Note on the Metamorphic Grade of the Mannville (Lower Cretaceous) Coal Deposits Near La Ronge, Central Saskatchewan J.E. Christopher 1 Christopher, J.E. (1996): Note on the metamorphic grade ot the Mannville (Lower Cretaceous) coal deposits near La Ronge, central Saskatchewan: in Summary of Investigations 1996, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 96-4. Immediately south of Lac La Ronge, in Tp 67 to 70, Rge 22W2 to 19W2, coal beds in the Lower Cretaceous Mannville Group lie subjacent to a relatively thin (6 to 24 m) Pleistocene drift. They occupy the western part of a compound, southerly plunging trough flanked by a low anticline at Rge 21 W2. The coals are near the base of the Mannville Group in the Cummings and Lloydminster formations, at the basal unconformity, where the Mannville erosionalfy overlaps from east to west the Devonian Winnipegosis and Meadow Lake formations (Figure 1 ). They are the most accessible of the many extensive Mannville coal beds to the south and west by virtue of their position at the up-dip edge of the formation and the Cenozoic erosion that has removed much of the overlying strata. Seams are 0.3 to 2 m thick, one to three in number, and form interbeds with carbonaceous shale and poorly indurated, argil- laceous quartzose sandstone. Data on these deposits are extracted from SEM Assessment Fite 731-14-0014, submitted by Brascan Resources. 1. Coal Rank From the proximate analysis (dry basis) of 42 samples, the volatile matter and carbon content of relatively clean coals lies between 39 and 42 percent, and the heating content of the coals overall, though inversely propor- tional to the ash percentage (8.9 to more than 55 percent), ranges up to 11,025 BTU/lb (British Thermal Units per pound) (Figure 2). Similar analyses (Montgomery et al., 1974; Luscar, Ltd., pers. comm., 1985) were run for coals from the Cummings Formation of the Mannville Group in the Kindersley-Lemsford district (Tp 22 to 34, Rge 22 to 28 W3) some 450 km to the southwest, where stratigraphi· cally equivalent coals lie at depths of 850 to 870 m below surface. Volatiles and carbon percentages are similar to those of the La Ronge coal basin. Likewise, the plot of the BTU/lb content versus ash content matches that of the northern deposit (Figure 2). Thus both coal deposits are of the same rank. Vitrinite reflectance in oil (RO) of the coal from the Kindersley (Gulf SHOP et al Druid 16-13-34-22W3) and Lemsford (CDR Lemsford 12-30-22-22W3) districts ranges between 0.38 and 0.40. Accordingly, both heating content and vitrinite reflectance indicate a sub- bituminous coal. (1) Geologica! Consultant, 252 ColdweU Road. Regina. SK S4R 4L2. 166 Coal rank, i.e. metamorphic grade, is generally taken to be a reflection of maximum depth of burial and geother- mal flux. Nurkowski (1984) discounts the effectiveness of the latter in the coalification process while emphasiz- ing pressure exercised by overburden and resultant dewatering of peat beds. This process requires a burial depth of 1176 m to produce a sub-bituminous C coal. The difference between this value and the thickness of extant overburden reflects the amount of strata removed by Cenozoic erosion. The La Range coal basin at an elevation of 340 m, occupies a lowland between the Thunder Hills (to the southwest) and the Wapawekka Hills (to the east) both at an elevation of 750 m above sea level. These hills, like others to the south, though veneered with glacial drift 100 to 200 m thick, are tablelands or large mesas, i.e. topographic outliers carved from Cretaceous strata that support the High Prairie. Thus by extrapolation, it is apparent that the La Range coal beds were overlain by a Cretaceous section at least 300 m thick. By projection southward across the southerly dipping Cretaceous strata to Tp 35 the missing beds above the Mannville would have been 620 to 650 m thick. At Gulf SHOP et al Druid 16-13-34-22W3, the strati- graphic section above the Cummings coal to the top of the Judith River Formation is 675 m thick, and at CDR Lemsford 12-30-22·22W3, 740 m thick. The increased overlying thickness southward between these wells is not reflected in the RO values. Southward