Regional paleoclimatic and stratigraphic implications of paleosols and fluvial/overbank architecture in the Morrison Formation (Upper Jurassic), Western Interior, USA

Abstract

Paleosols in the Morrison Formation (Upper Jurassic) from the Western Interior and Colorado Plateau regions occur in fluvial/overbank and marginal-lacustrine depositional facies associated with aggradational settings, and at sequence-bounding unconformities that mark divisions between major aggradational and degradational successions. Pedogenic features within these horizons preserve important contextual information about the local and regional paleoclimate and paleoenvironment in which the soils formed. Floodplain and lake-margin paleosols show evidence that most of the Morrison basin was characterized by a semi-arid to tropical wet–dry paleoclimate with fluctuating groundwater conditions, a low precipitation to evaporation ratio, and weak to moderately seasonal precipitation. Paleosol ichnofauna show evidence of a diverse and opportunistic flora and fauna that exploited changing conditions and existing nutrient and moisture regimes. Changes in paleosol type and degree of development over the basin indicate the overall regional paleoclimate was drier in the western and southern portions of the basin. Vertical trends indicate paleoclimatic conditions over the basin became steadily more humid through time. Laterally continuous, well-developed, deeply weathered paleosols formed during times of little or no deposition and mark regional unconformities. The paleosols at these sequence-bounding unconformities serve as useful regional stratigraphic markers to trace genetic packages across the Morrison depositional basin and to determine regional accommodation trends.