231-240. EARLY ORDOVICIAN THROMBOLITE REEFS, ST. GEORGE GROUP, WESTERN NEWFOUNDLAND. BRIAN R. PRATT1 AND NOEL P. JAMES2.
3 2 . Reefs, Canada and Adjacent Area H.H.J. Geldsetzer, N.P. James and G.E. Tebbutt, Editors Canadian Society of Petroleum Geologists Memoir 13. 1989. p. 231-240
EARLY ORDOVICIAN THROMBOLITE REEFS, ST. GEORGE GROUP, WESTERN NEWFOUNDLAND BRIAN R. PRATT1 AND NOEL P. JAMES2
General Location
- western Newfoundland
Age
- Ibexian(= Canadian; Early Ordovician)
Reef Type
- patch reefs
Dimensions
- 0.1-3 m wide domal mounds; larger banks by coalescence
Depositional Setting
- carbonate shelf
Tectonic Region
- Humber Zone, Appalachian Orogen
Crustal Position
- continental shelf
Foundation below Reef
- grainstone beds, locally hardgrounds, St. George Group
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HARE BAY
('',(6&29 (:(67
Bath> metric Range
- shallow subtidal
Reef-forming Process
- accretion and cementation of cyanobacterial
Dominant Organism(s)
mats - thrombolites, locally lithistid sponges and tabulate corals
Diagnostic Aspcct(s)
- framework inconspicuous; massive mottled lime mudstone/wackestone
PORT AU PORT PENINSULA CAMBRO-ORDOVICIAN SHELF CARBONATES
Fig. 1. Map of western Newfoundland showing coastal locations of well exposed thrombolite reefs.
INTRODUCTION
With the demise of the archaeocyathans at the end of Early Cambrian time, the reef-building niche was re-occupied by mats of micro-organisms, presumably mostly cyanobacteria (i.e., blue-green algae). By a combination of sediment binding, calcification and cementation, these mats built poorly laminated to unlaminated frameworks called thrombolites (Pratt and James, 1982, p. 545). Patch reef mounds and banks made up of thrombolites are abundant and well preserved in the St. George Group of western Newfoundland (Fig. 1) and have been the subject of detailed study (Pratt, 1979; Pratt and James, 1982; Kennard and James, 1986). Those in the upper half of the St. George accreted at a time just before the Middle Ordovician increase
in invertebrate diversity, so that there is a sporadic but minor contribution by encrusting metazoans such as tabulate corals and lithistid sponges. Nevertheless, these reefs are broadly similar in composition throughout the unit and this report summarizes their main attributes. STRATIGRAPHY
The St. George Group (Fig. 2) is a typical early Paleozoic sequence of peritidal, shallow water limestones and dolostones(Levesque, 1977; Pratt, 1979; Pratt and James, 1986; Knight and James, 1987). The subtidal environment is represented in these rocks by low energy, burrow-mottled
'Department of Geology, University of Toronto, Toronto, Ontario M5S 1A1 department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6 231
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B.R. PRATT AND N.P. JAMES
232
with the Catoche, a continental margin reef complex now exposed in Hare Bay developed further seaward (Stevens and James, 1976).
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REEF ANATOMY CYCLIC PERITIDAL LIMESTONES & DOLOSTONES
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Fig. 2. Stratigraphic column of the Lower Ordovician platformal sequence with nomenclature of Knight and James (1987).
lime mudstone and wackestone, whereas the high energy facies consists of thrombolite reefs and surrounding winnowed grainstone halos. These rocks were deposited on a relatively wide shelf that rimmed the continental margin in this area, the outer part of an epeiric sea that spread over much of the continental interior. This shelf can be envisaged during the Early Ordovician as a mosaic of tidal flat islands and intertidal banks (Pratt and James, 1986). The juxtaposition of St. George thrombolite reefs and peritidal facies indicates that these reefs formed in very shallow water, just below low tide but well within fair weather wave-base. Reefs occur throughout the St. George everywhere it is exposed. They tend to be largest in the Catoche Formation which was deposited when somewhat more stable subtidal conditions prevailed over most of the continental shelf of western Newfoundland. Correlative
SHAPE
Reefs in the St. George are exposed in a variety of ways, exhumed in their original distribution on bedding planes as smoothly washed or freshly broken vertical slices, and as differentially weathered bedding surfaces. They are generally hemispheroidal mounds (Figs. 3A, B) commonly rangingfrom0.4to 1.8 m in width and thickness, but occasionally as small as 0.1 m and as large as 3 m. Within individual beds, mounds tend to be similar in size and closely spaced, less than 1 m apart. In the Catoche and upper Boat Harbour formations they often coalesced laterally and vertically to form irregular, circular or linear banks (Figs. 3C-F). Mounds are flanked and separated by fossiliferous, bioclastic and peloidal grainstone halos or channel fills. The grainstones are locally bioturbated and cross-laminated and contain thrombolite fragments. The amount of bioturbation increases but bioclastic content decreases with distance away from mounds. Contacts with mounds are usually sharp and occasionally erosive. FRAMEWORK
The framework of these reefs is mainly unlaminated microbial structures, i.e., thrombolites, although lamination does occur locally such that stromatolites are recognized (Fig. 5 A). In the St. George this framework is darker colored than surrounding matrix but tends to be inconspicuous, especially when dolomitized. In vertical crosssection thrombolites are generally elongate and divergent upward and outward, branching and anastomosing in a highly variable pattern (Figs. 4,6). They lack distinct walls and their edges are irregularly invaginated. They are also variable in plan view, from subcircular to irregularly cuspate and interconnecting, and range from slightly less than 1 cm to about 3 cm across. When weathered, this pattern on bedding planes may be confused with burrow mottling. Thrombolite morphology and proportion relative to surrounding matrix seems to vary with reef size and shape. In very small mounds thrombolites have less vertical elongation and there is comparatively little matrix, so that the framework appears almost massive (Fig. 5B, 6H). In mounds larger than about 1 m there is less anastomosing and more vertical elongation. Frame-building metazoans are present locally, but are rarely volumetrically important, beginning in upper Boat Harbour time. The early tabulate coral Lichenaria occurs in two horizons of small mounds in the Boat Harbour Formation at Eddies Cove West and eastern Port au Port Peninsula (Figs. 6F, G). The problematical lamellar, spicular organism Pulchrilamina (see Toomey and Ham, 1967) was observed in one large bank in the Catoche Formation
233
THROMBOUTE REEFS, ST. GEORGE GROUP, NEWFOUNDLAND
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