Formation of Lobster-bearing Concretions in the Late Cretaceous Bearpaw Shale, Montana, United States, in a Complex Geochemical Environment01/01/2012
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 the lobster cuticle, and traces of worm (?) burrows and fecal pellets were preserved. The remainder of the concretion, the outer zone, formed under normal pH conditions and was probably induced by the chemistry of the core. Size of the concretions relative to the size of the enclosed lobsters, lack of evidence of a burrow complex in the surrounding sediment, and central positioning of the lobster remains within the concretions do not support the contention that the lobsters were entombed within a burrow.