ABSTRACT

The thermal maturation and burial history of the Paleozoic sedimentary sequence in southern Ontario is investigated by using colour alteration indices of conodonts and acritarchs. A total of 112 localities from the surface (92) and subsurface (20) in southern Ontario and Quebec provided 832 samples from Ordovician (735), Silurian (16) and Devonian (81) strata. These produced over 78,000 individual conodont elements for colour index identification. In the colour alteration of the leiosphere acritarchs, a total of 16 wells through various parts of the sequence provided 320 samples that yielded 240 slides productive in leiospheres. Nineteen surface samples were also processed for acritarchs.

The conodont data revealed three facies of thermal alteration. The first, in southwestern Ontario, extends from the top of the Paleozoic sequence down to Middle Ordovician (Trenton) strata. The conodont colour alteration index (CAI) is 1.5 and reflects maximum burial temperatures of less than about 60°C. Within this facies, acritarchs were found to change progressively in colour from light yellow to pale yellow through orange to brown, and an acritarch colour alteration index (AAI) is proposed. Within this first facies, AAI 2-4 are recognized.

The second thermal alteration facies occurs in the lower part of the Ordovician sequence in southwestern Ontario and in part of western Ontario. CAI values are 2-2.5 and suggest maximum burial temperatures of 60-90°C. At these higher temperatures acritarchs become degraded and AAI 5 is attained. In eastern Ontario and southern Quebec a third thermal alteration facies exists in which CAI values reach 3 along the Ottawa-Bonnechere graben and 4-5 in the vicinity of Montreal, reflecting maximum burial temperatures of 90-120°C and 185-300°C, respectively.

The depths of burial inferred from the paleotemperature estimates provided by CAI values indicate that the sedimentary sequence in southwestern Ontario underwent minimum burial. The thermal anomaly in the Ottawa Valley - Montreal area, however, requires deeper burial and/or higher temperatures. The anomaly is interpreted as resulting from high heat flow during Cretaceous - Early Tertiary rifting which also led to the Monteregian alkaline intrusions. The anomaly pattern may reflect the initiation and migration path of the Great Meteor Hotspot. The thermal maturation patterns are related to hydrocarbon occurrence in southwestern Ontario.

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INTRODUCTION

Potential hydrocarbon resources in southern Ontario are relatively small but the combination of escalating oil and gas prices, inexpensive exploration costs, proximity to markets and new discoveries in the Michigan Basin have resulted in a current upsurge in exploration. A century of hydrocarbon exploration has yielded significant information on the structure, stratigraphy and subsurface characteristics of the Paleozoic formations.

Recent methods of exploration include the determination of the level of thermal maturity attained by organic matter within sedimentary sequences. Such determinations are obtained through, for example, vitrinite reflectance (Hacquebard and Donaldson, 1970), phytoclast alteration (Bostick, 1974), palynomorph colour (Gutjahr, 1966), and conodont colour (Epstein et al., 1977; Harris, 1979). These techniques not only provide an interpretation of the degree of thermal alteration of organic material but, when complemented with organic-geochemical analyses (Deroo et al., 1977) and isotope studies (Stahl, 1977), they also provide a systematic approach to problems of hydrocarbon generation and occurrence.

The specific objectives of this study are:

1. To establish the thermal maturation levels of the Paleozoic rocks of southern Ontario through systematic study of conodont and palynomorph colour alteration.

2. To interpret burial temperatures and depth of burial attained by the Paleozoic sequence.

3. To investigate the relationship between regional maturation levels and oil and gas occurrences from thermal alteration maps compiled for southern Ontario.

Organic geochemical and isotope studies are being undertaken by T.G. Powell (Geological Survey of Canada, Calgary) and J.F. Barker, P. Fritz and R.W. Macqueen (University of Waterloo). Their results will form the basis of separate publications.

In addition to the implications for oil and gas occurrences, the thermal and burial history of the southern Ontario Paleozoic sequence has other important implications. Knowledge of the probable maximum burial depth of the sequence facilitates estimates of overburden thickness, and permits restoration of missing sections especially in areas extensively disturbed by tectonism. The validity of this approach has been documented by Epstein et al. (1977), Harris et al. (1978), Harris (1979), McArthur et al. (1980), Ogunyomi et al. (1980), and recently applied in connection with ancient hotspot paths by Crough (1981).

Despite the volume of information that has accumulated through hydrocarbon exploration in southern Ontario, the lack of published organic maturation studies precluded any attempt to assess the region's thermal and burial history. The general consensus, however, is that the relatively thin Paleozoic sequence has not been deeply buried, and that the lack of suitable structural and/or stratigraphic traps limits the size of hydrocarbon accumulations.

Recent organic maturation studies and their implications for oil and gas occurrence in the Appalachian Basin (Epstein et al., 1977; Harris et al., 1979) have verified that the entire Paleozoic section has potential for gas production, whereas only the western part of the northern Appalachian Basin has potential for oil production. The observed increase in thermal metamorphism and tectonic deformation substantially decreases the potential for oil and gas production in the eastern part of the basin.

Although published organic maturation studies are lacking for the Michigan Basin, the intervening area of southern Ontario, located updip from a large volume of potential source rocks in the Michigan and Appalachian Basins, is suited for entrapment of hydrocarbons. Southern Ontario has carbonate and clastic reservoirs that offer a variety of relatively small structural and stratigraphic traps.

The integration of conodont and palynomorph colour alteration techniques to investigate the thermal maturation history of the Ontario Paleozoic sedimentary sequence allows a

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test of previous estimates on subsidence and burial of these rocks. It also offers insight into temperature-depth relationships in the subsurface, and the results can continue to be used in future work on Ontario stratigraphy and hydrocarbon deposits.

STRUCTURE AND STRATIGRAPHY

With abundant literature available on the Paleozoic geology of southern Ontario, discussion herein is restricted largely to relating the structure and stratigraphy to the distribution of known hydrocarbon occurrences and possible source rocks. For readers unfamiliar with the regional stratigraphy, a brief supporting discussion is provided in Appendix 1.

The Paleozoic sedimentary sequence in southern Ontario exceeds 116,580 km² in outcrop areas and is from Cambrian to Early Mississippian in age (Hutt et al., 1973) (). The strata are relatively undisturbed and rest on an irregular-surfaced Precambrian basement.

Generalized geological map of southern Ontario and surrounding areas showing main structural elements (modified from Sanford, 1962).

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The Frontenac, Algonquin and Findlay Arches are positive structural features that amplify the erosional irregularities existing on the Precambrian surface. These basement structures () not only constitute the main tectonic elements but also are responsible for the present configuration of the southern Ontario depositional basin (Brigham, 1971; Winder and Sanford, 1972; Hutt et al., 1973).

The Frontenac Axis divides the southern Ontario basin into two segments: the southwestern Ontario sedimentary basin and the eastern Ontario sedimentary basin (the Ottawa Lowland or Embayment). Strata to the southeast of the Algonquin Arch are northwestern extensions of the Appalachian Basin, whereas those to the north belong to the Michigan Basin. The Findlay Arch trends northwest in the subsurface through Ohio and the southwestern tip of the Ontario Peninsula.

The Paleozoic sequence in southwestern Ontario ranges in thickness from 1 460 m under central Lake Erie and near Sarnia (Hutt et al., 1973) to 670 m along the Niagara Peninsula (Sanford, 1962; Brigham, 1971). The regional dip of the strata flanking the Algonquin Arch is about 5.5 m/km into the Michigan Basin, and about 8.5 m/km into the Appalachian Basin (Winder and Sanford, 1972).

In eastern Ontario, Paleozoic strata underlie an area of approximately 11 660 km² (Hutt et al., 1973) and reach a maximum thickness of 760 m (Sanford, 1961). In contrast to southwestern Ontario, the Paleozoic sequence in eastern Ontario has been disturbed by two sets of normal faults especially along its northern margins (Wilson, 1964; Kumarapeli and Saull, 1966) forming the Ottawa-Bonnechere graben. The principal set has a general southeasterly trend, whereas the lesser set has a northeasterly trend (Wilson, 1964) (). The faulting was related apparently to the rifting of the North Atlantic in Cretaceous and early Tertiary time (Kumarapeli and Saull, 1966).

Cretaceous-Tertiary faults and intrusions, eastern Ontario and southern Quebec. Monteregian Hills intrusions shown by triangles.

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Generalized stratigraphic column for southern Ontario and southern Quebec.

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The age of the sedimentary column in southwestern Ontario is from Late Cambrian to Early Mississippian, whereas in eastern Ontario the age range is Late Cambrian to Late Ordovician (). Similarity of facies and isopach data from both areas for most Ordovician formations suggests the Frontenac Axis has little effect on sedimentation in post-Chazyan time (Sanford, 1961; Rickard, 1973). The relative stability of the Michigan Basin contributed to the remarkable lateral continuity of many Paleozoic formations into Ontario. In contrast, in areas flanking the tectonically active Appalachian Basin, abrupt lateral facies changes are common. Periodic uplift and tectonic activity within the Appalachian Orogen resulted in widespread regional unconformities in southern Ontario ().

