Faults and Fractures in Carbonates
volume) in fine-grained, grain-supported carbonate rocks (grainstones, packstones), as well as by the good sorting and sphericity of individual carbonate grains. The influence of these factors in pressure solution development is recorded by longer, more connected and less spaced bed-parallel seams.
•
Bed-parallel CB formed only in poorly-cemented, porous carbonate grainstones (porosity >15 %). The dimensional parameters of the bands (i.e. length, spacing, thickness), however, were strongly controlled by specific pore types and distributions which, in turn, are affected by the skeletal grain assemblage of the rock.
In conclusion, based upon a pre-existing knowledge on (i) depositional environment, (ii) biota evolution, (iii) depositional processes, (iv) diagenetic evolution, (iv) rock components and (v) petrophysical rock properties, it is possible to infer (directly or indirectly) both type and distribution of the structural discontinuities pertaining to the background deformation and therefore assess the hydraulic properties of the carbonate rock masses.
Faulting of High-porosity Carbonates
Recently, a new faulting mechanism was documented in porous carbonate grainstones (porosity >15 %), which consists in strain localisation due to both compaction and shear along narrow tabular bands called compactive shear bands.10,11
In the field, these structures
are easily recognisable because they are light-coloured with respect to the parent rock and show a positive relief, due to their increased resistance to weathering, with respect to the host rock (see Figure 3A and B).
Starting from single shear bands, which resolve a few millimetres of displacement, these structures evolve due to slip accumulation in zone of shear bands and finally, in well-developed faults with discrete slip surfaces and fault rocks (breccia and gouge). Field data on their dimensional parameters show that single shear bands are generally characterised by given values of length (around 50 cm), displacement (around 6 mm) and thickness (around 6 mm). Differently, the zones of shear bands are characterised by a maximum displacement value in the middle, where a larger number of single shear bands are present. So, the increased displacement value is accompanied by an increased thickness of the zone of bands. Following what we observed in the field, we propose a conceptual model for fault growth in porous carbonate grainstones. Single shear bands can evolve to zones of shear bands and eventually, to well-developed faults or, alternatively, they can interact with each other and link to form larger single bands. Based upon the field data, interaction and linkage can occur at any deformation stage, so one single band can interact with an adjacent zone of shear bands or fault.
In conclusion, the following are the dimensional parameters we were able to document in the Pleistocene, coarse-grained carbonate grainstones of Favignana island:30
• •
Single bands are up to 50 cm long, 0.5 cm thick and solve up to 0.5 cm of displacement. The displacement/thickness (D/T) factor is therefore around 1.0.
Zones of compactive shear bands are up to 10 m long, 20 cm thick and solve up to 30 cm of displacement. The D/T factor is around 1.5.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124