GEOCHEMISTRY AND SEQUENCE STRATIGRAPHY OF THE MIDDLE CAMBRIAN WHEELER FORMATION: SUPPORT FOR TRILOBITE CHRONOSTRATIGRAPHY

The Middle Cambrian Wheeler Formation of western Utah was deposited in carbonate platform, slope, and basin environments of the House Range embayment (HRE), presumably, during a single 3rd order cycle. In the Drum Mountains, the Wheeler Formation (306 m thick) is dominated by carbonate slope and shallow platform deposits. At Marjum Pass, in the central House Range (40 km to the southwest), the Wheeler Formation (190 m thick) is dominated by basinal shale deposits. The Wheeler Formation is within the Bolaspidella trilobite range zone, but preserves only one biozone marker; the first appearance of Ptychagnostus atavus (P. atavus) replacing Ptychagnostus gibbus (P. gibbus). Lack of any other biozone or chronostratigraphic markers and the lack of distinctive stratal patterns in the basinal facies makes correlation across this platform-to-basin transect difficult. Therefore, carbon-isotope stratigraphy and total organic carbon analysis were used as intrabasinal correlation tools.

d¹³C_carbonate isotope values range from –1.7‰ to 0.07‰ (PDB) at Marjum Pass and –1.1‰ to 1.4‰ (PDB) in the Drum Mountains. Previously reported d¹³C_carbonate values in the Great Basin for this time interval range between –2‰ to 2‰ (PDB). In both localities, the isotopic values show small-scale variability and become more negative upsection. This small-scale variability was used to draw chemostratigraphic time-lines between the two sections. TOC values at both sections decrease upsection and define a distinct peak. This peak, along with other lithologic features, was used to define the maximum flooding zone (MFZ).

These platform-to-basin geochemical correlations suggest that the first appearance of P. atavus was coeval with transgression in both the Drum Mountains and Marjum Pass localities. P. atavus has been identified ~10 m below the MFZ in both sections. Therefore, the sequence stratigraphy of the Wheeler Formation supports the first appearance of P. atavus as a chronostratigraphic biomarker, strengthening the potential utility of P. atavus as a global correlation tool for the Middle Cambrian.