Allocyclic controls in volcaniclastic fluvial successions: lessons from aluvial architecture of Cretaceous Chubut Group, Patagonia

UMAZANO, A.M.; BELLOSI, E.S.; KRAUSE, J.M.

Congreso; VII Congreso Latinoamericano de Sedimentología y XV Reunión Argentina de Sedimentología; 2016

Alluvial architecture shows the spatial distribution and proportion of channel belt and floodplain deposits, as well as associated features, such as grain-size, bed geometry and dimensions. Such analysis is a powerful tool to assess the role of extrinsic factors, including tectonism, climate, and eustatic sea-level changes. In addition, in pyroclastic rich successions, the tephra influx must be evaluated. The Cretaceous Puesto La Paloma (PLPM) and Cerro Castaño (CCM) members of the Cerro Barcino Formation (Chubut Group) represent pyroclastic-rich, alluvial-aeolian successions deposited in the Somuncurá-Cañadón Asfalto Basin during sag conditions and ash-fall input. The PLPM (up to 42 m) comprises sheet-like tuffaceous strata dominated by sand-size deposits, whereas the overlying CCM (up to 215 m) includes sheet to ribbon/channel sandstone bodies intercalated within tuffaceous and finer-grained deposits. Both units were formed without oceanic connection. The goal of this contribution is to recognize allocyclic signals applying alluvial stratigraphy concepts. The study area is located in the western sector of the basin and includes the following localities: Huanimán, Tres Cerros, Puesto Mesa-Cerro León, La Payanca, La Madrugada, and La Juanita. Except for the Tres Cerros, the sections define a roughly N-S oriented transect. The methods included: i) measurement of sedimentary logs and correlation using the PLPM-CCM contact and two regional tufacceous markers up to 5 m thick; ii) facies and architectural analysis; and iii) definition of discrete stratigraphic intervals according to different facies associations.Six facies associations were defined, which consist of ash-falls, sheet-floods, shallow lakes, aeolian, fluvial channel-belts and reworked debris-flows. The four stratigraphic intervals, denominated 1 to 4 in chronological order of deposition, increase their thicknesses toward the Puesto Mesa-Cerro León site. Interval 1 (18-42 m) corresponds to the PLPM and includes numerous sheet-flood deposits, carbonate-rich lacustrine, aeolian sandy facies and ash-fall beds. This interval is interpreted as an ephemeral and unconfined alluvial system that interacted with aeolian dunes and dry interdunes. Interval 2 (20-47 m) represents the lower part of the CCM and includes the lower marker bed. It shows an alternation of fluvial channel-belt deposits and floodplain facies with sediments formed in sheet-floods, lakes, and few ash-falls and debris-flows. The density of channel-belt bodies ranges from 10 to 36%, reaching higher values in the northern part of the study area, where they are also thicker. Interval 2 represents a permanent, meandering or locally straight-like fluvial system, and displays both an increase of lacustrine facies and a decrease of ash-fall deposits. Interval 3 (7.5-27 m) corresponds to the middle part of the CCM, and lacks channel-belt bodies. It has the highest volume of sheet-flood and ash-fall deposits. This entire interval records a floodplain setting. In relation to interval 2, the volume of debris-flow deposits remains constant and lacustrine facies subtly increases. Interval 4 (18-148 m) represents the upper part of the CCM and includes the upper marker bed. It comprises an alternation of channel-belt bodies and floodplain facies, the latter being characterized by sheet-floods, lakes, debris-flows and volcanic ash rain deposits. The density of channel-belt deposits ranges from 6 to 32%. Interval 4 is related to channelized and perennial fluvial systems with meandering and locally straight-like styles. The highest density of thicker channel bodies is in the northern part of the study area. In Puesto Mesa-Cerro León locality, this interval is particularly the thickest one and has the highest density of thicker channel-belt bodies.We interpret these two architectural patterns as the response of alternated periods with high (intervals 1 and 3) and low (intervals 2 and 4) aerial pyroclastic sediment supply. Intrabasinal tectonic activity in the northern area is detected in the intervals 2 and 4. In accordance to proposed synorogenic sedimentation model, increased local subsidence and wetter conditions would have favored the expansion of lakes, which resulted in the abundant lacustrine deposits of interval 4.