FORMATION OF LOBSTER-BEARING CONCRETIONS IN THE LATE CRETACEOUS BEARPAW SHALE, MONTANA, UNITED STATES, IN A COMPLEX GEOCHEMICAL ENVIRONMENT

Abstract

Abstract Eleven concretions containing the nephropid lobster, Palaeonephrops browni (Whitfield, 1907), from the Upper Cretaceous (Campanian), Bearpaw Formation in northeastern Montana, were examined using visual and geochemical methods. The concretions were zoned, with an axial, phosphate-rich core also containing calcium surrounding the lobster remains and an outer, calcium-rich zone lacking phosphate. The overall composition documents these as carbonate concretions, not phosphatic concretions. Where visible, the inner zone is sheathed in a thin layer dominated by framboidal pyrite, suggesting formation by a microbial film. The different geochemical settings in the inner versus outer zones suggest reduced pH conditions during formation of the inner core and normal pH conditions resulting in formation of the outer zone. The pattern is suggestive of extremely rapid preservation of the lobster remains within a microbial sheath in which a calcium phosphate mineral, probably francolite, delicately replaced th...