Ottawa Embayment Research Articles

Sequence stratigraphy of a Middle to Upper Ordovician foreland succession (Ottawa Embayment, central Canada): Evidence for tectonic control on sequence architecture along southern Laurentia

AbstractThe Middle to Upper Ordovician foreland succession of the Ottawa Embayment in central Canada is divided into nine transgressive‐regressive sequences that defines net deepening of a platform succession over ~15 m.y. from peritidal to outer ramp settings, then a return to peritidal conditions over ~3 m.y. related to basin filling by orogen‐derived siliciclastics. With a backdrop of net eustatic rise through the Middle to Late Ordovician, there are several different expressions of structural influence on sequence development in the embayment. During the Middle Ordovician (Darriwilian), foreland‐basin initiation was marked by regional onlap with abundant synsedimentary deformation across a faulted trailing‐margin platform interior; subsequent craton‐interior uplift resulted in voluminous influx of siliciclastics contemporary with local structurally influenced local channelization; then, a formation of a platform‐interior shale basin defines continued intrabasin tectonism. During the Late Ordovician (Sandbian, early Katian), structural influence was superimposed on sea‐level rise as indicated by renewed local development of a platform‐interior shale basin; differential subsidence and thickness variation of platform carbonate successions; abrupt deepening across shallow‐water shoal facies; and, micrograben development coincident with foreland‐platform drowning. These stratigraphic patterns are far‐field expressions of distal orogen development amplified in the platform interior through basement reactivation along an inherited buried Precambrian fault system. Comparison of Upper Ordovician (Sandbian‐lower Katian) sequence stratigraphy in the Ottawa Embayment with eustatic frameworks defined for the Appalachian Basin reveals greater regional variation associated with Sandbian sequences compared to regional commonality in base level through the early Katian.

δ13C stratigraphy of a Turinian-Chatfieldian (Upper Ordovician) foreland succession, Ottawa Embayment (central Canada): resolving local and inter-regional isotope excursions in a tectonically active basin

Positive δ13Ccarb excursions are correlated through an upper Turinian to lower Chatfieldian carbonate-platform succession along the axis of the Ottawa Embayment and into outliers of the northern Ottawa-Bonnechere graben in central Canada. Successive Turinian excursions (E1 and E2) are lithostratigraphically constrained by erosional surfaces and hosted within the Watertown and overlying L'Orignal formations, respectively, the latter coeval with the Selby Formation in the adjacent northern Appalachian Basin. The excursions coincide with periods of regional transgression, but geographic patterns of 13C depletion versus enrichment coincide with structurally defined areas of stratigraphically condensed and preferentially thickened formation successions, respectively. Differential subsidence is interpreted to have created bathymetric variation resulting in intrabasinal restriction of seawater exchange between these areas, with preferential Corg recycling with stratigraphic condensation. By early Chatfieldian time, segmentation of the once regional carbonate platform (L'Orignal Formation) produced a regional mosaic of low-energy muddy carbonate banks (Rockland Formation) and a deeper water platform (lower Hull Formation) settings subject to fluctuating high to low energy current flow. Excursion E3 occurs in both successions, but 13C enrichment is associated only with the bank-top muddy facies. This may identify preferential photosynthetic drawdown of 12C across the bank tops due to limited seawater exchange across the bank-deeper platform boundaries. Excursions E1 to E3, and a younger excursion (E4) in the Hull Formation, are correlated with varying confidence with excursions across southern Laurentia, excursion E3 being the local expression of the Guttenberg δ13C excursion. Our study supports local modulation of regional, if not global, δ13C excursions arising from structurally controlled changes in oceanography and productivity.

