calculated from available well log checkshots provided by Arco Petroleum in 1992, International. 157. Department Dallas in 1984 and NZ Oil and Gas in 1984, ...
1
A Seismic Investigation into Structural Controls on Submarine
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Channel Distribution in the Taranaki Basin, New Zealand
3
Aisling Scully1
4
1
5
ABSTRACT
School of Biological, Earth and Environmental Sciences, University College Cork, Cork, Ireland
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Submarine or abyssal channels are formed primarily from turbidity currents in continental
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shelf margins. Still poorly understood, these channel systems can span up to hundreds of
8
kilometers and are economically appealing, being one of the main sediment transport mechanisms
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in deep water settings. Using high quality 3-D seismic reflection data, a series of these channels
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were mapped in the Parihaka Basin, offshore New Zealand. The Parihaka Basin has an aerial extent
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of 1520 km2, with a multiphase deformational history and is transected by the Parihaka Fault. This
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complex fault is approx. 50 km in length and has undergone several phases of activation leading
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to substantial throw displacements. Seismic coherence was used to record channel thalweg
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positions, at depth intervals of 25 ms (two-way travel time). The main parameters recorded were
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channel point location, sinuosity, vertical stacking and channel density. Supporting data collected
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included sediment thickness maps and fault throw measurements. In typical marine half-graben
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structures, the areas of maximum sediment supply and drainage are positioned mainly by the
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footwall scarp or in transfer/relay zones. While the data collected supported this, the results also
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showed concentration of channel points at sites of rupture associated with oblique deformation,
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particularly in the opposing sides of the main breached transfer zone. This was repeated numerous
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times in similar locations throughout the study area. Overall the results displayed a predictable
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pattern of sedimentation, which was directly controlled by this type of en echelon fault system,
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with concentration of sediment buildup and channel distribution at localized areas of subsidence
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and uplift, particularly in the zones of linkage between segments.
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INTRODUCTION
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One of the most important zones being targeted for hydrocarbon exploration at present is
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the deep-water continental slope margin. It is clear that a general understanding of the dynamics
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in these offshore areas has been greatly enhanced by 3D seismic investigation (Posamentier,
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2004). Increased detail in current studies shows far more complexity in these systems than
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previously thought and new methodologies have been constructed in order to classify factors such
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as morphology and depositional patterns in these submarine environments (McHargue et al., 2001;
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Deptuck et al., 2007).
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Sediment transport in continental shelf margins is largely provided by turbidity currents
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that migrate suspended material downslope (Bouma, 2004). The individual features of each
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deposit are constrained by physical parameters such as the steepness of the slope, the composition
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of the sediment and confining physical barriers within the basin topography (Posamentier and
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Walker, 2006). These transport flows lead to the development of a variety of characteristic features
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typical of such a setting, with submarine or abyssal channels and canyons covering vast distances.
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These channel systems can be either straight or highly sinuous depending on the gradient of the
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area, with higher sinuosity occurring in flatter terrain (Deptuck et al., 2003). Insight into the
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evolution of these abyssal channel systems, which may contain economically viable sand rich
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reservoirs, is of critical importance and can be reliant on a variety of controls. Studies have shown
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that tectonic activity can have a dominant influence on the patterns and rates of deposition and
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supply of sediment, particularly in the instance of fault related subsidence (Bouma, 2004; Lin et
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al., 2004; Shepard and Emery, 1973).
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Finding a consistent and repeatable array of sediment distribution in relation to specific
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areas on a particular fault system is an important topic for investigation. This is due to the
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prediction of potential hydrocarbon reservoirs being the main goal of exploration companies, with
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focus being placed on accurately identifying the optimal location to drill. Intrabasin transfer zones
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such as the one seen in the study area are often the site of hydrocarbon fields and classification of
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them has important implications for predicting reservoir location and size (Gawthorpe and Hurst,
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1993). This study aims to use high quality seismic reflection data and associated attributes in order
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to map the sediment transport surrounding the oblique normal system seen in the Parihaka Fault
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and determine the level of influence the fault system has on drainage and topography locally. It 55
also aims to ascertain any predictable
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localities of sediment build-up that
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relate to the en echelon structure by
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constraining the various segments
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connected to the transfer relay zones.
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GEOLOGICAL BACKGROUND
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The study area, which is known
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as the Taranaki Basin, is situated
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approximately 25 km offshore from the
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north-western
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Plymouth, New Zealand (Figure 1),
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encompassing ~1520 km2 of the North
67 Figure 1. Overview of the study area, highlighted in the black 68 polygon. Red and green polygons indicate previous
Taranaki Graben (Veritas, 2005). This
petroleum exploration zones. Modified from (Veritas, 2005).
boundary
of
New
entire region is divided into three
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separate sections, the Western Stable Platform, the Southern Inversion Zone and the Eastern
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Mobile Belt, where the North Taranaki Graben is situated. At present there is extension occurring
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in the Northern and Eastern sections, with compression happening in the South (King and
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Thrasher, 1996; Giba et al., 2010). The entire North Graben is divided down the centre by an
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extensive volcanic complex called the Mohakatino Volcanic Centre, comprising a series of
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submarine stratovolcanoes and intrusive bodies that permeate the region (King and Thrasher,
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1996). The Basin has changed over its duration from a post-rift basin during the Paleogene to an
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active foreland-basin in the Neogene. It is characterised largely by the main prograding sequence
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present, which is called the Giant Foresets Formation, a sedimentary section up to several
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kilometres in thickness (Stern and Davey, 1989).
