Pedogenic Mud Aggregates in Rift Sedimentation
Pedogenic mud aggregates are sand-size particles composed of clay flakes and silt grains, produced in vertic soils with expandable clays during seasonal cycles of wetting and drying. The aggregates are commonly reworked from floodplain soils and transported as bedload, but they tend to be destroyed by compaction during burial. Their identification is important for interpreting floodplain dynamics, i.e., bedload transport rather than suspension settling of clay. Expandable clays, principally smectite, are necessary for aggregate formation, and are commonly produced during weathering of mafic volcanicrocks and ash, which are prominent constituents of many rift-basin fills. To date, pedogenic mud aggregates have been documented from some ancient rift fills.
A new occurrence of mud aggregates is documented from the upper part of the Triassic-Jurassic New Haven Arkose of the Hartford rift basin, Connecticut. The sand- and silt-size aggregates are present in laminated and cross-laminated sandstones and siltstones, where the larger framework grains have protected the aggregates from destruction during compaction. Blue-light microscopy was especially helpful in aggregate identification. The host strata represent shallow bedload channels and floodplain sandstone sheets interbedded with thin red mudstones, interpreted as immature paleosols, and the aggregates are inferred to have originated principally from pedogenic activity in the mudstones and/or other adjacent paleosols. The high-energy floodplains were frequently reworked by overbank floods, and some mudstones may have been deposited by active flows capable of bedload transport of aggregates. Mobilization of soil aggregates is probably promoted in rift settings similar to that of the Hartford basin, where floodplains are small and frequently reworked by high-discharge events.
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Continental rift basins have long been of interest to sedimentologists. Of all the terrestrial settings, rift basins typically provide the greatest accommodation space, and consequently have some of the longest records of continental sedimentation. These records are a product of a complex interplay between several factors that include geological structure and tectonic activity, volcanism, climate and its temporal variability, hydrology, biology and time. Sedimentation in Continental Rifts is a timely update on this exciting interdisciplinary field and presents new approaches and insights into tectonic and structural controls of sedimentation. Other topics included are lacustrine and fluviatile depositional environments and some lesser-known settings, such as springs, wetlands, and paleosols. Several papers consider the behavior of silica in rift lakes, particularly the roles of microorganisms in silica precipitation, whereas others examine the paleoenvironmental importance of freshwater carbonates. The contents of the volume show that sedimentological research in rift basins has progressed beyond basic facies description and general models, and is now focused on understanding the integrative effects of physical, chemical and biological processes in rifts.