SEDIMENTOLOGY AND DEPOSITIONAL FACIES OF THREE NEWLY DESCRIBED MEMBERS OF THE MORRISON FORMATION IN THE WESTERN PANHANDLE OF OKLAHOMA

Basin margin sediments of the Morrison Formation in the Oklahoma panhandle are divided into three newly defined members. Each member is established by geological, geochemical, and petrographical characteristics with distinctive depositional facies and different climatic signals.

The Cimarron Member is composed of numerous thin bedded ripple-laminated sandstone beds representing small, shallow, ephemeral, clastic lakes at the distal edge of a distributive fluvial system (DFS) and was deposited under a seasonally wet climate. Mudstones are highly illitic (98.5%). Siltstone and sandstone beds have limited lateral extent and are composed of very angular (0.45), moderately well sorted (0.56) coarse-grained silt (4.20 Φ) to fine-grained sand (3.71 Φ) quartz arenites. The top of the member is defined by evaporite-associated cherts.

The Boise Member consists of thin pelmicrite or pelsparite beds which represent shallow, perennial, carbonate lakes. Mudstones are dominantly illitic (89.6%). The top of the member is defined by a regionally extensive couplet of thick bedded microbial limestone and a subangular (1.56), well sorted (0.41), fine-grained (2.38 Φ) quartz arenite. These units suggest a regionally extensive lake, herein called Lake Stovall. Lake Stovall lacustrine facies may have extended over 240 km² and indicate a wet climate.

The Kenton Member marks the disappearance of Lake Stovall and progradation of a DFS to the basin margin. Fluvial deposits consist of small isolated anastomosing channels and splays deposited on a mud-dominated illitic (75.4%) mudstone floodplain. Channel sediments are subangular (1.50), well sorted (0.41), fine-grained (2.33 Φ) quartz arenites. Splay sediments are subangular (1.36), very well sorted (0.32), fine-grained (2.60 Φ) quartz arenites. Near the top of the formation is a 7-m thick sandy braided fluvial sandstone bed. The braided channel sediments are subrounded (2.03), well sorted (0.39), medium-grained (1.63 Φ) quartz arenites. The channel bedload consists of pebble, granule, very coarse- and coarse-grained sediments (0.60 Φ), signifying the transport of coarse-grained sediments to the basin margin. The climate was seasonally dry.

Each member was deposited under distinct climatic conditions. Each is an excellent example of foreland basin margin sedimentation.