Origin of Dolomites in a Downslope Biostrome, Jefferson Formation (Frasnian), Central Idaho: Evidence From REE Patterns, Stable Isotopes, and Petrography

S. L. Dorobek

A completely dolomitized coral-stromatoporoid biostrome occurs at the top of the Dark Dolomite member of the Jefferson Formation (Frasnian) at Grandview Canyon, Lost River Range, central Idaho. The biostrome overlies a thick sequence of dolostones that were deposited in slope to deep ramp settings. The biostrome, therefore, formed in an open marine setting after shallowing of deep water environments.

Dolomite types can be subdivided according to their cathodoluminescent characteristics: homogeneous dully luminescent replacive dolomite (most common type), irregularly luminescent replacive dolomite, and zoned luminescent dolospar cement. Stable isotopes from replacive dolomites are -7.2 to -4.7^pmil ^dgr¹⁸O (average -5.8) and 0 to -0.4^pmil ^dgr¹³C (average -0.2), relative to PDB. Chondrite-normalized REE patterns from replacive dolomites show slight negative Ce and Eu anomalies and light REE enrichment. Ce anomalies in replacive dolomites become markedly more negative near an erosional surface that caps the biostrome.

Zoned dolospar cement fills dissolution vugs and tectonic fractures. Stable isotopes for zoned dolospar are -13.1 to -6.5^pmil ^dgr¹⁸O (average -11.5) and -1.5 to -0.1^pmil ^dgr¹³C (average -0.4). REE patterns for zoned dolospar have positive Ce anomalies, but total REE abundance is similar to REE abundance for replacive dolomites.

Stratigraphic occurrence in an open marine setting, stable isotopes, and REE patterns suggest replacive dolomite phases formed during shallow burial diagenesis with significant involvement of nonevaporated seawater. More negative Ce anomalies near the top of the biostrome suggest a diagenetic overprint by oxidizing meteoric waters. Zoned dolospar probably formed from warmer, reducing burial fluids. Carbon for zoned dolospar probably was recycled from preexisting dolomite. These data may be useful for interpreting the origin of other "anomalous" platform dolostones.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.