Abs. No. 50287

COMMON UNIONID BIVALVES IN A THIN SANDSTONE OF THE JURASSIC BRUSHY BASIN MEMBER OF THE MORRISON FORMATION IN THE NORTHERN SAN RAFAEL SWELL AREA OF UTAH

Author(s): BURTON, David K.,and Crisp, Edward L., Geology Department, West Virginia University at Parkersburg, Parkersburg, WV 26101, ecrisp@alpha.wvup.wvnet.edu; Stone, Dwayne D., Geology Department, Marietta College, Marietta, OH 45750

Keywords: bivalves, Jurassic, Morrison Formation

Although several authors have reported the presence, and even localized abundance, of unionid bivalves in the Jurassic Morrison Formation, there is only a small amount of literature dealing with the taxonomy and distribution of unionid bivalves within this stratiagraphic unit. Common external and internal molds of unionid bivalves occur in a thin olive gray sandstone in the Brushy Basin Member of the Morrison Formation located in the SE 1/4 NW 1/4 Sec. 20 T17S R12E Flattop Mtn. 7.5 min. Quad., Emery Co., Utah. The sandstone has a maximum thickness of 0.356 meters, a lateral extent of about 30 meters, and is stratigraphically 25.2 meters below the base of the Cretaceous Buckhorn Conglomerate Member of the Cedar Mountain Formation. Light green mudstone is present above and below the bivalve sandstone. There appear to be three distinct morphologic types of bivalves present. Some of the bivalve molds indicate that the shells were articulated when buried, others indicate disarticulation. Sand to pebble-sized mud and lime mud clasts occur in the basal portion of the sandstone along with a few fragments of dinosaur bone. The sandstone is a well indurated, texturally immature, fine grained litharenite and may represent the deposits of a small stream channel that cut into the muds of the floodplain or perhaps a small overbank crevasse splay onto the floodplain.

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