A GRAVITY SURVEY OF THE IGNEOUS BODY AT LITTLE MOUNTAIN, , B.C.t

D. A. SKETCHLEY and R. M. CLOWES

ABSTRACT I Little Mountain is a” igneous body which iterative modelling procedure was carried cmt forms the height of land within the City of Van- using computer programs based initially on two- couver. The outcrops there are part of the series dimensional and subsequently on three- of igneous rocks known as the Prospect Point dimensional algorithms. The final model ob- eruptives, which intrude the Lower Tertiary sed- tained suggests the igneous body may be likened iments in Vancouver and vicinity. Most of the to a broad, asymmetrical cone, roughly 500 m in sediments in the area are covered by a thick layer diameter, attached to the underside and near the of Pleistocene glacial deposits and interglacial edge of a polygonal slab at or near the surface and sand and clay. To better understand the origin of approximately 60 m thick. Consideration of this the igneous body which forms Little Mountain, a gross shape together with geological data gravity survey has been carried out over the im- suggests the igneous body probably results from mediateareasurroundingit. Standard reductions an upwelling of magma to produce a lava flow of have been applied to the data to obtain a terrain- small volume. corrected residual Bouguer anomaly map. An

INTRODUCTION GEOLOC~CAL SE’ITING One of the principal physiographie features of Figure I shows a geological sketch map of this Vancouver, , is the topog- general region and a north-south geological cross raphic high at Little Mountain. The high, section. The distribution of rock material is the around which Queen Elizabeth Park has been result of three distinct phases in the geological developed, is due to an igneous body. This evolution of the west coast. During early to paper reports the results of a detailed gravity mid-Cretaceous time, magma intruded volcanic survey which has been carried cmt over the islands and marine basins to form the coarsely body. crystalline rocks of the Coast Mountains: in late Cretaceous time, this Coast Crystalline Complex Little Mountain is located in the Burrard was uplifted above sealevel. From then to Lower Peninsula which forms part of the highland areas Tertiary time, the rocks were subjected to ero- of the general Fraser Valley lowland, “ear the sion, with the eroded material eventually cement- delta. The peninsula is bounded on ing into conglomerate, sandstone and mudstone. the south by the north arm of the Fraser River, on Small seams of lignite were formed from thin the north by and extends from Point layers of plant debris. Volcanic activity occurred Grey to , east of Bumaby (Figure 1). It sporadically throughout the Tertiary and con- includes the cities of Vancouver, New Westmins- tinued at least until a few thousand years ago ter and Bumaby. when there were eruptions on Mount Garibaldi

tManuscript received by the Editor July 12, 1976; revised manuscript received August 25, 1976. *Department of Geophysics and Astronomy, University of British Columbia, Vancouver, B.C. V6T lW5. 64 A GRAVITY SURVEY OF THE IGNEOUS BODY 65

3 1000 Norih Arm y 0 ~-1000 m N-S Geological Cross Section >

:s<* B- tRP r 0 UILN .A...:::. LL-.:::.:

FIG. 1. Geological sketch map and location mapof the greater Vancouver region. Shading. coded by number, shows the distribution of rock twes and unconsolidated sediments: (1) recent aravel. sand. clw wat: (2) alacial till, stoneyclay, interglacial sari& and clays; (3) basalt-andesite (dik&; plugs, fiows):‘(4) uppers-taceblis-lower Ter@y mudstone, sandstone. conglomerate: (5) granite, granodiorite. quwtz diorite, diorite: 6 volcanic and Sedlmentaw rocks (roof-oendantsl. 1% and (61 are oat of the Coast Cwstalllne Comolex. To the 12et of the mao~ a norlh-south’geolog& c&s seclibn‘(i& i& n ide& the map) is shown on an apb&%i~iyi :I~&& (Map and cross Section after Eisbacher. 1973). 66 D. A. SKETCHLEY and R. M. CLCIWES

