Paleokarst in the Lower Ordovician Beekmantown Group, Ottawa Embayment: Structural control inboard of the Appalachian orogen George R. Dix* Ottawa-Carleton Geoscience Centre and Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada George W. Robinson Seaman Mineral Museum, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931-1295 D. Colin McGregor Eastern Paleontology Section, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1N 0E8, Canada ABSTRACT and porosity development well inboard of the Mountjoy, 1994; Palmer, 1995). In eastern On- Appalachian orogen. tario and southwestern Québec, mineralized Two paleokarsts, different in age, character, (sulfide, sulfate, carbonate, quartz, minor hydro- and origin, occur in dolostones of the Lower INTRODUCTION carbon) and nonmineralized paleokarsts occur Ordovician Beekmantown Group of eastern within dolostones of the Early to Middle Or- Ontario, well within the interior of the Lau- Types and patterns of porosity in carbonate dovician Beekmantown Group. This region rentian paleoplatform. Surficial to shallow platforms help to define the history of platform forms part of the Ottawa Embayment, an interior subsurface (<2 m) epikarst formed during the aggradation, and constrain interpretation of part of the Laurentian paleoplatform, positioned latter stage (late Arenig to Llanvirn) of Sauk paleogeographic distribution of potential petro- well inboard of the structural front, or Logan’s platform development in this region, resulting leum and mineralization occurrences (Qing and Line, of the Appalachian orogen (Fig. 1). The as- from local change to base level and patterns of meteoric circulation likely initiated by reacti- vation of shallowly buried Precambrian struc- tures along a cratonic fault system, now de- fined by the Ottawa-Bonnechere Graben. Faulting was contemporaneous with initiation of the crustal forebulge within the distal, de- veloping Taconic orogen. Mineralized, in- trastratal, vuggy to local cavernous porosity composes a prominent second, more regional paleokarst. Dissolution followed burial chemi- cal compaction, but predated a history of fur- ther burial and tectonism defined by stages of geopetal cavity-fill sedimentation, cavity-fill mineralization, calcite veining, and renewed stylolitization. Regional paleokarst is inter- preted to have arisen from changes in pCO2 and H2S concentrations arising from mixing of continentally derived pore waters with brines derived from dissolution of Beekmantown evaporites. Compared to the region’s geologic history, a pre–late Paleozoic age for formation of the regional paleokarst and mineralization is likely. The two platform-interior paleokarsts demonstrate unexpected links between tecton- ism, changes in paleohydrological patterns, Figure 1. Location of the Ottawa Embayment and Beauharnois arch relative to the structural front (Logan’s Line) of the Appalachian orogen. Other patterned areas denote distribution of *E-mail: [email protected]. Precambrian rocks. Provincial boundaries of Québec (PQ) and Ontario (ON) are indicated. GSA Bulletin; August 1998; v. 110; no. 8; p. 1046–1059; 10 figures; 2 tables. 1046 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/8/1046/3382955/i0016-7606-110-8-1046.pdf by guest on 29 September 2021 ORDOVICIAN PALEOKARST, OTTAWA EMBAYMENT sociation of paleokarst, calcite veining, and re- gional metallogenic (including hydrocarbon) patterns within lower Paleozoic carbonate strata is well known along the length of the Appala- TABLE 1. LOCATIONS REFERRED TO IN THE TEXT Location* NTS map UTM coordinates† References chian orogen (e.g., Hoagland, 1976; Schrijver sheet Easting Northing et al., 1988; Kesler and van der Pluijm, 1990; 1 Kennedy’s Quarry (i) 31G 422800 5031500 Williams (1991) Knight et al., 1991; Gauthier et al., 1994; 2 Dunrobin (o/c) 31G 418000 5030000 This study Montañez, 1994; Dykstra and Longman, 1995; 3 GSC Lebreton (c) 31G 444900 5027500 Williams (1991) 4 Royel Quarry (i) 31F 410600 5001600 Forgrave (1995) Kesler et al., 1995; Paradis and Lavoie, 1996). 5 GSC Observatory Crescent (c) 31G 440470 5026700 Williams (1991) How mineralization in the platform-interior 6 Cumberland (o/c) 31G 463900 5040000 Williams (1991) strata in eastern Ontario is related in time and 7 Grant Quarry (a) 31G 459500 5014500 Derry et al. (1989) 8 Beaver Road Builders Quarry (a) 31G 456000 5016800 Williams (1991) space to orogen metallogenesis has received lit- 9 GSC Russell (c) 31G 469400 5017600 Bernstein (1991) tle attention (see Williams, 1991). 