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1

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Annual monsoon rains recorded by Jurassic dunes (2001)

Rowe, Clinton M. ; Joeckel, R. Matthew ; Loope, David B.

[s.l.] : Macmillian Magazines Ltd.

Nature 412 (2001), S. 64-66

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] Pangaea, the largest landmass in the Earth's history, was nearly bisected by the Equator during the late Palaeozoic and early Mesozoic eras. Modelling experiments and stratigraphic studies have suggested that the supercontinent generated a monsoonal atmospheric circulation that led to extreme ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/35083554

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2

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Anomalous 17 O compositions in massive sulphate deposits on the Earth (2000)

Thiemens, Mark H. ; Farquhar, James ; Campbell, Douglas A. ; [et al.]

[s.l.] : Macmillian Magazines Ltd.

Nature 406 (2000), S. 176-178

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] The variation of δ18O that results from nearly all physical, biological and chemical processes on the Earth is approximately twice as large as the variation of δ17O. This so-called ‘mass-dependent’ fractionation is well documented ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/35018052

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3

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Palaeo-yardangs: wind-scoured desert landforms at the Permo-Triassic unconformity (1992)

TEWES, DAVID W. ; LOOPE, DAVID B.

Oxford, UK : Blackwell Publishing Ltd

Sedimentology 39 (1992), S. 0

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ISSN: 1365-3091

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: In Canyonlands National Park, south-east Utah, at least 29 partly exhumed, aligned sandstone ridges trending generally N20°W occur at the upper unconformable surface of the Lower Permian (Leonardian) White Rim Sandstone. The ridges are at least 1·5 km long, 250 m wide and have up to 14 m of vertical relief (mean of 9 m). A thin lag of coarse sandstone that contains wind-ripple laminae and granule ripples directly overlies the ridges. Angular blocks of sandstone within the lag and sand-filled fissures immediately below the lag, within the ridges, attest to early cementation of the ridge-forming material. SE-dipping aeolian cross-strata within the White Rim Sandstone and within the lag closely parallel the ridge trend. The ridges are interpreted as wind-sculpted desert landforms (yardangs) that developed on the lithified upper surface of the White Rim Sandstone during an extended period of hyperaridity towards the end of the Permian.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-3091.1992.tb01037.x

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4

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Origin of extensive bedding planes in aeolian sandstones: a defence of Stokes' hypothesis (1984)

LOOPE, DAVID B.

Oxford, UK : Blackwell Publishing Ltd

Sedimentology 31 (1984), S. 0

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ISSN: 1365-3091

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-3091.1984.tb00728.x

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5

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Jurassic aeolian oolite on a palaeohigh in the Sundance Sea, Bighorn Basin, Wyoming (1997)

KILIBARDA, ZORAN ; LOOPE, DAVID B.

Oxford, UK : Blackwell Publishing Ltd

Sedimentology 44 (1997), S. 0

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ISSN: 1365-3091

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Aeolian limestones are widespread in the Quaternary record and have been identified in outcrops and cores of late Palaeozoic strata. These rocks have been interpreted as a low latitude signal of glacio-eustatic sea level fluctuations and have not been previously reported from the Mesozoic or from other episodes of earth history generally believed to have been non-glacial. Numerous lenticular bodies of cross-stratified oolite lie near the contact between the lower and upper members of the mudstone-dominated lower Sundance Formation (Middle and Upper Jurassic) in the Bighorn Basin of north-central Wyoming, USA. The lenses, up to 12 m thick, contain sedimentary structures diagnostic of aeolian deposition. Inversely graded laminae within thick sets of cross-strata were deposited by climbing wind ripples. Adhesion structures and evenly dispersed lag granules are present in flat-bedded strata at the bases of several of the oolite bodies. Thin sections reveal abundant intergranular micrite of vadose origin. The lenses appear to represent virtually intact, isolated aeolian bedforms that migrated across a nearly sand-free deflation surface. When the Sundance Sea transgressed the dunes, a thin (〈1 m thick), wave-rippled, oolite veneer formed on the upper surface of the aeolianite. Previous workers, primarily on the basis of sedimentary structures in the veneer, interpreted the oolite lenses as tidal sand bodies.The dunes provide clear evidence of widespread subaerial exposure on the crest and north flank of the Sheridan Arch. This structural high was delineated by previous workers who demonstrated thinning of pre-upper-Sundance Formation strata and localized development of ooid shoals. Ooids that formed in shoals on the windward (southern) side of the palaeohigh were exposed and deflated during lowstand. Thin, scour-filling ooid grainstone lenses that crop out in the southern part of the study area represent remnants of the marine beds that sourced the aeolianites. Farther north (down-wind), oolitic dunes prograded over thinly laminated lagoonal silts. When relative sea level began to rise, the uncemented dunes were buried under fine-grained marine sediment as the lee side of a low-relief island was inundated.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-3091.1997.tb01531.x

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6

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Deformation structures and an alteration zone linked to deposition of volcanogenic sulphate in an ancient playa (Oligocene of Nebraska, USA) (2005)

LOOPE, DAVID B. ; MASON, JOSEPH A. ; BAO, HUIMING ; [et al.]

