The complex origin of ooids in the Middle Jurassic Carmel Formation of southwestern Utah: Anna Cooke’s Senior Independent Study thesis

Editor’s Note: Independent Study (IS) at The College of Wooster is a three-course series required of every student before graduation. Earth Sciences students typically begin in the second semester of their junior years with project identification, literature review, and a thesis essentially setting out the hypotheses and parameters of the work. Most students do fieldwork or lab work to collect data, and then spend their senior years finishing extensive Senior I.S. theses. This year we have the COVID-19 pandemic to deal with in the spring, so our students have not had a chance to publicly present their hard work and scientific ideas. Some, then, will be writing blog posts like this. The text and images here are from Anna Cooke (’20) who is a member of Team Utah 2019. The picture above shows Anna and fellow team member Evan Shadbolt (’20) on the top of Angel’s Landing in Zion National Park. (Photo by Nick Wiesenberg.) Now Anna takes over —

The Carmel Formation (shown above) 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.

Cross-bedded ooid shoal deposit in the Carmel Formation.

Ooids in unit C/W-758A.

I have nothing but positive things to say about my I.S. experience at Wooster. Over the last three semesters, I have had the privilege of researching the Carmel, a formation in southwestern Utah that several other students have done research in. My focus was on ooids: tiny spherical grains composed of calcium carbonate which form in specific marine environments. I have learned so much about these amazing little grains, though at times they made me want to tear my hair out (I personally marked, counted, and recorded the nuclei of over 17,000 ooids)! Though I.S. is a process that comes with a certain amount of stress and frustration, it was also a rich and rewarding experience for me. I learned so much about geology, as well as fieldwork methods and research, writing, and presentation skills. My favorite part of this experience was the field work, which I conducted the spring semester of my junior year with the help of the rest of Team Utah 2019. I am so grateful to everyone who has helped me along in this process, especially my wonderful advisor Dr. Wilson! Independent Study is something I will no doubt remember fondly for the rest of my life!