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.