Sediments of the Middle Cambrian Burgess Shale, Canada D A V I D J. W. PIPER

Piper, D. J. W.: Sediments of the Middle Cambrian Burgess Shale, Canada. Lethaia, Vol. 5, pp. 169-175. Oslo, April 15th, 1972.

The Phyllopod Bed of the Burgess Shale, in which Walcott found the famous soft bodied fossils, consists of thin graded beds of calcareous siltstone and mud- stone, which are probably turbidites. The Burgess Shale was deposited on a reef front submarine fan, and the preservation of the fossils is probably due to rapid burial.

David J. W. Piper, Department of Geology, Sedgwick Museum, Downing Street, Cambridge CB2 3EQ, August loth, 1971.

The Middle Cambrian Burgess Shale of south-east British Columbia, Canada, is famous for the well preserved soft bodied fauna discovered and described by Charles D. Walcott in the earlier part of this century (Whit- tington, 1971). It is part of the Stephen Formation - a 200 to 400 m thick sequence of shales deposited in front of the Cathedral Formation reef (Fig. 1 and Fritz, 1971). In detail, there are a number of distinct shale, mudstone, and siltstone lithologies present, but their field distribution is as yet unknown. The soft bodied fossils are almost restricted to the 2 m thick ‘Phyllopod Bed’ exposed in Walcott’s quarry (Whittington, 1971). Most of the work described is based on specimens from the Phyllopod Bed and immediately overlying strata in this quarry, kindly made available by Prof. H. B. Whit- tington. This locality and other prcrts of the Stephen Formation have also been examined in the field.

Lithology The Phyllopod Bed consists of sharp based units of calcareous siltstone grading up through alternating laminae into mudstone (Fig. 2). Individual units are from 10 to 50 m thick, but typically in the lower part of this range. A similar lithology is found elsewhere in the Stephen Formation, notably in the section immediately above the Phyllopod Bed in Walcott’s quarry, and in a small part of the section near the reef front on Mount Stephen. However, in the localities where soft bodied fossils are found (Walcott’s quarry, and Raymond’s quarry 20 m above it), this graded cal- careous siltstone and mudstone tends to split along bedding planes, in con- trast to the other localities, where the fracture is more irregular and oblique to bedding. This bedding plane fissility in the Phyllopod Bed and the rocks

170 DavidJ. W. Piper

WEST EAST

- 1 - thick STEPHEN Fm --I __

500

m

L O 0 km 1 -

Fig. 1 . Schematic east-west cross section through the Burgess Shale and associated rocks showing distribution of shale and carbonate facies and location of Walcott’s quarry. Sim- plified from Fritz, 1971, Fig. 2.

Burgessio Morrello

Anomolocoris Ottoio

Woptio Ottoio Conodio Agnostids Brochiopods

Fig. 2. Schematic representation of the sequence of lithologies and fossils in graded beds of the Phyllopod Bed of the Burgess Shale. For fuller ex- Scenello Hyolithes

AgnoStids BrachioPods planation, see section on ‘The lami- nation’ in text.

of Raymond’s quarry probably results from the many streaks of carbonaceous material, visible in thin section parallel to the bedding, preventing diagenetic clay mineral bonding between laminae. Such carbonaceous streaks are not known in other developments of the calcareous graded siltstone and mud- stone. The preservation of carbon in the Phyllopod Bed probably reflects an anaerobic environment, as suggested by Whittington (1971).

The whole Stephen Formation within 500 m of the reef front has been protected from tectonic deformation by the massive limestones above, below, and to the east (J. D. Aitken, personal communication, 1970; cf. Fig. 1).

Burgess Shale 171

-

Fig. 3. Eroded base to a bed in the rocks immediately overlying the Phyllopod Bed of the Burgess Shale, southern end of Walcott’s quarry, at approximately the 17 feet level of Whit- tington, 1971. Scale bar is approximately 10 cm.

Scanning electron micrographs of the Burgess Shale show only a poor preferred orientation of clay minerals in the bedding plane ; this probably resulted from diagenetic growth of clays under conditions of reduced over- burden pressure, and may explain why most Stephen Formation lithologies have a poor bedding plane fissility.

Bioturbation, usually simple vertical burrows and horizontal tracks, is found in parts of the Stephen Formation, including in some of the graded siltstone and mudstone sequences, but is absent in the Phyllopod Bed, suggesting a lack of living benthos there.

