Lithostratigraphy of the basal seven formations (Figure 1-3) of the Nanaimo Group is described in this chapter. From the base upwards, they are the Comox, Haslam, East Wellington, Extension, Pender, Protection and Cedar District Formations. The Wellington Seam (Figure 2-1), which has the lithostratigraphic rank of bed and is the principal object of the present study, occurs near the base of the Extension Formation. The other six formations are of significance to this study in that they define the broader stratigraphic context of the Wellington Seam.
The basement paleosurface formed isolated hills with up to 220 metres of local relief during the deposition of the Comox, Haslam, East Wellington and Extension Formations (Buckham, 1947a). Buckham interpreted a prominent salient of Karmutsen volcanic rock between Wakesiah and East Wellington Collieries as an exposed headland during deposition of the Wellington Seam. An alternative explanation of this feature offered here, based on geological mapping of the exposed rocks and interpretation of boreholes and mine plans, is that the 'headland' is actually an overthrust block of basement, in tectonic rather than depositional contact with the coal measures. At least one inlier of basement rock has, however, been encountered by mine workings on the Wellington Seam, in the Lewis Heading District of Wakesiah Colliery, northeast of the Wakesiah shafts. Boreholes north and east of Wakesiah Colliery also suggest that a buried basement ridge extends north towards Northfield Colliery.
Bickford and Kenyon (1988) further refined the subdivision of the Comox Formation into three members: the basal Benson Member, and two new units, the coal-bearing Cumberland Member and the sandy Dunsmuir Member. Only the Benson and Dunsmuir Members are present in the western half of the Nanaimo Coalfield; all three members are present at depth in the vicinity of Harmac and Yellow Point in the eastern half of the Nanaimo Coalfield.
England (1989, 1990) rejected Muller and Jeletzky's (1967) extension of the name Comox to the basal conglomerate and sandstone of the Nanaimo Coalfield. On grounds of nomenclatural priority (North American Commission on Stratigraphic Nomenclature, 1983), England recommended the restoration of Clapp's (1912) Benson Formation. England also proposed the subdivision of the Benson Formation into two members: the basal conglomeratic Tzuhalem Member (which had been previously mapped by Muller and Jeletzky as the Benson Member of the Comox Formation) and the overlying sandy Saanich Member (which had been previously mapped by Bickford and Kenyon as the Dunsmuir Member of the Comox Formation). England's proposed revisions to the Comox Formation (in the sense used by Muller and Jeletzky) have not been adopted in this thesis because the usage of the term 'Comox Formation' has become well established in literature concerning Georgia Basin (Ward, 1978; Ward and Stanley, 1982; Pacht, 1984), and because the Comox Formation and its members can be readily mapped along the entire western erosional edge of Georgia Basin (Cathyl-Bickford and Hoffman, 1991).
Most of the Benson Member is conglomerate, consisting mainly of subangular to well- rounded pebbles, cobbles and boulders of volcanic rock, in an abundant matrix of coarse-grained, dark green, brown-weathering, volcanic sand, cemented by chlorite and calcite (Clapp, 1914). Most Benson conglomerates are very thick-bedded and framework-supported, although matrix- supported conglomerates and pebbly sandstones occur locally.
Lenses of thin to medium-bedded red shale are locally abundant near the base of the Benson Member, for example on the shoreline north of Horsewell Bluff, and in boreholes near Wakesiah Colliery. Thin to medium-bedded calcarenite and coquina occur in the basal 15 metres of Benson Member along the north shore of Departure Bay (Clapp, 1914; Mathews, 1947). These shell-bearing beds contain mostly broken and abraded valves of pelecypods and gastropods, with some fragments of bryozoans and echinoids.
The Dunsmuir Member consists of thick- to very thick-bedded, medium to coarse-grained, yellow-weathering, light grey, quartz-feldspar-volcanic sandstone. The Dunsmuir sandstones are well-indurated and tightly cemented by calcite.
Ward (1978) proposed the subdivision of the Haslam Formation into two members: the basal Haslam Creek Member, consisting of massive shales, and the overlying Cowichan Member, consisting of turbiditic shales, siltstones and sandstones. Inasmuch as the Cowichan Member has not been recognized in the Nanaimo Coalfield (Ward and Stanley, 1982), subdivision of the Haslam Formation has not been attempted in the present study.