HYDROCARBON OCCURRENCE AND DEVELOPMENT IN SOUTHERN ONTARIO

Exploration and development for hydrocarbons in southern Ontario have been confined to the southwestern segment of the region where most of the Paleozoic sedimentary sequence is still preserved (Rose et al., 1970). In eastern Ontario, the sequence is thin and nowhere has yielded commercial quantities of hydrocarbons, although oil and gas shows occur.

Commercial hydrocarbon production is derived from twelve stratigraphic units of Cambrian through Devonian age. Devonian strata yield 39.2 per cent of the total oil production, followed by those of the Silurian (36.7%), Cambrian (21.7%) and Ordovician (2.4%) (Ont. Min. Nat. Res., 1979). For gas production (Ont. Min. Nat. Res., 1979), 90 per cent comes from Silurian strata, with the remainder from Cambrian (6%) and Ordovician (4%) strata. The apparent absence of commercial volumes of natural gas in rocks of Devonian age and the presence of oil and gas in varying proportions in Cambrian, Ordovician and Silurian strata present a pattern of hydrocarbon occurrence which is not fully explained at present.

Cambrian Deposits

Hydrocarbon accumulation in Upper Cambrian rocks is confined to the southeastern flank of the Algonquin Arch bordering the Appalachian Basin. The main producing fields (), including the Willey, Clearville and Gobles, occur in permeable dolomite and sandstone reservoirs situated along the truncated edges of strata flanking the Algonquin Arch.

The source of Cambrian oil is uncertain due to lack of published organic geochemical studies on oil and potential source or reservoir rocks. On the basis of the lithologic characteristics, Pounder (1964) suggested two possibilities for the source of Cambrian oil: first, a Cambrian source provided by shale and dark grey carbonate of the Appalachian Basin, and second, the Appalachian Basin facies of the Ordovician Black River Group. Burgess (1962) postulated that updip migration of hydrocarbons into traps along the Cambrian erosional edge flanking the Algonquin Arch occurred during Late Ordovician time in response to regional tilting, with westward migration along the pinchout to more favourable structural and stratigraphic traps.

Ordovician Deposits

Ordovician strata include over 50 per cent of the sedimentary volume of southern Ontario (Sanford, 1961) but lack good reservoirs and have only small volumes of commercial hydrocarbons (Hutt et al., 1973). Most Ordovician production to date (Ont. Min. Nat. Res., 1979) has been from the Middle Ordovician carbonate sequence with the Meaford-Dundas Formation (), locally yielding small quantities of gas (Sanford, 1961). The productive fields; e.g., Colchester, Malden, and Dover (), occur in various stratigraphic and structural settings that coincide with dolomitized sections of the Middle Ordovician limestones along faults and anticlinal structures.

Silurian Deposits

Important Silurian oil and gas fields produce from both stratigraphic and structural traps that include blanket sandstone reservoirs, bioherm reefs and carbonate rocks in structures associated with underlying reefoid development (Koepke and Sanford, 1966; Sanford, 1969).

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Lower Silurian (Alexandrian - Lower Niagaran) strata of the Cataract Group () form the blanket sandstone reservoirs located in the Niagara Peninsula. These rocks dip southward into the Appalachian Basin, and include the Welland, Haldimand and Norfolk Fields and their offshore extensions (). Hydrocarbon production is stratigraphically controlled along porosity and permeability pinchouts (Koepke and Sanford, 1966). The Irondequoit Formation also yields substantial quantities of gas. Oil - and gas-bearing Silurian bioherms occur along the southeastern margin of the Michigan Basin and in southwestern Ontario (Cohee and Landes, 1958; Sanford, 1969; Brigham, 1971; Mesolella et al., 1974). Middle Silurian strata (Middle and Upper Niagaran) include important biohermal reservoirs, capped by Upper Silurian (Cayugan) evaporites, that are the sites of the Dawn, Enniskillen, Petrolia East, Oil Springs East and Brooke Fields.

Speculation on the origin of these hydrocarbons led Sharma (1966) to suggest the Lower and Middle Niagaran shales (Rochester and Cabot Head Formations) as the most likely sources. Burial and compaction of these argillaceous units presumably forced hydrocarbons into the overlying porous dolomites; from these the oil and gas migrated laterally into reef traps (Sharma, 1966). The possibility of in situ generation for most of the Silurian oil and gas was not considered, nor that most of the Silurian oil may have originated in situ whereas most of the gas was generated from the Upper Ordovician black shales upon burial.

Devonian Deposits

The major oil reserves are contained in rocks of Middle Devonian age that extend from the southern extremity of Lake Huron to Lake Erie (Rose et al., 1970). Active Devonian fields; e.g., Petrolia, Oil Springs and Rodney (), produce from shallow reservoirs ranging in depth from 90 to 150 m, which are related to porosity development along dolomitized zones in anticlines and domes (Ont. Min. Nat. Res., 1979). The features originated penecontemporaneously over Guelph reefs, and by differential compaction and

Active oil and gas fields in southwestern Ontario: 1) Willey, 2) Gobles, 3) Clearville, 4) Colchester, 5) Malden, 6) Dover, 7) Dawn, 8) Petrolia East, 9) Oil Springs, 10) Brooke, 11) Welland, 12) Haldimand, 13) Norfolk, 14) Rodney, 15) Petrolia. (Modified from Ont. Min. Nat. Res. Paper 78-1, 1979.)

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faulting resulting from the leaching of underlying Salina salt (Rose et al., 1970). Hydrocarbon production is principally from the Lucas and Dundee Formations ().

CONODONT AND PALYNOMORPH COLOUR ALTERATION STUDIES

Observations of the changes in conodont and palynomorph colour in response to increased temperature during burial provide the basis for evaluating the thermal and burial histories of the southern Ontario sedimentary basin.

Conodonts are minute tooth-like fossils composed of apatite (Lindstrom, 1964). They range from 0.1-3 mm in length and are usually translucent, straw yellow or brown (Lindstrom, 1964; Epstein et al., 1977). They are common in Paleozoic and Triassic marine rocks and now rank among the most useful fossils biostratigraphically. Palynomorph is a general term for a group of structured, chemically resistant, organic-walled microfossils chiefly of plant origin. In this project the palynomorphs studied were acritacrchs, a group of unicellular organisms of polyphyletic but largely unknown origin (Evitt, 1963; Eisenack, 1958; Loeblich, 1979).

Maturation determinations can be undertaken rapidly and inexpensively by using either or both microfossil groups. Such determinations are especially useful in southern Ontario where the application of conventional methods; e.g., vitrinite reflectance and spore translucency, is limited by the lack of abundant clastic rocks in the sequence, and the consequent unavailability of vitrinite and spores, as well as by much of the succession being older than the earliest known spores.

Paleozoic marine rocks commonly have an abundance of conodonts and palynomorphs and, although investigated separately, this study integrates the information obtained from both microfossil groups. For eastern Ontario, the thermal alteration data were derived exclusively from conodonts whereas, for the area west of the Frontenac Axis, the information was obtained from both groups.

Samples Studied

Sampling commenced in June 1978 and included surface (outcrop) samples, drill cuttings and core samples. With the exception of those derived from cuttings and cores, most of the surface samples used in the study were acquired through loans from institutions, and collections from previous investigations in the region. Surface samples from a total of 92 localities were used in this investigation. Abundant collections of Ordovician conodonts were already available from 70 localities in southwestern Ontario and the Ottawa Valley from earlier studies by Barnes (1964), Uyeno (1974), Brand and Rust (1977), Tarrant (1977), and undescribed collections of Barnes; and from four localities near Montreal (Sandi, 1978) -- all of which are shown on . Conodonts from 18 localities representing Silurian (3) and Devonian (15) strata in southwestern Ontario were obtained primarily from previous studies of Telford et al. (1977), collections of Uyeno (Uyeno et al., in press), and field collections by the authors.

Initially, drill cuttings from 13 wells in southwestern Ontario were acquired from borehole samples donated by the Ontario Ministry of Natural Resources, Petroleum Resources Division, as well as those provided by Consumer's Gas Limited from recent Lake Erie wells. Cuttings were available for the entire length of each well and provided continuous coverage within each stratigraphic unit. Well distribution afforded good regional control in the subsurface, based on relatively close sample intervals ranging from 9-20 m in 11 of the 13 wells; the remaining two wells were sampled at 30-60 m.

Core samples, kindly provided by Ontario Hydro, were available from six shallow boreholes at different locations. These core samples were taken at about 9 m intervals in each borehole. One sample was obtained from a cored section of a Lake Erie well made available by Consumer's Gas Limited.

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Ordovician sample localities in southern Ontario and southern Quebec.

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The close sample interval allowed good vertical control of the subsurface distribution of thermal alteration index values and ensured that the thermal facies boundaries could be drawn accurately.

The regional and stratigraphic distributions of the samples used in the study and their microfossil yields and colour data are documented in Appendix 2, including -.

Sample Processing

This study focused on the examination of two different microfossil groups, and different techniques were necessary to separate them.

