Sedimentology of Fraser River delta peat deposits: A modern analogue for some deltaic coals

International Journal of Coal Geology, 3 (1983) 101--143

101

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

SEDIMENTOLOGY OF FRASER RIVER DELTA PEAT DEPOSITS: A MODERN ANALOGUE FOR SOME DELTAIC COALS

W.B. S T Y A N *

and R.M. B U S T I N

Department of Geological Sciences, The University of British Columbia, Vancouver, B.C. V 6 T 2B4 (Canada) (Received October 14, 1982;revised and accepted April 4, 1983)

ABSTRACT Styan, W.B. and Bustin, R.M., 1983. Sedimentology of Fraser River delta peat deposits: a modern analogue for some deltaic coals. Int. J. Coal Geol., 3: 101--143.

O n the Recent lobe of the Fraser River delta peat accumulation has actively occurred on the distal lower delta plain, the transition between upper and lower delta plains, and the alluvial plain. Distal lower delta plain peats developed from widespread salt and brackish marshes and were not influenced appreciably by fluvial activity. Lateral development of the marsh facies were controlled by compaction and eustatic sea level rise. The resulting thin, discontinuous peat network contains numerous silty clay partings and high concentrations of sulphur. Freshwater marsh facies formed but were later in part eroded and altered by transgressing marine waters. The peats overlie a thin, fluvial, fining-upward sequence which in turn overlies a thick, coarsening-upward, prodelta--delta front succession. Lower delta plain--upper delta plain peats initially developed from interdistributary brackish marshes and were later fluvially influenced as the delta prograded. The thickest peats occur in areas where distributary channels were abandoned earliest.Sphagnum biofacies replace sedge-grass-dominated communities except along active channel margins, where the sedge-grass facies is intercalated with overbank and splay deposits. The peats are underlain by a relatively thin sequence of fluvial deposits which in turn is underlain by a major.coarsening-upward delta front and pro-delta sequence. Alluvial plain peats accumulated in back s w a m p environments of the flood plain. Earliest sedge~clay and gyttjae peats developed over thin fining-upward fluvial cycles or are interlaminated with fine-grained flood deposits. Thickest accumulations occur where peat fillssmall avulsed flood channels. Overlying sedge-grass and Sphagnum biofacies are horizontally stratified and c o m m o n l y have sharp boundaries with fine-grained flood sediments. At active channel margins however, sedge-grass peats are intercalated with natural levee deposits consisting of silty clay. These levees reduce both the number and size of crevasse splay deposits. Coal originating from peats of the different environments of the Fraser delta would vary markedly in character. Peats of the lower delta plain will form thin lenticular coal seams with numerous splits and have a high ash and sulphur content. Peats from the lower to upper delta plain will be laterally extensive and of variable thickness and quality. Basal portions of the seams will contain numerous splits and have a high sulphur content where*Present address: Shell Canada Resources, Calgary, Alta. T2P 2H5, Canada.

0166-5167183/$03.00

© 1983 Elsevier Science Publishers B.V.

102 as upper portions will be of higher quality. Peats from the upper delta plain--alluvial plain will form thick, isolated and laterally restricted coal seams characterized by low ash and sulphur contents.

INTRODUCTION The most recent lobe of the Fraser River delta complex is the product of active progradation of the Fraser River since the culmination of the last glacial stade 11,500 years ago. A large area of the modern delta is of extremely low relief and thus conducive to the development of marsh and raised bog deposits. Peats of approximately the same age initally began accumulating in several different depositional environments in response to a marked slowing in the rate of eustatic sea level rise around 4500 yr. B.P. (Fairbridge, 1976; Rampino and Sanders, 1981). On the inactive portion of the lower delta plain peats accumulated from salt and brackish marshes. On the lower-upper delta plain brackish and later freshwater peats were deposited in raised bogs and marshes between distributary channels whereas on the upper delta plain-alluvial plain peats, which originated from freshwater marshes, accumulated between and adjacent to channels and flood deposits. The distinct environmental settings of these different peat deposits provide an excellent opportunity to compare resulting differences in sedimentology, petrography and geochemistry. Because peat is the progenitor of coal, these differences can be extrapolated to produce models which may aid in the understanding of ancient coals and coalbearing strata. The purpose of this paper is: (1) to describe the bio- and lithostratigraphy and sedimentology of three representative deposits at Boundary Bay, Lulu Island and Pitt Meadows; (2) to outline the variation in clay mineralogy, ash content, pH and total sulphur of the peat; and (3) to develop models of peat deposition that may prove useful in understanding the distribution, thickness and quality of deltaic coal deposits. In an earlier paper we have described the petrography and decompositional pathways of Fraser Delta peats and have suggested possible coal-maceral and microlithotype precursors (Styan and Bustin, 1983). Previous studies of peat deposits have made significant contributions to the understanding of those factors that effect the distribution and quality of coal. In particular the work of Spackman et al. (1969, 1974), Cohen (1968, 1970, 1973), Staub and Cohen (1978, 1979) and Cohen and Spackman (1977, 1980) in the subtropical swamps of the southeastern United States have demonstrated the relationships between depositional environment, plant biofacies, ash and mineral matter content, and distribution:and thickr, ess of peat. Casagrande et al. (1977, 1979, 1980) and others have documented the relationship between depositional environments of subtropical peats and sulphur content which has provided a genetic model for predicting the distribution of sulphur in coal (Cecil et al., 1980). With the ex-

103

ception of the study by Allen (1978) of some coastal marshes of Delaware there has, however, been little investigation of temperate or sub-arctic peats from the perspective of coal geology. REGIONAL SETTING

The Fraser River is more than 1200 km long, and has a drainage area in excess o f 230,000 km 2 (Milliman, 1980). A total sediment load of between 12 and 30 million tons, consisting of equal amounts of silt and sand, and up to 10% clay, is carried to the river m o u t h annually (Mathews and Shepard, 1962) resulting in the progradation of the delta by as much as 9 m per year. The sand is deposited primarily in channels and in low islands at the mouths of channels. Part of the sediment is redistributed by longshore currents to form shallow bars. In areas of the delta front, n o t influenced by fluvial sedimetation, wave action winnows out finer sediments to produce sandy tidal fiats (Luternauer and Murray, 1973). Silts and clays accumulate with organic m u c k in interdistributary troughs and on the delta front. The modern delta presently covers an area of 975 km 2 and has an average thickness of 110 m (Mathews and Shepard, 1962). This lobe extends 31 km into the Strait of Georgia from a narrow gap in Pleistocene uplands at New Westminster and forms a perimeter greater than 27 km (Luternauer and Murray, 1973) (Fig. 1). The western perimeter is actively receiving sediment 123°00 '

FRASER RIVER DELTA .~

LEGEND Peat deposits ~

VANCOUVER

.~North . . . .

Deltaic sediments

~

Fraser R. NEW WESTMINSTER/l"~~~

) ~..

~: -Pitt Meadows Bog

Middle Lulu

INDEX MAP

Bounclary Bay

t23VO0* . 49000 ,

Point Roberts I

kilometres

Fig. 1. Location map of study areas, s h o w i n g recent deltaic s e d i m e n t s and peat deposits of the Fraser River delta.

104

from two distributary channels. Along the southern margin of the delta from Point Roberts to south of the Nicomekt River, a broad, shallow tidal flat has developed where only minor sediment is being added from the Serpentine and Nicomekl rivers and erosion of the Point Roberts Peninsula (Shepperd, 1981) The broad delta f r ont surrounding the perimeter has a slope of about 1.5 °, and is cut by a series of gullies leading to h u m m o c k y t o p o g r a p h y (Mathews and Shepard, 1962). Tidal range from mixed tides reaches a m axi m um of 5 m at the delta front. This strong tidal influence is felt over much of the delta t h r o u g h o u t the entire year, especially during the spring freshet (Milliman, 1980). A salt wedge extends as much as 20 km up river in winter, but seldom reaches past the distributary mouths between May and July, when r u n o f f peaks (Johnston, 1921; Swinbanks, 1979). Approximately one third of the m o d e r n delta is presently covered by peat deposits. Peat has accumulated on inactive port i on of the lower delta plain at Boundary Bay, on the lower and upper delta plains at Burns Bog and Lulu Island and on the upper delta plain and alluvial plain at Pitt Meadows and adjacent areas (Fig. 1). METHODS In order to determine lithofacies, biofacies, and peat stratigraphy in the Boundary Bay, Lulu Island and Pitt Meadows deposits three hundred and fifteen holes were drilled, using a hand-driven Hiller Corer. Due to variability in peat composition and depth, cores were spaced 100 m apart on a grid. For areas which had been disturbed by peat cutting, sections were restored to their approximate thicknesses before peat isopach maps were prepared. Surface lithofacies were also identified and mapped. Using information from cross sections and peat isopach maps, three uncompressed cores were selected and obtained from each of the Pitt Meadows and Lulu Island deposits, while a seventh was collected from the Boundary Bay peat. For about the inital meter and one-half, peat blocks measuring 20 cm X 150 cm X 50 cm were cut from the wall of a hole, using a machete. The remainder of the core was obtained using a t o o t h e d piston corer (Cohen, 1968). The piston was greased before each use and the barrel was pushed straight down without twisting. The core was then transferred to irrigation pipe of the same diameter and the ends sealed. This combination of m et hods avoided co mp action which resulted when ot h er coring devices were used. The cores were then logged and the peat subdivided into smaller homogeneous units. Samples collected for moisture, ash, and organic m at t er were stored in plastic containers. Water was added to saturate the samples. After equilibrium was attained, 30 g were placed in an oven at 105°C for 16 hours. The resulting weight loss was recorded, and the remainder of the sample was ashed in a muffle furnace at 550°C until no further weight loss occurred. The difference in weight of ash and water from the original sample provided a

105 measure of organic matter. Three grams of this mixture were used to determine pH (ASTM D 2976-71). Samples for sulphur analysis were reground and sieved at 100 mesh before analysis on a Fisher Sulphur Analyzer. The remainder of the core was utilized for pollen, petrographic, and clay mineral analysis. The less than 2 ~m fraction was separated from crevasse splay, overbank and underclay sediments for clay mineral analysis. An oriented slide was prepared, and the remaining clay fraction was saturated with a 1 M KCI solution for three days. After removal of the KC1 solution, another oriented slide was prepared. Both samples were subjected to X-ray diffraction using CuK a-radiation and a 002 graphite monochromator. The K-saturated slide was then heated to 300°C for 4 hours, cooled, and X-rayed again. A similar procedure was followed after reheating the slide to 550°C. The untreated slide was glycolated in a desiccator for one day before being analyzed a final time. Following identification of the clay minerals, the relative abundances were approximated using the methods of Bayliss et al. (1970). Pollen was isolated from peat samples using standard methods. After screening with 250 and 100 ~m sieves, samples were boiled in 5% KOH for 20 minutes and then water washed three times. This procedure was followed by two acetic acid washes and boiling in acetolysis solution (9:1 acetic anhydride; concentrated H2SO4) for 30 minutes. Prior to staining with 10% safranin solution, the acetolysis mixture was washed twice with both glacial acetic acid and water and was neutralized with 5% K2CO 3 solution. Slides were then prepared and pollen identified. RESULTS

