Pedofacies analysis; A new approach to reconstructing ancient fluvial sequences

Abstract

Paleosols formed on overbank deposits of the lower Eocene Willwood Formationin the Bighorn Basin, Wyoming, can be differentiated on the basis of pedogenic maturity. The least mature soils generally formed close to the channel margin where sediment accumulation rates were rapid, whereas the most mature soils developed on distal flood plains where accumulation rates were significantly slower. The term pedofacies is introduced to delimit laterally contiguous bodies of sedimentary rock that differ in their ancient soil attributes as a result of distance from areas of relatively rapid sedimentation.Vertical successions of Willwood overbank deposits show three orders of pedofacies sequences. Simple sequences consist of one or more paleosols bounded below and above by crevasse-splay deposits. They were generated by slow and sporadic alluviation and soil modification that were periodically interrupted by more rapid crevasse-splay deposition. On a larger scale, compound pedofacies sequences are composed of multistory paleosols sandwiched between channel sandstones. Pedogenic maturity of the paleosols progressively increases and then decreases upward in response to episodic channel avulsion. Development of compound and simple pedofacies sequences was largely controlled by local patterns of deposition that produced vertical variability in the rate of sediment accumulation.Superimposed on these smaller-scale cycles are pedofacies megasequences that are hundreds of meters thick. Megasequences show a distinct upward change in the overall maturity of their constituent compound sequences. They provide evidence for changes in the rate of sediment accumulation produced by allocyclic processes including varied tectonic activity. Comparison between Willwood deposits in the northern and central parts of the Bighorn Basin reveals that areally differing sediment accumulation rates, and thereby basin subsidence rates, can also be interpreted from large-scale pedofacies sequences. 143 on June 9, 2015 specialpapers.gsapubs.org Downloaded from 144 Kraus and Brown

Fluvial systems and their sedimentary models

“…RazdeUtev sedimentov in oblik zapolnitve rečne doline ali dolinskega zasipa (valley fill) so obravnavali številni raziskovalci v širšem ali ožjem pogledu (WoIman & Leopold, 1957;Leopold et al, 1964;Allen, 1965;Happ, 1971;Schümm, 1977;S e Hey, 1988). V zapolnitvi rečne doline, ki površinsko predstavlja nasipno naplavno ravnino (si.…”

“…Potek in oblika korit, ki se vijejo preko rečne naplavne ravnine, je zelo razhčen. Z njihovim opisom in klasifikacijo so se ukvarjaU številni raziskovalci (Leopold & Wolman, 1957;Schümm, 1960Schümm, , 1963Schümm, , 1968Schümm, , 1972Schümm, , 1977Schümm, , 1981Brice, 1964;Schümm & Khan, 1972;Miall, 1977;Rust, 1978a). Večina geomorfologov je za opis rečnega toka uporabljala parametre, kot so vijugavost (sinuosity) in prepletenost (braiding).…”

Lithofacies, palynofacies, and sequence stratigraphy of Palaeogene strata in Southeastern Nigeria

“…At the Okigwe-A section, the black shale is altered to nodular clay, which is mottled yellow, brown, and purple immediately below the sandstone, while at Umuasuwa, shale is abruptly truncated by a 50-cm thick ironstone. These features may record a palaeosol along the bedrock erosional surfaces of the palaeovalley (Wright, 1986;Retallack, 1988;Kraus and Brown, 1988), suggesting that the basal surface was subaerially exposed to weathering before deposition of the overlying valley-fill sediments. This surface is also considered a transgressive surface of erosion.…”

Soil taxonomy and paleoenvironmental reconstruction: A critical commentary

“…We consider work such as that of Kraus (1987;Kraus and Bown, 1988;Kraus and Asian, 1993) on the alluvial paleosols in northwestern Wyoming to be a more effective use of paleosols to illuminate past environments. The lateral and vertical relations of ancient soil-like features were studied to identify what is termed pedofacies in various eocene fluvial environments.…”