Relationship to Alluvial Stratigraphy and Depositional Systems
2013. "Relationship to Alluvial Stratigraphy and Depositional Systems", New Frontiers in Paleopedology and Terrestrial Paleoclimatology: Paleosols and Soil Surface Analog Systems, Steven G. Driese, Lee C. Nordt
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Modem sequence-stratigraphic theory has its foundation in the work of L.L. Sloss and W.C. Krumbein (1940s-1960s) and several Exxon researchers (1970s–1990s). This work largely focuses on the nature and origin of sedimentary cycles within marine stratal successions. More recently, sequence-stratigraphic concepts have evolved to include the analysis of terrestrial strata. Historically, the recognition of unconformity-bounded cyclic stratal units (such as sequences) has relied upon the geometric relationships of strata (i.e., onlap, toplap, truncation, and downlap) within two- and/or three-dimensional outcrop or subsurface successions. Oftentimes, however, outcrops or boreholes are isolated and do not preserve these diagnostic stratal relationships. In such instances, documentation of changes in the vertical, rather than lateral, succession of strata may allow reconstruction of the cyclic accommodation history and placement of associated bounding discontinuities. This technique, referred to as “stacking pattern” analysis, was originally developed for shallow-marine carbonate successions. More recently, the stacking pattern methodology has been similarly applied to alluvial successions and takes into account the unique processes of terrestrial deposition and pedogenesis. The most conspicuous and fundamental cyclic stratal units recognized within alluvial settings are fluvial aggradational cycles (FACs). Fluvial aggradational cycles are meter-scale, typically fining-upward successions that have a disconformable lower boundary and an upper boundary that either has a paleosol weathered into it or is disconformably overlain by the succeeding FAC without a paleosol. Fluvial aggradational cycles are thought to represent sediment accumulations during channel avulsion events that are subsequently weathered during the following period of channel stability. A thick succession of FACs indicates sediment accumulation during a prolonged episode of accommodation gain. Variations in the rate of accommodation gain (and loss) are interpreted to result in the organization of FACs into alluvial sequences and longer period composite sequences. Episodes of base-level rise result in relatively rapid rates of alluvial aggradation and less developed and more poorly drained paleosols. Associated FACs are thicker than average and transition from initially lower sinuosity, higher competence alluvial systems to comparably higher sinuosity, lower competence channel deposits. As base-level rise decelerates and initially falls, paleosols become increasingly well developed and better drained, and FACs are thinner than average and transition to even lower competence, higher sinuosity channel sandstones that are more extensive as a result of prolonged channel migration under low accommodation conditions. During base-level fall, the incisement of alluvial valleys produces sequence boundaries that are infrequently flooded across interfluve areas. Fluvial aggradational cycles across interfluve positions are much thinner than average and are characterized by the most well-developed and best-drained paleosols.
Application of the alluvial stacking pattern methodology is demonstrated within three case studies. Case study 1, from Big Bend National Park, Texas, considers a latest Cretaceous to earliest Tertiary passive margin and coastal plain succession and correlates alluvial sequences and associated climate and ecosystem changes to eustatic sea-level oscillations. Case study 2, from northern and northeastern New Mexico, documents a Late Triassic foreland basin succession in which tectonically induced accommodation events are correlated between isolated outcrop successions that are located 200 km apart. Case study 3, from central New York, demonstrates how stacking pattern analysis allows correlation of a Middle Devonian alluvial composite sequence with equivalent regressive-transgressive marine strata along a convergent plate boundary.
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New Frontiers in Paleopedology and Terrestrial Paleoclimatology: Paleosols and Soil Surface Analog Systems
After initial breakthroughs in the discovery of fossil soils, or paleosols in the 1970s and early 1980s, the last several decades of intensified research have revealed the much greater role that these deposits can play in reconstructing ancient Earth surface systems. Research currently focuses on terrestrial paleoclimatology, in which climates of the past are reconstructed at temporal scales ranging from hundreds to millions of years, using paleosols as archives of that information. Such research requires interdisciplinary study of soils conducted in both modern and ancient environments. These issues and many others were discussed at the joint SEPM-NSF Workshop “Paleosols and Soil Surface Analog Systems”, held at Petrified Forest National Park.