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The Taranaki Basin has undergone three main deformational phases, the first being a period
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of extension during the Late Cretaceous (approx. 84- 50 ma), driven mainly by the dissolution of
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Gondwana (Giba et al., 2010). The second phase was a period of compression between the Late
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Eocene up recent times (approx. 40-0 ma) and second extensional period spanning the Late
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Miocene to Recent (approx. 12-0 ma), largely initiated by subductive processes at the Hikurangi
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Trough and Pacific plate interaction zone, which caused back arc rifting to occur (Stern and Davey,
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1989). There was an overlap of compression and rifting between the Late Miocene to recent,
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attributed to subduction driven differences in tectonic behaviour on a regional scale (Giba et al.,
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2010).
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THE PARIHAKA FAULT
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The Parihaka Fault is the most prominent feature within the seismic survey, dividing the
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area from the centre up to the northern section (see Figure 2). The fault itself is a complex structure
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Figure 2. The sequence of images shown the left display the three main stages of activity in the Parihaka Basin, with Cretaceous extension show in image 1 and the Early Pliocene oblique reactivation shown in image 2. Current extension is shown in image 3. The sequence to the right displays the stages of fault linkage from hard at depth up to soft linkage in shallower horizons. Modified after (Giba et al., 2012). 92
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comprised of four individual fault segments, ranging between 8 and 15 km in length, soft linked
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in shallower sections by varying degrees of intensity and hard linked at depth. The fault has
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undergone several stages of activation and displacement and its unique en-echelon pattern is
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attributed to re activation of the fault during the Early Pliocene in a different direction to the
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original movement (Giba et al., 2012). It is the primary structural system that borders the western
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edge of the Taranaki Basin and has a total length of approx. 50 km. The fault trace displays a range
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of strikes, dependent on which horizon is viewed and its activation history. It is near-continuous
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at the Late Cretaceous level, with throws of approx. 1800 m and striking NNW to SSE (Giba et
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al., 2012). Changing at the Early Pliocene level, the four individual fault segments are noticeably
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separate and the fault strike in this location is approx. N-S to NE-SW, with vertical displacements
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of up to 1450 m (Giba et al., 2012). After the Miocene, both breached and unbreached relays divide
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the fault segments due to oblique reactivation of the original fault. These are hard linked to the
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basement fault, which strikes in an N-S direction (Giba et al., 2012).
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Giba et al. (2012) suggested that the relay zones of the Parihaka Fault support a coherent
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fault model, where the rapid formation of both fault segments and relay ramps occur geologically
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instantaneously, usually originating from a connection to a continuous basement fault at depth
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(Walsh et al., 2003). The Parihaka Fault reflects this, where it displays soft linkage at shallower
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horizons in the en echelon shaped sections, which combine at depth to the preceding unbroken
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fault trace at (Figure 2). The creation of these distinct fault segments was caused by reactivation
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around 3.7 Ma, at a 45-degree angle to the original extension during the Cretaceous. Although
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fault growth was instantaneous, further growth was accommodated by later displacement and
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rotation of the relay zones (Giba et al., 2012). The sedimentary record during the Late Cenozoic
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was exceptionally preserved due to high sedimentation rates, leading to the blanketing and
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conservation of the growth history of the main faults. When displacement analysis was undertaken
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on this fault, it was seen that movement post- Miocene was concentrated in the segments located
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at the north of the survey. The NE- SW fault segments underwent the most extension as they were
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the most optimally aligned to the direction of movement (Giba et al., 2012). Similar orientation in
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the fault strikes between the Cretaceous and Pliocene fault segments has produced an overall 3D
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fault shape that varies between sigmoidal and straight, with straight fault sections occurring in
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areas with particularly close alignment (Giba et al., 2012). Due to the sigmoidal shape that occurs
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in certain areas of the fault, the need for accommodation of volumetric strain has led to the
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formation of some smaller antithetic faults adjacent to a couple of the fault segments located in the
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hanging wall (Giba et al., 2012).
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METHODS
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3D Seismics
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3D seismic reflection consists of firing compressed air shots which generate sound waves
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that penetrate the subsurface. This seismic energy is produced in the form of wavelets, which
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reflect off different materials at a specific time interval. This reflected acoustic data is then
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processed into a three dimensional volume, which can be used to create images in both horizontal
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and vertical directions and then utilised for geological interpretation (Kearey et al., 2002).
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The Parihaka seismic survey was shot in the Taranaki Basin using Veritas Viking II
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between January 12th and February 24th, 2004 (Veritas, 2005). A total of eight streamers, each
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4500 meters in length and separated by 100 meters, were used in combination with alternating dual
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sources, laterally spaced at 50 meters. This combination resulted in a 60-fold subsurface
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illumination area of 400 meters by 2250 meters for each shot. The source arrays were fired
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alternately at 25.0 meter intervals, during which time 6.1440 seconds of seismic reflection data
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Figure 3. Image A displays a representative cross section through the inline of the survey area, complete with dated horizons and fault interpretation lines. Image B shows the coherence cube created of the study area, displaying the survey boundary and the four main fault segments that cut across it.