Boundary Bay a. Location and history The Boundary Bay peat is situated on the inactive margin of the delta between the Point Roberts Peninsula and the Serpentine and Nicomekl Rivers (Fig. 1). It extends from the foot of 112 Street towards Mud Bay and seaward into Boundary Bay. Although n o t verified, this peat may also extend shoreward and connect with Burns Bog through a series of channel fill peats (G.E. Rouse, pets. commun., 1981). The sedge-grass peats at Boundary Bay developed on the coastal portion of the lower delta plain, where it merged with the delta front. Prior to dyking, this area was flooded frequently because of high tides, runoff from the Fraser River freshet (Shepperd, 1981), and occasional storms. Erosion and alteration of the peat have continued on the seaward side of the dyke, producing a discontinuous horizon partially covered by silty sand (Fig. 2A). In some areas a more recent salt marsh peat has developed over the older peat, b u t it t o o is being eroded. The Boundary Bay tidal flats have been studied by Kellerhals and Murray

3~

107

(1969) and Swinbanks (1979). The only extensive study on the peats, however, is the palynological work by Shepperd (1981). She analysed pollen assemblages from the older peat at 112 Street, dated at between 3130+50 yr. B.P. (GSC 3202) and 3910+60 yr. B.P. (GSC 3183), and the much younger salt marsh peat at 64 Street, dated at 320+10 yr. B.P. (GSC-3186).

b. Lithofacies Peats at Boundary Bay form laterally extensive accumulations on inactive portions of the lower delta plain--delta front. Lithofacies include both marine and fluvial-derived sediments. Thickest peat accumulations overlie clean grey clay, which is confined primarily to upper intertidal and supratidai areas (Fig. 3). This clay grades over 0.5 m, from an overlying organic-rich sedge-clay, down into well-sorted silty clay, silt, and silty sand. Shell debris is absent, and only trace amounts

....

:

- -'----_-2-_---

lm

SCALE

LEGEND - SPHAGNUM

[

]

=

':i

- SPHAGNUM~EDGE

- SEDGE-GRA=

- SEDGE CLAY

- INORGANIC SILT CLAY - ORGANIC SILTY CLAY

I - - I

- BRACKISH-MARINE SILTY SAND

Fig. 3. Cross-section of the discontinuous peat horizons at 112th Street, Boundary Bay. The cross section is perpendicular to the shoreline. The thickest peats occur in the upper intertidal zone where they overlie fluvial sediments (BH 1--5). In the lower intertidal area thinner peats locally overlie algal mats (BH 8--14). The legend shown in c o m m o n to all cross-sections so that all facies shown in the legend m a y not appear on the diagram. Fig. 2A. Exposure of sedge-grass peat in the upper intertidal zone at Boundary Bay (112th Street). The peat has been highly eroded and altered from recent marine transgression. Peats are partially covered with silty clay and sand. The scale is 30 cm long. B. The fibrous texture of the sedge-grass peat is visible in this block. Horizontal components are completely decomposed, and those tissues which are visible are stems which have grown vertically through the earlier peat. The peat block is 25 cm wide. C. The highly reducing environment is exposed just beneath the sediment surface by this footprint. Zostera (Z) and pelecypods (P) are faintly visible on the sediment surface.

108

of organic matter are present. Beneath and between thinner peats in lower intertidal areas are grey to brown silty sand and fine sand. Variation in color is controlled by the relative a m o u n t of organic matter. These lithologies appear structureless and show only slight variation in grain size with depth.

silty sand transgressive marine units silty clay

brackish to freshwater

peat

flood or storm deposit

marine to brackish sedge peat

washover

deposit

silty sedge clay

Silt

Fig. 4. Photograph of core from Boundary Bay, showing peat stratigraphic units. The "flood or storm deposit" separates the brackish to freshwater peat biofacies from the marine to brakish sedge peat biofacies. The ruler is 30 cm long.

109 Directly beneath peats, however, faint black laminae of variable thickness occur. Shell fragments are present, b u t constitute a minor fraction of the sediment. Contacts between these massive units and the more shoreward graded units are sharp. A well-sorted medium sand, rich in oxidized organic matter, covers much of the upper intertidal zone. This unit has a variable thickness which seldom exceeds 20 cm and has a sharp basal contact.

c. Biofacies Two biofacies can be distinguished at Boundary Bay (Fig. 4). The lowermost unit is c o m p o s e d of massive grey to tan grey clay, and represents a transition from inorganic grey clay and silty clay. Sedge and grass leaf and stem tissue are b o t h vertically and horizontally oriented throughout. Vertically aligned plant tissues are often oxidized, leaving thin e m p t y tubes with dark brown rims. Black and yellow rootlets are pervasive. This unit is never more than 20 cm thick, and grades abruptly into a light chocolate brown peat, c o m p o s e d entirely of sedge and grass, leaves, stems, and roots {Fig. 2B). These tissues form horizontal laminae which parallel the base o f the deposit. Thin charcoal bands occur toward the middle and, less commonly, the upper sections of the peat. Degradation is high, and plant material cannot be identified at the generic level. Relatively large numbers of Chenopodiaceae pollen occur at the base of the peat, while an assemblage of dominantly Cyperaceae and Gramineae pollen occur throughout the remainder of the sedge-grass unit. e. Clay mineralogy and geochemistry Clays underlying sedge-grass peats at Boundary Bay are mainly kaolinite and illite, which together comprise, on average, a b o u t 80% of all analyzed samples (Table I). Smaller amounts of smectite, chlorite, vermiculite and mixed layer smectite-chlorite are also present. Sedge-grass peats at Boundary Bay are presently being oxidized by marine water, and as a result are highly decomposed. In the core taken from this environment (Fig. 5}, total sulphur values range from 5.2 to 6.1% in sedgegrass peat, b u t decrease abruptly to half this amount in sedge-clay peat. Dry ash values are correspondingly high, and vary from 22 to 51% througho u t the section. The highest ash content coincides with charcoal horizons at a depth of 0.3 meters. The pH of the peat averages 5.8. f. Depositional history Boundary Bay peats have a complex depositional history. Initial peats began developing between distributary channels and were freshwater in origin. After these channels were abandoned, a b o u t 4500 yr. B.P. a large portion of the delta margin became inactive. Brackish marshes then developed in lowlying areas over extensive areas of the lower delta plain. As the rate of eustatic sea level rise slowed, brackish marsh peats spread quickly over previously deposited freshwater marsh peats, floodplain silty clays, and sandy

freshwater under bog proper/organics freshwater under bog proper/organics freshwater n a t u r a l levee b o t t o m n a t u r a l levee m i d d l e n a t u r a l levee t o p freshwater crevasse s p l a y freshwater o v e r b a n k crevasse cse o v e r b a n k crevasse fine bottom bog proper/no organics freshwater brackish bog bottom top brackish bog bottom middle brackish bog bottom bottom marine bog bottom top marine bog bottom middle marine bog bottom bottom Pharo (1972) Fraser River a n d Strait o f G e o r g i a

PM P~C I

45 small

45

45

53

59

52

58 56

46

63 69 49

61

58

51

7° Kaolinite

41 38

30

43

30

27

28

25 27

29

28 19 36

27

2t

31

10: Illite

10 40

16

9

12

8

15

12 12

19

4 2 9

3

10

13

10°--17 ° Smectite

t unknown

¢

t

2

3

2

2 2

5

1 2 5

5

8

2

4 22

5

3

3

3

3

3 3

1

4 8 1

4

3

3

14 ° 14 ° Vermuculite Chlorite

*PM = Pitt M e a d o w s ; LIE = L u l u I s l a n d ; BB = B o u n d a r y Bay. P a n d C refer to s a m p l e (P) a n d core (C) labels.

BB P~C~ Average

BB PIC:

BB P~C~

LIE P4Cs

LIE P4C3

LIE P,C~

LIE P3C: LIE P3C~

LIE P3CL

PM P3 C, PM P3C6 LIE P~C:

PM P3C~

PM P~C~

Environment

Sample*

Clay m i n e r a l o g y

TABLE I

abundant noticed

abundant

abundant

abundant

small

small

t small

t

t t t

t

t

t

Mixed-layer Smectite-Chlorite

h.a O

iii MACROSCOPIC CONSTITUENTS

BB

/'E

% TOTAL SULPHUR

pH 3.0 4.0 5.0 6.0

% DRY ASH

2.0 4.0 6.0

10 15

20

Carex stem

0.2

K/

-Juncas stem

i

51

0.4 I

0.6

Sphagnum peat [ ~

sedge-Sphagnum peat

[ ~

sedge-grass peat sedge-clay peat

[ •

gyttjae

~

sand

clay

{ ~

petrography sample

silt

~

fire horizon,

~82

charcoal

Nuphar peat

Fig. 5. Peat profile from Boundary Bay, showing peat types, macroscopic plant constituents, and analysis of pH, sulphur, and dry ash. Maximum ash concentrations correspond to washover events and charcoal horizons where inorganic matter has been concentrated. Sulphur concentration decreases markedly in the underlying lithofacies. The legend shown is c o m m o n to all peat cores such that all facies shown on the legend may not occur in any one diagram.

marine silts. Marsh growth was pervasive, except in areas which were continuaUy influenced b y tidal activity. Accumulation of plant material initially kept pace or exceeded eustatic sea level rise, and isolated peats coalesced to form larger deposits. Freshwater peats gradually replaced brackish-water equivalents in areas where the substrate had been elevated above tidal influx. Throughout peat accumulation, storm events, Fraser River freshets, and extreme high tides intermittently deposited silt or silty clay into the peat as thin discontinuous lenses. Eventually the slower growth of the freshwater marsh peat, sediment compaction, and/or an increased rate of eustatic sea level rise allowed the peat to be inundated and covered b y silts. All growth ceased and the peat was (and presently is being) altered and eroded by wave and tidal action. Lulu Island a. Location and history The Greater Lulu Island Bog extends from near the centre of Lulu Island east to the main arm of the Fraser River (Fig. 1). It has been separated into t w o halves b y a northeasterly-trending channel {Fig. 6, Cf2) which was active throughout most of peat deposition. The eastern portion of this deposit was studied with 150 hand-driven Hiller cores (Fig. 1 ). The bog lies on the boundary between lower and upper delta plains. During the freshet flow in spring and early summer the area is influenced only b y fresh water. Throughout the remainder of the year, river flow is relatively low, and saline b o t t o m waters extend past the Lulu Island bog {Milliman, 1980). Mixing occurs such that even surface waters are slightly brackish for a period o f time.

112

i

CI~A~NFL

L

rILL :

INI E ~[)IST~IIBI~T~I~¥ CLA',

Fig. 6. Lithofacies map of the Lulu Island deposit. Dashed lines show linear channel markings visible from aerial photographs. Peat intercalates with silty clay overbank (Obl, Ob2) deposits along both the northeastern and northwestern margins. However abondonment of the northwestern channel (Cf2) has allowed peat to prograde over the former overbank deposits (Ob2, nL2) with sharp contact. Along the southern margin the Fraser River is presently eroding peat, former overbank deposits (Oh3) and interdistributary clay (IdC1).

