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.

Volcaniclastic lacustrine sedimentation in the Pleistocene Guayllabamba intermontane basin in the Ecuadorian Andes

In this work, we present the description of the sedimentary fill of a well-exposed lacustrine succession in the Ecuadorian Andes. The Guayllabamba basin is an intermontane basin located in the Andean range of Ecuador, and part of its sedimentary history is represented by a volcanically-influenced ∼100 m thick lacustrine unit of the Pleistocene age. We create a stratigraphic cross-section from the eastern to western lake margins and identify nineteen facies that were used to carry out a paleoenvironmental reconstruction. The Guayllabamba paleolake was developed in a tectonic depression surrounded by volcanoes and it was filled by sediments derived from the erosion of the volcanic edifices, the reworking of unconsolidated pyroclastic deposits, and deposition of pyroclastic currents into the lake. The lake shows a deepening trend, passing from shallow deltaic sedimentation to varved diatomites with turbidites. Abundant ash-fall beds, monolithological pumiceous deltaic sequences, and pumice-dominated thick ignimbrites show the impacts of volcanism on lacustrine sedimentation within this basin. Soft-sediment deformation and gravity flow deposits are common due to the intrabasinal tectonic activity and to the intrusion of a lava body. Aulacoseira-rich diatomites dominated the background lake sedimentation. The outcrops of the Guayllabamba basin are outstanding examples of the interaction between volcaniclastic and lacustrine sedimentation.

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.