Tectonism: the dominant factor in mid-Cretaceous deposition in the Jeanne d'Arc Basin, Grand Banks

Papers Tectonism: the dominant factor in mid-Cretaceous deposition in the Jeanne d'Arc Basin, Grand Banks lain K. Sinclair University of A b e r d e e n and C a n a d a - N e w f o u n d l a n d Offshore Petroleum Board, 140 Water Street, St. John's, N e w f o u n d l a n d A1C 6H6, Canada

Received20 February 1992; revised 14 September 1992; accepted 16 October 1992 The development of stratigraphic sequences has been demonstrated to be controlled by a set of factors including variations in subsidence, sediment input, eustatic sea level and physiography. Well and seismic data from the Jeanne d'Arc Basin, Grand Banks indicate that mid-Cretaceous tectonism controls at least three of these factors, namely subsidence, sediment input and physiography. North Atlantic rift tectonism was therefore the dominant factor in controlling the migration of coastal to shallow marine environments and the development of sequence stratigraphy in this basin during the mid-Cretaceous. The Avalon Formation represents a mainly Barremian to Early Aptian regressive phase of clastic, marine to marginal marine sedimentation. This followed the deposition of a thick sequence of mainly marine limestones and shales of the Whiterose Formation above a mid-Valanginian sequence-bounding unconformity. The increased clastic input and northward progradation of coastal environments represented by the Avalon Formation occurred during uplift of a basement arch to the south with subsidence of the basin increasing to the north, accompanied by only relatively minor faulting. These features indicate that a period of epeirogenesis was initiated during the Barremian. Continuing uplift over an expanding area at the southern end of the basin is interpreted to have resulted in the development of an angular unconformity with incised valleys. This mid-Aptian unconformity defines the top of the Whiterose/Avalon sequence. Initiation of brittle fracturing of the sedimentary package and underlying basement (i.e. rifting) in mid-Aptian times resulted in rapid fault-controlled subsidence and fragmentation of the Jeanne d'Arc Basin. This great increase in subsidence rate caused retrogradation of coastal environments across the previously developed sequence-bounding unconformity, despite continuing high rates of sediment input from the uplifted basin margins. The transgressive, siliciclastic Ben Nevis Formation comprises two separate but related facies associations. A locally preserved basal association represents interfingering back-barrier environments and is herein defined as the Gambo Member. An upper, ubiquitous facies association comprises tidal-inlet channel, shoreface and lower shoreface/offshore transition sandstones. This upper facies association onlapped marine ravinement diastems above the laterally equivalent back-barrier facies. The rapid fault-controlled subsidence and high sediment input rate of this mid-Aptian to late AIbian rift period resulted in the accumulation and preservation of very thick shoreface sandstones. The transgressive sandstones were buried by laterally equivalent offshore shales of the Nautilus Formation. Flooding of the basin margins induced by the onset of thermal subsidence in latest Albian or early Cenomanian times marks the top of the Ben Nevis/Nautilus syn-rift sequence. Keywords: Jeanne d'Arc Basin; Grand Banks; sequence stratigraphy; rifting

Introduction The dominantly siliciclastic Ben Nevis and Avalon Formations provide the most consistently hydrocarbonbearing reservoir in the Jeanne d'Arc Basin with accumulations encountered in nine discovery wells to date: Hibernia P-15, Ben Nevis 1-45, Hebron 1-13, Nautilus C-92, South Mara C-13, Whiterose N-22, West Ben Nevis B-75, North Ben Nevis P-93 and Springdale M-29 (Figure 1). Although the Ben Nevis and Avalon generally form a common reservoir, each formation occurs within a distinct sedimentary sequence. While eustasy was originally interpreted as providing the dominant control on the development of sedimentary sequences (Vail et al., 1977), subsequent work has demonstrated that the sequence stratigraphic

methodology need not be narrowly interpreted. Rather the interplay of eustasy, subsidence, sediment input and physiography is seen to dictate relative sea level change, accommodation space variation and sequence development for any given area (e.g. Hamblin and Rust, 1989; Posamentier and James, 1991; in press). The purposes of this paper are two-fold: first, to describe the sedimentary sequence patterns of the mid-Cretaceous hydrocarbon-bearing sediments of the Jeanne d'Arc Basin; and secondly, to provide an understanding of the dominant role that tectonism played in mid-Cretaceous sequence development through the control of subsidence, sediment input and physiography, overwhelming any effect of synchronous eustatic variations. A lithostratigraphic chart for the Jeanne d'Arc Basin

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is shown in Figure 2. The Gambo Member of the Ben Nevis Formation is formally proposed here as a new lithostratigraphic unit to assist in the discussion and understanding of this distinctive facies assemblage and its implications for well correlations, seismic sequence mapping and the quality and lateral extent of hydrocarbon reservoirs. The proposed Gambo Member sediments have been previously considered as part of the Ben Nevis Formation, as in North Ben Nevis M-61 (Harding, 1988; McAlpine, 1990), or as part of the Avalon Formation, as in Port au Port J-97 (McAlpine, 1990). The recognition of and regional correlation of the Gambo Member lithofacies and its boundaries are keys to understanding the new interpretation of the iithostratigraphy and structural evolution of the Jeanne d'Arc Basin presented here. Existing publications about the stratigraphy and genesis of the study sediments include Arthur et al. (1982), Grant et al. (1986), Tankard and Welsink (1987; 1988), Sinclair (1988a), Harding (1988), Tankard et al. (1989) and McAlpine (1990). Well data and formation tops for Jeanne d'Arc Basin wells are given in CNOPB (1990). Discussions of microfossil correlations and reservoir quality are provided in Sinclair (1988b and 1988c, respectively).

Review of tectonic setting interpretations There is broad agreement that the Jeanne d'Arc Basin formed in response to multiple rift events related to the opening of the North Atlantic Ocean. However, Figure 2 summarizes the considerable variation in published interpretations of the timing and type of tectonism active during Late Jurassic through Early Cretaceous times. A clear understanding of specific variations in tectonism is required to allow a logical assessment of the impact of those variations on sequence

T e c t o n i s m : s e q u e n c e stratigraphy: L K. Sinclair development. The comparative chart of Figure 2 attempts to accurately represent various interpretations in a consistent format so that areas of debate and interest can be quickly recognized. However, it is suggested that the reader review each referenced publication to be aware of the authors' own presentation, as each used different lithostratigraphic charts, time-scales and data sets. Where such differences were most difficult to resolve in terms of this chart, a slashed pattern represents the likely outer limits of the boundary in question. Figure 2 shows Barremian through Albian to be the period of least consensus concerning tectonic regimes in the Jeanne d'Arc Basin. The published interpretations will be briefly reviewed to summarize the range of opinion on the tectonic setting of this period. Jansa and Wade (1975) and Wade (1978) identified two Mesozoic rift phases affecting the Grand Banks. However, these papers were published before extensive drilling in the Jeanne d'Arc Basin and, therefore, dealt mainly with basins to the south. Hubbard et al. (1985) and Hubbard (1988) recognize two rift phases with the Barremian through Albian falling within the second major rift phase which began in the Tithonian. Howew.'r, these workers also recognize the early or mid-Aptian unconformity as having been formed at the onset of a period of intense rifting, marked by north-east-south-west oriented extension. Enachescu (1986; 1987) was the first to report that three distinct Mesozoic phases of rifting affected the Jeanne d'Arc Basin, with the final major rift phase occurring during Aptian to Albian times. The intended application of this three-fold division of rift stages with different vectors of extension was unclear, however, as the author grouped the first and second rift/ post-rift cycles into an extensional stage and the third rift/post-rift cycle into a thermal subsidence stage. Subsequently, Enachescu (1992; personal communication, 1992) interpreted the Aptian as the start of a post-break-up stage during which only thermal subsidence occurred in the Jeanne d'Arc Basin, following the end of extensional activity. Tankard and Welsink (1987; 1988) and Tankard et al. (1989) interpreted the Jeanne d'Arc Basin as having been structured by two Mesozoic phases of rifting, with the second phase (late Callovian-Barremian) being divided into three stages (Figure 2). Tankard and Weisink (1988) and Tankard et al. (1989) considered Aptian-Albian times to represent a post-break-up transitional stage in which the "memory of rift subsidence' persisted into the Albian, following a late Valanginian to Barremian period of decaying rift forces. However, Tankard et al. (1989) also related transtensional reactivation of basin margin faults in the Jeanne d'Arc Basin during the Aptian-Albian to extensional stresses centred in the Orphan Basin to the north. Although this mechanism for the activation of oblique-slip motion on north-east-south-west trending faults in the Jeanne d'Arc and Flemish Pass basins in Aptian-Albian times is supported in Sinclair (1988a) and Foster and Robinson (1993), the timing is difficult to reconcile with the contention of Tankard and Welsink (1989) that the Orphan Basin was formed largely by Late Cretaceous to Early Tertiary extension (see also comparative stratigraphic/tectonic chart of Tankard and Balkwill, 1989).