In 1927, Hugo Osvald observed the bog in a relatively undisturbed state, and described surface plant associations from num erous collections (Osvald, 1928, 1933, 1970). Hansen {1940) studied the paleoecology using pollen analysis from a core in the western por t i on of the bog. Little scientific study has occurred since. The deposit was ditched and subsequently mined for peat early in this century. Although large areas of peat were cut, primitive cutting m et hods prevented mining of large strips of the natural bog surface. Only in the northwestern area, where active mechanized mining is presently occurring, has the peat surface been d e s t r o y e d completely. Much o f the eastern portion o f the deposit is actively being covered with garbage and dredged fluvial sand. To the west and north, blueberry farms and nurseries make m ore conventional use o f the bog.

113

b. Lithofacies The Lulu Island peat developed within a fluvial-dominated lower delta plain environment. Through deltaic growth the fluvial network has gradually acquired the characteristics of an upper delta plain. Lithofacies deposited reflect these changes. Numerous small channels, and channel fill deposits which occur throughout the bog result in marked variation in peat thicknesses (Figs. 6--10). Most channel fills consist of sedge-peat and clay, and are flanked by levees composed of interlaminated silt and clay which in turn grade laterally into finer interdistributary clay units. In some channels, however, fining upward sequences of fine to medium sand and silt replace sedge peat and clay in the core. These clastic sediments grade upward into highly rooted clay and eventually peat. Near the transition to organic facies intercalated fine sand and silty crevasse units (Figs. 8--9) occasionally occur. These units are laterally extensive, becoming thinner and finer grained toward the center of the bog.

Fig. 7. Isopach map of Lulu Island deposit. Elongate lighter areas represent regions where channel activity was abandoned latest. As a result peat in these areas contain numerous overbank and crevasse splay deposits. Heavily printed numbers (1--3)indicate locations of major cores; letters show cross-section locations for Figs. 8 and 9.

114

Intensive rooting by later sedge-grass communities has destroyed most bedding structures and has added considerable organic content. Large channel deposits consisting of fining-upward sand and silt units have confined the lateral development of the bog (Cfl and Cf2, Fig. 6): These channels are mantled by up to 2 m of silty clay and are bounded laterally by broad natural levees consisting of interlaminated peat and silty clay. At depth, clay replaces peat which in turn is replaced by fine sand. Marginal to channels, laterally restricted crevasse splays composed of fine to medium C

C'

~ : - - = ~ - ~ - = - - ~

63

OLD _

CHANNEL

62

e

.o:-,~"

.'~'.-~,:~'~:~'~

.S

44

_-=-=-:

OLD

~2

4~

e,;,~

:--:-=--Z--:--2

lmI

. . . . . . . . . . . .

SCALE

LEGEND - SPHAGNUM - SPHAGNUM~EDGE

- SEDGE CLAY

- SEDGE-GRASS

- INORGANIC

- GYTTJA

- ORGANIC

SI LT CLAY

- FIRE

- BRACKISH-MARINE

SILTY

CLAY SILTY

SAND

Fig. 8. Cross-section C-C', D-D' from the Lulu Island deposit. Note small channel (C) within peat in cross-section C-C'. Note how small distributary channels in both cross sections reduce peat thickness. On deposit margins, where fluvial activity continued throughout peat deposition, peat intercalates with overbank deposits (BH 145-147, C-C~, and BH 63---61, D-D' ). Along these margins Sphagnum peats do not succeed sedge-grass peats; where sedimentation ceases prior to or during peat growth (BH 91--85, C-C') Sphagnum peats prograde over the sediments. Figure 7 shows location of cross-sections.

115

sand interrupt peat deposition at irregular intervals (Figs. 8--9). Such splays average 15 cm thick and thin rapidly over short distances within the peat. Fire splays consisting of isolated lenses of grey organic silty clay and clay occur occasionally within the sedge peat. Charcoal fragments are common within in the splays and increase in abundance towards a sharp base. Overlying contacts are highly rooted and gradational. E

FILL

~-----~-

FRASER

,my 100m

F

23

22

Z-~ ~' ,~-" ,~

~ ~

Zl

ZO

~,%~,~:

14

r~

I

:~

Z~-~.I0 ~'~

9

F'

SCALE

CHANNEL FRASER RIVER

LEGEND -

I

l

l

- -

1

I

I

SPHAGNUM SPHAGNUM~EDGE

~

- SEDGE CLAY

- SEDGE~}RAS8

~

- INORGANIC SILT CLAY

- GYTTJA

~

- ORGANIC SILTY CLAY

~

- BRACKISHJ~ARINE SILTY SAND

-

FIRE

Fig. 9. Cross-sections E-E', F-F' from the Lulu Island deposit. Dashed line on section E-E' shows former peat surface prior to mining. Note also truncation of the thickest peat section by the Fraser River at E', ~nd fire splay (Is) in section F-F' (BH 20,14). Both sections show that areas of thickest peat accumulation (BH 26,27,32, E-E'; B H 9--12, F-F') have been eroded by the main arm of the Fraser River. Figure 7 shows location of crosssections.

116

c. Biofacies Lulu Island biofacies depict a natural successional sequence leading to the f o r m a t i o n o f an oligotrophic raised bog (Styan and Bustin, 1981}. Sedge-clay peat represents the initial transition from fluvial to organic sedimentation. Accordingly, this peat is c o m p o s e d of bot h organic and inorganic components. The organic constituents include altochthonous w o o d and bark fragments and a u t o c h t h o n o u s Equisetum and sedge stem and r o o t tissues. Addition o f these materials to massive tan-grey clay produces a fibrous to granular sediment. Most plant c o m p o n e n t s of these peats are highly decomposed. As fluvial influence decreased and silty clay was prevented from entering the marsh, sedge-grass peats became d o m i n a n t (Figs. 10A and B). These peats are gold to tan brown, fibrous and c o m p o s e d o f Carex, Juncus, and Scirpus stems and occasional bands of charcoal. Culms of these sedges as well as Typha and Calamogrostis cut vertically through the peat (Fig. 10C) and rootlets are pervasive. Although the plant material is bet t er preserved than in sedge-clay peats, a b u n d a n t am or phous matrix attests to the high degree of decomposition. A

Fig. 10A. The modern brackish sedge-grass marsh developed between the two arms of the Fraser River on Lulu Island. This setting is analogous to the original depositional setting of peat in the Lulu Island deposit. B. A peat section at Lulu Island. At the base of the scale (30 cm), the boundary between Sphagnum and sedge-grass biofacies is visible. Textural differences in the peat types can also be observed. C. Large culms of Juncus, Carex and Typha occur throughout the fibrous sedge-grass peat. The color of this peat is golden brown to mustard yellow.

With the colonization o f Sphagnum spp., ericaceous shrubs replace sedgegrass co mp o n en ts. This transition occurs through a distinct sedge-Sphagnum peat. The addition o f small quantities of Ledum and Oxycoccus stem and r o o t tissues between Sphagnum pockets produces a red-brown coloured peat

117 with well developed bedding. Rhynchospora and Eriophorum culms are vertically oriented and cut obliquely across this stratification. Due to the presence of Sphagnum, (and therefore low pH which retards bacterial decomposition) most tissues, including some Oxycoccus and Ledum leaves, are well preserved. Ericaceous and pure Sphagnum peats represent climax biofacies (Fig. l l A ) . Vaccinium, Kalmia, and Ledum leaves, roots, and stems contribute significant amounts of lignin to a matrix of Sphagnum (Fig. l l B ) , resulting in a fibrous to granular textured peat. Charcoal fragments form discontinuous bands more frequently in the ericaceous Sphagnum biofacies, and are often closely associated with ericaceous layers. Bedding is consequently well developed. The presence of Sphagnum assures good preservation except near charcoal horizons where increased ash content neutralize acidic conditions. Nuphar-hollow peats result from the accumulation of detrital plant material, primarily Pinus and ericaceous tissues, in water-filled depressions. The growth and decomposition of Sphagnum, Nuphar, and liverworts within these pools adds to the complexity of the resulting peats. Degradation is almost complete for most tissues. The remainder are suspended in the resulting amorphous orange matrix. A poorly developed microbedding exists as a result of the presence of large liverwort thalli. Sedge-wood peats develop on the flanks of natural levees. These peats consist of allochthonous Betula stems interbedded with sedge peat. The presence of stumps is hard to detect using cores, and as a result sedge-wood peats are probably more extensive than initally evident. The matrix of the peat is dark brown and fibrous. Throughout this well-bedded substrate are scattered large Picea and Populus stumps in growth position (Fig. llC). Smaller stumps of Pinus replace these genera abruptly as Sphagnum biofacies are approached away from the levee. Preservation of lignified tissues is good.

d. Stratigraphy Earliest sedge-clay and sedge-grass peats grade downward to underlying grey silty clay and clay over short distances. These peats intercalate with thin, but laterally extensive, crevasse deposits of silt and silty clay, and are broken by small channels of dominantly silt and silty sand. These channels have positive relief, and thus reduce peat thickness (Figs. 8 and 9). Interlaminated clay and silty clay of overbank origin underlie these peats for several tens of meters laterally, along both sides of the channels. More recent freshwater sedge-grass peats are void of sediment and are horizontally stratified above these small channel-fill deposits (Cfl, Fig. 6). Along northern and eastern margins of the peat deposit, however, where sedge-grass facies have prograded vertically near larger channel-fill deposits, splays do occur (Figs. 8 and 9). Such splays are composed of medium to fine sand, and thin rapidly into the peat. Sedge-grass facies are horizontally stratified along the western edge of the bog (near Cf2, Fig. 6). Here, Sphagnum biofacies abruptly overlie

119 both levee and channel fill deposits of Cf2 (Fig.. 6). Splay deposits are absent from this section of the peat. To the south, the main arm of the Fraser River is presently eroding thick accumulations of peat.

e. Clay minerals and geochemistry Clay minerals in Lulu Island sediments (Table I) are characterized by the abundance of kaolinite, illite, and smectite. Vermiculite and chlorite are present in minor amounts. No significant variation in abundance was observed except in mixed layer smectite-chlorite, which increased from trace amounts in freshwater sediments to small amounts in marine and brackish influenced sediments. Three cores were analyzed for pH, total sulphur, and dry ash. Two of these cores were collected from undisturbed bog environments and a third was sampled from an eroded peat face which was exposed to the atmosphere at low tide and covered with river water during high tide. It is overlain with clay, silt, and silty sand from recent flood events. The analyses of one of the cores collected from the undisturbed bog environment are shown in Fig. 12. Sulphur compositions for Lulu Island sedge-Sphagnum and Sphagnum peats are similar to those obtained from comparable peats at Pitt Meadows described later. Values range from 0.15 to 0.23% and show no increase with depth. Ericaceous Sphagnum peats have slightly higher sulphur values. Sulphur contents for sedge-grass peats in the core taken from the eroded and exposed peat average 0.6%, and show little variation. Sulphur, however, increases markedly with depth, attaining values of between 3.0 and 6.0% near the b o t t o m of the undisturbed cores corresponding to the occurrence of brackish peat (Fig. 12). Nuphar hollows contain slightly higher concentrations of sulphur than surrounding Sphagnum biofacies. The a m o u n t of ash in Sphagnum peats is low, ranging from 0.5 to 1.5%. Little variation in the a m o u n t of ash occurs in vertical section until sedgegrass peats are reached where up to 4% ash occurs. Exceptionally large amounts of ash occur where either crevasse or fire splays have interrupted peat growth. In the eroded and exposed peat the average concentration of ash is 7.5%. Sedge-clay transition zones contain up to 48% ash. Average pH values for the undisturbed peat range from 3.5 for Sphagnum peat to 4.0 for freshwater sedge-grass peats. Charcoal horizons have a higher pH (Fig. 12). At depths, correlatable with the rise in sulphur concentration, Fig. 11A. The climax succesional stage of the peat biofacies is shown here. The community consists of Pinus contorta, Sphagnum ssp.,Pteridium and numerous ericaceous shrubs. B. Ericaceous Sphagnum peat with the distinguishing fine fibrous texture and flattened stems of Ledum (L). Roots and stems are well preserved as a result of the low p H (less than 4) created by Sphagnum. The peat block is approximately 15 c m wide. C. Erosion of the peat along the southern margin of the Lulu Island deposit reveals a Pinus stump in sedge-wood peat. Units of silty clay from earlieroverbank deposits (o) are intercalated with this peat as well.

