Paleosol evolution in a sequence stratigraphic framework, Cretaceous Ferron Notom Delta, south central Utah, U.S.A.



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Hydromorphic paleosols have been previously thought of as ever wet and poorly developed, and lacking useful paleoenviromental and paleoclimatic indicators. However, a suite of pedogenic features and geochemical attributes of hydromorphic floodplain paleosols of the youngest non-marine (100ka) sequence in the Cretaceous Ferron Notom Delta indicate the prevalence of shrink-swell processes, suggesting fluctuating soil moisture conditions. Alternating soil moisture conditions suggest seasonality in rainfall, linked to a paleoclimate with more wet periods than dry. Vertical organization of paleosols in the sequence reveals simple and compound vertical paleosol profiles that were integrated with channel-fill deposits to produce a high-resolution interpretation of the sequence. Detailed facies analysis reveal 11 depositional facies that built 33 fluvial aggradational cycles, 9 fluvial aggradation cycles sets, and 3 sequences, within what was previously classified as a single sequence. Fluvial aggradation cycles in the succession are either simple floodplain bedsets or single channel stories that are bounded by paleosols. They represent rapid depositional events that probably span less than 500 years when compared to modern analogs of the Ferron Notom Delta, and were followed by short hiatus (10 to < 500 years), possibly reflecting short-term avulsions. Fluvial aggradational cycle sets in the succession are aggregates of fluvial aggradational cycles, bounded by relatively mature paleosols or erosionally top-truncation. They represent multiple small-scale depositional events that span not more than 10Ka, followed by relatively long (1 to < 5Ka) hiati, which suggest relatively long-term river avulsions. Fluvial aggradational cycle sets are often capped by coals and carbonaceous strata. Associated marine trace fossils above coal beds suggest marine transgression due to compactional subsidence or eustatic sea-level rise. The similarity in the scale of cyclicity in fluvial aggradation cycle sets (10Ka) and marine parasequences of the Ferron Notom Delta (10 to 15Ka) may suggest a link between non-marine and marine cyclicity. Fluvial sequences are aggregates of fluvial aggradational cycle sets that are bounded by unconformities or their correlative conformities that represent relatively longer periods (5 to < 15Ka) of subaerial exposure and pedogenic development. Cyclicity on the sequence-scale in this succession is attributed to high-frequency Milankovitch-scale (~20Ka; 40Ka) fluctuations in sea level.



Paleosols, Ferron Notom Delta