The Haslam Formation in the Nanaimo Coalfield consists of thin- to medium-bedded dark grey to black mudstone and siltstone, with occasional thin to medium interbeds of fine-grained cherty sandstone. The upper 15 to 40 metres of the Haslam locally contains more frequent sandstone beds. The uppermost Haslam Formation is gradational by interbedding with the overlying East Wellington sandstone.
Outcrops of the East Wellington Formation are common in the Wellington and Extension areas, where the sandstone forms rimrock around southeast-plunging synclines. Elsewhere, the formation is often concealed beneath Pleistocene sediments and talus derived from the Extension Formation. The East Wellington Formation is 8 to 21 metres thick along its outcrop edge, and thins gradually downdip to the south and east. At the extreme northern end of the study area, the formation is thicker; boreholes near Lantzville have intersected up to 47 metres of East Wellington sandstone (Muller and Jeletzky, 1970).
The East Wellington Formation typically consists of fining-upward, fine to medium- grained, well-sorted, light grey quartz-feldspathic sandstone. The sandstone locally grades into fine pebble-conglomerate and grit (hence the old miners' term 'Millstone Grit' for this formation). The East Wellington coarsens and becomes cleaner towards its top, and is often coaly and rooty at its top contact with the Extension Formation.
The Northfield Member consists of thin- to medium-bedded brown and grey mudstone, sandy siltstone and coal. Lenses of coarse-grained clastic sedimentary rocks occasionally occur within the Northfield Member: near Departure Bay, a northeastward-thickening tongue of conglomerate, up to 25 metres thick, occurs in the middle of the member (Bickford, 1989), while at Wolf Mountain a fairly persistent bed of fine-grained, silty sandstone lies about 8 metres above the base of the member.
The Northfield Member contains several coal beds, some of which attain mineable thicknesses. The thickest and most extensive of these coals is the Wellington Seam, which usually lies at the immediate base of the Northfield Member, although a thin to medium bed of shale may occasionally intervene (Clapp, 1914; Buckham, 1947a) between the coal and the underlying East Wellington sandstones. The usual thickness of the Wellington Seam is 1.5 to 2.1 metres, although limited areas of very thick coal, in the 4 to 5 metre range, have been encountered in Extension, Wellington and Wolf Mountain Collieries.
Several thinner and less persistent coals occur higher in the Northfield Member. The No.2 Seam (locally known as the Little Wellington Seam) lies 5 to 10 metres above the top of the Wellington Seam. The No.2 Seam is thickest (60 to 85 centimetres thick) at the northwest end of Wellington Colliery, about a kilometre southwest of Long Lake. At Harewood (Figure 2-3), the No.2 Seam (known here as the Harewood Seam) is the stratigraphically-highest coal in the Northfield Member.
In Wellington and Northfield Collieries, two additional coal beds overlie the No.2 bed (Figure 2-2): the Northfield No.3 bed (40 to 65 centimetres thick) about 18 metres above the base of the Wellington Seam, and the Northfield No.4 bed (35 to 70 centimetres thick) about 8 metres further up (Buckham, 1947a).
The Millstream Member consists of very thick-bedded pebble-conglomerate, with interbeds of coarse-grained, gritty sandstone and greenish-grey siltstone, dark mudstone and dirty coal. The proportion of conglomerate in the Millstream Member increases southwards, from 60-75% at Northfield (Figure 2-2) to 80-85% at Harewood (Figure 2-3). Pebbles in the conglomerate are subrounded, consisting mainly of white quartz and dark grey chert with minor red chert, in a matrix of chlorite-rich quartz-feldspar sand.
Most of the Millstream conglomerates are framework-supported. Sandstone lenses are of similar composition to the matrix of the conglomerates, and contain floating pebbles and granules of quartz and chert similar to those found in the conglomerates. The conglomerates and sandstones are well-indurated and tightly cemented by quartz and calcite.
Siltstones of the Millstream Member are thin- to medium-bedded and locally contain rootlets. Mudstones of the Millstream Member are thin-bedded, locally silty or carbonaceous, and are often rooted.
Coal beds within the Millstream Member are usually thinner than those of the Northfield Member. Between Northfield and Departure Bay, one to three discontinuous coal beds (15 to 30 centimetres thick) occur within a shale unit which lies 15 to 25 metres above the base of the Millstream Member. At Extension, a discontinuous bed of up to 3 metres of interlaminated shale and coal (the Jacks coal bed of Clapp, 1914) lies 50 to 80 metres above the base of the member.