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(1988a)

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not differential, subsidence rates in the Jeanne d'Arc Basin during Albian times. They relate Albian subsidence to continuing extension in the area north of the Jeanne d'Arc Basin following break-up between Galicia Bank and the Grand Banks but before break-up between the Grand Banks and the Goban Spur-Porcupine Bank area. McAlpine (1990) interpreted the Jeanne d'Arc Basin to have been affected by two rift periods followed by a two-stage transition (latest Barremian through Palaeocene) to passive margin subsidence. McAlpine (1990) considered the Albian to be a period of prominent structuring and differential subsidence which he related to halokinetic deformation based on the presence of salt diapirs and a lack of evidence for basement-involved tectonism. Figure 3 is a wireline log section through six wells, based on the correlation of regional seismic data, lithological variations, wireline log response and biostratigraphic data from government and industry

phases

initially reported by Enachescu (1986; 1987), as well as onset warp stages preceding the latter two rift phases in the Jeanne d'Arc Basin. Tectonic stages were defined in terms of lithostratigraphic units and sequences and no detailed biostratigraphic data were provided to support the interpreted ages of lithostratigraphic sequences. Sinclair (1988a) characterized the mid-Aptian unconformity at the base of the Ben Nevis Formation as a 'rift-onser unconformity (sensu Falvey, 1974) in preference to the more common interpretation that this sequence boundary represents a 'break-up' unconformity. Hiscott et al. (1990) provide a comparison of the timing and effects of rifting on basins marginal to the North Atlantic. They interpreted Kimmeridgian to Aptian times as the second rift period to cause extension in the Jeanne d'Arc Basin with the greatest extension occurring as a pulse at the base of this time interval. Hiscott et al. (1990) describe accentuated, but

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reports. This diagram illustrates the gross division of mid-Cretaceous sediments by a sequence-bounding unconformity between the Avalon and Ben Nevis Formations, as well as regional variations in thicknesses and lithologies. The Avalon Formation consists of stacked regressive, dominantly siliciclastic cycles, whereas the Ben Nevis Formation is a transgressive cycle with two distinct facies associations. These units are discussed in greater detail in the following, concentrating on the Ben Nevis Formation which is the most widely preserved and cored of the mid-Cretaceous formations. Descriptions of lithologies and unit boundaries are followed by interpretations of environments of deposition. Discussions of the mid-Cretaceous lithostratigraphy are then followed by age interpretations using microfossil occurrence data from industry reports. Finally, well and seismic data are analysed to judge the type and timing of tectonism and its effect on sequence stratigraphy. Avalon

Formation

'A' Marker member Lithostratigraphy.

The base of the Avalon Formation is defined by the contact of silty shales of the Whiterose Formation with calcareous sandstones and limestones of the overlying 'A' Marker member across most of the Jeanne d'Arc Basin. The 'A' Marker member has not been cored but wireline logs and cutting descriptions indicate that this basal lithostratigraphic boundary is gradational (Figure 3). Locally, thin interbeds of very fine-grained sandstones and shales occurring immediately below the 'A' Marker member along margins of the Jeanne d'Arc Basin (e.g. Fortune G-57, Figure 3) may also be included in the Avalon Formation. The thickness and lithology of the 'A' Marker vary regionally across the basin. In the south, well cuttings and wireline logs show the 'A' Marker member to be a rather uniformly thick package of sandstones and limestones (e.g. Ben Nevis 1-45, 2874-2981 m RT,

metres below the rotary table, and Hebron 1-13, 1990-2091 m KB, metres below kelly bushing, Figure 3). The sandstones are quartzose, generally very fine to fine-grained and calcite-cemented, and contain abundant bioclastic debris. The limestones are bioclastic, pelloidal and/or oolitic and contain abundant quartz sand grains (Mobil Oil Canada, 1981). Locally, thin (about 3 m) interbeds of grey, red and green shales occur within and at the top of the thick 'A' Marker member sandstones and limestones, as in West Ben Nevis B-75 (2434-2437 m RT, Petro-Canada, 1987). However, along the basin margins to the north, the 'A' Marker member grades to thin, bioclastic, occasionally pelloidal and variably argillaceous limestones (e.g. Hibernia P-15, 2505-2520 m KB, Chevron Standard Ltd, 1980).

Depositional environments. The absence of core requires palaeoenvironmental interpretation of this unit to be based mainly on analysis of data from cuttings. For example, the high quartz sand component of the 'A' Marker member in the southern Jeanne d'Arc Basin relative to its low quartz sand component to the north is indicative of a clastic source located to the south or south-east. The common occurrence of oolites, consisting of calcite-coated quartz grains, is indicative of shallow, wave-agitated waters. Abundant bioclastic debris is supportive of a shallow marine or marginal marine environment. The 'A' Marker member in the south and south-east is characterized by abundant but low diversity foraminiferal assemblages such as those commonly found in high stress environments (Dennison et al., 1984). Additionally, the occurrence of arenaceous Choffatella decipiens and Trocholina infragranulata microfossils, shallow water ostracods and abundant gastropods and charophytes (freshwater algae) indicates a very shallow nearshore marine to a brackish and lagoonal depositional environment (Dennison et al., 1984) for the 'A' Marker member in the southern Jeanne d'Arc Basin. These data support the interpretation of the thick

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Tectonism: sequence stratigraphy: L K. Sinclair sandstones and limestones of the 'A' Marker as having been deposited as a progradational to aggradational package of broad, stacked shoals, barriers and lagoons of a mixed siliceous and oolitic/bioclastic nature. The occasional deposition of red and green shale interbeds is attributed to the local establishment of marsh environments fringing lagoons within and at the top of the regressive 'A' Marker member. The lack of core makes the study of sedimentary structures of the 'A' Marker impossible, but informative analogies may be drawn with the apparently similar settings which occur on the modern inner shelf of Mexico's Yucatan Peninsula (Ward and Brady, 1973) and in the ancient rock record of south-west Britain, as described from outcrop by Burchette et al. (1990). The thin, bioclastic, pelloidal limestones comprising the 'A' Marker member in the northern margins of the Jeanne d'Arc Basin are suggestive of a more distal, shallow marine environment of deposition.

Coarsening upward cycles Lithostratigraphy. Coarsening upward sandstone cycles are commonly present in Avalon Formation sediments overlying the sharp top of the 'A' Marker member with the thickest cycles preserved in wells located in the centre of the Jeanne d'Arc Basin. One of these cycles was cored in North Ben Nevis M-61 over the interval 3093-3127 m KB (3097.96-3131.44 m core depth, Figure 4). Very fine to fine-grained, silty, silica-cemented, commonly bioturbated sandstones interbedded with bioturbated, carbonaceous grey shales occur near the cycle base. These thin interbeds grade upwards into clean, fine to medium-grained, silica-cemented, occasionally bioturbated sandstones (Husky/Bow Valley, 1987). The lateral extent and upper boundary of the Avalon Formation are controlled by the mid-Aptian unconformity (discussed in the following).

diastems, also known as marine flooding surfaces, within the Avalon Formation is inferred solely on the basis of the interpreted rapid change from shallow to deep marine facies across cycle tops.

