LOWER DEVONIAN STROMATOPOROID REEFS, FORMOSA REEF ...

7 4 . Reefs, Canada and Adjacent Area H.H.J. Geldsetzer, N.P. James and G.E. Tebbutt, Editors Canadian Society of Petroieum Geologists Memoir 13, 1989, p. 506-509

LOWER DEVONIAN STROMATOPOROID REEFS, FORMOSA REEF LIMESTONE (DETROIT RIVER GROUP) OF SOUTHWESTERN ONTARIO BRIAN R. PRATT1

General Location

- southwestern Ontario

Age

- Emsian (Early Devonian)

Reef Type

- patch reefs

Dimensions

- up to 15 m thick and 900 m wide

Depositional Setting

- carbonate shelf

Tectonic Region

- Michigan Basin

Crustal Position

- intracratonic basin

Foundation below Reef

- sub tidal, bioclastic wackestone

Bathymetric Range

- shallow subtidal

Reef-forming Process

- growth of stromatoporoids and corals and binding of sediment

Dominant Organism(s)

- stromatoporoids, tabulate and rugose corals

Diagnostic Aspect(s)

- tabular stromatoporoids; primary porosity locally preserved

)LJ 1. Map of southwestern Ontario, showing exposure of the Formosa Reef Limestone.

INTRODUCTION

Small stromatoporoidal patch reefs are common in the Walkerton-Wingham area of southwestern Ontario. Although they have been encountered in the subsurface to the southwest, none produces hydrocarbons. However, these reefs are exceptionally well preserved and serve to illustrate framework relationships that may have application to their economic, dolomitized counterparts elsewhere. One reef, dissected along a roadcut 3 km north of the village of Formosa and west of Walkerton (Fig. 1), is the best exposed and is described in further detail here. STRATIGRAPHY

The most recent stratigraphic summaries of relevant Devonian rocks of this part of the Michigan Basin are presented by Uyeno et al. (1982) and Fagerstrom (1982). Patch reefs of the Formosa Reef Limestone occur in thin bedded, sparsely fossiliferous dolostones of the undifferentiated Detroit River Group; Freeman (1978), Klein (1980)

and Uyeno et al. (1982) extended the use of the Amherstburg Formation to this area as the lateral, off-reef equivalents. The Formosa Reef Limestone was formally erected as a stratigraphic unit by Fagerstrom (1961b), with its type section along the roadcut described here. Patch reefs range up to about 15 m in thickness and 900 m in diameter, although they are generally less than 100 m across. They appear to occur at different stratigraphic levels within the Amherstburg Formation (Fagerstrom, 1982). Macrofaunal studies by Fagerstrom (1961a) indicate that the Formosa reefs are stratigraphically equivalent to the lower Onondaga Formation of the Niagara Peninsula and adjacent New York State. The Edgecliff Member at the base of the Onondaga also hosts numerous bioherms (Lindemann, this volume) and has been tentatively assigned to the patulus Zone of the Emsian (late Early Devonian) (Ziegler and Klapper, 1985). Low diversity conodont faunas in the Detroit River Group of southern Ontario belong to the robustus Zone and suggest that the Formosa Reef Limestone is Emsian in age (Uyeno et al., 1982).

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Fig. 2. Vertically oriented polished slabs of boundstone; scale bar in cm divisions. (A) Alternating tabular stromatoporoids, fine bioclastic packstone and coarse pelmatozoan packstone, with scattered streptelasmitid rugose corals. (B) Thin, intergrown tabular stromatoporoids with bioclastic matrix. (C) Bioclastic matrix containing large zaphrentid rugose corals, overlain by a tabular stromatoporoid and branching rugose corals. In all three note unfilled shelter porosity and early fractures of compaction origin.

REEF LITHOLOGY

SHAPE

Reefs of the Formosa Reef Limestone are domal masses of resistant, light grey limestone within recessive, thin bedded, buff colored dolostones. Although the dominant current direction during the Early Devonian in this part of the Michigan Basin was probably from the present-day south and southwest, no corresponding elongation of the Formosa reefs is suggested by the outcrop maps of Klein (1980). The southern part of the reef exposed at the type section shows interfingering with bioclastic grainstone dominantly made up of pelmatozoan debris. Interfingering surfaces are relatively steep, on the order of 30°, and if they mark the reef margin, then topographic relief of the reef during growth was about 2 m.

