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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Petrology
Vol. 56 (1986)No. 1. (January), Pages 1-18

Fossil Soils as Grounds for Interpreting Long-term Controls on Ancient Rivers

G. J. Retallack


The main factors controlling fluvial systems are time, initial topographic relief, geology, climate, vegetation, base level, upland runoff, upland drainage network, hillslope morphology, downstream deliveries, channel behavior, and pattern of deposition. Similar factors control soil formation in drainage basins: climate, organisms, topographic relief, parent material, and time. Effects of these factors may be preserved as well or better in fossil soils of alluvial sequences, as in sedimentary structures, facies, and mineralogy of paleochannels. For example, former rainfall may be interpreted from depth to the top of the calcic horizon, from percent clay, from percent of different kinds of clay minerals, from quartz/feldspar ratio, from hue, and from identification of fossil soils.

Interpretation of the role of these factors in producing a particular stratigraphic section should take into account its resolution: the proportion of geological time represented (temporal completeness), of original sediment preserved (lithological completeness) and of original fossils preserved (paleontological completeness). Temporal and lithological completeness can be estimated by comparison with the most likely rates of sediment accumulation and most likely thicknesses (respectively) for pertinent sedimentary environments and time scales. Paleontological completeness can be estimated by comparing the preserved fossils with expectations based on former ecosystems indicated by fossil soils.

Likely controls and resolution of a sequence of Eocene and Oligocene fossil soils from Badlands National Park, South Dakota, were studied as a guide to the periodicity and causes of episodic sedimentation there. Short-term episodicity and filtering of sedimentation events of particular magnitude were apparent from superposition of paleosols, but resolution of the sequence was not sufficient to permit a detailed analysis of their individual causes. Long-term paleoenvironmental changes and four episodes of erosional downcutting could be recognized. All four occasions of landscape instability and erosion were times of drier climate and sparser vegetation than usual. Local uplift and supply of volcanic ash were contributing factors in some episodes of erosion. When allowances are made for time scales and completeness of sequences of fossil soils, they can be useful indicators of changes in former environment, biota, and fluvial regimen.

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