Bedding

Abstract: An experimental study of the physical properties of black, kerogen-rich shales, also including maturation analysis, scanning electron microscope (SEM) observations, and physical modeling, revealed fairly peculiar petrophysical parameters. Specifically, these rocks have very low porosity and density, but most importantly, both P and S ultrasonic velocities normal to bedding are extremely low, whereas they are much higher parallel to bedding, giving rise to a strong anisotropy even at high confining pressures. We found that these parameters primarily reflect kerogen content, microstructure, and maturation level of these rocks. We found also that microcracks inferred from ultrasonic velocity measurements occur only in mature shales. These microcracks are parallel to the bedding plane and further enhance strong intrinsic anisotropy, notably at low effective pressure. Our results show, that on a small scale, kerogen-rich shales are transversely isotropic rocks and can be effectively modeled using the thin-layer composite concept modified to account for the specific distribution of organic matter in the rock fabric.

Abstract: Some types of graded bedding, especially minor or isolated occurrences and varved "clays," can be readily accounted for by normal processes of sedimentation. Volcanic eruptions, dust storms, annual and longer climatic rhythms, rejuvenation of relief at the source or filling in of the sedimentary basin, churning up of sediment by storm waves, are among the more obvious potential causes of graded deposits. But the majority of occurrences cannot be explained by any of these processes. The authors believe turbidity currents of high density may be invoked to have supplied the sediment and deposited it in graded beds. The absence or insignificance of current bedding and ripple marking in graded deposits, the deposition of coarse material on the unconsolidated fine-grained top of the preceding bed, the enormous extent of each individual member without apparent change in thickness or character, and the frequent inclusion of angular fragments (even composed of clay) and of redeposited fossils are among the chief c...

Abstract: This study was undertaken to investigate whether sedimentary rocks can retain directions of magnetization through long intervals of geologic time. The method of investigation is to observe the angular relations of the directions of residual magnetization to the bedding in deformed structures, or in beds that were laid down in a random manner. Systematic relationships found indicate that the direction of magnetization in certain selected sedimentary rocks remains unchanged for at least 200 million years.

Abstract: Cyanobacterial films and mats syndepositonally influence erosion, deposition, and deformation of sediments. The biomass levels surface morphologies, and microbial mats stabilize depositional surfaces and shelter the sediment against erosion or degassing. Growing microbial mats dredge grains from their substrate upwards, whereas cyanobacterial filaments that are oriented perpendicular to the mat surface reach into the supernatant water and baffle, trap, and bind suspended particles. These and similar biotic-physical interactions are reflected in syndepositional formation of microbially induced sedimentary structures. We distinguish structures on bedding planes (leveled bedding surfaces, wrinkle structures, microbial mat chips, erosional remnants and pockets, multidirectional ripple marks, and mat curls) and internal bedding structures (sponge pore fabrics, gas domes, fenestrae structures, sinoidal laminae, oriented grains, benthic ooids, biolaminites, mat-layer-bound grain sizes). We propose to place this group of microbially mediated structures as a fifth category (bedding modified by microbial mats and biofilms) in Pettijohn and Potter's (1964) existing classification of primary sedimentary structures.