Abstract
AbstractCone-in-cone (CIC) and beef (BF) carbonate lenses ornament detachment zone faults underlying Triassic growth basins on Edgeøya. Field relationships place CIC and BF growth as during early diagenesis and a transition from hydroplastic to a later brittle-style of faulting that is marked by coarser calcite veining. Deformation is constrained to have occurred at <300 m depth. Multiple models exist for CIC formation. For the Edgeøya example, textural analysis of thin-sections suggests that small tensile fractures and carbonate shell fragments nucleated development of calcite aggregates with CIC and BF morphology within unconsolidated to poorly consolidated sediment to form asymmetric antitaxial tensile aggregates subparallel to bedding and fault surfaces. The conical forms result from differential growth on stepped, cleavage-parallel faces of fibres facing host sediment, with preferential inclusion incorporation at inner corners. The preferred directions of calcite growth are attributed to local stresses and seepage flow associated with pore pressure gradients. Substantial framboidal pyrite in the sediments represents an early phase of microbially driven sulphate reduction, which may have induced calcite mineralization. The transition to brittle-style faulting was marked by development of deformation twins in CIC/BF fibres, and a transition to coarse, blocky calcite growth in relay arrays of steeply oriented microveins. This indicates local fault-related stresses substantially changed during shallow diagenesis and lithification, an evolution attributed to changing pore pressures, seepage forces and material moduli. Calcite mineralizations at Edgeøya track the very significant changes in mechanical properties and stress states that occur during synlithification deformation at very shallow crustal levels.
Highlights
Previous work has identified a series of thin-skinned growth basins with a very large footprint (Fig. 1) in Upper Triassic strata of Edgeøya in the eastern Svalbard archipelago (Edwards, 1976; Osmundsen et al 2014)
Cobbold & Rodrigues (2007) explore how horizontal tensile fractures are mechanically enigmatic in a simple lithostatic stress field where vertical fractures are to be expected, but can be explained by considering how vertical seepage driven by pore-water-pressure gradients would influence fracture dynamics
Cobbold & Rodrigues (2007) address the enigma of how subhorizontal fractures with internal subvertical fibres could form without a compressive tectonic stress component. They attribute this morphology to vertical seepage forces which, if strong enough and especially at a shallow depth, could overcome the vertical lithostatic component
Summary
Previous work has identified a series of thin-skinned growth basins with a very large footprint (Fig. 1) in Upper Triassic strata of Edgeøya in the eastern Svalbard archipelago (Edwards, 1976; Osmundsen et al 2014). Associated fault zone deformation varies from being dominated by a soft-sediment style with particulate flow, hydroplastic folding and disaggregation bands, to brittle-style faults with fractured damage zones, breccias, coarse calcite veining and well-developed striae (Figs 2, 3). The top of a shale-dominated detachment zone with abundant shallowly dipping fault surfaces is well exposed in the cliff outcrops north of Kvalpynten (Fig. 1b). The associated diagenesis/lithification that drove the transition to brittle deformation is constrained to have occurred during faulting at a shallow level (
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