Criteria for Interpreting Paleoclimate from Red Beds - A Tool for Pangean Reconstructions
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Abstract Pangea represented an exceptional global paleogeography, characterized by a single supercontinent symmetrically disposed about the equator. This configuration had an extraordinary effect on global paleoclimate. The Permian to Triassic sedimentary section, with its distinct sedimentologic facies, provides important evidence of a unique global monsoonal climate. Historically, paleoclimate inferences have been attributed to red beds, but ferric oxides that color the strata form in early- to late-diagenetic oxidizing Eh-pH conditions that occur in a spectrum of wet to arid climatic settings. Thus the red color of these beds has no particular paleoclimate significance. However the assemblage of sedimentary and biogenic structures and depositional facies in both red beds and non-red beds provide criteria to assess the distribution, amount and frequency of precipitation and, ultimately, paleoclimate. Criteria that reflect the depth of the paleo-water table are keys to interpreting paleoclimate in Pangean marginal-marine and continental red beds. Paleo-water tables may be locally controlled by tectonic setting, drainage, or seasonal precipitation and temperature variations. Features present within geographically and temporally widespread strata provide the best climatic indicators. In playas, lakes, floodplains and tidal flats deep and wide desiccation cracks indicate previously water-saturated sediment, whereas thoroughly dry sediment that is subsequently only partially saturated will produce short, narrow, complex cracks commonly associated with vesicles. Evaporites in groundwater-saturated saline mudflats exhibit vertical decrease in crystal size and possibly changes in mineralogy and may be associated with sedimentary fabrics indicative of thick, efflorescent salt crusts. Evaporites formed in dry soils in arid or monsoonal-dry settings typically show an upward increase or no variation in crystal size and character, may be associated with rhizoliths and show no preference to depositional setting. Wind deflation and extensive eolianites related to restricted vegetation may indicate deep or saline paleo-water tables. Paleosols and pedogenic features such as mottling, nodules and soil fabrics may overprint any subaerially exposed continental or paralic strata and indicate paleo-water table position, fluctuation and degree of saturation. Biogenic structures such as rhizoliths and invertebrate ichnofossils can mark the sediment-water or wet sediment-air interface, or the vadose and phreatic zones. Sedimentary fabrics and biogenic structures are syndepositional features and are independent of rock color imparted by subsequent diagenetic alterations. These criteria, integrated with paleontologic data and compared to predictive climate models, are the basis for interpreting paleoclimate.Keywords:
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Foreword Introduction Marine Biotic Indicators of Paleoclimate Continental Biotic Indicators of Paleoclimate Marine Lithologic Indicators of Paleoclimate Continental Lithologic Indicators of Paleoclimate Integrating Marine and Continental Paleoclimatic Records Models Case Histories of Paleoclimatic Studies
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ABSTRACT ODP Leg 119 drilled a sequence of ?Permo-Triassic continental red beds 58 m thick containing paleosols in Prydz Bay, East Antarctica. The paleosols, mainly in overbank siltstones and mudstones, are characterized by extensive destruction of sedimentary structures and the presence of mottles, rootlets, and rare chalky caliche. Because of the shallow depth of burial and limited compaction and diagenesis, the paleosols have retained most of their original microstructure and fabric. Micromorphological analysis of the paleosols reveals the presence of phreatic monopodial root systems, nodules, and a variety of soil structures and fabrics as well as a micritic root structure with alveolar texture. Most of the paleosols are weakly developed, with small, incomplete ped structures and lack of illuviation. They resemble weakly developed alluvial soils with A-C and B(C) profiles, and are similar to modern base-deficient inceptisols. The nature of the paleosols suggests relatively high permeabilities and water circulation during soil formation and a relatively mobile water table, with little reduction of iron except locally on the less well drained parts of the alluvial plain. Extensive pedogenic carbonate formation was precluded by the soil-moisture regime and relatively wet climate. During Late Permian and Early Triassic times Prydz Bay, which lay some 30° south of the Equator, experienced climatic conditions broadly similar to parts of the contemporary tropics, dominated by a ubtropical high-pressure system, easterly winds, and seasonal rainfall.
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Numerous climate models predict that the geography of the supercontinent Pangea was conducive to the establishment of a "megamonsoonal" circulation. In general, geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the Triassic. Pangea in the Late Carboniferous had widespread peat formation in what is now central and eastern North America and Europe and relatively dry conditions on the Colorado Plateau. The equatorial region of the continent became drier through the end of the Carboniferous. By the Permian, the equatorial region of Pangea was dry, and indicators of aridity and rainfall seasonality became more widespread. Wind directions from Colorado Plateau eolian sandstones are consistent with an increasing influence of monsoonal circulation at this time. In the Triassic, climate in the Colorado Plateau region became relatively wet, though still seasonal, and the few eolian sandstones indicate a major shift in wind direction at that time. In addition, sedimentation in Australia, which was in relatively high latitudes, took on a much drier and more seasonal character. These two events support the hypothesis that the Pangean monsoon was at maximum strength during the Triassic. In the Early Jurassic, the Colorado Plateau region became arid again, but climate apparently became wetter in eastern Laurussia and Gondwana. Finally, drying occurred in Gondwana and southern Laurasia, indicative of the breakdown of the Pangean monsoon.
