| Moonstone Formation | |
|---|---|
| Stratigraphic range: Late Miocene to Early Pliocene | |
 | |
| Type | Formation |
| Thickness | Up to 400 metres (1,300 ft) |
| Lithology | |
| Primary | Sandstone, shale, claystone, limestone, tuff |
| Location | |
| Location | Granite Mountains, Wyoming, US |
| Coordinates | 42°28′N 107°30′W / 42.467°N 107.500°W |
The Moonstone Formation is a geological formation of late Cenozoic age, consisting of approximately 400 meters of fluvial (river-related) and lacustrine (lake-related) deposits. It is best known for mantling the Precambrian granites of the Sweetwater Arch in central Wyoming. The formation is significant for its rich vertebrate fossil record and its role in dating the subsidence and faulting that shaped the modern Wyoming landscape.
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.[1]
Lithology
The Moonstone formation is composed of sedimentary and volcanic rocks, including sandstone, shale, claystone, limestone, volcanic ash, and chaledony.[2][3]
Paleontology
Various fossils have been found in the formation. Some of those include exctinct: sabertooth cats, cricetid rodents, rabbits and beaver. Black shale in the formation contains pollen and megaspores indicating its age in the late Clarendonian period.[4]
See also
- Split Rock, which has evidence of the formation.
References
- ↑ "Vertebrate biostratigraphy and U-Pb zircon geochronology of the Moonstone Formation". Geological Society of America. Retrieved 2026-01-18.
- ↑ "Geology of the Rocks of the Granite Mountains, Wyoming" (PDF). USGS Publications Warehouse. Retrieved 2026-01-18.
- ↑ "Cenozoic Geology of the Granite Mountains Area, Central Wyoming". University of North Texas Digital Library. Retrieved 2026-01-18.
- ↑ "Magnetic stratigraphy and vertebrate biostratigraphy of the Moonstone Formation". Geological Society of America. Retrieved 2026-01-18.
External links
- Cenozoic Geology of the Granite Mountains Area, Central Wyoming (USGS Bulletin 1121-I)
- National Geologic Map Database – Moonstone Formation Details
- USGS Professional Paper 495-C: Cenozoic Geology of the Granite Mountains Area
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