Thermal maturity and burial history of Paleozoic rocks in western ...

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Thermal maturity and burial history of Paleozoic rocks in western Newfoundland1 S. Henry Williams, Elliott T. Burden, and P.K. Mukhopadhyay

Abstract: Palynomorphs and graptolites from Paleozoic strata in western Newfoundland are examined and correlated with previously published data to identify fossils which are characteristic of proven and suspected source rocks. Measurements of colour alteration of acritarchs and spores (acritarch alteration index and thermal alteration index), random graptolite reflectance, and vitrinite reflectance are applied to determine regional thermal maturation and burial history. General trends of increasing maturity from south to north along the Northen Peninsula and from west to east across the Port au Port Peninsula are observed. Within these general trends, a more detailed distribution of thermal maturities can be recognized. In the south, Upper Ordovician rocks of the Long Point Group, western Port au Port Peninsula, exhibit the lowest maturity values found in western Newfoundland and are considered immature or marginally mature. Middle Ordovician rocks of the Goose Tickle and Table Head groups and the Lower Ordovician St. George Group are marginally mature. Cambrian strata on the Port au Port Peninsula are mature. Maturation levels increase to the east; Goose Tickle Group black shales in the vicinity of Black Cove, east of Port au Port, are mature. Equivalent sediments extending for another 15–20 km to the east lie within the oil window. Beyond that area, the equivalent rocks are overmature. The best potential source rocks belonging to the allochthonous Cow Head Group contain abundant acritarchs and Gloeocapsamorpha sp. These rocks are marginally mature to mature within Gros Morne National Park; maturation levels increase farther north (e.g., Parsons Pond), becoming overmature somewhere south of Port au Choix. It is concluded that neither the allochthonous Ordovician rocks presently exposed in Gros Morne nor the autochthonous strata exposed on the Port au Port Peninsula have ever been covered by significant thicknesses of overburden (probably 3 km or less), either in the form of structural slices or other sedimentary units since their original deposition. Résumé : Les palynomorphes et les graptolites des strates paléozoïques, dans la partie occidentale de Terre-Neuve, ont été examinés et corrélés avec les données déjà publiées pour identifier les fossiles qui sont caractéristiques des rochesmères prouvées ou suspectées. Les mesures de l’altération de la couleur des acritarches et des spores (indice d’altération des acritarches et indice d’altération thermique), et de la réflectance aléatoire des graptolites et du pouvoir réflecteur de la vitrinite ont servi à évaluer l’état de la maturation thermique et l’histoire de l’enfouissement. On observe comme tendance générale un accroissement de la maturité du sud au nord le long de la péninsule Northern, et de l’ouest à l’est à travers la péninsule de Port au Port. Il est possible de reconnaître à l’intérieur même de cette tendance générale une distribution plus détaillée des maturités thermiques. Au sud, dans les roches ordoviciennes du Groupe de Long Point, la partie occidentale de la péninsule de Port au Port présente les valeurs de maturité les plus faibles qui ont été trouvées dans l’ouest de Terre-Neuve, elles sont évaluées comme immatures ou marginalement matures. Les roches des groupes de Goose Tickle et de Table Head de l’Ordovicien moyen, et les roches du Groupe de St. George de l’Ordovicien inférieur, sont marginalement matures. Les strates cambriennes dans la péninsule de Port au Port sont matures. Il y a accroissement du niveau de maturation vers l’est ; les shales noirs du Groupe de Goose Tickle aux environs de Black Cove, à l’est de Port au Port, sont matures. Les sédiments équivalents qui s’étendent pour un autre 15 à 20 km à l’est, apparaissent à l’intérieur d’une fenêtre d’huile. Au delà de cette région, les roches équivalentes affichent une surmaturité. Les roches-mères qui appartiennent au Groupe de Cow Head allochtone offrant le meilleur potentiel économique contiennent en abondance des acritarches et Gloeocapsamorpha sp. Dans la parc national de Gros Morne les roches varient de marginalement matures à matures; plus au nord les niveaux de maturation augmentent (ex. Parsons Pond), avec une surmaturité quelque part au sud de Port au Choix. Nous concluons que, ni les roches allochtones ordoviciennes qui affleurent présentement à Gros Morne et ni les strates autochtones exposées dans la péninsule de Port au Port, furent recouvertes dans le passé par des unités lithologiques sus-jacentes avec épaisseur significative (3 km ou moins), sous forme d’écailles structurales ou autres unités sédimentaires depuis leur dépôt original. [Traduit par la Rédaction]

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Received October 8, 1997. Accepted April 24, 1998. S.H. Williams2 and E.T. Burden.2 Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, NF A1B 3X5, Canada. P.K. Mukhopadhyay. Global Geoenergy Research Ltd., P.O. Box, 9469, Station A, Halifax, NS B3K 5S3, Canada. 1 2

Lithoprobe Publication 1017. Corresponding authors: S.H. Williams (e-mail: [email protected]); E.T. Burden (e-mail: [email protected]).

Can. J. Earth Sci. 35: 1307–1322 (1998)

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(i) autochthonous sedimentary rocks (Labrador, Port au Port, St. George, Table Head, and Goose Tickle groups) which were formed in a relatively shallow, nearshore marine environment; and (ii) allochthonous units (the Humber Arm Supergroup, made up of the Cow Head and Curling groups) developed under relatively deep, oceanic conditions originally some distance to the southeast. The allochthonous Cambrian–Ordovician ophiolite complexes evolved as arcrelated ocean crust within the Iapetus Ocean and were tectonically thrust onto the platform. Both allochthonous and autochthonous strata are overlain unconformably by Middle–Upper Ordovician parautochthonous sediments (the Long Point Group) and Upper Silurian to Carboniferous sediments (the Clam Bank Formation of the Port au Port Peninsula and Anguille, Codroy, and Barachois groups in the nearby Deer Lake Basin and the Bay St. George Subbasin). From a plate-tectonic standpoint, these rocks record the development and destruction of a passive continental margin starting from its early beginnings as a rift sequence with eventual shelf development and final destruction in the Taconic, Acadian, and Alleghenian orogenies. The stratigraphic and structural regimes have important implications in the thermal history of the region. The Lower Paleozoic rocks are broadly equivalent to petroliferous strata in Quebec (Dykstra and Longman 1995), Michigan (Longman and Palmer 1987), and Ohio (Cole et al. 1987). Each of these areas (Fig. 3) represents a part of a significant structural and stratigraphic petroleum province characterized by Lower Paleozoic shelf formation and orogenic destruction. Unlike the older established petroleum regions farther west, hydrocarbon exploration in the Paleozoic strata of western Newfoundland is still in its infancy.

