Moonstone Formation
Geologic formation in Wyoming, United States
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Stratigraphy and Age
The Moonstone Formation unconformably overlies the lower to middle Miocene Split Rock Formation. While early studies in the 1960s tentatively assigned it a Pliocene age, modern radiometric dating and mammalian biochronology have refined its placement to the late Miocene (late Barstovian to early Hemphillian ages). U-Pb zircon dating of interbedded volcanic ashes within the formation has yielded precise ages of approximately 8.39 to .24 Ma. This indicates that the formation was deposited between roughly 11.4 and 6 million years ago. The formation occurred as the Seetwater Arch subsided, forming an east to west syncline. Faulting in the region created the nearby Seminoe and Ferris Mountains to the south.
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Lithology
The Moonstone formation is composed of sedimentary and volcanic rocks, including sandstone, shale, claystone, limestone, volcanic ash, and chaledony.
Paleontology
Various fossils have been found in the formation. Some of those include extinct: sabertooth cats, cricetid rodents, rabbits, and beaver. Black shale in the formation contains pollen and megaspores, indicating its age in the late Clarendonian period.