Journal
MARINE AND PETROLEUM GEOLOGY
Volume 115, Issue -, Pages -Publisher
ELSEVIER SCI LTD
DOI: 10.1016/j.marpetgeo.2020.104250
Keywords
Soft-sediment deformation structures; Tectono-volcanic activity; Ordos basin; Trigger mechanism; Lake evolution
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Funding
- China National Science and Technology Major Project [20172X05009-002]
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The Ordos Basin, central China, is a large-scale Late Triassic intracratonic lacustrine basin, and in which the Upper Triassic Yanchang Formation comprises interbedded sandstones, mudstones and tuffs, representing deposits of a complete lacustrine life cycle. Based on observation of extensive well cores, various types of soft-sediment deformation structures have been identified in the lacustrine deposits of the southwest basin, including deformed lamination/bedding, load casts and flame structures, pseudonodules, microfaults, autoclastic brecciation, sand dykes, dyke-sill complexes, and sandy and muddy slumps. Liquefaction and fluidization of unconsolidated permeable sediments, together with brittle fracturing of semi-consolidated sediments, are the major deformation mechanisms that generated these structures. Both the scale and prevalence of the deformed sediments, as well as the geological setting of the lacustrine basin, suggest that seismic events (synsedimentary tectonic activity) were the main trigger to initiate deformation processes, although sedimentary overloading could also have been a minor localized trigger. Many volcanic tuffs preserved in both the primary undisturbed deposits and deformed sediments serve as support for synsedimentary tectono-volcanic activities that induced deformation in these lacustrine sediments. The deformation structures usually form vertical clusters of several different types, representing active syndepositional tectonic periods. Stratigraphic correlation shows three such deformation clusters to be widespread in the area detailed herein, and likely recorded three major tectonic periods during deposition. A thick extensive oil shale, with various deformation structures, also reveals another active tectonic period. These four intervals coincide well with the transition of the sedimentary architecture in the area, indicating four phases of most intense tectonic periods and their influence on lacustrine evolution.
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