Lithostratigraphy of the upper Turinian – lower Chatfieldian (Upper Ordovician) foreland succession, and a U–Pb ID–TIMS date for the Millbrig volcanic ash bed in the Ottawa Embayment

Three stages of carbonate-platform development are preserved in the upper Turinian – lower Chatfieldian succession of the Ottawa Group in the Ottawa Embayment and represent deposition along the Late Ordovician Taconic foreland interior of paleo-southern Laurentia. Compared with contemporary stratigraphy in the adjacent northern Appalachian (southern Ontario, New York state) and western Quebec basins, the intermediate Stage 2 succession, which brackets the Turinian–Chatfieldian boundary, preserves embayment-specific stratigraphic patterns. These include: (i) dramatic west-to-east thickening of the upper Turinian Watertown Formation that defines differential subsidence along the present axis of the embayment, (ii) post-Watertown base-level fall defined by appearance of shoreface siliciclastics, (iii) early Chatfieldian marine transgression represented by the proposed L’Orignal Formation that is coeval with but lithologically distinct from the Selby Formation in the northern Appalachian Basin, and (iv) platform segmentation that resulted in a depositional mosaic of shallow banks (Rockland Formation) and equivalent deeper water mico-seaways (lower Hull Formation). The latter event immediately follows accumulation of the Millbrig bentonite, here dated at 453.36 ± 0.38 Ma. Bracketing these local stratigraphic patterns are the bounding stages (1 and 3) represented by the upper Turinian Lowville Formation and middle Chatfieldian Hull Formation, respectively, that contain facies attributes in common with the adjacent basins and characterize inter-regional depositional systems of first warm, then cooler oceanographic conditions. Stage 2 identifies a structurally controlled transition between these end-member stages: a far-field response in the foreland interior, localized along the axis of a late Precambrian fault system, to contemporary change in subsidence rates and tectonomagmatic events along the Laurentian margin.

(U-Th)/He Thermochronology of the Ottawa Embayment, Eastern Canada: The Temperature-Time History of an Ancient, Intracratonic Rift Basin

AbstractThe Ottawa Embayment is an intracratonic rift basin in eastern North America that preserves a unique and eventful history through deep time. We report single-grain zircon and apatite (U-Th)/He and biotite 40Ar/39Ar data from samples collected along a 250-km transect across the crystalline rift flank and through the sedimentary sequences of the embayment. The data sets show significant intrasample age dispersions, and we investigate the influence of zircon crystallinity and nonideal crystal shapes on He diffusion through the application of Raman spectroscopy and X-ray microcomputed tomography. Ultimately, we utilize the relationship between radiation damage and helium diffusion to quantify observed age dispersion. We integrate our age data from the basement and overlying sedimentary unit of the embayment with regional geology to utilize multisample inverse modeling to reveal viable thermal histories that we then further evaluate by using the forward modeling method to improve our understanding of t...

Composition and Architecture of Braided and Sheetflood-Dominated Ephemeral Fluvial Strata In the Cambrian–Ordovician Potsdam Group: A Case Example of the Morphodynamics of Early Phanerozoic Fluvial Systems and Climate Change

Abstract: Two fluvial facies associations, braided and ephemeral fluvial, are recognized in strata of the Cambrian–Ordovician Potsdam Group in the Ottawa Embayment and Quebec Basin in northeastern North America. Braided fluvial sedimentation was dominated by the migration of coarse-grained sandy dunes and rare unit bars and local gravels that built up low-relief 0.4–5-m-thick downstream-accreting compound bars in very wide and shallow braided channels. Rare meter-scale confluence scours, incision, and filling of decimeter-thick bypass channels, and thin (≤ 7 cm) overbank deposition also occurred. Ephemeral fluvial sedimentation was dominated by the progradation of lobate terminal splays, most of which were deflated and/or reworked into thin (10–95 cm) sheets by eolian processes during the long recurrence intervals between floods. These strata were locally incised by rare, 0.3–1.9-m-thick scour-filling high-energy supercritical bedform strata and decimeter-thick bypass channels. Both braided and ephemeral fluvial deposits consist of meter-scale stacked, high-order stratal units separated by horizontal, laterally extensive surfaces recording the recurring buildup, avulsion, and finally abandonment of channel belts. In braided fluvial strata of the Potsdam the dominance of thin (≤ 2 m) elongated compound bars with low-relief (≤ 5°) bar margins and internal accretion surfaces (≤ 15°) suggest that braided channels and in-channel compound bars expanded laterally with negligible increase in flow depth during periods of increased discharge in the absence of bank-stabilizing vegetation, similar to other examples of the pre-vegetated “sheet-braided” style. However, in areas where bedrock topography inhibited the lateral expansion of channels, thicker compound braid bars (1.2–3.5 m) with higher-relief accretion surfaces (up to 35°), like those observed in post-Silurian and modern systems, preferentially formed in relatively deep channels. Ephemeral fluvial deposits here are similar to ephemeral examples throughout geologic history in that they mostly comprise poorly confined sheet-like terminal splays that accumulated in topographic lows. However, compared to several post-Silurian examples, strata in the Potsdam exhibit considerably more eolian reworking of splays and preservation of supercritical-bedform strata, a manifestation of greater rates of surface-water infiltration due to a lack of sheltering by rooted plants and propensity for unconfined flooding. Finally, in the upper part of the Potsdam two regional ephemeral fluvial units are interstratified with two braided fluvial units, providing evidence for shifts in regional climate. The sharp, areally extensive surfaces that separate these fluvial systems are interpreted to approximate time lines, and therein provide a means for relative and absolute chronostratigraphic correlation. These changes are correlated with documented Late Cambrian to Earliest Ordovician global climate fluctuations, with semiarid conditions and related ephemeral fluvial systems corresponding to global cooling events at ca. 491 and 487 Ma.

