Sedimentology and chemostratigraphy of the Grande Anse Formation, Cumberland Basin, eastern Canada: its relationship to salt tectonics and strata of the Joggins World Heritage Site
University of New Brunswick
The Pennsylvanian stratigraphy of the western Cumberland Basin has been influenced by salt tectonics, specifically the formation of the Minudie Anticline, a salt wall. South of the Minudie Anticline, in the Athol Syncline, the strata exposed along the Joggins UNESCO World Heritage shoreline have been extensively studied. Here, the Cumberland Group succession of strata younger than the Boss Point Formation is assigned to the Little River, Joggins, Springhill Mines, and Ragged Reef formations. In contrast, north of the Minudie Anticline, the little studied Grande Anse Formation lies in angular unconformity on the Boss Point and basal Little River formations. Investigation of the Grande Anse Formation sedimentology indicates four lithofacies associations: floodplain, braided channel, sheet flood, and debris flow deposits. A comparative lithostratigraphic study suggests two possible depositional models. One model has the Black Point sub-basin with its own hydrological system, completely separated from the Athol Syncline. The second model proposes that the strata north of the salt wall were exposed to erosion during accumulation of Joggins and Springhill Mines formations. Subsequently, the sediments of the lithologically similar Ragged Reef and Grande Anse formations, either (i) were deposited unconformably on all the underlying strata, or (ii) gradually onlapped to the north, becoming unconformable on the folded Boss Point Formation. To further assess the two models, forty sandstone samples from the strata above the Boss Point Formation were geochemically analyzed to determine major element compositions. The results show statistically significant differences between the Little River, Joggins, and Springhill Mines formations and the Grande Anse Formation, whereas the Ragged Reef and Grande Anse formations statistically show no significant difference. These latter two units show greater maturity likely due to more intense chemical weathering under sub-humid to humid climate. Thus, the deposition of the Ragged Reef Formation in the Athol Syncline may have gradually onlapped across the Minudie Anticline and then continued further north as the Grande Anse Formation. Petrographic investigations reveal that the Grande Anse Formation has undergone various diagenetic alterations, most extensively in the eodiagenetic near-surface sub-realm. Initially, dissolution of unstable minerals followed by precipitation of iron oxides/hydroxides and kaolinization occurred under slightly acidic porewaters of the vadose zone. Fluctuation to higher pH in the lower vadose zone and upper phreatic zone, due to close proximity of the salt wall, produced microcrystalline calcite, and silica phases comprising quartz overgrowths, macrocrystalline quartz, chalcedony, and pseudomorphous silica after gypsum. Later phases of high-Mn calcite and ferroan dolomite indicate anoxic phreatic conditions during shallow burial. The deep burial mesodiagenetic realm reflects only minor alterations including dickite and illitization of kaolinite.