Sedimentary Facies Analysis / Фациальный анализ осадочных пород

The Guaritas sequence is the uppermost stratigraphical level of the Camaqua Basin (southern Brazil) and comprises an alluvial, deltaic and aeolian continental facies association up to 800 m thick. Facies mapping of this unit has revealed a lateral association of tributary fans and trunk braided rivers developed under semi-arid conditions.
Two main regions (lobes) of alluvial fan development can be discriminated and the source points of both coincide with synforms in the nearby basement. This depositional system presents a normal down-fan facies change. An anomalous lateral change of facies within the trunk river system is interpreted as having been inherited from pre-existing alluvial fan deposits.
The main alluvial facies comprise trough cross-stratified (74%) and horizontally bedded (7%) sandstones, massive (16%) and tabular cross-stratified (2%) orthoconglomerates, and massive mudstones (1%).
Vertical aggradation of three-dimensional subaqueous dunes, followed by an upper flow regime plane-bed phase, characterized the depositional events of the sandy areas of the alluvial system. Diffuse gravel sheets and minor longitudinal and transverse bars were the main geomorphological features of the gravelly alluvial reaches. Fine-grained sediments represent temporarily abandoned areas within the braided channel network.