Tectonostratigraphy of the Chazyan (Late Middle-Early Late Ordovician) Mixed Siliciclastic-Carbonate Platform, Quebec Embayment

Abstract The Middle to lower Upper Ordovician (Chazyan) platform succession of eastern North America extends landward of the ancient Quebec Embayment along the Laurentian margin, part of a foreland basin peripheral to an arc-collisional plate boundary. The platform grades from seaward reefal carbonate (Chazy Group, Champlain Valley) into platform- and foreland-interior tide- and wave-dominated siliciclastics (Rockcliffe Formation) and restricted peritidal muddy carbonate (Hog's Back Formation). The latter two formations occur in the sedimentary Ottawa Embayment, which overlies the trace of a Neoproterozoic rift. Prominent tectonostratigraphic surfaces bound and subdivide the Chazyan platform-interior succession, and are characterized by abrupt marine transgressions, local synsedimentary faults, and seismogenic beds. The depositional history includes: (1) accumulation of shallow-marine siliciclastics (Rockcliffe Fm) across a differentially faulted paleoplatform; (2) abrupt regional retrogradation (∼ 100 km) of this siliciclastic system, replaced by an intraplatform shale basin (lower Hog's Back Formation); (3) basin fill and development of a low-energy muddy carbonate platform interior (upper Hog's Back Formation) coincident with regional fall in sea level and change in subsidence along the Laurentian margin; and (4) abrupt high-frequency base-level changes preceding regional onlap of Mohawkian (Turinian) shale contemporary with local synsedimentary deformation and rapid subsidence along the distal arc-collisional plate boundary. Structurally controlled sedimentation patterns in the platform (and foreland) interior document episodic tectonism coincident with relatively sustained deposition along the outer platform (the Chazy Group). This difference is interpreted to be a response to foreland-interior flexuring, modified by local reactivation of the Precambrian rift, that was coincident with preferential subsidence at seaward sites more proximal to structural loading along the arc-collision plate boundary.

The Role of Regional and Local Structure in a Late Ordovician (Edenian) Foreland Platform-to-Basin Succession Inboard of the Taconic Orogen, Central Canada

The Upper Ordovician (Edenian) Lindsay Formation of the Ottawa Embayment represents the final stage of carbonate platform development in the Taconic foreland periphery inboard of the northern Appalachian orogen. The succession overlies a narrow (~60 km) axis of a Neoproterozoic Laurentian rift extending across the Grenville orogen. The Lindsay Formation consists of a lower heavily bioturbated skeletal limestone that represents a warm-water shoal facies following an underlying outer ramp stratigraphy, and an upper division of renewed deep-water deposition with organic-rich shale and fossiliferous lime mudstone. Pyritic deep-water black shale of the westerly advancing Taconic foreland basin disconformably overlies this platform succession. Stratigraphic correlation through the central embayment identifies likely synsedimentary faults and seaward-directed erosion bounding the Lindsay Formation in a region of older Ordovician faults and a change in the lithotectonic character of the crystalline basement. The Late Ordovician shallowing and localization of structural/erosional features are interpreted to record a structural hinge: a local accommodation to, first, foreland periphery uplift, then rapid subsidence related to westerly diachronous foreland subsidence through the platform interior. Spatial association of structures of differing ages suggests that reactivation of inherited weakened crust influenced Late Ordovician sedimentary patterns.