10 Maple Grove Quarry (a) 31G 452000 4986200 Derry et al. (1989) 11 Highways 16 and 44 intersection (o/c) 31B 451900 4981200 This study We first discuss the origin of the two paleo- 12 Groveton (o/c) 31B 455000 4970800 This study karsts within Beekmantown Group dolostones in 13 Williamsburg (c) 31G 474200 4984800 Williams (1991) eastern Ontario and southwestern Québec to bet- 15 McCrimmon’s Corners (c) 31G 520100 5029900 Bernstein (1991) 14 Imperial Oil Laggan #1 (c) 31G 521200 5026300 Bernstein (1991) ter define the origin and distribution of available 16 Carillon Dam (o/c) 31G 548000 5045600 Bernstein (1992) porosity at the time of mineralization. Develop- 17 St. Justine Quarry (A) 31G 544000 5026300 Avramtchev (1994) ment of porosity is evaluated relative to discor- 18 St. Eustache Quarry (A) 31H 588200 5048500 Avramtchev (1994) 19 St. Chlotilde Quarry (A) 31H 608200 5002800 Bernstein (1991) dant relationships with cavity-fill sediment, stylo- Notes: NTS—National Topographic System; UTM—Universal Transverse Mercator. lites, mineralization, faults, and calcite veins. *Inactive (i) and active (a) quarry; outcrop (o/c); core (c). Placed in context of the early Paleozoic tectono- †Measured at scale of 1:250 000. stratigraphic framework of eastern North Amer- ica, separate stages of shallow and deep-burial dissolution are interpreted to reflect reorganiza- tion of paleohydrology linked to, but well inboard of, early to middle Paleozoic orogenic activity. Residue was placed on coverslips in cellosize and Taconic foredeep; foundering of the platform is affixed to glass slides with Elvacite. defined by deposition of synorogenic mudrock METHODOLOGY (Fig. 3) derived from the Taconic highlands GEOLOGICAL SETTING (Williams, 1991; Sanford, 1993b). Data were collected over the period 1994 to The Sauk-Tippecanoe sequence boundary in 1996 from active and abandoned quarries, drill The Ottawa Embayment is an intracratonic ex- the Ottawa Embayment is Middle Ordovician in cores, and outcrops from 19 localities in eastern tension of the central St. Lawrence Platform age (Bernstein, 1992), slightly younger than its Ontario and western Québec (Table 1; Fig. 2). In (Fig. 1; Sanford, 1993a). Although only Cam- late Early Ordovician equivalent along more addition to field observations and examination of brian–Ordovician strata are preserved today, seaward positions of the paleoplatform (Knight polished slab samples, powder X-ray diffractom- Middle Devonian rock fragments in Cretaceous et al., 1991). Recent work in eastern Ontario and etry (XRD) and scanning electron microscopy diatremes in the Montreal region (Fig. 2; southern Québec suggests that erosion ranged (SEM) were used to examine mineralogy and tex- Sanford, 1993b) suggest that substantial erosion from minor to diastemic (Bernstein, 1991; Dix tural characteristics of cavity-fill sediment and may have occurred; estimates of missing post- and Molgat, 1997). paleokarst wall rock. Oriented XRD sections of Ordovician strata range from ~2 to 7 km (Héroux The regional structure of Paleozoic strata in the mud fraction of unlithified cavity-fill sediment and Tassé, 1990; Héroux and Bertrand, 1991; eastern Ontario defines a mosaic of fault blocks at location 7, and rock powder of other mudrocks, Sanford, 1993b). (Fig. 2), part of a relatively narrow (100 km) cra- were obtained by centrifuging mud-water solu- Two carbonate platform successions are pre- tonic fault system manifest today by the Ottawa- tions for 10 s at 10 000 revolutions per minute to served in the embayment. The Early to Middle Bonnechere Graben (Kay, 1942). A seismic re- deposit the >2 mm size fraction (see Laver, 1981). Ordovician Beauharnois and Carillon Forma- fraction profile across the Ottawa-Bonnechere Aliquots of the remaining solution were placed on tions (Bernstein, 1992) of the Beekmantown Graben northwest of Ottawa (Mereu et al., 1986) glass slides to air dry. An XRD scan rate of 1° 2- Group (Fig. 3) contain heterogeneous peritidal lends support to the interpretation that many Ot- θ/min was used over the range 4° to 60° 2-θ. Cav- facies assemblages of siliciclastic, carbonate, and tawa-Bonnechere Graben faults are surface ex- ity-fill sediment and mudrock from locations 7, evaporite lithologies. The formations contain pressions of reactivated, older structures associ- 10, and 19 were processed for potential fossil pa- platform-interior carbonate facies of the Sauk ated with a cratonic fault system of Precambrian lynomorphs. Bulk samples were immersed in passive-margin succession (Sloss, 1963), which age (see Kumarapeli, 1985). Initial work (e.g., HCl (50% by vol) and heated for 1 hr at 65 °C. extends along the eastern margin of the North Kay, 1942) emphasized a Mesozoic age for The residue was transferred to concentrated HF American craton (James et al., 1989; Read, 1989; graben development; an interpretation influ- for two days, then placed