At Wolf Mountain, three coal beds (the No.3, No.4 and No.5 beds) lie 33, 60 and 80 metres respectively above the base of the member. The Wolf Mountain No.3 coal bed locally attains a thickness of 1.5 metres, inclusive of several thin to medium beds of carbonaceous to coaly mudstone; this coal bed may be correlative with the Jacks coal bed of the Extension area. The Wolf Mountain No.4 and No.5 coal beds each consist of less than 40 centimetres of coal without rock bands.
The Cranberry Member consists of distinctively dark greenish-grey silty mudstone, sandy siltstone and coarse-grained gritty sandstone, with occasional thin to medium beds of coal. The top part of the member is fairly well exposed, but the basal part is usually recessive, and may consist chiefly of dark grey and green shale, as reported in driller's logs of diamond-drill holes through the basal Cranberry. One fairly laterally continuous coal bed, the Cranberry bed, occurs 10 to 15 metres below the top of the Cranberry Member (Cathyl-Bickford and others, 1992). The Cranberry coal bed is 20 to 60 centimetres thick, and contains a few very thin bands of siltstone and coaly mudstone.
The upper 30 to 40 metres of the Newcastle Member consists of shale, siltstone and coal. 15 to 25 metres below the top of the Newcastle Member is the laterally-discontinuous but locally very thick (2.1 metres) Douglas Rider Seam. The Douglas Rider Seam consists of thinly- interbedded coal, black coaly mudstone and siltstone. The base of the fine-grained interval, 30 to 40 metres below the top of the Newcastle Member, is marked by thick (60 centimetres to 4.5 metres) coal of the Douglas Seam. The basal half of the Newcastle Member consists mainly of thick- to very thick-bedded, shell-bearing conglomerate and gritty sandstone, with coal of the Newcastle Seam (0.8 to 1.2 metres thick) at its base. The base of the Newcastle Seam marks the contact of the Newcastle Member with the underlying Cranberry Member. The Douglas Seam and the Newcastle Seam coalesce southeast of Nanaimo Harbour, forming the Douglas Main Seam, which is 0.3 to 21 metres thick (Cathyl-Bickford and others, 1992). The extreme variability of the thickness of the Douglas Main Seam is largely due to local folding of its floor (Graham, 1924).
Ridge-forming fine to medium-grained, light grey to white, clean, thick- to very thick- bedded sandstone constitutes most of the Cassidy Member. The sandstone consists of subequal amounts of quartz and feldspar, with minor hornblende. Medium to thick lenticular beds of fine- grained, quartzose pebble-conglomerate and cross-bedded grit occasionally occur near the base of the member.
The Reserve Member consists of thin- to medium-bedded, green to brownish-grey sandy siltstone and fine to medium-grained, medium- to thick-bedded, greenish-grey sandstone, containing abundant lenses and medium (0.10 to 0.30 m) interbeds of shaly coal.
The McMillan Member consists mainly of ridge-forming, coarse-grained, thick-bedded, light grey to white sandstone with occasional thin recessive interbeds of dark grey to greenish-grey sandy siltstone.
Dark grey to black shale and siltstone forms the bulk of the Cedar District Formation. Thin (5 to 10 cm) sandstone bands, together with sandstone dykes, are locally abundant in the middle of the formation. Near the base of the Cedar District Formation is a ridge-forming interval of thick-bedded, light grey sandstone similar to the sandstone of the upper Protection Formation. This sandstone is well exposed in a series of low hills south and west of Nanaimo Airport, in the extreme southwestern corner of the study area.
In considering the detailed stratigraphy of the Wellington Seam, it should be born in mind that not all its constituent coal beds or partings may be present in any given locality within the coalfield.
In the vicinity of Wellington Colliery, East Wellington Colliery and the northern end of Extension Colliery, the three coal beds coalesce and the Wellington Seam consists of a single very thick coal bed, 2.1 to 4.3 metres thick. Numerous unprofitable mining ventures have demonstrated that workable areas of Wellington Seam are bounded by split areas in which the Wellington Seam has split into two or three of its constituent coal beds, separated by thick clastic sedimentary rocks of the upper and lower partings.