Conodont samples ranging in weight from 20-100 g were dissolved in dilute (10-15%) acetic acid. Conodonts were then isolated from the -16 to + 150 mesh fraction of the residues by standard tetrabromoethane heavy-liquid and hand picking techniques. The conodont colour was then established by using a standard binocular microscope in conjunction with a visual colour comparison chart (Munsell Soil Colour Chart, 1975 edition).

Between 10 and 50 g of each palynomorph sample, depending on the amount of material available, was processed for acritarchs according to conventional methods described by Barss and Williams (1973). Samples were cleaned and crushed where necessary and treated with concentrated HC1 for up to twenty-four hours to dissolve carbonate material. After decantation and washing, the samples were treated with 52 per cent HF for three to four days. Washed residues were passed through a 180 µm sieve and fine screened with a 20 µm sieve in which the palynological material was retained. Oxidizing agents were not used.

At least four permanent slides of each sample were prepared from these concentrated residues with Canada Balsam or Elvanol-Elvacite as the mounting medium; the remaining material was stored in sealed glass vials. The specimens were then examined under a binocular microscope using transmitted light.

Conodont Colour Studies

The sequential colour changes exhibited by conodonts at increasing depth within a sedimentary sequence provide a valuable tool for assessing thermal maturation. Pioneering work by Epstein et al. (1977) included laboratory and field studies of changes in conodont colour. They observed the changes under short-term, high-temperature pyrolysis in a variety of open- and closed-system conditions obtained by using air, argon and then methane as pressure media. Their experiments were used to demonstrate that the conodont colour change can be treated as a simple chemical reaction that is consistent with first-order kinetics (described by the Arrhenius equation); that is, both temperature- and time-dependent. Their results were extrapolated to geological temperatures and times () and compared with empirical observations inferred from geological samples. The sequential change in conodont colour reflects the progressive alteration of trace amounts of organic matter within the microfossil (Epstein et al., 1977). It allowed a numerical representation on a 1 through 8 linear scale corresponding to a series of observable changes from yellow, brown through black, to grey and finally colourless crystalline.

Furthermore, Epstein et al. (1977) have concluded that:

1. The sequential colour change exhibited by conodonts in field collections is the same as that produced by heating alone, and this change is progressive, cumulative and irreversible.
2. Conodont alteration is time- and temperature-dependent.
3. Pressure neither retards nor accelerates colour alteration.
4. In an open system, heating in the presence of argon and methane neither retards nor accelerates colour alteration.

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5. Heating in a sealed system with argon and methane as the pressure media retards colour alteration by 50 per cent.
6. Colour alteration from pale yellow to black probably results from a carbon-fixing process in the conodont; at higher temperatures progressive colour alteration from black through grey and opaque white to crystal clear in the result of carbon loss and release of water through crystallization and recrystallization.
7. An Arrhenius plot of the experimental data and field data indicates that conodont colour alteration begins at about 50°C and continues into the garnet grade of metamorphism at approximately 550°C.

Regional investigations of conodont alteration using this information were completed for the Appalachian Basin (Epstein et al., 1977; Harris et al., 1978; Harris, 1979), the Valley and Ridge province in the southern Appalachians (Harris and Milici, 1977), the Bane Dome in the Alleghney Frontal Zone (Perry et al., 1979), and the Bjorne Peninsula, southwestern Ellesmere Island in the Canadian Arctic Archipelago (Mayr et al., 1978).

A set of standards of conodonts of different CAI was provided by Dr. Anita G. Harris of the U.S. Geological Survey. Additional sets of standards were assembled from the Ontario-Quebec collections. Representative Ontario-Quebec specimens were mounted on clean glass slides and photographed on Kodak Ektachrome 64 film. Because of the obvious contrasts that can result in colour reproduction, careful attention was paid to standardize the light source and the distance between the objective, the eyepiece and the specimen. Conodonts were photographed against the background of a Kodak Neutral Test Card, which provided the best assurance of consistency of colour tonality. In addition, a single standard specimen was included in all specimen groups photographed (, ).

Using this type of semiquantitative technique to determine the level of thermal alteration is not without its disadvantages, and it is useful to recognize its limitations before the data are presented and interpreted. Aside from being dependent on temperature and history of burial, the relationship between the conodont colour alteration index (CAI) and the degree of organic metamorphism is known to be related to variations in:

1. Genetic and structural differences of the neurodont conodonts when compared with the slender lamellar forms or with those forms with significant amounts of white matter (Barnes, 1964; Barnes et al., 1973; Epstein et al., 1977).
2. Shape, size, stage of growth, maturity and/or robustness of individual conodont elements (Epstein et al., 1977).
3. Host-rock lithology (Mayr et al., 1978).

The present conodonts were obtained mainly from carbonates, with minor collections from shales of Upper Ordovician (Whitby and Georgian Bay Formations) and Middle Devonian (Arkona and Hungry Hollow Formations) age. Conodonts from different stratigraphic levels in carbonate and shale samples of similar geological age in southern Ontario, especially those that occur within the temperature range defined by the CAI value of 1.5, exhibit only minor variations in colour of up to 0.5 unit of CAI.

Upper Ordovician and Middle Devonian conodonts from shale units do not reveal the straw yellow or pale yellow component that characterizes clean carbonates in a similar CAI range. Instead, the colours have a distinct dark brown component. Hence, colour ranges from straw yellow to pale yellow are more evident in clean carbonates than are changes from light brown through dark brown for the shales. In mixed lithologies (carbonates with interbedded shales), the amber - reddish brown colours are generally more characteristic.

As documented in experimental and field situations, conodont colour appears to be most diagnostic as a thermal indicator for temperatures ranging from 50° to 550°C (Epstein et al., 1977). However, the theoretical basis of the Arrhenius equation limits extrapolations of

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Arrhenius plots from pyrolysis experiments to geological systems over a wide range of time and/or temperature values. This is because the assumption of temperature independence of the constants in the Arrhenius equation is valid only over a short temperature range (Snowdon, 1979); hence, the time-temperature relation is not always linear.

As the conodont CAI divisions are subjective in practice, the use of colour as an organic maturation index of thermal alteration is of limited value in studies of sediments in the low-temperature range of organic metamorphism from 50° to 80°C. In areas of minimum burial (CAI 1.5) as is the case for all Ontario Silurian and Devonian localities (, ), it is impossible to calculate precise burial temperatures from the Arrhenius curve for CAI 1.5 (), as the critical time/temperature zone (>10⁷ years but <3 10⁸ years) for these Paleozoic strata is difficult to examine.

As the Silurian and Devonian sedimentary sequences are known to contain most of Ontario's oil and gas, the implications of low-temperature organic diagenesis suggested by the CAI data promoted the search for an alternative organic geothermometer that could discriminate temperatures in the range of 50-100°C. The thermal window for peak generation of oil is between 60° and 90°C and for peak generation of gas between about 90° and 120°C (Tissot and Welte, 1978). Hence, the prospect of refining the broad paleotemperature estimates through the use of an alternative organic maturation index (i.e., palynomorph studies) has immediate practical application to petroleum exploration in southwestern Ontario.

Palynomorph Colour Studies

Remains of organic-walled microplankton are abundant in Paleozoic strata of southwestern Ontario. Sporopollenin, the substance that constitutes the external membrane of many of these microfossils (Correia, 1970), is known for its physicochemical durability and provides a good thermal indicator of organic diagenesis in sediments that have not been intensively weathered or metamorphosed.

Various organic-walled microfossils chiefly of plant origin (spores and pollen) have been used to determine the thermal maturation history of sedimentary sequences: e.g., Gutjahr (1966), Staplin (1969), Correia (1969, 1970), Wilson (1971), Cramer and Diez (1975, 1976). Gutjahr (1966) showed that carbonization (i.e., light absorption, coalification, and blackening) of simple smooth-walled spores and pollen, and coal rank, are interdependent parameters. However, it must be noted that the short KOH treatment involved in his sample preparation introduced an unnecessary variable in his measurements, sufficient to cause selective loss of organic-matter particles and partial loss of colour in the microfossils.

Correia (1969, 1979), Staplin (1969) and Evans and Staplin (1971) have used colour changes in amorphous kerogen and palynomorphs as thermal indicators. The studies of Correia are perhaps the most detailed systematic investigations of the factors that affect kerogen and palynomorph colour changes. In terms of material best suited for use as organic thermal indicators, Correia (1969, 1970) favoured amorphous kerogen followed, in order of preference, by miospores, acritarchs, chitinozoans and wood-tissue fragments. Correia's sample processing technique, however entailed the use of HNO₃, an oxidant, and whereas Evans and Staplin (1971) did not fully document their sample preparation techniques, they did advocate that oxidative techniques be avoided. Hence, the colour changes observed by Correia may not be directly comparable with those reported by other workers.

Thermal alteration of organic microfossils follows a sequence that is essentially the same for all other organic material contained in sedimentary sequences. They exhibit diagenetic evolution by becoming progressively more opaque, and the colour darkens, passing from colourless through yellow, orange, brown to black; finally, the microfossils are completely destroyed. Correia (1970) observed that this colorimetric evolution manifests itself differently on different parts of the microfossil; hence, the appendages or membranous folds become brown more quickly than smooth areas. Palynomorphs are excellent thermal indicators in the low-temperature range of organic diagenesis that corresponds to the thermal window for oil generation (60-150°C).

