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Report from working group on alluvial pedogenesis
Part V - - Working Group Reports
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REPORT FROM WORKING GROUP ON ALLUVIAL PEDOGENESlS W.J. Autin,* A. Aslan,t E.A. Bettis:~ and P.M. Walthall§ *Institute of Environmental Studies, Louisiana State University, Baton Rouge, LA 70803, U.S.A. tDepartment of Geological Sciences, University of Colorado, Boulder, CO 80309-0250, U.S.A. J~IowaDepartment of Natural Resources, Geological Survey Bureau, 123 North Capital Street, Iowa City, 1,4 52242, U.S.A. §Department of Agronomy, LSU Agricultural Center, Louisiana State University, Baton Rouge, LA 70803, U.S.A.
Deposition of alluvium in fluvial and coastal settings occurs on every continent and is a relevant process throughout Earth history. Soil development on such landscapes is widely discussed by the investigators of alluvial sediments and soils. However, the relation of pedogenic processes to sedimentologic and geomorphic processes, and ground-water-surface-water interaction on active alluvial plains is poorly understood. There is a tendency among paleopedologists to apply axioms and inferences developed from the study of presently forming landscapes to ancestral soil forming environments. However, much of the current thinking about soil evolution on modern landscapes is developed from research on either upland settings, or terraced alluvial landforms affected by base level change. This attempt to translate the results from one landscape setting to a markedly different set of landscape positions has produced substantial misconceptions. Such correlation via analogue is fraught with uncertainty, especially in the rock record where deposits and paleosols have been affected by significant and very complex evolutionary changes. We wish to propose the formation of an Alluvial Pedogenesis Working Group to address ground issues of concern to research investigators of both currently evolving and ancestral alluvial settings. We suggest the following set of issues as a preliminary starting point for discussion. These uses arise from what may be a lack of communication between paleosol investigators and researchers of soils of the modern landscape. It is likely that an open discourse on these issues and others of relevance will foster an increased awareness of the complexity of alluvial pedogenesis in both Quaternary and pre-Quaternary settings.
ISSUE 1 Alluvial soils form in aggradational sedimentary sequences, therefore, alluvial paleosols are cumulative in nature and have depositional environment controlled vertical and lateral textural variability. Sedimentation is concurrent with pedogenesis, and pedogenic processes can operate in competition with sedimentation processes. Alluvial paleosols typically preserve a record of varying rates of aUuviation and soil evolution during the time interval of alluvial activity, plus the effects associated with subsequent alluvial system abandonment and preservation.
ISSUE 2
The hydrology of alluvial soils is complex and rarely unidirectional. Soil hydrology is governed by precipitation, stream flow dynamics, and ground water conditions in the alluvial aquifer. Moisture regime, as reflected by a paleosors preserved morphology, records the cumulative effects of these processes as they interact over time.
ISSUE 3 Paleosol morphology reflects processes of alluvial sedimentation, pedogenesis in an active alluvial environment, and complex alluvial hydrology. A paleosol can experience additional changes subsequent to its removal from the alluvial environment. Base-level change due to terracing or burial in a subsiding setting are common examples of such changes. Research in the Lower Mississippi Valley indicates that initial soil morphologic development and horizon sequences produced during incipient soil evolution affect the potential direction and magnitude of subsequent pedogenic (and possibly diagenetic) pathways available to a paleosol after base-level modification.
ISSUE 4 Processes that occur in alluvial soils and regulate their preservation as paleosols require careful investigation and a thorough understanding before chronologic or paleoenvironmental inferences can be applied. Pedogenic evolution through complex pathways can confuse applications of relative soil morphology, geochemical estimations of age (chronosequence analysis), and paleoclimate inferences extracted from soil morphologic properties (e.g. rubification, clay accumulation rates, carbonate accumulation).
SUMMARY These uses illustrate the complexity of alluvial pedogenesis as it relates to the analysis and interpretation of paleosols. Difficulties with interpretations of alluvial paleosols are probably greatest when applied to the preserved sedimentary record, where direct evidence of paleolandscape variability is scanty or lacking.
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Part V -- Working Group Reports
Traditional axioms applied to alluvial paleosols need re-evaluation as pedogenic systems that commonly have complex parent materials and intricate hydrologic evolution. Traditional models of pedogenic evolution do not automatically translate to alluvial landscape
settings that evolve under fundamentally different pedogenic pathways from upland settings. With this thought in mind, we wish to initiate an open dialogue among individuals with ideas relevant to this topic.