In some areas, one or two of the individual coal beds within the Wellington Seam has been mined singularly, the remainder of the seam being either absent or left unworked. For example, the very thick (1.8 m) coal worked at Timberlands Colliery appears to be the Wellington Main coal; its immediate siltstone roof is the upper parting. Isolated remnants of the Wellington Rider coal bed are present at Timberlands; most of the Wellington Rider coal has been removed by erosion prior to deposition of the overlying Millstream conglomerates. No.4 Mine of Extension Colliery may also have worked the Wellington Main coal, although here there is no sign of a rider coal above the bed which was worked. In White Rapids Colliery, the Lower Wellington coal was worked by itself, although it was of marginally mineable thickness (0.8 to 1.2 metres), and the proximity of the overlying coals resulted in very poor roof conditions.
Wellington coal is duller and has a higher ash content near basement highs, and locally grades into a black, carbonaceous or coaly mudstone. In some areas, for example at East Wellington and Wakesiah Collieries, the thickest and cleanest coal occurs a short distance from basement highs, which may have sheltered the coal-forming swamps from the influx of sediments (Buckham, 1947a). Perhaps here the coal-forming environment, being situated adjacent to emergent bedrock knobs, was topographically higher than the valley bottoms where sedimentation occurred.
The low sulphur content of coal from the Wellington Seam suggests that its precursor peat originated in fresh-water mires removed from the active shoreline, and was not subsequently in contact with sulphate-rich marine or brackish water (Cohen, 1974, 1984; Kalkreuth and Leckie, 1989).
A very thin (1 to 2 cm) persistent band of black arenaceous mudstone, termed 'pelletstone' by local miners (J. Perry, personal communication, 1989) occurs in the middle of the Wellington Rider coal. Where fresh, it has a uniform black colour and is difficult to distinguish from coal without close examination. Where weathered, it stands out as a light brown to brownish-grey band within the coal. It usually contains abundant wisps of bright coal, and is occasionally rippled. In thin section, this band consists of subangular grains of quartz, with rims of light brown carbonate (siderite or ferroan dolomite?) set in a matrix of dark brown to black carbonaceous mud. The origin of this band is uncertain: it may be an altered tuff, an eolian deposit, or a fire splay.
Partings 20 to 50 centimetres thick tend to be slightly coarser-grained than thinner dirt bands, ranging from silty mudstone to siltstone. Their colours are lighter, ranging from light brown to brownish-grey; they are usually rooted and massive, with no obvious vertical grain size gradations. Normally-graded partings are less abundant than massive partings; the graded partings tend to be greater than 40 centimetres thick, and typically consist of siltstone grading up to silty mudstone which in turns grades up to rooty carbonaceous mudstone. These fining-upward beds probably represent distal crevasse-splay deposits, while more uniform beds may represent levee deposits.
Partings 50 to 120 centimetres thick tend to consist either of massive, rooted mudstone and siltstone, or of silty sandstone. The sandstones often are crossbedded or rippled, and may contain thin crosslaminae of mudstone or muddy siltstone. The sandstones tend to be less silty at their bases, and locally scour down into the underlying coal. Such erosive-based sandstones commonly contain angular, twisted blocks of coal near their bases. Sandstones which contain mudstone laminae may be point bar deposits, while the erosive-based sandstones are probably proximal crevasse splays.
At Wellington, Northfield, Wolf Mountain and immediately to the north of Harewood Colliery (Figures 2-2, 2-3 and 2-5) the roof of the Wellington Seam consists of 5 to 11.5 metres of thin- to medium-bedded, brownish-grey or greenish-grey sandy siltstone and very fine- to fine- grained silty sandstone, overlain by up to 60 centimetres of light grey, rooted mudstone, which in turn is abruptly overlain by coal of the No.2 Seam.
At East Wellington and Wakesiah, the roof of the Wellington Seam consists of 5 to 9 metres of massive dark grey siltstone or soft silty mudstone with coaly streaks, overlain by coal of the No.2 Seam.
At Wellington, East Wellington, Northfield, Wakesiah (Figures 2-1 and 2-2) and the northern end of Harewood Colliery (Figures 2-1 and 2-3), the No.2 Seam is in turn overlain by dark grey and brown mudstone and siltstone with occasional thin coal beds (including the No.3 and 4 Seams) and lenses of sandstone and conglomerate. To the south and east of Harewood Colliery (Figure 2-3), at Wolf Mountain, and at the southeastern end of Extension Colliery, the fine-grained beds above the No.2 Seam are truncated by massive or trough cross-bedded conglomerates of the Millstream Member.
