Alternating coarse- and fine-grained sedimentation in Precambrian deep-water ramp (Apiúna Formation, SE of Brazil): Tectonic and climate control or sea level variations?

• Published in: Precambrian research 2018-07, Vol.311, p.211-227
• Main Authors: Basilici, Giorgio, Vidal, Alexandre Campane
• Format: Article
• Language: English
• Subjects: Apiúna Formation; Deep-water fine-grained ramp; Precambrian; SE Brazil
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2018.04.014

[Display omitted] •This is one of the few Precambrian deep-water systems described in literature.•Apiúna Formation is interpreted as fine-grained deep-water ramp.•Episodic coarse-grained input alternated with overall fine-grained sedimentation.•Unsheltered by rooted-vegetation clayey soils brought about high fine-grained input.•Tectonics and climate controlled the alternating phases of coarse and fine input. 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 Apiúna 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 Apiúna Formation; these enclose coarse-grained elements, constituted of lenticular sandstone or pebbly sandstone bodies. Isolated coarse-grained elements display parallel palaeoflow directions suggesting multi-point source of the sediment. Due to this last aspect and the mud-dominance, the Apiúna 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 Apiúna 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 clastic 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 coarse-gra