member (Long 1981 and in press).
Conglomerates and sandstones in the basal menber in Roberts Township interpreted as products of deposition in braided streams with characteristics of both Scott, and proximal Donjeck type rivers. Their irregular thickness and apparent low lateral continuity suggests that these conglomerates accumulated on an irregular topographic surface in both valley fill and alluvial fan settings.
Thin mudstone units associated with the conglomerates are interpreted as deposits of small flcodplain lakes or cut-off channels. Thicker sequences of fines may represent more extensive lakes. The upper member if interpreted as the product of deposition in proximal Platte type braided streams. Sane of the more massive and plane laminated sands near the base of the menber could represent distal sheet flood deposits of alluvial fans (cf Hooke 1967) .
Laterally discontinuous conglomerates and associated strata in the
basal member of the "Mississagi Formation" south of Roberts Township represent products of deposition fron Donjeck or proximal Platte type braided streams
locally in valley fill setting. Alluvial deposits are not important suggesting a more subdued topographic setting than in Roberts Township. Thicker argillaceous units may represent deposits of small floodplain lakes or ponds. The thick
package of argillaceous sandstones in the upper member is interpreted as the product of deposition in a more extensive (?) fresh water lake.
Distribution of conglomerates in the basal member in Grigg Township suggests that they are of valley fill origin. Conglomerates exposed on the present western margin of the paleovalley represent Scott type braided streams, with pebbly sandstones representing debris flow deposits. Conglomerates exposed
in the core of the paleovalley system represent a variant of the Donjeck type braided stream deposit with associated laminated silty sandstones and
mudstones representing pond deposits, which formed on the margins of the channel systems, or in channel cut-offs. Decrease in abundance and grade of conglomerates to the south, in Telfer and Fraleck Townships indicates a decrease in topographic relief in this direction.
Much of the basal member in Stobie Township formed in a valley-fill setting, with pronounced local relief. Mixtites which occur locally at the base of the sequence are interpreted as local debris flow deposits,
with the overlying laminated sequence, with scattered pebble horizons representing sheetflood deposits (cf Hook 1967) in a Bijou Creek (cf Miall 1978) type braided stream environment. Overlying strata are interpreted as the products of deposition in proximal Platte type braided streams.
Thick polymict conglomerates in the basal member of the "Mississagi Formation" in Turner, Seagram Dgnorest and Clery townships, may represent channel deposits of streams with characteristics intermediate in type between the Scott and Donjeck models of Miall (1978) . Conglomerates in the basal member contain clasts predominantly of local origin, are laterally discontinuous, and are not developed uniformally throughout the area. The irregular thickness of, and distribution of conglomerates within, the basal member may reflect deposition in valley fill environments. Mixtite in the basal member in Turner Township may represent debris flow events related to floods within the valleys, rather than floods confined to specific fan systems. This is reflected by absence of locally derived iron formation pebbles in this occurrence. Massive sandstones in the basal member may represent sheet flood deposits which accumulated in an overbank-floodplain environment. Intermittent exposure is indicated by the presence of dessication cracks in one example. As in other areas, the upper member is dominated by Platte type river deposits.
Conglomerates in the basal member in Janes, McNish Pardo and Clement townships form a relatively uniform sheet, 40 to 80 metres thick: there
is no evidence of pronounced topography at the base of the sequence, precluding a narrow valley fill origin. The conglomerates are interpreted as deposits of proximal Scott type braided streams with matrix supported conglomérâtes such as those at the base of the upper conglomerate representing in-channel debris flows. Most of the framework supported conglomerates are lenticular and represent channel filling by longitudinal bars. Thin sandstones may
represent bar-edge sand wedges. Sandstones separating the two main conglomerate zones in the basal member represent deposits of proximal Platte type rivers.
Thin conglomerate layers in the upper member may represent lag deposits, or minor longitudinal bars.
Distribution of basal conglomerates in Vogt Township indicates that they are of valley fill origin. They represent the deposits of proximal Scott type braided rivers. The overlying sandstones, and associated resistate conglomerates can be interpreted in terms of proximal, Platte type braided streams, with conglomerates accunulating as in-channel longitudinal gravel bars.
The dominance of traction current features in fluvial strata of the
"Jlississagi Formation" indicates deposition in a hunid climatic regime.
Vector analysis of crossbedding indicates a major swing in local paleo-slopes between the basal conglomeratic member and the overlying sandstone members. In places this reflects swamping of the original topographic
features by a more extensive sheet of Platte type braided stream deposits.
In other places this may also reflect local syndepositional tectonic activity.
A moving average map of inferred paleccurrents (Fig. 9) indicates that the preserved remnants of the "Mississagi Formation" may represent the products
>
— tyy.-FWFTvT-.-.1*.- .v.T.-T'TWr... .
Figure 9. Moving average paleocurrent map of Mississagi Formation in Southern Cobalt Plain, indicating possible drainage divides. Nunbers refer to uraniun showings referred to in text.
of accumulation in more than one distinct drainage basin. A possible drainage divide is inferred between Pardo and Vogt Townships; between Clement and Clary townships, and at least locally between Grigg and Roberts Township.
URANIUM SHOWINGS
Major uranium showings in the southern Cobalt Embayment are confined to the basal member of the "Mississagi Formation". They are not confined to pyritic quartz pebble conglomerates as are the major deposits in the Blind River - Elliot Lake area, but are more frequently associated with argillaceous sandstones and mudstones. Details of individual showings have been discussed, in a regional context by Thomson (1960) , Meyn (1979), Meyn and Matthews (1980 and in press) , and in a local context by Grant (1964) , Meyn (1970, 1971, 1972) and Dressier (1979). Pétrographie details of the
uranium mineralization are included in the work by Meyn and Matthews (in press): names for showings are taken from this report where applicable.
Roberts and Creelman Townships
Uranium mineralization occurs in three main areas within Roberts and Creelman Townships. These are: 1, the Roberts Lake area; 2, the
Central Roberts area; and 3, the Leslie showing (Fig. 9, locations 1, 2, 3) .
In the Roberts Lake area (Fig. 9, locality 1) polymict conglonerates at the base of the sequence in exposures adjacent to the Vermilion River have high background radioactivity. Upon analysis (Long, in press) those
In the Roberts Lake area (Fig. 9, locality 1) polymict conglonerates at the base of the sequence in exposures adjacent to the Vermilion River have high background radioactivity. Upon analysis (Long, in press) those