Accommodation / sediment supply fluvial deposition controlled by base level changes and relative sea level fluctuations in the Mata Amarilla Formation (Early Upper Cretaceous), Southern Patagonia, Argentina

AUGUSTO NICOLÁS VARELA

San Miguel de Tucumán, Argentina

Congreso; 9th International Conference on Fluvial Sedimentology; 2009

Universidad Nacional de Tucumán - Fundación Miguel Lillo

Accommodation / sediment supply fluvial deposition controlled by base level changes and relative sea level fluctuations in the Mata Amarilla Formation (Early Upper Cretaceous), Southern Patagonia, Argentina Augusto Nicolás Varela Centro de Investigaciones Geológicas, Universidad Nacional de La Plata – CONICET, calle 1 Nº 644 (1900) La Plata, Argentina. e-mail: augustovarela@cig.museo.unlp.edu.ar The aim of this contribution is to study the tectonic, eustatic and climatic controls over the Mata Amarilla Formation sedimentation. The area selected for the current study is situated along the southern margin of the Viedma Lake, where the unit has its maximum outcropping thickness, reaching about 350 metres. The Mata Amarilla Formation (Feruglio in Fossa Mancini, 1938) is composed of an alternation of grey, green greyish and dark mudstones with whitish sandstones deposited under continental to litoral environmental conditions. This unit date back to the Early Upper Cretaceous and is a key element for understanding the passage from the extensional Rocas Verdes basin to the foreland Austral Basin. The Mata Amarilla Formation is deposited over the deltaic Piedra Clavada Formation and is overlained by the shoreface facies of the Anita Formation. Detailed sedimentological logs, facies analysis and architectural parametres were useful to define thirteen sedimentary units. These sections are different in their accommodation/sediment supply rates. Paleosol horizons appear interbedding within the channel belt along the geological column. The lower section of Mata Amarilla Formation cropping out to the west shows distal fluvial mudstones with subordinated fine and medium sandstones. While at the eastern side, the deposits form a coarsening upward sequence consisting of estuarine mudstones, flaser and wavy heterolithic and herringbone sandstones, and bayhead delta sandstones. The medium section is mainly fluvial and shows, from west to east, a definite transition from conglomerate braided systems to sandy meandering and anastomosing systems at the farther end. The upper section shows fine grained distal fluvial deposits in the west and littoral setting in the east. In this manner, the lower and upper section should represent higher accommodation / sediment supply conditions, whereas the medium section should correspond to a lower accommodation / sediment supply stage. The three sections show evidence of consistent paleoclimatic conditions, so the variations in accommodation / sediment supply registered for the Mata Amarilla Formation are inferred to be promoted by base level changes related to relative sea level oscillations. The strong west to east evolution of the fluvial systems is in correspondence to the direction of propagation of the Austral Basin fold and thrust belt. In this way, the relative sea level changes of the Mata Amarilla Formation respond to a tectonic control over a pure eustatic fluctuation. Fossa Mancini, E., Feruglio E. and Yussen de Campana J. C.  (1938). Una Reunión de Geólogos de Y.P.F. y el problema de la Terminología Estratigráfica. Boletín de Informaciones Petroleras, 171, 31-95.