Mid-Aptian sequence boundary The base of the Ben Nevis Formation is defined by the widespread, angular mid-Aptian unconformity overlying Avalon or older formations. The character of this contact is variable across the basin but the unconformity is locally recognizable on a variety of data sets such as wireline log and lithostratigraphic correlations and seismic, dipmeter, microfossil occurrence and core data. For example, wireline log and lithostratigraphic correlations demonstrate the angular nature of this unconformity with erosion of underlying units increasing to the south and across the eastern margin of the basin (Figure 3). Drilling of Gambo N-70 confirmed the presence of the angular unconformity responsible for the truncation of seismic reflectors seen on Figure 5 and synthetic seismograms have been used to tie it to the base of the Ben Nevis Formation. The dipmeter log

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Depositional environments. The core study of Harding (1988) provides detailed palaeoenvironmental data and interpretations over one of the coarsening-upward cycles of the Avalon Formation in North Ben Nevis M-61 (Figure 4), though Harding included this unit within the Ben Nevis Formation. The base of the coarsening-upward cycle represents an offshore/shoreface transition with sharp-based, very fine-grained, storm-generated sandstones interbedded with moderately to heavily burrowed offshore siltstones and shales, as demonstrated by Harding. These interbeds grade upsection into mainly clean, horizontal to low angle cross-bedded, lower to middle shoreface sandstones, demonstrating progradation of coastal environments (Harding, 1988). The proximity of coastline deposits to these marine sediments is indicated by the common co-occurrence of carbonaceous debris with shell lags and bioturbation. The repetition of coarsening-upward and shoaling sandstones above the 'A' Marker member (Figure 4) is suggestive of episodic progradation or sedimentary switching of point sources and delta lobes due to avulsion. Tops of intra-Avalon Formation coarsening-upward cycles may correspond to marine ravinement diastems, each representing a rapid transgression before renewed deposition during progradation. The possible presence of ravinement 534

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Figure 4 South Mara C-13 to North Ben Nevis M-61 correlation showing shoaling-upward cycles of the Avalon Formation truncated at the mid-Aptian unconformity and the Gambo Member underlying a marine ravinement diastem. The Gambo Member extends from 2927 to 2958 m RT (2924.44-2956.5 m core depth) and from 3048 to 3093 m KB (3051.03-3097.96 m, core depth) in the two wells, respectively. Cored intervals have been adjusted to log depths

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can also be used locally to pinpoint the mid-Aptian unconformity, as in Gambo N-70 where structural dips increase by about 5° below 1642 m RT and in Archer K-19 (Figure 6). As the amount of erosion and the angularity of the unconformity decrease basinward, however, it becomes impossible to identify seismic reflector terminations or bedding dip changes at this sequence boundary. Fortunately, other data sets allow for continued correlation of the mid-Aptian unconformity into the centre of the basin. Microfossils Subtilisphaera perlucida, Choffatella decipiens and Muderongia simplex were reported by Dolby and Oliver (1984) to all have last occurrences at or just below 1984 m KB in Hebron 1-13. In contrast, these microfossils have widely spaced last occurrences below 2713 m RT in Ben Nevis 1-45, as reported in Dennison et al. (1984; Figure 7). The apparent simultaneous extinction of these Barremian to early Aptian microfossil organisms in Hebron 1-13 is interpreted to be the result of erosion at the mid-Aptian unconformity to a stratigraphic horizon in which the ranges of these microfossils overlapped (Sinclair, 1988b). Finally, the mid-Aptian unconformity has been cored in South Mara C-13 (Figures 4 and 8) where the base of carbonaceous conglomerates of the Ben Nevis Formation truncates very fine to fine-grained, burrowed sandstones of the Avalon Formation. The assignment of a mid-Aptian age to this unconformity is based on age interpretation of the top Avalon and base Ben Nevis in the centre of the basin where the sequence boundary approaches a correlative conformity (discussed in a subsequent section).

Ben Nevis Formation The sediments of the Ben Nevis Formation comprise mainly sandstones which can be divided into two units,

a locally preserved basal Gambo Member and a ubiquitous fining-upward sandstone sequence.

Gambo Member Name. The name Gambo Member is taken from the Petro-Canada et al. Gambo N-70 well where this unit is thickly preserved (1518-1642 m RT) above the clearly identifable mid-Aptian unconformity (Figures 3 and

5). Holostratotype section. The interval 2927-2958 m RT in Mobil et al. South Mara C- 13 (Figure 9) is herein designated as the holostratotype section in accordance with the guidelines of the International Subcommission on Stratigraphic Classification (1976). South Mara C-13 (Figure I) was drilled between 21 March and 16 October 1984 to a total depth of 5034. l m RT in 97.2 m of water (rotary table 25.6 m). South Mara C-13 is chosen as the type section well as the Gambo Member was continuously cored in this well and the upper and lower contacts are clearly identifiable. Longitudinal half slabs of South Mara C-13 core are available for viewing at the Canada-Newfoundland Offshore Petroleum Board's Core Storage and Research Centre in St John's, Newfoundland. Parastratotype section. The interval 3048-3093 m KB in Husky-Bow Valley et al. North Ben Nevis M-61 is proposed as a parastratotype section. Assignment of a parastratotype section is required to illustrate the dramatic lateral and vertical lithological changes which occur within the Gambo Member (Figure 4). Core through this section is also available for viewing as above. Lithology. The lithology of the Gambo Member is highly variable. Cores cut from the holostratotype section display multiple cycles of clean, medium to

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coarse-grained sandstones often with basal conglomerates containing common sideritic clasts and/or nodules, and white and black siliceous pebbles (Figure 9). The sandstones also contain coal laminae and debris, siliceous cement and rare feldspar grains. The sandstones are massive to medium angle planar and trough cross-bedded, fine upward and are occasionally overlain by thin, dark grey shale beds. Rip-up clasts of the thin shales (occasionally oxidized) are often found within the base of sandstone beds. Slumped bedding is also observed in shale and sandstone facies. Harding (1988) provided descriptions of highly carbonaceous mudstones and argillaceous siltstones, occasionally containing abundant mollusc assemblages, interbedded with thin, very fine to medium-grained sandstones from North Ben Nevis M - 6 1 core (3048-3081 m KB, Figure 4). Harding (1988) also identified dominantly trough cross-bedding with lesser

536

planar tabular cross-bedding and occasional weakly developed mud couplets within a medium to very coarse-grained and organic-rich sandstone in the interval 3081-3093 m KB. Cores containing red and green shales have also been cored from the Gambo Member as in Hibernia K-14 and B-27 (Chad du Toit, Chevron, personal communication, 1989). The petrographic report of Rodrigue (1985; his Figure 1, 1355 m sample) on the Port au Port J-97 well indicates that siderite was a major early cement in the Gambo Member (1330-1378 m KB) whereas calcite cement is absent. Therefore, the distinguishing lithological characteristics of the Gambo Member are diverse, including a wide range of clastic grain sizes, mainly quartzose but commonly polymictic grains and pebbles, coal laminae and debris, interbeds of carbonaceous, grey shales, occasional red and green shales, silica and siderite cements, and a general lack of calcite cement.

Boundaries. The base of the Gambo Member is defined by the mid-Aptian unconformity which was cored in South Mara C-13 (Figure 8) and described earlier. A sharp, erosive upper contact separates the coal-bearing, coarse-grained Gambo Member sandstones from overlying fine-grained, bioclastic-rich, calcite cement-rich sandstones of the balance of the Ben Nevis Formation in South Mara C-13. A thin, erosional lag deposit, comprising rounded and angular pebbles/cobbles, flattened shale clasts, abundant bioclastic debris and a fine-grained quartzose matrix immediately overlies this contact (Figure I0). A very similar erosive lag deposit was cored at the upper contact of the Gambo Member in North Ben Nevis M-61 (3048 m KB, Figure 4). Distribution. The Gambo Member is highly variable in its preserved thickness. Thickness ranges from zero in Hebron 1-13 to 124 m in Gambo N-70 (Figure 3). Anomalous thicknesses have been locally encountered filling palaeovalleys, as penetrated by Terra Nova K-17 (Figures 11 and 12). Age. The age of the Gambo Member in the South Mara C-13 holostratotype section is interpreted as early Aptian by the Geological Survey of Canada (1988). For most wells, however, the age of the Gambo Member is not closely constrained by microfossil occurrence data provided in biostratigraphic studies submitted to the Canada-Newfoundland Offshore Petroleum Board (e.g. Dolby et aL, 1984). The lack of firm biostratigraphic control is possibly due in part to the non-marine nature of this unit resulting in an impoverished in situ microfossil assemblage. Reworking of microfossils across the mid-Aptian unconformity (examples discussed later) adds to the uncertainty in biostratigraphic age interpretation. However, the occurrence of Gambo Member sediments above highly truncated strata along basin margins, as demonstrated by seismic and dipmeter data (Figure 5), indicates deposition synchronous with other post-mid-Aptian unconformity facies (Figure 3). Depositional environment. Gambo Member sediments are interpreted as having been deposited in various back-barrier environments on the basis of core and cuttings data as exemplified by the identification of

