4. NEW 1:4000 SCALE GEOLOGICAL MAPPING

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1 4. NEW 1:4000 SCALE GEOLOGICAL MAPPING 4.1 INTRODUCTION Accurate geological information is the foundation of all geotechnical investigations. It has been shown that weak geological units often have a direct correlation with the occurrence of landsliding. Within the Wollongong City Council (WCC) local government area (LGA) this correlation is most evident in the northern suburbs with the presence of landslides in areas largely underlain by the claystones of the Narrabeen Group (Bowman, 1972 and Pitsis, 1992). A direct high correlation between the incidence of landsliding and two geological intervals within the Newcastle Coal Measures has also been demonstrated in the Lake Macquarie City Council LGA (Fell and Flentje, 1991 and Flentje, 1992). The Newcastle Coal Measures outcrop on the northern side of the Sydney Basin, several hundred kilometres north of the subject area, and are laterally equivalent to the Illawarra Coal Measures, which outcrop within the subject area. It is very important, if not essential to place the geological data gained from a site investigation into not only a regional, but also a local geological setting. Indeed, such initial placement can help determine the nature of the challenge at hand as well as the scope of the detailed subsurface investigation required. Furthermore, accurate knowledge of the geological formations which underlie a site allows ready familiarisation with other sites in similar geological settings, and a more focused investigation from the outset. Therefore, accurate geological maps covering the subject area are vitally important. Geological mapping of the Wollongong area is currently available at several map scales, ranging from 1:6336 up to 1:250,000, as discussed in Chapter 3. There are some larger scale maps covering smaller areas. The commercially available mapping has been carried out and presented at small scales making them of limited use for geotechnical investigations of land instability. The 1:6336 scale maps of Bowman (1972), discussed in the previous chapter, suffer from contour and scale inaccuracies, such that they are of limited use today. The need for more accurate geological maps, at more appropriate scales, for investigations of land instability within the subject area has been highlighted. In addition, accurate geological maps are an essential component of the site 4-1

2 characterisation stage of the risk assessment procedure proposed in Chapter 2. One of the major decisions made at the outset of this research project was to undertake the preparation of accurate geological and land instability maps within the subject area. This chapter discusses the development of these new, flexible and accurate geological maps of the subject area. The development of new maps of land instability is discussed in the following chapter, Chapter 5. Chapter 6 highlights the compilation of the Geotechnical Landscape Map Series, which combines or overlays numerous Geographic Information System map layers, including both the geological maps discussed in this chapter, and the land instability maps discussed in Chapter WORK PRIOR TO MAPPING Prior to the commencement of the geological mapping of the subject area two important decisions had to be made. These decisions concerned; (a) the selection of map scale and hence base maps upon which to map, and (b) the mapping media. The provision by the WCC, as a major sponsor of this project, of access to the services and products of their Geographic Information System (GIS) Department and staff aided the decision making process in regards to both of these matters. The Central Mapping Authorities (CMA) 1:4000 scale ortho-photographic map series (contours superimposed on black and white aerial photography) with a 2m contour spacing, discussed in Chapter 1, characterise the study area very well. Larger scale maps, providing coverage of the whole subject area are not available, and smaller scale maps do not provide the topographic accuracy deemed necessary by the writer. Hence, it was decided to map the subject area at a scale of 1:4000. Geological mapping of the Wollongong area will always involve some interpretation of stratigraphic boundaries between areas of bedrock exposures, borehole locations across areas covered by colluvium. Consequently, there is always some uncertainty associated with the final product. As new and more accurate information becomes available, the geological maps will periodically require review and editing. This point is also relevant for maps of land instability, as discussed in chapter 5. Future flexibility of the map information is a most desirable attribute of a modern mapping procedure. Hence, instead of preparing manually drafted maps, the map information has 4-2

