Patrick, Doreena1, D. E. Grandstaff2
(1) University of Pennsylvania, Philadelphia, PA
(2) Temple University, Philadelphia, PA
ABSTRACT: Rare Earth Element (REE) Analysis of Fossil Vertebrates from the Pierre Shale, Central South Dakota, and Interpretation of the Original Paleoenvironment
REE concentrations were measured in fossils collected from five members of the Pierre Shale at localities near the Missouri River. Fossils from each member of the Pierre Shale have been shown to have different REE signatures that correspond to other fossils from the same member. The REE patterns can indicate the original pore water composition and can be interpreted as “fingerprints” for their units. Since REE signatures reflect the composition of early diagenetic waters they can be used for paleoenvironmental interpretation. Differences in REE signatures among members can be interpreted as related to differences in mixing of oxic, shallow seawater and anoxic, deep waters. If differences in mixing are interpreted as depth differences, the Sharon Springs Member was deposited in deep, anoxic water, with gradual shallowing through the Sharon Springs. The REE signatures indicate further subdivisions of the Sharon Springs into three previously identified submembers; Upper, Middle and Lower Sharon Springs. The Gregory was deposited in shallow coastal water, and the overlying Crow Creek, DeGrey, and Verendrye Members in progressively deeper marine waters. These interpretations are generally consistent with those based on faunal diversity and lithological interpretation of the members. We use ternary diagrams for interpreting variations in REE patterns in fossil vertebrate samples. Representative light (Nd), middle (Gd) and heavy (Yb) REE are at the vertices of the triangle, which allows the basic shape of the REE pattern to be represented. These ternary diagrams can then be compared with those of natural waters to infer the original paleoenvironmental conditions.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.