Oversteepened ripples, sedimentary pull-aparts with draped infills, and convoluted laminae in Paleoproterozoic intertidal deposits – all products of early post-depositional deformation, dewatering, liquefaction, and compaction – in this example possibly triggered by a seismic event. Fairweather Fm, Belcher Islands.
Soft sediment deformation
Deformation of sediment while it is soft or semi-consolidated, is common. The rock record is replete with folded and slumped strata, strata that slid in coherent packages, strata that lose their coherence during liquefaction or fluidization, displacement by faults where soft or plastic sediment seemingly acts like its brittle rock counterparts, or dyke-like injections where sediment is locally overpressured.
The term syn-sedimentary tends to be used rather loosely, as deformation that takes place during or soon after deposition; the ‘soon’ is the loose part of this broad definition. Sediment begins to compact almost immediately following deposition, where framework grains begin to move closer together. Interstitial water is expelled, and this process in itself can deform the sediment. Water expulsion in compacting deeper strata can also increase local pore pressures that in turn reduce sediment shear strength. Other common triggers are gravitational instability and seismic tremors. Coastal storm surges can also produce instability in sea floor sediments caused by rapid fluctuations in pore pressure.
There are several examples of soft- or syn-sedimentary deformed structures in the Atlas of submarine fans (not duplicated here).
Fluvial trough crossbeds here have been turned on end during early compaction and dewatering, producing what are commonly called ball and pillow structure. Proterozoic Loaf Fm. Belcher Islands.This sandstone dyke terminates in, and probably breached shallow intertidal deposits. The surrounding layers have been dragged upwards during sand intrusion. Structures like this commonly form during earthquakes when soft sediment is liquefied. Fairweather Fm, Belcher Islands, about 2 billion years old.Detached load casts in laminated (middle bed), locally rippled volcaniclastics. The wavy bedforms at top may be antidunes formed during supercritical flow. Flaherty (volcanic) Fm, Belcher Islands, about 2 billion years old.A sandstone dyke that originated from deformed – slumped sandy turbidites; the dyke intrudes a slope mudstone-siltstone succession, and extends about 40m up the exposed face. Both the slumping and sandstone intrusion are thought to have formed during a seismic event. Upper Jurassic, Tsatia Mt, Bowser Basin.Dololutite beds deformed while in a plastic state, are interbedded with and truncated by undeformed crossbedded (dolomitic) grainstone. These folds probably formed as packages of sediment were moved during karstification and limestone solution collapse. Some layers have detached from one another, forming voids that were eventually filled by aragonite-fibrous calcite – subsequently converted to dolomite. Both images from the Rowatt Fm, Belcher Islands, about 2 billion years old.Another example of synsedimentary deformation, possible during limestone solution collapse like the image above.Detached slump folded dololutite (brown)-calcilutite (grey). These thin carbonate beds were deposited on a Proterozoic slope (Costello Fm. Belcher Islands), outboard of really large, platform stromatolite buildups.Deformation of a turbidite package under different rheological conditions – brittle failure (beds pulled apart), ductile-plastic behaviour (flow folding), and liquefaction that produced disharmonic and incoherent folds. See also this page on outcrop details. The recumbent fold in this turbidite succession is detached from strata below along a relatively undisturbed glide plane. The lower limb is also cut by small faults. Waitemata Basin, Army Bay, Auckland.Folded, faulted, and discordant packages of turbidites, caused by synsedimentary sliding, slumping and faulting Highway roadcut, Albany, Auckland.Small, detached slump fold carried along the base of a turbidity current. Lower Miocene Waitemata Basin, Cockle Bay, AucklandFolded, and possibly thrust faulted turbidites, below undisturbed beds in the cliffs. Lower Miocene Waitemata Basin, Pukenihinihi Point, north Auckland.A pair of slump folds in Late Miocene Castaic Fm, Ridge Basin turbidites, probably initiated by a seismic events on the bounding San Gabriel strike-slip fault, that probably was an offshoot of the evolving San Andreas transform system.Syndepositional slumps and accommodation faults in thin delta front sandstones and prodelta or slope mudstone in the approximate centre of the basin. The units are part of a sediment wedge that prograde from the northeast margin. Marple Canyon Sandstone Member of the Ridge Route Formation, exposed along Templin Highway. I took this shot during a field trip led by Tor Nilsen.Folding and intrusion of liquefied sand in the Rosario Group, San DiegoLarge recumbent slump fold in a Late Miocene, basin floor submarine fan, Mt. Messenger Formation, North Taranaki, New Zealand. The lower limb lies along the glide plane at the top of white bed (a sandy turbidite) has also been deformed.Seriously deformed slump unit in Late Miocene, basin floor submarine fan, Mt. Messenger Formation, North Taranaki, New Zealand. The unit contains numerous, small, detached folds each with a different orientation. Strata below the detachment (glide plane) are not deformed.Detail of deformation associated with slumping – here, boudinage and tight recumbent folds in dark brown sandstone layers. Late Miocene, basin floor submarine fan, Mt. Messenger Formation, North Taranaki, New Zealand.The iconic ‘Jam Roll’ slump in Late Miocene, basin floor submarine fan, Mt. Messenger Formation, North Taranaki, New Zealand. Folded sandstone-mudstone almost completely encloses the structure.Detail of liquefaction and dewatering structures in the Jam Roll slump- here a modest size mud volcano.Highly fluid mud layers that flowed during liquefaction, exposed on top of the ‘Jam Roll”. Late Miocene, basin floor submarine fan, Mt. Messenger Formation, North Taranaki, New Zealand.Detached folds in ice-contact glacial outwash. Elevated pore pressures and deformation were probably caused by ice loading. Late Pleistocene, Ottawa.
