Emigrant Formation
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- (Outer Shelf) Mule Spring Limestone
- (Middle Shelf) Carrara Formation
| Emigrant Formation | |
|---|---|
| Stratigraphic range: | |
| Type | Formation |
| Sub-units | See: Members |
| Underlies | Palmetto Formation |
| Overlies |
|
| Lithology | |
| Primary | Shale |
| Other | Mudstone, Limestone, Chert, Sandstone |
| Location | |
| Region | Nevada |
| Country | United States |
| Type section | |
| Named for | Emigrant Pass |
The Emigrant Formation is a Cambrian to Ordovician geologic formation in Nevada. It preserves fossils dating back to the Cambrian period, as well as the Ordovician period.
Members
The Emigrant Formation is dominated by shales, with limestone common at the base and top of the formation. It is overlain by the Palmetto Formation, whilst it is underlain by the limestone dominated Mule Spring Limestone in the Outer Shelf of Nevada, and the siltstone and shale dominated Carrara Formation in the Middle Shelf of Nevada.[1][2]
The formation contains three members, which are as follows, in ascending stratigraphic order (lowest to highest):
- Limestone & Siltstone Member: This member is the thinnest of the three, only getting up to 150 m (490 ft) thick in some areas, and is dominated by lime mudstone. At the base of the member, it is entirely composed of shale, which gives way to calcareous sandstone, and then the common lime mudstones above that. Within these lime mudstones, there are occasional layers of more shale and chert.[1]
- Shale Member: This member can get up to 250 m (820 ft) thick in some areas, and is, as the name suggests, entirely dominated by shale, which may be faulted and repeated in many places.[1]
- Limestone & Chert Member: This member is the thickest of the three, getting up to 300 m (980 ft) thick in some areas, and contains a mixture of ribbon limestone, inter-bedded shales, as well as mega-breccia.[1]
The Emigrant Formation also contains lower Ordovician aged rocks above the Limestone & Chert Member, informally known as the "Platy Limestone Unit". This unit is primarily composed of fine-grained, platy limestone, which are inter-bedded with pale-red siltstone. The very top of the unit is not preserved, and contacts the overlying Palmetto Formation at a fault, and the unit itself is also difficult to age, although a lower Ordovician age is suggested, though more evidence will be needed to pinpoint the exact age of the unit. It has also been noted that this unit may represent a transitional bed between the Emigrant and Palmetto Formations.[3]