Western Newfoundland has been the focus of limited hydrocarbon exploration for over a century. Recently, its hydrocarbon potential has been reexamined in a number of studies (Sinclair 1990; Fowler et al. 1995) and the area is presently the target of both surface and subsurface activity by a number of Canadian and American companies. Large structures with closures and a multitude of clastic and carbonate plays may represent prospects for hundreds of millions of barrels of oil (Langdon and Hall 1994). Oil and gas shows have been known from natural seeps and boreholes in four regions of western Newfoundland for nearly 200 years (Fleming 1970; Government of Newfoundland and Labrador 1982, 1989), namely Parsons Pond and St. Pauls Inlet on the Northern Peninsula, the Port au Port region, the Deer Lake Basin, and the Bay St. George Subbasin (Fig. 1). The first two regions are associated with Lower Paleozoic rocks of both the Humber Arm Allochthon and the autochthonous Table Head Group (Cote 1962; Fleming 1970; Government of Newfoundland and Labrador 1989; Fowler et al. 1995). Wells in the latter two regions encountered significant gas shows in Carboniferous terrestrial deposits. In addition, Carboniferous oil shales occur in the upper part of the Rocky Brook Formation of the Deer Lake Basin (Hyde 1984; Macauley 1987a, 1987b) and the Barachois Group of the Bay St. George Subbasin (Solomon 1986). Potential hydrocarbon source beds have been recognized in western Newfoundland (Weaver and Macko 1987; Fowler et al. 1995), indicating the possibility of economic liquid hydrocarbon reserves. However, little is known about the geographic distribution of source rocks and their thermal maturities and burial histories other than in terms of broad, regional trends (Nowlan and Barnes 1987). Potential source rocks far to the west in the St. Lawrence Lowlands, between Québec City and Montréal, which are similar to those of western Newfoundland, are generally overmature (Dykstra and Longman 1995). Ordovician and Silurian units on Anticosti Island display significant variation in maturation levels (Bertrand and Héroux 1987; Nowlan and Barnes 1987). Farther east, in central Newfoundland, and south in Nova Scotia and New Brunswick, Lower Paleozoic strata have been metamorphosed and intruded by Upper Paleozoic granitic plutons. This study examines the thermal maturation and burial history of strata from western Newfoundland by combining new data with previously published conodont and fluid-inclusion studies.

Previous regional maturation studies based on conodont colour alteration indices (CAI) from western Newfoundland (e.g., Nowlan and Barnes 1987; Sangster et al. 1994) lacked precision in and around the oil window. To improve our knowledge of hydrocarbon potential and burial history and to determine thermal maturity levels for possible source strata in the Paleozoic of western Newfoundland, two major maturity parameters, the thermal alteration index of acritarchs (AAI) and random graptolite reflectance (GRorand), were measured and compared with other standard methods for establishing thermal maturation levels (e.g., thermal alteration index of spores (TAI), random vitrinite reflectance (VRorand), and maximum graptolite reflectance (GRomax)). This was then combined with previously published information on conodont alteration indices, fluid-inclusion data, and hydrocarbon indices (Nowlan and Barnes 1987; Saunders et al. 1992; Sangster et al. 1994; Fowler et al. 1995) and evidence from apatite fission track studies of the Precambrian Long Range Complex (Hendriks et al. 1993) to show how graptolite and acritarch data improve the resolution of thermal maturation distribution.

Geological framework for western Newfoundland The strata of western Newfoundland (Figs. 1, 2) are grouped into a number of discrete packages. Each was formed under different conditions and has a distinct tectonic history (James et al. 1988, 1989; Knight and Cawood 1991; Fowler et al. 1995). The earliest packages include Grenvillian Precambrian basement rocks which make up the granitic core of the Great Northern Peninsula, and nonconformably overlying Precambrian sedimentary and volcanic rocks which include the Bateau Formation and Lighthouse Cove Volcanics (the basal divisions of the Labrador Group). The Cambrian to early Middle Ordovician sediments in western Newfoundland are divided into two basic units:

Acritarch alteration indices (AAI) Because vascular land plants did not appear until the Silurian, it is generally assumed that any organic material in pre© 1998 NRC Canada



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Fig. 1. Location map after S.H. Williams et al. (1996) outlining the general geology of western Newfoundland and thermal maturity data not illustrated on other maps. Includes data from Nowlan and Barnes (1987), Saunders et al. (1992), Sangster et al. (1994), and Hamblin et al. (1995). Localities 13 and 14 are given in square brackets. FIT, fluid-inclusion temperature.

of late Paleozoic and younger strata employ spores, and relative scales, based on colour alteration, are now well established (Fig. 4). As the present work was restricted largely to pre-Silurian strata, spores could not be utilized in a comprehensive manner; instead, acritarchs were studied using similar techniques. At the outset of this study, several colleagues

Silurian rocks was marine in origin. Vitrinite in the strict sense did not exist, nor did other phytoclasts (trachids, cuticle, and cortex) or amber. Palynomorphs were predominantly acritarchs; rare trilete spores and tetrads in Ordovician strata were probably derived from advanced algal precursors of land plants (Grey and Boucot 1978; Grey et al. 1982). Most palynomorph-based thermal maturation studies

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Fig. 2. Generalized Paleozoic stratigraphic column of western Newfoundland. After Stenzel et al. (1990), S.H. Williams et al. (1996), and Quinn et al. (in preparation3).

the exception of data provided in a short abstract by Bertrand and Achab (1989), the concept of an AAI does not appear to have been much employed and the latter authors have since treated spore and acritarch measurements as essentially synonymous (A. Achab, personal communication, 1996). F. Goodarzi (personal communication, 1996), on the other hand, believes that the changes within the two groups are distinct, owing to differences in their organic structure,

commented to us that changes in acritarch colour, although broadly similar to those exhibited by spores, occurred at different paleotemperatures; thermal maturation studies should not, therefore, treat measurements obtained from the two groups to be equivalent. Legall et al. (1981) recognized that acritarchs underwent similar (although not identical) changes to spores, and proposed an acritarch alteration index (AAI) based on examination of a single genus Leiosphaerida. With 3 L.

