Tectonic Framework of Palaeozoic Formations in Manitoba

H. R. McCabe Tectonic Geocgist, Framework of MonitGba Dept. of Mhies and Natural Resources, Winnipeg, Man. Palaeozoic Formations in Manitoba Annual Western Meeting, CIM, Calgary, October, 1966 Transactions, Volume LXX, 1967, pp. 180-189 ABSTRACT The study of a detailed series of structure and isopach maps of the Palaeozoic strata of southwestern Manitoba T O indicates that the principal Palaeozoic tectonic elements I % — the Williston Basin mid Elk Point Basin — have been modified to a considerable extent by major tectonic fea tures of the Precambrian basement. The principal basement feature is the break between the Churchill and Supe nor provinces, which coincides roughly with the Manitoba- Saskatchewan border. East-west-trending orogenic belts within the Superior crustal block may also be reflected to some extent in later Palaeozoic movements. In addition, the Superior crustal block, as a whole, appears to have had a slightly different response to the tectonic forces, giving rise to both basin subsidence and basin uplift. During periods of deposition, especially during Ordovician time, the Manitoba portion of the basin appears to have undergone a relatively higher rate of subsidence, whereas during periods of erosbrn relatively greater amounts of uplift and truncation have occurred. As a result, the depo sitional and erosional patterns for most of the Palaeozoic formations in southwestern Manitoba are somewhat ano malous relative to the regional basin framework. Some prominent Palaeozoic structures probably are the Although the over-all structural pattern appears to result of salt solution and differential compaction caused be rather uniform, an examination of the isopach directly or indirectly by relatively minor tectonic move- maps for the various Palaeozoic formations shows ments related to Precambrian basement features. In par- ticular, the prominent hinge line at the edge of the De.vo that there are a considerable number of anomalous nian Prairie Evaporite salt basin may be due, in large features throughout the stratigraphic succession, and part, to salt solution. Numerous other small structural that the Manitoba portion of both the Williston and lows, structural highs and isopach thicks, probably re Elk Point basins appears to have formed an anomalous suiting from salt solution, occur along the salt edge and have been important in modifying and controlling Missis area within the regional basin framework. The writ- sippian oil accumulation in the Daly-Virden producing er believes that the tectonic framework has been di- area rectly affected by major structural features of the Precambrian basement (Figure 2) . The most promi nent basement feature and the one that appears to ‘ have had the greatest effect on the Palaeozoic tec tonic framework is the discontinuity or boundary Introduction zone between the Churchill and Superior Precambrian provinces. This zone is fairly well defined in the HIS study is based on the examination of a de Shield area, and its southwestward extension beneath I tailed series of isopach and structure contour the Palaeozoic cover is indicated by the associated maps* for all Palaeozoic formations of southwestern high and low gravity anomalies, as shown in Figure Manitoba, as well as all available geophysical data. 2. A prominent magnetic low is also seen to occur Selected references to the regional geologic setting are listed along this break in the southern part of the map-area. at the end. Southwestern Manitoba is located The Manitoba portion of the Williston and Elk Point on the northeastern edge of both the Williston and basins is thus underlain principally by rocks of the Elk Point Palaeozoic sedimentary basins (Figure 1). Precambrian Superior block, whereas Saskatchewan The regional structure contours on the Precamhan is underlain principaily by rocks of the Churchill basement reflect not only the composite effect of these block. two Palaeozoic tectonic elements but also later Meso zoic and Cenozoic subsidence. For Manitoba, As the Churchill and Superior blocks are known to only considerably about 20 per cent of the present structure is due to differ in age, lithologic character, tecto nic grain and even in average crustal thickness, it is Palaeozoic tectonic activity ; this makes direct inter- pretation of the tectonic framework difficult. possible that these two major Precambrian blocks may have had a somewhat different tectonic response to *Most of these maps are included in the Stratigraphic the forces giving rise to both basin subsidence and ba Map Series, issued by the Mines Branch. sin uplift. —1— A further and very important factor to consider in ferent, with the result that the utcrop-subcrop belts present structure con- ‘ evaluating the tectonic framework is the existence of are seen to be discordant to the prominent superficial or sedimentary structures which tours on the Palaeozoic erosion surface, and show a re not formed as a result of basement deformation, pronounced structural rise to th northwest. The stra but rather are the result of either salt collapse