The Harkless Formation is a geologic formation in Nevada and California. It preserves fossils dating back to the Cambrian period.[1][2]
It is named from exposures on a divide south of Harkless Flat in the southern half of the Waucoba Mountain 15-min quadrangle in the Inyo Mountains of central eastern California.[3]
The Harkless Formation is mainly composed of siltstones, with many layers of fine to medium-grained quartzites, with limestone within the lower parts of the formation, although differs in various parts of the formation.[1] In the Split Mountain area, the formation underlies the Mule Spring Limestone,[1][4] whilst in other areas it underlies the Saline Valley Formation.[1]
The formation contains two members,[4] which are as follows, in ascending age:
- Weepah Member: Near the Andrews Mountain, at the type area of the formation, the member is dominated by greenish-gray and light-olive-gray siltstone, alongside a few layers of yellowish-gray fine to medium grained quartzite. The quartzite layers are identical to the layers found in the Zabriskie Quartzite. The siltstone ranges from grayish-olive, pale-olive, and dark-greenish-gray, and is composed of silt-size grains within a matrix of muscovite and chlorite, and is thinly laminated. They have also slightly metamorphosed, resulting in phyllite or hornfels. The hornfels are primarily composed of muscovite, chlorite, biotite, and quartz. Trace fossils are relatively common within the type area.[1] Within the Esmeralda County, Nevada, the lower sections of the member is still dominated by siltstone, although this differs greatly from the siltstones higher up in the formation. This is due to the layers being quartzitic, coarser and more resistant to natural erosion. The sizes of the grains within these layers hovers around the boundary between coarse silt and very fine sand.[1]
- Alkali Member: The upper Alkali member is composed of limestone layers, up to 100 ft (30 m) thick, inter-stratified with siltstone, and contain an abundance of archeocyathid sponges. Further still there are light-brown limestone layers, only up to 1 ft (0.30 m) thick, that contain Salterella, and are also inter-stratified with the siltstone layers in the middle of the formation. More limestone can be found at the top of the member, being gray or locally yellowish-brown, and up to 40 ft (12 m) thick.[1] In some areas of the member, there are also tongues from the Zabriskie Quartzite, composed primarily of medium-grained quartzite, and can get up to 300 ft (91 m) thick in some places, and inter-stratify with the siltstones. The colour of the quartzite is commonly yellowish-gray or greenish-gray. It is also laminated to thin-bedded. Trace fossils are also common within these quartzite tongues, also appearing on the underlying siltstone layers.[1]
The Harkless Formation contains a number of sponge spicules, hyoliths, brachiopods, and other small shelly fauna.[5][2] Alongside this, there are a myriad of arthropods, namely trilobites,[6] including a single bradoriid, Mongolitubulus.[7]
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Uncertain or tentative taxa are in small text; crossed out taxa are discredited.
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| Renalcid-like[9]
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Cyanophyte cyanobacteria.
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