A MIDDLE ORDOVICIAN UNCONFORMITY IN THE ARCTIC LOWLANDS: A DISCUSSION
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ABSTRACT The well-documented Middle Ordovician regional unconformity between the Franklin Mountain and Mount Kindle Formations of the northern Interior Plains (NWT) can be continued into the Arctic Lowlands. Paleontological data exist that suggest Middle Ordovician strata are absent. A sedimentary hiatus is recognized between Lower and Upper Ordovician rocks on Somerset Island and is equated to the Middle Ordovician unconformity of the Interior Plains. A similar sedimentary break, described from Baffin Island, also may be the Middle Ordovician regional unconformity.Lower Paleozoic platform carbonate strata in northern Alaska (parts of the Arctic Alaska, York, and Seward terranes; herein called the North Alaska carbonate platform) and central Alaska (Farewell terrane) share distinctive lithologic and faunal features, and may have formed on a single continental fragment situated between Siberia and Laurentia. Sedimentary successions in northern and central Alaska overlie Late Proterozoic metamorphosed basement; contain Late Proterozoic ooid-rich dolostones, Middle Cambrian outer shelf deposits, and Ordovician, Silurian, and Devonian shallow-water platform facies, and include fossils of both Siberian and Laurentian biotic provinces. The presence in the Alaskan terranes of Siberian forms not seen in wellstudied cratonal margin sequences of western Laurentia implies that the Alaskan rocks were not attached to Laurentia during the early Paleozoic. The Siberian cratonal succession includes Archean basement, Ordovician shallow-water siliciclastic rocks, and Upper Silurian-Devonian evaporites, none of which have counterparts in the Alaskan successions, and contains only a few of the Laurentian conodonts that occur in Alaska. Thus we conclude that the lower Paleozoic platform successions of northern and central Alaska were not part of the Siberian craton during their deposition, but may have formed on a crustal fragment rifted away from Siberia during the Late Proterozoic. The Alaskan strata have more similarities to coeval rocks in some peri-Siberian terranes of northeastern Russia (Kotelny, Chukotka, and Omulevka). Lithologic ties between northern Alaska, the Farewell terrane, and the peri-Siberian terranes diminish after the Middle Devonian, but Siberian affinities in northern and central Alaskan biotas persist into the late Paleozoic.
Conodont
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Abstract Bennett Island stands alone in a remote part of the Arctic and information on its geology is essential to ascertain relations with other terranes in order to restore the early Palaeozoic Arctic palaeogeography. Lower Palaeozoic sedimentary rocks throughout the island were studied thoroughly for the first time. The Ordovician section (> 1.1 km thick) comprises three units: Tremadocian, lowest Floian black shale (130–140 m); Floian, lower Dapingian carbonate turbidite (> 250 m); and Dapingian, lower Darriwilian siliciclastic turbidite (> 730 m). Ordovician deposits conformably overlie Cambrian rocks deposited within the Siberian shelf, as shown earlier. Most of the Ordovician succession was formed in a deep trough that received carbonate debris from a nearby carbonate platform and silicate material from a distant landmass located to the NE (present coordinates). The Bennett Island Ordovician rocks have much in common with those of both the Central and Northern Taimyr belts. It could be tentatively suggested that both belts merged at their eastern continuation in the vicinity of De Long Islands. The whole system probably extends further eastwards. The Ordovician facies patterns and faunal assemblages in the New Siberian Islands are notably similar to those of northwestern Alaska, where the same lateral transition from turbidites to shelf limestones was reported.
Siliciclastic
Carbonate platform
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The Kennedy Channel and Ella Bay formations are the two oldest stratigraphic units exposed in the Franklinian margin sedimentary sequence in the Canadian Arctic Islands. An Early Cambrian age had previously been accepted by the occurrence of trilobites and small shelly fossils in the type section of the Kennedy Channel Formation. Reinvestigation of the area around the type section shows that several large strike-slip faults cut the succession and that the olenelloid trilobites are from an infaulted slice of a younger unit, the Lower Cambrian Kane Basin Formation. Thus, there is no unambiguous paleontological evidence for the age of either the Kennedy Channel or Ella Bay formations. However, the abundance of stromatolites, absence of trace fossils, and separation from overlying Lower Cambrian clastics by a regional angular unconformity indicate a probable late Neoproterozoic age for these two formations. The Ella Bay Formation likely correlates with the Portfjeld Formation in North Greenland, the Spiral Creek Formation in East Greenland, and the Risky Formation of the Mackenzie Mountains in northwestern Canada. The passive margin that existed in northern Laurentia during the early Paleozoic was, therefore, established in the late Neoproterozoic, and the onset of rifting must have preceded this, rather than occurring in the Early Cambrian as some authors have suggested.