The dominantly fine-grained strata of the Northfield Member, which immediately overlies the Wellington Seam, probably originated in a delta plain environment. Fining-upward sandy siltstones and silty mudstones represent overbank flood or crevasse splay deposits, while carbonaceous mudstones and coals represent swamp deposits. Isolated lenses of sandstone and conglomerate in the Northfield Member were probably deposited as fluvial bars or channel fills. Although the density of borehole and shaft sections through the Northfield Member is insufficient to conclusively prove that the sandstone and conglomerate bodies have the ribbon morphology typical of anastomosing stream deposits (Miall, 1992), the general scarcity of coarse clastic sediments in the Northfield Member suggests that widespread meander belts are absent in the Northfield Member. It is therefore more likely that the Northfield streams were anastomosing rather than meandering.
The conglomerates and associated sandstones of the Millstream Member, which overlies and locally truncates the fine-grained strata of the Northfield Member, represent deposits of gravel bed rivers (Miall, 1992), probably part of a coastal braid-plain delta. Laterally persistent, occasionally coal-bearing, fine-grained units within the Millstream Member probably represent floodplain and backswamp deposits which formed in areas between active river channels.
The basal 3 to 6 metres of the East Wellington Formation typically consists of very fine- grained, light grey, brown-weathering, medium- to very thick-bedded, concretionary sandstone with occasional thin interbeds of dark grey rubbly-weathering sandy siltstone similar to siltstone of the underlying Haslam Formation.
Gradationally overlying the concretionary sandstone is a unit, 1 to 5 metres thick, of fine- grained, light grey, light brown-weathering, clean, medium to thick-bedded sandstone. This sandstone unit locally contains medium to thick trough cross-beds with abundant coalified wood fragments and imprints of logs. Lenses of coarse-grained pebbly sandstone, gritstone and pebble- conglomerate (Clapp, 1914) also locally occur within this unit.
Gradationally overlying the medium to thick-bedded sandstone is a unit, 4 to 10 metres thick, of medium-grained, light grey, clean, low-angle cross-stratified, thick- to very thick-bedded sandstone. Trace fossils resembling Macaronichnus segregatis (Clifton and Thompson, 1978) are locally abundant near the middle of this unit.
The massive sandstone unit in turn grades up to 5 to 10 centimetres of dark grey to black, rooted, fine- to medium-grained carbonaceous sandstone, which generally forms the immediate floor of the Wellington Seam.
The contact of the East Wellington sandstone with the overlying coal is almost always abrupt, and is usually marked by small-scale undulations ranging in amplitude from 15 to 120 centimetres, known to coal miners as 'floor rolls' and 'swilleys'. Their characteristics and possible origins are discussed at length in Chapter III of this study.
In the northwestern corner of the Wellington area, near Little Ash Mine and Gillfillian Colliery (Entries 3 and 7a respectively on Map 1), the uppermost East Wellington sandstone grades upwards and southeastwards into dirty, sandy coal, up to 30 centimetres thick, which in turn grades up to clean coal of the Wellington Seam. Elsewhere in the Wellington area, a thin to medium bed of light to medium grey, soft mudstone lies between the top of the East Wellington sandstone and the base of the Wellington Seam. This mudstone (known to local miners as 'fireclay') lacks bedding or lamination, but is locally rooted.
The vertical sedimentological profile of the East Wellington Formation (Figure 2-4) resembles the Type A vertical sequence of sediments deposited along a wave-dominated, prograding coastline (Kalkreuth and Leckie, 1989). They interpreted their Type A section (coarsening-upward sandstone) as being representative of shorelines supplied by sand-dominated distributaries, and their Type B section (sandstone overlain by sharp-based conglomerate) as being representative of shorelines supplied by gravel-dominated distributaries.
The East Wellington Formation was therefore probably deposited on the shoreface of a prograding wave-dominated strandplain, fed by sand-dominated distributaries. The local southeastward intertonguing of the uppermost East Wellington sandstone with the basal Wellington coal, together with the general southeastward thinning of the East Wellington Formation, suggest that the East Wellington shoreface sands prograded to the northwest.
