Osman Salad Hersi

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.

Diagenesis of volcanic-rich tight sandstones and conglomerates: a case study from Cretaceous Yingcheng Formation, Changling Sag, Songliao Basin, China

Thick conglomerate and sandstone lithofacies of Yingcheng Formation in Changling Sag are tight gas reservoirs. The formation contains a fair amount of volcanic clasts and accumulated in a deltaic system that spans from delta plain to prodelta depositional setting. High-quality reservoirs are only a small fraction of these thick siliciclastic rocks. Petrographic analysis has been used to assess diagenetic processes, paragenetic sequence, and their effects on the reservoir qualities. The major loss of primary porosity is due to compaction along with grain-coating, pore-filling clay, and quartz cements. Alteration of volcanic rock fragments supplied alkali cations which favored formation of chlorite, smectite, and zeolite cements. The thickness of the chlorite coating decreases from the delta plain to the distal part of the delta front. On the contrary, the content of the zeolite cement increases toward the distal part of the delta front. Quartz and feldspar cements were ubiquitous due to high SiO2 concentration and alkali cations. Early dissolution of volcanic rock fragments and feldspars by freshwater increased porosity but did not contribute much to the permeability because pore-throats are predominantly occluded with clay and pseudomatrix. Late-stage dissolution of the zeolite cement was caused by organic acid expelled from maturation of organic matter. Although the reservoirs with chlorite coatings have moderate porosity, the permeability in situ is very low and pore-throat sizes are small. The high-quality reservoirs are medium- to fine-grained zeolite-dissolved sandstones developed in the distal part of the delta front and shallow lacustrine.

Biostratigraphic Constraints on Chronostratigraphic Intraformational Relationships within the Lower–Middle Ordovician Beekmantown Group, Laurentian Margin: Eastern Ontario and Southwestern Quebec, Canada

The Lower–Middle Ordovician Beekmantown Group of the St. Lawrence Lowlands (eastern Ontario and southwestern Quebec) consists of basal inner-shelf dolomitic sandstones (Theresa Formation) and inner- to middle-shelf carbonates (Beauharnois and Carillon Formations). The stratigraphic relationship of the three formations has been considered in the past as a normal vertical succession. Integration and reinterpretation of the existing lithostratigraphic and biostratigraphic data support a new chronostratigraphic relationship, particularly between the Theresa and Beauharnois Formations, as well as correlations with the outer-shelf deposits preserved in the Philipsburg tectonic slice (PTS). Variations in the lithostratigraphic pattern are pronounced along a cross sectional depositional profile from inner shelf to outer shelf. The data demonstrate that the Beekmantown Group is thickest in the region south of Montreal and gradually becomes thinner toward eastern Ontario (Ottawa embayment) and the region north of Montreal (Laurentian highs). The lower boundary of the Theresa Formation is diachronous, becoming younger toward the craton. In the region south of Montreal, Theresa Formation sandstones are temporally equivalent to the lower part of the middle-shelf carbonates assigned to the Ogdensburg Member of the Beauharnois Formation. The Theresa Formation sandstones pinched out toward the outer shelf and replaced by Ogdensburg-like facies. The latter apparently merges with the outer-shelf carbonates of the Wallace Creek and Morgan Corner Formations. The outer-shelf strata of the Hastings Creek and Naylor Ledge Formations correlate with the upper part of the Ogdensburg Member. The Naylor Ledge Formation is truncated by the Sauk-Tippecanoe unconformity and overlain by the lower Whiterockian (?Rangenian) Luke Hill Formation. The onset of the Sauk-Tippecanoe unconformity is older in the outer-shelf rocks of the PTS and becomes younger toward the inner shelf, where it separates the middle Whiterockian Carillon Formation from the Chazyan strata of the St. Lawrence Lowlands.