Changes in the Ichnologic, Pedologic, and Sedimentologic Characters During Aridification Between the Lower Jurassic Kayenta Formation and the Jurassic Navajo Sandstone, Colorado Plateau

The transition from the Lower Jurassic Kayenta Formation (Jk) into the Navajo Sandstone (Jn) represents as a major aridification period recorded by changes in the ichnologic, pedologic, and sedimentologic characters of interfingering fluvial-lacustrine (Jk) and eolian deposits (Jn). This transition began earlier in the eastern part of the Colorado Plateau and continued diachronously westward, where prominent interfingering is recorded by the Lamb Point Tongue (Jnl) of the Jn and the Tenney Canyon Tongue (Jkt) of the Jk in southcentral Utah.

A hydroclimatic shift towards drier conditions––lower ratio of precipitation to evaporation (P/E)—likely began within the upper part of Jk in the thinner eastern as well as the thicker western parts of the basin. This transition is recorded by changes in the freshwater aquatic and terrestrial plant and animal trace-fossil assemblages from comparatively high diversity to low or no diversity recorded in the strata. In general, ichnodiversity decreased from the Jk to the Jn, with the exception of higher diversity in sedimentary facies recording wetter settings. Paleosols formed in sedimentary facies where (1) pedogenesis outpaced sedimentation during periods of nonsteady sediment delivery and (2) the P/E ratio was sufficient to stabilize the surface with vegetation. Evidence for pedogenesis is the co-occurrence of plant, invertebrates, and vertebrate terrestrial traces and an array of ped features. Weak to moderate pedogenesis occurred during wetter periods, whereas drier periods recorded little or no pedogenesis due to the lack of freshwater to support life, except for wetter intervals during the drier phases.