To the south, at Wellington and Northfield, folds plunge to the southeast and east and have slightly steeper limb dips, up to 12 degrees. Two major faults (the Harbour and Jingle Pot Faults) parallel the general northwestward strike of the coal measures. Each fault is bounded by a narrow zone of drag folding. Neither fault is exposed at surface, but their position and vertical displacement have been proven by mining.
The Harbour Fault forms the northeastern boundary of the No.1 Slope workings of Wellington Colliery, and cuts through the centre of Northfield Colliery. In terms of the Norris (1958) classification of faults, the Harbour Fault is an extension fault in the Wellington-Northfield area, where it has a steep northeastward dip and vertical displacement of 15 to 20 metres down to the northeast. To the southeast, along the southwestern shore of Nanaimo Harbour, the Harbour Fault is associated with a belt of easterly-verging asymmetric folds which was encountered in the workings of Nanaimo Colliery (Buckham, 1947a). The Harbour Fault eventually ceases to be recognizable as a discrete fault near the mouth of Chase River.
The Jingle Pot Fault forms the southeastern boundary of Wellington and West Wellington Collieries, and cuts across the southwestern sides of East Wellington and Wakesiah Collieries. The Jingle Pot Fault is a thrust fault; it dips at 30 to 50 degrees southwestward, and has a vertical displacement of 30 to 100 metres. Considerable folding is developed adjacent to the Jingle Pot Fault where it crosses East Wellington Colliery (Buckham, 1947a). At this locality, the roof of the Wellington Seam is folded into several recumbant, north-northeast-verging asymmetric anticlinal folds (termed "rolls" by Clapp, 1913), while the floor is essentially undeformed. As a result of the structural disharmony of its roof and floor, the Wellington Seam pinches and swells across the rolls. The top part of the coal bed, and the basal roof, are sheared and slickensided, while the floor is essentially undeformed. Sheared, dirty coal (termed "rash" by Clapp) occurs in thin, pinched portions of the Wellington Seam, while clean, blocky coal occurs in the thick, swelled areas.
Northeast-trending extension faults cross the western workings of Wellington Colliery. One of these faults, which passes just north of the confluence of Tunnel Creek and Millstone River, cuts both the Jingle Pot and Harbour Faults, and has an apparent sinistral offset of 200 to 400 metres.
The crest of the Extension Anticline is poorly exposed. Isolated outcrops of Millstream conglomerate north and southeast of Extension village show very steep northeastward dips, suggesting the core of the anticline may be structurally complex.
The northeastern limb of the Extension Anticline dips to the northeast at 10 to 15 degrees. Just northeast of the anticlinal crest, the northeast-dipping Lakes Fault cuts down to the northeast through the coal measures, and forms the northeastern boundary of No.4 Mine, Extension Colliery. The surface trace of the Lakes Fault is marked by a prominent southwestward-facing cuesta scarp, the base of which is followed by Stark Creek. The Lakes Fault cannot be traced north of Extension Prospect Mine, nor south of Nanaimo River. At the north end of its trace, the Lakes Fault appears to be underlain by a footwall lateral ramp in the Millstream Formation. Along Nanaimo River, the Lakes Fault may pass into a zone of bedding-plane detachment and folding.
On the southwest side of the Extension Anticline, extensive workings of Extension Colliery have clearly defined the structure of the coal measures. Three southwest-verging asymmetric, southeast-plunging synclines are present in this area. The synclines have broad northeast limbs which dip 10 to 15 degrees southwest, and narrow, steep southwest limbs which dip 35 to 65 degrees northeast. The synclines are bounded by four southwest-dipping, northeasterly-directed contraction faults: the Timberlands, White Rapids, Extension Main and Village Faults. Subparallel to each fault is a belt, up to 50 metres wide, of bedding-plane shear and crumpling in the weak rocks of the Northfield Member.
The Extension fold-thrust belt continues southeastward beyond Nanaimo River to Haslam Creek. The fold-thrust belt cannot be traced far south of Haslam Creek, and appears to be terminated by the Cassidy Fault. The Cassidy Fault is best exposed in the workings of Granby Colliery, where it consists of a 150 to 250 metre wide shear zone with a cumulative southeastward downthrow of 200 to 240 metres to the southeast.