Quinn, A. Bashforth, E.T. Burden, H. Gillespie, R.K. Springer, and S.H. Williams. The Red Island Road Formation: Early Devonian foreland basin in the Humber Zone of western Newfoundland. (In Preparation.) © 1998 NRC Canada



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Fig. 3. Generalized comparison of western Newfoundland Cambro-Ordovician stratigraphy with that of analagous, representative hydrocarbon-yielding areas of eastern Canada and United States.

Lycopodium spores and dissolved in HCl and HF acids. Lycopodium spores are used to determine fossil concentrations, therein providing a proxy indication of the quantity of structured organic algal material and source rock potential. After washing to remove the acid, slides were made of the unsieved residues. Later, after sieving with a 10 µm screen, additional slides were prepared for sieved and unoxidized organics. Slides were scanned and fossils identified and counted in alternating fluorescence and transmitted light to obtain an indication of the palynomorph species and abundances, TAI and AAI, and fluorescence properties. Fossil assemblages and concentrations were identified for source rock potential, and our numerically coded colours were matched to a colour chart of maturation indices (both AAIs and TAIs) calibrated with vitrinite reflectance (VRo) results from other published (Raynaud and Robert 1976; Staplin 1982; Waples 1982; Pearson 1984) and unpublished studies (E.T. Burden and R.S. Hyde, personal observations) of spores and acritarchs.

and advocates the use of two separate schemes. This is also suggested in a nonqualitative figure reproduced by Traverse (1988, p. 432), showing acritarchs to remain apparently unaffected by low temperatures which cause changes in spores and other palynomorphs. With spores, there are considerable variations in colour changes dependent on the size and the thickness of the organic wall (Staplin 1977; Jansonius and Schwab 1996); our study suggests that similar variation also occurs with acritarchs of different taxa. Notwithstanding the complexity involved in our comparing individual measurements from markedly different taxa, we believe that carefully measured acritarch assemblages do follow a predictable maturation path (Fig. 4). In recognizing that the maturation paths for some acritarch taxa are different from spores in immature strata, assemblages show little difference from spore TAI within the oil window. Assemblages provide a clear indication of burial and maturation history. We therefore employ in our work a more broadly based and therein distinctive AAI numerically similar to the four-point index used for spores and different from that proposed by Legall et al. (1981).

Vitrinite (VRo) and graptolite (GRo) reflectance Unlike strata of later Paleozoic age where true vitrinite is present, the precise determination of thermal maturity of early Paleozoic sediments in and around the oil window has historically been extremely difficult. The use of other organic maturity measurements (bitumen reflectance, fluorescence of algae, etc.) provides the most suitable proxy data to define exact maturity of those rocks (Mukhopadhyay 1992, 1994, and references therein). Land plants first evolved dur-

Acritarch alteration index (AAI) methodology Thermal alteration index (TAI and AAI) and fluorescence studies of palynomorphs require a modified palynomorph preparation technique that reduces or eliminates any acids which may damage the fossil walls. For this study, crushed and weighed samples were spiked with a known number of

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Fig. 4. Correlation of AAI and graptolite reflectance with other thermal maturation techniques (includes data from Staplin 1977; Waples 1982; Bertrand 1991; and Gentzis et al. 1996). Absolute temperatures are based on work from spores in western Canada.

Care should thus be taken when interpreting GRo data from lithologies other than black shale.

ing the Lower or middle Silurian, but did not become abundant until the Devonian Period. Vitrinite (telocollinite or gelocollinite) in the strict sense does not, therefore, exist in sediments deposited before that time. Graptolite workers have, however, realized for some time that the fossilized skeletal remains of these hemichordates differ in appearance between those preserved in relatively undeformed strata and those which have been subjected to low-grade regional metamorphism. Nonmetamorphosed graptolites are black with a lustrous sheen. In contrast, those occurring in black shales of zeolite or prehnite–pumpellyite facies have a graphitic or metallic lustre. This change has been investigated quantitatively and results have been published by a number of authors (e.g., Bertrand and Héroux 1987; Bertrand 1991, 1993; Malinconico 1992, 1993; Goodarzi 1984; Goodarzi and Norford 1985; Hoffknecht 1991; Wang et al. 1993; Gentzis et al. 1996). Based on work by these and other authors, the use of graptolite reflectance studies appears to be a valuable technique in understanding the burial and structural history of Lower Paleozoic sediments. The present study contributes towards resolution of a calibration scale for graptolites by comparison with other techniques, including vitrinite reflectance, organic colour alterations (AAI, TAI), and conodont colour alteration index (CAI) (see Fig. 4). True graptoloids (planktonic graptolites) are most commonly preserved in black shales and are restriced in age from Ordovician to Devonian; they are, however, occasionally also found in other lithologies including sandstones and limestones. Although reflectance measurements are possible for the latter lithologies, the graptolite peridem is often less well preserved due to irregular compaction combined with oxidation and bacterial breakdown prior to fossilization.

