Syndepositional Lateral Migration of Hyperpycnal Lobes in Submarine Ramp Systems. Early Miocene, Austral Basin, Argentina

Juan José Ponce, Eduardo B. Olivero, and Daniel R. Martinioni
Laboratorio de Geología Andina - Centro Austral de Investigaciones Científicas (CADIC-CONICET). Bernardo Houssay 200. (V9410CAB) Ushuaia, Tierra del Fuego, Argentina

During the Cenozoic the foredeep of the Austral Basin in Tierra del Fuego evolved as a ramp system until the Middle Miocene. This ramp system controlled in several ways the hyperpycnal lobe accumulation features in the Upper Cabo Ladrillero Beds (Figure 1; Ponce et al., in review). If compared to more restricted slope breaks, e. g. in some deltaic systems, the larger length of the slope in ramp systems promotes the sustained acceleration of hyperpycnal flows, allowing for the transfer of a large volume of sediments to the deep sea and favoring the deposition of thick sedimentary successions in internal areas of the basin. The main objective of this study is to document the internal arrangement of these hyperpycnal lobe systems -which are the dominant depositional elements within this unit- and to discuss the variability in geometry and sedimentary facies produced by the lateral migration of lobes during one hyperpycnal discharge.

The lobe deposits of the Upper Cabo Ladrillero Beds consist dominantly of thick sedimentary packages characterized by an initial coarsening-thickening trend, followed by a fining-thinning trend. These packages show transitional and recurrent passages of sedimentary structures -without sharp rheological boundaries-, and high concentration of Nothofagus leaves and phytodetritus. Sedimentary successions with similar internal trends were assigned to “typical hyperpycnites” by Mulder et al. (2003). Minor channel systems and slide blocks have been recognized also at the toe the depositional slope of these lobes at Cabo Ladrillero.

Individual hyperpycnal lobe packages are up to 3-m thick, with sharp base and coarsening-thickening trends followed by fining-thinning trends. The initial coarsening-thickening deposits, accumulated during the waxing stage of the hyperpycnal flow, consist of rhythmical successions of heterolithic, thinly bedded fine-grained sandstones with ripple cross-lamination, wavy and lenticular bedding, and massive mudstones. Dewatering structures and high phytodetrital concentration are commonly observed. The following fine-grained sandstones record transitional passage between horizons with climbing ripples, parallel lamination, and climbing dunes. The deposition of thick massive sandstone beds, with well-preserved Nothofagus leaves occurs when hyperpycnal flows in continuous acceleration are at or above maximum capacity. When maximum flow velocity is attained numerous erosive surfaces are produced. The following fining-thinning succession, originated during the waning stage of the hyperpycnal flow, is characterized by a transitional passage between fine-grained massive sandstones and sandstones with climbing dunes, parallel lamination, and climbing ripples. Bioturbated mudstones are usually at the uppermost part of this package.

In addition to these coarsening-thickening and then fining-thinning successions, which are interpreted to reflect the waxing-waning stages of the flow, there are other seemingly "anomalous" arrangements that cannot be interpreted as the "normal" succession between the waxing-waning stages of hyperpycnal flows. These "anomalous" arrangements are characterized by thinly bedded, usually convoluted heterolithic horizons that are often truncated by vanishing erosive surfaces. The common transition between these heterolithic horizons with thick, massive sandstones or sandstones with upper-flow regime sedimentary structures, e. g. climbing dunes and parallel lamination, is striking. Transitions between these contrasting lithologies and sedimentary structures are interpreted as the results of the lateral migration of depositional lobes (Fig. 2A, B). The overall geometry of the lobe deposits is similar to that resulting from compensation cycles. The latter, however, are generated by different sedimentation events while the lateral migrations of depositional lobes with the described "anomalous" sedimentary arrangements are thought to be formed during one hyperpycnal event. (Figs. 1, 2B).

Bibliografía

Mulder, T., Syvitski, J.P.M., Migeon, S., Faugères, J.C., Savoye, B., 2003. Marine hyperpycnal flows: initiation, behavior and related deposits. A review: Marine and Petroleum Geology 20, 861-882.

Ponce J.J., Olivero E.B., and Martinioni D.R, in review. Upper Oligocene-Miocene clinoforms of the foreland Austral Basin of Tierra del Fuego, Argentina: Stratigraphy, depositional sequences and architecture of the foredeep deposits. Journal of South American Earth Sciences.

Figure 1. Idealized representation of hyperpycnal lobe arrangements in a submarine ramp (plane view). Note that the different time slices (t1-t3) indicate the lateral migration of lobes during the same hyperpycnal event.