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were recorded at 2 milliseconds sampling rate. This data was then processed and filtered to create
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a seismic volume.
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The seismic attribute generated from this volume was a coherency cube. This was produced
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using Petrel 2014 by Schlumberger. Coherency is the measurement of similar adjacent traces
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which are located within a specific area by changing the volume from continuity to discontinuity.
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This makes it useful for interpretation of any disruptions within the volume which may represent
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features such as faults, channel points, sedimentary structures or unconformities. It can also be
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used in order to observe stratigraphic changes such as parallel or chaotic bedding and mapping the
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lateral extent of a certain reflector.
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Data Analysis
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All interpretation, calculations of area and modelling were carried out using Petrel 2014
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Software package by Schlumberger. The fault interpretation tool was used to map each individual
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fault segment and two main horizons were mapped using the picking tool, selecting the strongest
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reflectors above and below the section where channels were located. These two main reflectors
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will be referred to as the Top Chanel Horizon and the Bottom Channel Horizon. The seafloor and
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basement horizons were also mapped in order to accurately date the various layers and features.
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Data from previously completed depth conversion graphs (Giba et al., 2012), which were
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calculated from available well log checkshots provided by Arco Petroleum in 1992, International
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Department Dallas in 1984 and NZ Oil and Gas in 1984, provided a time to depth reference for
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the seismic lines (Fig 3A). The biostratigraphic dating of each horizon, which was completed by
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Giba in 2012, was utilised and compared to the corresponding TWTT lines in every seismic section
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produced, in order to date the different layers (Figure 3A). A seismic coherence cube was created
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(Figure 3B) and the polygon feature was used in order to create channel points. This was carried
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out by analysing a plan view of the coherence image and working at regular intervals of 25 ms
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between the depths of -864 ms and 1248 ms, the first and last appearance of channels in the images.
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The channels were traced by placing points at a fixed distance along each channel thalweg. The
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thalweg is the central, deepest section of the submarine channel. The point data, coordinates and
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horizon grids were converted into a shapefile and exported into ARC GIS for analysis.
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ARC Map was used for further analysis of channel morphology. The channel length was
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measured by tracing each individual channel along its thalweg and measuring the entire distance
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(Appendix II). Valley length was also recorded, a parameter defined as the straight line distance
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between final points at each side of the channel (Flood and Damuth, 1987). These two datasets
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(Appendix II) were recorded in order to determine the sinuosity of each channel, which was
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calculated by dividing channel length by valley length. Meander wavelength was measured by
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counting the number of meanders in each channel, then calculating the ratio of number of meanders
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to valley length.
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The supporting maps that were attached to the seismic survey were modified using ARC
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Map. A channel point density map was also created using this software by exporting the shapefile
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from Petrel and calculating the seismic point data using GIS.
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RESULTS
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Overview
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The survey field is separated into four zones for analysis, displayed in Figure 4B. Zone A
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is the topmost section of the fault, encompassing both hanging wall and footwall and highlights
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the area around the main unbreached relay ramp. The seismic section S-A (Appendix I) is a
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representative slice through this feature. Zone B1 and Zone B2 span the central section of the
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survey, with B1 displaying the footwall features and B2 the hanging wall. The seismic slice S-B
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(Appendix I) is a section through the fault which transects this part. Zone C is the bottom sector
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of the survey, with the seismic slice S-C (Appendix I) displaying features throughout the fault in
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this location.
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Channel point distribution on the footwall of the Parihaka Fault is displayed in Figure 4A,
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at depths of between -864 ms to -1248 ms, measured in Two Way Travel Time (TWTT). The
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submarine channels initiate at number of locations along the main fault boundary and fan out in a
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general N-W trend. Channel initiation occurs at both the inside and outside bend of the sigmoidal
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S shape of the fault trace. There is also some channel point data seen within the fault link zones
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and horsetail splay features. The channel systems on this side of the fault are widely spread and
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display moderate sinuosity, spanning distances of up to 200 kilometres. Directly east of the
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Parihaka Fault, channel point data on the hanging wall (Figure 4C) can be found between depths
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of -1248 to -1600 ms in TWTT. Submarine channel initiation in this sector of the survey area
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appears much more concentrated to the central section of the main fault, with three distinctive
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localities heavily populated by points. There is no visible trend of channel direction in the hanging
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wall as the data points are so tightly packed. However, the overall profile appears to be a
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hemispherical pattern, spreading from the centre of the main fault.
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Zone A
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This section is located to the Northern half of the survey area and the main focus is the
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relay ramp feature. As seen in the seismic section S-A (Appendix I), which is vertical profile
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through the relay itself, the channel points can be observed on both sides of the central ramp. There
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is an element of drag displayed in the beds that feed into the ramp and the channel points in these
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horizons follow this trend accordingly. The channels exit on the hanging wall in the same horizon
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as the footwall, however, the throw appears to be moderate due to rotation of the beds within the
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ramp, also highlighted in the seismic profile.
