Ashton F. Embry

Sequence boundaries and sequence hierarchies: problems and proposals

Significant problems are being encountered by stratigraphers as they attempt to apply Exxonian sequence analysis to the depositional record. The most serious problem is one of consistent and objective boundary recognition. The unconformable portion of the boundary usually can be recognized with reasonable objectivity but a major problem occurs when the boundary is a “correlative conformity”. The Exxon model defines such a surface as the depositional surface which existed at the time of the end of base level (relative sea level) fall. In many cases this theoretical surface has no apparent lithologic expression and cannot be recognized objectively. Thus correlation of a depositional sequence throughout a basin is either impossible or is an extremely subjective exercise. To remedy the problem of boundary recognition it is proposed that a sequence boundary be placed at the subaerial unconformity and at the correlative transgressive surface. The transgressive surface is ideal for the conformable portion of a sequence boundary because: (1) it is very distinctive lithologically and occurs in both ramp and shelf/slope settings; (2) it has only minor diachroneity in most cases; (3) it merges with the basinward termination of the unconformable portion of the sequence boundary. This methodology results in a practical, genetic unit (T-R sequence) which can be objectively correlated. A second problem with current sequence stratigraphic practice is the use of a sequence hierarchy scheme which is based on frequency of boundary occurrence. This system is very subjective in nature and is prone to circular reasoning. To counter the hierarchy problem, a hierarchical arrangement of T-R sequence boundaries has been established using boundary characteristics which include: (1) extent of the boundary, (2) extent of the unconformable portion of the boundary, (3) degree of deformation of strata directly underlying the boundary, (4) magnitude of deepening across the boundary, (5) degree of change of the depositional regime across the boundary, and (6) degree of change of the tectonic regime across the boundary. These characteristics reflect the magnitude of base level changes which generate sequence boundaries and this linkage allows the establishment of a hierarchy. Five distinct orders of sequence boundaries are recognized in the hierarchy and vary from 1st order boundaries which are widespread subaerial unconformities associated with significant deformation, to 5th order boundaries which are transgressive surfaces which can be correlated only locally.

The breakup unconformity of the Amerasia Basin, Arctic Ocean: Evidence from Arctic Canada

To delineate more closely the age and evolution of the Amerasia Basin of the Arctic Ocean, a breakup unconformity has been identified in sedimentary basins along the Canadian margin of the basin on the basis of one or more of the following criteria. (1) Strata underlying such an unconformity are cut by major normal faults which extend into the basement, whereas strata overlying the unconformity are relatively unfaulted. (2) A major decrease in subsidence rate in the marginal basins coincides with the time of breakup and the formation of the unconformity. (3) Volcanic rocks occur beneath the unconformity. The widespread late Albian-Cenomanian unconformity is interpreted to be the breakup unconformity and thus this time interval would coincide with the initiation of sea-floor spreading in the Amerasia Basin. Sea-floor spreading and the opening of the Amerasia Basin by the counterclockwise rotation of northern Alaska and adjacent northern Siberia away from the Canadian Arctic Islands are interpreted to have occurred during Late Cretaceous time and to have ceased near the Cretaceous-Tertiary boundary when the active plate margin switched to the site of the present Eurasia Basin.

Detrital zircon geochronology and provenance of the Neoproterozoic to Late Devonian Franklinian Basin, Canadian Arctic Islands

More than 1800 detrital zircon uranium-lead (U-Pb) ages collected from Franklinian Basin sedimentary strata of the Canadian Arctic Islands provide important insights into the depositional and tectonic evolution of the northern margin of Laurentia from the late Neoproterozoic to the Late Devonian. The Franklinian Basin succession is composed of strata with three distinctly different U-Pb age provenance signatures, which have implications for the tectonic and paleogeographic evolution of the entire Arctic region. Neoproterozoic and Lower Cambrian formations contain detrital zircon populations of 1750–1950 Ma and 2650–2800 Ma, which are consistent with derivation from Archean to Paleoproterozoic gneisses and granites of the west Greenland–northeast Canadian Shield. The Lower Silurian to Lower Devonian Danish River Formation contains a dominant population of 900–2150 Ma detrital zircons with scattered Archean ages. The 900–2150 Ma zircons were likely transported axially along the foreland basin of the East Greenland Caledonides (Caledonian orogen) and deposited in a deep-water basin between the Pearya terrane and northern Laurentian margin. Middle Devonian to Upper Devonian strata contain detrital zircon populations of 900–2150 Ma, similar to the Danish River Formation, but these units also contain 370–450 Ma and 500–700 Ma detrital zircons. The 900–2150 Ma zircons were likely derived from the East Greenland Caledonian Mountains, the uplifted foreland of the East Greenland Caledonides, and the Pearya terrane. The population of 370–450 Ma detrital zircons potentially comes from uplifting granites in the Caledonian Mountains and Pearya terrane. The 500–700 Ma detrital zircons were likely derived from the continental landmass responsible for the Ellesmerian orogen. The 500–700 Ma age of the zircons suggests that the northern landmass likely had a connection to rocks of the Timanide orogens, located in the Timan Range of northwestern Russia. A dominant population of 365–450 Ma and 500–700 Ma ages in Upper Devonian strata suggests that the Pearya terrane and the northern continental landmass became the dominant source by the end of Franklinian Basin sedimentation. Because detrital zircons are often recycled from older strata into younger deposits, these data provide the basis for understanding the sedimentary provenance of younger units of the Sverdrup Basin and sedimentary wedges along the present Arctic continental margin.