85
REPORT FROM WORKING GROUP ON ALLUVIAL PEDOGENESlS W.J. Autin,* A. Aslan,t E.A. Bettis:~ and P.M. Walthall§ *Institute of Environmental Studies, Louisiana State University, Baton Rouge, LA 70803, U.S.A. tDepartment of Geological Sciences, University of Colorado, Boulder, CO 80309-0250, U.S.A. J~IowaDepartment of Natural Resources, Geological Survey Bureau, 123 North Capital Street, Iowa City, 1,4 52242, U.S.A. §Department of Agronomy, LSU Agricultural Center, Louisiana State University, Baton Rouge, LA 70803, U.S.A.
Deposition of alluvium in fluvial and coastal settings occurs on every continent and is a relevant process throughout Earth history. Soil development on such landscapes is widely discussed by the investigators of alluvial sediments and soils. However, the relation of pedogenic processes to sedimentologic and geomorphic processes, and ground-water-surface-water interaction on active alluvial plains is poorly understood. There is a tendency among paleopedologists to apply axioms and inferences developed from the study of presently forming landscapes to ancestral soil forming environments. However, much of the current thinking about soil evolution on modern landscapes is developed from research on either upland settings, or terraced alluvial landforms affected by base level change. This attempt to translate the results from one landscape setting to a markedly different set of landscape positions has produced substantial misconceptions. Such correlation via analogue is fraught with uncertainty, especially in the rock record where deposits and paleosols have been affected by significant and very complex evolutionary changes. We wish to propose the formation of an Alluvial Pedogenesis Working Group to address ground issues of concern to research investigators of both currently evolving and ancestral alluvial settings. We suggest the following set of issues as a preliminary starting point for discussion. These uses arise from what may be a lack of communication between paleosol investigators and researchers of soils of the modern landscape. It is likely that an open discourse on these issues and others of relevance will foster an increased awareness of the complexity of alluvial pedogenesis in both Quaternary and pre-Quaternary settings.
ISSUE 1 Alluvial soils form in aggradational sedimentary sequences, therefore, alluvial paleosols are cumulative in nature and have depositional environment controlled vertical and lateral textural variability. Sedimentation is concurrent with pedogenesis, and pedogenic processes can operate in competition with sedimentation processes. Alluvial paleosols typically preserve a record of varying rates of aUuviation and soil evolution during the time interval of alluvial activity, plus the effects associated with subsequent alluvial system abandonment and preservation.
ISSUE 2
The hydrology of alluvial soils is complex and rarely unidirectional. Soil hydrology is governed by precipitation, stream flow dynamics, and ground water conditions in the alluvial aquifer. Moisture regime, as reflected by a paleosors preserved morphology, records the cumulative effects of these processes as they interact over time.
ISSUE 3 Paleosol morphology reflects processes of alluvial sedimentation, pedogenesis in an active alluvial environment, and complex alluvial hydrology. A paleosol can experience additional changes subsequent to its removal from the alluvial environment. Base-level change due to terracing or burial in a subsiding setting are common examples of such changes. Research in the Lower Mississippi Valley indicates that initial soil morphologic development and horizon sequences produced during incipient soil evolution affect the potential direction and magnitude of subsequent pedogenic (and possibly diagenetic) pathways available to a paleosol after base-level modification.
ISSUE 4 Processes that occur in alluvial soils and regulate their preservation as paleosols require careful investigation and a thorough understanding before chronologic or paleoenvironmental inferences can be applied. Pedogenic evolution through complex pathways can confuse applications of relative soil morphology, geochemical estimations of age (chronosequence analysis), and paleoclimate inferences extracted from soil morphologic properties (e.g. rubification, clay accumulation rates, carbonate accumulation).
SUMMARY These uses illustrate the complexity of alluvial pedogenesis as it relates to the analysis and interpretation of paleosols. Difficulties with interpretations of alluvial paleosols are probably greatest when applied to the preserved sedimentary record, where direct evidence of paleolandscape variability is scanty or lacking.
86
Part V -- Working Group Reports
Traditional axioms applied to alluvial paleosols need re-evaluation as pedogenic systems that commonly have complex parent materials and intricate hydrologic evolution. Traditional models of pedogenic evolution do not automatically translate to alluvial landscape
settings that evolve under fundamentally different pedogenic pathways from upland settings. With this thought in mind, we wish to initiate an open dialogue among individuals with ideas relevant to this topic.