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In the coherence image displayed in Figure 4A, the channel points plotted on the footwall
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show a general west- southwest trend, and are highly sinuous. The channels in this sector can be
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seen feeding into the relay ramp, however there is also a channel initiating from the fault block
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behind the main fault line which appears to bisect intrusive bodies. Point data on the hanging wall
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propagates from one distinct area, where the channels cross the relay ramp and follow the same
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general trend as the foot wall. Again the channels are sinuous and concentrated from the central
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point of the C shape of the fault block.
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Zone B1 and Zone B2
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Zones B1 and B2 cover the footwall and hanging wall sectors of the centre of the survey
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area. The seismic slice S-B in Appendix I is a vertical profile through both Zone B1 and Zone B2
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across the main fault. In the seismic profile S-B (Appendix I) that intersects both Zones B1 and
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B2, there is a clear difference between sediment thicknesses on both sides. A number of layers on
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the footwall pinch out as they near the fault surface, whereas on the hanging wall there are many
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extra layers of deposition between the top and bottom horizon. There is limited vertical stacking
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of channels on the footwall side of approx. 200 ms thickness and they are situated directly adjacent
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to the fault. On the hanging wall, vertical channel accretion is double that of the footwall with
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vertical thickness of 700 ms. The throw in this central fault segment is 250 ms in difference
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between footwall and hanging wall, particularly on the bottom horizon.
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In Zone B1, which is the footwall portion of the central area, channels initiate in a number
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of distinct locations. The fault in this section comprises a number of fault segments, connected by
230 Figure 4. Submarine channel point data is shown displaying extents and trends of channels in the survey area. A) Channel distribution located on the footwall. B) Overview of the area, showing Zones A, B1, B2 and C. Also displays the channel trends compared to individual fault strikes and the central hanging wall depocentre. C) Channel distribution on the hanging wall. 231
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relay ramps, observed in plan view and in the seismic profile. In Figure 4A, there are three separate
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fault segments each joined by a breached relay ramp with channel initiation occurring at each
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linked zone. The channels in this area are moderately sinuous and spread out, with large visible
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branching. The trend of channel direction varies depending on the fault trace strike. In Segment 1,
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which has an N-S strike, the channels are running at a 45-degree angle to the south-west. However,
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in Segments 2 and 3, which both have an N/NE-S/SW strike the channels also travel at a 45-degree
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angle from the main fault strike, trending directly west. Zone B2, or the hanging wall section of
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this area (Figure 4C), has a much more concentrated distribution of channels. Where the fault
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segment traces display a sigmoidal profile, channel point initiation occurs mainly on the curvature
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maximum of the “S” shape. This is repeated in three separate locations in this section. The channel
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points in this location are highly sinuous and tightly packed near the area of initiation. There is
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also a large quantity of channels in this area, as seen both in the plan view and in the seismic
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profile.
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Zone C
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In the seismic profile for Zone C, S-C (Appendix I), throw between footwall and hanging
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wall is minimal. The fault profile itself is much more steeply dipping in this section compared to
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the other zones. Channel distribution on the footwall side is small, only visible directly adjacent to
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the fault surface. Channel distribution on the hanging wall side is more prominent with repeated
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vertical stacking occurring in this part of thicknesses up to 500 ms.
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In Zone C, there is one main fault segment which displays the sigmoidal profile in plan
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view. Channel point initiation on the footwall occurs mainly on the inside bend of this outline,
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whereas on the hanging wall it occurs on the opposite bend of this sigmoidal fault trace. The
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channels are concentrated largely on the hanging wall side and follow an E-W trend. There is also
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some channel initiation occurring in the horsetail splay features at the very bottom of the main
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fault. Channel initiation that is visible in the central area of the hanging wall is occurring adjacent
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to the intrusive body which is situated to the south east of the coherence image in Figure 4A.
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The channel point density map created for the entire area displays a number of features
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(Figure 5B). Overall trends suggest three main lines of heavy population which transect the survey
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area. Each is adjacent to the areas where the fault segments are connected, either by soft linked
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relay ramps or where they have fully been breached and classified as transfer zones. The red and
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yellow zones in the channel density map highlight the areas of greatest point concentration. This
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is also directly related to vertical stacking as the channel density calculation takes into account
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every point across all depth layers. There is a significant concentration of channels in the central
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area of the hanging wall, this is also the location with the most vertical stacking, although some
266
can be seen on the foot wall as well.
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An isochron sediment thickness map was also created (Figure 5A), with the top channel
268
horizon and bottom channel horizon being used as parameters. The constraining horizons selected
269
above and below the submarine channel appearance in the seismic imagery, are dated at 2.0 and
270
3.4 Ma respectively, during the Pliocene-Pleistocene syn-rift sequence and were used in order to
271
calculate sediment thickness. When compared with the channel point density map, the isochron
272
displayed a similar trend, with thickness maximums focused in the hanging wall across all three
273
zones (Figure 5B).