Stratigraphic restriction of stromatolites in a Middle and Upper Ordovician foreland-platform succession (Ottawa Embayment, eastern Ontario)

Stromatolites are abundant and widely distributed within two narrow stratigraphic intervals in Middle (Darriwilian) and Upper (lower Turinian) Ordovician strata of the Laurentian foreland interior in the Ottawa Embayment, eastern Ontario. These lithostratigraphic markers coincide with rapid, tectonically driven flooding of the foreland interior and may identify an opportunistic microbial response to nutrient loading with shallow (peritidal, subtidal) marine reworking of terrestrial or nutrient-rich coastal systems. The remaining (Chatfieldian–Edenian) foreland-platform succession represents deeper-water deposition in response to elevated subsidence rates along the Laurentian margin. Stromatolites are absent in this higher-energy setting, but microbial calcite is preserved as coccoid-like microencrustations on skeletal debris. Stromatolites of Darriwilian age are dolomitic, occur in the Carillon Formation, and are part of a regional (200+ km) onlap of peritidal sediment during onset of Taconic orogenesis. Stratiform to large (2 m diameter) low-relief domal stromatolites contain rhythmic laminations of inclusion-rich and -poor dolomicrospar grouped by Fe-oxide-stained erosional surfaces. Domal forms also contain thrombolitic microstructure. Patterns of lamination and stable (C, O) isotopes suggest a balance between abiotic and microbial carbonate production, likely influenced by water depth and temperature. Stromatolites of Turinian age are calcitic and form a regional (80+ km) thin (<8 m) subtidal biostromal unit in the lowermost Pamelia Formation. Their occurrence defines an abrupt vertical transition from initial intrabasinal transgressive subtidal phosphatic siliciclastics and carbonates to an interbasinal stratigraphy of peritidal lime mudstone. Arrhythmic laminations of microbial peloid packstone, with possible eukaryote alga moulds, and locally evaporitic spongiostromate microstructure identify salinity and energy as primary depositional controls. The stromatolite–?eukaryotic association is similar to some modern subtidal microstructures and is part of the reef-community diversification in the Ordovician.

Late Ordovician (Chatfieldian) catastrophic volcanism and abrupt carbonate platform-interior subsidence: A tectonic link across a Taconian foreland basin (Quebec Embayment) due to inherited crustal weakness

The Late Ordovician Trenton Group of New York State and equivalent strata in south-central Canada peripheral to the Quebec Embayment form a regional carbonate platform succession within the Taconian foreland basin. The upper and lower group boundaries record geodynamically controlled changes in regional subsidence, whereas a higher-order allocyclic stratigraphy traced south along the foreland interior of the eastern United States may identify shorter-term eustatic controls. A tectonomagmatic event extending across the foreland basin is recorded a few million years prior to the regional collapse of the foreland platform in the Quebec Embayment, within the lower C. spiniferus (graptolite) Zone. Platform-margin retreat (∼50 km) in southern Quebec is contemporaneous with a shale-rich limestone succession that thins (∼30–10 m) westward over ∼300 km into the Ottawa Embayment, eastern Ontario, west of the Appalachian structural front. This stratigraphic record coincides with catastrophic volcanism within the once distant volcanic arc terranes as recorded by a bentonite traced geophysically from the platform margin to the Michigan Basin. Core from the Ottawa Embayment demonstrates western-directed transport of lithic-bearing ash. The lithic material, representing reworked deep-water sedimentary and/or volcaniclastic sediment, was entrained by eruption of calc-alkaline felsic to basaltic andesitic, and trachyandesitic magmatic sources. The above geological history documents a short period of diminishing influence of subsidence moving into the platform interior—an event that enhanced, briefly, cross-platform oceanographic circulation as recorded by elevated shale abundance and change in macrofaunal assemblages. Short-lived differential subsidence of the continental crust in this region, in response to distal tectonism, may have been predicated by inherited weakness of a shallowly buried intracratonic (>400km) Precambrian fault system, manifest today by the Ottawa-Bonnechere Graben. The resulting thin (<10- to 30-m) stratigraphic record of the tectonomagmatic event is of similar thickness to high-order allostratigraphic patterns within the Taconic foreland interior of the eastern United States. Because there is a heterogeneous distribution of Precambrian fault systems underlying the Taconian foreland interior of eastern Laurentia, differential structural response of the foreland interior can be anticipated. The apparent regional allostratigraphy may contain subtle facies differences along the strike of the foreland basin that reflect this tectonic component.

