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Ancient Floodplain Evolution and Techniques for Analysis |
1 Department of Earth Sciences, University of Western Ontario, London, Ont. N6A 5B7, Canada
2 Department of Geology and Geophysics, and Geophysical Institute, University of Alaska, Fairbanks, AK 99775-5780, USA mccarthy{at}gi.alaska.edu
Floodplains represent both sedimentological and pedological entities that record a unique blend of potential environmental indicators in the rock record. Because floodplains develop in aggradational environments with a high preservation potential, they preserve a detailed record of the local pedosedimentary history. Assemblages of micromorphological features present in floodplain palaeosols record, with a high degree of temporal and spatial resolution, specific pedological processes that operated under particular environmental conditions. Detailed micromorphological analyses of floodplain deposits, in conjunction with their regional distribution, are required for accurate palaeoenvironmental reconstruction of these environments.
Micromorphological features are described from floodplain palaeosols in the Upper Cretaceous (Cenomanian) Dunvegan Formation in northwestern Alberts and northeastern British Columbia, Canada. The palaeosols developed on low-lying and generally poorly drained floodplains during a time when the tectonic subsidence rate was very low. High-frequency fluctuations in base level were superimposed upon this tectonic signature, resulting in at least seven phases of valley cutting and the corresponding development of mature interfluve palaeosols. The major pedogenic processes operating were clay illuviation, redox processes, precipitation of soluble salts (primarily FeCO3) and pedoturbation. Palaeopedological features in the Dunvegan Formation are consistent with features developed in modern cool temperate soils with a non-seasonal precipitation distribution, such as Brunisols and Luvisols. The presence of embedded grain argillans, compound silt-clay coatings and rounded aggregates suggests at least the possibility of limited frost penetration in inland soils. Floodplains of the mid-Cretaceous Dunvegan Formation (palaeolatidude c. 65°N) record evidence for cool temperate conditions that supports other palaeobotanical and sedimentological evidence for cool Cretaceous palaeoclimates at high latitudes.