Oxford, UK : Blackwell Science Ltd

Sedimentology 52 (2005), S. 0

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ISSN: 1365-3091

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Historic, sulphur-rich volcanic eruptions have altered global climate for as much as five years, and much larger events are known from the geologic record. At Scotts Bluff, Nebraska, Early Oligocene strata of the lower Arikaree Group contain a tephra bed with abundant calcite pseudomorphs after gypsum. Previous work has shown sulphate from the pseudomorphs in this tephra bears a high 17O anomaly indicative of oxidation of sulphur gases by ozone or hydrogen peroxide in the atmosphere. Possible sources of the tephra were caldera eruptions at about 28 Ma in the San Juan volcanic field of south-western Colorado (∼500 km SW of the study site) and the eastern Great Basin (∼1000 km WSW). The present sedimentological study shows that tephra and volcanogenic sulphate were deposited and preserved within a small, surface-discharging playa that developed on the irregular upper surface of aeolian siltstones of the subjacent White River Group. Sulphate solutions (including perhaps sulphuric acid) percolated downward within the vadose zone, dissolving early formed smectite cement within underlying volcaniclastic sandstones, reddening these rocks along an irregular alteration front. Preserved fine-scale stratification within the sandstones precludes the possibility that reddening took place during pedogenesis. Displacive growth of gypsum at the playa centre folded tephra beds and forced tephra into underlying sandstones, forming elongate cones. The large mass fraction of gypsum (now replaced by calcite) in the playa sediments suggests a huge, long-distance delivery of sulphate aerosols. Some of the sulphate and tephra may have come from the same eruption, or the fine-grained tephra may simply have aided preservation of dry-fog sulphate derived from an unrelated, effusive eruption of lava.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-3091.2004.00689.x

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7

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Tropical westerlies over Pangaean sand seas (2004)

Loope, David B. ; Steiner, Maureen B. ; Rowe, Clinton M. ; [et al.]

Oxford, UK : Blackwell Science Ltd

Sedimentology 51 (2004), S. 0

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ISSN: 1365-3091

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Cross-equatorial, westerly winds are key features of tropical circulation in monsoonal regions. Although prominent in numerical climate models of Pangaea (the supercontinent straddling earth's equator, Late Palaeozoic to Early Mesozoic), such flow has not been confirmed previously by migration directions of ancient dunes. Wind-blown sandstones that span 100 million years of earth history are widely exposed in south-western USA. If recent palaeomagnetic data from the Colorado Plateau are used to correct Mesozoic palaeogeographic maps, the Plateau is placed about 10° further south than previously assumed, and the prevailing north-westerly surface winds recorded by dune-deposited sandstones are explicable as cross-equatorial westerlies – the hallmark of modern monsoon circulation. Permian to Early Jurassic dunes were driven by north-westerlies produced by a steep pressure gradient spanning the supercontinent during December–January–February. Although winds are light in most modern, near-equatorial settings, the East African Jet accounts for more than half the cross-equatorial flow in June–July–August. The thicknesses of annual depositional cycles within the Navajo Sandstone indicate that the near-equatorial, north-westerly winds that drove these particular dunes were stronger than the modern East African Jet. The Early Jurassic dunes that deposited the thick cycles were positioned west of the dominant (southern hemisphere) thermal low and against highlands to the west – a setting very similar to the East African Jet. The mountains along the western coast of Pangaea not only enhanced wind strength, but also cast a rain shadow that allowed active dunes to extend very close to the palaeoequator.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1365-3091.2003.00623.x

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8

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Holocene history of lacustrine and marsh sediments in a dune-blocked drainage, Southwestern Nebraska Sand Hills, U.S.A. (1997)

Mason, Jon P. ; Swinehart, James B. ; Loope, David B.

Springer

Journal of paleolimnology 17 (1997), S. 67-83

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ISSN: 1573-0417

Keywords: paleohydrology ; paleoclimate ; sand dunes ; eolian ; Holocene ; Nebraska

Source: Springer Online Journal Archives 1860-2000

Topics: Biology , Geosciences

Notes: Abstract As many as 2500 interdune lakes lie within the Nebraska Sand Hills, a 50 000 km stabilized sand sea. The few published data on cores from these lakes indicate they are typically underlain by less than two m of Holocene lacustrine sediments. However, three lakes in the southwestern Sand Hills, Swan, Blue, and Crescent, contain anomalously thick marsh (peat) and lacustrine (gyttja) sediments. Swan Lake basin contains as much as 8 m of peat, which was deposited between about 9000 and 3300 years ago. This peat is conformably overlain by as much as 10.5 m of gyttja. The sediment record in Blue lake, which is 3 km downgradient from Swan lake, dates back to only about 6000 years ago. Less than two m of peat, which was deposited from 6000 to 5000 years ago, is overlain by 12 m of gyttja deposited in the last 4300 years. Crescent Lake basin, one km downgradient from Blue Lake, has a similar sediment history except for a lack of known peat deposits. Recently, a 8-km long segment of a paleovalley was documented running beneath the three lakes and connecting to the head of Blue Creek Valley. Blockage of this paleovalley by dune sand during two arid intervals, one shortly before 10 500 yr BP and one in the mid-Holocene, has resulted in a 25 m rise in the regional water table. This made possible the deposition of organic-rich sediment in all three lakes. Although these lakes, especially Swan, would seem ideal places to look for a nearly complete record of Holocene climatic fluctuations, the paleoclimatic record is confounded by the effect dune dams have on the water table. In Swan Lake, the abrupt conversion from marsh to lacustrine deposition 3300 years ago does not simply record the change to a wetter regional climate; it reflects the complex local hydrologic changes surrounding the emplacement and sealing of dune dams, as well as regional climate.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1023/A:1007917110965

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