Tectonostratigraphic framework of Upper Ordovician source rocks, Ottawa Embayment (eastern Ontario)

Abstract Two intervals of Upper Ordovician organic-rich (gas-prone) source rocks are preserved in a regional fault block of approximately 1000 km2 within the Ottawa Embayment (eastern Ontario). They bracket an apparent conformable mid-Katian (late Edenian to early Maysvillian) transition from a relatively shallow-water carbonate platform setting to a deep-water shale basin within the local Taconic foreland basin. The platform succession is represented by the Lindsay Formation, and records deepening and replacement of a once skeletal-rich, productive platform environment by fossiliferous shale and limestone facies association (10 to 18 m thick) comprising the Eastview Member. Nodular and bedded crinoidal lime mudstone is intercalated with fossiliferous (trilobite, brachiopod) organic-rich (2–6% TOC) shale. The upper 2 m of the unit contain shale with dispersed to compacted accumulations of trilobites and brachiopods. The Eastview Member thickens to the southeast, and is overlain with apparent conformity by organic-rich (2–4+% TOC) shale of the Billings Formation. Regionally, this latter unit is approximately 20 m thick, and accumulated in an anoxic to dysoxic deep-water basin. Thin laminae of siltstone to fine-sandstone herald the later, principal, phase of basin fill by sandstone and shale that characterize the overlying Carlsbad Formation. Geophysical (gamma-ray) transects restricted to the western embayment reveal km-scale spacing of high-angle faults and small-scale synsedimentary horsts and grabens that formed after shallow (3–5 m) burial of the Eastview Member. One small-scale graben illustrates a near doubling (<40 m) of the Billings Formation. A tectonostratigraphic framework for source rock accumulation includes a first-order geodynamic control driving regional platform foundering, then a higher-order structural response that enhanced local thickness of organic shale and created faults subject to reactivation in post-Ordovician time.

A new geological framework for the Middle Ordovician Carillon Formation (uppermost Beekmantown Group, Ottawa Embayment): onset of Taconic foreland deposition and tectonism within the Laurentian platform interior

The Carillon Formation underlies the Ottawa Embayment, a craton-interior extension of the regional Laurentian paleoshelf, defined today by the Ottawa–Bonnechere Graben. Facies within the embayment during the Middle Ordovician identify a mixed-sediment (carbonate, siliciclastic), tide-influenced, peritidal and schizohaline basin, host to a mosaic of dolomitic tidal mudflats, shallow lagoons or ponds, and channels. Skeletal metazoans were rare, but bioturbation increased with episodic influx of less saline water carrying sand and mud, possibly in response to regional climate change. Stratigraphic analysis reveals that the formation coincides with a third-order eustatic sea level cycle, but higher order stratigraphic patterns show no obvious eustatic link. Instead, a stepwise onlap geometry coincided with local faulting that produced widespread syndepositional sediment deformation. In particular, stages of faulting and post-Carillon erosion are mapped along the present-day trace of a northwest–southeast-oriented lineament, the Gloucester Fault, within the present graben. Other present-day northwest–southeast-oriented faults of the graben may also possess Middle Ordovician histories. Platform-interior tectonism and development of the Carillon Formation coincided with the onset of Taconic deformation along the outer paleoshelf (western Newfoundland). The formation defines the initial stage of foreland basin sedimentation in the embayment. Platform-interior faulting identifies a predisposed structural weakness associated with underlying Neoproterozoic structure, possibly enhanced by a regional structural connection (an ocean-to-onland transform fault system) with the distal (~1200 km), convergent, plate boundary.