120

pH values decline to near 3.0 in these cores, pH m e a s u r e m e n t s from the exposed and eroded peat are more alkaline and do n o t b e c o m e m o r e acidic at depth, as a result o f invasion and c o n t a m i n a t i o n by river water at the e r o d e d peat surface. MACROSCOPIC LIE

1

% TOTAL

CONSTITUENTS 3.0

40

5.0

6.0

0

0 I

.

02 04

o~co¢¢~s

Ledum roots P~nus seeds

C~8 [,edur~ stems and r ~ c t Rhy~cnospora and JL~nC¢tS

10 I 2

]

Juntas

i ,

0

6.0

'

5

I

t,I

i

I.

I

Care~ c u l m s

' •

! I ,f I I I

I

8etula stem t,agmen~

' ° "

\

[

22

i .

5

Betula or A~r1~s wood fr t#m[tnt

i,

r:.paa / s!*.m

l/

;/

24 2~

20

I

I I t3

15

t

14 ~,

ASH

,I

I I

aria C~lrex culms

~0

t I

1II

''

i ' ''

4.0

I I t I I I f

,unne,s

I

(q 6

20

f

P1er~,um rh,zomes !

% DRY

SULPHUR

pH

60

\

\

i! \

2~

\ \

:3 (:'

I I I I I I

2 S~. ea stem~ 3 4 3

~ • .

sedge

Sphag .......

L

j

.q

• .j ., *

~. . . .

~

,

sedge q

grass

sedge-clay

peat peat

•

]

petrograo

............

_

,._

j

silt

~

q

Nuphar

L

hre horizon,

char{:o,~l

peat

Fig. 12. Peat profile from bulu Island showing peal types, macroscopic plant constituents and analysis of pH, sulphur and dry ash. High ash values at a depth of 2.5 m result from incorporation of splay sediment into the peat. pH values increase in the sedge-grass biofacies untill horizons influences by brackish water are reached. At this depth, more acidic conditions together with an increased sulphur concentration occurs.

f. Depositional history Sedge-grass peats began accumulating above interdistributary silty clays and clays o f the lower delta plain e n v i r o n m e n t 4 6 8 5 yr. B.P. ( T e l e d y n e 1 - 1 1 - 7 4 2 , 1 9 8 1 } . As a result o f a n e t w o r k o f intervening channels, early peats were d i s c o n t i n u o u s and c o n t a i n e d n u m e r o u s fine grained splay de-

121 posits. The gradual a b a n d o n m e n t of these small channels occurred as major distributaries became established. Isolated peat-forming marshes then coalesced, producing continuous peat horizons. The formation of both levees along major channels and an acidic environment within the marsh limited the extent of clastic material entering into later peat facies. Because of the influence of marine and brackish water, the transition from sedge-grass to Sphagnum biofacies occurred only after the substrate had been raised substantially by brackish sedge-peat accumulation. Along northern and eastern boundaries of the deposit, however, sedge-grass peats have not been replaced by Sphagnum biofacies, but have prograded vertically in response to continual fluvial activity. Sphagnum peats are thin in these areas as a result. A thick mantle of silty clay which overlies coarser channel fill (Cfl, Fig. 6) sediments further suggests that the channel remained active at least during flood periods until very recently. The channel on the western margin (Cf2, Fig. 6) appears to have been abandoned sooner, about 3000 yr. B.P. (Teledyne 1-11-743, 1981). Here Sphagnum peats have prograded over old channel-fill deposits. The growth of the south arm of the present Fraser River was coincident with the abandonment of the major channel to the north and east of the deposit. In recent years this channel has meandered slowly northward, cutting through previously deposited overbank (Ob3, Fig. 6) and peat deposits and has replaced them to the south with coarse-grained channel-fill deposits.

Pitt Meadows a. Location and history The Pitt Meadows peat deposit is situated on the Fraser River alluvial plain, east of the confluence of the Pitt and Fraser Rivers (Fig. 1). Several other peat bogs have developed within this alluvial plain to the north along the Alouette River and to the east along the Fraser River. Most are relatively small peat accumulations, confined by flood plain sediments. However, a large freshwater marsh has developed south of Pitt Lake, where deltaic lake sediments gradually have been reclaimed by sedge and grass communities. A lateral biofacies zonation can be traced to the south in this deposit where the peat thickens. Climax communities similar to those in other raised bogs overlie areas of maximum peat thickness. A radiocarbon date of 1860+80 yr. :B.P. (WAT 651) has been obtained from the base of this peat at a depth of 140--145 cm (Lyngberg, 1979). Previous study of the Pitt Meadows peat deposit is limited to a single profile reconstructed by Rigg and Richardson (1938). Mining of Sphagnum peat from this bog occurred as early as 1920. When mining was finally abandoned around 1960, up to 80 cm had been cut off certain areas (C. Bacus, pets. commun., 1980). Many areas along the margins have been reclaimed for agricultural use, primarily as blueberry and cranberry farms.

122

b. Lithofacies The Pitt Meadows peat deposit has accumulated within the Fraser River alluvial plain. Fluvial lithofacies of similar origin but of different ages have interacted to produce a complex peat-forming environment (Figs. 13, 14 and 15).

From air-photo interpretation and 150 Hiller cores, remnants of old fluvial channels can be recognized surrounding much of the deposit (Fig. 13). To the north and east, thick accumulations of grey silt and silty clay overlie fining-upward sequences of fine to medium sand (Cfl, Fig. 13). The sand grades laterally into interlaminated silts and clays which thin away from channels and are eventually replaced by massive grey clay and lenticular units of dark brown organic clay. The boundary between clay and overlying gyttja and sedge-grass peat is gradational over short distances (Fig. 16). Channel-fill deposits on the west (CfP, Fig. 13) are covered by a thin layer of silt which grades downward over short distances into well-sorted medium

400 m

L~f ~ J

//" /// Cf 2

/ '\

~-:i

A:i:i

NATURAl LEVEE

.........

,

\

\

N A T U R A L LEV([: 2 ( h

~,LLUVIAL

\

\

\

I ,).~i

,L~, <

C.,~r~L,,[L :, . . . .

\

\

--

LITHOFACIES PITT

MEADOWS

PLAIN ('LAy AL

Fig. 13. Lithofaeies map of the Pitt Meadows deposit. Dashed lines show linear channel features visible from aerial photographs. Peat progrades over channel-fill sediments ( C f l ) and associated overbank deposits ( O b l ) w h i c h were deposited prior to peat accumulation along the northern and easter margins. To the south and west peat either intercalates with overbank deposits (ObP) or is eroded by later channel meandering (CfP). As shown, peat overlies flood or alluvial plain clay (Apc) in the remainder o f the deposit.

123 and coarse sand. Along this western margin, interlaminated fine-grained sediments are discontinuous and abruptly terminated by coarser clastic units. To the south, most recent levee sediments consist of interlaminated silty clay, peat, and allochthonous wood fragments, which grade at depth to clay and silty clay {Fig. 20). Several smaller channels are recognized by marked lateral increases in peat thicknesses (Figs. 14, 15, and 16). Such depressions are filled by sharply b o u n d e d units of either sedge-clay, gyttja, and sedge peat, or highly organic fining-upward cycles of fine sand, silt, and clay.

PITT MEADOWS

Fig. 14. Isopach map of the Pitt Meadows deposit. Darker areas show increased peat thickness as a result of infilling of former avulsed channels which trended approximately east-west. Roman numbers (1--3) indicate the location of major cores; letters show crosssection locations of Figs. 15 and 16.

Thin, areaUy restricted crevasse splays of fine sand and silt are confined to the earliest stages of peat development. Such clastic sediments have sharp contacts with both peat and surrounding sediment and are generally massive, pervasively rooted and locally graded. Coarser sediments grade laterally to clay away from charcoal-rich levees.

124

C

~

!

/

-

_-_-- ----.....

V

E

E

T- -

lOOm

/

," /

1 ii1

I SCALE

OLD

_

CHANNEL

F'C-~--~.~" ",,.Z.).=,~[Z--~ - .

"" " - ~ ;

• . . . . .

• ~

-~

,i-

,.. . . .

~

. . . . . . . . .

LEGEND - SPHAGNUM

I

I

- SEDGECLAY

- SPHAGNUM-SEDGE SEDGE-GRASS GYTTJA

- FIRE

I-Z--z-----Z-~Z~

- INORGANIC

~ - _ ~ .

-

SILT CLAY

~

- BRACKISH-MARINESILTY SAND

ORGANICSI L T Y

CLAY

Fig. 15. Cross-sections C-C', D-D' from the Pitt Meadows deposit. Small flood channels (C) are more evident in these two cross-sections. Both cross-sections show small flood channels (C) which have been infilled by sedge-gress, gyttjae, and sedge-clay peats; These sections also show erosion by later channel meandering (BH 112,52, C-C'; B H 101,60, D-D') and progradation of peat over previously deposited sediment (BH 40, C-C'; B H 114, D-D' ). The location of the cross-sections is shown in Fig. 14.

Fig. 16. Cross-sections E-E', F-F' from the Pitt Meadows deposit. On both cross sections, at E' and F', interlamination of peat and overbank silty clay form the natural levee facies. Note the horizontal stratification of peat biofacies except along the margins. Figure 14 shows location of cross-sections.