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Figure 5. A) Ischron sediment thickness map produced of the survey area, calculated between the Top Channel Horizon and the Bottom Channel Horizon. B) Channel point density map created displaying concentration of channel points and vertical stacking. 275
276
Channel Sinuosity
277
When longitudinal profiling was carried out on a representative selection of channels, in
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both the hanging wall and footwall, a number of trends were observed (Figure 6A). In the footwall,
279
sinuosity varied depending on proximity to the fault. In Zones B1 and C, sinuosity was classified
280
as straight (1.5) at the furthest distance
282
away from the fault. The average sinuosity in zone B1 was 1.18 next to the fault zone and 1.9 in
283
the section to the west of survey area. Averages in Zone C showed a similar gradient, moving from
284
1.2 near the fault and 1.8 away from it. This varied slightly in Zone A, which displayed some
285
moderate sinuosity in channels located immediately next to the fault and transfer zone, measured
286
at 1.46 and 1.8, fault adjacent and non-fault adjacent respectively. The sinuosity variation between
287
different depths in the footwall was negligible.
288
In the hanging wall, however, these trends were reversed (Figure 6B). Instead, sinuosity
289
was lower away from the scarp area, becoming less sinuous as proximity to the fault lessened. The
290
most notable characteristic of the parameter found in the hanging wall section was the variance
291
with depth. Channels changed morphology completely, moving from straight to sinuous to
292
meandering as depth decreased (Figure 7). There was little variation within zones in the hanging
293
wall, however, most channels were concentrated around the central B1 Zone on this half of the
294
study area.
295 296
Overall, the trends varied completely between foot wall and hanging wall sections, displaying opposing sinuosity measurements and behaviour.
297 Figure 6. Sinuosity profiles for the hanging wall and footwall channels. A) Overview showing different zones of sinuosity. B) Hanging wall distribution over depth changes. C) Footwall distribution based on zone and proximity to the fault system.
298 299 300
Figure 7. Changes in hanging wall sinuosity throughout a variety of depths. A) Sinuosity at
301
deepest level, 1584 ms, average of 1.10. B) Sinuosity at mid-level, 1464 ms, average of 4.50. C)
302
Sinuosity at shallowest level, -1320 ms, average of 10.85.
303
304
DISCUSSION
305
Central Hanging Wall Depocentre
306
Standard normal fault growth models suggest that displacement is highest in the centre of
307
a fault and decreases gradually towards the tips (Walsh and Watterson, 1987, 1988). It has also
308
been observed that values for throw or rates of localized depression and uplift may change along
309
the length of a fault (Gawthorpe et al., 1994). While that concept of fault growth is quite basic,
310
this study displays a strong correlation to that idealized type of model, particularly on the hanging
311
wall side. Offset between horizons varies from zone to zone, with the largest throw recorded in the
312
central Zone B1 at 250 ms, while measurements taken in the north section Zone A and the south
313
section Zone C are minimal in comparison, although in Zone A it can be attributed to intense
314
hanging wall deformation. An isochron sediment thickness map produced between the top and
315
bottom channel horizons (Figure 5A) displays deposition maximums or red zones in the central
316
section of the main fault, with thickness gradually decreasing along the length of the fault towards
317
the ends, indicating the accommodation space available is controlled by fault activity. This pattern
318
is consistent with displacement analysis undertaken on this fault system by Giba et al., in 2012,
319
which indicates throw maximums in the centre of the fault, decreasing at greater distance away
320
from the centre. This has implications correlating throw displacements to sediment thickness, as
321
both follow the same trend when moving from fault centre to tips. A 3D model created of the
322
bottom channel horizon, complete with structural contours (Figure 8A), clearly shows a large
323
depression in the centre of the hanging wall and provides a visual approximation of the topography
324
at this level. The extent of this basin and volume of sediment accumulation observed are
325
constrained by the fault type present and its related activity, as is typical of marine half-graben
326
systems (Leeder and Gawthorpe, 1987).
327
While significant subsidence occurred during the Cretaceous–Eocene extension period,
328
attributing to the total displacement figure of 1800 m (Giba et al., 2012), this study has focused on
329
the syn-rift deposition occurring during the Pliocene reactivation period. It is during this time
330
frame that the submarine channel system was formed, providing insight into the tectono-
331
sedimentary facies type present adjacent to this fault. This study shows a significant concentration
332
of channels in the central hanging wall area, with two localized zones of focus near the two main
333
transfer zone/relay ramps as seen in the channel density map produced. This has implications for
334
the type of drainage occurring in this system, as it supports a model put forward by Leeder and
335
Gawthorpe in 1987 of a continental half-graben with transverse through drainage. Several key
336
indicators support this type of facies model, firstly, the two main sites of channel concentration
337
surrounding the transfer zone and relay ramp, suggesting drainage initiating between the en
338
echelon segments and moving down into the sub-basin, while the presence of transverse migrating
339
channels, which produced both vertical and lateral channel stacking throughout the sequence, was
340
observed repeatedly throughout the seismic images. Sinuosity profiles completed on the hanging
341
wall channel systems at a range of depths support both the normal fault model and the facies model
342
proposed and provide evidence to help constrain slope and direction of flow in the hanging wall.
