The Carmel Formation formed in a shallow inland sea during the Middle Jurassic and is located in parts of Utah and Arizona. It can be broken into four distinct members, one of which, the Co-op Creek Limestone Member, contains ooid shoals. The ooids in these shoals are calcitic with radial crystals and sparry cement. Several noteworthy features are found in the Carmel ooids, such as delamination, pressure solution, and microborings created by the cyanobacteria: Hyella sp. and/or Solentia sp. Foraminifera are sometimes incorporated into ooids as their nuclei. Seventeen of 21 Carmel thin sections contain foraminiferans inside or outside of ooids. Of these 17, 16 thin sections (94%) show more foraminiferans inside ooids than outside, meaning that ooids can act as taphonomic engineers, preserving what might otherwise not be preserved in the rock record. These foraminiferans likely belong to genera Turrispirulina and/or Ammodiscus. Eolian quartz silt is common in the Carmel shoals. The hypothesis of this study is that a pulse of quartz silt provided nuclei for the formation of the shoals and extinction of the shoals occurred when another pulse smothered it. This is partially supported by point counts, used to determine the percentage of each individual component of these limestones, and nuclei counts, used to determine the percentage of each type of nucleus found in these ooids. The locality that supports this hypothesis most strongly is C/W 142 EMR, which shows three distinct pulses of quartz accompanied by an inverse effect on the percentage of quartz nuclei. Locality C/W 757 DV is also of note, displaying a large amount of quartz early in the shoal’s life, decreasing over time. The percentage of ooids in the shoal shows the inverse. However, other shoals show no such pattern; one method of formation cannot be attributed to all of the Carmel Formation’s shoals, and even those geographically close show marked differences.
Cooke, Anna M., "Ooids of the Middle Jurassic Co-op Creek Limestone Member in the Carmel Formation, Southwestern Utah" (2020). Senior Independent Study Theses. Paper 8981.
Geology | Sedimentology
geology, ooids, Jurassic, Carmel Formation
Bachelor of Arts
Senior Independent Study Thesis
© Copyright 2020 Anna M. Cooke