Normal fault geometry related to sediment compaction and burial

Abstract

Field observations are presented on meso-scale normal faults and sandstone dikes which cross-cut turbidites in the petroliferous Rec6ncavo Basin situated in Bahia, NE Brazil. Two discrete phases of normal faulting occurred. It is argued that the first was probably active during the first 200 m of burial. Compaction estimates from folded sandstone dikes and the accommodation patterns of shale bedding planes around ball-and-pillow structures were made. First-phase fault profiles were initially reasonably straight but refraction on the fault planes increased due to the differing compaction between sandstone and shale. The later phase of faulting occurred at a greater depth, with steeper fault dips and strong refraction of fault planes at lithological boundaries. This greater fault refraction is probably the result of burial and diagenesis, which increased the difference in theological properties between sandstone and shale before faulting occurred. Thus, complex superposed fault geometries may be developed by changing theological properties and stress conditions during progressive burial. It is shown that sandstone dikes were not always intruded vertically. Injected dikes cannot be used in the same manner that Neptunian dikes are conventionally used to estimate tectonic strain, if independent evidence of the original orientation of the dike is not available.