Bedrock Geology Compilation Map Series: Grand Rapids NTS
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Manitoba Industry, Trade and Mines Geological Survey MARGINAL NOTES B.G.C.M. Series Grand Rapids, NTS 63G, 1:250 000 The Grand Rapids map sheet (NTS 63G) is underlain by Paleozoic carbonate formations, with the QUATERNARY exception of minor Precambrian bedrock exposures in the northeast corner. The Paleozoic strata Pervasive glacial erosion carved the Grand Rapids area into an outcrop-dominated landscape. Sediment O O represent a portion of the northeastern flank of the Williston Basin and dip gently to the southwest. Recent 100 0 O 98 0 4 000 ’ 0 ’ 0 5 000 cover is restricted to the south along The Pas Moraine and, most likely, to the area north of Lake 50 30 99 0 30 50 glaciation has modified the exposed Paleozoic bedrock. The rocks outcrop in a region known as the O O Interlake, and the Grand Rapids area is where many of Manitoba's caves and karst features are located. Winnipeg, where peatlands dominate. No information is available within the area regarding preLate 54 0 54 0 LEGEND ISOPACH MAPS STRUCTURE CONTOUR MAPS SML Wisconsinan glaciation, although multiple glacial striations and discontinuous older sediments may be SA N The map sheet is blanketed by up to 60 m of glacial till, lake sediments and glacial outwash deposits (see SA present. SA 'Surficial Geology' and 'Overburden Thickness' inset maps). The north basin of Lake Winnipeg occupies N Little Playgreen WESTERN CANADA SEDIMENTARY BASIN McMartin (2000) documented two late Wisconsinan glacial advances in the Flin Flon area, first towards SML Lake a significant portion of the sheet. Stratigraphic coreholes, mineral-exploration drillholes, Water SFB SML SEA SFB G Williston Basin Resources Branch testholes and Manitoba Hydro testholes provide data sufficient to estimate the the south-southeast (315°) and then towards the southwest (045°). The interlobate position between Talbot SML OS SFB Interlake Group Bedrock Topography Kiskittogisu R. 100 O O 100 O O regional structure of the sedimentary strata. Using these data, formation contacts were extrapolated from these ice lobes fluctuated greatly from east to west, and the first of these advances extended to an SA Paleozoic 98 98 54 O 54 O 54 O 54 O known well locations to their intersection with the buried bedrock surface (see 'Bedrock Topography'inset unknown position to the east, with little or no evidence of the advance in the Grand Rapids area. The Lake SA SML Mesozoic 260 280 second advance terminated at The Pas Moraine and deposited the uppermost till, the calcareous silt-rich SA 20 270 map). SA ORR 270 220 220 SML Lower Cretaceous 10 200 Clearwater till in this area (Nielsen and Groom, 1987). The till surface is commonly streamlined SA SFB N 280 270 Large expanses of Paleozoic bedrock are exposed, or partially covered, in the Grand Rapids Uplands 30 260 SML OS 270 due to thin or scattered surficial deposits. Frequent forest fires that have swept through this area have (McMartin, 2000). McMartin (2000) suggested that the Clearwater till may be partly derived from the Ch. Swan River Formation (0- 25m): sandstone to SEA 270 Hudson Bay Lowlands, with a greater amount of locally derived constituents in areas of high rock A shale; kaolinitic; minor lignite; presently known 30 270 280 S William River 30 280 resulted in excellent bedrock exposures. Bedrock exposures also occur as shoreline outcrops along the SML Sepastak 40 10 SEA A PLAYGREEN mainly as outliers within channel and/or karst fill 270 210 plateaus of Paleozoic age. S McLeod Island 270 260 many lakes in the area. SA Little Ch. in the Paleozoic outcrop belt 30 290 250 The Pas Moraine is the most prominent glacial feature in the area. It extends more than 300 km from SML N 40 230 210 Sub-Phanerozoic mineral-exploration drilling for Ni-Cu mineralization along the Superior Boundary Zone SA L a k e 240 L a k e 210 Clearwater Lake, in the north, to Long Point on Lake Winnipeg, in the east. The moraine is a thick wedge SEA SA 50 260 270 (SBZ) in NTS 63G has provided abundant Phanerozoic drill core for stratigraphic evaluation. Core WILLIAM SA OSM Interlake Group 40 280 290 of Clearwater till (McMartin, 2000), up to 20 km wide, that rises as much as 60 m above the surrounding Sepastak 30 logging of these drillholes revealed the existence of numerous outliers of Cretaceous age and unknown SEA W i n n i p e g W i n n i p e g 20 230 220 terrain. The proximal, northeastward-facing slope of the moraine is a gentle ramp (~3 m/km) with SA G Silurian 50 30 270 extent. Many shoreline exposures are now flooded in the Cedar Lake area due to the Grand Rapids OS 260 streamlined features, perpendicular to the moraine trend, which curve to merge with the lobe-shaped SOUTH MOOSE Flett Cedar Lake