Marine and Petroleum Geology, 1993, Vol 10, December

Tectonism: sequence

stratigraphy: I. K. Sinclair

BEN NEWS l-45 m

/

/

l P neocomica

2713

DATUM

l H herlertonensir

t M. stouroto

Figure 7 Correlation of highest occurrences and abundances of microfossils (solid lines) between Hebron l-13 and Ben Nevis l-45. Fossil data is taken from Dolby and Oliver (1984) and Dennison et al. (1984), respectively. Cuttings samples were composited and examined every 15 m for microfossils, supplemented by a few sidewall cores in Hebron l-13, in the two studies cited. Lithostratigraphic correlations are shown by broken lines

Harding (1988) of thin (l-3 m) interbeds of lagoon, marsh, sand flat, crevasse/bay fill, washover and tidal channel facies (North Ben Nevis M-61, 3048-3081 m KB, Figure 4). The sandstone at the base of the Gambo Member in North Ben Nevis M-61 (3081-3093 m KB) is herein interpreted to represent estuarine channel deposition based on Harding’s recognition of fining-upward cycles, dominant trough cross-bedding, mud couplets (Boersma and Terwindt, 1981) and high clay and organic content (Matlack et al., 1989). The stacked cycles of massive to medium-angle planar and trough cross-bedded, carbonaceous, coarse-grained to conglomeratic sandstones, which occasionally are overlain by thin, dark grey shales (i.e. South Mara C-13, Figure 9), are interpreted to be representative of stacked fluvial channels overlain, in places, by floodplain and lacustrine shales. The Texas Gulf coast provides a number of modern analogues where a variety of back-barrier/lagoonal facies pass Marine

laterally and landward into fluvial/deltaic facies filling incised palaeovalleys (LeBlanc and Hodgson, 1984) similar to the interpreted facies variation between North Ben Nevis M-61 and South Mara C-13 (Figure 4). Ubiquitous jining-upward

sandstone

Lithostratigraphy. Fining-upward sandstones occur either above the Gambo Member (e.g. Ciambo N-70, 1378-1518 m RT), or directly overlie the mid-Aptian unconformity (Ben Nevis I-45, 2377-2713 m RT, Figure 3). These sandstones were cored extensively in North Trinity H-71 (Figure 13). Core 4 was cut from massive quartzose sandstones characteristic of the base of the fining-upward sequence. Sandstones in core 4 are non-argillaceous, fine- to medium-grained and occasionally coarse-grained and contain finely ground bioclastic and carbonaceous debris as well as glauconite and Petroleum

Geology,

1993, Vol 10, December

537

Tectonism: sequence stratigraphy: I. K. Sinclair to be representative of a transgressive shoreface based on core and cuttings lithologies, accessories, cementation and sedimentary structures, wireline logs and microfossil assemblages. For example, these sandstones contains abundant shell debris and associated calcite cement, suggestive of a shallow to marginal marine environment. The abundance and variety of marine dinoftagellates and foraminifera seen throughout these sediments also indicate a marine depositional environment (Dennison et al., 1984). The following are interpretations of depositional environments from the base of the fining-upward sandstones through the transition into the overlying Nautilus Formation. The clean sandstones occurring at the base of the fining-upward marine sequence are massive and vary from weakly bedded to low angle cross-bedded and often contain pebble lag deposits. These characteristics are consistent with deposition on upper to middle shorefaces and in tidal inlet channels of a basin-wide barrier island/beach system (Sinclair, 1988a; his Figure 9). The common occurrence of carbonaceous debris in marine Ben Nevis sediments is suggested to be the likely result of erosion and reworking of the laterally equivalent and terrestrial organic-rich Gambo Member sediments at the high energy foreshore and upper shoreface and at the scouring base of migrating tidal inlet channels. This transgressive reworking is the mechanism interpreted to have resulted in the coarse polymictic lag deposit seen above the previously Figure8 Arrowhead marks the mid-Aptian unconformity in South Mara C-13 at 2958 m RT (2956.5 m core depth). Here carbonaceous conglomerates of the Gambo Member overlie a truncated section of very fine to fine-grained, burrowed sandstones of the Avalon Formation

GR 30 r

(Petro-Canada, 1985a). Weakly cemented, friable sandstones alternate with heavily calcite-cemented sandstones. In places, the calcite-cemented zones are associated with beds of shell debris (coquina beds). Elsewhere, the cemented zones apparently contain no shell debris and form globular concretions. The massive clean sandstones seen in core 4 grade upsection to thin interbeds of (1) clean sandstones, (2) argillaceous, highly burrowed sandstones and siltstones and (3) shales (cores 1-3, Figure 13). The thin sandstone interbeds of the upper Ben Nevis Formation are fine to very fine-grained, commonly glauconitic and have common layers of abundant shell debris and associated heavy calcite cementation following bedding. Bjorkum and Walderhaug (1990a; 1990b) provide studies of the genesis and lateral extent of similar calcite-cemented sandstones of north-west Europe. The upper limit of the Ben Nevis Formation is defined in the well bore as the contact between the top of the fining-upward sandstone sequence and the lowest consistent shale beds of the Nautilus Formation (e.g. 1860 m KB in H e b r o n 1-13, Figure 3). However, this contact is conformable and gradational. Core 1 from Hebron 1-13 (1828.8-1845.85 m KB) shows the Nautilus Formation in this well to comprise monotonous grey shales with rare and very thin interbeds of very fine-grained sandstones.

Depositional environments. The fining-upward sandstones of the Ben Nevis Formation are interpreted 538

Marine and Petroleum

Geology,

LITHOLOGY 120 1

LEGEND

vf f m c vc

o

p e b b l e s , cobbles

•

shale clasts

•

c a r b o n a c e o u s debris

bioclastic debris

i

coal laminae

----

or thin beds

~-

calcareous cement

argillaceous m a t r i x

A

siliceous cement m e d i u m angle p l a n a r bedding m e d i u m angle

cross bedding trough cross b e d d i n g ~,

flaser bedding

soft s e d i m e n t deformation burrowing

Figure 9 Gamma ray and lithology over the Gambo Member in the holostratotype section of South Mara C-13

1993, V o l 10, D e c e m b e r

Tectonism: sequence stratigraphy: L K. Sinclair described sharp, erosive contact (ravinement diastem) capping the Gambo Member (Figures 4 and 10).

Figure 10 A r r o w h e a d marks the r a v i n e m e n t diastem separating Gambo M e m b e r back-barrier facies from shoreface sandstones of the remainder of the Ben Nevis Formation in South Mara C-13, core 1, at 2927 m RT (2924.44 m core depth). A coarse lag deposit, comprising rounded and angular pebbles/cobbles, flattened shale clasts and bioclastic debris in a fine-grained sandstone matrix i m m e d i a t e l y overlies this scoured surface

Nummedal and Swift (1987) describe the formation of transgressive erosional ravinements in the Holocene and Cretaceous stratigraphic record of North American basins and demonstrate the cross-cutting relationship between time-planes and these surfaces. Demarest and Kraft (1987) highlight the importance of, and difficulties in, recognizing the difference between such ravinement surfaces and sequence-bounding unconformities in the Quaternary stratigraphic record of the American eastern and Gulf coasts. Waning energy levels, representing increasing water depths and distance from the shoreline, are evidenced by fining-upward grain sizes and an increase in argillaceous content with an associated increase in gamma ray activity upsection in the Ben Nevis Formation (Figures 4 and 13). The gradual increase in water depths is also reflected in an upsection increase in microfossil species diversity and numbers of planktonic foraminifera documented in Dennison et al. (1984). Ichnofossil diversity and abundance also increase upward in the Ben Nevis Formation with higher energy forms (e.g. vertical burrows) grading upward into lower energy forms (e.g. horizontal feeding and dwelling structures, C. du Toit, personal communication, 1989). Sandstones located about midway in the fining-upward sequence of the Ben Nevis Formation display thin, parallel bedding (core 3, North Trinity H-71, Figure 13). This combination of features (energy levels, accessory minerals, bedding style and fossil species diversity and abundance) is indicative of deposition on the middle to lower shoreface. The uppermost sediments of the Ben Nevis Formation are characterized by thin, alternating cycles of dark grey, mottled, argillaceous sandstones