343
Channel sinuosity profiles were classified as straight at the base of the sequence, flowing down off
344
the footwall scarp and into the basin, while alternative drainage was recorded from channels which
345
emerged from the east of the survey area and ran into the central depression zone adjacent to the
346
fault. This provides two key details with regards the morphology of the sub-basin; that the slope
347
was relatively steep coming down off the fault scarp, as sinuosity decreases with increased slope.
348
349 350 351
Figure 8. Layer images of the Bottom Channel Horizon and Top Channel Horizon. A) Bottom
352
Channel Horizon illustrating the hanging wall depocentre. B) Top Channel Horizon showing the
353
en echelon morphology on footwall and structural dip for the area.
354
It also represents an element of initial steepness in the hanging wall dip slope. This would support
355
the coherent fault model suggested by Giba et al., in 2012, as it subscribes to the idea of
356
instantaneous fault growth, with maximum displacement occurring over a brief period of time, and
357
a gradual levelling out of the hanging wall slope, a concept which might justify such steep hanging
358
wall dip followed by rapid infilling. That the channels increase in sinuosity as the sequence moves
359
up depth, is consistent with the evolution of these types of submarine channel systems, changing
360
from immature, linear channels in high gradients to mature, meandering channel systems with a
361
complex geometry (Gee and Gawthorpe, 2007) and displaying vertical accretion as seen in this
362
study. This is due to slope levelling out as the sequence thickens, a concept which is familiar in
363
the infilling of half-graben hanging wall structures (Gawthorpe and Leeder, 2000).
364
Transfer Zone/ Relay Ramp Footwall Controls
365
Displacement accommodation between individual fault segments is often aided by the
366
formation of intrabasin transfer zones or relay ramps, particularly where the fault shape follows an
367
en echelon side stepping pattern (Leeder and Gawthorpe, 1987). These intrabasin transfer zones
368
have a strong influence on sedimentation trends on a localised scale and may act as a pathway for
369
turbidity currents or footwall deposits, especially in transverse drainage facies models (Gawthorpe
370
and Hurst, 1993). Individual transfer zone morphologies are poorly constrained at the subsurface
371
level (Gawthorpe and Hurst, 1993), as the majority of seismic surveys are imaged parallel to the
372
strike of these areas (Gibbs, 1989).
373
In this study area, there are two separate transfer zones within the main fault system. The
374
first transfer zone is located in Zone A, and can be classified as an unbreached relay ramp. The
375
submarine channel data which was recorded surrounding the ramp, shows a clear convergence of
376
footwall drainage moving down through the relay ramp and feeding into the hanging wall sub-
377
basin. The second transfer zone, located in Zone B, also acts as a conduit for sediment flow into
378
the hanging wall and is fully breached. Both of these factors are consistent with the transverse
379
drainage facies model (Leeder and Gawthorpe, 1987) suggested in this study already from hanging
380
wall observations. This theory is backed up by the results of the channel point density map which
381
indicates a build-up of sediment in two distinct lines of concentration focused around each transfer
382
zone from across the footwall.
383
There are a number of notable trends found within this footwall channel data which may
384
have implications for the en echelon segment morphology. The first element for consideration, is
385
how the flow direction of an individual channel follows a repeatable pattern depending on the
386
particular point of initiation along the sigmoidal “S” fault trace. This infers separate zones of
387
subsidence and uplift which are directly controlled by the fault profile. In the lower half of each
388
en echelon segment, channels initiate in the central internal point of curvature, then run off at
389
approx. 45° to the fault strike, this angle does not change throughout the entire fault system, and
390
when the fault strikes vary from south to north, the direction of channel movement maintains the
391
same angle and varies accordingly. As fluid flow of any type migrates downslope taking the path
392
of maximum gradient, this behaviour in has implications for topographical high zones within the
393
segments of this fault type, suggesting an area of uplift is situated in the lower half of the en echelon
394
unit. Previous research indicates that certain structures observed in these typical relay ramp
395
transfer zones hold similarities to strike slip fault systems, particularly the alternating zones of
396
extension and compression (Reading, 1980; Gawthorpe and Hurst, 1993), an idea that appears to
397
be supported by the behaviour of the fault system examined in this study. In the upper section of
398
the same fault, where channels initiate at the point of maximum external curvature, the angle is
399
observed to be between 85°-90° from the fault strike and channels flow in the opposite direction
400
to the lower half. This implies that the topography in this half of the en echelon segment slopes
401
down towards the relay transfer zone. This result corresponds with the characteristic elevation
402
decrease towards the ramp area associated with these type of intrabasin transfer zones (Gawthorpe
403
and Hurst, 1993).