-~------~

~

~

~

9

8

7b'

7

6 y-

~

- BRACKISH-MARINE SILTY SAND

- GY1-1"JA

- FIRE

- INORGANIC SILT CLAY - ORGANIC SILTY CLAY

AVULSED FLO00 CHANNEL

I

Im /

B

LTURAL EVEE

SCALE

lOOmf

_ . ~ . ~ . ~ . ~ ~_

- - ------------ -------------

- SEDGE-GRASS

L

- SPHAGNUM3EDGE

- SEDGE CLAY

~ IL

- SPHAGNUM

LEGEND

tR

3~

126

c. Biofacies Pitt Meadows biofacies are similar to those already described for the Lulu Island bog apart from the absence of Nuphar peats and the presence of gyttja. Sedge-grass peats of Pitt Meadows, however, originate entirely from freshwater rather than brackish marshes. As a result, ericaceous shrubs like Ledum and Spirea contribute higher concentrations of woody tissue to sedge-grass peats. Ledum continues this trend in the lower portions of Sphagnum peat. The sedge-wood facies, which is exposed along the Fraser River at Lulu Island, was not recognized at Pitt Meadows, but should be extensively developed around much of the bog perimeter on natural levees. Gyttja peat, which is a red-brown to black organic muck, overlies clay or sedge-clay in lenticular beds. This biofacies represents the accumulation of plant debris in shallow pools prior to sedge-grass peat development. Consequently, material is so highly degraded that diatoms and detrital wood and bark fragments are the only components recognizable in microtome section (Styan and Bustin, 1983). d. Stratigraphy Cyclic units of fining upward silt, silty clay, and clay underlie much of the peat deposit. Thin lenses of brown organic clay intercalate with these sediments, forming distinct but gradational contacts. To the north these lithologies are transitional to finely interlaminated clay and silty clay. Within both lithofacies are sharply bounded troughs filled with either organic rich, silty sands or sedge-clay, gyttja, and sedge-grass peats (Fig, 15). Both channel-fill deposits grade into overlying sedge-grass and Sphagnum peats, increasing peat thickness in long linear bands. With the exception of gyttja, which fills small depressions beneath sedge-grass peats, later biofacies are generally horizontally stratified and of nearly constant thickness. Sedge-grass and Sphagnum peats prograde over underlying lithofacies along northern and eastern margins of the deposit. Here, surface peats form sharp and steep contacts with thick mantles of grey, silty clay which overlie coarser channel-fill sediments (Cfl, Fig. 13). To the west, peat intercalates with similar lithofacies over short distances (Cf2 and Ob2, Fig. 13). In some areas, however, these finegrained lithologies are replaced near the surface by beds of well-sorted medium to fine sand which coarsen sharply downward. Deposition of these channel fill units is irregular. In the south, like the west, peats and fluvial silty clays are intercalated (Cf2, Fig. 13). The zone of intercalation, however, is laterally extensive, and progrades vertically toward the present channel position. Much allochthonous woody debris has been incorporated into these sediments. In a similar manner, sedge-grass peats prograde vertically near the channel margin. e. Clay mineralogy and geochemistry Underclay and natural levee deposits at Pitt Meadows are comprised mainly of kaolinite (60%), with minor illite and smectite and smaller amounts of

127

vermiculite and chlorite (Table II). Only trace amounts of mixed-layer smectite-chlorite occur. Near the top of natural levee sediments the percentage of kaolinite increases at the expense of illite. Three cores were obtained for geochemical analysis and petrographic study. Two of these cores, PM 1 and PM 2 (Fig. 17 and 18) were collected from peat-forming environments, while the third was obtained from a natural levee. Several centimeters of Sphagnum peat have been removed by mining from the top of core PM 1. The remaining upper section of this peat has also been desiccated for extended periods of time because of ditching. PM 2, however, is from an undisturbed area of the bog, and represents a natural peat section. Sulphur concentrations are less than 0.5% for all of these cores, and show only a slight increase with depth. In cores PM 1 and PM 2, ash content varies from 0.5 to 1.5% in Sphagnum peats and from 1.0 to 3.0% in sedge-grass peats. Concentrations of ash of up to 7.0% occur in charcoal horizons within highly ericaceous Sphagnum peat. Near the base of the deposit ash values of between 25 and 75% occur in transitional sedge-clay peats.

PM 2

MACROSCOPIC CONSTITUENTS 3.0

0.2

* o o

0.4 ~ . ~ 0.6 0.8

"1-

o o oO

0 °

o oo

1.0

*~<

1.2

~,

1.4

~ o~

1.6"

o;

~'

t.8 - ~

2 . 0 . ~ .^ ^ ^ ^ ^ 2.2.

% TOTAL SULPHUR

pH

4.0

5.0

6.0

O

2.0

4.0

% DRY 6.0

0

ASH 15

20

,!

÷

Ledum stems and roots Oxycoccua leaves Pinus root

10

k

>

f

Carex cuim

Rhync~olpo~ stem Ledum stems and roots

~,'

Ox~coccus runners Ledum stems and leaves Oxycoccus leaves

D

_Carex and Rhynchospora culms O~FOCCUl runners an'-' leaves Rhyncholpora stems Ledum ~tems and roots Carex Spirea ? stems

Care x. Juncas, stems Equisetum rhizomes Spirea ? stems

\

Equisetum Spirea ? stems

Sphagnum p e a t

~

gyttjae

sedge-Sphagnum peat

~

clay

sedga-g. . . . paat

~

s.t

s e d g e - c l a y peat

~

Nuphar peat

~

sand

~)etrography sample

[ ~

fire horizon, charcoal

Fig. 17. Peat profile from Pitt Meadows, showing peat types, macroscopic plant constituents, and analysis of pH, sulphur and dry ash.

128

Where peat has prograded over natural levee silty clay, ash contents average 5% (Fig. 18). Values increase to between 33 and 68% for the finely interlaminated peat and silty clay beneath these zones. pH measurements from each of the Pitt Meadows cores show decreased acidity with depth (Figs. 17--18), with values approaching 5.0. Minor increases in pH occur in charcoal horizons throughout the three cores. MACROSCOPIC CONSTITUENTS

PM 1

0~ o.2 . ' i

':c

0.6

~

o.8

:.~

1.0

% TOTAL SULPHUR

pH 3.0 4.0 2.0 6.0

Ledum branches fern rhizomes

% DRY A S H

b"

Rhyn~hoq)ora stems Bet u l ~ branches

,=

Ledum roots and b r a n c h e s

2

Ox~coccus runners PJnu~ root

KalmLa root Va~cj~ium slems

t

Rhyl~cholpora stems

Ox¥coccu$ runners C.arex culm r~own

~

I

= Typha culm ?

1.4 ~ t,6-

q"e

'\

seeds

stem and root ~quisetum rhizomes

)=.

Carex culm

1.8 LEGEND

~

S~ha~num peat

gyttjae

~

sand

sedge-Sphagnum peat

clay

~- ~

petrography sample

[?.~. ,.go-g .... ~

.ea,

s e d g e - c l a y pe at

~s,, [:~

f---~,,reho.,. . . . .

h. . . . . '

. o . h . . poat

Fig. 18. Peat profile from Pitt Meadows, showing peat types, macroscopic plant constituents, and analysis of pH , sulphur and dry ash. This profile represents the natural levee, and therefore ash contents are very high as a result o f intercalated overbank deposits. Ash content also increases at charcoal horizons.

f. Depositional history Peats near Pitt Meadows initially accumulated in shallow depressions and small flood channels within the alluvial plain of the Fraser River. The growth of these early peats was likely coincident with a declining rate of eustatic sea level rise. At this time, channels which appear to have been active in the north (Cfl, Fig. 13} gradually began infilling with fine sand and silt, and were replaced to the south by the present channel (Cf2, Fig. 13) of the Fraser River. After establishment of this channel, flooding occurred less frequently, thus allowing previously confined peat lenses to spread laterally over the flood plain. Sphagnum colonized the freshwater sedge-grass marshes soon after establisment of the channel. The highly acidic conditions produced by Sphagnum

129 resulted in flocculation of clay minerals at bog boundaries and increased peat accumulation through reduced decomposition. Fluvial activity was thus further restricted, which allowed later peat biofacies to succeed earlier sedgegrass peats and expand the bog environment laterally. Peats prograded over flood sediments on inactive boundaries and intercalated with deposits on active margins. Limited meandering by the present channel has eroded natural levee and peat facies on the western boundary and replaced them with coarse clastic channel-fill deposits. DISCUSSION

Lithofacies and depositional setting Peats began accumulating in quiet facies of several different environments of the Fraser River delta between 4300 and 4800 yr. B.P. (Luternauer and Murray, 1973i Hebda, 1977; Teledyne #1-11 742, 1981). At present, peat deposits extend over approximately one third of the delta surface. Although similarities exist between individual deposits, lithofacies exhibit differences resulting from unique depositional settings developed within the delta (Table II). Boundary Bay peats have accumulated on an inactive portion of the lower delta plain, marginal to the delta front. Unlike the peats of Pitt Meadows and Lulu Island, those of Boundary Bay were not influenced to any great extent by fluvial activity. Rather, they developed from salt and brackish marshes on the broad expanse of inactive tidal flats. Coleman and Smith (1964) have used the term "blanket peats" for these extensive but thin and discontinuous units. Sediment compaction and the rate of eustatic sea level rise control the margins of individual peat horizons. In areas where peat accumulation exceeds sea level rise, peats coalesce into larger "peat islands" (Staub and Cohen, 1979) dominated by glycophyte communities. Otherwise, peat development is restricted or terminated by transgressive marine sediments. Addition of sulphur, increased degradation, and partial erosion may have occurred prior to burial. Thin intercalated units of silt and silty clay occur throughout the peat section at Boundary Bay as a result of the complex interaction of spring freshets, storms, and tides. The occurrence of these washover deposits increases toward the base of the peat, where it grades into underlying fluvial sediments or delta front silty sand. Both of these units overlie an extremely thick sequence of coarsening-upward prodelta silty clay and clay (Blunden, 1973; Fig. 19). Both Lulu Island and Pitt Meadow peats have formed in fluvially-dominated environments (Figs. 20 and 21, and Table I). Earliest peats of these deposits overlie fining-upward sequences of silty sand, silt, and silty clay, and are confined laterally by channel-fill sands and silts. Thin splay units composed of fine sand, silt, and clay intercalate near the base of the peats,

interdistributary clays,

Lithofacies:

Clay minerals:

Peat quality : Sulphur: Ash :

mixed-layer chlorite-smectite common kaolinite/smectite ratio lower

high, pervasive high, pervasive s t o r m s - tides wash over deposits

laterally extensive, thin and irregular

Peat thickness and extent:

grass (fresh) grass (brackish) grass (marine) clay

sedge sedge sedge sedge

Biofacies:

beach sands, channel-fillsilts

thin fining-upward sequence over thick coarsening-upward prodelta sediments

Depositional setting:

Boundary Bay Delta front--lower delta plain

Summary of depositional parameters

T A B L E II

mixed-layer chlorite-smectite at base only kaolinite/smectite ratio moderate

high at base only high at base only crevasse splays floods

laterally extensive thick but variable

nuphar hollow (sedge wood) sedge Sphagnum sedge grass (fresh) sedge grass (brackish) sedge clay

Sphagnum

interdistributary clays, natural levee silty clays, channel-fill sands, silts, crevasse splays, fire splays

thick fining-upward cycle over major coarsening-upward prodelta sediments

Lulu Island Lower delta plain--upper delta plain

kaolinite/smectite ratio high

no mixed-layer chlorite-smectite

low except near base floods

low

laterally restrictive, moderately thick -- constant

sedge grass (fresh) gyttjae sedge clay

Sphagnum sedge Spagnum

interdistributary clays, natural levee silty clays, channel-fill sands, silts, crevasse splays

several small, fining-upward cycles in a major, fining-upward sequence

Pitt Meadows Upper delta plain--alluvial plain

131

though less commonly than at Boundary Bay. Lulu Island sedge-grass peats are different, however, from those of Pitt Meadows, having accumulated under the influence of brackish water and tidal activity in an environment transitional between upper and lower delta plains. As a result, peats deposited initially between distributary channels at Lulu Island are more degraded and contain higher concentrations of both ash and sulphur than analagous freshwater peats at Pitt Meadows. Maximum peat thickness of over 4.0 m occurs in interdistributary regions of the Lulu Island deposit, where channels were abandoned earliest (Fig. 20). In contrast, the thickest accumulation of Pitt Meadows peat occurs where avulsed flood channels have been filled with hypautochthonous plant material. Sphagnum-dominated peat facies succeed earlier sedge-grass and sedgeSphagnum peats in both the Pitt Meadows and Lulu Island deposits. Although of similar generic composition, peats in each deposit interact differently with the surrounding lithofacies. At Lulu Island, where channels were active throughout peat accumulation, overbank silty clays intercalate with sedge peat and are disrupted by occasional splay deposits of sand and silty sand. Such flood deposits are rare in Pitt Meadows peats as a result of welldeveloped natural levees along active channels {Fig. 21). Along inactive margins, sharp boundaries reflect the active progradation of Sphagnum lithofacies and the effectiveness of acidic bog water in flocculating suspended clays (Staub and Cohen, 1978).

