Grant Wach

The last 100 million years on the Scotian Margin, offshore eastern Canada: an event-stratigraphic scheme emphasizing biostratigraphic data

In order to provide a detailed stratigraphic framework for the Late Cretaceous and Cenozoic basin fill of the Scotian Margin (the continental shelf and continental slope off Nova Scotia, eastern Canada), we have developed an event-biostratigraphic scheme based mainly on new analyses of several exploration wells. These include the following shelf wells — Demascota G-32, Hesper I-52, Onondaga E-84 and Wenonah J-75 — as well as Shelburne G-29 and Shubenacadie H-100 on the slope. Several fossil groups are involved in this study, most notably dinoflagellate cysts (dinocysts) and nannofossils, with the former generally providing more diverse assemblages and the latter more precise calibration with global correlations. Because most of the material studied is from cuttings samples, we have relied largely on Last Appearance datums (LADs). The scheme incorporates information from about 250 microfossil species (or groups of species), delimiting about 180 events. Not all events occur in all wells and the scheme needs to be tested against future observations: nevertheless, it represents a considerable advance on previous schemes, which were based mostly on broad zones rather than detailed events. Moreover, it allows for a more refined assessment of ages of geological events, such as the early Eocene gamma spike and the incision of the Wenonah Canyon. And it provides an accurate age context for ongoing stratigraphic, sedimentological and paleoenvironmental studies, and ultimately for a fuller understanding of petroleum systems on the Scotian Margin. RESUME Pour fournir un cadre stratigraphique detaille du remplissage du Cretace tardif et du Cenozoique du bassin de la marge Neo-Ecossaise (plateau continental et pente continentale au large de la Nouvelle-Ecosse, est du Canada), nous avons mis au point une formule evenementielle-biostratigraphique principalement basee sur de nouvelles analyses de plusieurs puits d’exploration. Ces derniers comprennent les puits Demascota G-32, Hesper I-52, Onondaga E-84 et Wenonah J-75 sur le plateau continental ainsi que Shelburne G-29 et Shubenacadie H-100 sur la pente continentale. L’etude touche plusieurs groupes de microfossiles, plus particulierement les kystes de dinoflagelles (dinokystes) et les nannofossiles, les premiers procurant generalement des assemblages plus diversifies et les derniers, un etalonnage plus precis avec des correlations mondiales. Comme la majeure partie du materiel etudie provient d’echantillons de deblais, nous nous sommes largement appuyes sur les plans de reference des dernieres manifestations. La formule incorpore des donnees provenant d’environ 250 especes (ou groupes d’especes) de microfossiles delimitant quelque 180 phenomenes, dont environ 50 % sont bases sur des nanofossiles, quelque 46 % sur des palynomorphes et 4 % sur des foraminiferes. Les phenomenes en question ne se manifestent pas tous dans tous les puits et il faut encore soumettre la formule a des essais en vertu des observations futures: elle represente neanmoins un progres considerable par rapport aux formules anterieures, qui etaient surtout basees sur des zones etendues plutot que sur des phenomenes detailles. Elle permet de plus une evaluation plus raffinee des âges des phenomenes geologiques, comme les pics gamma du Paleocene tardif et l’incision du canyon Wenonah. Elle procure par ailleurs un contexte de datation exact pour les etudes stratigraphiques, sedimentologiques et paleoenvironnementales courantes, et elle permettra en fin de compte une comprehension plus complete des systemes petroliers le long de la marge Neo-Ecossaise. [Traduit par la redaction]

Heterogeneous mixed-layer clays from the Cretaceous greensand, Isle of Wight, southern England