Laurentia
Passive margin
Stratigraphic section
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FIELD TRIP OBJECTIVES During latest Ordovician to earliest Silurian time, deposition and biotas were influenced globally by environmental changes that led to glaciation and subsequent deglaciation in southpolar Gondwana. At the centre of equatorial Laurentia, the Williston Basin provides a record of global and regional change in an epicontinental sea. Classic localities in southern Manitoba will be compared with sites north of Grand Rapids and northeast of The Pas, including the only known exposures of the Ordovician – Silurian boundary in this vast basin. New research, integrating lithostratigraphy, biostratigraphy, and C-isotope stratigraphy, suggests that deposits of latest Ordovician (Hirnantian) age are present and that the Ordovician – Silurian boundary is at a higher position than previously thought. Strata exposed in southern and west-central Manitoba provide a superb opportunity to examine this extraordinary time in Earth’s history. This field trip will focus on the Stony Mountain, Stonewall (Figs. 1, 2), and Fisher Branch formations. These predominantly carbonate strata record environments and events leading up to and following the major Hirnantian glacial episode. The succession of fossils represents the culmination of the ‘Great Ordovician Biodiversification Event’, the Late Ordovician mass extinction, and the Early Silurian recovery. The Williston Basin outcrop belt in Manitoba provides a dip section, from relatively basinal deposits in the south to more marginal in the north. On Day 1, we will visit the type sections of the three formations in the southern part of the outcrop belt. Exposures of correlative strata in two areas within the northern part of the outcrop belt will be examined on Days 2 and 3. This field trip complements several GAC–MAC 2013 Special Sessions, including Williston Basin and Other Intracratonic Basins – Sedimentology, Stratigraphy, Paleontology and Resources; Advances in Phanerozoic Geoscience in the Hudson Bay Basin, Foxe Basin and Hudson Strait; and Evolving Carbonate Environments from Precambrian to Modern: Sedimentological and Geochemical Perspectives; and the symposium on Life and Times of Phanerozoic Seas (in Honour of Rolf Ludvigsen).
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Outcrop
Lithostratigraphy
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Abstract The lower Paleozoic in Greenland is most fully represented in northern Greenland where extensive outcrops of platform and deep-water sediments represent the continuation of the Arctic Platform and Franklinian Trough of adjacent Ellesmere Island. The Cambrian platform sequence attains a thickness of 1.5 km and includes, in addition to the Lower and Middle Cambrian, a thick Upper Cambrian (Dresbachian, Franconian, Trempealeauan) record. Ordovician platform deposits (1 km) include evaporites equivalent to the Baumann Fiord and Bay Fiord formations of Arctic Canada. In the Silurian, a complex of carbonate mounds developed in a variety of environmental situations near the junction between the platform to the south, and the Franklinian Trough to the north. The latter contains a mainly clastic sequence at least 7.5 km thick of probable Late Proterozoic-latest Silurian age, and now forms the locus of the North Greenland fold belt. About 4 km of Cambrian and Ordovician platform sediments occur within the Caledonian fold belt in East Greenland, but Silurian deposits are also affected by this orogenesis in the northern part of the fold belt in northern Greenland. Traces of Ordovician platform deposits demonstrate a former lower Paleozoic cover to the Precambrian Shield in West Greenland.