Graptolite (GRo) reflectance methodology Graptolites display strong optical anisotropy, the maximum reflectance occurring in bedding-parallel polished sections (Wang et al. 1993). Such observations have now been quantified through the use of reflectance studies of graptolite periderm, of which the maximum reflectance in oil and bireflectance are commonly taken to be the most diagnostic indicators for maturity (Goodarzi et al. 1988; Hoffknecht 1991; Gentzis et al. 1996). Rather than using oriented specimens which are difficult to collect and process for the GRomax values reported in a number of previous graptolite reflectance studies (e.g., Goodarzi et al. 1988), this study uses techniques which are more in keeping with general practice in vitrinite analyses, and similar to those of Bertrand (1991) and Bertrand and Héroux (1987). Here, fragments of rock chosen for analysis of graptolite reflectance are not oriented parallel to bedding (cf. Goodarzi and Norford 1985; Wang et al. 1993; Gentzis et al. 1996). Instead, the rocks were crushed to –20 mesh size and unoriented strew mounts were impregnated in a cold-set epoxy resin and polished according to the standard procedure followed for vitrinite reflectance (ASTM 1991; Stach et al. 1992; Mukhopadhyay 1992). In incident white light, thick cortical tissues of graptolite show grey colour and finely laminated textures which bear a close resemblance to true vitrinite (telocollinite) grains. The random reflectance of graptolite fragments (corresponding to Rograp random of Goodarzi et al. 1988) was determined by means of a Zeiss Axioskop incident light microscope with MPM 03 photomultiplier and MPM 21 microprocessor using © 1998 NRC Canada



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an oil immersion objective. Usually 50 grains were measured to determine the mean random reflectance of graptolite fragments (percent GRorand).

Graptolites As with acritarchs, attempts have been made previously at calibrating graptolite reflectance (GRo) against conodont alteration indices (CAI), true vitrinite reflectance (VRo), scolecodont and chitinozoan reflectance, and absolute maximum temperatures to which the rocks have been subjected (e.g., Goodarzi and Norford 1985; Bertrand and Héroux 1987; Bertrand 1991, 1993; Malinconico 1992, 1993; Wang et al. 1993; Gentzis et al. 1996). Goodarzi and Norford (1985, 1989), Goodarzi et al. (1988), Hoffknecht (1991), and Wang et al. (1993) suggest that VRo values are significantly lower than GRo values. Goodarzi and Norford (1985) indicate that a VRo of 2.5% corresponds to the semianthracite stage and a temperature higher than 130°C; however, for maximum graptolite reflectance (GRomax), a value of 2.5% represents a rather lower temperature of about 100°C. Hoffknecht (1991) and Wang et al. (1993) suggest a relationship between GRomax and VRo which increases with the level of thermal maturation. Thus, for a VRo of 1.0, GRomax is about 2.5; when VRo is 3.5, GRomax is nearly 10.0. When random graptolite reflectance (GRorand) measurements are employed (Bertrand and Héroux 1987; Bertrand 1991; this study), values do not seem to differ much from those expected by VRorand. Goodarzi et al. (1988) and Gentzis et al. (1996) noted a similar trend in measurements of GRomax and conodont CAI in rocks which were thermally altered to levels well above the oil window (CAI 3+). Taken together, these studies show graptolites to be sensitive thermal maturation indicators that respond more rapidly than conodonts to changes in the temperature of strata within and immediately below the oil window. By and large, our GRorand and AAI data span the other end of the spectrum, that is in thermally immature strata and strata in the oil window (Fig. 4). Fluid-inclusion data from nearby localities support the degree of acritarch alteration measured at many sites, and indicate fluid-inclusion paleotemperatures from about 60 to 130°C. Correlated plots of GRorand with AAI indicate that changes in GRorand are subtle in strata above and within the oil window, suggesting that at this level graptolites are less reliable than acritarch AAI measurements, but are marginally better than conodonts in recording general maturation trends. Thus, for strata in western Newfoundland, acritarchs and graptolites provide the most sensitive maturation indicators for burial history, with the acritarchs accurately delineating shallow burial and low burial temperatures and graptolites showing deeper burial and higher, low-grade metamorphic temperatures.

Acritarchs Correlation of AAIs with other measures of maturation is based on a number of direct observations. Conodonts are widely reported in western Newfoundland where CAIs have been regionally mapped (Stouge 1986; Nowlan and Barnes 1987; Sangster et al. 1994). However, local and regional studies (Epstein et al. 1977; Nowlan and Barnes 1987) indicate conodonts are unresponsive to subtle changes from shallow burial and heating. Inasmuch as conodonts are better suited for studies of strata lying beneath the oil window, AAIs apparently provide much more sensitive indicators of changes in strata as they pass through the oil window. Conodonts may prove to be more useful within this interval if recently proposed methods by Deaton et al. (1996) which employ spectral reflectance measurements become accepted practice. Calibration of AAIs with TAIs is achieved by examining and comparing thermal indicators in nearby Carboniferous beds with our Cambro-Ordovician rocks. In some of our sections, Carboniferous shales and limestones directly overlie Cambro-Ordovician rocks. Here, yellow spores (TAI 2.4, 2.5, and 2.1) which fluoresce amber and red contain vitrinite (VRo 0.58, 0.52, and 0.65), indicating marginally mature rocks. These values suggest paleotemperatures less than 100°C. Nearby beds of shale and limestone contain extremely well preserved Cambro-Ordovician acritarchs which are still transparent, pale yellow in colour (AAI 1.0, 1.4, and 1.8), and very brightly fluorescing. Fluid-inclusion paleotemperatures from nearby and diagenetically related Ordovician mineral prospects show temperatures of 67–77°C (Saunders et al. 1992). Elsewhere in our study area, pale yellow and yellow Cambro-Ordovician acritarchs from Green Point (AAI 1.3 and 2.1) which fluoresce yellow and amber come from oily, marginally mature source rocks of the Cow Head Group (Weaver and Macko 1987; Fowler et al. 1995). By comparing our control points with spore TAI, acritarch alteration colours appear to be shifted towards pale colours and bright fluorescence in the upper part of the oil window, changing rapidly through the middle part of the oil window and becoming essentially equivalent to spore colours before the bottom of the oil window is reached. Thus, while the top of the oil window is characterized by a TAI of 2.0 and AAI of approximately 1.5, the lowermost limit of oil production (Staplin 1977; Pearson 1984; Batten 1996) is marked by both TAI and AAI of approximately 3.0, with brown to dark brown fossils exhibiting dull brown fluorescence. Our conclusions are similar to those given by Bertrand and Achab (1989), who concluded that for equal rank, AAIs have lower values than those for TAIs based on spores. The brief report of Jansonius and Schwab (1996) shows there is clearly a need for further critical comparison of the various organic walled fossil groups.