Turbidite origin of the Phyllopod Bed sediments The laminated calcareous siltstonks of the Phyllopod Bed are interpreted as turbidites on the following grounds: (1) Parts of the Stephen Formation contain allochthonous masses of shallow water limestone (Rasetti, 1951 : 45 and P1. VI), suggesting that the visible facies change from reef limestone to shale was accompanied by a consid- erable relief difference in middle Cambrian times. Fritz (1971) achieved accurate correlation by fossils of the limestone and shale facies, and showed that at this time the escarpment between the two was at least 680 f t high. Thus the minimum depth of water in the Stephen Formation was around 200 m. (2) Within the Stephen Formation there is little material to form the source of carbonate for the calcareous laminae of the Phyllopod Bed (which includes

172 David J. W. Piper

Fig. 4. Thin sections of graded beds from the Phyllopod Bed of the Burgess Shale. Scale bar 1 mm. A. thin section DJWP 3/015; B. 3/043; C. 3/017. All from field number 74995, level 8 feet 7 ins - 8 feet 10 ins of Whittington, 1971.

fecal pellets and bioclastic debris). These carbonates could be readily derived from limestones of the Cathedral Formation facies. (3) The orientation of Marrella (Whittington, 1971) and other fossils in the Phyllopod Bed suggests that they were catastrophically buried in sediment. (4) Although the Phyllopod Bed contains algae, they are associated with fossils showing catastrophic burial, and are thus presumably redeposited. (5) The calcareous siltstone beds seen in the field are invariably sharp based. One bed in the rocks overlying the Phyllopod Bed in Walcott’s quarry has an elongate scour on its base, oriented NW-SE (Fig. 3). (A NW-SE pre-

Burgess Shale 173

ferred orientation of silt grains in the bedding plane was found in the only available oriented specimen from Walcott’s quarry). (6 ) Conical shells of Scenella are common in the lowest parts of the coarser siltstone beds and, although mainly oriented convex up (Fig. 4A), a propor- tion are oriented concave up. This presence of concave up specimens is suggestive of rapid deposition, rather than winnowing (cf. Middleton, 1967 : 229). (7) There is an overall grading of the beds from coarse siltstone through into fine mudstone. The well developed lamination in the graded sequence, with well sorted silty laminae, is similar to that seen in other fine grained turbidites (Piper, in press).

The lamination

Fig. 2 shows a typical graded laminated unit from the Phyllopod Bed. Many beds do not show this complete sequence: the lower parts of the sequence, and less commonly, the upper fine mudstone, may be absent.

The lowest parts of coarse graded beds are of calcareous siltstone, with distinctly irregular laminae (Figs. 4A, 4C, lower parts). Mudstone laminae are absent.

This is overlain by alternating laminae of mudstone and well packed cal- careous siltstone, with an upward decrease in the thickness, coarseness, and packing of the siltstones, and a parallel increase in the thickness of mudstone laminae (Figs. 4A, 4B, middle parts). Individual laminae may have sharp or gradational upper and lower boundaries, but most have increasing con- centrations of silt grains passing up through the laminae, and have sharp tops. The mudstones contain a little silt sized carbonate material.

There then follows mudstone laminae alternating with carbon rich lami- nae with some calcareous silt sized clasts (Fig. 4B, upper part). The amount of carbon gradually increases upwards through a lamina; the tops of the silty carbon rich laminae are sharp, while the bases are gradational through from mudstone. Where the base of a graded laminated unit is in this carbon rich lithology, there is an upward decrease in the amount of carbon in the lowest of the laminae.

The highest part of a graded bed consists of mudstone, with a little silt sized carbonate, without any visible lamination.

No way has been found of distinguishing hemipelagic deposits in the Phyllopod Bed. The mudstones, whether carbonaceous or not, have sensibly constant clay mineral compositions, consisting mainly of mica group clays. Large, demonstrably diagenetic muscovite is found, and the whole clay mineral assemblage has probably been profoundly affected by diagenesis. Diagenetic chlorite is seen in thin section (and confirmed by x-ray diffraction) to be common in the calcareous lithologies. Small amounts of diagenetic apatite and dolomite have been found in some calcareous siltstones. Some detrital quartz is found in the calcareous siltstones.

174 Daniel J. W. P@er

Fig. 5. Inferred depositional environment of the Burgess Shale. For explanation of A and B, see text.