Precambrain Fault Systems as Control on Regional Differences in Relative Sea Level Along the Early Ordovician Platform of Eastern North America

Abstract This study proposes that there was regional variation in Early Ordovician subsidence histories along eastern Laurentia controlled by broad differences in the susceptibility of inherited Precambrian crustal structure to reactivation with changing ocean-basin tectonism. Two continental-scale Phanerozoic fault systems (Southern Oklahoma Fault System, Ottawa–Bonnechere Graben) extend into the craton interior from the apices of the Ouachita and Quebec embayments or reentrants, respectively. Their traces identify reactivated Proterozoic fault zones. Previous studies show that rates of subsidence for Early Ordovician sedimentation along the axis of the Southern Oklahoma Fault System are elevated compared to regional rates beyond this structural corridor. A similar history applies to Early Ordovician sedimentation within the Ottawa Embayment (eastern Ontario), along the present axis of the Ottawa–Bonnechere Graben. The Theresa Formation (lower Beekmantown Group) contains a third-order peritidal (siliciclastic-dominated) sequence. Evidence for local tectonism prior to and during its development within the embayment includes: (1) Late Cambrian (pre-Theresa) preferential uplift, differential erosion, and subsidence along the eastern structural boundary of the embayment; (2) differential intrabasinal subsidence during Theresa deposition; (3) an apparent offset in timing of peak flooding within the embayment to that along the St. Lawrence Promontory, ~ 1500 km distant; and (4) absence of a subaerial unconformity in the embayment that otherwise separates eustatic cycles on the promontory, and defines a likely sea-level fall below the shelf margin. In comparison, subsidence within the embayment appears to have matched (at least) the rate of the eustatic fall. Regional differences in Early Ordovician subsidence histories along this margin may illustrate that the Precambrian fault zones were structurally more sensitive than promontories to the developing reconfiguration of oceanic lithospheric elements that would lead to the Taconic orogeny.

The Potsdam–Beekmantown Group boundary, Nepean Formation type section (Ottawa, Ontario): a cryptic sequence boundary, not a conformable transition

There are two unreconciled interpretations for the age and character of the boundary separating the Cambrian–Ordovician Potsdam and Beekmantown groups that underlie the Ottawa Embayment in eastern Ontario. These stratal groups consist of interior facies of the central Laurentian Platform. As exposed in the type section of the Nepean Formation (upper Potsdam Group), located in the City of Ottawa, the boundary was previously interpreted to be conformable and of Early Ordovician age. This intepretation was of enormous impact on subsequent regional geology compilations that showed a diachronous boundary across the platform interior. From recent subsurface analysis across eastern Ontario, the contact was interpreted to be disconformable, a sequence boundary separating Late Cambrian and Early Ordovician strata. This paper reexamines the type section. Lithologically, the group boundary should be repositioned downsection by ~1.5 m. The contact now lies coincident with a disconformity that has a paleorelief of &lt; 10 cm. The proposed revision is geologically significant. Previous collections of Early Ordovician conodonts from the type section, used to define the age of what had been interpreted to be upper Potsdam strata, now fall entirely within the lower Beekmantown Group. Nepean (Potsdam) strata exposed in the type section remain undated. Regional correlation of the disconformity across the Laurentian platform suggests that Nepean strata at the type section are likely of Late Cambrian age. There now exists a regionally coherent separation of Cambrian and Ordovician sedimentation patterns in the Ottawa Embayment.

The geochemical record of hydrothermal mineralization and tectonism inboard of the Appalachian Orogen: the Ottawa Embayment