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A succession of continental red beds in the Paleogene Carroza Formation, northeastern Mexico, contains an assemblage of evaporite paleosols previously unknown in pre-Neogene strata that record the syndepositional exposure of nearby diapiric evaporite and a climatic shift to increasing aridity. Carroza red beds were deposited in an ephemeral braided-fluvial system in a high-accommodation setting. Paleosols developed in nearly all depositional settings, including channels, crevasse splays, and floodplains, and contain salic/natric, gypsic, baritic, and calcic horizons. Calcic paleosols are limited stratigraphically to the lowermost part of the formation in oyster-bearing estuarine strata and yield upsection to evaporitic paleosols, thus providing a record of increasingly arid conditions as the Paleogene marine shoreline shifted eastward, toward the Gulf of Mexico Basin. The increase in aridity reduced vegetation and residuum thickness on the exposed diapiric salt, consequently increasing the influx of evaporitic minerals into the basin, and driving the development of salic/natric, gypsic, and baritic horizons in all depositional environments. Evaporitic paleosols of the Carroza Formation have characteristics similar to soils forming today in climates with annual precipitation ranging from <80 mm/yr to as much as 450 mm/yr, in apparent conflict with estimates of subhumid to subtropical conditions from Carroza fossil leaf data. Because evaporitic paleosols are persistent throughout the Carroza section, we infer that a combination of spring-fed, high water tables, augmented by flood-basin inundation from high-discharge seasonal fluvial flood events sustained perennial woodlands, and sodium-caused clay dispersion created poor drainage in topographically low parts of a rapidly subsiding salt-withdrawal basin.
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Research Article| March 01, 1967 Formation of Red Beds in Modern and Ancient Deserts THEODORE R WALKER THEODORE R WALKER University of Colorado, Boulder, Colorado Search for other works by this author on: GSW Google Scholar GSA Bulletin (1967) 78 (3): 353–368. https://doi.org/10.1130/0016-7606(1967)78[353:FORBIM]2.0.CO;2 Article history received: 28 May 1965 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation THEODORE R WALKER; Formation of Red Beds in Modern and Ancient Deserts. GSA Bulletin 1967;; 78 (3): 353–368. doi: https://doi.org/10.1130/0016-7606(1967)78[353:FORBIM]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract New evidence supports the theory that the hematite pigment in many red beds, particularly those associated with evaporites and aeolian sandstones, formed after deposition in hot arid or semiarid climates. The evidence includes field, petro-graphic, and chemical data collected from studies of two stratigraphic sequences that contain red beds: (1) Recent, Pleistocene, and Pliocene deposits in the Sonoran desert of northeastern Baja California, Mexico, and (2) late Paleozoic deposits in Colorado.The sequence in the Sonoran desert contains examples of red beds forming today in a hot dry climate. They are associated with bedded evaporites and occur where regional faunal, floral, and pedological evidence indicates rainfall has been low throughout the postdepositional history of the sediments. The facies associations reflect deposition in fluvial and fluvial-marine transitional environments. Red arkose fanglomerates occur on the flanks of the highland source areas, and red muds, probably of intertidal and shallow subtidal origin, occur in the transition sediments. Both of these red-stained facies show progressive stages of in situ alteration of nonred sediments to hematite-stained red beds, and in each, the iron in the stain is derived from intra-stratal alteration of iron-bearing detrital grains, particularly iron silicates such as hornblende and bio-tite.The late Paleozoic red beds of Colorado contain rock types and facies associations similar to those of the Sonoran desert; they are interpreted as ancient counterparts of those red beds. Numerous lines of field and petrographic evidence indicate that the hematite pigment in the late Paleozoic red beds formed in place after the sediments were deposited in desert basins. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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ABSTRACT Lower Permian strata have been extensively cored in the subsurface of the Bravo Dome field, northeastern New Mexico. Analysis of core indicates that these strata consist of conglomeratic and sandy fluvial deposits and volumetrically significant eolian silt (loessite). Fluvial facies dominate the lower half of the study interval and include matrix-supported, massive conglomeratic debris-flow units and laminated arkosic sandstone, whereas loessite dominates the upper half of the study section and consists of massive, well-sorted quartzose siltstone that locally reaches thicknesses as much as 120 m in the greater study region. Paleosols are present throughout the study interval and consist of protosols and dolosols, commonly exhibiting vertic features. Dolomite that is interpreted to be of pedogenic origin is an unusual but volumetrically significant component in these paleosols. Paleogeographic reconstructions and paleomagnetic data indicate that these strata accumulated at equatorial (3-8°) latitudes, but depositional and pedogenic evidence both suggest seasonally wet to markedly arid conditions from early Wolfcampian to early Leonardian time. The loessite covers a substantial area (> 6000 km2), making this the largest pre-Cenozoic loess accumulation yet documented. This is significant, because loess generally suggests arid to semiarid conditions. Intercalated paleosols in the loessite section record repeated cessation of silt influx coupled with landscape stability, which we relate to high-frequency oscillation between dry and slightly wetter conditions, possibly attributable to glacial-interglacial climatic conditions that prevailed at low latitudes. At a lower frequency, the evolution from a predominance of fluvial to primarily eolian strata, in tandem with changes in pedogenic character, reflect a long-term aridification for the study interval. These data corroborate independent inferences of monsoon-induced equatorial aridity in western Pangea and help constrain the timing of the zonal-to-monsoonal transition to earliest Permian time.
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