Recurrent Early Triassic ocean anoxia

The Early Triassic record, from the Smithian stratotype, shows that the organic carbon isotope record from northwest Pangea closely corresponds to major fluctuations in the inorganic carbon records from the Tethys, indicating truly global perturbations of the carbon cycle occurred during this time. Geochemical proxies for anoxia are strongly correlated with carbon isotopes, whereby negative shifts in δ 13 C org are associated with shifts to more anoxic to euxinic conditions, and positive shifts are related to return to more oxic conditions. Rather than by a delayed or prolonged recovery, the Early Triassic is better characterized by a series of aborted biotic recoveries related to shifts back to ocean anoxia, potentially driven by recurrent volcanism.

Triassic transgressive-regressive cycles in the Sverdrup Basin, Svalbard and the Barents Shelf

Nine transgressive-regressive (T-R) cycles have recently been recognized in the Triassic succession of the Sverdrup Basin, Arctic Canada (Embry, 1988). The transgressions which initiated the cycles are dated as earliest Griesbachian, earliest Smithian, late Smithian, earliest Anisian, early Ladinian, earliest Carnian, mid-Carnian, earliest Norian, latest Norian and earliest Jurassic.

Sedimentary Cover of the Canadian Shield through Mesozoic Time Reflected by Nd Isotopic and Geochemical Results for the Sverdrup Basin, Arctic Canada

The Sverdrup Basin of the Canadian Arctic Islands contains a sedimentary record, with only short breaks, from Early Carboniferous to Late Cretaceous time and can be used to document the nature of sediments delivered from northern Canada and Greenland. Sm‐Nd isotopic analysis of 72 sedimentary rock samples from the Sverdrup Basin, coupled with trace element characterization, shows that for most of Carboniferous to Late Cretaceous time, the sediment supply in the northern part of North America was dominated by a single broad provenance; 56 of the 72 samples lie squarely within the Nd isotopic evolution of a clastic sedimentary cover delivered to the region following 450–350 Ma Caledonian and Franklinian mountain building in Greenland and the Canadian Arctic Islands. Cratonic Shield sources in Greenland and Canada are hardly evident in the record, and significant contributions to the sediment budget from any source other than the post‐mid‐Paleozoic orogenic cover occurred only during four relatively short periods. First, during Carboniferous time, pre–Late Ordovician rocks of the Franklinian orogen contributed to alluvial clastic rocks in small rift basins in northern Ellesmere Island. Second, during Early Cretaceous time, Shield basement contributed to more widespread deltaic deposits in central and southern Ellesmere Island. Third, minor volcanic contributions to much of the basin occurred during Late Triassic–earliest Jurassic time and also, fourth, during Late Cretaceous time. Sedimentary materials from Caledonian and Franklinian mountains dominated the provenance of the continental and continent‐margin sedimentary system for at least 370 m.yr., a period of time extending far beyond the existence of the mountains themselves. This dominance was achieved by recycling of widespread Middle and Upper Devonian strata into Mesozoic units in the Canadian Arctic and Cordillera. We assess the extent to which the results call for cover of the Greenland‐Canadian Shield from 450 to 80 Ma and conclude that while much of the Shield was probably covered by Ordovician to Middle Devonian carbonate units, the northerly derived Upper Devonian clastic sedimentary rocks probably covered about one‐half of the Shield in its western and northern portions. This cover was progressively removed through Mesozoic time.

Arctic petroleum geology

The vast Arctic region contains nine proven petroleum provinces with giant resources but over half of the sedimentary basins are completely undrilled, making the region the last major frontier for conventional oil and gas exploration. This book provides a comprehensive overview of the geology and the petroleum potential of the Arctic. Nine papers offer a circum-Arctic perspective on the Phanerozoic tectonic and palaeogeographic evolution, the currently recognized sedimentary basins, the gravity and magnetic fields and, perhaps most importantly, the petroleum resources and yet-to-find potential of the basins. The remaining 41 papers provide data-rich, geological and geophysical analyses and individual oil and gas assessments of specific basins throughout the Arctic. These detailed and well illustrated studies cover the continental areas of Laurentia, Baltica and Siberia and the Arctic Ocean. Of special interest are the 13 papers providing new data and interpretations on the extensive, little known, but promising, basins of Russia. A DVD is provided inside the back of the book, that contains PDFs of all papers plus all related Supplementary Publications.