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Groups of conodonts illustrating Conodont Colour Alteration Index (CAI) 1.5, from different stratigraphic levels and localities in southwestern Ontario. All magnification ( 100). Panderodus sp. (CAI 1.5, Pratt Ferry Fm., Alabama) is shown in all photographs to ensure standard tonality.

Fig. A. CAI 1.5: Middle Devonian Ipperwash Formation, Ipperwash, Ontario. Sample No. I-1 (8083).

Fig. B. CAI 1.5: Middle Devonian Dundee Formation. St. Mary's, Ontario. Sample No. DU-2 (8076).

Fig. C. CAI 1.5: Middle Silurian Goat Island Formation, Hwy. 403 roadcut, Hamilton, Ontario. Sample No. GO-1 (8071).

Fig. D. CAI 1.5: Middle Silurian Gasport Formation, Hwy. 403 roadcut, Hamilton, Ontario. Sample No. GA-1 (8069).

Fig. E. CAI 1.5: Lower Silurian Reynales Formation, Hwy. 403 roadcut, Hamilton, Ontario. Sample No. RE-1 (8064).

Fig. F. CAI 1.5: Upper Ordovician Georgian Bay Formation, Meaford, Ontario. Sample No. EMC-11 (5626).

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Groups of conodonts illustrating different values of the Conodont Colour Alteration Index (CAI). Conodonts illustrated are from different stratigraphic levels and localities in Paleozoic outliers and areas in eastern Ontario. All magnification ( 100). Panderodus sp. (CAI 1.5, Pratt Ferry Fm, Alabama) is shown in all photographs to ensure standard tonality.

Fig. A. CAI 1.5: Middle Ordovician Lowville Formation, Little Manitou Island, Lake Nipissing. Sample No. LM-4.

Fig. B. CAI 2: Middle Ordovician Black River Group, Eganville, Ontario. Sample No. EG-3.

Fig. C. CAI 2.5: Middle Ordovician Black River Group, Ottawa. Sample No. Section 3, #1.

Fig. D. CAI 3: Middle Ordovician Black River Group, Douglas, Ontario. Sample No. DG-4.

Fig. E. CAI 4-5: Lower Ordovician Beauharnois Formation (Huntingdon Mbr.), Caughnawaga, Quebec. Sample No. ES-14.

Fig. F. CAI 4-5: Lower Ordovician Beauharnois Formation (Huntington Mbr.), St. Jean County, Quebec. Sample No. 3Q.

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In southwestern Ontario, acritarchs provided an excellent alternative organic maturation index to that based on conodonts. Their colour change is similar to that observed in some spores and pollen, but the degree of colour alteration is different from that of pollen and spores and therefore they require separate thermal calibration. In this study a single acritarch genus, Leiosphaeridia Eisenack 1958 emend. Downie and Sarjeant 1963, was chosen for study primarily because of the relatively large size (40-200 µm) of the specimens, the relative structural simplicity, and the fact that this genus is fairly common in the Ordovician to Devonian units (, ). Furthermore, the specimens selected were: a) larger than 40 µm and unornamented, b) not pyritized, and c) the least altered among the floral assemblages present.

Of the twenty documented species of leiospheres (Eisenack et al., 1979), only four were selected for actual colour identification: L. baltica Eisenack 1958, L. fastigatirugosa (Staplin) Downie and Sarjeant 1964, L. major (Staplin) Downie and Sarjeant 1964, and L. wenlockia Downie 1959. The morphology of these species is relatively simple. They are spherical, but many appear disc-shaped because of compression during burial, and have no ornamentation. The size and wall thickness vary among these species: L. fastigatirugosa is thin-walled, averaging 120-150 µm in diameter. L. baltica is also thin-walled (3-8 µm) with an average diameter of 80-140 µm. L. wenlockia ranges from 20 to 50 µm in diameter, with wall thickness of about 1-3 µm, whereas L. major is relatively thin-walled with an average diameter from 55 to 85 µm (Eisenack et al., 1979). Their relatively large size as compared with most other acritarch genera provides larger surface areas over which reliable colour observations can be made.

No published experimental studies have been undertaken to test the validity of the colour alteration of acritarchs for use in thermal maturation studies. Ideally, thermal maturation studies using acritarchs require calibration of the colour index against either another known organic indicator (usually kerogen) which in turn may be correlated with quantitative data derived from vitrinite reflectance, or artificially obtained kerogen colours (through pyrolysis) which in turn can be correlated with an independent vitrinite reflectance scale.

In this study, however, the general lack of vitrinite in the samples precluded the use of this quantitative technique as an aid in calibrating a suitable thermal alteration scale compatible with the observed changes in the acritarchs. Instead, the acritarch colour changes were calibrated against the conodont colour alteration at a specific CAI and temperature range. Two problems are inherent in this approach. First, burial temperatures in the CAI range 1.5-2 overlap considerably, and the limitations imposed by the Arrhenius plot for CAI 1.5 make it difficult to obtain reliable temperature estimates for this CAI. As the data were from strata within the CAI range of 1.5-2, estimating a burial temperature range for the acritarchs is difficult. Second, the number and distribution of samples used in the architarch investigation also limit regional interpretations of the data. A more serious problem, however, lies in calibrating a suitable temperature and thermal index scale compatible with each of the observed colour changes in the acritarchs.

The solution to the first problem of erecting a temperature scale of the acritarchs (leiospheres) lies in first empirically documenting their colour changes (), which are interpreted to represent different thermal levels. The presence of degraded leiospheres in Ordovician strata, that are known to have attained burial temperatures in the range of 50-90°C (CAI 2-2.5), suggests that these acritarchs do not survive much higher temperatures and are gradually destroyed (). This observation allows the establishment of a temperature range using 90°C as a maximum thermal limit.

A greater problem is to calibrate a suitable thermal alteration scale compatible with leiosphere colour changes, and to provide estimates of the paleotemperature range for each of the observed acritarch colour changes (-). However, by using concepts expressed in previous publications of thermal alteration of palynomorphs (Staplin, 1969), "hystrichospheres" (Correia, 1969, 1970) and kerogen (Burgess, 1974) together with observations of the colour of kerogen in the samples (, , ), and by working within the

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framework established through previous organic maturation studies in the adjacent Appalachian Basin (Epstein et al., 1977), the relative colour changes observed in the leiospheres were calibrated against a thermal index scale of 1-5.

In constructing a suitable scale for colour change in leiospheres, it was necessary to ensure some degree of consistency in the standardization and indexing procedures. Representative specimens were photographed on a Leitz Orthoplan microscope equipped with an automatic and adjustable stabilizing transformer and quartz-iodide lamp, and using Kodak Ektachrome (ET-160) film.

The lack of experimental data for the leiospheres precludes direct estimates in the paleotemperature range associated with each alteration index. It is possible, however, to derive minimum paleotemperature estimates indirectly through visual examination of the kerogen colour associated with leiospheres of different values of thermal alteration (). Thus, a minimum threshold temperature of 60°C was established for the youngest Paleozoic strata in southwestern Ontario.

Although the inferred 60°C minimum was not derived from experimental studies of leiospheres, it is not an unreasonable estimate for several reasons. First, it is well within the predicted (50-90°C) temperature range given by Epstein et al. (1977) for Paleozoic strata in the southwestern Ontario portion of the Appalachian Basin. Second, available data on the distribution of hydrocarbons in southwestern Ontario, and information from previous studies regarding temperatures attained in Paleozoic rocks (Pusey, 1973; Deroo et al., 1977), indicate that minimum temperature estimates of 60-68°C are reliable. Pusey (1973), on the basis of several studies, listed a minimum temperature of 65°C for the liquid hydrocarbon window in Paleozoic sequences, and Deroo et al. (1977) showed that 88 per cent of the initial oil deposits of Alberta were generated at paleotemperatures ranging rom 68° to 116°C.

The initial transition from colourless to light yellow in the leiospheres, not apparent in samples from southwestern Ontario and not indicated in , indicates minimum burial temperatures, probably less than 60°C, and represents acritarch colour alteration index 1. This index is referred to as AAI herein. AAI 2 corresponds to the transition from light yellow to pale yellow, and the change from pale yellow to orange has been indexed at 3. The colour change from orange to dark brown was indexed at 4, and it is this alteration zone that lies within the upper limits of the temperature range derived from CAI 2-2.5. At higher burial temperatures, corresponding to the maximum temperature defined by CAI 2-2.5 (90°C) observed in a number of Middle Ordovician localities in southwestern Ontario, leiospheres are generally absent in the samples and it is uncertain whether their absence is due to thermal destruction or strong facies control documented by Jacobsen (1979).