Port au Port Peninsula Nowlan and Barnes (1987) found little evidence of thermal alteration on the Port au Port Peninsula, with CAIs from the autochthonous Ordovician strata consistently measuring 1 (Fig. 5). Fluid-inclusion paleotemperatures from the east and west ends of the Port au Port Peninsula suggest heating to 67–77°C in mineral prospects in Ordovician strata in the west and 80–120°C in Carboniferous strata in the east © 1998 NRC Canada



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(Saunders et al. 1992). Both sets of paleotemperatures fall within the oil window (Staplin 1977, 1982). However, some care should be taken in interpreting these fluid-inclusion data; the higher temperatures from the eastern sample are from a hypothesized hydrothermal vent system (von Bitter et al. 1990) and may not be representative of the regional history. Autochthonous Lower–Middle Ordovician rocks on the Port au Port Peninsula and adjacent mainland have graptolite reflectance varying randomly between 0.67 and 0.85% GRorand which suggests that the strata are marginally mature to mature (Table 1). VRo measurements from Carboniferous sediments of the Codroy Group on the eastern end of the peninsula, and in the vicinity of proposed hydrothermal vents, are 0.58 and 0.65%, and only slightly lower than the graptolite data. Other vitrinite measurements from Early Devonian plant remains in sandstones of Red Island Road formation (Quinn et al., in preparation3) come from Red Island off the west coast of the Port au Port Peninsula (Fig. 5). These plants gave a VRo of 0.52% which is lower than that from the Carboniferous rocks on the eastern end of the peninsula and likely indicates immature or marginally mature strata. The TAI measurements from Red Island are not from the most suitable lithologies. One sample with a spore-based TAI of 2.5 and dull red fluorescence comes from sandstone clasts in a conglomerate; the other with an alteration index of 4.0 on scattered (carbonized) debris and with bright fluorescence on amorphous matter comes from a cross-bedded red sandstone; VRo is 0.52. Blackened debris indicates that the bulk of the organic matter in these Devonian samples is oxidized and these samples are thus of little use in determining the local thermal history of this island. Acritarchs from the Port au Port Peninsula present a more complete picture of thermal maturation and source rock changes. Autochthonous Middle Cambrian sediments of the Port au Port Group at March Point contain abundant acritarchs. These include Tasmanites sp. and ?Gloeocapsamorpha sp. clusters, which are commonly found in relatively productive Lower Paleozoic type I or type II source rocks (Jacobson et al. 1988). The AAIs lie between 2.2 and 2.9 and show brown and dull brown fluorescence characteristic of mature strata. Acritarch-rich strata of the Middle Ordovician Table Head and Goose Tickle groups on the Port au Port Peninsula have AAIs of 1.0–1.8 and mostly bright yellow fluorescence, suggesting that the majority of the Table Cove, Black Cove, and Cape Cormorant formations on the Port au Port Peninsula are marginally mature or immature. The Winterhouse Formation is a rich source of acritarchs; it is the lateral and nearshore equivalent to the organic-rich Utica and Macasty formations of the northeastern United States and eastern mainland Canada which are considered to be important source rocks (Sinclair 1990). However, with an AAI of 1.0 and bright yellow fluorescence, the rocks appear to be immature or only marginally mature. Spores from the Carboniferous Codroy Group near Aguathuna have TAIs of 2.4 and amber fluorescence, confirming vitrinite reflectance measures of marginally mature strata.

Autochthonous and allochthonous strata, Stephenville area Samples of the Black Cove and Mainland Sandstone formations at Black Cove just northeast of the Port au Port isthmus (Fig. 5) contain abundant amorphous matter and pyrite. In addition, Black Cove samples contain diverse and relatively abundant palynomorphs, including Tasmanites? sp., whereas the Mainland Sandstone Formation has a small, low-diversity assemblage. The AAIs of 1.9–2.5 and red to brown fluorescence colours (Table 1) indicate that these samples are mature. Correlation of this locality with strata on the Port au Port Peninsula indicates that thermal maturity increases from west to east. Fossil assemblages suggest that the Black Cove Formation is potentially a type I or type II source rock, as also suggested by Fowler et al. (1995); the Mainland Sandstone Formation may be a reservoir. The sample from the Humber Arm Allochthon south of Point au Mal is a bituminous siltstone of unknown age or stratigraphic affinity, with a strong petroliferous odour; before processing, the oil had to be removed with solvents. After processing, the sample was seen to contain abundant brown amorphogen and no fluorescence. This suggests that the rock has passed into the lower part of the oil window; we suspect that oil in this rock probably migrated from another source. The Goose Tickle Group at Cold Pond, north of Stephenville, contains dispersed amorphous organic material, pyrite, and a low-diversity assemblage of corroded and thin-walled sphaeromorph acritarchs (Fig. 1). The AAI of 2.6 for these fossils and lack of fluorescence indicate that these rocks are at or near the bottom of the oil window. The amorphous matter in the sample suggests that this is potentially a type II source rock. GRorand values from the Cold Pond locality increase to 1.35%, suggesting mature to overmature strata. Continuing inland, samples from an unknown formation near Gallants and probable Table Cove Formation strata near Georges Lake, northeast of Stephenville (Fig. 1), contain carbonized organic material. Both samples are thought to be metamorphosed with an alteration index (AAI) of 4.0 for scattered bits of amorphous material and zooclasts; they are unlikely candidates for source rocks. A nearby fluidinclusion paleotemperature from the Petit Jardin Formation at Dons Brook, south of the Bay of Islands, indicates local temperatures between 99 and 118°C (Saunders et al. 1992). The increased thermal maturity passing eastwards from the Port au Port Peninsula to Georges Lake, indicated by higher alteration indices and fluid-inclusion paleotemperatures, is as would be expected from the more intense structural deformation experienced towards the Long Range Mountains. The exposed fold and thrust belts of this region sampled for the study probably lie outside the oil window. Allochthonous and autochthonous strata, Northern Peninsula In regional studies by Nowlan and Barnes (1987) and Sangster et al. (1994), conodont data indicate increasing values of CAI from 1.0–1.5 in the south to 5 in the north, together with an increase from west to east in the south. A sample from Lower Cambrian shales and sandstones in the southern part of the Northern Peninsula, south of Rocky © 1998 NRC Canada