from Lemsford one can account for an additional 450 m of missing section that includes the upper portion of the Judith River Formation and the thickened Bearpaw Formation of the Cypress Hills region (Whitaker, 1976). Thus the 1176 m depth of maximum peat burial required by Nurkowski's (1984) criterion would be met at Lemsford by simple addition of the missing section (450 m) and the extant overburden (740 m). Elevation of the pre-Quaternary land surface at the Lemsford site would have been 1020 m. well above the KB elevation of 879 m but below the 1190 m elevation on the Bearpaw Formation in the Cypress Hills. The southward rise on the Bearpaw Formation reflects Late Cenozoic uplift of the Cypress Hills region. In spite of the north- easterly decline of the present topographic surface, add- ing the 1176 m section of missing strata to the 340 m elevation of the La Ronge coalfield would create a Summary of Investigations 1996
Transcript
Note on the Metamorphic Grade of the Mannville (Lower Cretaceous) Coal Deposits Near La Ronge, Central Saskatchewan
J.E. Christopher 1
Christopher, J.E. (1996): Note on the metamorphic grade ot the Mannville (Lower Cretaceous) coal deposits near La Ronge, central Saskatchewan: in Summary of Investigations 1996, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 96-4.
Immediately south of Lac La Ronge, in Tp 67 to 70, Rge 22W2 to 19W2, coal beds in the Lower Cretaceous Mannville Group lie subjacent to a relatively thin (6 to 24 m) Pleistocene drift. They occupy the western part of a compound, southerly plunging trough flanked by a low anticline at Rge 21 W2. The coals are near the base of the Mannville Group in the Cummings and Lloydminster formations, at the basal unconformity, where the Mannville erosionalfy overlaps from east to west the Devonian Winnipegosis and Meadow Lake formations (Figure 1 ). They are the most accessible of the many extensive Mannville coal beds to the south and west by virtue of their position at the up-dip edge of the formation and the Cenozoic erosion that has removed much of the overlying strata. Seams are 0.3 to 2 m thick, one to three in number, and form interbeds with carbonaceous shale and poorly indurated, argillaceous quartzose sandstone. Data on these deposits are extracted from SEM Assessment Fite 731-14-0014, submitted by Brascan Resources.
1. Coal Rank
From the proximate analysis (dry basis) of 42 samples, the volatile matter and carbon content of relatively clean coals lies between 39 and 42 percent, and the heating content of the coals overall, though inversely proportional to the ash percentage (8.9 to more than 55 percent), ranges up to 11,025 BTU/lb (British Thermal Units per pound) (Figure 2).
Similar analyses (Montgomery et al., 1974; Luscar, Ltd., pers. comm., 1985) were run for coals from the Cummings Formation of the Mannville Group in the Kindersley-Lemsford district (Tp 22 to 34, Rge 22 to 28 W3) some 450 km to the southwest, where stratigraphi· cally equivalent coals lie at depths of 850 to 870 m below surface. Volatiles and carbon percentages are similar to those of the La Ronge coal basin. Likewise, the plot of the BTU/lb content versus ash content matches that of the northern deposit (Figure 2). Thus both coal deposits are of the same rank. Vitrinite reflectance in oil (RO) of the coal from the Kindersley (Gulf SHOP et al Druid 16-13-34-22W3) and Lemsford (CDR Lemsford 12-30-22-22W3) districts ranges between 0.38 and 0.40. Accordingly, both heating content and vitrinite reflectance indicate a subbituminous coal.
(1) Geologica! Consultant, 252 ColdweU Road. Regina. SK S4R 4L2.
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Coal rank, i.e. metamorphic grade, is generally taken to be a reflection of maximum depth of burial and geothermal flux. Nurkowski (1984) discounts the effectiveness of the latter in the coalification process while emphasizing pressure exercised by overburden and resultant dewatering of peat beds. This process requires a burial depth of 1176 m to produce a sub-bituminous C coal. The difference between this value and the thickness of extant overburden reflects the amount of strata removed by Cenozoic erosion.
