Slump Structures

Introduction

Slump structures are the sedimentary record of mass movement - sediment that moved downslope while still soft, driven by gravity, and preserved the chaotic or folded internal geometry of that movement. They are found across a huge range of environments and scales, from thin beds on tidal flats to units tens of metres thick on continental slopes. What unites them is rapid deposition on oversteepened slopes, and the triggering of wholesale movement before the sediment had time to consolidate.

Definition and Two Types of Movement

Slump structures is the general term for structures produced by penecontemporaneous deformation resulting from movement and displacement of unconsolidated or semiconsolidated sediment, mainly under the influence of gravity. Two structural end-members are recognised. Pervasive movement involves the interior of the transported mass, producing a chaotic mixture of different sediment types - such as broken mud layers embedded in sandy sediment. Décollement movement concentrates lateral displacement along a sole, producing beds that are tightly folded and piled into nappe-like structures. ^[1]

The distinction between the two types matters for interpreting the intensity and style of the mass movement. Pervasive movement means the mass completely lost coherence and mixed internally - debris flow behaviour. Décollement movement means the mass stayed relatively coherent but slid as a sheet along a basal detachment, folding as it went - more like a slide. Both products can occur in the same slump complex.

Thickness, Faulting, and Field Recognition

Slump structures may involve many sedimentation units and are commonly faulted. Reported thicknesses of slump units range from less than 1 m to more than 50 m. Slump units may be bounded above and below by strata that show no evidence of deformation. ^[1]

The sharp boundary between deformed and undeformed strata is both diagnostic and potentially confusing. In some stratigraphic successions it can be difficult to distinguish slump units from incompetent beds such as shale deformed between competent sandstone or limestone beds during tectonic folding. The synsedimentary origin is supported by geometry - individual convoluted units that are cleanly capped by undisturbed overlying beds - and by the absence of the through-going faults that would be expected in tectonic settings.

Environments

Slump structures typically occur in mudstones and sandy shales, and less commonly in sandstones, limestones, and evaporites. They are generally found in rapidly deposited units where oversteepened slopes led to instability. Documented occurrences span glacial sediments, varved silts and clays of lacustrine origin, eolian dune sands, turbidites, delta and reef-front sediments, subaqueous dune sediments, and sediments from the heads of submarine canyons, continental shelves, and the walls of deep-sea trenches. ^[1]

The sheer breadth of environments reflects the two common conditions - rapid deposition and oversteepened slope - that can be met in many different settings. Continental slopes and submarine canyon walls provide the steepest gradients; delta fronts and reef fronts build sediment piles that regularly exceed the angle of repose; and turbidite successions deliver large sediment volumes rapidly enough to create the pore-water pressures that promote instability.

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