Date of Award:

1987

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

Peter T. Kolesar, Jr.

Abstract

The Middle Cambrian Langston and Ute formations were studied in the northern portion of the Bear River Range and the south-central part of the Portneuf Range in southeastern Idaho.

The rocks of the Langston and Ute formations were divided into 17 different rock types. The 17 rock types were formed within four recognizable lithofacies: I) Shoal - or Coastal -Peritidal Carbonate Complex; 2) Agitated Shoal; 3) Inner Marine Shelf; and 4) Outer Marine Shelf. Clastic sediments belonging to the Spence Shale Member of the Langston Formation (outer marine shelf) were deposited over the carbonate complex . A transgressive sequence marks the base of both formations.

Paleomagnetic evidence suggests that during the time of the deposition of the two formations, the study area was located near the outer reaches of an equatorial epeiric sea. Clay mineralogy of insoluble residues suggests a warm, humid, tropical climate.

Eogenetic diagenetic features include compaction, cementation, aggrading, and degrading neomorphism, birdseye structures, and initiation of dolomitization . Mesogenetic diagenetic features include dolomitization and pressure solution. Telogenetic diagenetic features are confined to fracturing and subsequent calcite infilling, and the oxidation of pyrite.

Massive dolomitization in the northernmost section is believed to be the result of a downward flux of fluids originating as hypersaline brines. The nonconformable lower contact, conformable upper contact, the vertical and lateral extent of dolomitization, and the general association with coastal-peritidal facies have led to this conclusion. Two other subordinate types of dolomite are believed to be the result of: 1) the release of magnesium caused by the decomposition of magnesium-rich organic matter; and 2) the formation of a secondary ferroan dolomite as Fe/Mg ratios in the precipitating fluids increased under reducing conditions.

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