A few thin sections contain features which do not fit with the simple model of graded calcareous siltstone to mudstone. These include thin cal- carenite laminae apparently unrelated to graded bed sequences, but since the hand specimens from which these slides are cut are small and not precisely located, it is difficult to determine their megascopic relations. They could be accounted for by a number of processes, including ordinary trac- tional bottom currents, but insufficient data are available.

A model of Phyllopod Bed deposition The calcareous siltstones of the Phyllopod Bed (and other parts of the Stephen Formation) were deposited from turbidity currents derived from the nearby Cathedral Formation reef, possibly forming a deep sea fan-like topography from apices at gaps in the reef front (Fig. 5). The carbonate reef was not the direct source of the large amounts of mudstone deposited in the same graded laminated beds as the calcareous siltstones. Neither was it a likely source for most of the fossil animals of the Phyllopod Bed, a number of which are identical to species autochthonous to other parts of the Stephen Formation (Fritz, personal communication, 1970). The clay and animals were probably eroded from deposits at intermediate depths by the turbidity currents. Such a process occurs at the present’ day on La Jolla deep sea fan (Piper, 1970), where turbidity currents begin entirely as sand from the canyon heads, but erode and ultimately deposit much clay, and even in some cases deep water brachiopods as well (Piper, 1970 : 223). The eroded autochthonous muds accumulated from clays swept over the reef

Burgess Shale 175

front by wave action and settling out in the area immediately in front of the reef. Possibly analogous bioturbated clays are now found very close to the reef front in the Stephen Formation. The soft bodied animals living in these muds were destroyed by the normal processes of postmortem decay, except for those swept into the anaerobic Phyllopod Bed environment. There is no reason to suppose that turbidity currents carrying only clay and silt are sufficiently turbulent (except at their head) to break up soft bodied animals transported only a short distance.

There are two possible types of anaerobic environment in this geological setting. One is in a shallow ponded basin, perhaps trapped between two coalescing fans and the reef front, with stagnant bottom waters depleted in oxygen (A in Fig. 5). Present day turbidity currents entering anaerobic basins (such as the Santa Barbara Basin; Hulsemann and Emery, 1961) replenish the basin with aerated water, and permit the introduction of benthos, until the oxygen is again used up.

Alternatively, deposition in a fan valley may be sufficiently rapid to in- hibit bioturbation and decay (B in Fig. 5). Unbioturbated graded muds from La Jolla Fan valley (Piper, 1970: 217) are found in a parallel modern environment. A shallow surface layer of bioturbated mud may be eroded by the turbidity current before depositing. The proximity of the Phyllopod Bed to the reef front favours this interpretation.

The postulated presence of thick hemipelagic muds near the reef suggests that much of the unlaminated mudstone in the upper parts of beds in the Phyllopod Bed is also hemipelagic.

Acknowledgements. - I thank J. D. Aitken and W. H. Fritz for hospitality and discussion in the field; Prof H. B. Whittington for samples; and Whittington and C. P. Hughes for discussion. Prof. B. M. Funnel1 provided scanning electron microscope facilities at the Univer- sity of East Anglia. This work was carried out during the tenure of a Research Fellowship from Jesus College, Cambridge. Travel funds were provided by Jesus College and the Depart- ment of Geology, Cambridge.

R E F E R E N C E S

Fritz, W. H. 1971 : Geological setting of the Burgess Shale. North Am. Paleont. Convention, Chicago, 1969, Proc. I , 1155-1 170. Lawrence, Kansas.

Hulsemann, J. & Emery, K. 0. 1961 : Stratification in Recent sediments of Santa Barbara Basin as controlled by organisms and water character. your. Geol. 69, 279-290. Chicago.

Middleton, G. V. 1967: The orientation of concavo-convex particles deposited from ex- perimental turbidity currents. Jour. Sed. Petrology 37, 229-232. Menasha, Wisconsin.

Piper, D. J. W. 1970: Transport and deposition of Holocene sediment on La Jolla deep sea fan, California. Marine Geol. 8, 21 1-227. Amsterdam.

Piper, D. J. W. In press: Turbidite origin of some laminated mudstones. Geol. Mag. 109. Rasetti, F. 1951 : Middle Cambrian stratigraphy and faunas of the Canadian Rocky Moun-

tains. Smithsonian Misc. Collns. 116, 5. 277 pp. Washington. Whittington, H. B. 1971 : The Burgess Shale: history of research and preservation of fossils.

North Am. Paleont. Convention, Chicago, 1969, Proc. I , 1170-1 201. Lawrence, Kansas.