Quaternary loess-Paleosol sequences as examples of climate-driven sedimentary extremes; Extreme depositional environments; mega end members in geologic time

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Extreme depositional environments: mega end members in geologic time

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Loess is a widespread, wind-transported, silt-dominated deposit that contains geologic archives of atmospheric circulation and paleoclimate on continents. Loess may cover as much as 10% of the Earth's land surface. It is composed mainly of quartz, feldspars, and clay minerals, with varying amounts of carbonate minerals. The geochemistry of loess differs from region to region, depending on source materials, but all loess is very high in SiO (sub 2) with lesser amounts of other major elements. Trends in loess downwind from source areas include systematic decreases in thickness and amounts of sand and coarse silt, and increases in amounts of fine silt and clay. Loess particle size also varies at a given locality over time within individual depositional packages. This variability may be a function of changing wind strengths, different source sediments, or some combination of the two. The classical concept of loess is that is a product of glacial grinding, with subsequent entrainment by wind from outwash deposits. However, it is now known that other processes contribute to silt particle formation, including frost shattering, salt weathering, fluvial and colluvial comminution, eolian abrasion, and ballistic impact. Much debate has taken place over the concept of "desert" (nonglaciogenic) loess, which is widespread in some regions but of limited distribution elsewhere. Nevertheless, glacial silt production probably exceeds the amount of silt generated by all other processes. Much of the loess in or adjacent to deserts may be inherited silt-sized particles from silt-stone, mudstone, shale, and volcanic ash. In many regions, loess is near dune fields or eolian sand sheets. A question that arises from this geographic association is whether or not eolian sand and loess should be considered facies of the same depositional unit. There are regions such as China where these deposits are interbedded, which supports the facies concept. In other regions, such as North America, detailed geochemical and isotopic analyses show that the majority of loess particles were derived from a different, and more distant source than eolian sand. Key to understanding loess stratigraphy and interpreting environments of the past is the recognition of buried soils (paleosols). Ancient soils can be recognized by their distinctive morphological features and by vertical changes in particle size, chemistry, and mineralogy. Paleosols represent past periods when loess sedimentation rates decreased to zero or slowed significantly. Thus, loess and their interstratified soils represent end members of a continuum of sedimentary extremes: high rates of sedimentation yield relatively unaltered loess in the stratigraphic record, whereas low or episodic rates of sedimentation commonly leave a record of buried soils. The shift between these sedimentary extremes is preserved in the long-term glacial-interglacial record of the Quaternary. Although it is now known that not all eolian silt is glaciogenic, in almost all loess regions, eolian sedimentation rates were much higher during glacial periods than during interglacial periods. Drier, colder climates, a decrased intensity of the hydrologic cycle, stroinger or more persistent winds, increased sediment supply, decreased vegetation cover, and increased sediment availability all probably contributed to the sedimentary "extreme" of rapid loess accumulation during the last glacial period. The present interglacial period represents an opposite sedimentary "extreme" of minimal loess sedimentation and is characterized primarily by soil formation within loess deposits of last glacial age.

Published Article/Book Citation

Chan, Marjorie A, and Allen W. Archer. Extreme Depositional Environments: Mega End Members in Geologic Time. Boulder, CO: Geological Society of America, 2003. (Special Paper - Geological Society of America, 370) pp.53-74.

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