southern Rocky Mountains, Alberta. - Ibexian (= Canadian; Early Ordovician). - patch reefs. - up to 2 m thick and 10-15 m wide. - open carbonate shelf or sub- ...
2 9 . Reels, Canada and Adjacent Area H.H.J. Geldsetzer, N.P. James and G.E. Tetabutt, Editors Canadian Society of Petroleum Geologists Memor 13, 1989, p. 213 217
EARLY ORDOVICIAN CRYPTALGAL-SPONGE REEFS, SURVEY PEAK FORMATION, ROCKY MOUNTAINS, ALBERTA BRIAN R. PRATT1
General Location
- southern Rocky Mountains, Alberta
Age
- Ibexian (= Canadian; Early Ordovician)
Reef Type
- patch reefs
Dimensions
- up to 2 m thick and 10-15 m wide
Depositional Setting
- open carbonate shelf or sub-basin
Tectonic Region
- Bow Platform
Crustal Position
- continental margin
Foundation below Reef
- Survey Peak Formation; grainstone and rudstone tempestites
Bathymetric Range
- moderately deep—below fair weather wavebase, within storm wave-base
Reef-forming Process
- accretion and cementation of cyanobacterial mats and growth of lithistid sponges
Dominant Organism(s)
- cyanobacterial mats, lithistid sponges
Diagnostic Aspect(s)
- framework inconspicuous; massive lime mudstone and wackestone
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the elements form a thrombolite mound or reef. This paper is the first where detailed observations on "type" thrombolite reefs are presented.
INTRODUCTION
The suite of patch reefs in Cambrian and Ordovician limestones of the southern Rocky Mountains was the inspiration for Aitken's (1967) pioneering study of the reef building role of cyanobacteria (blue-green algae) in the early Paleozoic. Small patch reefs occur abundantly in the "upper massive member" of the Lower Ordovician Survey Peak Formation. Sponges are important accessory frame-builders in these reefs (Rigby, 1965), presumably reflecting initial stages of diversification of reef building metazoans at this time. These reefs were called "cryptalgal" because they were constructed by mat-forming micro-organisms (mainly cyanobacteria by inference) which have left no direct fossil remains (Aitken, 1967). Individual reef mounds were termed "thrombolites", the framework elements of which were termed "clots". These terms have been modified slightly to conform with parallel usage of terms for laminated cryptalgal structures, i.e. stromatolites, such that thrombolite now refers to the individual framework elements (Pratt and James, 1982). These elements often exhibit a microstructure of clotted micrite; together,
STRATIGRAPHY
The Survey Peak Formation consists of variably interbedded shale, calcareous siltstone, bioclastic grainstone, intraclastic rudstone, lime mudstoneand thrombolite boundstone at the type section at Mount Wilson (52°00'N, 116°45'W; Fig. 1). The formation has been subdivided informally into four members: the basal silty member, putty shale member, middle member and upper massive member (Aitken and Norford, 1967). Shale and siltstone dominate the lower two members, whereas the upper two are more calcareous. Small thrombolite reefs occur sporadically in lower beds, but they dominate the upper massive member and their occurrence in this unit is the subject of this description. The upper massive member encompasses Zones E and F of the Ibexian trilobite biostratigraphic zonation of Utah (Dean, 1978). Overall, the Survey Peak Formation comprises a "Grand Cycle",
'Department of Geology, University of Toronto, Toronto, Ontario M5S 1A1 213
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B.R. PRATT
214
representing a large scale shallowing upward event (Aitken, 1966, 1978). Although Aitken and Norford (1967) and Aitken (1978, p. 540) interpreted the whole formation to have been deposited under peritidal conditions, more recent investigation (Westrop, 1984; Pratt, pers. obs.) shows, rather, that shallow water indicators are absent. The Survey Peak accumulated on a subtidal shelf largely below fairweather wave-base but within storm wave-base. Background sedimentation resulted in shale, siltstone and lime mudstone, whereas grainstone and rudstone were deposited by storms as tempestites. The sediments enclosing thrombolite reefs in the upper massive member are mainly thin bedded lime mudstone, grainstone and rudstone with dolomitic partings. They contain less terrigenous material than the underlying member and were probably deposited under somewhat shallower conditions. The reefs do not exhibit halos of reef-derived grainstone, indicating that they did not form constantly winnowed shoals. The environment of deposition is thus envisaged as relatively deep subtidal but well within storm wave-base. The lack of slumps and other gravity features shows that these reefs accumulated on a horizonal shelf.
micrite as opposed to the light colored, locally burrowed, bioclastic, coarser crystalline micrite or microspar of matrix and geopetal internal sediment. A variety of microstructures is present, from "massive" micrite to densely clotted micrite, to clotted micrite with abundant small
REEF ANATOMY
SHAPE
Reefs of the Survey Peak consist dominantly of coalesced mounds and are up to 2 m thick and about 15 m wide; isolated mounds, less than 1 m thick and in diameter occur locally. Coalescence is mainly lateral, but vertical stacking does occur (Fig. 2A). They appear as massive lime mudstone, in contrast to the enclosing rocks which are thin bedded and drape markedly over reef tops (Fig. 2B); reef tops may form overhanging protuberances (Fig. 2C). FRAMEWORK
The thrombolite boundstone is massive and mottled in hand specimen (Fig. 3). Macroscopic lamination is not present. Whereas in shallower water reefs, such as those of the St. George Group of western Newfoundland (Pratt and James, 1982, this volume), individual thrombolites are clearly recognizable as upward-branching columns surrounded by bioclastic matrix and comprise on the order of 50% of the rock, thin sections of Survey Peak boundstones reveal that in these reefs thrombolites are more tabular and less columnar, and comprise more than 80% of the rock (Fig. 4). Geometrically the cryptalgal component is much like the massive framework in deep water Paleozoic mud-mounds (Pratt, 1982b). Lithistid sponges as spicular networks and rarely root tufts occur sporadically intergrown within the thrombolite framework. In thin section view (Figs. 4, 5), the thrombolite framework consists of dark colored, dense, finely crystalline
)LJ 2. Vertically oriented outcrop photographs; upper massive member, Survey Peak Formation. (A) Thrombolite reefs as a series of thick, superimposed massive beds, grading laterally to thin bedded flanking grainstones and lime mudstones. Mount Wilson; person standing at lower right for scale. (B) Tops of two small thrombolite reef mounds with draping flank beds. Wilcox Pass, Columbia Icefields, 40 km NW of Mount Wilson; hammer is 33 cm long. (C) Top of large thrombolite reef showing overhanging boundstone knob and flank beds. Mount Wilson; lens cap is 6 cm across.