/i

[

F

[i~iiiiii! Delta Front Sand

m

ProdeltaClay

BOUNDARY BAY

Fig. 19. Summary model of peat relationships with other lithofacies within the distal lower delta plain environment. Peats overly a thick sequence of coarsening-upward sediment capped by a thin fining-upward fluvial unit. Transgressing marine units erode and alter the already thin and discontinuous peat network.

132

LEGEND Peat Organic Clay Interdistributary Clay Overbank Silty Clay Channel Sand Delta Front Sand

m

LULU I

Prodelta Clay

D

Fig. 20. S u m m a r y model of peat relationships with other lithofacies within the transitional lower delta plain--upper delta plain environment. Peats overly fluvial fining upward sequences which in turn are underlain by a thicker prodelta sequence. Small distributary channels reduce peat thickness and produce numerous overbank and splay deposits while larger fluvial channels erode the total peat section and replace it with channel-fill deposits.

LE Peat

~,~\~ ~.<~! ~ ~,~: ~~ ,

Organic C l a ~ Interdistributary Cla'

~

Oo°°

~:

Z-Z

'°o°O =E_-C- --~--

-

Overbank Silty Clay Channel Sand

De.a Front Sand

PITT MEADOWS

Fig. 21. S u m m a r y model of peat relationships with other lithofacies within the upper delta plain--alluvialplain environment. Peats represent the culmination of a fining upward sequence of fluvial sediments. Unlike lower delta plain peats, these peats infill small avulsed channels and thus are thicker.

133

Biofacies and peat stratigraphy The biofacies of the Lulu Island, Pitt Meadows, and Boundary Bay deposits are related through a c o m m o n successional sequence. This sequence originates from either pioneering marine or freshwater sedge grass communities, and culminates with a climax c o m m u n i t y dominated by Sphagnum, Ledum, and Pinus {Osvald, 1933). External disruptions can interrupt the sequence permanently, as the marine transgression has done to the sedgegrass communities at Boundary Bay, or they can cause temporary reversals, such as those resulting from fires in the Sphagnum communities of both Lulu Island and Pitt Meadows. In all instances, sedge-grass communities initiate the successional sequence by occupying wet disturbed niches and forming marshes. Species composition varies according to salinity, substrate elevation, and pH (Envirocon, 1980). Any sediment influx is trapped b y the rigid n e t w o r k of stems and is quickly stabilized by the extensive root system. Gradually the substrate is built up sufficiently to resist all intrusions of sediment-laden water, and a transition to organic sedimentation occurs. Eventually peat accumulation is sufficient to change the regime from brackish to freshwater conditions and allow the colonization of Spagnum spp. The growth of Sphagnum restricts water flow, reduces pH and limits nutrient supply. These conditions are unfavourable for the continued development of sedge-grass communities, and they are replaced by Pinus and ericaceous shrubs such as Ledum, Vaccinium, and Kalmia. Once this climax stage is reached, fire controls species composition and distribution (Hebda, 1977). Although most peat facies are c o m m o n to each deposit, variation in thickness, lateral extent, and geometry do occur. These differences arise as a result of changes in climate and sedimentary regime throughout the delta. At Lulu Island, due to the influence of brackish water, Sphagnum was unable to colonize early sedge-grass peats. As a result, thick accumulations of sedge-grass peat were required here before the substrate was raised sufficiently to allow other biofacies to succeed. The continued movement of tidal water through the early marshes may also have restricted the development of a gyttja biofacies. The freshwater environment of the Pitt Meadows deposit n o t only allowed the transition to Sphagnum biofacies to occur quickly near the base of the deposit, b u t also allowed a discontinuous gyttja horizon to develop prior to continuous sedge-grass peat accumulation. Sedge-grass peats are thin as a result. The vertical migration of sedge-grass biofacies occurs almost exclusively at the Lulu Island deposit as a result of continual fluvial activity along deposit margins throughout peat accumulation. Correspondingly, the only margin at Pitt Meadows where sedge-peats have prograded vertically is to the south. All other margins are horizontally stratified, similar to the remainder of the deposit. The alluvial plain near Pitt Meadows receives approximately twice the average rainfall of the delta plain, where both Lulu Island and Boundary Bay

134 peats are located. The peat surface at Pitt Meadows is seldom desiccated for extended periods of time, and in comparison, peat facies from this deposit are better preserved, with fewer observable fire horizons, than those peats of the delta plain. Woody material and sedge culms are also more pervasive in Sphagnum and ericaceous Sphagnum peats. During the summer, desiccation of the peat surface at the Lulu Island deposit is severe enough to allow deep and laterally extensive burning. In the winter months, areas where deep burning has occurred fill with water to produce Nuphar hollows (Osvald, 1933). The depressions accumulate allochthonous plant debris and form unusual peats which are characteristic of the delta plain. Although the confinement of the sedge-wood biofacies to the Lulu Island deposit may be caused by the limited exposure of these peats, it may also be related to the degree of natural levee development. The large supply of nutrients necessary to support the large biomass of this c o m m u n i t y could only be provided by~ occasional flooding. The poorly developed natural levees at the Lulu Island deposit have allowed small amounts of clays and nutrients into this facies. This is confirmed by the high ash content of these peats. Peats marginal to channels at Pitt Meadows, in contrast, are low in ash (Figs. 17--18). A combination of well-developed natural levees and highly acidic bog conditions may have restricted the sedge-wood facies to a narrow band along the levee. Here, conditions were not favourable for the preservation of plant material.

Clay mineralogy Only small variations of clay mineral composition occur within bog lithofacies (Table I). Kaolinite and illite are the dominant clay minerals, comprising between 75 and 85% of the total sample, while smectite, vermiculite, and chlorite occur in minor amounts. Kaolinite is enriched in samples affected by low pH conditions, including those from underclay, crevasse splay and natural levee lithofacies (Table I). However, sediments from beneath marine-influenced peats which have been affected by more neutral pH conditions, have smaller concentrations of kaolinite. The variation in kaolinite content within the natural levee lithofacies can be attributed to a response to pH conditions (Fig. 18). In those samples showing the largest kaolinite content, there is a corresponding decrease in smectite. This relationship is most prominent when clay mineral assemblages from peat-forming environments are compared to those reported from the Fraser River and Strait of Georgia by Pharo (1972). Although of similar provenance, these marine-deposited clays contain 30% more smectite and 40% less kaolinite. Smectite must therefore be breaking down under the highly acidic conditions of the Pitt Meadows and Lulu Island bogs and forming kaolinite. A possible reaction is (Berner, 1971): 4Nao.s All.s Mg0.s Si4010 (OH)2 (montmoriUonite) + 6H2CO3 + 19H20 -~ 3A12Si2Os(OH)4 (kaolinite) + 2Mg 2+ + 2Na÷ + 6HCO- + 10H~SiO4

135 Staub and Cohen (1978) describe a similar transformation in the Snuggedy Swamp, South Carolina, and Huddle and Patterson (1961) and others have come to analogous conclusions from study of coal deposits. A mixed-layer smectite-chlorite occurs in brackish influenced clays and increases in abundance in marine influenced sediments of Boundary Bay. This mixed-layer clay was noted by both Pharo (1972) and Mackintosh and Gardner (1966) in sediments collected from the Fraser River. Although the formation of this mixed-layered clay may be dependent on marine conditions, it is likely that it was also present in the peat-forming environment and it t o o was converted to kaolinite.

Geochemistry Measurement of pH in the three peat-forming environments of the Fraser Delta supports earlier research by Staub and Cohen (1978, 1979) and others that near neutral to slightly alkaline pH conditions are associated with marine-influenced peats (Fig. 5), whereas freshwater environments, especially those which contain Sphagnum, exhibit pH values as low as 3.0. In Pitt Meadows cores the underlying freshwater sedge-grass peats are less acidic (Figs. 17--18) however, in cores from Lulu Island, this trend stops and reverses itself in the underlying brackish-water, high-sulphur peats (Fig. 12). The pH decreases to values representative of Sphagnum peats in areas where sulphur concentration is highest. It is suggested that H + ions, released when H2S undergoes further reduction to form amorphous iron sulphide minerals, cause the increased acidity. Measurements of pH from the Boundary Bay core (Fig. 5) show no variation with depth. Total sulphur analyses from the various biofacies of the three peat deposits (Figs. 5, 12, 17, 18 and Tables II and III), indicate that the large differences in sulphur concentration observed are controlled by the environment of deposition. The association of high sulphur concentrations with marine environments is well d o c u m e n t e d (Williams and Keith, 1963, and others). The reason for such a relationship is not only the high concentration of dissolved SO~- in marine water, but also elevated pH conditions which allow bacterial reduction of the sulphate ion (Berner, 1971). Sulphur in marine peats is also concentrated, often by t w o orders of magnitude, above those of freshwater peats (Casagrande et al., 1977) by the continual growth of pyritic sulphur from bacterially reduced H2S. Both Cecil et al. (1980), from studies of Appalachian coals, and Casagrande et al. (1977), from studies of Okefenokee peats, suggest that pH is the controlling parameter in this process. The acidic conditions of freshwater peats ( p H < 4 . 5 ) d e creases the activity of Desulfivibrio spp. (Zobell, 1963), correspondingly reduces H2S production, and lowers the concentration of sulphur incorporated into the peat (Casagrande et al., 1980). Sulphur distribution in Fraser delta peats reflects these principles. Samples from marine influenced peats (Fig. 5) and brackish influenced peats, cores LIE 1 and LIE 2 (Fig. 12), which had

136 'FABLE [II Sulphur content in peat facies Peat facies

Pure Sphagnum Ericaceous Sphagnum Sedge-grass freshwater Sedge-grass brackish Sedge-grass marine Nuphar hollow Sedge-Sphagnum Sedge-clay freshwater Sedge-clay marine

Percent total sulphur (dry weight) Range

Average

0.12--0.19 0.15---0.23 0.14--0.77 0.64--1.50 5.3 --6.3 0.21--0.32 0.19 0.13--0.16 3.3 --5.9