In spite of the emphasis on the usefulness of organic-walled microfossils as thermal indicators (Gutjahr, 1966; Staplin, 1969), few detailed studies have been attempted. This is not surprising, as the methodology involved is time-consuming and requires careful observation and standardization. Also, there has been some scepticism concerning the potential use of acritarchs as indicators of thermal diagenesis (e.g., Gunther, 1976).

This method provides only a crude evaluation of the burial temperatures, because choosing the species of acritarch and its colour involves some subjectivity. It has merit in that reworked material can be selectively eliminated. By confining observations of colour change to a single genus, it was hoped that most of the discrepancies in colour changes that can be attributed to the differing size, ornamentation and wall thicknesses of different genera could be eliminated. This restriction does not, however, eliminate the variation in colour changes that occurs even at the species level within a single genus.

In this study, general observation of selected species of Leiosphaeridia indicates that size and wall thickness and stage of alteration determine the extent of coloration of the leiospheres. Thin-walled species (e.g., L. fastigatirugosa) are consistently lighter in colour than thicker-walled species (L. wenlockia) in the same thermal alteration zone. Furthermore, at any stage

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Acritarchs illustrating different values of the Acritarch Colour Alteration Index from different stratigraphic levels in southwestern Ontario. Magnification ( 100).

Fig. A. Alteration Index 2. Leiosphaeridia major Staplin 1961, Devonian Ipperwash Formation, Ipperwash, Ontario. Sample No. I1-8083D.

Fig. B. Alteration Index 2. Leiosphaeridia wenlockia Downie 1959. Middle Devonian Widder Formation, Arkona, Ontario. Sample No. W1-8081D.

Fig. C. Alteration Index 2. Leiosphaeridia fastigatirugosa Staplin 1961. Lower Middle Devonian Bois Blanc Formation, Innerkip, Ontario. Sample No. BB1-8073.

Fig. D. Alteration Index 3. Leiosphaeridia major Staplin 1961. Middle Silurian Goat Island Formation, Hwy. 403 Roadcut, Hamilton, Ontario. Sample No. GO1-8D.

Fig. E. Alteration Index 3. Leiosphaeridia major Staplin 1961. Middle Silurian Gasport Formation, Hwy. 403 Roadcut, Hamilton, Ontario. Sample No. GA1-8069B.

Fig. F. Alteration Index 3. Leiosphaeridia major Staplin 1961, Middle Silurian Rochester Formation, Hwy. 403 Roadcut, Hamilton, Ontario. Sample No. RO1-8068D.

Fig. G. Alteration Index 4. Leiosphaeridia baltica Eisenack 1958. Middle Ordovician Trenton Group. Sample No. PEW13-9468B (40 m).

Fig. H. Alteration Index 4. Leiosphaeridia sp. Middle Ordovician Trenton Group. Sample No. WT2-9458A (27 m).

Fig. I. Alteration Index 4. Leiosphaeridia sp. Middle Ordovician Trenton Group. Sample No. LI 2 9444 B (35 m).

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Acritarchs illustrating different values of the Acritarch Colour Alteration Index from different stratigraphic levels and the colour of amorphous organic matter (kerogen) in selected Silurian and Devonian strata.

Fig. A. Leiosphaeridia fastigatirugosa Staplin 1961, Middle Devonian Dundee Formation, St. Marys, Ontario. Sample No. IMP#68 9541C. (155 m).

Fig. B. Amorphous organic matter (yellow), Middle Devonian Dundee Formation, DU2-8077D.

Fig. C. Leiosphaeridia fastigatirugosa Staplin 1961, Middle Silurian Reynales Formation, Hwy. 403 Roadcut, Hamilton, Ontario. Sample No. 8064B.

Fig. D. Amorphous organic matter (reddish brown), Middle Silurian Reynales Formation, Hwy. 403 Roadcut, Hamilton, Ontario. Sample No. RE1-8064B.

Fig. E. Leiosphaeridia sp., Middle Ordovician Trenton Group. Sample No. LI-2 9442B.

Fig. F. Amorphous organic matter (brown), Middle Silurian Reynales Formation. Sample No. RE-1 8064B.

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of low-temperature organic metamorphism, and particularly at temperatures ranging from 60° to 90°C, leiospheres exhibit variations in colour that become less pronounced with increasing temperature.

In the thermal alteration of the leiospheres through organic diagenesis, important morphological structures become increasingly obscure, and pitting and fragmentation precedes eventual disintegration at temperatures greater than about 90°C. The loss of important morphological characteristics that accompanies the colour changes observed at higher temperatures decreases the likelihood of making accurate taxonomic identification of the selected species of Leiosphaeridia in the upper regime of the low-temperature field for organic diagenesis, corresponding to CAI 2-2.5.

REGIONAL COLOUR AND THERMAL ALTERATION TRENDS

Regional outcrop and subsurface stratigraphic variations in conodont colour for most of the Paleozoic sedimentary sequence in southern Ontario were determined from CAI indices (, in Appendix 2). For southwestern Ontario, the regional trend suggested by the CAI of conodonts was integrated with the AAI exhibited by the leiospheres to evaluate thermal alteration trends in the basin. For eastern Ontario and southern Quebec, only conodont data are available. The colour alteration data allow the construction of thermal alteration maps for southwestern Ontario () and eastern Ontario and southern Quebec ().

Southwestern Ontario

CAI 1.5 outlines a broad area in southwestern Ontario that includes all Silurian and Devonian strata (-), together with Middle (Trenton) and Upper Ordovician strata (), and extends eastward over the Algonquin Arch (). The considerable overlap of temperatures in the conodont CAI range 1-2 (Epstein et al., 1977) serves to emphasize the difficulty in estimating paleotemperatures within this CAI range. Hence, acritarch coloration becomes extremely useful to subdivide the broad thermal alteration zone defined between CAI 1.5-2.

The leiosphere colour changes range from light yellow in Middle Devonian strata, increasing to dark yellow in basal Middle Devonian and to orange in Lower and Middle Silurian strata. Upper and Middle Ordovician leiospheres generally appear orange to dark brown. These changes are important, since they are unique for these microfossils within the temperature range suggested by the conodont CAI data. In addition, the specimens generally are well preserved, with no evidence of mechanical deformation except that normally attained through burial and compaction. The minor amount of pyrite limits the role of pyrite oxidation in contributing to discoloration of the leiosphere.

The leiosphere data indicate the existence of three distinct thermal alteration zones in the Middle Devonian through Middle Ordovician stratigraphic interval in the temperature range 60-90°C (). The first zone corresponds with the light yellow to pale yellow colour changes (AAI 2, -), and includes most Middle Devonian formations. Although the data are sparse from the number of wells sampled, this zone apparently persists to depths of about 220 m below the surface. The second zone represents the pale yellow to orange colour changes (AAI 3, -) and includes basal Middle Devonian and Lower and Middle Silurian strata to depths ranging from more than 520 m in wells from the extreme southwest of the peninsula, where the strata attain maximum thickness, to about 100 m in the central part and 90 m in the east. The third zone represents the orange to dark brown colour change (AAI 4, -), and includes the upper part of the Middle Ordovician succession. Attempts to establish the continuity of the colour sequence were hampered by the paucity of leiospheres in the underlying Middle Ordovician strata that lie within the CAI range 2-2.5; hence AAI 5, corresponding to black and degraded leiosphere, is not represented.

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This paucity of leiospheres in Middle Ordovician strata in the St. Lawrence Lowlands and New York State has been attributed to facies control (Jacobsen, 1979). Although the susceptibility of acritarchs (leiospheres) to environmental control has been documented (Staplin, 1969), the fact that the leiospheres are present in rocks of similar facies in Kentucky and Ohio (Jacobsen, 1979), but absent in equivalent Middle Ordovician strata in New York and southern Ontario, indicates the possibility that their distribution results from different degrees of thermal alteration rather than facies control (Harris et al., 1978, Map 1-917-E, #1).

The general thermal alteration trend in the subsurface, for any particular borehole, is compatible with colour alteration with increased depth of burial. On a regional scale,

Silurian and Devonian sample localities in southern Ontario, with superimposed acritarch alteration indices (AAI).

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Ordovician thermal alteration map based on conodont alteration indices (CAI).

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however, the position of a particular thermal boundary varies because of the occurrence of higher alteration indices eastward along outcrops or at shallower depths within the same stratigraphic unit, as demonstrated by the acritarch alteration indices of . This pattern of thermal alteration is important, in that it contradicts that expected for the area on the basis of the present distribution and thickness of Paleozoic strata.

Eastern Ontario and Southern Quebec

Conodonts provided the basis for paleothermal reconstruction in eastern Ontario, and for the outliers in northern Ontario and adjacent parts of Quebec (). Ordovician outliers in northern Ontario were indexed at 1.5 () whereas outliers in adjacent parts of Quebec and along the western limit of the region were indexed at 2 (). However, one locality at Douglas on the Bonnechere River () was indexed at 3, possibly close to a fault plane. Several locations along the Ottawa-Bonnechere graben have CAI values ranging from 2.5-3. In the Ottawa-Hull region and most outlying areas to the south, the conodonts were indexed at 2.5 (). East of Ottawa, Black River strata have been indexed at 3 for over 300 km along the Ottawa Valley. CAI values increase to 4-5 in the vicinity of Montreal (-).

