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The Placentian Series: appearance of the oldest skeletalized faunas in southeastern Newfoundland
- Ed Landing, Paul Myrow, Alison P. Benus, Guy M. Narbonne
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- Journal:
- Journal of Paleontology / Volume 63 / Issue 6 / November 1989
- Published online by Cambridge University Press:
- 19 May 2016, pp. 739-769
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The lowest Cambrian of Avalon, or Placentian Series, is a relatively thick sequence (1,400 m) in southeastern Newfoundland. A newly proposed body fossil zonation supplements an existing trace fossil zonation of the lower part of the Placentian Series and includes strata to the top of the sub-trilobitic Lower Cambrian.
The Sabellidites cambriensis Zone brackets the Precambrian–Cambrian boundary and comprises peritidal and wave-influenced subtidal facies deposited during deepening through the lower part of the Chapel Island Formation. Younger “Ladatheca” cylindrica Zone strata include the deepest facies of the Chapel Island Formation. The base of the overlying Watsonella crosbyi Zone (a post-Nemakit Daldyn and pre-Tommotian equivalent) is significantly diachronous because the diagnostic mollusks were preferentially preserved in pyritiferous offshore muds rather than in coeval nearshore muds. High diversity, upper Watsonella crosbyi Zone faunas (18 species) are limited to peritidal limestones of member 4 and are dominated by calcareous small shelly fossils. A thick interval (ca. 430 m) without body fossils and an important episode of block faulting that led to 750 m of differential erosion preceded deposition of the lower part of the Bonavista Group (=Sunnaginia imbricata Zone, an interval considered to be largely older than the Tommotian). Although much Early Cambrian time may be lost as a result of erosion at Random Formation–Bonavista Group unconformities, many Watsonella crosbyi Zone species reappear in the Sunnaginia imbricata Zone. Shoaling accompanied the immigration event defining the base of the Camenella baltica Zone, and an unconformity following regional offlap marks the top of the Placentian Series.
Calcareous, and not phosphatic, composition is most common in earliest Cambrian shelly remains. Little evidence suggests that a global, Precambrian–Cambrian boundary interval “phosphogenic” event either resulted in deposition of local phosphate deposits in the Tethyan region or had a role in the appearance of mineralized skeletons.
Twenty metazoans and problematica and an alga are illustrated from the Chapel Island Formation. Bemella? vonbitteri Landing n. sp. and Halkieria stonei Landing n. sp. are described. The monoplacophoran Archaeospira? avalonensis Landing n. sp. has right-and left-handed conchs comparable to those of Archaeospira (=Yangtzespira) from China. Anabarites is the senior generic synonym of Tiksitheca.
Travertine deposits from along the South Tibetan Fault System near Nyalam, Tibet
- R. ZENTMYER, P. M. MYROW, D. L. NEWELL
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- Journal:
- Geological Magazine / Volume 145 / Issue 6 / November 2008
- Published online by Cambridge University Press:
- 09 September 2008, pp. 753-765
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A newly investigated travertine deposit in southern Tibet provides a window into Holocene hydrological, geomorphic and climatic processes near the boundary of the Tibetan Plateau and High Himalaya. Travertine, deposited as a result of the degassing of CO2-rich groundwater as it emerges on the Earth's surface, is in many cases formed along the trace of major crustal-scale faults in primarily extensional tectonic regimes. A travertine platform measuring roughly 1 km by 0.5 km exists near the town of Nyalam in southern Tibet along a major Himalayan down-to-the-N normal fault, the South Tibetan Fault System. A wide variety of travertine depositional textures and features are recorded in the platform on a series of terraces. Active travertine deposition was observed from spring mounds and seeps along the base of the platform at the modern river level. Palaeotemperatures of spring water, calculated from δ18O of the travertine, range from 9 to 25 °C, which closely matches the temperatures recorded from modern springs in the area. A complex geomorphological landscape records interaction between growing alluvial fans, travertine accumulation, and a rapidly down-cutting river with associated fluvial terraces. River incision was contemporaneous with travertine deposition, as indicated by cemented fluvial river gravel layers interbedded with travertine. High 87Sr/86Sr ratios in the travertine (mean of 0.7168) indicate subsurface fluid interaction with radiogenic crystalline rocks of the underlying Greater Himalaya. Uranium-series ages of the travertine platform range from 5400 a (±950 a) to 11600 a (±1000 a), and indicate a younging progression from higher terraces near the valley wall to lower terraces at present-day river level. Travertine that overlies a river gravel terrace 18 m above river level formed at 11600 a. This date yields a local incision rate of 1.6 mm a−1, consistent with estimated fluvial incision rates in the High Himalaya. The range of our U-series ages coincides with an interval of higher precipitation associated with greater intensity of the Indian monsoon, which led to elevated spring discharge and carbonate precipitation in this part of the High Himalayas.