3 been digitised 1 and stored as computer files, or map layers within the WCC GIS, from where the map can be printed. Having the information stored digitally facilitates future modifications as more accurate information becomes available. The Wollongong City Council Geographic Information System computer package was adopted as the main tool for the production of base maps, electronic data storage, and final map presentation. Mapping of the geology, discussed below, and land instability, discussed in Chapter 5, has been carried out at a scale of 1:4000 as stated earlier. The GIS department prepared 1:4000 scale base maps upon which the geological mapping was carried out. The base maps included the CMA contour information, cadastral boundaries which are regularly updated by the WCC, and as discussed below, the GEOWELLS, FAULTS and MINOR FAULTS map overlays. As discussed in Chapter 3, a large percentage of the slopes of the escarpment within the subject area are covered by thick deposits of colluvium and geological mapping represents a considerable challenge. In the field, bedrock stratigraphy is only visible in isolated bedrock outcrops and exposures, which occur in: cliff lines along the coast, the upper escarpment, and various intermediate cliff lines water courses which have scoured down through the colluvium mantle into the bedrock sequence occasional spur lines which have not been inundated with colluvium, or alternatively those that have been denuded of colluvial cover by erosion and man made excavations such as road side and railway line cuttings and building site excavations. Often the outcrops and/or exposures are of limited spatial extent and it is difficult without other supporting evidence to correlate the exposed sequence to its correct position within the stratigraphic column of the region (Figure 3.4). An important source of supporting evidence are borehole logs. 1 Digitising is a process of computer data capture whereby map based information is converted into digital information which can be stored as a computer file. Manual digitising uses a digitising tablet like a drafting table that is equipped with a cursor or stylus for tracing and hence recording the information. The operation is controlled by a computer program. 4-3

4 4.2.1 Borehole database - BOREHOLES. One of the ways to view the geological sequence underlying an area, other than in outcrop or exposure, is to drill a borehole. During the drilling of a borehole, a log is compiled which documents the geological layers encountered amongst other subsurface conditions. The log may comprise a summary of the cuttings or wash/mud returned to the surface in the drilling medium (which may be a chemical mud, water or air) or alternatively it may be a detailed engineering geological description of recovered core. Boreholes are commissioned for a wide variety of purposes within the subject area, including; (a) geological exploration by the Geological Survey of New South Wales in the early stages of exploration of the area, (b) coal exploration by many companies, and (c) geotechnical investigations on behalf of many government authorities, private companies and/or individuals. The writer undertook a research task of developing a borehole database after collecting borehole logs from coal mining companies and other companies involved in geotechnical investigations to include in a borehole database. The boreholes were primarily coal exploration boreholes and were researched from Broken Hill Proprietary Ltd, Kembla Coal and Coke Pty Ltd and Shell Pty Ltd libraries. However, some geotechnical investigation boreholes drilled by the Railway Services Authority (RSA), the Roads and Traffic Authority (RTA), Longmac Associates Pty Ltd and Coffey Partners International Pty Ltd have also been incorporated in the database. All the boreholes selected from geotechnical investigation work have been prefixed with the letter G. Information for boreholes located between the coastline and two kilometres west of the escarpment has been collected. Boreholes considered as suitable for inclusion in the database contain accurate stratigraphic and survey information. As a minimum requirement, stratigraphic information must have included the intersection of a least one confirmed stratigraphic horizon. Accurate survey required the availability of coordinates, preferably to Integrated Survey Grid (ISG), for the location of the borehole, and reduced ground level to the Australian Height Datum (AHD). For some boreholes, site plans were considered sufficient where the writer could determine the required information on the basis of the 1:4000 CMA ortho-photographic map series and the site plan. The database references each borehole record with 27 fields of data, as shown in 4-4

5 Table 4.1. Initially, this information was entered into the Wollongong City Council computer Ingress database. More recently, this information has been copied to a Microsoft Access database using a personal computer. An example Borehole Data Sheet, for one borehole in the Borehole database is shown in Figure 4.1. Field name Data Type Size (bytes) Well Identity Text 255 Well name Text 255 Colliery Text 255 Data source Text 255 units Text 255 drill date Text 255 ISG east Text 255 ISG north Text 255 Collar RL Text 255 End Of Hole Text 255 colluvium Text 255 Base Hawkesbury Text 255 base Rnbh Text 255 base Rnb Text 255 base Rnsp Text 255 base Rns Text 255 base Rnw Text 255 base Rnc Text 255 top Bulli Text 255 base Bulli Text 255 base Balgownie Text 255 base Wongawilli Text 255 base American Text 255 base Tongarra Text 255 base Woonona Text 255 base Figtree Text 255 base Unnaderra Text 255 Table fields in GEOWELLS database. The boreholes logs vary in drill date (year) from 1901 through to The geological profile recorded from each borehole was dependent on the desired purpose of the borehole and experience of the logger, be that of a modern geologist experienced in Sydney Basin geology or a turn-of-the-century drilling hand. Where possible, the most recent boreholes with detailed logs were given preference over older boreholes. Consequently the writer had to exercise considerable judgement. Validation of the borehole data included a comparison of the borehole ground level reported in the log, with the ground level indicated at the same location on the CMA 1:4000 contour plans. A difference of more than 2m in this data resulted in 4-5

