Pedogenic Mud Aggregates and Paleosol Development in Ancient Dryland River Systems: Criteria for Interpreting Alluvial Mudrock Origin and Floodplain Dynamics

摘要

Mudrocks with well preserved pedogenic mud aggregates in the alluvial Upper Triassic Lunde Formation, northern North Sea, provide information about preservation potential in alluvial sediments, basin infill dynamics, and paleosol development in ancient dryland river systems. Sand-size to millimeter-size pedogenic mud aggregates composed of densely packed clay and silt dominate the floodplain facies. These aggregates are present both in situ in paleovertisol profiles and as reworked aggregates which form thick floodplain units. In situ aggregates are characterized by homogeneous composition, closely associated with pedogenic slickensides and crystallaria, and display evidence of translocation of clays around them. The reworked mud aggregates are heterogeneous and are mixed with reworked carbonate fragments, rip-up clasts, and large, fresh unweathered micas. These pedogenic mud aggregates have survived compaction down to at least 2500-3000 m of burial without any significant overpressure. High rates of sedimentation resulted in only poor to moderate pedogenic overprinting, which could have destroyed the aggregate texture. The preservation of the aggregates was also related to early carbonate cementation and the formation of robust aggregates suitable to resist disintegration during both reworking and burial. The presence of such mud aggregates emphasizes the importance of mud transported as aggregates and not as suspended load in ancient dryland river deposits. Superimposed pedogenic structures on the reworked aggregates indicate a highly dynamic system with pedogenesis, erosion, reworking and redeposition. In floodplain mudrocks, where the texture of reworked aggregates is lost, the mudrock beds attain a massive and structureless morphology. Such units can be confused with more developed paleosol horizons, and in the present study this process is termed pseudo-pedodestratification. Neglecting or overlooking the presence of the mud aggregates also results in the misinterpretation of alluvial mudrocks and paleosol development, and criteria are presented for recognizing in situ and reworked aggregates.