(about 80 km north). The third phase is the effects Well-glaciated outcrop at Little Mountain sub- overthelast millionyearsofvolcanicactivity, ice stantiates this view. sheets, and water: these formed the glacial till, The Kitsilano formation is overlain by the gravel, sand, clay and peat. Boundary Bay formation, which is presumed to As shown by Figure 1, the Burrard Peninsula is be of the same age as the Prospect Point erup- formed primarily of glacial drift with some Ter- tive. Evidence is provided by the volcanic ash tiary bedrock and recent deltaic material. The which occurs with the relatively unconsolidated bedrock forms the northern part of two easterly sandstones and shales. trending ridges which are separated by the west- The Upper Tertiary sediments are separated draining and the east-draining Still from Quaternary deposits by a major unconfor- Creek, Lake and sys- mity. They consist primarily of Pleistocene gla- tem. The northern ridge extends from Stanley cial deposits and recent deltaic and alluvial de- Park, where bedrock outcrops are well exposed, posits with some interglacial sediments. The lack to , the height of land on the of significant outcrop and non-exposure of con- peninsula. The southern ridge extends from Point tacts with sedimentary deposits at Little Moun- Grey to the Fraser River, with good exposure of tain is due to this layer of overburden. Only on Tertiary bedrock along . Little the northern side, where two quarries have been Mountain is the height of land along this southern excavated, are exposures good (Figure 2). ridge. The rock which outcrops at Little Mountain is The outcrop at Little Mountain is part of the a medium to fine grained, vesicular, augite- series of igneous rocks known as the Prospect olivine basalt. The vesicules are generally few Point eruptives (Johnston, 1923) which intrude and small, although larger ones with a tilling of the Lower l’ertitiry sediments in Vancouver and vicinity. These sediments are divided into the natrolite have been noted (Wooton, 1959). Com- position is about 50% plagioclase, with abundant Burrard, Kit&no and Boundary Bay forma- small grains of augite. Secondary minerals in- tions. cludeepidote, magnetite and purite. Although no The Burrard formation unconformably over- contacts have been exposed, they are reflected lies the Coast Crystalline Complex. No contacts by sandstone xenoliths and inclusions of sand are known in the map area. It consists of a basal with igneous material in the matrix (Wooton, conglomerate and an overlying series of 1959). sandstones and shales with a vertical thickness of A prominent feature of the larger quarry is its at least 600 m. A disconformity separates the well-developedcolumnarstructure. Thecolumns Burrard and the overlying Kitsilano formation. are nearly vertical, up to several feet in diameter The slight erosion surface is reflected in the com- and several tens of feet high. The convergent position of the Kitsilano formation, which varies arrangement of the columns is possibly due to a from a basal conglomerate to sandstones and rapid cooling or radiation of heat from a focal shales, with a total vertical thickness of at least point (Johnston, 1923). The rock exposed in the 300 In. smaller quarry is predominantly massive. Al- A number of intrusive contacts with the Pros- though nearly vertical, the columns do show a pect point eruptives are known to occur in the minor inclination to the south. Johnston (1923) Burrard and Kit&no formations. These igneous has attributed this to a tilting of the body with rocks consist of a series of basic dykes. sills or enclosing sediments to form a gentle monoclinal flows and volcanic tuffs. The largest dyke occurs structure in the Vancouver area. at Prospect Point in . A number of smaller dykes and volcanic tuffs intruded by A small outcrop also occurs on Cambie Street dykes occur at other locations in the Vancouver about 500 m south of the nearest quarry. Survey area (Wooton, 1959). The igneous body at Little evidence presented in this report indicates it is Mountain is considered to be a sill or flow by not a dyke offshoot of the main body as Johnston (1923). while Armstrong (1954) has considered by Wooton (19591,but is more likely suggested it is a laccolithic body. According to formed by the southwest edge of the main Wooton (1959), the igneous activity took place body. over considerable time, possibly extending from SURVEYMEASUREMENTSANDTHEIRREDUCTION the Mid-Eocene into the Miocene with major ac- The gravity survey was carried out with Wor- tivity being post-Eocene but pre-Pleistocene. den gravimeters. Most of the observations were A GRAVITY SURVEY OF THE IGNEOUS BODY

