Arkose
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Arkose (IPA: /ˈɑː(r)kəʊz/) is a detrital sedimentary rock containing roughly 60% quartz sand and 25% feldspar, typically grey to reddish in colour. The sand may range from fine to very coarse, but tends toward the coarser end of the scale. Arkose usually contains small amounts of calcite cement, which causes it to effervesce slightly in dilute hydrochloric acid; sometimes the cement also contains iron oxide. Similarly, arkosic sand is sand that is rich in weathered feldspar.
Arkose is generally formed from the weathering of granitic rocks, which are primarily composed of quartz and feldspar. Occasionally it contains fragments of mica from the parent rock. These sediments must be deposited rapidly and/or in an arid environment such that the feldspar will not undergo chemical weathering and decompose; therefore arkose is designated a texturally immature sedimentary rock. Common facies in which arkose is found include conglomerate deposits with granitic source rocks and above unconformities in granitic terrain.
Due to these necessities of the depositional process, fossils are rare in arkose, though bedding is frequently visible.
The famous central Australian monolith Uluru (Ayers Rock) is composed of Proterozoic arkose, deposited in the marine Amadeus Basin and uplifted during the middle Paleozoic.
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