Lapas attēli
PDF
ePub

banks and shifting of the river channels; for the material dredged from the bottom of the river is usually waterworn, whereas that dredged from the sand banks is not, and the currents in the channels are too strong to permit of formation of the material. A careful search of the sand banks exposed at low tide failed to reveal any of the sandstone in place. It probably, therefore, forms below the level of low tide but at a depth of only a few feet below that level. It can not possibly be derived from erosion of older formations for no such deposits are being eroded by the river, and it occurs in the seaward part of the Recent delta to which no material larger than fine gravel is being transported by the river.

The specimens of the material obtained show that it consists in part of sandstone of which the cementing material is calcium carbonate, in part of sandy or silty and shelly limestone, and in part of concretionary limestone. A partial analysis of one sample of the material, made by Mr. R. D. McLellan of the Department of Mines, Canada, gave the following results:

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small]

The material of this specimen is composed of sand grains cemented together by calcium carbonate, which constitutes 50 per cent of the rock. The proportion of lime varies considerably in different specimens, the concretion-like specimens being largely composed of lime; others are composed largely of shells with a mixture of sand and silt cemented by lime. The material is largely calcareous sandstone. It occurs as irregularly shaped masses which are quite consolidated when brought out of the water. Marine shells partly dissolved and fragments of wood only slightly altered are nearly always associated with the occurrences and usually form parts of the material. In the vicinity, where the material occurs there is usually an escape of gas which is inflammable and is probably marsh gas. During the freshet stage of the river, (May, June, and July,) the sand banks are covered for the greater part of the time by fresh river water. During the low-water stage sea-water covers the banks

most of the time. At certain stages of the river, particularly during the times intermediate between high and lowwater stages of the river, there are daily oscillations of sea-water and river-water over the banks, because of the tides. During the greater part of the year, therefore, the sand banks are saturated with sea-water and during the freshet months they are saturated with river-water. The river-water during the freshet is 2 to 4 degrees C. warmer than the sea-water. The temperature of the river-water during the freshet months varies from 12 degrees C. in May to 18 degrees C. in July or August. The temperature of the surface sea-water in the Strait of Georgia, near the mouth of the river, varies from 12 to 14 degrees C.; at a depth of 25 feet it is 10 to 11 degrees C. and at depths of 50 to 100 feet, 9 to 10 degrees C.

The mode of formation of the calcareous material as suggested by the mode of occurrence and the conditions under which it occurs is as follows: The lime which forms a considerable part of the material is probably derived from the shells, for the shells are partly dissolved, they usually form part of the material, and the river-water does not contain an excess of lime, the average of 22 analyses made by the Department of Mines showing only 11.85 parts per million of calcium. The analyses were made from composite samples of the river-water taken tri-weekly at New Westminster for the period of one year from May 5, 1919, to May 5, 1920. The seawater, which contains much less lime than the river-water and is probably rendered acid by the gas formed from the decay of the wood, tends to dissolve the shells. The river-water, which displaces or mixes with the sea-water in the sand banks, tends to cause deposition of the bicarbonate of lime in solution because of the higher temperature and higher lime content of the river-water. The shells are partly aragonitic in character and hence are readily dissolved. The lime is deposited as calcium carbonate and is not readily redissolved. There is thus a mass action in the direction of deposition of the lime.

The occurrence is an unusual one and differs from the well-known "stone or rock reefs" in that the material is formed below the permanent water level. It shows that, in exceptional circumstances such as obtain in the seaward part of the Fraser delta, lithification, to some extent, of the sediments and the formation of sandy and shelly limestone and concretionary limestone may take place below the permanent water level.

ART. XXXIV.-The Age of the Recent Delta of Fraser River, British Columbia, Canada;1 by W. A. JOHNSTON.

One of the problems studied in connection with an investigation of the characteristics of Fraser river, British Columbia, by the Geological Survey, Canada, in cooperation with the Department of Public Works, in 1919 and 1920, was the question of the age or time which has elapsed during the period of formation of the Recent or modern delta of the Fraser. This is of interest not only in itself, but because there has been much dispute in recent years as to whether uplift has continued into Recent time; in regions such as the Fraser Delta region where post-glacial uplift is known to have taken place.