Knowledge of the conodont CAI, together with estimates of the duration of burial for portions of Paleozoic sedimentary sequence from existing geological data, permits the calculation of burial temperature from the Arrhenius plots of Epstein et al. (1977). These plots of the reciprocal absolute temperature, calibrated to degrees Celsius along the X-axis and logarithmic time along the Y-axis, illustrate the relationship between temperature, duration of burial, and the CAI (see ). The duration of burial for specific stratigraphic

Arrhenius plots of the experimental data of Epstein et al. (1977) and maximum and minimum temperature ranges of conodont samples in southern Ontario and southern Quebec.

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intervals was taken as the difference between the time (in Ma) of initial burial of the sequence and the maximum and minimum times for unloading, determined from the regional geology of the Michigan and Appalachian basins. Minimum duration of burial gives maximum temperature estimates, and maximum duration of burial yields minimum temperature estimates.

In eastern Ontario, Lower and Middle (Black River) Ordovician conodonts would have a maximum duration of burial of 440-480 Ma. The area, however, clearly has been uplifted and Poole et al. (1970) indicated that the latest possible time for unloading of these rocks is Late Mississippian or Early Pennsylvanian (280 Ma), since by this time continental conditions were established across eastern Canada. Crough (1981) argued that the Ontario - New England region was uplifted and eroded in Early Triassic - Late Jurassic time (225 Ma). Estimates of duration of burial of 160-215 Ma for Middle Ordovician and 200-255 Ma for Lower Ordovician strata are consistent with these observations.

Burial temperatures for Middle Ordovician localities in the CAI range 2-2.5, estimated by projecting the 2.-2.5 field segment of the 215 Ma line to the X-axis (), range from 50° to 85°C; for 160 Ma of burial the temperature range is 55-90°C. The temperature range for this CAI range is 50-90°C. The burial temperatures estimated for the CAI 2.5-3 over a duration of 160 Ma range from 90° to 120°C and from 85° to 115°C for a duration of burial corresponding to 215 Ma. In the vicinity of Montreal, where CAI values in Lower Ordovician strata range from 4 to 5, the estimated temperature range is 190-300°C for 200 Ma of burial and heating, whereas for 255 Ma of heating the temperature range of organic metamorphism is 185-295°C. Thus, in the Montreal area alone, the inferred paleotemperature range is 185-300°C ().

GEOTHERMAL HISTORY

The inferred minimum burial temperature of 60°C for Middle Devonian strata correlates well with the low level of organic metamorphism observed in these rocks, indicating that the rocks were subjected to temperatures at the lowest or threshold limit of the liquid window of Pusey (1973). The maximum temperature of 90°C is calibrated against the maximum temperature estimates for CAI 2.5 (85-90°C), and is justified by the fact that disintegration of the acritarchs apparently occurs in this temperature range () within a matrix of organic matter that appears dark brown.

The close correlation of the temperature ranges, derived from conodonts and acritarchs throughout the area west of the Frontenac Axis and in Paleozoic outliers to the north, implies the existence of a low, relatively constant, long-lived geothermal gradient in southern Ontario during Paleozoic and most of Mesozoic time, that was later interrupted by higher heat flow in areas east of the Frontenac Axis. This higher heat-flow event is interpreted as being related to local intensive faulting and intrusive igneous activity during Cretaceous and Early Tertiary time. The anomalous paleothermal values of the Ottawa-Bonnechere graben and those in the vicinity of Montreal appear to reflect areas of high heat flow related to major rifting and contemporaneous emplacement of the Monteregian intrusives, as briefly discussed below.

BURIAL HISTORY

Approach

Burial depths of portions of the Paleozoic sedimentary sequence of southern Ontario and Quebec were evaluated through their relationship with probable geothermal gradients and the thermal maturation indices. Assessment of overburden thickness from the inferred paleotemperatures requires the use of assumed geothermal gradients and estimates of the average surface temperature at the time of deposition. Rather than use an average geothermal gradient for the Paleozoic section, a series of curves representing gradients from 20° to 50°C/km was constructed, with an assumed average surface temperature of 20°C (). The

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range of geothermal gradients selected facilitates comparison of different temperature - burial depth relationships. This approach can be applied to southern Ontario for differing thermal regimes, as the evidence from thermal alteration suggests that eastern Ontario and adjacent parts of Quebec have experienced locally higher heat flow during the Cretaceous

Temperature - burial depth relationships for southern Ontario and southern Quebec.

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and Early Tertiary, related to rifting and intrusive igneous activity. The choice of an average surface temperature of 20°C is supported by the paleocontinental reconstructions of Ziegler et al. (1977) and Morel and Irving (1978), indicating that Ontario lay within the southern tropics during Silurian and Devonian time.

As the organic thermal indicators record the highest temperature attained by the sedimentary sequence, the burial depths estimated from these paleotemperatures have inherent limitations, for they do not allow for variations in the geothermal gradient over time. For example, if maximum heating of the Paleozoic sedimentary cover in eastern Ontario occurred during a Late Cretaceous - Early Tertiary thermal event related to continental rifting, the burial temperatures inferred are maximum values that occurred at higher geothermal gradients operative in the sedimentary cover only during this relatively brief interval. If, as seems likely, burial temperatures were lower during Paleozoic and most of Mesozoic time, then a combination of lower thermal gradient and lower burial temperatures can be used to evaluate the thickness of sedimentary section during the period of low heat flow. As most of the heat flow in the southern Ontario region throughout Paleozoic and Early Mesozoic time is likely to have been of geothermal origin, overburden estimates can be based on continuous burial and assumed geothermal gradients of 20-30°C/km. According to Klemme (1975), a geothermal gradient of 20-25°C/km is normal for cratonic basins.

Southwestern Ontario

Using the minimum paleotemperature estimate of 60°C for the top of the Middle Devonian interval in southwestern Ontario, indicates that the top of the Middle Devonian interval, now at or close to the surface, formerly reached burial depths of from 1300 to 2000 m for geothermal gradients ranging from 20° to 30°C/km. The depth of burial to the base of the Silurian, where the minimum paleotemperature estimate is about 80°C, ranges from 2000 to 3000 m for geothermal gradients of 20-30°C/km. The depth to the top of the Middle Ordovician (Black River) strata in southwestern Ontario with a minimum paleotemperature of 90°C must likewise have reached at least from 2400 to 3500 m, using 20-30°C/km geothermal gradients. Shallower burial at the same minimum paleotemperatures yields higher geothermal gradients; higher paleotemperatures would yield higher geothermal gradients, especially where the burial history requires an unreasonable thickness of sediments. Although it is instructive to make comparisons of this nature as a convenient means of visualizing temperature-depth relationships, it must be remembered that the geothermal gradients, represented as straight lines, probably were more complex through geological time. One main point emerges, however: the Paleozoic strata of southwestern Ontario formerly were more deeply buried than they are at present, on the basis of the reasonable assumptions made with respect to past geothermal gradients operative in the area.

The broad regional extent of Paleozoic strata with CAI below 2 () shows that the Algonquin Arch has had no major control on the thermal history of the sequence. The thermal and burial history was controlled by large-scale Paleozoic sedimentation patterns, dominated by progradational, thick, clastic wedges from an eastern source area that increased the burial depth of the little-deformed platform sequence in southwestern Ontario.

What can be established regarding the former thickness of post - Middle Devonian strata in southwestern Ontario? Reconstruction of the original configuration of the sedimentary cover in southwestern Ontario is hampered by the virtual absence of post - Upper Devonian strata in the region. Nevertheless, the continuity of the Paleozoic section in the Michigan and Appalachian basins, and the information accumulated from organic maturation studies, allow a more systematic evaluation of the overburden thickness. In addition, paleogeographic concepts related to Mississippian-Pennsylvanian stratigraphy and sedimentation patterns in Michigan (Ells, 1979), Pennsylvania and New York (Edmonds et al., 1979), Ohio (Collins, 1979), and the adjacent areas of Illinois (Atherton and Palmer, 1979), and Indiana (Gray, 1979), also help to estimate the former thickness of post - Middle Devonian strata in southwestern Ontario.

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Upper Devonian strata are thin (60 m) and cover a small part of southwestern Ontario. They are well represented in northwestern Michigan, where they are about 330 m thick. In the Appalachian Basin, Upper Devonian strata have been mapped by Rickard (1964).

In the Michigan Basin, the subsurface thickness of Mississippian-Pennsylvanian strata is about 1100 m. Mississippian rocks of marine origin and subsurface data indicate a major erosional unconformity with a relief of from ten to hundreds of metres separating Pennsylvanian strata of largely terrestrial origin (Ells, 1979). In Ohio, Mississippian strata are predominantly clastic and formed a northwestward-thickening wedge that is 300 m thick in the subsurface; however, the wedge is of limited lateral extent and is absent in the northern part of the state adjacent to the study area. Likewise, Pennsylvanian strata averaging 300 m thick in the subsurface are restricted to the southeast. In Pennsylvania and New York, Mississippian and Pennsylvanian strata averaging 300 m thick in the subsurface are restricted to the southeast. Also in Pennsylvania and New York, these strata are dominantly clastics and range from 700 to 3200 m in southwestern Pennsylvania (Edmonds et al., 1979).