C

C, Terra Nova

Terra Nova

K-17

K-07

Tops

Two Tin

(m s,

1.(:

I

m lerker mbr.~)

2.(

I<

Gambo Member paleovalley fill

>]

~

Scale

t.o Km

\ Figure 11 Migrated three-dimensional seismic section Track 345 (Petro-Canada Inc., 1984) across locally thickened Gambo Member sediments filling a palaeovalley. Faulting which occurred synchronously with erosion on the Aptian unconformity and deposition of the Ben Nevis Formation may have affected, in part, the location of the palaeovalley

Marine and Petroleum Geology, 1993, Vol 10, December

539

Tectonism: sequence stratigraphy: L K. Sinclair TERRA NOVA K - 0 7

TERRA NOVA K-17

GR !

,s~

~0o SONIC ,0o,

1380 1364 1433 1420 1501 1480

1579

1542

1633

1680

Datum

1713 1698

1748

1735

1867

Figure 12 Gamma ray and sonic log correlation between Terra Nova wells illustrating the presence of thickened Gambo sediments,

filling a palaeovalley over a truncated 'A' Marker member. Note the sonic logs illustrate a lack of cement in the Gambo Member relative to under- and overlying sediments

separated by erosive contacts from overlying clean, massive, very fine-grained sandstones (Figure 14). Thin, subparallel laminations gradually become prominent upsection in the clean sandstones, followed by minor burrowing, disturbing the laminations. The laminated clean sandstones grade upsection until they are again completely burrowed and highly argillaceous. These cycles of sedimentary structures are interpreted to represent bioturbated offshore-shoreface transition sandstones deposited during fair weather, interbedded with clean, storm-generated sandstones. The interpretation of storm beds is supported by the observation of Dennison et al. (1984) that the association of shallow water ostracods and pelecypods with the abundant planktonic assemblages in the uppermost Ben Nevis Formation may have resulted from downslope transportation of sediments during storms. These storm beds are similar to those recognized by Harding (1988) at the base of core 3 in North Ben Nevis M-61. Planktonic foraminifera exhibit a gradual increase upsection through the siltstones and shales of the Nautilus Formation (Dennison et al., 1984), indicative of a continued deepening trend and conformable transition to a marine offshore environment. The presence of rare thin sandstone interbeds, as in core 1 of Hebron 1-13, indicates that distal storm-driven turbidites may be locally deposited within the offshore environment.

Age of formations Microfossil occurrence data are taken from charts provided in Dennison et al. (1984) and Dolby and Oliver (1984) with correlations and analyses taken from Sinclair (1988b).

Avalon The top of the Whiterose Formation, where it lies 540

immediately below the 'A' Marker member of the Avalon Formation, is characterized by the last occurrence in time (i.e. highest in well bore) of microfossils such as Buccicrenata italica and Lenticulina guttata (Figure 7). These last occurrences of fossils, where found in association with the more complete assemblage listed in Table 1, are indicative of a late Hauterivian to early Barremian age. Where sandstones of the Avalon Formation occur just below the 'A' Marker member, they are the lateral time-equivalents of the upper Hauterivian to lower Barremian shales of the uppermost Whiterose Formation. The 'A' Marker member of the Avalon Formation is commonly characterized by the foraminifera Trocholina infragranulata and by an abundance of Choffatella decipiens foraminifera. These fossils are indicative of a Barremian age where associated with the last occurrences of others such as Heslertonia heslertonensis and Muderongia staurota (Table 1). Microfossils with last occurrences recorded upsection in the Avalon Formation commonly include

Muderongia simplex, Pseudoceratium pelliferum, Choffatella decipiens and Subtilisphaera perlucida. The first two microfossils indicate that Avalon Formation deposition continued in Barremian times and the latter two indicate that the top of this formation is of late Barremian to early Aptian age.

Ben Nevis There are relatively few microfossil last occurrences which are consistently identified in the Ben Nevis Formation. Of these few last occurrences, Phoberocysta neocomica and Kleithriasphaeridium simplicispinum suggest an early or late Aptian age for the base of this unit, whereas the last occurrences of Chlamydophorella nyei and Verneuilinoides subfiliformis indicate an Aptian to early Albian age for the rest of the formation in the centre of the basin (Table 1). The Ben Nevis Formation, including the

M a r i n e a n d P e t r o l e u m G e o l o g y , 1993, Vol 10, D e c e m b e r

o"

3

o

o

3

t--

8

"z Q_ "0

Q)

age

m

\

I I

m

I I

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'A'

Nautilus

Whiterose

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P

Ben Nevis

Z

Lithostrat.

D F F

Kleithriasphaeridium corrugatum

Lenticulina busnardoi Epistomina tennuicostata

F F F F F

D D D F

F D F

F D D D D D F D D D F

F D F

Epistomina hecti Buccicrenata italica Lenticulina guttata Conorboides valendisensis Lenticulina saxonica bifurceila

Muderongia staurota Cribroperidinium sepimentum Caucasella hoterivica

Heslertonia heslertonensis Epistomina caracolla Dingodinium cerviculum

Epistomina spinulifera Cerbia tabulata Gavelinella barremiana Verneuilinoides subfiliformis Chlamydophorella nyei Apteodinium grande Phoberocysta neocomica Kleithriasphaeridium simplicispinum Subtilisphaera perlucida Choffatella decipiens Pseudoceratium pelliferum Muderongia simplex Subtilisphaera perlucida abundant Choffatella decipiens abundant Trocholina infragranutata

Microfossils

reported

age

o! microlossil

occurrence

late Valanginian (Ascoli, 1976) Hauterivian (Ascoli, 1976)

iate Valanginian (Ascoli, 1976) early Barremian (Davey, 1974); middle BarremiaLq (Davey, 1979); lower middle Ban'amian (Ouxbufy, 1960)

Hauterivian (Ascoli, 1976; Jenkins and Murray, 1981) Valanginian (Ascoli, 1976; Jenkins and Murray, 1981; Williamson, 1987)

Valanginian (Ascoli, 1976)

Barremian (Millioud, 1969); late Barremian (Davey, 1974) early Barremian (Sargeant, 1966); middle Barremian (Davey, 1974) late Barremian (Ascoli, 1976) late Barremian (Ascoli, 1976); Barremian (Jenkins and Murray, 1981)

Aptian (Ascoli, 1976); late Barremian or early Aptian 0Nilliamson, 1987) late Barremian (Davey, 1974) early Barremian (Bartenstein and Bettenstaedt, 1962); late Barremian (Ascoli, 1976) Aptian (Williams, 1974); Barremian (Bujak and Williams, 1978)

early Aptian (Millioud, 1989); Berriasian-Valanginian 0Nilliams, 1974); earliest Aptian (Duxbury,1978) late Barremian (Davey and Williams, 1966; Sargeant, 1966; Davey, 1974); late Al~ian (Duxbury, 1978) Barremian (Alberti, 1961); Aptian (Bujak and Williams, 1978) Aptian (Ascoli, 1976); late Barremian or early Aptian 0Nilliamson, 1987) Barremian (Millioud, 1989; Williams, 1974; Bujak and Williams, 1978); late Barremian (Davay, 1979) Barremian 0Nilliams, 1974; Bujak and Williams, 1978); late Hauterivian (Davey, 1979)

late Aptian (Duxbury, 1983) early Aptian (Bartenstein and Bettenstaedt, 1962); Aptian (Ascoli, 1976; Williamson, 1987) early-middle Albian (Bartenstein and Bettenstaedt, 1962); early Albian (Ascoli, 1976) Cenomanian (Cookson and Hughes, 1964); late Aptian (Duxbun/, 1983) Cenomanian (Cookson and Hughes, 1964; Bujak and Williams, 1978); late Albian (Davay and Verdi~r, 1973)

Aptian (Ascoli, 1976); Albian (Jenkins and Murray, 1981); middle-late Aptian 0/Villiamson, 1987)

Youngest

Table 1 Youngest reported age of microfossil occurrences and interpreted ages of lithostratigraphic units. Microfossils are plotted at the approximate lithostratigraphic interval where the fossils are first encountered in the wellbore. Column 4 represents microfossil type: F = foraminifera; D = dinoflagellate. A summary of many Lower Cretaceous dinoflagellate age ranges is given in Heilmann-Clausen (1987)

Haut,

Barr.

e. Apt,I, Barr,

Aptian

/

Albian

Interp.

