Stromatactis cavities in sediments and the role of coarse-grained accessories

Abstract

The analysis of previous experimental work made on the simulation of stromatactis-like cavities in rapidly settling suspensions of particulate matter substantiates the continuation of experiments toward identifying the conditions of cavity formation. It has been suggested that the most promising directions in this experimental work involve the simplification of complex factors (e.g., variables that derive from the properties of particles and media, as well as the sedimentation dynamics of slurries). Our new hydrodynamic concept of stromatactis formation addresses the traditional key arguments of previous authors on the origins of stromatactis systems. The direct production of stromatactis-type cavities during the sedimentation of fine particulate, polydisperse, multimodal aquatic suspended matter can be characterized in terms of competition between fluids escaping from compressed, diluted domains, and the dynamic effects of the dense packing of solid particles on their boundaries, the latter gradually overtaking from the former, until a middle layer of sediment is sufficiently stabilized and the first internal sedimentation from residual suspensions begins. With the earliest stabilization of the grain-supported, skeleton-like structures in the sediment, low domical but surprisingly stable vaults develop above the cavity zones. Underneath the coalescing arched structures, there often remain places in which grains can still be fluidized, and which consequently enable the further widening of these primary cavities. The specific grain size distribution is derived from natural counterparts, an attribute combined with the high internal friction angle, and increases the final sediment cohesion and stability. This process is particularly effective when highly polydisperse-multimodal sediment materials have highly angular, rugged, or potentially cohesive grains. When the relatively coarsest-and finest-grained fractions are present in increased amounts, the energy dissipation of the dense turbulent slurry is enhanced, and the stromatactis-producing mid-layer is gradually sealed by a relatively impermeable, non-stromatactis, very fine-grained cover in the upper part of the deposit. Two categories of experiments are discussed in this paper. The first is aimed at explaining how moderately large particles can interact with each other. In this category of experiments, moderately graded tridisperse mixtures of angular or highly textured particles tended to produce firmly packed clusters with ensuing domical vaulting above cavities. The second group of experiments is based on combinations of bidisperse mixtures of large grains with polydisperse nearly-unimodal matrices of small angular grains. These two components, if used separately, have close to zero capacity for producing any type of stromatactis-like cavities. However, once they were combined, even modest amounts of these large grains led to the growth of spacious cavity systems, particularly if artificial crinoid columnals were present. The comparison of our experimental r...

Possible origin of stromatactis-like cavities in Upper Jurassic sediments from the Wielkanoc quarry near Gołcza (Kraków–Częstochowa Upland, southern Poland)—experimental studies

“…The presence of stromatactis in mud mounds formed beneath the storm-wave base, as described by Matyszkiewicz (1993), Krause (2001), and Boulvain et al (2004), suggests that internal erosion of sediments caused by migrating waters is not a decisive factor in the formation of stromatactis. Recently a new sedimentary hypothesis of stromatactis origin has been presented by Hladil (2005Hladil ( , 2007 and Hladil et al (2006) who suggested that these structures originated during turbulent deposition and separation of highly unsorted clastic material from strongly dispersed suspension cloud.…”

Early Middle Frasnian platform reef strata in the Moravian Karst interpreted as recording the atmospheric dust changes: the key to understanding perturbations in the punctata conodont zone

“…The stromatactis limestone lies directly on truncated rocks. The surface of this limestone was again truncated and covered by a scatter of debris (Hladil et al 2006b). Crinoidal debris deposited in thin lenses or spray was delivered by storm flooding from the outer areas, locally from the E. Above these anomalous beds, the upper part of the E-MF consists of thin grainstone layers and algal bindstones.…”

“…In fact, they are not completely absent, but they are minimized, due to absence of typical large scale or wide ramp environments. This change of knowledge regarding Moravian stromatactis was caused by recognition of the localization of cm-sized stromatactis in limestone lenses along the damaged reef margins of mid-E-MF times (J. Otava, Svážná studna; Hladil et al 2006b;Fig. 6).…”

“…It is remarkable, that the limestones with stromatactis are always very pure, with the lowest contents of non-carbonates. Therefore, the characteristic stromatactis-forming material (Hladil et al 2006b, which was hydrodynamically separated and finally dumped from the medium dense collapsing mixtures on its downslope trajectory, had to be originally collected in very pure parts of submerged highs and on the upper slope.…”

Late Jurassic facies architecture of the Złoczew Graben: implications for evolution of the tectonic-controlled northern peri-Tethyan shelf (Upper Oxfordian–Lower Kimmeridgian, Poland)

“…Such open spaces might have formed at the contacts of loose particles (ooids, oncoids, bioclasts, or intraclasts) and partly lithified microbial crusts during sedimentation (cf. Hladil et al 2006). According to Schmid et al (2001), the presence of small-scale discontinuities within sediment together with an unstable shelf position may led to gravitational slumping.…”