PETROLOGY OF EVAPORITE-ASSOCIATED CHERTS OF THE UPPER JURASSIC MORRISON FORMATION, OKLAHOMA PANHANDLE AND THEIR CLIMATIC IMPLICATION

The Morrison Formation of Oklahoma can be divided into three informal members based on sedimentology and depositional facies. The lower member is comprised of numerous shallow ephemeral clastic lakes. The middle member consists of large shallow perennial algal carbonate lakes. The transition between the two members is marked by a series of 10‒20 cm thick chert beds that are stratigraphically 14‒17 m above the Lower Jurassic Exeter Sandstone. The lowest chert beds are one meter above the regional Morrison chronostratigraphic illite/smectite clay change.

Magadi-type cherts, which form in association with sodic evaporites, have been reported from the formation in Colorado and Wyoming. However, the Oklahoma evaporite-associated cherts formed in a shallow acidic (pH > 9) phreatic zone as a result of silicification of sulfate evaporites. The chert is white with red or gray-blue chalcedony spherules, and occasionally mimics chicken-wire anhydrite. It consists of silica and authigenic minerals including dolomite, barite, and celestine. Anhydrite inclusions also occur. The silica fabric consists of euhedral quartz (> 2 mm), megaquartz grains, chalcedony spherules, length-slow and zebraic chalcedony, and “primrose” quartz. These silica structures form by infilling anhydrite dissolution cavities. The large euhedral grains, megaquartz, and length-slow chalcedony indicate a high Si concentration in the phreatic brines during crystallization. Many of the euhedral quartz, chalcedony spherules, and “primrose” quartz display growth bands, signifying changes in phreatic water chemistry or pH. Late in the diagenetic history, the chert was fractured, resulting in sheared grains and chalcedony. Sparry calcite cement infilled voids and fractures.

Sediments record wet paleoclimatic trends separated by a dry interval. The antecedent lacustrine evaporites designate a period of increased evaporation, due to either increased temperature or decreased precipitation. The chert zone and the large alkaline Lake T’oo’dichi of the southern Colorado Plateau are both stratigraphically just above the clay change. If a chronostratigraphic correlation could be established, it would indicate that the regional dry climatic interval defined by Lake T’oo’dichi extended to the southeastern margin of the Morrison foreland basin.