Print this pageAbstract: Sedimentology of Morrison Formation in Southern San Juan Basin
C. L. Sandusky
A major climatic change took place in the southern San Juan basin within Jurassic time. With the advent of Morrison sedimentation, a transition from the northeast tradewinds to the westerlies can be seen as a change from dominantly arid, eolian sediments to more humid, fluvial deposits.
In early Morrison time, Recapture sediments reflect the braided-stream pattern of the wadi channels and predominantly oxidized silts and clays. An increase in the uplift of the source area accompanied by an increase in moisture during Westwater Canyon sedimentation resulted in an enhancement in the rate of weathering. This, in turn, allowed an increase in the rate of sedimentation and subsequent burial of the sediments and associated organic material. Deposited under reducing conditions, the Westwater Canyon sands exhibit the traits of a dominantly coarse-grained meander belt, whereas the Brushy Basin sediments show a sedimentation pattern more typical of meandering streams. Locally, a fine-grained meander belt was responsible for the deposition of the Jackpile sand. A slight tilting
nd beveling of Morrison sediments closed the Jurassic Period and allowed the 
introduction
of oxygenated waters.
Only by considering the processes and resultant environments can the exploration geologist properly evaluate a formation's potential as a uranium host.
AAPG Search and Discovery Article #90979©1975 AAPG – SEPM Rocky Mountain Sections Annual Meeting, Albuquerque, New Mexico