CRYPTALGAL-SPONGE REEFS, ALBERTA
fenestral pores; clots may be interconnected to form serpentine to roughly tabular sheets and probably rows. Poorly preserved Girvanella filaments can be identified locally within the dense micrite framework. A very vague laminoid fabric may be discernible, mainly due to horizontally elongate fenestrae. Most fenestral pores are lined with cream-colored synsedimentary microspar cement and filled with colorless coarser spar. Matrix and internal sediment contain quartz silt, peloids and intraclasts of thrombolite fragments, and disoriented bioclasts including trilobite and pelmatozoan debris, sponge spicules, rare gastropods and nautiloids. Relationships with flanking sediment indicate that Survey Peak reefs had a relief above the sea floor of a metre at most. Domal protuberances on reef tops and orientation of thrombolites in hand specimen imply formation of boundstone "heads" locally within these reefs. Presence of small growth-framework cavities and sponges
)LJ 3. Vertically oriented polished slab of boundstone showing mottled aspect from micritic thrombolites and lime mudstone matrix. Boundstone is composed of vertically elongate thrombolites (lower right), flanked and overlain (left and upper part) by more laminar thrombolites. Upper massive member, Survey Peak Formation, Mount Wilson; scale bar in cm divisions.
215
and orientation of bioclasts suggest the individual growing surfaces possessed a synoptic relief of less than one or two centimetres.
216
BR. PRATT
)LJ 4. Low-power thin section photomicrograph of boundstone showing lithistid sponge (spicular network at lower right), thrombolite framework (dark-colored, "microfenestrate" mass making up most of the view), and burrowed matrix lime mudstone as internal sediment (lighter-colored pockets). This sediment is locally intraclastic (e.g., below sponge) and bioclastic (e.g., trilobite fragments, and nautiloid between two lobes of sponge). Arrow points upward; upper massive member, Survey Peak Formation, Mount Wilson; scale bar is 5 mm.
CRYPTALGAL-SPONGE REEFS, ALBERTA
217
)LJ 5. Thin section photomicrographs of cryptalgal fabrics in thrombolites. Upper massive member, Survey Peak Formation, Mount Wilson; scale bar for all is 500 (j,m. (A) Massive micrite (lower right) overlain by fenestral, interconnected, densely clotted micrite; matrix lime mudstone (as microspar) at lower left. (B) Massive to densely clotted micrite (lower centre and upper right) and interconnected, loosely clotted micrite (centre); matrix lime mudstone and spar fill growth-framework cavity. (C) Massive and densely clotted micrite sheltering growth-framework cavity floored with geopetal lime mudstone and filled by spar. (D) Matrix wackestone of bioclasts (trilobite fragment in upper left) and cryptalgal intraclasts (massive micrite) in a burrowed lime mudstone (microspar) matrix, overlying massive and clotted micritic thrombolite (lower right).
REFERENCES
Aitken, J.D. 1966. Middle Cambrian to Middle Ordovician cyclic sedimentation, southern Rocky Mountains of Alberta. Bulletin of Canadian Petroleum Geology, v. 14, p. 405-441. 1967. Classification and environmental significance of cryptalgal limestones and dolomites, with illustrations from the Cambrian and Ordovician of southwestern Alberta. Journal of Sedimentary Petrology, v. 37, p. 1163-1178. 1978. Revised models for depositional Grand Cycles, Cambrian of the southern Rocky Mountains, Canada. Bulletin of Canadian Petroleum Geology, v. 26, p. 515-542. Outram southern Alberta.Peak Bulletin and formations, Norford, B.S. 1967. Rocky Lower Mountains OrdovicianofSurvey and of Canadian Petroleum Geology, v. 15, p. 150-207.
Dean, W.T. 1978. Preliminary account of the trilobite biostratigraphy of the Survey Peak and Outram formations (Late Cambrian, Early Ordovician) at Wilcox Pass, southern Canadian Rocky Mountains, Alberta. Geological Survey of Canada, Paper 76-34. Pratt, B.R. 1982. Stromatolitic framework of carbonate mud-mounds. Journal of Sedimentary Petrology, v. 52, p. 1203-1227. and James, N.P. 1982. Cryptalgal-metazoan bioherms of early Ordovician age in the St. George Group, western Newfoundland. Sedimentology, v. 29, p. 543-569. Rigby, J.K. 1965. Stratigraphy and Porifera of Ordovician rocks near Columbia Icefields, Jasper National Park, Alberta, Canada. Brigham Young University Geology Studies, v. 12, p. 165-184. Westrop, S.R. 1984, Late Cambrian and earliest Ordovician trilobites, southern Canadian Rocky Mountains, Alberta. Unpub. PhD thesis, University of Toronto.