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0 100 200 300

Meters Outcrop Quarry Sample Location

Fig. 2. Location map of the area within Vancouver in which the gravity survey was carried out. Queen Elizabeth park is outlined by the heavy line, Solid dots show the locations at which gravity readings were taken. Extent of the known o”tcrops at Little Mountain, position of the two quarries, and sample Iocations for the igneous body are indicated. Samples of the other rock materials were taken at locations where e~powies were good. 68 D.A. SKETCHLEY and R. M. CLOWES recorded on a Prospector Model with dial con- The maximum and minimum density contrasts stant of 0.0908 mgals per scale division although of the unconsolidated material and the basalt some preliminary measurements utilized an Edu- were averaged to obtain a figure of 0.7 gmicc, the cational Model with a dial constant of 0.4201 density contrast used in modelling. This im- mgals per scale division. plicitly assumes that the unconsolidated material A base station was established and gravity has a depth extent of greater than 100 m. Evi- measurements were made on loops from this lo- dence supporting this may be found in thegeolog- cation, with repetition every two hours to enable ical studies of the Vancouver area discussed pre- calculation ofdrift corrections. Station sites were viously. distributed in favorable and convenient locations To enable a direct comparison between the to enable a uniform density of coverage (Figure survey data and the Bouguer anomaly map of 2). Inside the park area, the stations are approxi- southwestern B.C. (Walcott, 1967). the survey mately 63 m apart and were located by chain and base point was tied to National Gravity Network compass measurements, then spotted on a Van- base station no. 9279.60, located at the Univer- couver Parks Board map of Queen Elizabeth sity of British Columbia. After application of cor- Park. Elevations of stations obtained from this rections for instrument drift, the complete map are accurate to 0.3 m while locations are Bouger anomaly was calculated on the basis of accurate to within 3 m. Outside the park, station the Geodetic Reference System (19671 and a spacings average about 125m. The stations were Bouguer density of 2.67 gmicc. The program located on a map of the City of Vancouver which described by Ager (1972) was used to make the provides elevations accurate to 0.8 m and loca- calculations and included terrain corrections for tions accurate to within 30 m. Figure 2 shows the distances to 600 m around each station. specific area within which the survey was made. The Bouger anomaly map showed a regional Rock samples representing the igneous body trend which was removed by visual smoothing. (Figure 2), the underlying Burrard and Kit&no A map of this trend was drawn by extrapolation formations (from exposures in the Vancouver of the contour curves, from areas where the area) and samples of the unconsolidated till and effects of the anomalous body were minimal, sand surrounding the body were collected for through the region where its effects were density determinations. Density values were ob- obvious. This map of the regional field was tained for the basalt and sandstone samples with subtracted manually from the Bouguer anomaly a Jolly balance and for the till and sand by water map to yield the residual Bouguer gravity displacement. These resultswereaveraged toob- anomaly map shown in Figure 3. tain an overall representative density value for The major source of error in the survey is that each of the four major rock types. This minimizes involved with elevations. For all stations inside the variations due to weathering of the samples the park boundaries, elevations are known to a and the finely vesicular nature of the basalt. The precision of 0.3 m, assuming the maps are near results are summarised in Table I. perfect. This yields a combined free air and TABLE 1. Representative density values for materials in thr region of Little Mountain. Density contrast No. of Density with basalt Material samples Wed (gdcc) Basalt (ProspectPoint Eruptives at Little Mountain) 25 2.75 Sandstone(Burrard and Kitsilana formations) 15 2.55 0.20 Unconsolidatedtill to I.92 0.83 Unconsolidated sand 5 2.17 0.58 Average density contrast of unconsolidatedmaterial and basalt is 0.71 pm/cc. A GRAVITY SURVEY OF THE IGNEOUS BODY 69

Bouguer correction accuracy of ?0.06 mgals. INTERPRETATION Outside the park boundaries, elevations are known to within 0.8 m so that the combined free air and Bouguer corrections are accurate to Interpretation of the data ntilized computer within ?O. I5 mgals. The reading accuracy of the based iterative model studies. An initial model Prospector gravimeter is within ?O.Ol mgals and determinedfromanomaly half-widths,geological that of the Educational model within kO.04 reasoning and an assumed density contrast was mgals. established. This model was refined using the two-dimensional computational procedures of The most difficult error to evaluate is that due Talwani, Worzel and Landisman (1959). Final to the terrain corrections. A major portion of it results were obtained by three-dimensional mod- may be contributed by the uncertainty in the de- elling procedures utilizing the computational nsity estimates. However, because Little Moun- technique described by Talwani and Ewing tain is located in an area of very low local relief, (1960). excepting the immediate park area where relief is less than 31 m, the correction factor is generally less than0.2 mgals. In the fourth correction zone To facilitate calculation of the gravitational at- (300 m) the correction factor is less than0.03 traction of a two-dimensional body of arbitrary mgals. From this it is concluded that the terrain shape, the body cross section is approximated by correction factor is reasonably accurate and an appropriate polygon. The periphery of this probably contributes less than i-O.05 mgals in polygon is defined by specifying the co-ordinates error to the survey. Based on the foregoing dis- of adjacent points at vertices of the body. CaIcu- cussion the residual Bouguer anomaly map is l&ion of the vertical component of gravitational considered to be accurate to within to.25 mgals. attraction at any external point may be made by