The history of mineralization of the Ottawa Embayment includes migration of basin brines within the confines of an Early Paleozoic basin, and cross-stratal fracture flow linked to structural reactivation of an underlying Neoproterozoic failed-rift system. Paleoporosity consists of paleokarst (PAL) in Lower Ordovician dolostone (Beekmantown Group), and fractures (now veins) that cross-cut both Lower Paleozoic (PL) and Precambrian (PC) strata. A series of contrasting mineral assemblages, which define changing fluid compositions, fill paleokarst cavities and rare interconnected paleofractures (now veins). First, there is an early-PAL succession of quartz, saddle dolomite, then calcite, with accessory hydrocarbon as well as sulphide and sulphate minerals. The quartz defines migration of saline (>20 wt.%) silica brines and hydrocarbon that spans a temperature increase (defined by fluid inclusions) from <100 to <200 °C. This gradient is registered also by a change in state, from liquid to methane, of hydrocarbon trapped within the quartz crystals. The succession of saddle dolomite, then calcite associated with liquid hydrocarbon, marks an overall cooling to <110 °C. δ 13C of the dolomite identifies strong sedimentary rock–water interaction, but a depleted 13C composition (∼−14‰) for the calcite illustrates that, with cooling, paleokarst cavities were sites of bacterially mediated sulphate reduction. Remaining paleokarst porosity is filled with a late-PAL mineral assemblage that contains low-temperature (mostly <60 °C) calcite with a varied assemblage of sulphide and sulphate minerals. However, a similar mineral assemblage occurs in regionally abundant PL calcite veins that, locally, visibly intersect paleokarst void space. Furthermore, PL veins are also connected to PC veins. Thus, the late-PAL period of mineralization is linked to regional fracture-controlled cross-stratal fluid migration, which locally intersected paleokarst. Isotope (S, O, C) geochemistry, mineral assemblages, and fluid inclusions from vein and late-PAL minerals identify mixing among four endmember solutions: evolved hydrothermal (<130 °C) basement-derived brines; cool (<60 °C) meteoric-derived fluids; dissolved Middle Ordovician evaporite; and residual basin brines. The age of mineralization awaits future analysis, but patterns and compositions of fluids in this interior basin are consistent with being far-field effects of Paleozoic and Mesozoic tectonism in the Appalachian orogen.

Late Ordovician platform foundering, its paleoceanography and burial, as preserved in separate (eastern Michigan Basin, Ottawa Embayment) basins, southern Ontario

Comparison of litho-, bio-, and chemostratigraphy in two cores from the northeastern margin of the Michigan Basin (Manitoulin Island) and from within the Ottawa Embayment (eastern Ontario) identifies interbasinal differences of Late Ordovician platform foundering linked to Taconic orogenesis. Graptolite biostratigraphy defines an east-to-west younging (late Edenian to early Maysvillian) of platform burial. A regional unconformity likely caps the platform succession. In both basins, an increased supply of mafic material appears during the final stages of platform collapse, with the accumulation of organic-rich (&lt;8%), petroliferous shales (Collingwood Member — Michigan Basin; Eastview Member — Ottawa Embayment). Both units preserve evidence for deposition coincident with increased dysoxic to possible anoxic bottom-water conditions, but the Collingwood Member accumulated under a relatively stable paleoceanographic environment. Rhythmic interbedding with platform limestone in eastern Ontario, combined with evidence for fluctuating paleoproductivity, suggests the depositional environment of the Eastview Member was more sensitive to higher order controls affiliated with tectonic, oceanographic, and (or) sea level variation. Such interbasinal differences likely reflect a greater rate of subsidence in the Manitoulin region transforming platform sedimentation to a distal ramp facies. In eastern Ontario, a lesser rate of subsidence maintained a shallower water, but open margin, setting. Burial of the Upper Ordovician platform, as preserved in eastern Ontario, occurred during peak dysoxic conditions, with deposition of a hemipelagic facies (Billings Formation) that marks the peak supply of clay-size mafic-derived sediment. Bottom-water ventilation occurred only with appearance of abundant Taconic-derived distal turbidites. An equivalent hemipelagic facies appears to be absent from the Manitoulin region. However, equivalent resedimented deposits are represented by the Blue Mountain Formation.

ABSTRACT: The Ottawa Embayment: A Giant, Structurally-Controlled Lagoon During Early Blackriveran Stage (Upper Ordovician), Eastern Ontario-Southwestern Quebec

ABSTRACT: The Ottawa Embayment: A Giant, Structurally-Controlled Lagoon During Early Blackriveran Stage (Upper Ordovician), Eastern Ontario-Southwestern Quebec

Blackriveran (lower Mohawkian, Upper Ordovician) lithostratigraphy, rhythmicity, and paleogeography: Ottawa Embayment, eastern Ontario, Canada

