This chapter is divided into two parts. The first section addresses the methodology used to collect field data and how the data was processed. The second section reviews the understanding of sedimentation within rotated fault blocks.
3.2 Data Collection and Mapping 3.2.1 Pre-Field Work
In order to address the questions that this thesis is attempting to answer, the sub-horizontal layer had to be mapped. Previous mapping of the area was conducted on a large scale, and did not offer enough detail of the two fault blocks, Kalavryta and Kerpini, that were being studied. Prior to travelling to Greece for the field work, a thorough map study was conducted.
Attempts were made to use a DEM within ArcGIS; however the resolution of this DEM (30m) was not adequate to isolate smaller features in the terrain. Therefore the primary tool used was Google Earth. Google Earth provided an excellent platform to view the terrain from various angles and to pinpoint possible locations that might have sub-horizontal layers.
3.2.2 Field Work
The majority of the mapping conducted during a 10-day fieldtrip was done on foot. The mapping involved identifying lithology and faults, and locating possible contacts between the sub-horizontal layers and basement/dipping sediment. Dip angles and dip directions were measured using a “Silva Expedition w/clinometer” compass. This compass allows for a fairly accurate measurement of exposed bedding. In order to avoid measuring apparent dips an effort was made, where possible, to view the bedding from various angles to get the most accurate reading. This was often not possible and therefore there is some uncertainty in the measurements.
Every point that was measured was recorded on a geological map and by a GPS waypoint.
The data collected for flow direction within the sediment was done by searching for clast imbrication within the tabular clasts. The consolidated tabular and disc shaped clasts that showed a 3:1 length-width ratio were used to judge flow direction. Several sections of the outcrop were observed to look for trends in the various layers, and, based on that, note down the direction of
15 flow. Although this might not be the preferred method of obtaining flow direction, it did provide a flow trend within nearly all the conglomerate outcrops visited. Figure 3-1 is an image taken at Unit C (See Chapter 4), the purpose of which is to briefly show the method used to obtain a flow trend within the conglomerate outcrops. The image shows the tabular clasts highlighted with red circles.
Figure 3-1: This image is taken at Unit C (See Chapter 4). It shows how the imbrication was observed at the various outcrops. Note that a few of the tabular clasts are highlighted with a red oval.
With regards to clast and grain size, clast measurements were conducted by determining a 1x1m section of the outcrop and measuring the ten largest clast sizes within that 1x1m area. An average was then taken based on these measurements. Where possible an attempt was made to measure at various height intervals, however this was often difficult due to steep terrain. In order to review the data locations at a later date, a total of 2262 images were taken. All images were taken using a Nikon D800 at various focal lengths.
3.2.3 Post-Field Work
Geological maps created in the field were digitized into ArcGIS, during which special care had to be taken to ensure that the maps where correctly referenced. Once correctly geo-referenced, the GPS data points were imported into ArcGIS. Quality checks were conducted on a
16 random selection of data points. As each data point had a coinciding image, the image was reviewed to ensure that the data point was placed correctly on the map. All images taken were processed in Adobe Photoshop. Minor corrections were made to exposure and sharpness. Finally the processed images were annotated and interpreted in CorelDraw Graphics Suite.
3.3 Sedimentation within Rift Basins 3.3.1 General
When discussing sedimentation within rift basins it is important to separate between rifting and faulting. Although one may refer to sediment as being pre-rift, a basin within the rift area may be bounded by faulting. Therefore in this thesis when discussing pre-rift, it is discussing the Gulf of Corinth rifting and references to pre-, syn- or post-faulting will be tied specifically to a fault by name.
Figure 1-2 showed a generalized conceptual sketch of sediment infill within a series of rotated fault blocks controlled by a series of normal faults. As the displacement of the normal fault increases, it will also increase the accommodation space for sediment. The sediment that is deposited within the fault process can then be broken down into three phases.
3.3.2 Sedimentation Phases within Fault Blocks 3.3.2.1 Pre-Fault
As the name suggests this is sediment that is deposited before any movement of the fault.
What is common in pre-fault deposits is a symmetrical trend within dip angles and thickness, i.e.
they have not been affected by faulting.
3.3.2.2 Syn-Fault
Syn-fault sediment is deposited during the course of the movement of the fault. These can be further subdivided into: early, mid and late syn-fault. Certain traits that are commonly found within syn-fault deposits are: decreasing dip angle (towards the controlling younger fault) from older to younger sediments, as one moves away from the controlling faults’ footwall the thickness of the sediment becomes thinner, similarly they are thicker on the controlling younger faults’ hanging wall.
17 3.3.2.3 Post-Fault
Lastly there is the post-fault sediment; these are usually deposited once the fault is considered “dead”, in other words when the fault is no longer active. Commonly these show a consistent dip angle within bedding as there is no longer any tectonic movement to displace beds.
3.3.3 Extensional Tectono-stratigraphic Models
Gawthorpe and Leeder (2000) proposed an evolutionary model in their paper for continental environments. The intermediate stage and the final stage, “fault death”, model will be shown.
3.3.3.1 Intermediate Stage
Figure 3-2: This image represents the intermediate stage with in a tectono-sedimentary continental environment evolution. At this stage in the evolution there is lateral progradation and interaction between the fault segments. As a fault becomes inactive the sediment in the basin adjacent to this fault (red square) becomes buried and persevered or they can become uplifted, incised and reworked. The green square shows a diverted river through a fault segment. (Modified after Gawthorpe and Leeder (2000)).
Figure 3-2 is an image from Gawthorpe and Leeder (2000) and shows the intermediate stage in the evolution of a normal fault continental environment. Although there is no scale to this image, it is there to show that in a classical model the sediment influx may arrive from
18 several areas within the rotated fault blocks. The image shows sediment from the uplifted footwall flowing “down” into the basin, gathering at the main depocentre in the basin, and fans that have come off the fault face (red square). More importantly it displays an antecedent river (green square) as it is diverted through two fault segments creating a fluvial system running almost parallel with the fault. This diversion of the fault segments could be viewed as a possible ramp structure between the two fault segments.
3.3.3.2 Final Stage
In the final stage the main fault has died, become inactive, and there is a shift in sediment transportation running more parallel in the basin created by the main fault. The previous antecedent river shown in Figure 3-2 is now depicted as a fan coming off the inactive fault face (red square, Figure 3-3).
Figure 3-3: The final stage of the tectono-sedimentary continental environment evolution model. At this stage the fault segments have linked and the controlling fault has become inactive. As the fault segments have linked the previous antecedent river is now depicted as a fan coming off the inactive fault (red square). The green square shows that the sediment transport is now moving more parallel with the inactive fault within the basin. (Modified after Gawthorpe and Leeder (2000)).
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