Evidence for Early Diagenesis in Presence of Soft Parts from Panopea Occidentalis (Mollusca:Pelecypoda) in the Fox Hills Fm. (Maastrichtian:Late Cretaceous), North Dakota

During the Late Maastrichtian, a back-barrier bay or lagoon along the Dakota Isthmus was the site of an extensive intertidal, muddy sand flat occupied by a population of the deep-burrowing pelecypod, Panopea occidentalis Meek and Hayden. This population was covered by a storm-generated overwash fan trapping these large, poorly mobile, clams in their burrows by a sheet of impenetrable sand. Population dynamics were studied by Palmateer (1996) and taphonomy was reported by Erickson and Palmateer (1996). Fifty-nine individuals were collected in living position, most encased in iron-rich concretions representing the deep burrows maintained by each clam into which storm generated sediments had infiltrated.

Implications of the burial scenario include recognition that tissues of these large bivalves, once expired, gravitated to the bottom half of each vertically oriented specimen, becoming infiltrated by entombing sediments. Over twenty years since being collected, a pattern of museum weathering has been recognized in several specimens suggesting localized, early mineralization within confines of the valves. In some, it appears that reducing conditions were created during tissue decay creating diagenesis of pyrite or marcasite which subsequently became unstable reacting with and damaging the shell from the inside out in stereotypical locations on the valves. Encasing sandstone is grain-supported very fine and fine quartz sand whereas the sand filling the interior is matrix-supported often with angular quartz grains floating in clay matrix which includes the cement. To investigate the diagenetic process we are using EDAX to identify chemical signatures throughout the fabric of a cross section of the specimen. Relative values are then contoured. In the first specimen, which has approximately 2 percent early siderite cement, maps of carbon and phosphorous suggest distributions controlled by presence of soft parts, whereas calcium does not. Additional data are being generated. Field work under NSFURPGY-9761 and EDAX under NSFDBI-0116408 to SLU.