The earliest Phanerozoic carbonate hardground (Cambrian Stage 5, Series 3): Implications to the paleoseawater chemistry and early adaptation of hardground fauna
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- The earliest Phanerozoic carbonate hardground (Cambrian Stage 5, Series 3): Implications to the paleoseawater chemistry and early adaptation of hardground fauna
- Lee, Jeong-Hyun
- Physical Geography; Geology; Paleontology
- Hardground; Calcite sea; Cambrian Series 3; North China Platform
- Issue Date
- Lee, Jeong-Hyun, Jitao Chen, and Jusun Woo. 2015. The earliest Phanerozoic carbonate hardground (Cambrian Stage 5, Series 3): Implications to the paleoseawater chemistry and early adaptation of hardground fauna. Palaeogeography, Palaeoclimatology, Palaeoecology, 440: 172？179.
- Carbonate hardgrounds are lithified seafloors formed by synsedimentary cementation of carbonate sediments,
which dominantly occur during the period of calcite seas. The earliest typical Phanerozoic hardground known
until now was reported from the Furongian of USA, which was suggested to indicate onset of the early Paleozoic
calcite sea period. In this study, we report hardgrounds from the early and middle parts of the Cambrian Series 3
(Stage 5 and Drumian) of the North China Platform, which predate previously reported hardgrounds. The
hardground surfaces developed on oolitic grainstone, oncolitic wackestone, and microbialite (thrombolite and
dendrolite),which sharply truncate the underlying deposits. The radial fibrous calcite cements between the carbonate
grains below the hardground surfaces indicate that the cements formed by early marine cementation.
EPMA analysis reveals that the fibrous cements typically consist of low-Mg calcite. The hardgrounds are sometimes
encrusted by microbialites and coated by hematite, suggesting long exposure to the open seawater after
formation of the surface. In addition, detailed review on the sedimentological studies of Cambrian Series 3 to
Furongian deposits throughout the world reveals that there may be several other hardgrounds during these
times, which could have been overlooked. The abundant occurrence of hardgrounds in Cambrian Series 3 deposits
suggests that the general paleoseawater chemistry was suitable to induce synsedimentary cementation
of low-Mg calcite, implying that seawater chemistry would have changed fromthe aragonite to calcite seas during
the Cambrian Series 3 or even earlier period. Metazoan encrustors and macroborers possibly could not have
adapted to the newly appeared substrate condition yet, until the latest Cambrian Series 3.
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