Fig. 3. Residual Bouguer gravity anomaly map of the Little Mountain region. %a!, circles are gravity stations. Contour intervals is 0.2 mgal. AN and BE%are lines of cross sections as shown in Figures 5 and 6. 70 D. A. SKETCHLEY and R. M. CLOWES using the following expression (TaIwani, Worzel Analysis of the anomaly half-widths indicated the and Landisman. 1959): width of the feeder pipe to be of the order of 100 m. Thedensitycontrast was taken to be0.7gmicc ng=zp YR. based on the sample density measurements. j=, ’ A series of two-dimensional models were gen- erated and the associated anomalies were com- G ,= universal constant of gravitation pared with profiles taken from the residual P = YOlUmedensity Of the body Bouguer map. The results implied that the lower order portions of the anomaly may be attributed ” = number Of sides Of pOlygOnat cross Sectio” to a thin, shallow slab approximately 500 m in width. The higher order portion or peak most Ri = an expression involving the co-ordinates of each Of the polygon YertiCeS relative to likely results from a feeder pipe offset to one side the gravity station of the slab. Using the two-dimensional models as the basis To make numerical calculations on a computer, for defining an initial three-dimensional model, the position of points at which the magnitude of additional calculations using the three- the gravitational attraction is desired, the coordi- dimensional algorithm were carried out. The nates of the vertices of the body, and the thickness of the layers specified for the models density contrast need to be specified. ranged from 5 to 40 m, depending upon their For calculations of the gravitational attraction depth. For the numerical integration there was a of a three-dimensional model, the body volume is linearinterpolation toinfinitesimally thinlaminae divided into thin horizontal polygonal laminae, within the specified layers. each having a specific depth extent. The gravita- Each computer-calculated anomaly that re- tional anomaly caused by each individual lamina sulted from a specific model was contoured and may be computed for any external point. By a compared with the residual Bouguer anomaly system of linearinterpolation betweenlayers de- fine-d by the laminae, combined with numerical map. Systematic perturbations were made to integration, the gravity anomaly caused by a each subsequent model, resulting in the consid- three-dimensional body can be calculated ac- eration of more than twenty different models. cording to the following expression (Talwani and While it is realired that the procedure is non- Ewing, 1960): unique and depends to some extent on the choice of a starting model, it is felt that a reasonable and %p simple model consistent with the observations K = I Y dr; where Y = hi, 4, was achieved by this approach. ‘bottom A plan view ofthe “best tit” three-dimensional 1 = Vertical co-ordinateais model obtained in this manner is shown in Figure Y = vertical cmlp’ment0‘ the ya”it?tional dttrdction 4. It suggests that the igneous body may be y;;;:;it thick”eSsOf B thin holll0”t.l po,ygma, likened to a broad, asymetrical cone attached to the underside and near the edge of a thin poly- G = ““i”WSd, CO”sta”tOf glalitdtio” gonal slab. The density contrast of this model is 0 = “alme density Of the 1amina 0.7 gmicc. n = “YmbePOf IidS 0‘ the polygonal,amina The gravity anomaly computed from this model is shown in Figure 5. This map agrees Qi = an eyrerrion containingthe ~o.o’d’“dfer Of the reasonably with the overall character of the re- Yeltlce5 Of the polygonal1am,na sidual Bouguer anomaly map (Figure 3). The The coordinates of the laminae which detine the anomaly peak, which is catered over the cone, is body, the elevation of the top surface of each flankedon the southwest side byasteepgradient, lamina, the density contrast and the coordinates and on the northeast side by agentle gradient that of the points at which the anomaly is to be deter- results from an offset of the cone from the centre mined form the input data necessary for compu- of the slab. Slight differences arise from low- ter calculations. order flank anomalies and perturbations due to Consideration of geological data suggested the other irregularities which cannot be considered initial modelcouldconsistofatube-like structure easily in a model. forming a feeder system. overlain by a more mas- To obtain a better understanding of the com- sive structure formed by the upwelling of magma. parison between the actual and calculated A GRAVITY SURVEY OF THE IGNEOUS BODY 71

anomalies, two cross sections were constructed ing features on the body’s upper surfxe and “ar- (Figures 6 and 7). The sections emphasize the ying density within the body. It should be noted, offset nature of the anomaly peak and corres- however, that the procedure involved in direct pondingly that of the thin polygonal slab and modelling is essentially an infinite process. It has cone. The agreement between the residual been arbitrarily terminated at a point where Bouguer anomaly and the calculated anomaly is further perturbations of the model to obtain a in most cases within the error bounds of the sur- better match with the residual anomaly are not vey. A better fit probably could be obtained by significant enough to affect the interpretation. further refinements ofthe models, such as includ-

Fig. 4. Plan view of the “best fir’ three-dimensional model. Contours are in metres with mean sea level equal to 0 rn Area within zero contour is stippled Note that due to shape of body, the -25 m and ~50 m Contours and the +128 m contour are superposed on the Stippling. Density contrast = 0.7 Qmlcc~ AA’and BB‘ as in Figure 3, t

D. A. SKETCHLEY and R. hf. CLOWES

‘the model shown in Figure 4. Contour interval is 0.2 mgal, AK and BB A GRAVITY SURVEY OF THE IGNEOUS BODY

Fig. 6. Lower: Section AA across the model of Figure 4. Upper : Comparison of calculated gravity anomaly with the residual Bouguer anOmalY.