Presence of lateral zonation of framebuilding organisms has been argued by Roper (in Stumm, 1969) and Klein (1980, p. 38). They noted that bedding appears more massive and large (up to 2 m in diameter) favositid tabulate corals become more common toward the southern end of the outcrop, whereas tabular stromatoporoids become less abundant; they therefore suggested that the windward edge of the reef was to the south. Klein (1980) also claimed that a vertical succession of ecologic stages was present, complying with the theoretical model of Walker and Alberstadt (1975). However, vertical changes in composition are not readily apparent at the type section. FRAMEWORK

Boundstones of the Formosa Reef Limestone domi-

508

nantly consist of superimposed tabular stromatoporoids up to about 5 cm thick and 0.5 m in diameter (Figs. 2A, B). Hemispherical favositid coral colonies up to 2 m across are common and in-place fasciculate rugose corals up to 0.5 m across occur locally. In addition to the large framebuilders, diverse, smaller solitary rugose and domal to branching tabulate corals occur abundantly (Fig. 2C). These skeletons are usually prostrate or clearly toppled. Microbial (cryptalgal) structures (i.e. stromatolites and thrombolites) seem to be absent. The stromatoporoid fauna is diverse and different Formosa reefs exhibit differing relative abundances of species (Fagerstrom, 1982, p. 46). The sediment between the stromatoporoids and corals consists of alternating layers of fine bioclastic packstone and coarse pelmatozoan packstone. This matrix is richly fossiliferous, containing trilobites, ostracodes, bryozoans, brachiopods, gastropods, rostroconchs and cephalopods (Fagerstrom, 1961a; Ludvigsen, 1987). Except for crinoid and trilobite disarticulation, fossils are whole and not broken or abraded within the reef. The coarser sediment probably reflects deposition during episodic high energy events when flanking calcarenites were cast over the reef surface. Extensive galleries do not seem to have formed under the living framework, but small cavities under stromatoporoids were present and are locally preserved free from internal sediment and cement. These exhibit an encrusting fauna offavositid and auloporid corals (Fig. 3) and bryozoans. Laminoid, centimetre-sized, blocky spar-filled sheet cracks are common. These are not growth-framework, stromatactislike cavities, but are probably due to sediment settling or shrinkage. Formosa reefs are undolomitized. Fibrous submarine cements occur locally (Fig. 4A) but are not abundant. This suggests that the reef was not exposed to a high degree of

B.R. PRATT

continuous water turbulence, a factor known to promote widespread synsedimentary cementation. This also implies that the framebuilding organisms were responsible for holding the reef together, and that the tabular morphology of the stromatoporoid fauna was a response to relatively low energy conditions (Steam, 1982, p. 236). Cementation by blocky calcite cement has occluded most primary porosity, but preservation of intraskeletal pores (Figs. 2C, 4B) and compaction-induced fractures (Fig. 2A) is common. Conodont and acritarch color alteration studies indicate that these rocks were at one time buried under 1.3-2.0 km of sediment (Legall et ah, 1981).

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LOWER DEVONIAN STROMATOPOROID REEFS, ONTARIO

REFERENCES

Fagerstrom, J.A., 1961a. The fauna of the Middle Devonian Formosa Reef Limestone of southwestern Ontario. Journal of Paleontology, v. 35, p. 1-48. , 1961b. Age and stratigraphic relations of the Formosa Reef Limestone (Middle Devonian) of southwestern Ontario. Geological Society of America Bulletin, v. 72, p. 341-350. , 1982. Stromatoporoids of the Detroit River Group and adjacent rocks (Devonian) in the vicinity of the Michigan Basin. Geological Survey of Canada, Bulletin 339. Freeman, E.B. (Ed.), 1978. Geological Highway Map, Southern Ontario. Ontario Geological Survey, Map 2418. Klein, K.P. 1980., The lithofacies and biofacies of the Formosa Reef Limestone (Eifelian) in Bruce and Huron Counties of southwestern Ontario. Unpublished M.Sc. thesis, University of Windsor. Legall, F.D., Barnes, C.R. and Macqueen, R.W., 1981. Thermal maturation, burial history and hotspot development, Paleozoic strata of southern

509

Ontario-Quebec, from conodont and acritarch colour alteration studies. Bulletin of Canadian Petroleum Geology, v. 29, p. 492-539. Ludvigsen, R., 1987. Reef trilobites from the Formosa Limestone (Lower Devonian) of southern Ontario. Canadian Journal of Earth Sciences, v. 24, p. 676-688. Stearn, C.W., 1982. The shapes of Paleozoic and modern reef-builders: a critical review. Paleobiology, v. 8, p. 228-241. Stumm, E.C., 1969. Devonian bioherms of the Michigan Basin. University of Michigan, Contributions from the Museum of Paleontology, v. 22,p. 241-247. Uyeno.T.T., Telford, P.G. andSanford, B.V., 1982. Devonian conodonts and stratigraphy of southwestern Ontario. Geological Survey of Canada, Bulletin 332. Walker, K.R. and Alberstadt, L.P., 1975. Ecological succession as an aspect of structure in fossil communities. Paleobiology, v. 1, p. 238-257. Ziegler, W. and Klapper, G., 1985. Stages of the Devonian System. Episodes, v. 8, p. 104-109.