0.16 0.19 0.35 1.12 5.9 0.27 0.19 0.15 4.3

less acidic depositional environments, have characteristically high sulphur concentrations. Correspondingly highly acidic freshwater peat environments (Figs. 14 and 15), have peats of low sulphur content. In less acidic environments where pH does not control the a m o u n t of reduced sulphate, Berner (1971) suggests that the a m o u n t of organic substrate available for bacterial c o n su mp tio n may control the a m o u n t of pyrite and thus total sulphur. Such a relationship may explain the decrease in total sulphur in sediments beneath the peat at Boundary Bay (Fig. 5), where organic m a t t e r concent rat i on is much less than in the overlying peats. Small variations in sulphur with depth in freshwater peats of Lulu Island and Pitt Meadows can be attributed to differences in sulphur concent rat i on in specific biofacies (Table III). S p h a g n u m peats contain the smallest a m o u n t of sulphur, while sedge-wood peats contain the largest concentration. Woody tissues and charcoal cause slight increases in sulphur content. Fires may simply concentrate sulphur in ash, but also the more alkaline conditions associated with the fire horizons may p r o m o t e reduction of SO~- by bacteria. Work by Casagrande et al. (1980) suggests that much of the sulphur of freshwater peats is organic, with a major fraction occurring as ester sulphate. Amounts of pyrite and H2S are correspondingly low. Complete sulphur analysis on these peats has n o t yet been c om pl et ed to confirm these trends. Both the Pitt Meadows and Lulu Island freshwater peats contain small amounts o f ash. Increased concentrations of ash appear to result from w o o d or charcoal in s e d g e - S p h a g n u m and ericaceous S p h a g n u m biofacies and also from crevasse splay silty clays in sedge-grass biofacies. Facies transitional to fluvial sediments, the sedge-clay and gyttja peats, contain large quantities of ash. The high co n cent r a t i on o f ash in the marine derived and influenced peats o f Boundary Bay was caused by the occasional flooding o f this deposit during accumulation and as a result of concent rat i on of organic ash by

137

decomposition of the peat. Suspended clay and silt were deposited after inundations by some annual high tides, the Fraser River freshet, and/or extreme storms. The lower pH of the marsh environment may furthur have assisted in the deposition by causing flocculation of clay materials (Staub and Cohen, 1978}. Fraser River delta peats as coal deposits

Fraser River delta peats will eventually transform into coal seams. Considering a compacting ratio of approximately 10:1 for peat to subbituminous coal {Ryer and Langer, 1980}, the present thickness of peats at Lulu Island could produce a coal seam 40 cm thick. The accumulation of peat at both Pitt Meadows and Lulu Island would certainly have continued, were it not for the recent intervention of man. Each of the three peat-forming environments studied has produced deposits with a similar successional sequence of plant communities. The effects of the physical environment, however, have modified surrounding lithofacies, peat stratigraphy, and ultimately deposit size and shape. Boundary Bay peats will produce thin, discontinuous seams of coal. These seams will be interbedded with thin units of fossiliferous sandy siltstone and silty mudstone. Coal lenses will become more numerous and thicken over interlaminated fluvial mudstone and silty mudstone, until eventually they grade into thick interdistributary seams of the delta plain. Underlying these thin fluvial units will be a thick sequence of coarsening-upward, prodelta silty mudstones, siltstones, and fine-grained sandstones. Boundary Bay coals, unlike those of freshwater origin, will contain large amounts of b o t h ash and sulphur. The ash will be composed primarily of the clay minerals illite and smectite. Sulphur will occur dominantly as pyrite. Both Lulu Island and Pitt Meadows deposits will produce relatively thick coal seams as compared to Boundary Bay. In contrast to the alluvial plain seam at Pitt Meadows, the delta plain coals of Lulu Island will form a laterally extensive n e t w o r k of seams. Individual seams will be separated by fluvial channel-fill units of graded sandstone and siltstone. Coals proximal to channels will contain numerous partings of silty mudstone which ultimately will reduce seam thickness. Seams will also thin appreciably over a number of smaller silty mudstone distributary channels, cresting along narrow want areas. Coals between these want areas will be underlain b y mudstones composed mainly of kaolinite and illite, and because of their brackish water origin, they will l~ave high pyrite and ash contents. The remainder of the seam will contain better quality coal. The Pitt Meadows peat will form a small isolated seam, surrounded by channel-fill sandstone and associated levees of argillaceous siltstone. Thinly bedded mudstones underlying the seam will be composed chiefly of kaolinite, and will grade downward into well-sorted siltstones and fine-grained sandstones. In contrast to the seams of Boundary Bay and Lulu Island, those

138 of Pitt Meadows will contain few partings, and will maintain a nearly constant thickness. Concentrations of sulphur and ash will be low, and as a result, coals produced will be of better quality, Due to seam characteristics, the Pitt Meadows coals will be the easiest to mine but may also be the hardest seams to locate unless initially exposed at the surface. The thick sequence of sandstone which will surround the Pitt Meadows seam is not broken by sections of finer marine sediment, as they are on the delta plain. The formation of coals within this sequence is unpredictable, as numerous environments similar to that at Pitt Meadows contain no peat. In general the peat-forming environments of the Fraser Delta are not unlike those of the Mississippi Delta outlined by Fisk (1958) and Frazier and Osanik (1969) or other subtropical and tropical peat-forming areas (Anderson, 1964; Staub and Cohen, 1978). The principal difference is the nature of the biofacies and the interaction of biofacies and lithofacies. The Fraser Delta peats are characterized by the paucity of arborescent (woody) vegetation whereas most tropical and sub-tropical swamps (even those that are oligotrophic) contain a significant arborescent component. The resulting coals from the Fraser Delta peat may thus be relatively impoverished in thick vitrain (bright) bands as compared to coal formed from sub-tropical or tropical peats. An additional difference is the nature of the transition between biofacies and between biofacies and lithofacies. The peats from Lulu Island and Pitt Meadows change facies over short distances laterally and vertically in response to edaphic controls and fires. The vegetation is capable of controlling the water table level and thus substrate elevation such that formation of raised bogs is facilitated. The peat progrades over overbank, levee and channel deposits at relatively high angles. The topography of the peat may thus exert control over the distribution of lithofacies and laterally confine channels and extent of overbank flooding. Such control of the lithofacies distribution may occur where ever raised bogs form. Although raised bogs are not restricted to cool climates (see Anderson, 1964) they are typical of peat deposits of cool climates. The net accumulation rate of the Lulu Island and Pitt Meadows peats, based on very few radiocarbon dates, is considered approximately 1 mm/yr. Such a rate is similiar to that reported from some swamps of Florida (1.3 m m / y r ) and the Mississippi Delta (1 m m / yr) by Fisk {1960} and somewhat lower than reported from tropical peats of Borneo {3--4 m m / y r ; Anderson, 1964). The same general rates of net accumulation of the Fraser Delta peats and the tropical and subtropical peats supports the argument that, although the growth rate o f vegetation is much slower in cool climates, the rate of decomposition is also slower, such that the net accumulation rate of peat from the different climates may be similar. An additional factor that should be considered in comparing peats but has not been quantified is the degree of post-depositional compaction. Peats of the Fraser Delta, which lack significant amounts of w o o d y material would undoubtedly undergo greater compression with burial and thus form less coal/metre of peat then peats with a higher wood component.

139 SUMMARY AND CONCLUSIONS

Three peat-forming environments were studied from different depositional settings on the Recent lobe o f the Fraser River delta (Table III). Peats which have accumulated on the inactive portion of the distal lower delta plain developed from widespread salt and brackish marshes. These peats were not influenced appreciably by fluvial activity, allowing the lateral development of marsh facies to be controlled by a combination of compaction and eustatic sea level rise. As a result, an extensive thin but discontinuous peat network developed, containing numerous intercalated silty clay laminae and high concentrations of sulphur. These peats overlie either fining-upward silty sand, silt and silty clay of fluvial origin or homogeneous fine and silty sand of marine origin. Underlying both of these thin units is a major coarseningupward sequence of prodelta silty clay and silt. Peat deposits transitional between the lower and upper delta plain have accumulated in fluvial-dominated environments. The relatively thin fluvial units are underlain by a thick sequence of prodelta sediments similar to those accompanying distal lower delta plain peats. Earliest peats of these deposits, having developed from interdistributary brackish marshes, contain large amounts of sulphur and numerous fine-grained splay deposits. Individual peat horizons are underlain by a fining-upward sequence of fine sand, silt, and silty clay, and are confined laterally by silt and silty clay of small distributary channels. Thickest peats occur in areas where these channels were abandoned earliest. Except near the margins of active channels or marine transgressive areas, Sphagnum-dominated biofacies gradually replace sedge-grass peats upsection. These later peats contain less sulphur and fewer splays. Peat deposits which have developed near the boundary between the alluvial and upper delta plain represent the culmination of a major fining-upward sequence. Unlike lower delta plain environments, an underlying sequence of prodelta sediments is absent. Initial sedge-grass peats overlie interlaminated silt and clay or fining-upward cycles of silty sand, silt, and clay of flood origin. These peats have not been influenced by brackish water, and as a result, contain low concentrations of sulphur. The presence of well-developed natural levees reduces the number and size of splays, restricting them to sedge-grass peats marginal to active channels. O n other boundaries, peats prograde over silty clay with sharp contacts. Thickest peats occur in belts where small avulsed flood channels have been filled with sedge-clay, gyttja, and sedge-grass peat. Biofacies present in each of the three peat-forming environments are related through a c o m m o n successional sequence. The sequence is initiated when pioneering marine or freshwater sedge-grass communities occupy wet, disturbed niches and modify them sufficiently to restrict clastic sedimentation. Provided the substrate has been raised above brackish water influence by accumulating organic matter, Sphagnum soon colonizes the habitat. Once

140 established, it creates acidic, oligotrophic conditions which allow only acidophiles to survive. Ericaceous shrubs, Pteridium and Pinus contorta, form the balance of the climax successional stage. In upper delta plain--alluvial plain environments where peats originate from freshwater marshes, the substrate requires little modification before Sphagnum is able to colonize. As a result, sedge-grass peats of alluvial plain deposits are much thinner than similar facies originating on the delta plain. Nuphar peats occur only in Sphagnum and ericaceous Sphagnum biofacies of delta plain peats. Their exclusion from alluvial plain peats results from greater precipitation in this region. Gyttja peats are absent from lower delta plain deposits because tidal activity prevents the accumulation of organic matter in pooled water niches. Distal lower delta plain peats will produce laterally extensive but thin and lenticular coal seams. These coals will contain, in addition to large concentrations of pyritic sulphur, high ash contents resulting from numerous thin splits. Consequently, both coal quality and calorific value will be low. Numerous thick coal seams will be developed from transitional lower delta plain-upper delta plain peats. Individual seams will be separated by channelfill deposits, with coals being thinned appreciably by numerous splits marginal to these channels and by smaller channel-fill units throughout the remainder of the seam. Coal quality at the base of seams is poor, comparable to that present in distal lower delta plain coals. The remaining upper portion of the seam, however, will contain only small amounts of both sulphur and ash. Alluvial plain peats will produce isolated thick coals surrounded by channel-fill sands. Seam thickness will remain nearly constant throughout, with very few splits. As a result of low ash and sulphur contents, a good quality coal will be produced. ACKNOWLEDGEMENTS We wish to thank Drs. G.E. Rouse and W.C. Barnes for their assistance in plant anatomy, palynology and geochemistry and for m a n y useful comments on an earlier draft of this paper, Financial support was received from the University of British Columbia and Natural Sciences and Engineering Research Council Grant 67-7337.