Fig. 7. Lower: Section EB’ across the model of Figure 4, Upper: Comparison of calculated gravity anomaly with the residual Bouguer anomaly. 74 D. A. SKETCHLEY and R. M. CLOWES

CONCLTJSION The final model interpreted from gravity data amounts of water or other sources of gases in or takenaround the igneous body at Little Mountain near the flow. Columnar jointing is commonly suggests the body may be likened to a broad, found in flows, although more usually in vertical asymmetrical coneattached to the undersideand positions. A convergent structure may be attri- near the edge of a thin polygonal slab. The cone buted to a rapid cooling or radiation of heat from a axis underlies the southwestern edge of Queen focal point, but this does not necessarily imply a Elizabeth Park. The slab, of average thickness 60 sill or a similar subsurface origin. It is reasonable m, located at or near the smface, covers the to assume that rapid cooling takes place in a flow, majority of the northwestern half of the park and and, due to varying thicknesses of lava, there extends across Cambie St. on its southwestern may be uneven cooling or radiation of heat from a edge. The lenticular shape of the upper surface focus. suggests either a flow or a sill with the cone part forming a feeder system. The shape of the final three-dimensional model suggests the igneous body at Little Mountain has Because several periods of extensive erosion been formed by the upwelling of a small volume and glaciation and the absence of visible contacts ofbasic magma, most likely through a weak point have reduced the amount of geological informa- in the crustal surface. The main flow appears to tion available, there is little evidence to show have been to the northeast, with a minor amount whether the body is a sill or a flow. Johnston to the northwest and southeast. The lack of sig- (1923) states that “the smooth, rounded upper nificant change in the local structure of the host surface of the rock - which, however, may be rock also suggests an extrusive origin. Further- due toglaciation-its finely vesicular character, more, traces of volcanic ash found in the Bound- and the convergent columnar structure possibly ary Bay Formation, which is considered to be of suggest that it is a sill rather than a flow.” Since the same geological age, also suggest a flow two periods of extensive erosion (in&dine elaci- rather than a sill. ation) have taken place in the time since &-rock was formed, it is reasonable to assume the ACKNOWLEDGEMENTS smooth upper surface of the outcrop is a result of The authors wish to thank Cominco Limited these processes. The vesicular nature of igneous for the loan of their Worden gravimeter. Compu- rocks is due to expanding, escaping gases and terprograms provided by Dr. C. A. Agerassisted volatiles. Although a finely vesicular nature may in the reduction of the gravity data. Mr. R. suggest an intrusive origin, a flow may be finely Clayton and Mr. E. Waddington kindly provided vesicular if there does not exist appreciable the routines on which the modelling was based.

REFERENCES

Ager, C. A. 1972. A gravity model for the computation of gravitational attraction of Guichon Creek batholith. Unpubl. M.Sc. three-dimensional bodies of arbitrary shape. thesis, Univ. of British Columbia. Geophysics 25, 203.225. Armstrong, J. E. 1954. Preliminary map Van- Talwani, M., Worzel, J. L. and Landisman, couver North, British Columbia. Geological M. 1959. Rapid gravity computations for Survey of Paper 53-28. two-dimensional bodies with application to Eisbacher, G. H. 1973. Vancouvergeo1ogy.a the Mendocino Submarine Fracture Zone. J. short guide. Cordilleran Section, Geological Geophys. Res. 64, 49-59. Association of Canada. Walcott, R. I. 1967. The Bouguer Gravity Geodetic Reference System 1967. Special Pub- Anomaly Map of southwestern British Col- lication No. 3, Int. Assn. Geod., Paris, Fr- umbia. Scientific Report No. IS, Institute of tT”CC. Earth Sciences, Univ. of British Columbia. Johnston, W. A. 1923. Geology of Fraser Wooton. A E. 1959. The Tertiary eruptives at River Delta Map Area, British Columbia. Vancouver, B.C. Unpubl. B.Sc. thesis, Univ. Geological Survey of Canada Memoir 135. British Columbia. Talwani, M. and Ewing, M., 1960. Rapid