Fjord
Devonian
Outcrop
Baltic Shield
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In the Mount Jolmo Lungma region, fossiliferous Ordovician, Silurian and Devonian rocks were discovered for the first time. The Mesozoics and Cenozoics were classified in detail. A more complete stratigraphical column is thus established.According to the sedimentary development and distribution, the region here concerned may be divided into two parts along the Kangma-Sakya-Sutso line. In the southern part, the strata ranging from Lower Ordovician (possibly from Cambrian) to Eocene (possibly even younger) are mostly marine and well exposed. They are probably platform deposits. In the northern part, only Permo-Carboniferous and Mesozoic were found, they are probably geosynclinal deposits.
Devonian
Mount
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Diamictite
Lithostratigraphy
Laurentia
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Abstract The Murzuq Basin is one of the most petroliferous basins of North Africa. Its remote eastern flank has been largely ignored since early reconnaissance work in the 1950s and 1960s. This article presents new stratigraphic and sedimentological data on the Neoproterozoic through Devonian succession from the Mourizidie and Dor el Gussa regions. The Neoproterozoic to Cambrian Mourizidie and Hasawnah formations in the eastern part of the Mourizidie region dip to the east and north‐east, resting directly on late Precambrian metasediments and granitoids. These strata record the initial progradation of sand‐dominated braidplain systems upon peneplained Precambrian basement. Rhyolite clasts in the Hasawnah Formation may record tectonically driven uplift and unroofing in the southern Tibesti Massif or tectonomagmatic rejuvenation to the south of this massif. In the western part of the Mourizidie region, Late Ordovician through Silurian strata (Mamuniyat and Tanezzuft–Akakus formations) directly overlie late Precambrian metasediments and granitoids, and dip at a low angle towards the west into the Murzuq Basin. Elsewhere at the eastern Murzuq Basin flank, in Dor el Gussa, Late Ordovician glaciogenic sediments rest with angular unconformity upon shallow marine sandstones of Cambrian–Ordovician age. This angular unconformity may also occur in the Mourizidie region and indicates widespread tectonism, either as a result of a Middle–Late Ordovician orogenic event, far‐field tectonism related to the opening of the Rheic Ocean along the northern margin of Gondwana or alternatively crustal depression associated with the growth of Late Ordovician ice sheets. Unconformity development was also probably associated with glacial incision. Following ice sheet retreat, isostatic rebound during deglaciation resulted in uplift of tens to hundreds of metres, locally removing all Cambrian and Ordovician formations. Rising sea levels in the Silurian led to deposition of the Tanezzuft Formation on Precambrian basement in the northwestern Mourizidie region.
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Devonian
Basement
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Carbon isotopes are analyzed for the first time from the Ordovician–Silurian boundary interval in the Williston and Hudson Bay basins of Manitoba, revealing a prominent positive excursion. The nature of this excursion and its co-occurrence with the appearance of the Ozarkodina hassi Conodont Zone and a distinctive coral-dominated macrofauna indicate that this is the upper portion (Metabolograptus persculptus Graptolite Zone) of the globally recognized Hirnantian isotopic carbon excursion (HICE). As a result, the Ordovician–Silurian boundary is placed at a higher position than previously thought, at the disconformable Stonewall – Fisher Branch formational boundary in the Williston Basin, and probably the Port Nelson – Severn River formational boundary in the Hudson Bay Basin. Disconformities within sections suggest periods of nondeposition due to subaerial exposure as sea level fluctuated during the Late Ordovician glaciation. Latest Ordovician (Hirnantian) deposition was far more widespread on the Laurentian craton than realized previously. Age determination and correlation having unprecedented precision are attainable within and between the Williston and Hudson Bay basins, and beyond.
Excursion
Conodont
Chemostratigraphy
Subaerial
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The Jarlsbergian unconformity at the Late Proterozoic-Early Cambrian boundary, is expressed in the Hecla Hoek Succession of South Spitsbergen as a regional low-angle unconformity, the result of folding and subsequent erosion of the Late Precambrian Jarlsbergian Basin deposits. The unconformity pre-dates the Bonnia-Olenellus trilobite zone; the sedimentary hiatus covers the lowest Cambrian Fallotaspis and Nevadella trilobite zones, and a closer undefined uppermost part of the Late Proterozoic. There are no Varangian (latest Proterozoic) tillites present in south Spitsbergen at the top of the Late Proterozoic metasediment column which is represented by the Gashamna Formation phyllites and associated rocks.
Trilobite
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