6 exclusion of the borehole from the database. An additional check involved a visual examination of the reported borehole location on the CMA 1:4000 ortho-photographic maps to determine whether the site was suitable for the drilling of a borehole. For example, a steep cliff or a steep inaccessible gully would not be suitable for drilling a vertical borehole. Well Identity: C0013 Well name: Data source: KCC KCC Colliery: Units (M or I): COALCLIFF m ISG easting (m): ISG northing (m): Drill date: Collar RL: End of Hole: Base of colluvium: - Base of Hawkesbury Sandstone: - Base Baldhill Claytone: Base Bulgo Sandstone: Base Stanwell Park Claystone: Base Scarborough Sandstone: Base Wombarra Claystone: Base Coalcliff Sandstone: Top Bulli Coal Seam: Base Bulli Coal Seam: Base Balgownie Coal Seam: Base Wongawilli Coal Seam: Base American Creek Coal Seam: - Base Tongarra Coal Seam: - Base Wonoona Coal Seam: - Base Figtree Coal Seam: - Base Unanderra Coal Seam: - Figure 4.1. Example Borehole Data Sheet for borehole C0013, one of the 154 boreholes in the BOREHOLE database and the GIS based GEOWELLS overlay. This multi-faceted process of validation resulted in the exclusion of approximately 100 boreholes from the database. The completed and validated database includes 154 boreholes. A map of the locations of all the boreholes in the BOREHOLE database, entitled GEOWELLS was generated for the writer by the WCC GIS department. This map includes a borehole symbol centred at the ISG location of the borehole with a label 4-6

7 showing the data from the Well identity field of the borehole database. This map overlay was included on the base maps used for the geological mapping, discussed below. This allowed for easy reference to nearby boreholes to assist the correct stratigraphic positioning of any bedrock outcrops and exposures. This borehole database was also used as one component in the computer modelling of a small area of the escarpment, discussed below. A reduced scale plan, showing the distribution of boreholes in the BOREHOLE database, and major faults, discussed below, is shown as Figure 4.2. Figure 4.2. GIS generated summary plan showing distribution of boreholes in BOREHOLE database and location of the major faults included in the FAULTS overlay. 4-7

8 4.2.2 Geological faults Geological faults are extremely difficult to map because of the masking effect of the colluvium cover. Only in areas of extensive bedrock outcrop, such as the coastal cliffs between Clifton and Coalcliff could faults (the Clifton Fault, the Jetty Fault and the Harbour Fault) be accurately mapped along the ground surface. Hence an alternative source of structural information was required. The Australian Coal Industry Research Laboratory (ACIRL) has compiled a series of 1:25000 Structure Contour plans (showing folding and faulting) of the floor of the Bulli and Wongawilli coal seams (ACIRL 1989). This map series represents a compilation of mine record tracings at a uniform datum (10000 metres below AHD). The location of faults and dykes on these plans, covering the area between the coast and several kilometres inland have been digitised by the writer into the WCC GIS computer package. Seven laterally extensive faults, mapped during coal extraction from the Bulli and Wongawilli coal seams (ACIRL 1989), have throws in excess of 20m, namely the Metropolitan Fault (to 65m), Clifton fault (to 66m), Scarborough Fault (to 60m), North Bulli Fault (to 60m), Bulli Fault (to 90m), Corrimal Fault (to 28m max) and the Wongawilli Fault (to 50m). These faults have been digitised and saved as a GIS-based map layer named FAULTS. Smaller faults and dykes have been digitised and saved as a GIS based map layer named MIN FAULTS. Coal mining operations extend eastwards often to near the outcrop of the coal seam. Consequently, fault locations are considered to be accurate, at the level of the Bulli and/or Wongawilli Coal Seams and above, but are not clearly defined eastwards of the outcrop of the coal seam. Hence, some extrapolation by the writer has been required for some faults (for example the location of the Scarborough Fault on Map G11, is not known east of the railway line, other than is indicated by some indirect evidence based on bedrock outcrops). A plan showing the distribution of the major faults comprising the FAULTS overlay is shown in Figure 4.2. In addition to these faults, some additional faults have been mapped and/or inferred at ground level during the field mapping stage of this work. For example, the Harbour Fault, on the coast just south of Coalcliff has been mapped, after Pitsis 1992, as having a throw of approximately 20m. The major faults have been used to generally subdivide the subject area into fault 4-8

