High-resolution record of tectonic and sedimentary processes in growth strata

Abstract

a b s t r a c tGrowth strata are used to determine the kinematics of synsedimentary structures such as faults. Classical methods of analysis such as thickness versus throw plot consider that the available space created by fault slip in the hanging wall of faults is instantaneously filled up by sediments. This has lead many previous works to identify a cyclic activity for growth faults. Here we perform a careful analysis of the variation of strata thicknesses on each side of a very well documented normal growth fault in the Niger delta. We show that these thickness variations are induced by the alternation of sedimentary processes during continuous fault slip. Suspended-load processes induce either uniform or slightly variable thickness of a large majority of mudstone layers. Bedload processes result in a preferential thickening of sand layers in the hanging wall. These high quality data thus provide strong grounds for doubting the polycyclic growth diagnosed for some faults at the scale of sedimentary cycles and supports the notion that fault displacement rates can be very well behaved. Our study emphasizes the important conclusion that stable fault growth, and related displacement rates, can appear to be punctuated when viewed at the scale of sedimentary cycles. It follows that care should be taken when attempting to derive displacement rates on temporal scales equivalent to those of alternating sedimentological cycles.

Deep-seated faults and hydrocarbon leakage in the Snøhvit Gas Field, Hammerfest Basin, Southwestern Barents Sea

“…These plots provide insights about potential reactivation of fault by dip linkage (Mansfield and Cartwright, 1996;Omosanya and Alves, 2014;Tvedt et al, 2013), and also distinguishing faults that developed through synsedimentary activity from blind or radial propagation of tips (Childs et al, 2003;Omosanya and Alves, 2014). Expansion Index (EI) has been used to define periods of most significant fault growth form normal faults (Beach, 1984;Bischke, 1994;Gibbs, 1983;Mansfield and Cartwright, 1996;Pochat et al, 2009;Thorsen, 1963) but does not contain any information about absolute slip rate (Cartwright et al, 1998). Expansion indices and t-z plots can be used to constrain timing of fault activity.…”

“…Growth index records can indicate the timing and depict the growth history of faults (Henriksen et al, 2011;Hongxing and Anderson, 2007;Pochat et al, 2009). It is a measure of relative throw rate to the sedimentation rate in the footwall.…”

Evolution and character of supra-salt faults in the Easternmost Hammerfest Basin, SW Barents Sea

“…The throw is the difference between hangingwall and footwall cut-offs. Expansion index (EI) is the ratio of thickness between the layers in the hangingwall to the thickness in the footwall of a fault (Pochat et al, 2009;Thorsen, 1963) while the growth index is calculated as the difference in thickness between the hangingwall and the footwall of an interval divided by the thickness of the interval in the footwall (Childs et al, 2003a,b). Errors in throw estimate were dependent on the vertical sampling rate of 4 ms (see Baudon and Cartwright, 2008b).…”

“…Displacement plots such as displacement vs. distance (t-x) (Cartwright and Mansfield, 1998;Baudon and Cartwright, 2008a), displacement vs. depth (t-z) Peacock and Sanderson, 1991;Mouslopoulou et al, 2007), expansion and growth indices (Pochat et al, 2009;Thorsen, 1963), and cumulative throw vs. age (Omosanya and Alves, 2014) can provide information on fault nucleation, propagation, segmentation and linkage. Despite these numerous works and tools, salt-influenced normal faulting remains an area of continuing interest and research to both the academia and exploration industry.…”

Geometry, kinematics and dynamic characteristics of a compound transfer zone: the Dongying anticline, Bohai Bay Basin, eastern China

“…Thus, assuming that the sedimentation rate is constant over the footwall, variable values of EI (i.e., increase or decrease of HWt relative to FWt) can be directly related to variations of the fault movement rate [31].…”