Outcrop location
The Hopeman sandstone is comprised mainly of Permian age aeolian sand dunes and is located on the southern shores of the Inner Moray Firth Basin (Fig. 1). This studied fault zone is located on the Cummingstown foreshore. Permian Sandstones are present offshore in the Moray Firth and observations here are relevant for prospects offshore, although it has more often been used as an analogue for the age equivalent Leman Sandstone in the Southern North Sea.
Fig. 1. Location of the Hopeman Sst outcrop on the southern shores of the Inner Moray Firth. (Map by Henk Kombrink, GeoExpro)
Conjugate Faults
Faults often form in crossing pairs known as conjugates. Normal faults initiate at 60 degrees and are separated by a 60 degree angle (Fig. 2). Sigma 1 is vertical, Sigma 2 is in the plane of the intersection of the 2 faults and Sigma 3 is perpendicular to the fault strike. Reverse faults and strike slip faults can also form as conjugate pairs but to my knowledge are not present in this outcrop. These crossing conjugates are common when there is a high density of faults e.g., in a rollover anticline to a listric fault.
Cross Cutting Relations and Timing
These normal fault pairs have a dip of 60 degrees and are separated by an angle of 60 degrees. Often, 1 of the faults will displace the other fault, meaning that it is the slightly younger member of the pair. Note how the green faults have been displaced by the red faults meaning that they are the slightly older member of each pair (Fig.2).
Fig. 2. Crossing conjugates in the Permo-Triassic Hopeman Sandstone outcrop. Note the compound zone of deformation bands in the foreground.
Deformation Bands
These fault like structures are present in this outcrop. Individual strands coalesce to form compound zones with slip surfaces (highlighted yellow in Fig 2), resembling faults. The fault conjugates here are in fact made up of deformation bands (Fig. 2) so each fault could actually be described as a compound zone of deformation bands. A map view of the compound zone in Fig 2 is shown in Fig. 3. Note the sharp transition from the compound zone with many deformation bands to the host sandstone which only has a few of these structures.
These structures can reduce the permeability of otherwise excellent quality aeolian sandstones by a few orders of magnitude. Therefore, they are often bad news for oil and gas production.
Fig. 3. Looking down on a compound zone of deformation bands in the Hopeman Sandstone. This is the yellow polygon area in Fig. 2.
Concluding Remarks
Faults often form in conjugate pairs. Crossing conjugates are common when there is a high density of faults in outcrop or in the oil field. Often, 1 fault member of the pair is older than the other and identified by the fact that it has been displaced by the other member (younger) fault. The conjugate faults in the Hopeman Sandstone outcrop example are actually made up of deformation bands so each fault could be described as a compound zone of deformation bands.