or tigraphic succession and formalion names are noted differential compaction. Differentiation between su in the legend. The structure an isopach patterns for perficial structures and true tectonic structures pre each of these units will be discassed briefly. sents a major problem in the map-area. Most of the Palaeozoic formations of southwestern Manitoba are para-time-stratigraphic units delimited Precambrian by marker horizons, and conseçuently the isopach of such units reflects the relative amount of tectonic The structure map on the Precambrian surface subsidence and the structure contours reflect true (Figure ) shows a relatively uniform dip to the structural deformation. southwest, ranging from 17 to 50 feet per mile. Al- though most of this reflects post-Palaeozoic deforma Ordoviciai tion, any Palaeozoic tectonic features should neverthe . Ordovician strata comprise the first record of Pala less show on the structure map, but the relatively high eozoic sedimentation in Manitoha except for a small superimposed dip coupled with the sparse deep well the Prov it difficult to delineate any but the most area in the extreme southwestern corner of control make present. The To- prominent structures. The principal feature shown by ince where Cambrian strata may be tcLl Ordovician isopach (Figure shows a relatively this map is the pronounced high in the vicinity of 4) Lake Winnipeg narrows, uniform and rather rapid thi±ening to the south, Lake St. Martin, west of the thickness variations. This mdi- granitic and volcanic rocks are exposed at sur with surprisingly few where very smooth, flat Precambrian Available data indicate that this feature is a cates deposition on a face. surface, with differential subidence apparently hay- topographic high on the erosion surface with a local that there has been ing occurred along an east-trending axis. The Willis- relief in excess of 700 feet, and the principal tectonic element associated post-Precambrian tectonic de ton Basin, however, was little or no and the pattern of subsidence in south- formation. The volcanic rocks, however, are very fresh at this time, the typical green- western Manitoba is markedly discordant to the re looking and not at all similar to isopach pattern province, which suggests that gional Williston Basin trends. The stones of the Superior Manitoba appears to result from a may possibly represent late Precambrian or even of southwestern they greater amount of subsidence in the Manito younger activity. Two radioactive age determinations relatively portion of the basin. The area of thickening is de these volcanics, by the K-Ar whole-rock method, ba for limited approximately by the Churchill-Superior boun gave an age of only 200-250 million years. However, the and vesicular nature of the dary zone, and the isopach trends are similar to because of the crystalline the belts within the Precambrian these results may be totally unreliable trend of orogenic material used, Superior province. and need to be checked. The associated granitic rocks gave a normal “Superior” age of approximately 2.4 Although the Total Ordovicisn isopach is uniform, billion years. Assuming that the basement high is of the isopachs of the constituent anits are not, and most Precambrian age, it must have remained emergent if not all of the isopach anona1ies shown by these throughout Ordovician time, and all of the Ordovician units can be attributed to differential compaction of strata must pinch out against the high. the basal Ordovician Winnipeg formation (Figure 5), which shows rapid lateral lithologic changes from sand Another prominent feature on the Precambrian map to shale. The thickest and most persistent sandstone is the northeast-trending Moose Lake syncline or flex- the north end of unit within the Winnipeg is the Carman Sand, which ure located immediately northwest of Win- This pronounced flexure also shows up occurs as an east-trending bar-dike body south of Lake Winnipeg. compaction around this area of prominent bend in the outcrop belt of Ordovician nipeg. Differential as a sand content has given rise to the isopach thick Silurian strata, as shown in Figure 3. Because of high and area as well as the stractural high associated extensive post-Palaeozoic erosion in this area, the age in this that it with the isopach thick. It is ‘possible, however, that of this feature cannot be determined except flexure the youngest strata in the area and con- slight tectonic movement, as evidenced by the has affected the Precambrian structure contours, may have sequently in post-Middle Silurian. The synclinal flex- of helped to localize deposition of the Carman sand body. to be coincident with the boundary zone ure is seen The easterly trend of the Carman Sand is seen to pa- between the Churchill and Superior Precambrian prov the trace of the gravity anomalies rallel the trend of the Precambrian orogenic belts, al inces and with magnetic data are available to determine this boundary zone extends beneath the Palaeo though no where if the sand body is coincident with a Precambrian zoic cover.

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