The Recent, or modern, delta of Fraser river, British Columbia, is for the most part sharply delimited from the raised delta and marine deposits formed during the period of uplift of the land at the close of the Pleistocene. The surface of the Recent delta is all, except in a few places where the surface of peat bogs is a few feet above the general level, below the level of high tide; and the delta land high enough to be reclaimed is diked to exclude the flood-tidal and freshet waters. The head of the Recent delta, as defined by the point where the first distributary is given off, is at the city of New Westminster, 19 miles upstream or east from the seaward front of the delta in the Strait of Georgia. At New Westminster the river is confined between drift ridges or upland areas, which rise 200 to 300 feet above the river; and the river has occupied the valley between these ridges throughout the time of formation of the Recent delta. The upland area south of the river marks the inner edge of the delta, and extends from a point on the river 31⁄2 miles below New Westminster nearly straight south to Boundary Bay. The delta is bounded on the north by the highland area extending from New Westminster nearly west to Point Grey. In its seaward part on the south side it is interrupted by the highland area of Point Roberts, an island-like drift hill which has been joined to the mainland by the construction of the delta. Above New Westminster there is a large area extending from the south side 'Published by permission of the Director of the Geological Survey, Canada.

of Fraser river north to Pitt lake, the surface of which is largely below the level of high tide and, therefore, may be in part considered as belonging to the Recent delta of the Fraser. A considerable part of the filling of this area, however, is stratified clay deposited during the period of uplift of the land, and parts of the delta area above New Westminster are a few feet above sea-level. It is probable, therefore, that the great part of the delta deposits of the Fraser, above New Westminster, were formed during the period of uplift following the final retreat of the Pleistocene glaciers from this region, and that the great part of the delta deposits below New Westminster have been formed during Recent time when the sea and land had their present relationship or very nearly so.

The delta is building out into fairly deep water in the strait of Georgia, and in structure presents the forms characteristic of a high-grade delta. The fore-set beds are well developed and extend from the 3-fathom line to about the 30-fathom line, and have an average dip of about 10 degrees. Below the 30-fathom line the beds slope more gradually seaward, the 100-fathom line being reached at from 1 to 2 miles from the outer edge of the sand banks, which form the seaward part of the delta. The sand banks are in large part exposed at low tide and extend seaward on an average of 4 to 5 miles from the higher delta land which is diked. The delta is building out into fairly deep water in spite of the facts that the river is tidal for a considerable distance above its mouth, with a mean tidal range of 6.4 feet and a maximum range of 15 feet at its mouth, and that the seaward front of the delta is swept by fairly strong tidal currents. The out-building occurs because of the dominance of the river currents over the tidal currents. The flood-tidal currents in the strait of Georgia run north and are the dominant tidal currents. They have the effect of giving the larger part of the subaqueous front of the delta a smooth, curved outline lacking the finger-like projections characteristic of many deltas. The steep under-water face of the delta is a characteristic feature and extends along the whole of the seaward front of the delta from the highland area of Point Grey on the north to the highland area of Point Roberts on the south, a distance of 14 miles.

The thickness of the Recent delta is known approxi

mately at one point by a well boring at Steveston on Fraser river, 5%1⁄2 miles upstream from the seaward front of the delta. The man who drilled the well stated that sand was passed through for a depth of 700 feet from the surface, a bowlder 10 feet in diameter was penetrated at 710 feet, and the first stratum of hard shale was encountered at 860 feet. The Recent delta is probably, therefore at least 700 feet thick at this point.

An estimate of the yearly rate of seaward advance of the delta for the past 60 years has been made by Mr. W. H. Boyd, Chief Topographer of the Geological Survey Branch, Department of Mines, Canada, by a comparison of the soundings made in 1859 and shown on the 1860 chart with those made in 1919 by the Hydrographic Survey of Canada. The advance seaward of the bottom of the steep under-water face of the delta, which is marked approximately by the 30-fathom line, was determined by a comparison of soundings made in 1859 with those made in 1919. The rate of advance thus determined was taken as the rate of advance of the delta. The results showed that there has been no advance in the southern (one-third) part of the delta front, the reason for this being that the flood-tidal current sweeps this part of the delta and comparatively little sediment has been delivered by the river to this part. In the central part, for a distance of 41⁄2 miles, the rate of advance is considerable, the average of all the rates of advance, as determined at different points, being 26 feet a year. At one point the rate of advance was found to be 50.6 feet a year. The rate of advance at this point, however, is probably representative of only a very small part of the delta front. Discarding it, the average rate of advance a year of the central part of the delta is 20 feet per year. The northern (one-third) part of the delta front has also advanced because the entrance of the North Arm of the Fraser is in this part of the delta and the flood-tidal currents tend to carry northward part of the sediment brought down by the main Fraser. The average rate of advance of the northern part of the delta is probably about half that of the central part but is not definitely known because of the lack of sufficient soundings for purposes of comparison. The average rate of advance of the delta as a whole is probably, therefore, about 10 feet a year.

The age of the Recent delta may be approximately

« iepriekšējāTurpināt »