Cashman and Cashman (2000) and Du Bernard et al (2002) reported multiple sets of deformation bands in poorly consolidated sand, gravel, and mud deposit of late Pleistocene age exposed in the 83 ka Savage Creek marine terrace near McKinleyville,NW of the town of Arcata in northern California (see Figure 1 for location map). The bands occur in the footwall of the McKinleyville thrust: Two apparent conjugate sets of bands dipping approximately 30 degree in opposite directions and a third set of nearly sub horizontal bands (Figure 2).

Figure 1. Location map of the Savage Creek marine terrace exposed on a cliff face, McKinnleyville, NW of the town of Arcata in northern California.

Figure 2. Photo and map of the Savage Creek marine terrace cliff exposure showing two major sets of deformation bands with thrust sets of offset (view to the east). From Eichhubl et al. (2003) and Du Bernard et al. (2002).

The two sets of inclined shear bands consistently offset bedding laminations in a reverse sense. Single shear bands are a few grain diameter thick with slip of 1 to 5 mm. Also exist zones of shear bands with tens of centimeters offsets. The third set of sub-horizontal bands is parallel or shallowly inclined to bedding along a direction that roughly bisects the conjugate sets of shear bands (Figures 2, 3 and 4). The sub-horizontal bands are 1 to 2 mm or 5 to 10 grain diameter thick. They are always spatially associated with shear bands, frequently originating at their tips and extending into their extensional quadrants (Figures 3a, b and Figures 4a and b). Sub-horizontal bands in some places cut across but do not offset inclined markers such as cross beds, unconformities and channel fills indicating that the bands form in predominantly opening-mode in response to slip along the inclined shear bands. These are identified as 'Dilation Bands' by Du Bernard et al. (2002) and thus being the type locality for this type of localized deformation.

Figure 3. (a) Two sets of thrust faults and an associated sub horizontal dilation band set in splay orientation. Also shown are thrust offsets (the blue lines showing the offset markers). (b) The interpreted dilation bands sub-parallel to the slightly inclined cross-beds. Revised from Eichhubl et al. (2003) and Du Bernard et al. (2002).

Figure 4. (a) Map showing two sets of conjugate shear bands with reverse sense of slip exposed on a sea cliff of a poorly consolidated deposits of the Savage Creek marine terrace, McKinleyville, California. (b) A close up photograph of one of the shear bands composed of three segments with splays nearly parallel to the local bedding, which are in the orientation of the greatest compression as inferred from the conjugate pattern in (a) and the sense of their offsets. (c) Photomicrograph of a thin section of one of the splay bands in (a) and (b). The blue epoxy corresponds to the pore space which is noticeably more abundant in the matrix adjacent to a thin zone corresponding to the macroscopic band which is highlighted by a relative concentration of darker cement composed of clay minerals, iron oxide and organic matter. (d) Frequency diagram of rock constituents and pore space. Solid red line (marked as A) means abundance of detrital grains and solid white line (marked as B) means residual porosity. The relative percentage of grains larger than 4 (mu)m is lower inside the band, but the residual porosity maybe higher as much as 10% (7% plus-minus 3%) within the band. From Du Bernard et al. (2002).

In polished thin sections, shear bands are characterized by grain rotation and translation and little grain fracturing. See the photomicrograph presented under 'Mechanisms and Mechanics of Shear Bands'. Grain rotation and translation resulted in porosity reduction and preferred orientation of elongate grains parallel to the shear bands. Remaining pore space is preferentially filled with clay, predominantly kaolinite, chlorite, iron oxides, and organic matter. Sub-horizontal bands are identified in thin-section by an increased abundance of pore filling clay forming meniscus cement bridges across neighboring grains (Figures 4c and d). It is determined by point-counting, the abundance of detrital grains within sub-horizontal bands decreases by up to 7%±3% relative to the grain abundance outside of bands marked by line 'A' in Figure 4d. The 7% reduction in detrital grain abundance correlates with an increase in clay abundance of 9%±3% with respect to the clay abundance outside the bands (line 'B'). Based on intact clay bridges across grains in sub-horizontal bands, it was inferred that the clay cement is largely a diagenetic product resulting from mineral alteration and infiltration that occurred after band formation. The subsequent precipitation and infiltration of clays may have prevented collapse of deformation-induced dilatancy after reverse slip on the inclined shear bands has seized.