A sedimentary structure occurring on the base of some sandstones which are interbedded with mudstones, and characterized by globular protrusions and isolated pillows of sandstone found in the underlying mudstone. These structures form by the differential settling of the unconsolidated sand into less dense mud below.
This chapter summarizes the properties of the materials involved in soft-sediment deformation, framed in terms of the history of possible states over the duration of the process. Soft-sediment deformation structures tell more about the early consolidation history of sediments than about their depositional environment; very few varieties seem specific to a particular depositional setting. The structures are most prevalent, however, in water-laid sediments and arise most frequently where high sedimentation rates prevail to promote the loose packing of sands and silts. Some kinds of soft-sediment deformation dependent primarily on liquefaction involve the production of regular folds, either within one sedimentation unit or at the interface between two, the process being wholly internal to single or paired layers. Soft-sediment deformations arise when liquidized, hydroplastic, and sometimes more competent sediments are stressed during or shortly after deposition. Many soft-sediment deformation structures are related to the presence of a gravitationally unstable profile of bulk density within a sedimentation unit or sequence of unitsfor example, convolute lamination, load casts, pseudo-nodules, ball-and-pillow, heavy mineral sags and drops, and dish structures.
The images shown here illustrate some of the outcrop features associated with fine-grained fluvial deposits; In most cases they relate to ancient high sinuosity (meandering) channel systems and floodplains.
The first three diagrams show some basic sediment descriptors and terminology, and a typical stratigraphic column drawn from outcrop data. These are your starting points for describing and interpreting sedimentary rocks and sedimentary structures in outcrop, hand specimen, and core.
The literature on fluvial systems and their deposits is vast (actually, this also applies to most other sedimentological domains); the accumulation of two centuries of knowledge and excellent science. The few cited here provide a taste of this talent.
J.S. Bridge, 2006. Fluvial facies models: Recent developments. In: Posamentier H W, Walker R G. (Eds.), Facies models revised. SEPM Special Publication 84, 2006: 85170. This is a revision of the iconic volume Facies Models, originally published by the Geological Association of Canada (Geoscience Canada) (1976-79). Currently Open Access
S.K. Davidson, S.Leleu, and C.P. North (Eds.). 2011. From River to Rock Record: The preservation of fluvial sediments and their subsequent interpretation. SEPM Special Publication 97. 21 papers on many aspects of fluvial sedimentology.