The sea-cliffs of the Isle of Wight were deposited during a period of overall sea-level rise starting in the Barremian (Lower Cretaceous) and continuing into the Aptian and Albian. They consist of fluvial, coastal and lagoonal sediments including greensands and clays. Numerous episodes of erosion, deposition and faunal colonization reflect condensation and abandonment of surfaces with firmgrounds and hardgrounds. This study focused mainly on shallow marine cycles where variations in clay mineralogy would not be expected, because overall system composition, sediment source, and thermal history are similar for all the samples in the studied section. Instead we found a wide variety of clay assemblages even in single samples within a 200 m interval.In this interval, distinct clay mineral assemblages were found and can be described as consisting of Al-rich, Fe-richand intermediate Fe and Al compositions withrespect to 2:1 and 1:1 layers in mixed-layer arrangements. Nearly pure glauconite-nontronite clays exist in the <2 µm fraction only when the bulk rock is free of K- and plagioclase feldspar. Conditions favorable to glauconite-nontronite formation are interpreted to result from a hiatus in volcanoclastic sedimentation, thus providing a stable substrate for glauconitization.The Fe-bearing mixed-layer clay assemblages consist of glauconite, nontronite and berthierine-like layers in various proportions withseveral mixed-layer clays often coexisting in the same sample. In different samples, Al-richand Fe-Mg-richmixed-layer clays are similar in their content and distribution of 1:1 and 2:1 layers. This suggests that the original clay assemblages were similar and later diagenesis affected certain horizons resulting in substitution of Al by Fe + Mg while preserving the original layer structure and arrangement.Structural formulae for the berthierine-like phase and berthierine-like layers in these mixed-layer clays show their layer cation composition is intermediate between odinite and standard berthierine. The total sum of octahedral cations varies from 5.26 to 5.55 whereas the amount of Fe2+ cations varies from 2.12 to 2.22 per O10(OH)8. A feature of the berthierine-like phase as well as of berthierine-like layers is that they are di-trioctahedral and Fe2+ and Fe3+ are the prevalent cations. Moreover, in these berthierine-like components, the amount of Fe2+ is greater than that of Mg (in contrast to odinite) and Fe3+ cations prevail over Al (in contrast to berthierine). The presence of authigenic ferrous Fe clays and the relationship between glauconite-nontronite and bulk mineralogy has implications for sedimentological processes and geochemical conditions during and shortly after deposition.

Provenance of the Ordovician–lower Silurian Tumblagooda Sandstone, Western Australia

Detrital zircon U–Pb ages and heavy mineral assemblages provide conflicting evidence of the provenance of the Ordovician–lower Silurian Tumblagooda Sandstone, a fluvial to shallow marine, red-bed succession over 2000 m thick, within the northern Perth and Southern Carnarvon basins in Western Australia. Tourmaline composition indicates a main provenance from interior continental terranes dominated by ‘Li-poor granitoids, pegmatites and aplites’ and ‘Ca-poor metapelites, metapsammites and quartz-tourmaline rocks,’ akin to the Yilgarn Craton to the east of outcrop of the Tumblagooda Sandstone. Other possible source areas include orogens mostly to the south but lack tourmaline analyses for comparison. Taking into account the lack of garnets—a conspicuous component of the adjacent Proterozoic Northampton Inlier—the limited zircon data are compatible with the Albany–Fraser and Pinjarra orogens along the southern and western margins of Australia and/or terranes in or adjacent to East Africa and/or Antarctica, as ultimate source regions with a minor contribution from the Yilgarn Craton, as with other Phanerozoic strata in Western Australia. Whereas the textural and mineralogical maturity of the sandstone could be explained by derivation from such regions, it is more likely that the source was relatively local and that the sediment passed through several phases of reworking. The main source of ilmenite and hematite, by comparison, may have been mafic–ultramafic rocks and/or banded iron formations within the Archean Yilgarn Craton to the east or the Pilbara Craton to the northeast, mobilised by acidic meteoric waters. Iron oxides forming the earliest cements may have been derived from the oxidation of detrital hematite and ilmenite grains concentrated along some bedding laminae or transported in solution from beyond the zone of deposition. Whereas the detrital iron oxides most likely come from the craton to the east of outcrop of the Tumblagooda Sandstone, the sand grains appear to have originally come from a relatively local orogenic source.

Provenance, diagenesis and reservoir quality of the Upper Triassic Wolfville Formation, Bay of Fundy, Nova Scotia, Canada

Abstract The reservoir characterization and provenance of the Wolfville Formation were investigated using petrography, heavy minerals and microprobe analysis of tourmaline and garnet. Sandstone samples were taken from exposures at Rainy Cove and Cambridge Cove, and from the subsurface at Chinampas N-37 well beneath the Bay of Fundy. The surface and subsurface rocks have differences in their relative content and/or type of detritus, cement and heavy minerals (opaques, garnet, scheelite, tourmaline, rutile, apatite and others). These sandstones have continental block provenance in the subsurface rocks and recycled orogen provenance in the exposures. The main sources of the exposed Wolfville Formation sediments were the Palaeozoic rocks of the Meguma Supergroup, South Mountain Batholith, Horton and Windsor groups; meanwhile the subsurface sandstones might have been derived from the same sources or the Avalon Terrane and/or Gondwana. The sandstones were deposited during early stages of rifting post-dating earlier Palaeozoic collision orogenies that culminated with the Appalachian orogeny. The exposures of Wolfville Formation have low porosity (c. 6%) that diminishes to negligible in the subsurface. The Wolfville Formation has a considerable thickness beneath the Bay of Fundy where it overlies the Horton Bluff Formation, Meguma and/or Avalon terranes. However, its reservoir potential is not encouraging.