The Ottawa Embayment contains erosional remnants of a shallow-water carbonate platform (St. Lawrence Platform) of Late Ordovician (Caradocian) age. Stratigraphy of three Blackriveran formations - in ascending order, Pamelia, Lowville, and Chaumont - documents regional changes in continuity and types of depositional facies. The Pamelia Formation contains two members, each containing basin-wide basal siliciclastic units overlain by interbedded limestone, dolostone, and shale. An alternate division recognizes six shallowing-upward units; regionally extensive, intertidal to supratidal, dolomitic sandstone and (or) sandy dolostone define their tops. Sedimentary structures and isotope (C, O) geochemistry support a syngenetic origin of this stratigraphic (bedded) dolomite. The Lowville Formation contains two facies associations: subtidal to lower intertidal bioclastic and oolitic packstone - grainstone followed by lagoonal to intertidal mudflat facies. Lateral facies continuity is reduced compared to the Pamelia Formation. The Chaumont Formation contains thick beds of burrowed, bioclastic, peloidal mudstone to packstone, and minor shale. No rhythmic pattern is recognized in these subtidal facies. Upsection decrease of rhythmic sedimentation, with a decrease in lateral facies continuity of the studied strata reflects a progressive increase in net accommodation space related to Taconic transgression. Higher order rhythmicity of dolostone and sandstones of the Pamelia Formation can be traced into adjacent regions (New York and Kingston, Ont.). Dolomitic units may identify basin-wide chronostratigraphic markers, potentially useful for future sequence analysis. Regional correlation reveals a good oceanographic linkage between the Ottawa Embayment and the Appalachian Basin during Pamelia time and a restricted access across a paleohigh in the Montréal region. By the time of Lowville deposition, Taconic transgression had breached this restriction.

Blackriveran (lower Mohawkian, Upper Ordovician) lithostratigraphy, rhythmicity, and paleogeography: Ottawa Embayment, eastern Ontario, Canada

Blackriveran (lower Mohawkian, Upper Ordovician) lithostratigraphy, rhythmicity, and paleogeography: Ottawa Embayment, eastern Ontario, Canada

Character of the Middle Ordovician Sauk-Tippecanoe sequence boundary in the Ottawa Embayment (eastern Ontario): possible evidence for platform-interior, Taconic tectonism

In eastern Ontario, a Middle Ordovician disconformity that separates predominantly sandstone of the Rockcliffe Formation from underlying dolomite of the Carillon Formation (Beekmantown Group) has, historically, been considered equivalent to the Sauk-Tippecanoe sequence boundary. The character of the formation contact varies considerably, and includes (1) rare, serrated micropinnacles (centimetres in height); (2) smoothed, irregular to undulose erosional bedforms along shallow (&lt;30 cm) incised channels at least ten's of metres in width; (3) a planar boundary with no obvious evidence of erosion; (4) compaction effects across the boundary, as well as within uppermost Carillon and lowermost Rockcliffe strata; and (5) interbedding of Carillon- and Rockcliffe-like lithologies. The latter two observations suggest that, locally, the disconformity represents a relatively minor gap in time. Local, irregular paleotopography is interpreted as some combination of paleokarst, mechanical erosion, and differential sediment loading. The Carillon-to-Rockcliffe transition, from peritidal carbonates to marine or estuarine sandstones, is punctuated by local, anomalous structural and stratigraphic features: ptygmmatically folded fractures, local disconformities, and deposition, then differential erosion of high-energy fluvial-dominated sediments. Differential erosion, fracturing, and paleokarst are local, anomalous attributes of the pre-Carillon paleosurface as well. Combined, these events bracket Carillon deposition, and are interpreted to reflect local platform-interior tectonism coincident with onset of distal Taconic orogenesis. Platform-interior expression of distant, developing changes in plate-boundary dynamics may be linked to incipient basement reactivation along an underlying, shallowly buried Proterozoic fault system, later manifest as the Ottawa-Bonnechere Graben.

Character of the Middle Ordovician Sauk-Tippecanoe sequence boundary in the Ottawa Embayment (eastern Ontario): possible evidence for platform-interior, Taconic tectonism

Character of the Middle Ordovician Sauk-Tippecanoe sequence boundary in the Ottawa Embayment (eastern Ontario): possible evidence for platform-interior, Taconic tectonism