Paleoenvironmental interpretation of terrestrial deposits: The Cap-aux-Os and Fort Prevel members, Battery Point Formation (Early-Middle Devonian), Gaspe Bay, Quebec.

摘要

The Battery Point Formation of Gaspe Bay, Quebec, is an Early to Middle Devonian prograding succession of siliciclastic deposits shed in response to the Acadian Orogeny. Within the Battery Point Formation are three members, of which, the Cap-aux-Os and Fort Prevel Members are represented by a variety of terrestrial depositional environments, including: coastal-margin, transitional, palustrine-lacustrine, and alluvial-plain environments. These deposits and their paleoenvironmental designation are based on paleosol structures and features, plant and root trace fossils, invertebrate fossils, and features from associated nonpedogenic deposits.; Paleosol maturity and colonization of soil substrates by early land plants was a function of paleoenvironmental stresses resulting from base-level fluctuation and sedimentation rate. The best developed paleosols formed along the coastal-margin and transitional environments during intervals of low sedimentation rates; the weakest developed paleosols formed in alluvial-plain environments, during high rates of sedimentation. Early vascular land plants were most effective in promoting the physical weathering of substrates located in low sedimentation rate environments such as the coastal-margin and transitional settings. In these environments, they modified soil structure from platy mudcracked parent material into blocky-angular or fine blocky peds. This process increased the soil surface area exposed to weathering, increased alteration depth, and allowed greater oxidation to occur deeper in Early Devonian paleosols. Plants in the alluvial-plain environment did not modify their substrate greatly due to high-energy ephemeral flood events, which resulted in halted or retarded weathering.; In addition to root traces, paleosols occurring in the coastal-margin to alluvial-plain environments contained pedogenic carbonate; the range of delta 13C from pedogenic carbonate used in this study was --3.4 to --5.6‰ delta13C. Using Ceding's (1991) soil carbonate paleobarometer, it is estimated that Early Devonian CO2 levels were 8.3 to 14 times (2,200 to 4,200 ppmV) modern levels (300 ppmV). Because paleoenvironmental stresses induced by changing sedimentation rate prevented plants from greatly modifying substrates, weak to moderate soils developed. Due to these conditions, plants did not greatly modify the substrate or surface area, therefore, the rate of weathering induced by plants was insufficient to remove CO2. Plants probably benefited from elevated CO 2 levels, which may have supported their rapid evolution, diversification and expansion into new environments.