Heavy mineral record of Andean uplift and changing sediment sources across the NE margin of South America: a case study from Trinidad and Barbados

Abstract The heavy mineral compositions of sandstones in Trinidad and Barbados record the onset of Andean-related erosion and a reduction of craton-derived sediments into NE South America. The changing provenance was deduced by comparing heavy mineral assemblages interpreted from ancient sandstones with associations recognized in modern sands that can be reasonably correlated to existing tectonic domains. The impact of the Andean orogeny across the margin was to introduce a suite of minerals characteristic of low-temperature metamorphism that today is prevalent adjacent to the Caribbean Mountain belt and differs from the zircon-rich assemblage produced within cratonic plains. Twenty-one Paleocene–Late Pliocene sandstone samples from Trinidad revealed systematic changes in mineral diversity and maturity that recorded this provenance transition, and suggests Andean erosion during deposition of the Late Oligocene Nariva Formation. Similar to Palaeogene sandstones of Trinidad, four Eocene Scotland Formation samples from Barbados support craton derivation, but with additional evidence of minor Andean input probably due to the proximity of the Scotland Formation delivery systems to an earlier uplift episode. By the Late Miocene, most of the sediments delivered into Trinidad basins were supplied from the Andean orogeny as suggested by the relative abundance of minerals of this affinity. The heavy mineral records of Trinidad and Barbados are similar to that described across northern South America from both modern and ancient environments that collectively mark the uplift of the Andean mountain belt, with its strong influence on drainage patterns and reservoir provenance along this sector of the continental margin. Supplementary material: Sample location coordinates, sample and outcrop photographs, and summary outcrop sections are available at http://www.geolsoc.org.uk/SUP18728.

The detrital record of Cretaceous to Pliocene sandstones across the NE South American margin

The Cretaceous to Pliocene erosional history of the NE South American margin depicts the increasing influence of the uplifting Caribbean Mountain belt. Petrographic analyses of 59 Cretaceous to Late Pliocene sandstone samples derived from across the islands of Trinidad and Barbados, demonstrate systematic mineralogical changes whereby Early Cretaceous arkosic arenites gave way to mature quartz arenites by the Late Cretaceous and Caribbean Mountain-derived lithic-rich arenites during the Neogene. Changes in the framework compositions are accompanied by changes in textural attributes, with Caribbean Mountain detritus being notably finer grained than cratonic input. Silica cements predominate in quartzose sandstones and sericitic and clay cements within litharenites. The changing mineralogical compositions record the gradual deformation of the northern South American passive margin and initiation of the Andean–Caribbean Mountain foreland basin system, with incipient orogenic detritus evident from the latest Eocene and unambiguous orogenic input by the Late Oligocene. The preponderance of finer grain sizes within deltaic Pliocene sandstones records the later fill of an established foredeep with sediments sourced primarily from erosion of the Andean hinterland. These interpretations agree with heavy mineral compositional changes and the timing of uplift from geo- and thermo-chronological data across the margin. Supplementary material: Qualitative descriptions, sample coordinates and raw point counts are available at https://doi.org/10.6084/m9.figshare.c.3659381

Integrated Assessment of Risks Associated with Exploration Offshore Eastern Canada

Research in the Basin and Reservoir Lab of the Dalhousie University focus on petroleum system and risk elements analysis on the Nova Scotia, Newfoundland, and Labrador shelves and deepwater offshore. Research includes the observation and geochemical study of selected cores to improve source rock understanding, reservoir outcrop analogs, overpressure and integration of these data with time-equivalent sedimentary series in eastern Canada and associated conjugate margins (Morocco, Portugal, Spain, UK and Ireland). The extensive source rocks and reservoir analogs cropping out on the Western European and African conjugate margins suggest that exploration for hydrocarbons can test alternative (and new) play concepts, improving chances of success.

Baffles and barriers; lessons from the Lower Cretaceous of the Channel Basin and Isle of Wight (southern England)

Numerous episodes of erosion, deposition and faunal colonization reflect multiple erosion/bypass events that culminate in condensation and abandonment of surfaces in the Aptian of the Lower Greensand of Southern England. These surfaces form a range of baffles and barriers to flow that cap reservoir quality sands. Baffles form readily at flooding surfaces on the parasequence scale. Flooding surfaces on parasequence sets form effective barriers and seals to flow at the local (field scale) and these are prone to lithification due to significant exposure and bioerosion on the sea floor. Amalgamated sequence boundaries and flooding surfaces can create barriers that are effective regional seals, mappable over a wide area.