Deformations of weakly lithifed rocks of the Bazhenov-Georgievsky complex

Abstract

Research subject. Deposits of the Upper Jurassic Bazhenov-Georgievsky complex situated in central West Siberian regions. At a frst approximation, the deposits are represented by interlayering pure or carbonatized radiolarites and organic-clay mixites. Materials and methods. A collection of core samples taken from the deposits was analyzed to study the geological and geophysical features of borehole sections, photographs of core samples, and micrographs of thin section. Soft sediment deformations, which appeared within the Bazhenov-Georgievsky complex as a result of earthquakes, were investigated using data obtained by Chinese researchers. Results. It is shown that, at different stages of diagenesis, layers with sharply differing density and mechanical properties came into contact, which led to various deformations of weakly lithifed layers during earthquakes. In the early and middle diagenesis, mobile carbonated radiolarites deformed plastic clay interlayers with the formation of carbonate nodules. In the middle diagenesis, in radiolarites, areas of weak lithifcation and mobile radiolarites coexisted. Along with plastic deformations, the processes of autobrecciation and autofluid fracturing of radiolarites, abrasion erosion of the boundaries of clayey varieties took place. In the late diagenesis, lithifed varieties predominated in radiolarites and clayey silicites; as a result of deformations, radiolarites and the argillaceous bridges separating them were transformed into fragments, polished surfaces, and pellets. If, under the influence of differential loads, fluidization (softening) and movement of weakly lithifed radiolarites occurred, then the rates of such movements in adjacent layers inevitably differed. Each of the described cases is demonstrated by the core material of specifc boreholes and petrographic thin sections. The scale of the phenomenon is shown from the signs of loading of millimeter interlayers to pressure shafts of brecciated limestones with a thickness of 7–10 m. Conclusions. The West Siberian Plate is epicontinental; therefore, the most intense earthquakes occurred at consolidated basement block boundaries during isostatic subduction and compensations in the Neocomian clinoform ultrafast sedimentation zones. Soft sediment deformations occurred within the Bazhenov-Georgievsky layers recorded the rock history of intense seismic events of the past, their temporary and spatial distribution.