File:Tidalites (Elatina Formation, Neoproterozoic; Pichi Richi Pass stream cut, South Flinders Ranges, South Australia) 2 (30441202460).jpg
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Tidalites in siltstone in the Precambrian of South Australia. Tidalites (tidal rhythmites) are rhythmically laminated sedimentary rocks that record monthly tidal depositional cycles. They can occur in siliciclastic or carbonate rocks. Well-preserved tidalites will readily show neap tides & spring tides and can allow days-per-month counts and days-per-year counts. This information can tell us hours-per-day in the ancient past. Early in Earth’s history, the number of days per year was much higher than the current 365. The number of hours per day was also significantly less than 24. How come? Earth's rotation rate has slowed over time. The South Australian tidalites shown above are probably the most famous examples on Earth. These are part of a 10 meter section of gently dipping, reddish-brown siltstones of the Elatina Formation. Tidalites form in facies at or very close to sea level. Laterally, along strike, the Elatina Formation has glaciofluvial sandstones and tillites. Paleomagnetic analysis has shown that this site was within 5° of latitude of the paleoequator. In other words, glaciers were present at sea level at the equator. That can only happen during an extreme ice age. Stratigraphic evidence from around the world indicates that two or three global or near-global ice ages occurred near the end of the Precambrian. These are called the Snowball Earth Glaciations. They were the most significant ice ages that Earth ever experienced. The most extreme models describing Snowball Earth have glacial ice completely covering all continents and all oceans, even at the equator. Some models, called “Slushball Earth”, have Earth’s equatorial oceanic areas not completely frozen over. Each Snowball Earth Glaciation was followed by a super-greenhouse climate. The resulting sedimentary record of these “freeze-fry” events typically consists of glacial tillites and overlying cap carbonates. These units are preserved at many localities on Earth. Stratigraphy: Elatina Formation, Marinoan, upper Cryogenian, middle Neoproterozoic Locality: outcrop along a dry creek on the eastern side of Rt. 47 at Pichi Richi Pass, southern South Flinders Ranges, South Australia (vicinity of 32° 25.266’ South latitude, 137° 58.338’ East longitude) |
| Date | |
| Source | Tidalites (Elatina Formation, Neoproterozoic; Pichi Richi Pass stream cut, South Flinders Ranges, South Australia) 2 |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/30441202460 (archive). It was reviewed on 6 December 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 02:12, 6 December 2019 | | 3,008 × 2,000 (5.34 MB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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