Small-Scale Soft-Sediment Deformation Structures in the Cross-Bedded Coconino Sandstone (Permian; Arizona, United States); Possible Evidence for Seismic Influence

The Permian Coconino Sandstone of northern Arizona contains numerous small-scale, soft-sediment deformation structures (SSDSs). These novel structures may be indicators of paleoenvironment or sedimentary processes. These SSD are generally shallow and occur on the surfaces of cross-beds, in contrast to convoluted bedding up to tens of meters thick commonly observed in some other eolian sandstones. These differences in structures imply differences in the processes that formed the Coconino Sandstone, or differences in the underlying depositional conditions. These SSDSs occur in outcrops at the Grand Canyon, and farther south in quarries near the towns of Seligman and Ash Fork. Size, orientation, structure, sedimentary context, clay content, and porosity of the structures are described. The SSDSs occur as small folds and ridges on the paleo lee side of otherwise undisturbed cross-beds. Some are associated with small rotated sandstone blocks within the cross-beds. The structures are exposed on bedding plane surfaces and in cross-section on vertical quarry walls. A few SSDSs up to a meter thick also occur in the Coconino Sandstone, but the others are only up to a few cm thick, 2–10 cm wide, and 20 cm to 10 m long. Evidence is presented that liquidization (as fluidization or liquefaction) may have been involved in producing these features, implying a high water content in scattered locations at time of deformation, but this process also requires some stressor to trigger the deformation. Seismic events may provide a triggering mechanism. The Coconino Sandstone SSDSs represent unusual or previously overlooked small-scale features related to individual foreset surfaces.

Framboid Shapes

The original idea that framboids were generally spherical was due to the limitations of the contemporary optical microscopic methods. Later scanning microscopic investigations showed that many framboids were at least partly faceted and some display polygonal icosahedral forms. This is significant since the assumption of framboid sphericity informed earlier explanations of how they could form. It cannot be assumed, for example, that framboids necessarily require a precursor template, such as a spherical space or spherical organic globule, to develop. There is a continuum in original framboid shapes between ellipsoid, oblate spheroids, prolate spheroids, and spheroids. Irregularly curved shapes are common, especially in clusters of framboids, and result from deformation under the influence of gravity, analogous to soft sediment deformation. Framboidal icosahedra have varying triangular faces and are similar to the pseudo-icosahedral habit of pyrite macrocrystals. Framboids with mixtures of curved and faceted faces are common and these may result in part by local organized internal microcrystal domains. Various framboid clusters have been described as polyframboids, but the term is strictly reserved to spherical clusters of framboids. The constituent framboids may number 100–200 in these polyframboids, and they commonly show evidence of soft-sediment deformation.

The Use of Soft-Sediment Deformation Structures As Proxies For Paleoseismic Activity And Shaking: A Review

Quantifying the magnitude of an earthquake is very important for long-term and medium-term earthquake prediction, post-earthquake emergency rescue and seismic hazard assessment. Paleoseismology is the investigation of past earthquakes in the geological record, in particular their location, timing and size. Uncertainties remain in the paleoearthquake magnitudes determined by traditional surface rupture parameters, especially because most seismic events do not result in surface ruptures. In order to address the problem of magnitude evaluation of earthquakes that did not reveal major dislocations, this paper deals with the methods used to determine the seismic shaking intensity based on the types and forms of soft-sediment deformation structures, including maximum liquefaction distance, thickness of disturbed layer, empirical formulae, and thickness of rapidly deposited sand layer. Then we discuss and analyze these methods in terms of their theoretical basis, advantages and disadvantages, accuracy, applicability and problems. We chose two case studies: first, a typical seismics-related deposit (liquefied layer and dsirupted layer) represented by a seismite in the late-Pleistocene Lake Lisan section near Masada in the Dead Sea Basin; and second, the liquefied diapir triggered by an earthquake in the late-Quaternary lacustrine sediments at Luobozhai in the upper reaches of the Minjiang River, east Tibet. The six methods listed above are employed to determine earthquake magnitudes associated with the seismics-related deposit and liquefied diapir, yielding magnitudes of 5.5-6.5 and 6-7, respectively. The combination of the six methods, provided a new and relatively convenient method for determining seismic shaking, especially in lacustrine sediments. This study can serves as a valid reference for comparing methods of calculating the magnitude of a paleoearthquake based on surface rupture parameters, and provides a better understanding of the long-term seismic activity and risk in tectonically active regions.