REFERENCES Allen, E.A.D., 1978. Petrography and Stratigraphy of Holocene Coastal-marsh Deposits along the Western Shore of Delaware Bay. Ph.D. Thesis, Univ. Delaware, Department of Geology, 287 pp. Anderson, J.A.R., 1964. The structure and development of the peat swamps of Sarawak and Brunei. J. Trop. Geogr., 18: 7--16. Bayliss, P., Levinson, A.A. and Klovan, J.E., 1970. Mineralogy of bottom sediments, Hudson Bay. Bull. Can. Pet. Geol., 18: 469--473.

141 Berner, R.A., 1971. Principles of Chemical Sedimentology. McGraw Hill, New York, N.Y. N.Y., 240 p. Blunden, R.H., 1973. Urban geology of Richmond, British Columbia. Adventures in Earth Sciences Series no. 15, Department of Geological Sciences, The University of British Columbia, Vancouver, 13 pp. Casagrande, D.J., 1970. Geochemistry of Amino Acids in selected Florida Peats. Ph.D. Thesis, The Pennsylvania State University (unpubl.). Casagrande, D.J. and Erchull, L.D., 1976. Metals in Okefenokee peat-forming environments: relation to constituents found in coal. Geochim. Cosmochim. Acta, 40: 387-393. Casagrande, D.J. and Ng, L., 1979. Incorporation of elemental sulphur in coal as organic sulphur. Nature, 282: 598--599. Casagrande, D.J. and Park, K., 1978. Muramic acid levels in Okefenokee peat: the role of microorganisms in the peatforming sy stem. Soil Sci., 125: 181--183. Casagrande, D.J., Seifert, K., Berschinski, C. and Sutton, N., 1977. Sulfur in peat-forming systems of the Okefenokee Swamp and Florida Everglades: origins of sulfur in coal. Geochim. Cosmochim. Acta, 41: 61--167. Casagrande, D.J., Idowu, G. and Friedman, A., 1979. H2S incorporation in coal precursors: origins of organic sulphur in coal. Nature, 282: 599--600. Casagrande, D.J., Gronli, K. and Sutton, N., 1980. The distribution of sulfur and organic matter in various fractions of peat: origins of sulfur in coal. Geochim. Cosmochim. Acta, 44: 25--22. Cecil, C.B., Stanton, R.W. and Dulong, F.T., 1980. Geological controls on sulfur content in coal. Bull. Am. Assoc. Pet. Geol., 64: 689--690. Cohen, A.D., 1968. The Petrology of Some Peats of Southern Florida (with Special Reference to the Origin of Coal). The Pennsylvania State University, Department of Geology, Ph.D. Thesis, 352 pp. Cohen, A.D., 1970. An allochthonous peat type from southern Florida. Geol. Soc. Am. Bull., 81 : 2477--2482. Cohen, A.D., 1973. Petrology of some Holocene peat sediments from the Okefenokee swamp-marsh complex of southern Georgia. Geol. Soc. Am. Bull., 84: 3867--3878. Cohen, A.D. and Spackman, W., 1977. Phytogenic organic sediments and sedimentary environments in the Everglades-mangrove complex, Part II. The origin, description, and classification of the peats of southern Florida. Palaeontographica, 162B: 71--114. Cohen, A.D. and Spackman, W., 1980. Phytogenic organic sediments and sedimentary environments in the Everglades-mangrove complex, Part III. The alteration of plant material in peats and the origin of coal macerals. Paleontographica, 172B: 125--149. Coleman, J.M. and Smith, W.G., 1964. Studies of Quaternary sea level. Coastal Studies Institute, Louisiana State University, Tech. Rep. 20, pp. 833--840. Envirocon, 1980. Fraser River Estuary habitat development program, Criteria Summary Report. Prepared for Department of Supply and Services, Fisheries and Oceans, Canadian Wildlife Service and Public Works Canada by Envirocon Limited, Vancouver, 147 pp. Fairbridge, R.W., 1976. Effects of Holocene climatic change on some tropical geomorphic processes. Quat. Res., 6: 529--556. Fisk, H.N., 1958. Recent Mississippi River aedimentation and peat accumulation. In: E. van Aelst (Editor), Congres pour l'Advancement des Etudes du Stratigraphique et de G~ologie du Carbonif~re. 4th, Heerlen, Compte Rendu, 1: 187--199. Frazier, D.E. and Osanik, A., 1969. Recent peat deposits; Louisiana coastal plain. In: E.C. Dapples and M.E. Hopkins (Editors), Environments of Coal Deposition. Geol. Soc. Am., Spec. Pap., 114: 63--86. Given, P.H., 1972. Biological aspects of the goechemistry of coal. In: Advances in Organic Geochemistry. Int. Series of Monographs in Earth Series, 33: 69--92.

142 Given, P.H. and Miller, R.N., 1971. Distribution of forms of sulphur in peats from saline environments in the Florida everglades. Abstr. Geol. Soc. Am. Meeting Miami, p. 580. Gleason, P.J., Piepgras, D., Stone, P.A., et al., 1980. Radiometric evidence for involvement of floating islands in the formation of Florida Everglades tree islands. Geology, 8: 195--199. Hansen, H.P., 1940. Paleoecology of two peat bogs in southwestern British Columbia. Am. J. Bot., 27: 144--149. Hebda, R.J., 1977. The Paleoecology of a Raised Bog and Associated Deltaic Sediments of the Fraser River Delta, British Columbia. Ph.D. Thesis, The University of British Columbia, Department of Botany, 200 pp. Horn, J.C. and Fern, J.C., 1979. Carboniferous Depositional Environments in the Appalachian Region. 760 p. Huddle, J.W. and Patterson, S.H., 1961. Origin of Pennsylvanian underclay and related seat rocks. Geol. Soc. Am. Bull., 72: 1643--1660. Johnston, W.A., 1921. Sedimentation of the Fraser River Delta. Geol. Surv. Can., Mem. 125, 46 pp. Johnston, W.A., 1922. The character of stratification of the sediment in the Recent delta of Fraser River, British Columbia, Canada. J. Geol., 30: 115--129. Kellerhals, P. and Murray, J.W., 1969. Tidal flats at Boundary Bay, Fraser River Delta, British Columbia. Bull. Can. Pet. Geol., 17: 67--91. Koch, J., 1966. Petrologische Untersuchungen an jungpleistozanen Schieferkohlen aus dem Alpenvorland, der Schweiz und Deutschlands mit Vergleichsuntersuchungen an Torfen. Diss. Technische Hochschule Aachen, 186 pp. Koch, J., 1970. Petrologische Undersuchungen an niedersachsischen Torfen und Welchbraun Kohlen. Geol. Mitt., 10: 113--150. Lavkulich, L.M., 1978. Methods manual, Pedology Laboratory. Department of Soil Science, The University of British Columbia, Vancouver, 224 pp. Luternauer, J.L. and Murray, J.W., 1973. Sedimentation on the western delta-front of the Fraser River, British Columbia. Can. J. Earth Sci., 10: 1642--1663. Lyngberg, E., 1979. The Palynology and Vegetative Succession of a Peat Bog located at northern Pitt Meadows, British Columbia. B.Sc. Thesis, The University of British Columbia, Department of Botany, 34 pp. MacKintosh, E.E. and Gardner, E.H., 1966. A mineralogical and chemical study of Lower Fraser River alluvial sediments. Can. J. Soil Sci., 46: 37--46. Mathews, W.H. and Shepard, F.P., 1962. Sedimentation of Fraser River Delta, British Columbia. Bull. Am. Assoc. Pet. Geol., 46: 1416--1438. Milliman, J.D., 1980. Sedimentation in the Fraser River and its estuary, southewestern British Columbia. Estuarine Coastal Mar. Sci., 10: pp. 609--633. Osvald, H., 1928. Mossar ach mosskulture I Nordamerika. Sven. Mosskulturf'6ren. Tidskr., Vols. 42 and 43. Osvald, H., 1933. Vegetation of the Pacific Coast bogs of North America: Acta Phytogeographica Sueica, v. Almkvist and Wiksells, Botrycheri-A.B., Uppsala, 34 p. Osvald, H., 1970. Vegetation and Stratigraphy of Peatlands in North America. Uppsala, Vetenskapssocieteten, Stockholm, Almkvist och Wiksell, 96 pp. Pharo, C.H., 1972. Sediments of the Central and Southern Strait of Georgia, British Columbia. Ph.D. Thesis, Department of Geology, The University of British Columbia, 290 pp. Rampino, M.R. and Sanders, J.E., 1981. Episodic growth of Holocene tidal marshes in the northeastern United States: a possible indicator of eustatic sea level fluctuations. Geology, 9: 6 3 - 6 7 . Rigg, G.B. and Richardson, C.T., 1938. Profiles of some Sphagnum bogs of the Pacific coast of North America. Ecology, 19: 408--434. Ryer, T.A. and Langer, A.W., 1980. Thickness changes involved in the peat-to-coal transformation for a bituminous coal of Cretaceous age in central Utah. J. Sediment. Petrol., 50: 987--992.

143 Shepperd, J.E., 1981. Development of a Salt Marsh of the Fraser Delta at Boundary Bay, B.C., Canada. M.Sc. Thesis, The University of British Columbia, Department of Geological Science, 143 pp. Spackman, W., Reigel, W.L. and Dolsen, C.P., 1969. Geological and biological interactions in the swamp-marsh complex of southern Florida. In: E.C. Dopples and M.E. Hopkins (Editors), Environments of Coal Deposition. Geol. Soc. Am., Spec. Pap., 114: 1--36. Spackman, W., Cohen, A.D., Given, P.H. and Casagrande, D.J., 1974. The comparative study of the Okefenokee Swamp and the Everglades mangrove swamp-marsh complex of southern Florida. Field Guide Book. Geol. Soc. Am., Meeting Miami, p. 265. Staub, J.R. and Cohen, A.D., 1978. Kaolinite-enrichment beneath coals: a modern analog, Snuggedy Swamp, South Carolina. J. Sediment. Petrol., 48: 203--210. Staub, J.R. and Cohen, A.D., 1979. The Snuggedy Swamp of South Carolina: a backbarrier estuarine coal-forming environment. J. Sediment. Petrol., 49: 133--144. Styan, W.B. and Bustin, R.M., 1981. Sedimentology, petrography and geochemistry of some peat deposits of the Fraser River Delta. Program with Abstracts, Joint Annual Meeting, Geol. Assoc. Can.; Mineral. Assoc. Can., 6: 54. Styan, W.B. and Bustin, R.M., 1983. Petrography of some Fraser Delta Peat Deposits: coal maceral and microlithotype precursors in temperate-climate peats. Int. J. Coal Geol., 2: 321--370. Swinbanks, D.D., 1979. Environmental Factors controlling Floral Zonation and the Distribution of Burrowing and Tube-dwelling Organisms on Fraser Delta Tidal Flats, British Columbia. Ph.D. Thesis, The University of British Columbia, Department of Geological Sciences and Institute of Oceanography, 274 p. Williams, E.G. and Keith, M.L., 1963. Relationship between sulfur in coals and the occurrence of marine roof beds. Econ. Geol., 58: 720--729. Zobell, C.E., 1963. Organic geochemistry of sulfur. In: I.A. Breger (Editor), Advances in Organic Chemistry. Pergamon Press, New York N.Y., pp. 573--578.