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MOrrison Formation

(Tidewell Member, Salt Wash Member, Brushy Basin Member)

Age: Late Jurassic (Kimmeridgian) 155-148 Ma

Depositional Environment: Alluvial plain, fluvial channels and floodplains with paleosols


                     Subduction to the west created a back arc rift basin (between Morrison basin and paleo Pacific Ocean).

                     Mountain ranges (rift shoulder) to the west were source for clastic sediment

                     Calderas in rift basin provided abundant ash fall during Brushy Basin deposition



                 Located ~32° N (modern southern AZ)

                 Prevailing easterly winds (present day NE due to          
                 rotation of plate)

                 Warm, dry climate with high evaporation



Morrison Formation - 180-200 m thick

        Tidwell Member  (oldest) 

                Alluvial plain – streams, overbank deposits, paleosols;
                locally (in Capitol Reef area)

               gypsiferous, hyper-saline lagoons

               Varicolored mudstone with interbedded sandstone,
               limestone, gypsum

        Salt Wash Member (middle)

               Fluvial channel deposits, floodplain deposits, crevasse

               Predominately fine/medium sand - coarse sand/ pebble
               conglomerates; trough stratification,

               fining upward

        Brushy Basin Member (youngest)

                Lacustrine/ wetlands; local fluvial channels

                Varicolored mudstone

                Mostly ground and surface water flowing to the east
                (present day NE)

                Losing streams with associated riparian environments  
                prograding to the east

                Floodplains with paleosols; grassy savannahs


Figure 1: Paleogeographic map of the Middle Jurassic, Page Sandstone, Carmel Formation, Entrada Sandstone, Curtis Formation, and Summerville Formation. (Blakey, 2008)









Figure 2: Channel in Salt Wash Member (outlined with dotted line).

Last Updated: 5/14/21