404
Sinuosity profiles completed on the channels in the footwall section show some
405
corresponding trends. General patterns in sinuosity show low to zero sinuosity directly adjacent to
406
the fault segments and moderate to high sinuosity as distance from the fault gradually increases
407
towards the west. This type of progression provides evidence of footwall backtilting, which is
408
frequently observed in other half graben systems, however the direction of channel flow indicates
409
the oblique nature of this particular fault controls the orientation of this slope. When the channels
410
which are fault adjacent are examined in greater detail, there are slight variations in sinuosity
411
between the upper and lower halves of the en echelon segments. Channels which run off the lower
412
half of the unit, display zero to low sinuosity until they reach a moderate distance away from the
413
fault. Alternatively, the channels which propagate in the upper half of the segment show low to
414
moderate sinuosity as they feed down into the transfer relay ramps. This substantiates the
415
hypothesis that there are localised zones of interlinked uplift and depression associated with this
416
style of fault. The variation in sinuosity also helps constrain the different levels of elevation on
417
opposing sides of the en echelon segment, inferring a steeper slope which runs at 45° to fault strike,
418
dipping south from the inside bend of the “S” and a gentler slope which runs between 85°-90° to
419
fault strike dipping north from the outside bend of the “S” and leading into the transfer/relay zone.
420
There is a slightly higher grade of sinuosity in Zone A which does not fit in with the rest of the
421
model, however this is most likely attributed to the fact that the northern section of the fault system
422
underwent the least amount of displacement, due to the angle of extension being sub-optimal in
423
this area. This factor led to repeated rotation of beds in the relay ramp and significant hanging wall
424
deformation (Giba et al., 2012), which may account for the lower gradient slope and offset in this
425
sector.
426
By combining submarine channel flow direction with sinuosity gradient it is possible to
427
postulate a model of the morphology of this fault system and consequently how it might control
428
sediment supply to the surrounding areas.
429
CONCLUSIONS
430
Outlined are the main conclusions drawn from this study;
431
Submarine channels and other fault scarp and transfer zone drainage observed on the
432
hanging wall are directed towards the sub- basin located at the centre of the fault system,
433
where the accommodation space was created by fault activity and sediment thickness maps
434
correlate with fault throw, decreasing along the fault length from middle to tip.
435
Sinuosity profiles in the hanging wall sub-basin indicate an evolution of the channel system
436
present, from immature, high gradient channels to mature, meandering systems as depth
437
decreases. This type of high sinuosity profile and vertical channel accretion combined with
438
en echelon fault transfer drainage indicates a facies model of a continental half graben with
439
transverse through drainage.
440
Channel drainage patterns in the footwall collaborate with sinuosity profiles in order to
441
constrain the topography produced by the en echelon fault shape. The data supports the
442
concept of intra basin highs and lows associated with transfer zones and provides an
443
indication of the associated slopes of each.
444 445
Sinuosity gradient in the footwall provides evidence of footwall back tilting, the orientation of which is controlled by the oblique fault system.
446
While sedimentation patterns on a regional level are controlled by overall topography and
447
tectonics, the trends found in the Parihaka Basin are influenced locally by the main oblique normal
448
fault. Accommodation space created by the original faulting in the area has resulted in a significant
449
hanging wall depocentre, while the complex zones of uplift and subsidence associated with the en
450
echelon fault system has controlled distribution surrounding the transfer zones and relay ramps
451
between segments. Syn-rift abyssal channel development on the footwall, initiated during the
452
oblique reactivation of the fault system, follows a predictable and repeatable pattern, with channel
453
initiation and flow direction being strongly influenced by specific areas in each fault zone.
454
APPENDIX Table of Contents Appendix I: Seismic Sections A) Seismic cross section through the complete survey area, inline vertical section from N-S. B) Representative Seismic Slice S-A, vertical section through the fault at Zone A. C) Representative Seismic Slice S-B, vertical section through the fault at Zone B1 and B2. D) Representative Seismic Slice S-C, vertical section through the fault at Zone C. Appendix II: Channel Measurement Data A) Representative image of the type of channel that was mapped, complete with the set of measurements taken. B) Footwall Channel Measurement Data C) Hanging Wall Measurement Data
APPENDIX I Seismic Cross Sections A) Seismic Section Inline Area
Appendix I (A). Seismic cross section from N-S encompassing the whole area. Annotations include the main features and the relative ages of the layers. B) Seismic Section S-A
Appendix I (B). Representative cross section through the main fault in Zone A of the study area, complete with annotations of the main features.
C) Seismic Section S-B
Appendix I (C). Representative cross section through the main fault in Zone B1 and B2 of the study area, complete with annotations of the main features. D) Seismic Section S-C
Appendix I (D). Representative cross section through the main fault in Zone C of the study area, complete with annotations of the main features.
APPENDIX II A) Representative Channel Image
Appendix I (A). Example of the submarine channels mapped in survey area. Annotations include the relevant parameters that were measured and an example of a thalweg.