Nd isotopes, geochemistry, and constraints on sources of sediments in the Franklinian mobile belt, Arctic Canada

The Franklinian mobile belt of Arctic Canada and Greenland was formed by plate convergence of an unknown landmass with North America during Silurian and Devonian time. Clastic sedimentary units of the thick passive- to convergent-margin sequence seen in the Franklinian mobile belt have been characterized for Nd isotopes and trace element geochemistry. Samples of Lower Cambrian to Upper Devonian sedimentary rocks from Ellesmere, Bathurst, and Melville Islands show no trends in trace elements either in space or time, but do display an abrupt shift in ϵNd values at ca. 450 Ma. Samples older than Late Ordovician have initial ϵNd values of −17 to −25, consistent with derivation from Archean to Early Proterozoic shield terrane of Greenland and Canada. Silurian and Devonian samples have less negative ϵNd values of −5 to −13, requiring detrital input from a source with a significantly younger mantle extraction age. The shift to a younger provenance occurred first in condensed deep-water sediments, whose deposition predated the arrival of voluminous clastic materials. For the massive quantities of Silurian and Devonian sedimentary rocks that are present in the Franklinian belt, the only plausible ultimate source lies in the Caledonian orogenic belt, which extends into the Arctic region along the east coast of Greenland. Sediments from these Caledonian mountains, which were probably first shed around the time of the Ordovician–Silurian boundary, were deposited in the Franklinian trough at ca. 440 to 435 Ma. As deformation in the Franklinian orogen progressed, this material was in turn recycled during Middle to Late Devonian time into the large clastic wedge of the Franklinian foreland basin. This propagated southwestward to give rise to the thick Upper Devonian Imperial Assemblage of the northern Cordilleran miogeocline, as well as to thinner Devonian clastic rocks over a wide region of the miogeocline and continental interior, at least as far south as Alberta. In all these regions, our previous work identified an anomalously young ϵNd signature similar to that seen in the post–450 Ma Franklinian rocks. Clastic material with this signature continued to dominate miogeoclinal sedimentation in Alberta until Late Jurassic time. Thus, the Franklinian orogen was a throughway for voluminous sediments derived from Caledonian mountains to reach the western side of the North American continent.

Regional maturity as determined by organic petrography and geochemistry of the Schei Point Group (Triassic) in the western Sverdrup Basin, Canadian Arctic Archipelago

Abstract Maturity and source rock potential of organic rich beds in the Triassic Schei Point Group in the Sverdrup Basin, Arctic Canada have been investigated using reflected light microscopy and the results are compared with other maturity parameters determined geochemically (i.e. Rock Eval, and biomarker maturation parameters). The samples evaluated belong to the Eden Bay Member of the Hoyle Bay Formation and contain a predominance of marine algal material, in the form of Tasmanales , and dinoflagellates, along with mixed terrestrial organics. The rock matrix is dominantly carbonate with some shaly input, indicating that the rocks were deposited in an iron-poor highly euxinic environment. With few exceptions there is good agreement between parameters,determined using microscopy; namely % R o , λ max and R GQ and geochemical parameters, T max , HI , Dia C 27 Dia C 27 + reg C 27 steranes, S (S+R) C 29 steranes. The ternary diagram showing the abundance of normalized C 27 , C 28 and C 29 regular steranes indicates a mostly marine depositional environment for the Schei Point source rock. This is confirmed by the abundance of marine fauna and flora in these samples. Analytical results from several different techniques indicate that the source rocks become more mature from the margin towards the axis of the Sverdrup Basin. This is due, in part, to the increase in overburden of sediments in the axis of the basin. Also the high thermal conductivity of salt has strongly influenced the maturity of Schei Point source rocks over the crest of the salt cored structures, i.e. Well Hazen F-54, and the proximity of salt has enhanced maturation levels at Well Rock Point J-43. The sections investigated were also considered to have an excellent gas potential due to their higher than average TOC content.

Correlation of Upper Palaeozoic and Mesozoic sequences between Svalbard, Canadian Arctic Archipelago, and northern Alaska

The Upper Palaeozoic and Mesozoic succession of the Sverdrup Basin of the Canadian Arctic Archipelago is very similar to that of both northern Alaska to the west and Svalbard to the east. These areas were tectonically linked throughout the Upper Palaeozoic and Mesozoic, and were joined by a seaway throughout much of this time. Numerous unconformity-bounded sequences, each with characteristic facies associations, can be correlated across the entire region. The unconformities are interpreted to be tectonic in origin and are possibly related to episodic plate-tectonic re-organizations. Two sequential tectonic regimes affected the Upper Palaeozoic — Mesozoic succession of Arctic Euramerica and each regime had three phases; early rifting, main rifting and thermal subsidence. The first regime lasted from earliest Carboniferous to early Middle Jurassic and was characterized by the formation and development of basins along the former Caledonian—Ellesmerian Orogenic Belt. The second regime is early Middle Jurassic — latest Cretaceous in age and is related to rifting and seafloor spreading in the Amerasia Basin.