I. Knight

Dolomitization of the Lower Ordovician Aguathuna Formation carbonates, Port au Port Peninsula, western Newfoundland, Canada: implications for a hydrocarbon reservoir

The Lower Ordovician Aguathuna Formation (∼100 m thick) is formed of shallow-marine carbonates, which constitute the uppermost part of the St. George Group of western Newfoundland. Sedimentation was paused by a major subaerial exposure (St. George Unconformity), which likely developed a significant pore system in the underlying carbonates by meteoric dissolution. The sequence has been affected by multiphase dolomitization that caused complex changes in the rock porosity. The Aguathuna dolomites are classified into three main generations ranging in crystal size between ∼4 µm and 2 mm. The occurrence of fabric-retentive dolomicrites implies that dolomitization likely started during the early stages of diagenesis. Although dolomitization is pervasive in the upper part of the formation and significantly occludes the pores, some intervals in the lower part have higher porosity. The development of lower permeable layers overlain by an impermeable (seal) cap suggests a possible potential diagenetic trap. Unlike sabkha deposits, the Aguathuna carbonates do not have evaporite interlayers. Furthermore, the low Sr contents (∼96 ppm) and the δ18O values of earlier dolomites (–3.3‰ to –6.9‰ VPDB (Vienna Pee Dee Belemnite)) are also difficult to reconcile with a brine origin. The Sr/Ca molar ratios (0.0067–0.0009), calculated for the earliest dolomitizing fluid, suggest a modified seawater origin, likely mixed sea and meteoric waters. The least radiogenic 87Sr/86Sr values of the earliest dolomite are consistent with those of early Ordovician seawater, which supports an early-stage diagenesis. Petrography, geochemistry, and fluid inclusions of the late dolomites suggest precipitation at higher temperatures (∼73–95 °C) in deeper burial environments from hydrothermal solutions.

The Great American Carbonate Bank in Eastern Canada: An Overview

The postrifted margin of Laurentia in eastern Canada had a rugged paleomorphology, with major salients and recesses formed during the long-lasting (Ediacaran to late Early Cambrian) breakup of Rodinia. After short-lived carbonate production during the Early Cambrian, the great American carbonate bank (GACB) was firmly established in the earliest Middle Cambrian as the last rift-related event (Hawke Bay event, late Early Cambrian), and was followed by mostly passive thermal subsidence of the continental crust of Laurentia. Middle to Upper Cambrian carbonates are well preserved in the Port au Port Group in western Newfoundland (St. Lawrence promontory). Scattered outcrops of upper Middle to Upper Cambrian sedimentary rocks are found in southern and eastern Quebec (Quebec reentrant), although most of the preserved Upper Cambrian facies in the reentrant consist of nearshore to fluvial clastics unconformably overlying the Grenvillian basement. The Cambrian shallow-marine carbonates are dominated by high-energy facies with significant thrombolite reefs at the platform margin. The succession consists of large-scale transgressive-regressive cycles known as Cambrian grand cycles. Some anomalies in stacking patterns are suggestive of local tectonic events that were hypothesized based on the nature (facies and age) of carbonate clasts that accumulated on the continental slope. The Cambrian–Ordovician transition occurred at a time of a major sea level lowstand that resulted in a significant unconformity in southern Quebec and Ontario. In western Newfoundland, this sea level fall is recorded in the regressive facies of the last Cambrian grand cycle but did not culminate in subaerial exposure. The duration of the depositional hiatus at the Cambrian–Ordovician transition increases toward the west from an early Skullrockian gap in the Philipsburg thrust slice in southeastern Quebec; the hiatus covered the entire Skullrockian in eastern Ontario. A major sea level rise at or near the base of the Ordovician resulted in sedimentation on an extensive peritidal, mud-dominated, low-energy carbonate platform. This platform is known as the St. George Group (western Newfoundland), the Beekmantown Group (southwestern Quebec and Ontario), the School House Hill Group (southeastern Quebec), and the Romaine Formation (Anticosti Island). The carbonate facies are characterized by large- and small-scale depositional cycles. Two third-order cycles are well documented in western Newfoundland. The presence of such cycles is also proposed farther south, although their precise character still has to be documented. Multiple fifth-order meter-scale peritidal-dominated cycles have been documented in the Lower Ordovician carbonates. A diachronous change in depositional style occurred along the margin of Laurentia near the base of the Middle Ordovician. Facies patterns became controlled by faulting and accumulation rates increased significantly. These changes occurred first in the late Ibexian in southeastern Quebec and in the early Whiterockian elsewhere. At most localities, this transition is also expressed in a significant subaerial unconformity that is recognized along the entire eastern (paleosouthern) margin of Laurentia. This subaerial event is interpreted as resulting from lithosphere upwarping in front of the migrating Taconic orogenic wedge. The west-directed migration of the tectonic peripheral bulge resulted in the final destruction of the GACB as sedimentation resumed in a tectonically active foreland basin.