The La Range coal basin at an elevation of 340 m, occupies a lowland between the Thunder Hills (to the southwest) and the Wapawekka Hills (to the east) both at an elevation of 750 m above sea level. These hills, like others to the south, though veneered with glacial drift 100 to 200 m thick, are tablelands or large mesas, i.e. topographic outliers carved from Cretaceous strata that support the High Prairie. Thus by extrapolation, it is apparent that the La Range coal beds were overlain by a Cretaceous section at least 300 m thick. By projection southward across the southerly dipping Cretaceous strata to Tp 35 the missing beds above the Mannville would have been 620 to 650 m thick.
At Gulf SHOP et al Druid 16-13-34-22W3, the stratigraphic section above the Cummings coal to the top of the Judith River Formation is 675 m thick, and at CDR Lemsford 12-30-22·22W3, 740 m thick. The increased overlying thickness southward between these wells is not reflected in the RO values. Southward from Lemsford one can account for an additional 450 m of missing section that includes the upper portion of the Judith River Formation and the thickened Bearpaw Formation of the Cypress Hills region (Whitaker, 1976). Thus the 1176 m depth of maximum peat burial required by Nurkowski's (1984) criterion would be met at Lemsford by simple addition of the missing section (450 m) and the extant overburden (740 m). Elevation of the pre-Quaternary land surface at the Lemsford site would have been 1020 m. well above the KB elevation of 879 m but below the 1190 m elevation on the Bearpaw Formation in the Cypress Hills. The southward rise on the Bearpaw Formation reflects Late Cenozoic uplift of the Cypress Hills region. In spite of the northeasterly decline of the present topographic surface, adding the 1176 m section of missing strata to the 340 m elevation of the La Ronge coalfield would create a
Summary of Investigations 1996
Quaternary land surface at an elevation of 1516 m, well above the 1368 m of the Cypress Hills. This is unlikely. The Cretaceous depocentre in Saskatchewan, from the base of the Joli Fou to the top of the Bearpaw Formation , is in the southwest on the Swift Current Platform where these strata have thickened under tectonic subsidence and thinned depositionally toward the northeast. Thus the outlying La Ronge coal deposit was never buried deeply enough for conversion to subbituminous C coal by overburden pressure.
The other factor invoked for coalification is geothermal flux. Formation water under the head provided by the Rocky Mountain system, acquires heat from the deeper Paleozoic formations of the basins and transfers them to the Mesozoic system of the Prairies enroute to
,4
Meadow Lake Fm ~Winnipe~osis Fm
~(.J l;:i
discharge points along the Phanerozoic subcrop (Hitchen, 1984). As examined by Majorowicz et al. (1986) and Osadetz et al. (1990) for the Western Canada Sedimentary Basin , heat flow density in the Paleozoic of Saskatchewan is greater than that in the Mesozoic, south of a line drawn from Kindersley to south of Yorkton. North of that line, heat f low density of the Mesozoic is greater (Figure 3). Data is sparse in central Saskatchewan, but an extrapolation of the high heat flux region depicted for central Alberta to eastern Saskatchewan would be consistent with the hydrodynamic regime of the basin. In this context, Mannville coalfields of the Kindersley district, which lie to the south of the line, are associated with a lower geothermal flux. but were buried more deeply than the La Ronge coal district; where the coals lie at a shal
lower depth, but benefitted from a higher geothermal flux.
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36 31
e oil show
Elevated paleotemperatures at the La Ronge site are also indicated by traces of galena and sphalerite in the solution breccias of the Middle Devonian Meadow Lake Beds beneath the Mannville strata. Fluid inclusion homogenization measurements made on dolomite crystals show a population of inclusions that homogenized at 100°C and another that did not homogenize even at 180°C. Both populations indicate a hydrothermal origin for the dolomite (Campeau and Kissin, 1988). Mineralization of this type occurs elsewhere along the northern belt of the underlying Paleozoic, as exemplified by lead and zinc at OMA Smoothstone 3-13-72-6W3, and copper and nickel at DMR La Loche 2-21-85-21W3 (Fuzesy, 1980). Because the underlying Cambre-Ordovician Deadwood sandstones reveal anomalous uranium and radiogenic lead, a migration of brines from deep in the basin is also postulated by Campeau and Kissin (1988).
