Aseismic Tectonism-Induced Soft-Sediment Deformation in a Tranquil Palaeogeography: Chikkshelikere Limestone Member, Proterozoic Kaladgi Basin, Southern India

Abstract

The wide spectrum of synsedimentary deformation products occurring almost at all stratigraphic levels within the ~40 m-thick section of the Chikkshelikere Limestone Member of tentative Mesoproterozoic age in India is evaluated for its origin. Among the two principal facies components, both carbonate, of this Member the dark micritic facies generally underwent brittle deformation, and the light microsparry facies responded in ductile fashion to the same deformational stress. Breccia patches, hardly having any boundary, abound at almost every stratigraphic level within the Chikkshelikere Limestone Member. The third facies constituting less than 3% by volume of the Member is of laterally persistent carbonate intraclastic conglomerate beds. The dark facies is of massive micrite, while the light facies is made up of interlocking microspar crystals, but bears minor wave-current structures, and rare minute erosional features at its base. Non-luminiscent character of the former under CL is reminiscent of oxidizing basin-floor environment, while the bright orange luminescence of the latter testifies pervasive burial recrystallization. The dark micritic facies is interpreted as indigenous and the light microsparry facies as allochthonous, possibly laid by highly energy-depleted storm wave-cum-current. Mineralogical as well as geochemical analyses indicate preferred dolomitization and carbon enrichment in the dark micritic facies. Selective pyritization is also observed along the base of the same facies. These features collectively suggest selective microbial mat proliferation within this facies. Despite early induration being the rule for carbonate sediments, microbial mat growth apparently enhanced its rate within the dark micritic facies and caused a viscosity contrast between the two principal facies. Both the facies underwent ductile deformation in slump folds generated on the sediment surface, but then acquired and accentuated viscosity contrast led them later to respond differently to similar deformational stress. However, in micro-graben structures both the facies underwent brittle deformation indicating pervasive cementation with longer residence time. Breccia patches, where present, elicit liquefaction, occasionally followed by fluidization, rarely the porewater is poured out on the sediment surface. The intraclastic conglomerate beds are massflow products. Indentation on the bed-roofs and shear fold on their tops elicit subsurface occurrence of the flows. Only those intraclastic beds without indentation on their roofs, but with eroded bases were possibly surficial products. Frequent liquefaction and fluidization without any stratigraphic selectivity and lateral continuity is hardly attributable to seismicity. Aseismic tectonism such that relates to geoidal tilt possibly accounts for the small-scale SSDs distributed all over the Chikkshelikere Limestone Member better. Slow warping of strata engendered frequent pore-water overpressuring that caused the synsedimentary deformation including intraclastic conglomerate beds emplaced under and above the sediment surface.