Cone-in-cone and beef mineralization associated with Triassic growth basin faulting and shallow shale diagenesis, Edgeøya, Svalbard

Abstract

-Cone-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.

Characterization of Bedding-Parallel Fractures in Shale - Morphology, Size Distribution and Spatial Organization

“…This evolution contributed to changing pore pressures, seepage forces and material moduli. Maher et al (2016) contend that the calcite mineralization tracks the very significant changes in mechanical properties and stress states that occur during synlithification deformation at very shallow crustal levels. This question will be explored more fully in the discussion chapter of this thesis.…”

“…Measured in outcrop, bedding-parallel fractures can be a few millimeters to at least tens of meters long, with their thickness ranging from 10s of microns to several centimeters (Rodrigues et al, 2009;Smith et al, 2014;Maher et al, 2016) (Figure 1-1). Because these features are so large-comparable to the dimensions of beds-their presence may impact fluid flow in hydrocarbon reservoirs (Curtis, 2002;Chen et al, 2009;Ding et al, 2011;Duhalan et al, 2013;Zanella et al, 2015).…”

“…Aydin et al (2014) conclude that thermal maturation provides a mechanism for continually renewing pore pressure despite leakage during faulting, and is the source of the low effective stress, during foreland deformation. A recent study by Maher et al (2016) proposes a mechanism on how beef mineralization is associated with growth faulting and shale diagenesis. The local fault-related stresses substantially changed during shallow diagenesis and lithification.…”

“…Existing publications focus on characterizing vertical fractures (Gale et al, 2007;Gale et al, 2014 for review; Kim et al, 2015;). Publications on bed-parallel fractures are mostly based on outcrop studies in Europe and South America (Rodrigues et al, 2009;Maher et al, 2016), and describe the occurrence of this type of fracture mainly. Systematic size and spatial distribution data have not been presented in published works.…”

“…Preliminary examination of bed-parallel fracture examples in the literature (Rodrigues et al, 2009;Cobbold et al, 2007Cobbold et al, , 2012Cobbold et al, , 2013Maher et al, 2016) is the deepest, from around 6200ft to 7000ft (see Table 3 …”