9 confined discrete geological blocks, to assist the mapping exercise. Whilst the geology in each block is similar to the next, a change in relative levels typically exceeding 20m, of the geological sequence occurs due to the relative vertical displacement across the fault(s). During the modelling and field mapping work, discussed below, the faults have been shown as vertical. The faults are shown at the surface in the same position as which they have been encountered at depth in the Bulli/Wongawilli Coal seams, unless field evidence suggested otherwise. Field evidence reveals that many of the faults in the study area are typically near vertical, hence this assumption is, in many cases, acceptable. This ACIRL (1989) map series has also been quite useful in helping to determine the location of the gentle fold axes and the dip of the geological sequence, which are shown by the contours of the base of the Bulli/Wongawilli Coal Seam. 4.3 MAPPING TECHNIQUES The 1:4000 scale geological maps prepared during this research project have been produced using an integrated and often iterative series of techniques which comprise; i) geological field mapping, ii) computer modelling, and, iii) inference from existing maps. Each of these techniques is discussed below Geological Field Mapping The writer spent many months during the first two years of this research project in the field walking the length of most of the water courses, coastal cliff lines, roads and the railway line within the subject area, mapping the exposed geology. As discussed in the following chapter, some mapping of land instability was also carried out during this time. Field mapping was carried out directly onto the GIS prepared base maps, which included the contours generated photogrametrically by the CMA, the WCC cadastre, and the GEOWELLS, FAULTS and MINOR FAULTS map overlays discussed above. In addition to the lack of outcrop, discussed above, geological mapping is hampered by dense vegetation, often steeply sloping ground and limited access. The dense vegetation which most often limits or prohibits access is lantana, an introduced 4-9

10 species on the east coast of Australia which has become rampant and is now regarded as a noxious weed. There are many areas of steeply sloping ground, including near-vertical cliffs, particularly on the upper sections of the escarpment. Limited access arises due not only to the dense vegetation and steep slopes, but is also a result of privately owned land in the developed areas. Field positioning on the 1:4000 base maps was facilitated by the following: the form of contour lines on the base maps cadastral map boundaries and existing fence lines house locations in the field and on the 1:4000 scale CMA ortho-photographic map sheets Garmin Global Positioning System (GPS) 38 personal navigator hand held compass. During the field mapping work the writer marked the location of many of the outcrops inspected on the field maps, and annotated them with a series of field notes. Stratigraphic boundaries (tops/bottoms) of the formations summarised in Chapter 3, section 3.6, have been identified in the field, where possible. Alternatively, the location of formations tops has been estimated using the location of outcrops, nearby boreholes and the average or typical formation thickness information discussed in Chapter Computer Modelling. It was initially hoped that computer modelling using the WCC GIS computer package incorporating information from the BOREHOLE database and the CMA 1:4000 scale contours would be sufficient for generating a geological map of the study area. Due to some large gaps in the coverage of the borehole data and contour maps, computer modelling of the escarpment geology, and hence computer generation of geological maps of the subject area had only a limited success. The gaps in the data coverage were the result of irregular and often widely spaced borehole locations (each borehole having a varying amount of stratigraphic detail), and the fact that at the outset of this research project (in 1993) the WCC did not have access to the 1:4000 scale contours for some of the 1:4000 scale map areas. The WCC did use alternative 1:25000 scale, and hence less accurate, contour information to cover these areas. However, during the course of this research project, the WCC has extended its data coverage of the 1:4000 scale CMA contour maps. The CMA, however, has not completed its coverage of the WCC LGA 4-10