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co

¢b

Tectonism." sequence stratigraphy: I. K. Sinclair GR(API) DTO~e/m) GRAIN

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DEPTH I

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of the Avalon Formation. Similarly, a single specimen of the dinoflagellate Kleithriasphaeridium corrugatum (early to middle Barremian extinction, Table 1) was found in the 2430-2445 m RT sample of Ben Nevis 1-45 in the upper sediments of the Ben Nevis Formation, although consistent identification did not occur until 3225-3240 m RT, well below the 'A' Marker member of the Avalon Formation.

1950

Role of tectonism in sedimentary sequence development oj..

The base of the Whiterose/Avalon sequence is defined by the mid-Valanginian unconformity which Sinclair (1988a) correlated with the onset of a phase of thermal subsidence following Late Cimmerian rifting. Late Valanginian and Hauterivian times were dominated by

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Figure 13 Gamma ray, lithology and sonic logs over the Ben Nevis Formation in North Trinity H-71. Cores are plotted by drill depths (i.e. not adjusted to log depths)

Gambo Member, is dated Aptian to Albian in the southern end of the basin (Port au Port J-97; Dolby et al., 1984). The paucity of recorded age-diagnostic microfossils and the common reworking of older fossils into the Ben Nevis Formation often makes age determination of the formation difficult. This fossil reworking probably accounts for the occasional assignment of a Barremian age to sediments of the Ben Nevis Formation. However. reworking can sometimes be identified by a review of the microfossil occurrence charts included with some biostratigraphic reports (e.g. Dennison et al., 1984). For example, a single specimen of Subtilisphaera perlucida was recorded for the 2370-2385 m RT sample at the top of the Ben Nevis Formation in Ben Nevis 1-45. This dinoflagellate with a late Barremian to early Aptian extinction (Table 1) was not found consistently until 2745-2760 m RT at the top 542

Figure 14 Interbedded fair weather and storm cycles of the offshore/shoreface transition in the uppermost Ben Nevis Formation, North Trinity H-71. Core depth is about 2050.92051.25 m

M a r i n e a n d P e t r o l e u m G e o l o g y , 1993, V o l 10, D e c e m b e r

T e c t o n i s m : s e q u e n c e s t r a t i g r a p h y : I. K. S i n c l a i r

marine limestone, siltstone and shale deposition within a sag basin (Sinclair, 1988a). Barremian to early Aptian times, however, are marked by significant changes in subsidence patterns and sediment input. The introduction of large amounts of very fine- to medium-grained siliciclastic detritus into the basin, the northward progradation of coastal environments and the development of incised valleys suggest the uplift and creation of a clastic source area to the south. A rejuvenated Avalon Uplift trend at the south end of the Jeanne d'Arc Basin is the most obvious source area candidate (Avalon Uplift first identified in Sherwin, 1973: Jansa and Wade, 1975). The uniformity of thickness, lithology and palaeoenvironmental facies of the "A" Marker member from Hebron 1-13 to Ben Nevis 1-45 (Figures 3 and 7) demonstrates an absence of active faulting during mid-Barremian deposition. Variations in the thickness of Avalon sediments above the 'A' Marker member (Figure 15) can generally be accounted for by a combination of increasing uplift and erosion toward the Avalon Uplift area with synchronous subsidence of the basin increasing to the north during the late Barremian to early Aptian. Only the Hibernia area adjacent to the Murre fault shows any evidence of active faulting during Avalon Formation deposition (Figure 15). Movement along the Murre fault during this period was indicated by Tankard and Welsink (1987). The general lack of synchronous faulting (De Silva, 1987b; Sinclair, 1988a) during this Barremian to early Aptian period of basin subsidence, margin uplift and increased clastic input is indicative of epeirogenic conditions.

49'10'

Figure 15 Gross thickness map of those Avalon Formation sediments occurring above the 'A" Marker member. Contours represent approximate isopachs (50 m intervals), based on integration of seismic mapping (CNOPB, 1989) and wellbore thicknesses. Abbreviations: E = section eroded; ND = section not deposited; NR = section not reached; SD = encountered salt dome

Marine

Figure 16 Gross thickness map of the combined Ben Nevis and Nautilus formations demonstrating basinwide tectonism synchronous with mid-Aptian to Albian deposition. Contours represent approximate isopachs (100 m intervals) based on integration of seismic mapping (CNOPB, 1989) and wellbore thicknesses. Abbreviations as for Figure 15. Map also shows the locations of basin margin seismic sections comprising Figures 17 and 18

During the following mid-Aptian to late Albian period, however, conditions in the Jeanne d'Arc Basin changed dramatically. A rough isopach contour map of the combined thickness of the Ben Nevis and Nautilus Formations (Figure 16) was derived from a combination of wellbore thicknesses and a time thickness map from CNOPB (1989). The rapid thickness changes across north-west-south-east faults are indicative of fault growth during Ben Nevis and Nautilus deposition (Figure 17: De Silva. 1987a; 1987b; Sinclair, 1988a) in contrast with the lack of active faulting during Avalon Formation deposition (compare Figures lO and 15). Figure 17 also shows that the Egret and related faults were not active during Tithonian to mid-Valanginian (TmV) times as demonstrated by the gradual southward thinning of tlhe sedimentary sequence deposited during that rift period and the lack of thickening of the TmV sequence toward the north-west-south-east trending faults. Regional mapping of the oldest intra-Mesozoic reflectors across the southern Jeanne d'Arc Basin has demonstrated that the throw along portions of major north-west-south-east trending faults is sufficient to offset both the Upper Triassic-Lower Jurassic Argo Formation salt horizon and the likely top of basement during mid-Aptian to late Albian times (e.g. Egret fault, Figure 17), though the basement top does not provide a strong seismic reflector that can be directly mapped with confidence. Other synchronous, but subordinate, faults sole out within Jurassic shales or Argo Formation salt (Enachescu, 1987; Tankard et al., 1989; McAIpine, 199(/). Studies of extension in layered and Petroleum

Geology,

1 9 9 3 , V o l 10, D e c e m b e r

543

Tectonism: sequence stratigraphy: I. K. Sinclair physical models using materials of different shear strength has produced similar patterns. The sand model with an internal silicone layer of Vendeville et al. (1987) has shown that, although the presence of an intervening weak (i.e. mobile) layer will partially isolate extensional faulting in overlying and underlying brittle layers, a few major faults can penetrate the entire 0 t,

D

layered model. In contrast, the faults which offset the Base Tertiary unconformity on Figure 17 and flatten within shales of the Dawson Canyon Formation are the result of gravity-driven failure of the shelf slope. These relatively young intra-sedimentary faults are unrelated to and post-date basement extension. Mid-Aptian to late Albian age faulting across the

t

2 II

I

3 km

I

D

5

sea floor

Tertiary uric,

Cenomanian unc.

