Changes in the architecture of the fluvial systems of Paganzo Group (Late Paleozoic): an example of sea level control over coastal fluvial systems.

TEDESCO, A.M.; CICCIOLI, P.L.; SURIANO, J.; LIMARINO, C.O.

Bariloche

Congreso; IV Congreso Latinoamericano de Geología y XI Reunión Argentina de sedimentología; 2006

Asocición Argentina de Sedimentología

The idea of establishing a direct and unequivocal connection between sea level changes and specific configurations of alluvial systems tripped on the fact that other allocyclic controls, as tectonics and climate, also exert an important role in the nature of fluvial deposits. Therefore, the relative importance of each one of the allocyclic controls depends on such factors as distance of fluvial system to the sea, tectonic activity of the basin, climatic history, basin subsidence rate, etc. The examined Huaco section consists of Late Carboniferous and Early Permian deposits belonging to the lower and upper Paganzo Group, respectively. Even though this group records a very complex climatic history, the studied time interval was characterized by a uniform wet and temperate climate, and very low subsidence rate, with tectonic activity being limited to local movements along the basin margins. Additionally, the Huaco section was located close to the sea during the Late Carboniferous and Early Permian. Taking all of these factors into account, the considered time interval represents, a good opportunity to analyze the effect of sea level changes on fluvial systems. Five types of fluvial systems (FS) have been recognized in the studied interval. The lowermost (FS1) one rests on a low relief surface worked on marine and estuarine deposits of the Guandacol Formation. FS1 is made up by a complex of sandy channel belts (up to 8 m thick) characterized by DA architectural element without fine-grained floodplain deposits. FS1 is interpreted as large braided alluvial plains dominated by channel-avulsion sedimentation. FS2 consists of sandy multistory channel deposits up to 1,5 m thick, which are characterized by CH and SB architectural elements. One of the most conspicuous features of FS2 is the presence of two groups of floodplain deposits, including lenticular beds of coals and organic-rich mudstones with abundant plant remains and metric-scale successions of interbedded fine-grained sandstones and mudstones. An anastomosed channel network with broad vegetated and poorly drained flood plains is supposed to be the origin for FS2. FS3 occurs intercalated between two marine transgressive deposits and is almost entirely composed of coarse-grained sandstones and a few conglomerate beds, forming a complex of multistory channels that are up to 3m thick and characterized by CH architectural elements. Each complex channel is bounded by a 5 order planar surface, but in some cases complex channels are amalgamated, when 5 order surfaces are not clearly noted. A distinctive feature of the FS3 is the presence of higher order planar surfaces (6-order) originated by long-term stabilizations of the alluvial plains. DA and SB architectural elements were recognized in this FS. Thin intercalations of flood plain or top bar deposits occur sporadically. FS3 is likely to have been formed in braided alluvial plains that showed channels dominated by transversal bars. FS4 covers the Sakmarian transgression deposits. This unit shows two types of channel deposits: principal channels (up to 6 m, frequently with DA element) bounded by a low-relief 5-order surface and individual channels up to 0.7 m thick and displaying SB elements. Flood plain deposits (up to 1 m thick) are plant-bearing shales that might show poorly developed paleosols. FF and CS elements can be recognized. A model of anastomosed fluvial system is believed for FS4. This unit is transitionally overlain by FS5, which displays sandy channel deposits mainly composed of LA architectural element, and muddy and sandy flood plain deposits with FF, CS and CR architectural elements. A conspicuous feature of this FS is the development of fining-upward cycles ranging from 3 to 5m thick. FS5 is interpreted as a meandering fluvial system. Changes in the architecture of the fluvial systems have been correlated with sea level variations. Thus, FS1 is related to the lowstand stage and FS2 is linked to the transgressive stage of the Stephanian sea level rise. Small decrease in sea level produced incision of the underlying Stephanian marine deposits. Later, the incised surface was filled by FS3 sediments related to the Sakmarian transgression. Despite lacking marine transgression younger than Sakmarian in Huaco, FS4 probably correlates with transgressive marine facies of the neighboring upper Cerro Agua Negra Formation, while FS5 represents highstand conditions.