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Fig. 5. Geology and thermal maturity data for rocks in the Port au Port Peninsula. Includes data from H. Williams and Cawood (1989), Nowlan and Barnes (1987), Saunders et al. (1992), and Sangster et al. (1994). Localities are given in square brackets.

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Locality No.

Lithostratigraphy

Lithology

Autochthonous rocks, Port au Port Peninsula 1 Sandstone boulders on beach Grey sandstone 1 Sandstone boulders on beach Grey sandstone 1 Silt interbeds in situ Grey sandstone 2 Cape Cormorant Fm. Black shale 2 Cape Cormorant Fm. Black shale 2 Cape Cormorant Fm. Black shale 2 Cape Cormorant Fm. Black shale 2 Cape Cormorant Fm. Black shale 3 Winterhouse Fm. Black shale 4 Black Cove Fm. Black shale 4 Black Cove Fm. Black shale 4 Table Cove Fm. Black shale 4 Table Cove Fm. Black shale 5 Table Cove Fm. Black shale

10

5

Table Cove Fm.

Black shale

6 7 7 8 8 9 10

Codroy Gp. Codroy Gp. Codroy Gp. March Point Fm. March Point Fm. Petit Jardin Fm. Watts Bight Fm.

Calcareous Calcareous Calcareous Calcareous Calcareous Calcareous Calcareous

Age

Location

TAI

Lower Devonian Lower Devonian Lower Devonian Llanvirn Llanvirn Llanvirn Llanvirn Llanvirn Caradoc Llanvirn Llanvirn Llanvirn Llanvirn Llanvirn

Red Island Red Island Red Island Mainland Mainland Mainland Mainland Mainland Lourdes West Bay Quarry West Bay Quarry West Bay Quarry West Bay Quarry Shore below West Bay Quarry Shore below West Bay Quarry Boswarlos Aguathuna Aguathuna Degras Degras March Point Isthmus Bay

2.5

Llanvirn shale shale shale shale shale shale shale

Visean Visean Visean Mid-Cambrian Mid-Cambrian Mid-Cambrian Tremadoc

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Autochthonous rocks, Northern Peninsula 15 Hawkes Bay Fm.? Grey shale 15 Hawkes Bay Fm.? Grey shale 16 Black Cove Fm. Black shale 17 Table Cove Fm. Black shale 17 Aguathuna Fm. Dolomitic shale

Lower Cambrian Lower Cambrian Llanvirn Llanvirn Lower Arenig

Black Cove oil tanks Black Cove oil tanks Black Cove oil tanks Black Cove oil tanks Black Cove oil tanks Point au Mal Cold Pond Cold Pond Trans Canada Highway, Georges Lake South of Mill Brook South of Mill Brook Spudgells Cove Table Point Table Point

4.0

Fluorescence

VRo (%)a

Brown

?Reworked 0.52 (0.09) ?Reworked

1.4

Bright yellow Bright yellow

1.0

Bright yellow

GRo (%)a

0.72 (0.09) 0.74 (0.06) 0.70 (0.07) 1.0 1.5

Bright yellow Bright yellow

1.5

Bright yellow

1.8

Yellow

0.73 (0.12) 0.67 (0.10)

0.85 (0.08) 2.4 2.1 2.0 2.6 2.2 2.9 2.3

Red Orange Amber Dull brown Dull brown Black Dull brown

2.0

Red

2.5

Dull brown

1.9 2.7 2.6

Brown Black Black

4.0

Black

4.0 4.0

Black Black

0.58 0.65

0.54 (0.11) 0.70 (0.11)

1.35 (0.16)

Almost barren Almost barren 0.96 (0.15) 1.39 (0.14) 1.90 (0.15)


Autochthonous (and allochthonous?) rocks, Stephenville area 11 Black Cove Fm. Black shale Llanvirn 11 Black Cove Fm. Black shale Llanvirn 11 Black Cove Fm. Black shale Llanvirn 11 Black Cove Fm. Black shale Llanvirn 11 Mainland Sandstone Grey siltstone Llanvirn 12 Bituminous silt–shale Grey siltstone ? 13 Goose Tickle Gp. Grey siltstone Llanvirn 13 Goose Tickle Gp. Grey siltstone Llanvirn 14 Table Cove Fm.? Grey shale ?Llanvirn

AA1


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Table 1. Thermal maturation data collected during the present study (see Figs. 1, 5, 6, and 7 for locations).

Lithostratigraphy Table Point Fm. Table Point Fm. Catoche Fm. Catoche Fm., Hor. H Boat Harbour Fm.

11

Age Llanvirn Llanvirn Lower Arenig Lower Arenig Tremadoc

Location Point Riche Point Riche Laignet Point Laignet Point Barbace Cove

Allochthonous rocks, Northern Peninsula 21 Cow Head Gp. Black shale 21 Cow Head Gp. Black shale 22 Cow Head Gp. Black shale 22 Cow Head Gp. Black shale 23 Cow Head Gp. Black shale 23 Cow Head Gp. Black shale 23 Cow Head Gp. Black shale 24 Cow Head Gp. Black shale

Upper Cambrian Tremadoc Arenig Arenig Upper Cambrian Upper Tremadoc Upper Arenig Upper Arenig

25

Cow Head Gp.