~,

DC Cenomanian correlative conformity

BNN

2-

3A A 4"

near top of basement

basalt

Figure 17 Migrated two-dimensional section 80-1307A (Petro-Canada, 1980) across the Egret and related north-west-south-east trending faults of A p t i a n - A l b i a n age. Though many of these faults detach within the sedimentary column, t h r o w along the main portion of the Egret fault is sufficient to offset the likely top of basement, demonstrating synchronous rift-induced extension of the basement. Abbreviations: B - Banquereau Formation; DC = Dawson Canyon Formation; BNN = Ben Nevis/Nautilus sequence; R = Rankin Formation; I = Iroquois Formation; and A = Argo Formation

544

Marine and Petroleum Geology, 1993, Vol 10, December

Tectonism: s e q u e n c e stratigraphy: L K. Sinclair during the Aptian/Albian had created a peneplain (e.g. south of the Egret fault, Figure 17). In contrast, areas which were highly uplifted during Aptian/AIbian times may not have subsided completely below sea level until early Tertiary times (e.g. footwall block of Figure 18). Within most of the Jeanne d'Arc Basin, however, the correlative conformity of the sequence-bounding Cenomanian unconformity (Figures 3 and 17) is marked only by increases in water depth (Dolby et al., 1985) and carbonate content, and a corresponding decrease in elastic content. These three trends are all considered to be responses to continued subsidence over the area of the preceding rift basin, combined with the retreat of coastlines well beyond the limits of that older basin.

north-east margin of the Jeanne d'Arc Basin is demonstrated by the very thick Ben Nevis/Nautilus sequence on the south-western down-dropped block of Figure 18. The rotated margin of the high block shows the effects of extensive synchronous footwall uplift and erosion. This footwall uplift during extension may be attributable to tectonic unloading and the flexural cantilever mechanism which Kusznir et al. (1991) have mathematically modelled. The uplifted and tilted north-eastern margin alongside the rapidly subsiding basin illustrates the dramatic effect tectonism had on the physiography, sediment input and subsidence of the Jeanne d'Arc Basin during the mid-Aptian to late Albian. The offset of the top of basement by the north-west-south-east trending Egret fault (Figures 16 and 17) and prominent footwall uplift on the northeastern basin margin demonstrate this tectonism was driven by north-east-south-west oriented extension of the basement (i.e. rifting) during mid-Aptian to late Albian times. Sedimentary sequence development in the Jeanne d'Arc Basin responded to this mid-Aptian/late Albian rift-induced deformation. Retrogradation of the coastline occurred in response to the rapid faultcontrolled subsidence of the basin and despite massive sediment input from the uplifted margins. The interplay of these two competing mechanisms (subsidence versus sediment input) resulted in the deposition and preservation of the very thick, transgressive back-barrier/shoreface sandstones of the Ben Nevis Formation and offshore shales of the Nautilus Formation. Initiation of thermal subsidence in the latest Aibian or early Cenomanian (Hubbard et al., 1985; Hubbard, 1988; Sinclair, 1988a) caused flooding and onlap of the sequence-bounding Cenomanian unconformity on the basin margins. Onlap was rapid in areas where erosion 0 |

1

2

Role ofeustasy in sediment sequence development The age of the unconformities and their correlative conformities which bound the mid-Aptian to late Albian sedimentary sequence in the Jeanne d'Arc Basin closely approximate times of two major eustatic fluctuations interpreted by Haq et al. (1988). The mid-Aptian and latest Albian to Cenomanian unconformities could be correlated within the abrupt eustatic sea-level falls which Haq et al. (1988) interpret at their 112 and 98Ma second-order sequence boundaries. Additionally, Haq et al. (1988) indicate that a long-term eustatic sea-level rise occurred throughout most of the mid-Aptian to late Albian period of transgression in the Jeanne d'Arc Basin. Given this apparent correlation, the question arises 'Did variations in eustatic sea level act as a major influence on mid-Cretaceous deposition in the Jeanne d'Arc Basin?' The first concern has to be the precision of biostratigraphic age dating which must form the basis 3 !

km E"

2

3'

Figure 18 Migrated two-dimensional seismic section 6313-85 (Petro-Canada, 1985b) across the north-eastern fault margin of the Jeanne d'Arc Basin displays dramatic footwall uplift during Aptian-Albian times. Abbreviations as for Figure 17 plus F = Fortune Bay Formation. Footwall area has been mapped by Mclntyre (1991)

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Tectonism: sequence stratigraphy: L K. Sinclair for any comparison of mid-Cretaceous sequence boundaries of the Jeanne d'Arc basin with sequence boundaries of Haq et al. (1988). Correlation and analysis of industry-generated microfossil occurrence data (Table 1; Sinclair, 1988b) indicate the correlative conformity to the mid-Aptian unconformity falls within Lower Aptian sediments, giving a good match with the 112 Ma sequence boundary. However, Table 1 also shows the mid-Aptian unconformity to fall between the youngest occurrences of the dinoflagellates Cerbia tabulata and Pseudoceratium pelliferum. The biochronostratigraphic column of Haq et al. shows these marker fossils to bracket their 109.5 Ma sequence boundary, near the base of the late Aptian. Therefore, microfossil data available to the author are inadequate to constrain the lower sequence boundary more precisely than mid-Aptian and, it follows, are inadequate to differentiate between the 112 and 109.5 Ma sequence boundaries derived by Haq et al. (1988). Though planktonic fossil assemblages are abundant in the deep water sediments bracketing the correlative conformity to the latest Albian to Cenomanian unconformity, microfossil occurrences from drill cuttings are not uniformly consistent across this uncored interval (Sinclair, 1988b). Therefore, this sequence boundary, defined on well and seismic data, cannot be correlated with a high level of confidence to any one of the five sequence boundaries between 94 and 99 Ma on the charts of Haq et al. (1988). The second concern is whether the eustatic variations of Haq et al. ( 1988), assuming they are accurate for the purposes of this discussion, could control to a major degree the formation and character of mid-Cretaceous sequence boundaries, lithofacies trends and sequence architecture in the Jeanne d'Arc Basin. The short-term eustatic curve of Haq et al. shows a drop and then rise of about 75 m occurring in less than one million years at the 112 Ma sequence boundary. The scale and length of this interpreted eustatic sea-level fall are inadequate to account for the degree of erosion seen at the Gambo N-70 and Archer K-19 sites (i.e. hundred of metres, Figure 3). Only tectonism could account for both this high degree of erosion and the different levels of erosion on adjacent fault blocks which were synchronously rising and subsiding (Figure 18). The long-term eustatic rise of about 75 m over 14 million years during the mid-Aptian to late Albian estimated by Haq et al. is inadequate to account in a major way for either the deposition of the great thicknesses of retrogradational sediments preserved in the Jeanne d'Arc Basin or the nearly complete restriction of deposition to within the fault-bounded basin. For example, compacted mid-Aptian to late Albian sediments are interpreted to thicken southward to over 800 m adjacent to the Egret fault (Figures 16 and 17) and are over 2851 m thick at the Mercury K-76 well site on the basin's western faulted margin (Figure 16). Fault block subsidence and rotation clearly created most of the accommodation space required for the deposition of such great and varied sediment thicknesses. The continued rise of eustatic sea level into Late Cretaceous times shown by Haq et al. cannot account for the change in styles of deposition across the latest AIbian to Cenomanian unconformity. Though a continued rise in eustatic sea level could result in flooding of eroded basin margins, eustasy could not

546

account for another feature of Late Cretaceous deposition in the Jeanne d'Arc Basin. That feature is the end of fault-controlled subsidence and rotation (Figure 17). If subsidence had remained relatively constant from the preceding period and eustatic sea level had risen to the point of flooding the basin margins, sediments would have either thickened into the more rapidly subsiding basin or been condensed in a deep-water basin not seen on the margins. Figure 17 shows that neither of these patterns developed. Therefore, the end of faulting coincident with flooding of the basin margins is interpreted to indicate the onset of post-rift regional subsidence of both basins and basin margin platforms. The apparent coincidence of mid-Cretaceous sequence boundaries in the Jeanne d'Arc Basin with short-term drop in the eustatic curve of Haq et al. (1988) may be less an indication of the effect of world-wide eustatic variations on the Grand Banks area than a recognition that a high percentage of data from tectonically linked basins of the North Atlantic margins were used in the construction of the eustatic curve. A similar conclusion was presented by Underhill (1991) with respect to the Late Jurassic portion of Exxon's global cycle chart.