Formation: dolomite, yellow buff, Hydro Dam. SEA Island N 280 crest, whereas the distal slope is relatively steep (>20 m/km; Matile and Keller, 2002). SEA CL finely crystalline, thin bedded, stromatolitic, 260 210 The Phanerozoic rock sequence overlies the Precambrian and the SBZ. The SBZ is the juncture of the SA Russell S 50 280 SEA 270 Lake SA fragmental, micritic, biohermal, biostromal, 40 30 Superior and Churchill Precambrian cratonic blocks, and is the site of the Thompson Nickel Belt (TNB). The process that led to the formation of The Pas Moraine may have been similar to the model proposed SA SA Baker reefoid for ice streams, past and present, in Antarctica. Seismic studies on the Antarctic ice stream B showthat Langlois 260 270 220 SML Lake Islands East Arm Formation (5- 8m): dolomite, light to 270 the glacier is riding on a 6 m layer of saturated glacial debris (Alley et al., 1987). It was concluded that Trout SEA 210 BEDROCK TOPOGRAPHY Island LAKE LAKE G dark grey, finely crystalline, stromatolitic, SEA 270 most of this deformation till is derived from an up-ice source and not locally. The deformation till is LAKE 260 Bedrock topography in the Grand Rapids map area (see 'Bedrock Topography' inset map) exhibits SML fragmental, oolitic, fossiliferous; containing 40 continuously being carried toward the ice margin and, if the ice-marginal position was maintained in the SCL SFB Limestone Bay SEA SEA argillaceous/arenaceous marker beds 50 260 considerable irregularity. The main upland area consists of dolomite pavement and is bounded to the east vicinity of The Pas Moraine for some time, deformation till would accumulate at the ice margin and SCL 60 by the north-trending Silurian escarpment. The escarpment trends parallel to the east side of Provincial SA SCL eventually avalanche down the distal slope (Fig. 1). This process would have generated a stratigraphic SA Atikameg Formation (3- 7m): dolomite, buff 210 Highway 6 (see the map and cross-sectionA-A' for the location of the escarpment). SA 50 210 record similar to that of a delta, but composed of unsorted debris: what could be referred to as a till delta. OSM orange, massive, vuggy; may contain fossils 40 SEA SFB Using seismic reflection data obtained in the Lake Winnipeg study, Todd et al. (1998) identified the Lower Streak 60 Lake 250 Lake 40 Paleozoic section (Winnipeg and Red River formations) as a prominent, buried escarpment, up to 40 m OS G 30 260 SCL 20 high, within Lake Winnipeg. This escarpment represents the Precambrian-Paleozoic boundary (G. SCL SCL SA Moose Lake Formation (0- 10m): dolomite, 210 SCL Davidson SML 250 220 200 Matile, pers. comm., 2002; see cross-section B-B'). SA SFB stromatolitic, micritic 53O 53O 53O 53O O O O 210 O SA 100 98 100 98 The 'Bedrock Topography' inset map shows an irregular surface characterized by erosional lows more SEA SML Limestone than 50 m in depth. These lows may be the result of karst topography (i.e., sinkholes), channel erosion or Fisher Branch Formation (0- 15m): dolomite, Lake SFB structural faulting. Some of these deeply incised topographic lows in the Paleozoic bedrock have been Little fossiliferous SA infilled by Cretaceous outliers. These outliers occur throughout the area of the map sheet; eighteen are Two Mile Stonewall Formation Overburden Thickness Channel known to exist. At one time, the area may have been covered by Mesozoic sediments that were 0 0 O O 45 ’ SEA 45 ’ Ordovician and Lower Silurian 100 98 O 100 98 O subsequently eroded. Stonewall Formation (0- 20m): dolomite, yellow 54 O 54 O 54 O 54 O 0 0 5 Creek B’ SA SA grey, finely crystalline, fragmental, sparsely The 'Bedrock Topography' inset map reflects regional trends in the rock surface and does not necessarily OS 20 18 0 10 20 20 account for local fluctuations. In areas with Paleozoic bedrock, local fluctuations commonly occur as fossiliferous, interrupted by argillaceous zones 0 0 0 5 karst depressions, resulting in anomalous depth to bedrock values. Alternatively, in areas underlain by SCL SEA and marker beds (T- zone demarcates the 20 Ordovician/Silurian boundary zone); argillaceous 5 5 Precambrian bedrock, east and north of Lake Winnipeg, local relief on the bedrock surface can be as SA SEA 22 10 dolomite at the base (Williams Member) 0 much as 20 m and the depressions are commonly filled with Quaternary sediments (see cross-section B- SML 18 59 22 Griffin 5 10 50 B'). 000 Lake Ordovician SEA 1 0 000 50 Cypress 22 L a k e 0 L a k e Bracken Ck. Stony Mountain Formation (0- 40m): dolomite, 0 PRECAMBRIAN UNCONFORMITY 59 SCL 24 20 OSM yellow brown, nodular, colour- mottled in part, 22 5 22 W i n n i p e g 0 W i n n i p e g 26 5 The Precambrian strata are unconformably overlain by basal Paleozoic sandstone of the Middle massive 24 2220 Ordovician Winnipeg Formation.