Distinct styles of fluvial deposition in a Cambrian rift basin



Process-based and facies models to account for the origin of pre-vegetation (i.e. pre-Silurian) preserved fluvial sedimentary architectures remain poorly defined in terms of their ability to account for the nature of the fluvial conditions required to accumulate and preserve architectural elements in the absence of the stabilizing influence of vegetation. In pre-vegetation fluvial successions, the repeated reworking of bars and minor channels that resulted in the generation and preservation of broad, tabular, stacked sandstone-sheets has been previously regarded as the dominant sedimentary mechanism. This situation is closely analogous to modern-day poorly vegetated systems developed in arid climatic settings. However, this study demonstrates the widespread presence of substantially more complex stratigraphic architectures. The Guarda Velha Formation of Southern Brazil is a >500 m-thick synrift fluvial succession of Cambrian age that records the deposits and sedimentary architecture of three distinct fluvial successions: (i) an early rift-stage system characterized by coarse-grained channel elements indicative of a distributive pattern with flow transverse to the basin axis; and two coeval systems from the early- to climax-rift stages that represent (ii) an axially directed, trunk fluvial system characterized by large-scale amalgamated sandy braid-bar elements, and (iii) a distributive fluvial system characterized by multi-storey, sandy braided-channel elements that flowed transverse to the basin axis. Integration of facies and architectural-element analysis with regional stratigraphic basin analysis, palaeocurrent and pebble-provenance analysis demonstrates the mechanisms responsible for preserving the varied range of fluvial architectures present in this pre-vegetation, rift-basin setting. Identified major controls that influenced pre-vegetation fluvial sedimentary style include: (i) spatial and temporal variation in discharge regime; (ii) the varying sedimentological characteristics of distinct catchment areas; (iii) the role of tectonic basin configuration and its direct role in influencing palaeoflow direction and fluvial style, whereby both the axial and transverse fluvial systems undertook a distinctive response to syn-depositional movement on basin-bounding faults. Detailed architectural analyses of these deposits reveal significant variations in geometry, with characteristics considerably more complex than that of simple, laterally extensive, stacked sandstone-sheets predicted by most existing depositional models for pre-vegetation fluvial systems. These results suggest that the sheet-braided style actually encompasses a varied number of different pre-vegetation fluvial styles. Moreover, this study demonstrates that contemporaneous axial and transverse fluvial systems with distinctive architectural expressions can be preserved in the same overall tectonic and climatic setting.