B) Footwall Channel Data
CHANNEL NUMBER
FAULT SECTION
SURVEY ZONE
DEPTH
CHANNEL LENGTH
VALLEY LENGTH
-936 -936 -936 -936 -936 -936 -936 -936 -936 -936 -936 -936 -936 -936 -936 -936 -936 -936 -936 -936
m 1449.84 3210.56 479.19 595.84 2119.77 2051.05 3757.40 1384.24 1367.36 2581.28 3174.34 2506.35 7229.85 4259.88 8834.38 1189.98 2155.08 1326.68 1847.44 1660.55
m 1377.29 3005.80 469.94 575.98 1443.76 1442.09 2651.66 1060.73 1238.57 2298.13 2919.40 1513.93 3962.57 3425.77 7507.27 1033.11 1953.30 1181.98 1468.51 1530.45
1 2 3 4 5 6 4 5 6 7 8 9 10 11 12 13 14 15 16 17
FW FW FW FW FW FW FW FW FW FW FW FW FW FW FW FW FW FW FW FW
ZONE C C C C C C C C C C C C B B B A A A A A
FAULT /NON ADJACE NT FA FA FA FA NA FA FA FA FA FA FA FA NA FA NA FA FA FA FA FA
18 19 20 21 22 23 24 25 26 27 28 29 30
FW FW FW FW FW FW FW FW FW FW FW FW FW
C C B B B B B B B A A A A
FA NA FA FA FA NA NA FA NA FA FA FA FA
-984 -984 -984 -984 -984 -984 -984 -984 -984 -984 -984 -984 -984
4413.23 10901.95 4863.14 6084.16 3012.41 9643.29 10821.82 1726.64 11313.20 5455.10 3977.03 4704.75 5664.31
3929.30 7346.25 4141.95 4708.56 2242.29 6303.12 4979.55 1661.07 2250.96 4806.92 3720.17 3165.12 917.71
31 32 33
FW FW FW
C C B
FA FA FA
-1008 -1008 -1008
7854.60 6821.28 4497.21
5040.61 4921.51 3336.28
CHANNEL NUMBER
FAULT SECTION
SURVEY ZONE
DEPTH
CHANNEL LENGTH
VALLEY LENGTH
FAULT /NON ADJACE NT NA
-1008
m 11397.54
m 8574.52
34
FW
ZONE B
35 36 37 38 39 40
FW FW FW FW FW FW
B B A A A A
NA NA FA FA FA NA
-1008 -1008 -1008 -1008 -1008 -1008
17497.46 12787.83 2497.01 5133.71 5375.37 5692.16
9199.64 9908.92 2264.57 4231.23 4543.58 3042.36
41 42 43 44 45 46 47 48 49 50 51 52
FW FW FW FW FW FW FW FW FW FW FW FW
C C C B B B B B A A A A
FA NA NA FA NA FA NA FA FA FA FA FA
-1032 -1032 -1032 -1032 -1032 -1032 -1032 -1032 -1032 -1032 -1032 -1032
5823.83 15376.24 4100.16 8920.92 16851.57 6066.54 11788.78 5806.75 4148.00 4612.17 2001.57 3026.41
5647.75 9803.60 1289.86 7876.85 10438.99 5696.20 7029.97 5000.76 3644.08 3475.55 1449.82 2654.62
53 54 55 56 57 58 59 60
FW FW FW FW FW FW FW FW
C C B B B B A A
FA FA NA FA FA FA FA FA
-1056 -1056 -1056 -1056 -1056 -1056 -1056 -1056
6112.42 5701.24 15280.74 4501.72 9666.93 6366.20 3653.53 1738.59
3385.60 4457.92 8847.03 3945.30 8345.79 5704.61 2960.86 1603.64
CHANNEL NUMBER
1 2 3 4 5
1.05 1.07 1.02 1.03 1.47
>1.15 STRAIGHT * * * *
1.15-1.5 SINUOUS
*
MEANDER
WAVE LENGTH
1 4 0 1 6
0.00073 0.00133 0.00000 0.00174 0.00416
1.15 STRAIGHT 6 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
1.42 1.42 1.30 1.10 1.12 1.09 1.66 1.82 1.24 1.18 1.15 1.10 1.12 1.26 1.09 1.12 1.48 1.17 1.29 1.34 1.53 2.17 1.04 5.03 1.13 1.07 1.49 6.17 1.56 1.39 1.35 1.33 1.90 1.29 1.10 1.21 1.18 1.87 1.03 1.57 3.18 1.13 1.61 1.07 1.68
1.15-1.5 SINUOUS * * *
MEANDER
WAVE LENGTH
4 3 2 3 8 3 5 12 3 7 2 2 1 1 0 3 8 5 4 1 14 11 1 15 6 1 4 7 5 6 4 12 10 13 1 7 4 9 2 10 5 5 15 2 7
0.00277 0.00113 0.00189 0.00242 0.00348 0.00103 0.00330 0.00303 0.00088 0.00093 0.00194 0.00102 0.00085 0.00068 0.00000 0.00076 0.00109 0.00121 0.00085 0.00045 0.00222 0.00221 0.00060 0.00666 0.00125 0.00027 0.00126 0.00763 0.00099 0.00122 0.00120 0.00140 0.00109 0.00131 0.00044 0.00165 0.00088 0.00296 0.00035 0.00102 0.00388 0.00063 0.00144 0.00035 0.00100
1.15 STRAIGHT 48 49 50 51 52 53 54 55 56 57 58 59 60
1.16 1.14 1.33 1.38 1.14 1.81 1.28 1.73 1.14 1.16 1.12 1.23 1.08
1.15-1.5 SINUOUS *
MEANDER
WAVE LENGTH
4 3 6 1 3 4 3 17 4 6 3 4 1
0.00080 0.00082 0.00173 0.00069 0.00113 0.00118 0.00067 0.00192 0.00101 0.00072 0.00053 0.00135 0.00062