These observations indicate that the thickness of Mississippian strata can be extrapolated reasonably well across the intervening area between the Appalachian and Michigan basins. The existence of Lower Mississippian strata in extreme southwestern Ontario also indicates a definite extension of these strata into the region. From the thicknesses of Mississippian and Pennsylvania strata in New York, at least 500-700 m of post - Devonian strata can be accounted for in southwestern Ontario, but it is unlikely that the total thickness of post-Devonian strata exceeded 1000 m. Post-Devonian strata, however, probably had limited areal extent over southwestern Ontario because of Late Mississippian uplift and erosion. This uplift, that affected northwestern Pennsylvania and adjacent New York, must have been responsible for significant removal of sediments over southwestern Ontario.

The inferred thickness of post-Devonian sedimentary cover, calculated by assuming a subsurface temperature of 60°C, ranges from 1300 to 2000 m under low geothermal gradients (20-30°C/km) or 800 to 1200 m assuming higher geothermal gradients (35-50°C/km) (). The range of burial depths estimated from the temperature-depth plots appears significantly greater than the burial depths estimated from paleogeographic reconstruction (only 500-700 m).

The fact that these discrepancies exist is significant. Burial depths of less than 1000 m for the southwestern Ontario region would demand unreasonably high geothermal gradients (45-50°C/km) to attain temperatures of 60°C. Even at a lower subsurface temperature (50°C), the geothermal gradient would have to have been in the range of 35-50°C/km for burial depths of less than 1000 m. Since there is no evidence to support the existence of such high geothermal gradients in the Paleozoic strata in southwestern Ontario, the top of the Paleozoic sedimentary sequence there must have been more deeply buried, possibly under a Mesozoic nonmarine sequence, since removed.

Eastern Ontario and Southern Quebec

Estimating burial depth of the Paleozoic section in eastern Ontario and southern Quebec is more difficult than for southwestern Ontario. Less of the Paleozoic sequence is preserved, and the area probably underwent greater burial during the Paleozoic, related to the Taconic and Acadian orogenic events and their clastic wedges. Furthermore, paleotemperatures inferred from the thermal alteration studies are related to localized thermal factors within the platform. The drastic increase in burial temperature from 90°C in eastern Ontario to 185-300°C in southern Quebec over a distance of 600 km indicates the presence of above-normal geothermal gradients in areas east of the Frontenac Axis, especially along the Ottawa-Bonnechere graben and in the vicinity of Montreal.

The pattern of CAI isograds along the Ottawa-Bonnechere graben () indicates anomalously high heat flow in this area. Although the concordance of the CAI 2 isograd with overburden isopach trends cannot be established in the absence of post-Ordovician isopachs for the area, the eastward increase in the CAI to 2.5 in localities south of the graben may

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partly reflect the composite effect of the eastward thickening of Upper Ordovician, Silurian and Devonian clastic wedges now mainly removed by erosion. Although the preponderance of faults in the region have substantial vertical displacements, it is only along the main downfaulted block that the magnitude of vertical displacement (420 m) could be significant enough to affect the conodont colour. The increase in the CAI along the graben is probably a reflection of the high heat flow that occurred after the main burial phase of alteration was established (i.e., in Late Cretaceous - Early Tertiary time). This latter contention is supported by the fact that the general range of CAI values observed in these areas is higher than that indicated by Harris et al. (1978). Although the eastern Ontario area was not extensively sampled in the investigations of Epstein et al. (1977), Harris et al. (1978) and Harris (1979), the general trend of the CAI values found by Harris et al. (1978) for this region is in the range of 2.5-3. The values obtained in this study, however, indicate a higher range (2.5-5). The CAI 3 isograd protrudes from localities south of Montreal and along the Ottawa-Bonnechere graben as far west as Ottawa. Furthermore, in the vicinity of Montreal the high values of CAI 4 and CAI 5 correlate well with known areas of Cretaceous igneous activity (i.e., Monteregian Hills).

Sikander and Pittion (1978) conducted reflectance studies on organic matter in the St. Lawrence Lowlands. Among their results, they showed that Beekmantown carbonates (Ordovician) attained mature to postmature levels of organic epimetamorphism. Furthermore, of the six wells studied from the Lowlands, Well 1 near Montreal showed high reflectance values (Ro of 2-3%) due to the proximity of the Monteregian intrusives.

In the absence of overburden isopach maps for the eastern Ontario region, useful estimates of the thickness of post-Trenton strata in the area can be derived from a series of papers by Clark (1972), Hoffman (1972), Clark and Globensky (1975, 1976, 1977) and Globensky (1978) on the regional geology of adjacent Paleozoic localities around Montreal and the St. Lawrence Lowlands of Quebec. In the vicinity of Montreal, Cretaceous igneous intrusions provide an indirect but useful approximation of the overburden thickness in the area. Investigation of the minimum and maximum thickness of the Paleozoic sedimentary cover intruded by the Mount Royal Pluton (Clark, 1972, table 21, p. 120) has been estimated at between 1230 m and 2900 m. This maximum figure does not include Silurian strata that may have been present, nor the thickness of Devonian limestone, remnants of which are preserved in volcanic pipes on St. Helens Island (Clark, 1972), and which has been estimated at over 150 m thick.

Estimates of the thickness of Silurian strata removed in the region are speculative. Twenhofel (1928) reported 370 m of Silurian strata on Anticosti Island, whereas over 915 m have been recorded by Sikander and Pittion (1978) on the Gaspe Peninsula. In southwestern Ontario between 450 and 600 m of Silurian strata are present (Sanford, 1969). With the presence of Silurian strata in the Lake Timiskaming outlier, in northern Ontario - Quebec, there seems little doubt that most of the southern Canadian Shield probably was covered by Silurian strata, possibly about 400 m in thickness.

By using the minimum estimate of approximately 1230 m given by Clark (1972) for the Montreal area, an additional minimum of 500 m can be included that would represent the cumulative thickness of Silurian and Devonian strata, yielding a total of over 1700 m of overburden removed from the region.

If credence is to be placed on attributing the higher CAI indices along the graben entirely to higher heat flow related to Cretaceous rifting and intrusion, then the vertical movement along the faulted zone alone should compensate for the observed temperature increase reflected by the CAI 3 isograd. The burial depth indicated by CAI 3 (120°C) is between 3300 and 5000 m for geothermal gradients between 20° and 30°C/km, and 2000 and 2900 m for gradients from 35° to 50°C/km.

If the higher indices along the graben are due entirely to burial with no increased or anomalous heat flow, however, the vertical movement along the faulted zones should

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compensate for the observed increase reflected by the CAI 3 isograd. Assuming an initially low geothermal gradient (25°C/km) for this area (Epstein et al., 1977) and a minimum overburden estimate of 2800 m for CAI 2.5, the amount of vertical displacement along the graben needed to generate temperatures compatible with CAI 3 is 1200 m. The maximum vertical displacement reported along the graben is, however, only 420 m (Wilson, 1964). Therefore, burial alone with no heat influx does not appear to have been responsible for the higher CAI values along the graben. Similarly, in the vicinity of Montreal, where maximum burial temperatures were in the range of 185-300°C, an overburden thickness of 4700 to 8000 m is required, with geothermal gradients of 20-30°C/km. These estimates can be offset only by higher temperatures. Therefore, a combination of burial and localized higher heat flow during the Cretaceous - Early Tertiary seems the most plausible explanation for the observed CAI trend in eastern Ontario and southern Quebec.

STRUCTURAL AND THERMAL EVOLUTION OF THE SOUTHERN ONTARIO SEDIMENTARY BASIN, AND HOTSPOT DEVELOPMENT

Information discussed above on the thermal and burial history of the Paleozoic sedimentary sequence in southern Ontario provides the opportunity to examine, in detail, the various hypotheses that have been proposed to elucidate the structural and thermal evolution of the basin. Most relevant are the studies by Kumarapeli and Saull (1966) for the St. Lawrence Valley and by Epstein et al. (1977) and Harris et al. (1978) for the Appalachian Basin, as well as the recent studies related to Mesozoic hotspot epeirogeny in eastern North America (Crough, 1981).

Structural and seismic evidence (Kumarapeli and Saull, 1966) indicates that the eastern Ontario area, part of the more extensive St. Lawrence Valley rift system, is located in a region that experienced general doming, normal faulting and alkaline intrusive activity (Monteregian Hills) (). In the absence of precise dating on the timing of the doming and faulting, they suggested (op. cit.) that the igneous activity and the normal faulting were synchronous and genetically related to tectonic processes occasioned by the rifting of the North Atlantic Ocean during Cretaceous - Early Tertiary time. Recent investigation of hotspot phenomena in eastern North America (Crough, 1981) has provided considerable insight into the regional tectonics of the Ontario - New England region during Mesozoic time, and provided a useful framework for assessing the vertical motions of the craton in affecting the thermal history and subsidence-sedimentation relationships within the cratonic interior. In essence, Crough (1981) proposed domal uplift to remove up to 4 km of the Paleozoic sedimentary sequence as the North American continent moved northwestward over a mantle plume (Great Meteor Hotspot) in Jurassic-Cretaceous time. The important aspect of Crough's hypothesis is that doming, differential uplift and erosion that accompany passage of the continent over a mantle hotspot are controlled in the first degree by the thermal history of the underlying lithosphere. Behaviour of the lithosphere provides a major constraint on the pattern of sedimentation, uplift and erosion, controlling such variables as the maximum thickness, horizontal extent of sediment accumulation, stratigraphy and likelihood of evaporite formation.

