科研之友
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摘要
Are the depositional processes and architecture of Precambrian deep-water deposits the same of the Phanerozoic time? Few examples of Precambrian deep-water deposits exist in the literature. This paper describes the Apiuna Formation, a Neoproterozoic deep-water succession, c. 4500 m thick, located in SE Brazil. Detailed facies analysis, six large logs, 2000-3150 m thick, and a geological map have been applied to define types and geometrical distribution of three fine-grained and four coarse-grained architectural elements. Fine-grained elements (mudstone and fine-grained sandstone) constitute more than 70% of the thickness of the Apiuna Formation; these enclose coarse-grained elements, constituted of lenticular sandstone or pebbly sandstone bodies. Isolated coarsegrained elements display parallel palaeoflow directions suggesting multi-point source of the sediment. Due to this last aspect and the mud-dominance, the Apiuna Formation is interpreted as fine-grained ramp depositional system characterised by episodes of coarse-grained sedimentation. Nevertheless, this unit diverges from the Phanerozoic models for having (1) a wide range of grain-size, (2) small channel-levee systems, (3) immature clastic material and (4) irregular distribution of the architectural elements. In Apiuna sedimentary succession, five phases of coarse-grained sedimentation alternate to six phases of fine-grained sedimentation. In unvegetated earth, different rates of chemical weathering and mechanical erosion of the soils, controlled by climate and tectonic activity, can explain these variations of the elastic input. In rainy and warm climate conditions, Precambrian soils on silicate rocks formed thick clayey profiles. During quiescent or low tectonic activity, a dynamic equilibrium between weathering and erosion by runoffs on unprotected by rooted-vegetation soils produced high quantity of fine-grained material. During high tectonic activity and drier climate conditions, clayey soils did not form or were very reduced and mechanical erosion of the silicate rocks produced coarsegrained sediment. Thereby, high input of fine-grained deposits can be considered a peculiar characteristic of Precambrian deep-water systems.
- 出版日期2018-7
