Deformation Structures

Created when soft sediments are compressed, sheared, or disrupted by ice, water, or gravity.

Understanding Deformation in Soft Sediments

Deformation structures form when soft sediments are subjected to stress during or shortly after deposition. These features reflect compressional, extensional, or shear forces acting on unconsolidated or partially lithified materials—often influenced by glacial loading, meltwater pressure, or gravity-driven mass movements.

In glacigenic and sediment gravity flow environments, deformation can be widespread and highly variable, producing structures such as convolute bedding, folds, faults, shear planes, boudins, injections, and glaciotectonites. These features often record subglacial shearing, mass transport events, or soft-sediment slumping, offering valuable insight into sediment dynamics and depositional history.

This section presents visual examples and interpretations of deformation structures preserved in glacially influenced deposits, helping users recognize the processes responsible for their formation and understand their significance in stratigraphic and paleoenvironmental reconstruction.

Deformed Sand Blocks

Deformed sand blocks are isolated or semi-coherent masses of sand that have been disrupted, tilted, or folded within a finer-grained matrix. These blocks often retain internal bedding but appear out of place or rotated within surrounding sediments.

Formation

These features form when underlying or adjacent sand bodies are dislodged and displaced by glacial loading, mass movement, or fluid overpressure. They may be rafted by debris flows or dislocated by glaciotectonic thrusting and folding, often in water-saturated settings.

Significance

Deformed sand blocks indicate episodic disturbance in the sedimentary record and are commonly associated with glaciotectonites, slump deposits, or debris flow margins. Their presence highlights instability during or shortly after deposition and helps reconstruct processes like ice-push deformation, mass wasting, or rapid burial events.

San Juan Province, Argentina

Deformed Sand Block with Wisps

Description:
-A disrupted block of sand preserved within a finer-grained matrix
-Characterized by sharp contacts, internal contortion, and possible minor folding
-Surrounded by wispy extensions or streaks of sand trailing from the block into the surrounding material

Interpretation:
-The block shows signs of displacement or rotation, likely during or shortly after deposition
-Wisps are interpreted as drag features—sediment pulled or injected during the block’s movement
-Deformation likely reflects localized overpressure, differential loading, or liquefaction

Significance:
-Represents a moment of soft-sediment deformation in progress
-Wisps capture subtle fluidization or strain transfer from adjacent moving material
-Useful for identifying zones of instability and internal reorganization within rapidly deposited sequences

San Juan Province, Argentina

Rotated Sand Block

Description:
-A semi-coherent block of sand preserved within a finer-grained or deformed matrix
-Internal bedding is visible but offset from surrounding stratigraphy
-Exhibits clear signs of rotation and displacement

Interpretation:
-Formed through soft-sediment deformation shortly after deposition
-Likely driven by liquefaction, shearing, or local slope instability
-May occur as part of a larger deformation zone (e.g., slump or debris flow)

Significance:
-Indicates mass movement within an unconsolidated sediment package
-Rotation provides evidence of directional stress and flow reorganization
-Useful for reconstructing post-depositional deformation or slope failure events

Injection Structures

Injection structures form when liquefied sediment is forced into overlying or adjacent layers. These features range in scale from large clastic dikes and sand sills that cut across bedding, to small-scale structures like flames where tongues of sediment intrude upward into overlying layers in flame- or cusp-like patterns.

Formation

These structures develop under conditions of high pore fluid pressure, often caused by rapid sediment loading, meltwater influx, or seismic activity. Fluidized sands or silts are injected into weaker or overlying beds, disrupting the original stratification. In glacial and paraglacial environments, triggers include meltwater overpressure, ice loading, and mass transport events.

Significance

Injection structures are key indicators of fluidization and instability in sedimentary environments. Their presence reflects dynamic depositional settings and processes such as rapid burial, seismic shaking, and debris flow impact. In glacigenic systems, they can signal moments of sudden pressure release and help identify disturbed or remobilized intervals within the stratigraphy.

San Juan Province, Argentina

Flame Structures

Description:
-Upward-pointing, tongue-like protrusions of mud or silt injected into an overlying sand layer
-Typically occur at the interface between a coarser overlying unit and a finer underlying bed
-Flames are often irregular, with cuspate to lobate geometries, and may be symmetrical or show a preferred direction

Interpretation:
-Formed by soft-sediment deformation during or shortly after deposition
-Caused by instability at the density boundary, where denser overlying sediment sinks into water-saturated mud below, displacing finer sediment upward
-May result from rapid sediment loading, pore pressure increase, or vibration/seismic shock

Significance:
-Indicators of instability and rapid deposition in water-saturated settings
-Common in fluvial, deltaic, glaciolacustrine, and gravity flow environments

