10.2: The McCloud Limestone- An Ancient Coral Reef
- Page ID
- 21519
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Coral Reefs Around Volcanic Islands
During the Permian Period, somewhere in the vast expanse of the Panthalassa Ocean, there was a balmy tropical island arc. Like many tropical islands today, these islands were ringed with coral reefs. Today, fringing reefs form around volcanic islands and grow into barrier reefs, and atolls in response to a combination of island subsidence and sea level fluctuations ( Figures \(\PageIndex{1}\) and \(\PageIndex{2}\)).
The McCloud Fauna
From a distance, Permian coral reefs might have looked similar to modern coral reefs ( Figure \(\PageIndex{2}\)), but up close, Permian reef communities looked rather different than those present today ( Figure \(\PageIndex{3}\)). The end-Permian mass extinction, sometimes called the “Great Dying” was the largest mass extinction in earth history and it hit coral reefs hard. The leading interpretation for the cause of the extinction is an increase in greenhouse gasses globally related to large volcanic eruptions in Siberia. The increase in greenhouse gasses is thought to have corresponded with rapid warming, ocean acidification and decrease in oxygen (see 17.2: The Past). More than eighty percent of marine species went extinct. When coral reefs returned, the species that evolved to fill the open niches were very different.
The fossils of the McCloud Limestone have helped geologists piece together the history of the Eastern Klamath Terrane. Paleontologists have observed that coral and fusulinid species of the McCloud Limestone are distinctly different from the species found in rocks that were part of mainland Pangea at the time, such those found in the Basin and Range Province (see 8.6: The Oldest Rocks in the Basin and Range). This has been used as evidence to argue that the Eastern Klamath Terrane was of exotic origin, and must have been several thousand kilometers offshore during the Permian to prevent mixing of species with the Pangean coast. Review your knowledge of exotic terranes in Query \(\PageIndex{1}\).
Near Lake Shasta, a network of caverns has formed in the McCloud Limestone. Caverns form in limestone because it is soluble in acidic water. Natural rainwater is slightly acidic and therefore, water percolating through the ground can dissolve limestone. Groundwater can eat away large cavities of limestone. If the water table is then lowered, due to uplift or stream base level change, these cavities become caves or caverns. Continued flow of water through these caverns dissolves and reprecipitates the limestone creating features called speleothems. Stalactites and stalagmites are examples of speleothems.
There are many other limestone and marble caverns in the Klamath Mountains. Since coral reefs and other carbonate rocks often occur in island arcs, many of the Klamath terranes contain limestone and marble. Many of the caves in the Klamath Mountains are remote and only accessible to experienced spelunkers, but others have manufactured tunnel entrances and other infrastructure, which make them accessible to the general public. Oregon Caves (in Oregon) is a national monument (shown on map in Figure \(\PageIndex{2}\) and may be accessed by guided tour. Lake Shasta Caverns are privately owned and also shown to visitors by tour.
If you would like to learn more about how limestone caverns form, watch this video, “Karst”, which explains the process in greater detail.
References
- Belasky, P., Stevens, C., & Hanger, R. (2002). Early Permian location of western North American terranes based on brachiopod, fusulinid, and coral biogeography. Palaeogeography, Palaeoclimatology, Palaeoecology, 179, 245–266. https://doi.org/10.1016/S0031-0182(01)00437-0
- Harden, D. (2003). California Geology (2nd edition). Pearson.
- Hickey, H. (2018, December 6). What caused Earth’s biggest mass extinction? Stanford Doerr School of Sustainability. https://sustainability.stanford.edu/news/what-caused-earths-biggest-mass-extinction
- King, E. (2015, April 18). Ocean acidification triggered mass extinctions 252 million years ago. Climate Home News. https://www.climatechangenews.com/2015/04/18/ocean-acidification-triggered-mass-extinctions-252-million-years-ago/
- Lake Shasta Caverns National Natural Landmark. (n.d.). Sierra Nevada Geotourism Developed in Associattion with National Geographic. Retrieved July 17, 2024, from https://sierranevadageotourism.org/entries/lake-shasta-caverns-national-natural-landmark/969cc90d-b65b-40e4-bd7c-46d3f3bd1c97
- Stevens, C. H., Yancey, T. E., & Hanger, R. A. (1990). Significance of the provincial signature of Early Permian faunas of the eastern Klamath terrane. In D. S. Harwood & M. M. Miller (Eds.), Paleozoic and Early Mesozoic Paleogeographic Relations; Sierra Nevada, Klamath Mountains, and Related Terranes (Vol. 255, p. 0). Geological Society of America. https://doi.org/10.1130/SPE255-p201
- Wignall, P. B., & Bond, D. P. G. (2024). The great catastrophe: Causes of the Permo-Triassic marine mass extinction. National Science Review, 11(1), nwad273. https://doi.org/10.1093/nsr/nwad273
- Zubin-Stathopoulos, K. D., Beauchamp, B., Davydov, V. I., & Henderson, C. M. (2013). Variability of Pennsylvanian–Permian Carbonate Associations and implications for NW Pangea Palaeogeography, east-central British Columbia, Canada. Geological Society, London, Special Publications, 376(1), 47–72. https://doi.org/10.1144/SP376.1