Doctoral Dissertations

Date of Award

12-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Steven G. Driese

Committee Members

Kenneth R. Walker, Claudia J. Mora, Craig E. Barnes

Abstract

The Battery Point Formation of Gaspé Bay, Québec, 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 Prével Members are represented by a variety of different 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.

Coastal-margin, and transitional environments of the Cap-aux-Os Member, which had their origins in the eastern part of the study area, were characterized as having experienced alternating oxidizing-reducing conditions, containing Entisol and Inceptisol paleosols, and containing abundant, well-preserved plant fossils, root traces, and signs of invertebrate activity. This range of paleosol types indicate that subaerial exposure surfaces in this environment were weakly to moderately affected by soil forming processes. The presence of early vascular land plant and marine invertebrate fossils and traces associated with this environment suggests episodes of submergence by brackish-water. The transitional environment was less affected by flooding due to a greater distance from the shoreline. Palustrine-lacustrine deposits in the Capaux-Os Member were also affected by fluctuating water-level. In this environment, freshwater limestone, characterized by diffuse color mottling, fenestrae, pedotubules, pseudo-microkarst and collapse breccia, experienced subaerial exposure during low water levels, resulting in pedogenic modification of the carbonate. The degree of soil maturation in these different settings was strongly controlled by changes in local base-level.

The alluvial-plain environment of both the Cap-aux-Os and Fort Prevel Members is characterized as high-energy oxidized deposits, within which weakly developed Entisols to Inceptisols formed. These paleosols were influenced by periodic avulsion and overbank sedimentation. Plant and root traces preserved in this environment, especially in the Fort Prevel Member, are larger and more resilient than previously documented, and represent the colonization of new and more stress-filled upland environments.

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 granular 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. Plant traces from the alluvial-plain environment did not modify their substrate greatly due to high-energy ephemeral flood events, which halted or retarded weathering.

In addition to root traces, paleosols occurring in the coastal-margin to alluvial-plain environments contained pedogenic carbonate. Using Cerling's (1991) soil carbonate paleobarometer, it is estimated that Early Devonian pC02 levels were 8.3 to 14 times (2,200 to 4,200 ppmV) modern levels. Because paleoenvironmental stresses induced by changing sedimentation rate prevented plants from greatly modifying substrates, weak to moderate soils developed. Under such conditions, plants were unable to remove significant amounts of CO2 from the atmosphere. Plants probably benefited from elevated pCO2 levels, which may have supported their rapid evolution, diversification and expansion into new environments.

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