San Juan Province, Argentina

Diamict with Liquefied Substrate Incorporation (Deformed Basal Contact)

Description:
-A massive, poorly sorted diamicton overlies and incorporates fragments of the underlying stratified sediment
-The contact between the diamict and substrate is irregular, marked by entrained blocks, ripped-up clasts, or fluidized wisps of finer material
-Underlying beds show signs of partial liquefaction, with internal disruption but limited folding or shear continuity

Interpretation:
-Reflects a free-slip basal interaction during flow emplacement
-Liquefaction of the substrate likely occurred due to rapid loading, elevated pore pressure, or seismic agitation, allowing substrate material to be entrained into the base of the flow
-The absence of a well-developed basal shear zone suggests limited mechanical coupling between flow and substrate

Significance:
-Indicates soft-sediment deformation by liquefaction and sediment incorporation, rather than ductile shear
-Supports interpretation of a high-mobility, substrate-decoupled flow (e.g., fluidized debris flow or water-rich MTD)

Folds

Folds are curved or bent layers of sediment that reflect deformation under compressional or shear stress. They range in scale from gentle undulations to tight, overturned, or recumbent structures. In soft sediments, folds may appear as isolated features or as part of larger glaciotectonic or slump-related deformation zones.

Formation

Folds form when cohesive, unconsolidated sediments are subjected to compressional or tangential forces before full lithification. In glacigenic and mass transport settings, folding may result from ice-push deformation, glaciotectonic thrusting, subglacial shearing, or slope failure and slumping. Fold geometry often reflects the style and direction of applied stress.

Significance

Folds are valuable indicators of subsurface stress orientation and deformation intensity. In glacial environments, they often record ice movement direction, bed conditions, or reorganization of sediment during mass transport. Recognition of folding; especially when combined with associated features like faults or shear planes—can help interpret depositional and post-depositional processes within dynamic sedimentary systems.

University of Wisconsin–Milwaukee’s Sedimentology

Recumbent Fold (Hand Sample)

Description:
-A tight, overturned fold with limbs oriented nearly horizontal
-Fold hinge is often curved or distorted, and the axial plane lies at a low angle to bedding
-Internal layering shows symmetrical or asymmetrical curvature, often with associated minor shears or disrupted bedding continuity

Interpretation:
-Formed through compressional or shear stress applied to soft, unconsolidated sediment
-Typically develops in environments where horizontal stress exceeds vertical resistance, such as in slumping, glaciotectonic thrusting, or subaqueous mass movement
-Indicates that deformation occurred prior to full lithification, likely during or shortly after deposition

Significance:
-A classic indicator of soft-sediment deformation under horizontal compression
-Often associated with mass transport deposits, glaciotectonic deformation, or slope failure in water-saturated environments
-Fold orientation and asymmetry can be used to infer stress direction and kinematic conditions during deformation

Shear Planes

Shear planes are discrete surfaces within sediments where material has been laterally displaced due to stress, often appearing as zones of fissility, alignment, or localized grain reorientation. When deformation is more distributed, these may develop into broader shear zones with a penetrative internal fabric.

Formation

Shear planes and zones form through ductile deformation of soft, water-saturated sediments, often under conditions of subglacial drag, glaciotectonic compression, or within slow-moving debris flows. In glacial environments, shear can occur at the ice-bed interface or within subglacial deforming layers.
A key feature of many shear zones is the development of C–S fabric, where C-surfaces (shear planes) represent the direction of simple shear and S-surfaces (foliation or layering) develop oblique to it. This microstructural pattern provides insight into the strain path and kinematic conditions of deformation.

Significance

Shear planes and C–S fabrics are important indicators of directional stress and progressive strain. In glacigenic systems, they can reveal ice movement direction, bed deformation styles, and subglacial water content. Their recognition is critical for interpreting deformation histories in tills, glaciotectonites, and soft-sediment thrust complexes.

San Juan Province, Argentina

C–S Fabric

Description:
-A microstructural fabric defined by two intersecting planes:
-C-planes (cisaillement): shear planes, parallel to the direction of overall shear
-S-planes (schistosity): foliation or alignment surfaces that form oblique to the shear plane
-Often appears as fine-scale alignment of clasts, grains, or foliated laminae, creating a characteristic angular relationship (typically 10–35°)

Interpretation:
-Formed by ductile shear within soft or partially consolidated sediment
-C–S fabrics reflect simple shear deformation, with S-planes rotating toward C-planes as strain accumulates
-Indicates direction and sense of shear—the acute angle between C and S planes points toward the direction of displacement

Significance:
-Key indicator of progressive strain and subglacial or mass flow deformation
-Used to infer shear direction and deformation regime in subglacial tills, deformed diamicts, and shear zones
-Often preserved in hand samples, thin sections, or outcrops with well-defined planar fabrics