Architectural analysis of fluvial deposits, Guará Formation (Upper Jurassic – Lower Cretaceous), southeastern Paraná Basin, RS, Brazil

The Guará Formation (Upper Jurassic) crops out in the western portion of Rio Grande do Sul along a track with north-south orientation, of which the northern part essentially consists of fluvial deposits indicative of... [ view full abstract ]

The Guará Formation (Upper Jurassic) crops out in the western portion of Rio Grande do Sul along a track with north-south orientation, of which the northern part essentially consists of fluvial deposits indicative of palaeodrainage to southwest. Despite the existence of outcrops with good vertical exposure and high lateral continuity, detailed analysis of the fluvial architecture of this unit has not previously been undertaken. Facies analysis has involved the measurement of six vertical logs (132 m total thickness), plus four architectural panels. Nine lithofacies are identified and these form the fill of eight architectural elements. These elements are grouped into sandstone bodies of two fluvial styles: (1) Deep perennial braided rivers are composed of simple and compound downstream-accretion elements (DAS and DAC), small and large-sized hollows (SHO and LHO), trough cross-stratified sets (TCS) and overbank sand and fine facies (OS and OF); (2) Poorly channelized ephemeral braided rivers are characterized by horizontally stratified sandstones (HSS) and trough cross-stratified sets (TCS). Two or more sandstone bodies of the same style amalgamate into 10 to 15 m-thick packages, each package representing an interval in which dominates one of the river styles. These packages of amalgamated sandstone bodies alternate vertically and are separated by bounding surfaces with lateral extents of several hundred meters. The packages reflect intervals with larger and more continuous water discharge (packages that amalgamate sandstone bodies of deep perennial rivers) or with high energy episodic discharge (packages amalgamating sandstone bodies of poorly channelized ephemeral rivers). The river systems of the northern proximal portion of the Guará Formation record low-frequency discharge variations controlled by the climate. Similarly, climatically controlled depositional shifts have been reported in the southern (distal) portion of this unit. Here, ephemeral fluvial deposition alternates with eolian dune and sandsheet deposits to form wetting-upward cycles. Both the proximal and the distal parts of the succession might be responding to the same climatic driver.