Rgure 1 - Structure map at the sub-Mannvifle unconformity in the Lac La Ronge coal basin, central Saskatchewan. The Devonian Winnipegosis Formation underlies the Mannvil/e to the east of the hatched line; the Devonian Meadow Lake Formation to the west. Numbered section grid equals 1 square mile.
Oil shows are reported in the Brascan report in samples taken from four boreholes in the Winnipegosis and one in the glacial drift at the subcrop edge of the La Ronge trough (Figure 1). These may be vestiges of an oil migration that originated in the deeper sedimentary basin to the southwest. Their stratigraphic proximity to the lower Winnipegosis bituminous member also may indicate a local source, especially if the conclusion that the district was subjected to a high heat flux is valid.
Saskatchewan Geological Su,vey 167
10
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8 " 6
5 !:ii
4
2
+
+
&+ + . . . + • \:+
+ + . . . .+ ... . . ..
flTL'ih , I 000
€ 60p--1_0 __ ,j" ' l 6sj~~-t:I 74-~
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Figure 2 - Plot of ash percentage versus BTU/lb of Mannville coal samples: dots, Lac La Ronge coal district, Tp 68 and 69, Rge 22W2, central Saskatchewan; and crosses, Kindersley-Lemsford district, Tp 30 to 34, Rge 22 to 28W3, west-central Saskatchewan. Insets: Plots of ash content versus BTU/lb of Mann ville coal seams in representative samples of the Lac La Ronge deposit.
UNJTf:D STATES OF AMEl{l(A
Figure 3 - Contour plot of the differences between the heat flow density values (mW!m2) for the Mesozoic+Cenozoic and the Paleozoic formations in the Western Canada Sedimentary Basin (adapted from Majorowicz et al., 1986).
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2. References
Campeau, R. M. and Kissin, S.A. (1988): Pro;ect WAPA: Evaluation of a lead-zinc occurrence in Middle Devonian carbonates of northern Saskatchewan; Geosci. Can., v15, p106-108.
Fuzesy, L. M. (1980): Geology of the Deadwood (Cambrian), Meadow Lake, and Winnipegosis (Devonian) formations in West-Central Saskatchewan; Sask. Miner. Resour., Rep. 210, 64p.
Hitchon, B. (1984): Geothermal gradients, hydrodynamics and hydrocarbon occurrences, Alberta, Canada; AAPG Bull. , v68, p713-743.
Majorowicz, J.A., Jones, F.W., and Jessop, A.M. (1986): Geothermics of the Williston Basin in Canada in relation to hydrodynamics and hydrocarbon occurrences; Geophys., v51, p767-779.
Montgomery, W.J., Nandi, B.N., and Montgomery, D.S. (1974): Analysis of five coal samples from a core of the basal part of the Lower Cretaceous Blairmore Formation in west-central Saskatchewan; Energy, Mines and Resources, Mines Branch, Ottawa, Fuels Research Centre, Divisional Report FRC 74/41 RCCC.
Nurkowski, J.R. (1984): Coal quality, coal rank variation, and its relation lo reconstructed overburden, Upper Cretaceous and Tertiary plains coals, Alberta, Canada; AAPG Bull., v68, p285-295.
Osadetz, K.G., Pearson, D.B., and Stasiuk, L.D. (1990): Paleogeothermal gradients and changes in the geothermal gradient field of the Alberta Plains; in Current Research, Part D; Geol. Surv. Can., Paper 90-10, p165-178.
Whi1aker, S.H. (1976): Geology and groundwater resources of the Cypress Hills area (72F), Saskatchewan; Sask. Resear. Counc., Geol. Div., Map 32.