Conclusions The mid-Cretaceous shallow marine to coastal sediments of the Jeanne d'Arc Basin are amenable to multidisciplinary study using some of the methodologies of sequence stratigraphy beginning with Vail et al. (1977). The definition and correlation of genetically related depositional sequences using seismic and well data are especially useful in offshore basin analysis. The assumption of eustasy being a dominant factor in sequence stratigraphic development, however, is clearly not viable in basins undergoing extensive rifting such as characterized the mid-Aptian to late Albian in the Jeanne d'Arc Basin. As noted by Eyers (1991), the identification of extensional tectonic control on sedimentary sequence development during the mid-Aptian to late Albian in southern Britain challenges the 'traditional' view of this time as a period of passive thermal subsidence (see also Hesselbo et al., 1990; Ruffell and Wignall, 1990; Ruffell and Wach, 1991; and contrast with Chadwick et al., 1989). This statement could equally be applied to the Jeanne d'Arc Basin.

Acknowledgements I am grateful for the support of the Canada-Newfoundland Offshore Petroleum Board and the University of Aberdeen for this research. I thank S. Harding (Husky Oil), P. A. Bjorkum (Rogaland Research Institute, Norway) and A. H. Ruffell (Queen's University, Belfast) for helpful discussions; John Harper (Memorial University), Charl du Toit and S. W. Garret (Chevron) and Tony Jenkins (Associated Biostratigraphic Consultants Ltd) for useful critical reviews; and Petro-Canada Inc, Mosbacher Operating Ltd, and Husky Oil Operations Ltd for their permission to use confidential checkshot data from the Petro-Canada et al. Gambo N-7(t well.

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T e c t o n i s m : s e q u e n c e s t r a t i g r a p h y : L K. S i n c l a i r

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Petro-Canada (1985a) Well History Rep. Petro-Canada et al. North Trinity 1-t-71" Petro-Canada (1985b) Marine Reflection Seismic, Gravity and Magnetic Survey, South Grand Banks Regional Survey, Offshore Newfoundland, CNOPB Project No. 8624-P28-81E Petro-Canada (1987) Well History Rep. Petro-Canada et al. West Ben Nevis B-75" Posamentier, H. W. and James, D. H. (1991) Variations of the sequence stratigraphic model: past concepts, present understandings, and future directions [abstr.] Bull. Am. Assoc. Petrol GeoL 75, 655-656 Posamentier, H. W. and James, D. H. An overview of sequence-stratigraphic concepts: uses and abuses. In: Spec. PubL Int. Assoc. SedimentoL No. 18, in press Rodrigue, G. (1985) Petrographic Analysis, Canterra et al. Port au Port J-97, Petro-Canada Resources Report, Well History Rep. Canterra PCI et al. Port au Port J-97" Ruffell, A. H. and Wignall, P. B. (1990) Depositional trends in the Upper Jurassic-Lower Cretaceous of the northern margin of the Wessex Basin Proc. GeoL Assoc. 101,279-288 Ruffell, A. H. and Wach, G. D. (1991) Sequence stratigraphic analysis of the Aptian-Albian Lower Greensand in southern England Mar. Petrol GeoL 8, 341-353 Sargeant, W. A. S. (1966) Further dinoflagellate cysts from Speeton Clay (Lower Cretaceous), studies on Mesozoic and Cainozoic dinoflagellate cyst Bull. Br. Mus. Nat. Hist. (GeoL) 3, 199-213 Sherwin, D. F. (1973) Scot/an Shelf and Grand Banks. In: Future Petroleum Provinces of Canada - - Their Geology and Potential (Ed. R. G. McCrossan), Can. Soc. Petrol GeoL Mem. No. 1, pp. 519-559 Sinclair, I. K. (1988a) Evolution of Mesozoic-Cenozoic sedimentary basins in the Grand Banks area of Newfoundland and comparison with Falvey's (1974) rift model Bull. Can. Petrol GeoL 36, 255-273 Sinclair, I. K. (1988b) Biostratigraphic Correlations Over the Avalon, Ben Nevis, Nautilus and Dawson Canyon Formations of the Jeanne d'Arc Basin, CNOPB Rep. GL-CNOPB-88-1, 19pp, I chartt Sinclair, I. K. (1988c) Avalon, Ben Nevis, and Nautilus Formations of the Jeanne d'Arc Basin, CNOPB Rep. GL-CNOPB-88-2, 58ppt Tankard, A. J. and Balkwill, H. R. (1989) Extensional tectonics and stratigraphy of the North Atlantic margins: introduction. In: Extensional Tectonics and Stratigraphy of the North Atlantic Margins (Eds A. J. Tankard and H. R. Balkwill), Am. Assoc. Petrol. Geol. Mem. No. 46, pp. 1-6 Tankard, A. J. and Welsink, H. J. (1987) Extensional tectonics and stratigraphy of Hibernia oil field, Grand Banks, Newfoundland Bull. Am. Assoc. Petrol Geol. 71, 1210-1232 Tankard, A. J. and Welsink, H. J. (1988) Extensional tectonics, structural styles and stratigraphy of the Mesozoic Grand Banks of Newfoundland. In: Triassic-Jurassic Rifting: Continental Breakup and the Origin of the Atlantic Ocean and Passive Margins, Part A (Ed. W. Manspeizer), Dev. Geotect. No. 22, Elsevier, Amsterdam, pp. 129-165 Tankard, A. J. and Welsink, H. J. (1989) Mesozoic extension and styles of basin formation in Atlantic Canada. In: Extensional Tectonics and Stratigraphy of the North Atlantic Margins (Eds A. J. Tankard and H. R. Balkwill), Am. Assoc. Petrol GeoL Mem. No. 46, pp. 175-195 Tankard, A. J., Welsink, H. J. and Jenkins, W. A. M. (1989) Structural styles and stratigraphy of the Jeanne d'Arc Basin, Grand Banks of Newfoundland. In: Extensional Tectonics and Stratigraphy of the North Atlantic Margins (Eds A. J. Tankard and H. R. Balkwill), Am. Assoc. Petrol GeoL Mem. No. 46, pp. 265-282 Underhill, J. R. (1991) Controls on Late Jurassic seismic sequences, Inner Moray Firth, UK North Sea: a critical test of a key segment of Exxon's original global cycle chart Basin Res. 3, 79-98 Vail, P. R., Mitchum, R. M., Jr, Todd, R. G., Widmier, J. M., Thompson, S., Ill, Sangree, J. B., Bubb, J. N. and Hatfield, W. G. (1977) Seismic stratigraphy and global changes of sea level. In: Seismic Stratigraphy-Applications to Hydrocarbon Exploration (Ed. C. E. Clayton), Am. Assoc. Petrol GeoL Mem. No. 26, pp. 49-212 Vendeville, B., Cobbold, P. R., Davy, P., Brun, J. P. and Choukroune, P. (1987) Physical models of extensional tectonics at various scales. In: Continental Extensional Tectonics (Eds M. P. Coward, M. P. Dewey and P. L. Hancock), Spec. PubL GeoL Soc. London No. 28, pp. 95-107

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Tectonism: sequence stratigraphy: L K. Sinclair Wade, J. A. (1978) The Mesozoic-Cenozoic history of the northeastern margin of North America Proceedings of the Tenth Annual Offshore Technology Conference, Houston, TX, USA, 8-11 May 1978, Vol. 3, pp. 1849-1858 Ward, W. C. and Brady, M. J. (1973) High-energy carbonates on the inner shelf, northeastern Yucatan Peninsula, Mexico Trans. Gulf Coast Assoc. GeoL Soc. 23, 226-238 Williams, G. L. (1974) Dinoflagellate and spore stratigraphy of the Mesozoic-Cenozoic, offshore eastern Canada. In: Offshore Geology of Eastern Canada (Eds W. J. M. van der Linden and J. A Wade), GeoL Surv. Can. Pap. No. 74-30, 2, pp. 107-162 Williamson, M. A. (1987) A quantitative foraminiferal biozonation of the Late Jurassic and Early Cretaceous of the East Newfoundland Basin Micropaleontology 33, 37-65

*Released well history and other reports are available for viewing at the offices of the Canada-Newfoundland Offshore Petroleum Board, Fifth Floor, TD Place, 140 Water Street, St John's, Newfoundland. Requests for reproduction of such data may also be sent to the Exploration Manager at the same address 1Copies of the Schedule of Wells and CNOPB reports are available by writing to the Public Affairs Manager at the above address

M a r i n e a n d P e t r o l e u m G e o l o g y , 1993, V o l 10, D e c e m b e r

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