Black shale

Upper Tremadoc

25

Cow Head Gp.

Black shale

Upper Tremadoc

26 27 27 28 29 29

Cow Cow Cow Cow Cow Cow

Black Black Black Black Black Black

Upper Upper Upper Upper Upper Upper

Green Point Green Point, GP26 Green Point, GP47A Green Point, GP47A Martin Point, MPS36D Martin Point, MPS42B Martin Point, MPS58 Western Brook Pond, WBS52A Western Brook Pond, WBN4 Western Brook Pond, WBN4 St. Paul’s, SPS 7 or10 St. Paul’s, SPN43A St. Paul’s, SPN43A Cow Head, CHS13.6 Cow Head, CHN32 Cow Head, CHN32

Head Head Head Head Head Head

Gp. Gp. Gp. Gp. Gp. Gp.

Lithology Calcareous shale Calcareous shale Grey limestone Grey limestone Grey limestone

shale shale shale shale shale shale

Arenig Tremadoc Tremadoc Arenig Tremadoc Tremadoc

TAI

AA1 3.8

Fluorescence Black

3.0

Black

3.4

Black

1.2

Bright yellow

2.1

Yellow

1.4

Bright yellow

VRo (%)a

GRo (%)a Almost barren 1.77 (0.05) 1.11 (0.08) Almost barren

0.51 (0.06) 0.69 (0.08) 0.56 (0.10) 0.55 (0.08) 0.63 (0.05) 2.1

Red 0.59 (0.06) 0.53 (0.05)

2.3

Brown 0.53 (0.11) 0.56 (0.07)

1.7

Amber 0.57 (0.04)

a

Standard deviations for VRo and GRo values are shown in parentheses.


Locality No. 18 18 19 19 20

Williams et al.


Table 1 (concluded).

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Fig. 6. Geology and thermal maturity data for rocks in the Port aux Choix – Daniel’s Harbour area. Includes data from H. Williams and Cawood(1989), Nowlan and Barnes (1987), Saunders et al. (1992), and Sangster et al. (1994). Localities are given in square brackets; the data for most of these localities are from a small fossil count.

ganic matter also fluoresces bright yellow. Palynomorph diversity and abundance are relatively high; most samples have Tasmanites? sp., and one has Gloeocapsamorpha sp. As with the graptolites, no quantifiable systematic variation is demonstrated by AAI (1.2–2.3) values between thrust slices. The AAIs and fluorescence colours suggest that the samples are marginally mature or immature. With sufficient burial depth, these are likely to be excellent candidates for oil-prone type I or type II source rocks. Qualitative observations of graptolite periderm by S.H. Williams and Stevens (1988) during the study of isolated graptolites from the Cow Head Group in and around the Gros Morne National Park (Fig. 7) appeared to suggest a slight increase in thermal maturity from west to east. The overall range of GRorand values for the Cow Head Group (0.51–0.69%) suggests that the outcrops are marginally mature. The large overlap of graptolite reflectance values and

Harbour (Fig. 7), is barren of palynomorphs; scattered carbonized organic debris has an AAI of 4.0, indicating metamorphosed sediments. Nowlan and Barnes (1987) considered the CAIs of 4.5–5 in Ordovician autochthonous and allochthonous strata in the Bonne Bay area of the Northern Peninsula to have been related to intense structural deformation and ophiolite emplacement in that region. Nowlan and Barnes (1987) reported conodont alteration indices of 1–1.5 for four samples of Cow Head Group strata, concluding that these rocks were neither deeply buried nor covered by the ophiolite, and heated to less than 80°C. The role of structure and stratigraphy in creating reservoirs for the light, waxy hydrocarbons found at Parsons Pond has yet to be determined (Fleming 1970; Fowler et al. 1995). Samples from the Cow Head Group in and around Gros Morne National Park on the Northern Peninsula contain abundant and fluorescing bitumen. Dispersed amorphous or-

© 1998 NRC Canada



Williams et al.

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Fig. 7. Geology and thermal maturity data for rocks in the Cow Head – Gros Morne area. Includes data from H. Williams and Cawood (1989), Nowlan and Barnes (1987). Localities are given in square brackets.

consistently average above 120°C and may average as high as 187°C (Fig. 1). All surface data suggest that in the region north of Daniel’s Harbour, including Table Point and Port aux Choix, the autochthonous Ordovician strata are overmature for liquid hydrocarbon generation. Graptolite reflectance for the strata of the St. George and Table Head groups in the Daniel’s Harbour – Port aux Choix region (Fig. 6) varies from 0.96 to 1.90%; these are, therefore, demonstrably higher than those on the Port au Port Peninsula, and similar to those from near Cold Pond, north of Stephenville. The results suggest that exposures of potential source rocks in the autochthon in this region are mature to overmature.

AAIs between thrust slices means, however, that no change in thermal maturity levels between thrust slices is quantitatively demonstrable. Samples from the Lower–Middle Ordovician St. George, Table Head, and Goose Tickle groups in the Daniel’s Harbour to Port aux Choix region (Fig. 6; Table 1) have AAIs of 2.9–3.8 and are nonfluorescent, indicating thermally altered, overmature strata. CAIs from the same units in this area are 2 and 2.5, in broad agreement with the AAI and GRorand values. Strata from farther north on the Northern Peninsula were not examined for AAI; conodonts (Fig. 1) follow an orderly increase in maturity, reaching a maximum of 5 in the north (Nowlan and Barnes 1987). Fluid-inclusion paleotemperatures from the Daniel’s Harbour Zinc Mine (Lane 1990) suggest an average temperature of 140°C. On the coast, just north of Port aux Choix, fluidinclusion paleotemperatures from three sites average 108, 121, and 141°C (Fig. 1). Continuing north, towards the tip of the Northern Peninsula, fluid-inclusion paleotemperatures

