Rapid Deformation-Induced Calcite Precipitation in Siltstone from IODP Hole U1581B, Transkei Basin

The IODP 392-U1581B borehole was drilled in 2022 about 300 km from the South African coast, at ~4500 m water depth in Transkei Basin. The recovered stratigraphic section includes Cenozoic, mostly carbonaceous, rocks and upper Cretaceous (Maastrichtian and Campanian) siltstones with a low calcium carbonate content (<5 wt%). The lowermost part of the recovered Campanian strata, located between 870 and 1000 m below the seafloor, includes 5-10 cm thick beds of stiff low-porosity (<10%) mudstones. In these beds, we observed enigmatic zebra-like textures including subparallel light-coloured bands (1-5 mm wide) and feather-shaped criss-crossing veins.

Here we present a set of multidisciplinary investigations aiming to define the origin of these enigmatic structures. The SEM-EDS and XRD analyses indicate that the light-coloured bands and veins mainly consist of calcium carbonate, with 10-20% of quartz, clay minerals, siderite, and pyrite. The SEM images reveal microstructures of shear deformation within the bands. The shear plane and transport direction identified based on flow indicators, implies a displacement of ~0.1 mm. In thin sections, the narrow axial zone of the band appears like a void filled with siliciclastic matrix indicating that the crystallization front developed from the fracture wall inwards. The XRD analysis shows that the matrix is composed of quartz, muscovite/illite, kaolinite and some scarce detrital minerals, including siderite, pyrite, plagioclase and others. The siderites grains range in size from 10 to 15 μm, while framboidal pyrite forms small aggregates with a diameter of ~1 μm, often nucleating on top the siderite crystals. Inside the veins, the grain size of the calcite filling is smaller than 1 μm indicating a short crystallization time.

We constrained the origin of the calcium carbonate deposited in the veins using isotopic analyses. ⁸⁷Sr/⁸⁶Sr = 0.708-0.709 is close to the isotopic composition of modern seawater. Depletion of ¹⁸O (δ¹⁸O = -9‰ to -11‰) implies deposition at elevated temperatures. The negative δ¹³C = -11‰ to -13‰ remains unclear but could be associated with oxidation of methane. Since no evidence of recrystallization was observed in the Campanian strata, these isotopic ratios would rule out that the zebra textures were formed during burial.

The deformation microstructures indicate that calcite precipitated concurrent to dilatant shear fractures. There is no evidence of post-Campanian tectonic events in the Transkei Basin. If such deformation had existed, some deformation indicators outside the siderite-rich layers should be visible. It is well known that extensional disc fractures and other deformation structures can form in core samples during drilling and core recovery. The geometric relations of fractures to core irregularities also imply that the zebra textures can be induced. However, the precipitation of calcium carbonate in the induced fractures would require super-saturation of the fluid, high reaction rates, the source of calcium and carbon. An elevated pH and temperature conditions, mixed oxidizing and reduced fluids, and rapid decompression on core retrieval could potentially drop the solubility of calcite but it is unknown what would drive precipitation calcite on a time scale of minutes to hours.