11 with its 1:4000 scale 2m contours. Due to the considerable effort devoted to this aspect of research, it is worthy of being reported in this thesis. The procedure used is summarised in the following paragraphs. To trial this procedure, a small section of the study area was selected. The area selected covered the map sheets N6, N6, O5, O6 and O7. This area was selected because it contained a relatively high density of borehole data, and it is situated within one fault-confined geological block. To simplify this initial trial, three stratigraphic horizons were selected for mapping, (a) the base of the Stanwell Park Claystone, (b) the base of the Bulli Coal Seam, and (c) the base of the Wongawilli Coal Seam. Computer generated stratigraphic surfaces for the three formation tops were produced for the writer, by staff of the WCC GIS department, by passing a surface through the relevant formation tops in each borehole (requiring a minimum of three boreholes) within the one fault isolated block. This involves only interpolation (no extrapolation) between known real points, employing computer techniques known as kriging and then triangulated irregular network (TIN) modelling. Each of the three stratigraphic surfaces were then examined by an array of cross sections as shown in Figure 4.3. The cross sections have been aligned approximately perpendicular to the escarpment at 500m separations. On graphical workstations with high resolution computer screens, the cross sections, an example of which is shown as Figure 4.4, have been examined whereby an extrapolated intersection with the contour defined surface of the escarpment was digitised, as shown on Figure 4.3. This technique is repeated for each cross section for each selected stratigraphic surface within a given fault isolated block. The digitised points are then plotted out in plan form. The points provide a guide to stratigraphic positioning at 500m intervals along the escarpment. Each consecutive point is joined by an outcrop/subcrop line typically running parallel or sub-parallel to the contours. This technique has been completed for the five 1:4000 scale map sheets, N6, N7, O5, O6 and O7. However, the four map sheets N6, O5, O6 and O7 have not been the subject of any further geological mapping work. The computer generated subcrop lines were used as a guide to the other means of geological mapping for map N Inference In areas where field mapping was not possible due to inaccessibility, where no 4-11

12 geological exposures were observed, and when there was no supporting borehole information, the positions of geological boundaries have been inferred on the basis of either previous mapping by other workers, or inferred along slope between areas of known geology by following the form of the contours. Figure 4.3. Computer model of geology within area of clipped contours, bounded by boreholes in BOREHOLE database. Stratigraphic surface generated for the Base of the Wongawilli Seam. Surface examined by cross sections at 500m spacing, orientated approximately perpendicular to the escarpment. North is vertical, and the map is at a scale of 1:50,000. Mount Kembla is situated within the area of closed contours on the right hand side of the map. Mapping work of other authors has been discussed in Chapter 2. Previous mapping by Bowman (1972) at the scale of 1:6336, was frequently referred to during this stage of the mapping exercise. In addition to Bowmans work, the mapping work of Pitsis (1992) is also worthy of note in this regard. Pitsis presented two 1:4000 scale map sheets depicting the geology and land instability of the Stanwell Park to Clifton area. These sheets are referred to during this research project and by the WCC as map sheets E12 and F12. Although modified and extended the mapping work of Pitsis has been incorporated as part of the series of maps prepared during this research work. 4-12

13 Figure 4.4. Cross section WongC8 (fifth from the left hand side of Figure 4.3) of the computer model of the base of the Wongawilli Coal Seam. The interpolated intersection point with the face of the escarpment is digitised and plotted in plan, as shown by the blue crosses in Figure DIGITISING AND CHECK PLOTTING Upon completion of the geological mapping work, the mapped geological formation boundaries were digitised by the writer into the Wollongong City Council Geographic Information System computer package using the Genasys digi routine. The digitising procedure has been summarised briefly in a previous footnote in this chapter. The digitising involved the use of an AO sized Calcomp digitising tablet and a Hewlett Packard graphical workstation. 4-13

14 Each of the thirty one 1:4000 map sheets were digitised individually, and great care was taken to ensure geological boundaries were matched exactly across the map sheet boundaries. This was an important aspect of the digitising process, as the GIS computer package can do a great deal of analysis and other land management work with area features, provided they are closed or topologically correct, as opposed to just drawing lines on a map. Quantitatively assessing the GIS based data discussed in this and the following chapter, is another aspect of the work carried out during this research project, and is discussed in Chapters 7 and 10. The area features formed during the digitising work have each been tagged with a label representing the abbreviated geological name identified in brackets in section 3.6 in Chapter 3. The digitised formation boundary lines on the geological maps have been variably classified as either reliable - a solid black line, or inferred - a dashed black line, based on the judgement of the writer. Following the digitising of the boundaries and labelling of the area features and line types, the maps were plotted at a scale of 1:4000 for checking. Where necessary, some editing of the data was required. The digitised geology data now comprises a digital layer, or a specific data-set entitled stratigraphy in the WCC GIS computer package. This digital layer of data has been incorporated into a GIS produced multilayered map series, entitled the Geotechnical Landscape Map series, which is discussed fully in Chapter 6. Three example Geotechnical Landscape Map Series map sheets, maps E12, G11 and L8 (refer to Figure 1.3 for an index and reference) are included in Appendix 3 in the pocket inside the rear cover of this thesis. These example maps include the GEOWELLS, FAULT and MIN FAULT overlays and the geological map layer STRATIGRAPHY, discussed in this chapter. These example maps also include the map component of the landslide inventory which is discussed in the following chapter, Chapter