Because the concept of hotspot epeirogeny makes specific predictions of the spatial distribution and timing of continental uplift (Crough, 1979, 1981), the data from conodont geothermometry can be used to test the validity of this concept. If differential uplift resulting from the hotspot swell controlled local continental elevation, thermal history and sedimentation patterns on the craton, and if the horizontal dimensions of the hotspot were of the magnitude proposed by Crough (1981), then the hotspot would have been the site of localized higher heat flow and it should be possible to trace the outline of such a feature by using organic thermal indicators. Whereas the conodont CAI data of Epstein et al. (1977) support the contention that the geothermal gradient in the Appalachian Basin never had substantial variation during the Paleozoic and most of the Mesozoic, the data from conodont geothermometry in this study indicate a local thermal event in eastern Ontario and in the vicinity of Montreal,

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where paleotemperature estimates range up to 185-300°C. This evidence supports the existence of a higher-than-normal geothermal gradient in the area, and is the basis of the interpretation that only the area east of the Frontenac Axis was affected by the hotspot swell.

The information most critical in interpreting the structural evolution of the eastern Ontario region is whether the St. Lawrence rift system (including the Ottawa-Bonnechere graben) was produced by the hotspot swell or by other processes occurring at the time of rifting (McHorne, 1981). Abundant evidence (Kinsman, 1975) suggests that rift valleys bounded by large marginal faults and a series of subparallel faults that locally extend beyond the rift itself (as is the case in eastern Ontario) are developed across domes activated by mantle plumes. It is quite conceivable that rifting along the swell may have triggered the initial rifting of the Ottawa-Bonnechere graben and the St. Lawrence system. However, the width of the increased CAI anomaly in the Ottawa Valley area is narrow () in contrast to the width of doming of 1200 km, as proposed by Crough (1981) along a hotspot track. It is possible that, after hotspot initiation in the Montreal area in the Cretaceous, the westerly drift of Canada resulted in the relative easterly shift of the hotspot centre through Vermont and into the newly forming Atlantic Ocean. There is no evidence, from the colour of the conodonts in collections from the Hudson Bay basin and the Lake Timiskaming outlier, that the hotspot was initiated earlier and had tracked toward the Montreal area from the northwest, as predicted by Crough (1981, ).

THERMAL MATURATION AND HYDROCARBON OCCURRENCE IN SOUTHERN ONTARIO

In an attempt to evaluate the significance of the thermal alteration data derived in this study to known hydrocarbon occurrences, the maturation indices were interpreted within the framework of organic metamorphism proposed by Heroux et al. (1979). correlates

Correlation of the thermal alteration indices, burial temperatures and stages of hydrocarbon generation, southern Ontario.

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the conodont CAI and acritarch AAI data with the inferred burial temperatures. In addition, the organic epimetamorphic facies were compared with the main stages of hydrocarbon generation.

Generally in southwestern Ontario, the distribution of the main oil and gas occurrences strongly reflects the regional organic maturation trends. The Paleozoic rocks of southern Ontario lie mainly within the thermal maturation facies associated with the main phase of liquid hydrocarbon generation (60-150°C), indicating that the region experienced ideal conditions for oil generation, given potential source rocks. However, the fact that exploration and development of the hydrocarbons is confined to southwestern Ontario is related primarily to the thickness of the Paleozoic sedimentary cover in the region. In eastern Ontario, the sedimentary cover is too thin and lacks suitable reservoirs.

Arcitarch alteration indices 2-4 outline the main areas of oil and gas production in southwestern Ontario. Burial temperature estimates (derived from CAI and AAI patterns) for the youngest strata enclosed by the alteration indices are less than 60°C, and define a zone of incipient oil generation. The liquid window conditions of Pusey (1973) do not appear to have been fully realized for Middle Devonian strata, and the occurrence of oil without substantial quantities of gas is probably due to Devonian gas having migrated out of the reservoirs, since they occur near the surface.

Middle and Lower Silurian strata lie within the liquid window of Pusey (1973) (). The relatively large volumes of natural gas obtained from the Silurian fields along the flanks of the Appalachian Basin is in keeping with the observations of Epstein et al. (1977), who verified that the entire Paleozoic section of the basin (and its extension in southwestern Ontario) has a potential for gas production. Furthermore, the gas production from these clastic rocks indicates that most of it was generated in deeper sections of the Appalachian Basin, possibly from the Upper Ordovician black shale facies.

Although Ordovician strata lack significant quantities of hydrocarbons, the Upper Ordovician black shale sequence is known to be locally bituminous with a high organic-matter content -- characteristics that are typical of source rocks. Paleotemperatures indicated for the Upper and Middle Ordovician (primarily Trenton) in southwestern Ontario are 60-90°C. The implication is that Ordovician and Cambrian oil was generated in deeper sections of the Appalachian Basin, where the strata are more organic-matter-rich, rather than locally derived from Ordovician source rocks.

East of the Frontenac Axis, Middle Ordovician localities in the CAI range 2.5-3 with estimated paleotemperatures from 90° to 120°C are well within the liquid window. However, the lack of significant traps, and thin sedimentary cover, have considerably diminished the region's oil-producing potential. Furthermore, the increase in thermal maturation eastward related to Cretaceous igneous activity has increased the level of organic maturation in these areas. In the vicinity of Montreal, where the burial temperatures range from 185° to 300°C, the Lower Ordovician carbonates are overmature.

CONCLUSIONS

On the basis of the data derived from the thermal alteration of conodonts and acritarchs, it is possible to make tentative conclusions on the thermal and burial history of the Paleozoic sedimentary sequence in southern Ontario, and the maturation history of the contained hydrocarbons.

1. Based on the conodont data, three facies of organic metamorphism have been recognized in southern Ontario. The first of the thermal alteration zones extends from the top of the Paleozoic sedimentary sequence to include Middle Ordovician (Trenton) strata. In this zone the conodont colour alteration index (CAI) is 1.5 and reflects burial temperatures of less than 60°C. Systematic refinement of the thermal facies boundaries within this CAI range is

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provided by the use of acritarchs. Within this zone, acritarchs (leiospheres) exhibit colour changes from light yellow to pale yellow through orange to brown in the direction of increasing temperature and depth of burial (AAI of 2-4). The broad spectrum of colour changes observed in the acritarchs indicates the likelihood that paleotemperatures exceeded the 60°C limit imposed by the conodont CAI data in the zone.

The second zone includes the remainder of the Ordovician section in southwestern Ontario and part of the Ottawa Valley in eastern Ontario. The CAI values for this zone lie in the range 2-2.5 and suggest burial temperatures of about 60-90°C. Superimposed on this broad-scale thermal alteration pattern that reflects burial depth are zones of higher alteration index (CAI 3) that extend along the Ottawa-Bonnechere graben and outlying areas immediately to the south. In these areas, the paleotemperatures attained by the Paleozoic sequence range from 90° to 120°C. Paleozoic strata in southern Quebec also have attained burial temperatures of 90-120°C. Acritarchs become degraded at these higher temperatures (AAI 5). Maximum CAI indices (4-5) that were observed in the vicinity of Montreal reflect higher paleotemperatures (185-300°C).

2. The depths of burial inferred from the paleotemperature estimates provided by the conodonts and palynomorphs indicate that for most of Paleozoic and Mesozoic time the sedimentary sequence in southwestern Ontario underwent minimal burial. The thermal alteration indices observed along the Ottawa Valley and in the Montreal area require a combination of deeper burial and higher temperature. This condition was effected by rifting related to the opening of the North Atlantic Ocean during Cretaceous - Early Tertiary time and, more especially, by high heat flow resulting from the Cretaceous alkaline intrusions (Monteregian Hills).

3. The extensive faulting that is observed in the eastern Ontario region may have been produced by rifting related to the initiation of a hotspot under the platform as indicated by Crough (1981). With westward migration of Canada in post - Middle Cretaceous time, the apparent site of this hotspot migrated eastward and is possibly seen today as the Great Meteor Hotspot in the North Atlantic.

Further research by colleagues on Ontario hydrocarbon deposits utilizing organic-geochemical and isotope studies is aimed at providing hydrocarbon deposit - source rock relationships by comparing petroleum and natural gas fractions with in situ - bitumen fractions and kerogen. Current carbon isotopic studies of natural gas will aid in determining a model for the generation of hydrocarbon deposits in southwestern Ontario that should prove useful in future exploration programs.