Clay gouge

Dykes often act as barriers to groundwater flow so that the water table on one side may be higher than on the other. Fault planes occupied by clay gouge may have a similar effect. Conversely, a fault plane may act as a conduit where it is not sealed. 

Most of these clays were formed by the alteration of volcanic material and basic igneous rocks. A few analyses of gouge clay and hydrothermal alteration deposits are included. Volcanic-derived montmorillonites have been formed in marine and lacustrine environments and in a variety of weathering and terrestial environments. A... 

Weir and Green-Kelley (1962) Black Jack Mine, Beidell, Colo., U.S.A., Gouge clay. 

There appears to be some relation between composition and mode of origin but with the present data, it is difficult to deduce. In general, the high-Mg, low-Al montmorillonites are most likely to have formed by the alteration of volcanic material and the high-Al beidellite type, to have been of hydrothermal origin or to have been formed as gouge clay and soil clay. In the soil clays much of the excess A1 is present as hydroxyl A1 in the interlayer position. 

Fig. 1. Classification of fault seal processes. Fault seals are divided into two types juxtaposition fault seals and fault gouge seals (e.g., cataclasis and clay smear). One fault may have a combination of different sealing processes affecting its sealing capacity.

The second stage of static fault seal analysis is the evaluation of the properties of the fault gouge (Fig. 2). In the presence of clay layers, the introduction of clay into a fault is one way of strongly increasing the capillary entry pressure. A common process is the... 

Fig. 3. Outcrop picture of a clay smear. Photograph was taken in a lignite quarry in SB Germany, throw ca. 10 cm. It shows the cumulative effect of two thin clay layers. The deformation occurs by a combination of squeeze flow, as indicated by thinning of the source bed, and simple shear as evidenced by the sharp contacts between fault gouge and undeformed rock.

The alluvial and fluvial strata were flooded by meteoric pore water and got cemented prior to faulting. While clay smear and intergranual shear fractures dominate in fine-grained and poorly cemented intervals (Fig. 7a), well cemented intervals cut by faults display fault gouges characterized by brecciation and granulation (Fig. 7b). Within these (brittle) fault... 

The general observation for all three rock types is decreasing bulk flow with increasing Oj lo ratio, and some effect of the fracture shear displacement on the bulk flow. For the clay containing YBS, bulk flow is actually reduced by 90% for o la = 1.3. The strongest rock (YBS) shows the most pronounced cross-flow reduction, whereas the weakest rock (LC) shows the least reduction. The explanation for the limited cross-flow reduction for the LC sample lies in the low matrix permeability in combination with its special mineralogical composition (>90% carbonate), which reduces the effect of a low permeability gouge layer on the bulk flow. 

We distinguish clay smear from fault gouge in accordance with Smith (1980). We identify clay smear based on the description of Lehner and Pilaar (1997), our own observations of the Rhine graben browncoal mines and the results of our numerical modelling. In accordance with Smith (1980), clay smears are understood to be clay fault filling derived from bedded material. 

Fig. 8. Movement on a fault plane may result in juxtaposition of high and low permeability sandstones, or reconnection of two high poroperm sandstones, previously disconnected. Conversely microfabric rearrangements of clay minerals, grain breakage and fault gouge formation may reduce the relative permeability for oil versus water across and in the fault zone. Burial-induced diagenesis in the fault zone, as in the deeper parts of Halten Vest, will eventually reduce/constrain/disrupt fluid flow (flux) communication and induce regional fault controlled pressure cells (cf. Figs la 5).

X-ray diffraction (XRD) analysis reveals that the fault gouge is mainly composed of calcite, quartz, feldspar, and a few clay minerals (Figure 3, Table 2). Clay mineral is dominated by kaolinite. Quartz is about ten times more than that in the host rock. 

Buatier, M.D., Chauvet, A., Kanitpanyacharoen, W., Wenk, H.R., Ritz, J.F. Jolivet, M. 2012. Origin and behavior of clay minerals in the Bogd fatilt gouge, Mongolia. Journal of Structural Geology 34 77-90. 

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