Analytical Structure of Soft Sediment Deformation |
TheChaibasa Formation in Eastern India, which was deposited between 2100 and 1600million years ago, shows deformations that must have formed when the sedimentswere not yet consolidated. Some of these deformation structures have never beendescribed before. Here they are described, depicted and their origin isanalysed. We show that they must be the result of shocks, which can only beexplained satisfactorily as triggered by earthquakes. The layers containingthese deformation structures are termed bseismitesQ. They are among theearliest records of earthquakes known in the Earth’s history.Penecontemporaneous decimetre-scale soft-sediment deformation structures arereported from the basal part of the Upper Jurassic–Lower Cretaceous Vaca MuertaFormation, in the Malargüe–Las Leñas area of the back-arc Neuquén Basin(Mendoza Province, Central Andes). The deformed interval (Amarillas bed) isonly 0.3 to 0.9 m thick but occurs in a wide area, larger than 1500 km2. Itsage, determined by ammonite biostratigraphy, is Early Tithonian. Thesoft-sediment deformation structures were generated in finely laminated,partially consolidated, organicrich, carbonate microbialites that weredeposited in open-marine, poorly oxygenated settings, apparently devoid of anysignificant slope. Those structures include boudins of different sizes andcomplexity, a variety of folds, normal (listric) dm-scale faults,sub-horizontal detachment surfaces and other features, which are part ofseveral larger-scale, complex slump structures. Deformation was dominantlyplastic but near to the ductile–brittle field transition. Severaltypes of syndepositional deformation structures contain strain localizationstructures known as disaggregation bands. Abundant field examples from Utahshow that such bands can be related to vertical movements linked to loading andfluid expulsion, forming a pre-tectonic set of strain localization structuresin deformed sandstones that can easily be overlooked or misinterpreted astectonic structures in petroleum reservoirs. Plugmeasurements and thin-section investigations show that they have little or noinfluence on fluid flow. In contrast, disaggregation bands formed as a responseto tectonic stress at higher confining pressures (depths) in the same lithologyshow up to 3–4 orders of magnitude reduction in permeability. This makes itimportant to distinguish between synsedimentary and tectonic deformation bands.They should also be separated because only bands formed in relation to tectonicstress can be used to predict nearness to important faults and to assess theextent of faulting in a reservoir. Thestandard explanation for soft sediment deformation is associated with overturnof inverted density gradients. However, in many cases, observations do notsupport this interpretation. Here we suggest an alternative in which stablystratified layers undergo a shear instability during relative sliding via theKelvin–Helmholtz Instability (KHI) mechanism, triggered by earthquake shaking.Dead Sea sediments have long stood out as a classical and photogenic examplefor recumbent folding of soft sediment. These billow-like folds are strikinglysimilar to KHI structures and have been convincingly tied to earthquakes. Ouranalysis suggests a threshold for ground acceleration increasing with thethickness of the folded layers. Themaximum thickness of folded layers (order of decimeters) corresponds to groundaccelerations of up to 1 g. Such an acceleration occurs during largeearthquakes, recurring in the Dead Sea.