| Johnnie Formation | |
|---|---|
| Stratigraphic range: Ediacaran [1] | |
| Type | Geologic formation |
| Sub-units | See: Members |
| Underlies | Stirling Quartzite Formation |
| Overlies | Noonday Dolomite |
| Area | 10,000 sq mi (26,000 km2) [2] |
| Thickness | 0–5,000 feet (0–1,524 m) [2] |
| Lithology | |
| Primary | Shale |
| Other | Siltstone, Quartzite, Conglomerate, Limestone and Dolomite |
| Location | |
| Region | Mojave Desert, California and Nevada |
| Country | United States |
The Johnnie Formation is a geologic formation in the northern Mojave Desert of Inyo County, California and Nevada. [1]
It can be seen in the Providence Mountains and Kelso Mountains. [1] It also preserves microbial mat structures dating back to the Ediacaran period of the Neoproterozoic Era. [1]
Due to the large area and thickness of the formation, the Johnnie Formation primarily consists of shales, siltstones, quartzites, conglomerates, limestones and dolomites, outcropping in the Death Valley, Kingston Range, the Funeral Mountains, as well as Nevada, in the Spring Mountains, Nevada Test Site and Desert Range. It is also highly variable compared to other formations within the wider area, making it hard to correlate it to said other formations, which are more uniform. [2]
The Johnnie Formation is composed of six members in the Kingston Range of the Death Valley, listed here in ascending age:
This member is up to 485 ft (148 m) thick, and is dominated by very fine, medium-gray crystalline dolomite and sandy dolomite, which can weather to a pale-orange. The sandy dolomites contain fine to very coarse grains of quartz, and both the crystalline and sandy dolomite rocks contain scattered pebbles and granules of quartz. They are also inter-stratified with fine to coarse grained yellow gray quartzite, and greenish-gray siltstone, yellow gray shale and ark yellowish brown or dark gray argillite, making up 25% of the member. Algal structures are also known from this member. [2]
This member is up to 480 ft (150 m) thick, and as the name suggests, is dominated by yellowish-gray, pale-brownish-gray and pinkish-gray fine to coarse-grained quartzite rock. In some parts of the member, this quartzite is also conglomeratic, containing granules and pebbles of quartz. There are also small amounts of dolomitic sandstone, dolomite, and siltstone. [2]
This member is the thinnest member of the six, only getting up to 177 ft (54 m) thick, but is poorly defined. As the name suggests, it is composed of carbonate layers. In one part of the member, there are very fine medium-gray crystalline laminated to thin-bedded dolomite rocks up to 8 ft (2.4 m) , alongside a 25 ft (7.6 m) unit of dolomite, sandstone and shale. In most other areas of the known member, there are light-gray, olive-gray and pale-yellowish-brown dolomite and sandy dolomite, including small amounts of quartzite, dolomitic sandstone, and siltstone, up to 100 ft (30 m) thick. It also contains light-medium-gray massive dolomite, also up to 100 ft (30 m) thick. [2]
This member is up to 585 ft (178 m) thick, and as the name suggests, is primarily composed of siltstones, and sometimes either sandstone or quartzite. The siltstone is composed of fine to coarse silt, and is a moderate yellowish-brown, light-greenish-gray, or yellowish-gray. In some areas of the member, the siltstone has been metamorphosed, turning into a phyllitic siltstone or argillite. The siltstone is inter-stratified with pale to dark-yellowish-brown sandstone or quartzite, usually very fine grained and laminated to thin-bedded. There may also be tuffaceous rocks, although multiple studies have struggled to pin-point exactly what it is, possibly due to the rock being highly altered, or the amount of tuffaceous material is too low to be read. [2]
This member is up to 590 ft (180 m) thick, and is composed of dolomite, quartzite, and siltstone. The dolomite rocks are the distinctive feature of this member, being medium-gray, yellowish-gray or olive-gray on freshly revealed surfaces, weathering to a moderate-yellowish-brown or grayish-orange over time. It is aphanitic to very fine crystalline and is laminated to thin-bedded, and also contains irregular layers of gray chert in some areas. It also contains very fine to fine grains of quartz to dolomitic sandstone. As for the quartzite and sandstone layers of the member, they are yellowish-gray to pale-yellowish-brown, ranging from very fine to fine grained, rarely fine to medium grained, and laminated. The siltstone layers are greenish-gray to pale-yellowish-brown. All these layers are evenly proportioned in thickness at about 50 ft (15 m) at their thickest, although in some areas the dolomite layers can get as low as 10 ft (3.0 m). [2]
This member is the thicket member out of the six, getting up to 1,136 ft (346 m) thick, and is the most widespread, outcropping in the Death Valley Kingston Range area, the Spring Mountains, Nevada Test Site and the Desert Range, and is consistent in its rock types throughout its entire area. It itself is composed of three units, from bottom to top informally known as the "Siltstone Unit", "Carbonate Unit", and "Siltstone and Quartzite Unit", which consist of the following rocks: [2]
The Johnnie Formation is mostly noted for containing microbial laminations, [2] alongside Stromatolite structures. [1]