Controls on thermal maturity Possible controls that might have had an influence on thermal maturities of rocks examined during the present © 1998 NRC Canada



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within the oil window (Hamblin et al. 1995). Any elevated thermal levels would presumably have also had an effect on these Carboniferous sediments, and we thus consider the maximum temperatures to have occurred during the Paleozoic prior to that time. Furthermore, normal orogenic processes, both in terms of burial and high thermal gradients, are quite capable of generating the kinds of hot mineralizing fluids and higher levels of metamorphism reported from the Northern Peninsula (Saunders et al. 1992) during Paleozoic times without needing to invoke Mesozoic processes. The simple modelling exercise presented here employs a generalized geothermal gradient of 25°C per kilometre burial; we have no evidence, however, as to whether this remained constant through time; higher and lower gradients may also have occurred, particularly during orogenic episodes. Locally, Von Bitter et al. (1990) suggested that hydrothermal vent communities lived in fissures in the Aguathuna region of the eastern Port au Port Peninsula during the Carboniferous; this may explain the slight anomaly for the TAI of spores and fluid-inclusion values in that area, and implies the existence of higher than normal heat flow at that time.

study include sedimentary burial, structural burial, duration of heating, hydrothermal systems, and hotspots. On the Port au Port Peninsula, low GRorand and AAI values close to the ophiolite suite indicate that sediment burial depths for the Late Ordovician to Carboniferous strata were shallow, and probably never more than 3 km. Likewise, for the older Cambrian strata deposited on the Port au Port Peninsula before the Taconic orogeny, the AAIs show no abnormal patterns beyond what would be expected from burial. If a typical geothermal gradient of about 2.5°C per 100 m of burial is adopted for the 2–3 km of Cambro-Ordovician autochthonous shelf strata, the maturation values fall broadly within a range that would be expected for simple burial. From a local perspective, this suggests that no thick sedimentary packages were deposited and subsequently eroded from the Port au Port Peninsula. In addition, structural burial was apparently of little consequence in modifying the thermal history of the Port au Port Peninsula. Only a relatively thin structural allochthon slice, possibly some 1 km in thickness or less, could ever have been present on the Port au Port Peninsula. Regionally, the Port au Port thermal maturity data indicate that (i) autochthonous shelf strata were just entering the oil window at the onset of Taconic orogenesis, (ii) the limit of significant Taconic influence on burial and thermal maturation lies some distance to the east of the Port au Port region, and (iii) subsequent sedimentary and structural history is very important to locating sites for petroleum generation and entrapment. Higher thermal maturity values in autochthonous strata east of Stephenville are probably due more to structural control than sedimentary burial. The emplacement of thickened, complexly deformed allochthonous thrust sheets containing the ophiolite suite buried most of the autochthon beneath the oil window. Surface samples of autochthonous strata carried in some thrusts indicate that little potential remains for the preservation of hydrocarbons generated in CambroOrdovician strata east of our collection site at Cold Pond, and north of Stephenville. Elevated thermal values which occur in Lower Paleozoic strata north of Deer Lake and south of Green Point are related to intense structural deformation associated with deep burial of the region. All lines of evidence, including AAI, CAI, and organic geochemistry, indicate that allochthonous slices of the Cow Head Group to the north of this region were never deeply buried. In addition, maturation indices presented here do not provide conclusive evidence that thermal maturity increases towards the front of the Long Range mountains; it is, therefore, not possible at the present time to infer the maturity levels of any Lower Paleozoic strata which might lie beneath the thrusted Grenvillian Long Range Mountains. The gradual increase in thermal maturity north of the Gros Morne region, demonstrated by AAI, CAI, and fluidinclusion data, was considered by Nowlan and Barnes (1987) to reflect the passage of the area over a hot spot during the Mesozoic. Hendriks et al. (1993) also discussed the possibility of late cooling of the Long Range Complex during mid to late Mesozoic times, possibly related to rifting of the North Atlantic. Evidence from Carboniferous strata at Conche (Fig. 1), however, apparently contradicts these models, TAI, and vitrinite reflectance, showing them to lie

In combination, acritarchs and graptolites have been shown to provide a powerful tool in the study of the thermal history of Lower Paleozoic strata. By calibrating and correlating thermal maturation indices from these fossils, together with evidence from conodont and fluid-inclusion data, it is possible to produce a burial history and to determine potential source rocks in western Newfoundland. Plots of thermal alteration indices suggest that the rocks have been heated primarily from burial with secondary heating from tectonism. The effects, if any, of hypothesized hydrothermal vents and hot spots have not been confirmed from this work. Structural or depositional burial has never been significant on the Port au Port Peninsula; deeper burial levels are found farther east and north of Stephenville. The rocks around Gros Morne have been structurally buried more deeply than those of the Port au Port, but never more than a few kilometres. Farther north on the Northern Peninsula, either burial depths were much greater (presumably related to thrust stacking) or thermal gradients were considerably higher during pre-Carboniferous times. From an exploration standpoint, our results agree well with those of Fowler et al. (1995) in suggesting that the most promising regions for economic hydrocarbon deposits are the Port au Port Peninsula and areas in and around the Gros Morne National Park from north of Rocky Harbour to south of Daniel’s Harbour. Potential source rocks include the allochthonous Green Point Formation of the Cow Head Group and autochthonous Black Cove and Cape Cormorant formations. The Forteau and March Point formations may represent locally developed Cambrian sources, but their potential has yet to be demonstrated in terms of total organic carbon (TOC) data, as has that of the Catoche and Winterhouse formations. All autochthonous Lower Paleozoic strata from north of Daniel’s Harbour on the Northern Peninsula are thermally overmature and unlikely candidates for liquid hydrocarbon production. © 1998 NRC Canada



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This study was funded through a contract between the authors, the Centre for Earth Resources Research (Memorial University), and Mobil Oil Canada. We thank these organizations and acknowledge particularly the interest and support given by Dennis Price of Mobil Oil Canada. Earlier versions of the paper were improved and substantially modified based on the comments of reviewers.

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