Deep-Marine Hyperpycnal Channel-Levee Complexes in the Miocene of Tierra Del Fuego, Argentina: Architectural Elements and Facies Associations

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

1. Introduction

In the Miocene Cabo Viamonte beds, Austral Basin of Tierra del Fuego, deep-marine channel-levee complexes (CLC) were recognized at the toe of depositional slope breaks in sigmoid clinoforms (Ponce et al., in review). In contrast to published models of submarine fans, outcrop studies and inspection of seismic lines in Tierra del Fuego indicate that these CLC are not associated with canyons or other similar flow conduits. The channel deposits show complex internal geometries characterized by multiple, large-scale, lenticular cut-and-fill structures. Individual depositional units bear abundant fossil leaves and phytodetritus, and show recurrent lateral transition of current sedimentary structures that reflect fluctuations in flow velocity. These features suggest that the deposits of the CLC of the Cabo Viamonte beds accumulated from sustained and pulsating gravity flows generated by extraordinary fluvial discharges entering into the sea as hyperpycnal flows. The main objectives of this paper are to characterize the architectural elements, sedimentary facies, and facies associations of the hyperpycnal CLC systems from Tierra del Fuego and to discuss the sedimentary mechanisms involved in the generation of their complex fill.

2. Architectural elements

Based on the morphological, geometrical, and lithological features of the Cabo Viamonte beds, two architectural elements that include multistory channel and levee deposits are recognized. Multistory, hyperpycnal channels constitute a characteristic architectural element located at the base of the slope of depositional breaks in major sigmoid clinoforms. Maximum dimensions of individual channels are usually about 500 m in width and 40 m in depth. The channel fill is aggradational, with its basal and upper parts characterized by distinct lithologies. The basal part consists mainly of sandy conglomerates and pebbly sandstones, with subordinated large (up to 1 m in diameter) intraformational boulders, blocks glided from the channel walls, and slump deposits. The upper part of the channel fill is completely aggradational and consists of graded tabular beds, which onlap markedly the erosive channel margins (Figure 1). The levee deposits are wedge-shaped, with major thickness in the proximity of channel margins and thinning away distally. The levee deposits have a minimum observed lateral extension of 250 m in one direction away from the channel and consist mainly of rhythmically interbedded heterolithic mudstones and fine sandstones, with subordinated isolated, slump deposits (Figure 1). The wedge-shaped levee deposits can be amalgamated with the tabular upper channel fills.

3. Facies and facies associations

Three facies associations are recognized in the hyperpycnal CLC of the Cabo Viamonte beds.

a)- The lower channel fill facies association consists of very thick packages, up to 15 m thick, of sandy conglomerates, pebbly sandstones and sandstones, with a coarsening-thickening trend followed by a fining-thinning trend. Each package consists of poorly sorted, structureless sandy conglomerates and pebbly sandstones with diffuse, parallel to undulating lamination, climbing dunes, and cut-and-fill structures. Thick packages of massive sandstones and pebbly sandstones are usually not separated by distinct rheological boundaries. Commonly, sedimentary structures are laterally transitional within the same horizon and vertical successions of these horizons are also not separated by sharp rheological boundaries. Contorted bedding, convolute lamination, and dewatering structures, such as pillars and dish structures, are common. Occasional intercalated silty, fine-grained sandstone beds show soft-sediment deformation structures. In the pebbly sandstones, pebbles are usually imbricate and have their largest axis oriented normally to the flow direction. In the basal part of each package, slump deposits, resedimented boulders up to 1 m in diameter, and armored clasts aligned along diffuse bedding planes, are also common.

b)- The upper channel fill facies association consists of tabular, graded sandstone and mudstone beds, less than 50 cm thick, with sharp, slightly erosive basal contacts. The sandstones are structureless or normally graded with parallel lamination, ripple lamination, and climbing ripples. The mudstones are laminated and usually include high concentrations of plant detritus.

c)- The levee facies association consists of rhythmical successions of heterolithic, thinly bedded fine-grained sandstones and mudstones, in part bearing abundant carbonized plant debris. The heterolithic packages could be either sandstone- or mudstone-dominated. The fine-grained sandstone beds or laminae record climbing ripples, wavy bedding, or delicate, superb examples of lenticular bedding (Figure 1.3). The primary lamination in mudstones could be destroyed by intense bioturbation. Isolated horizons of slumped beds are occasionally intercalated within these heterolithic packages.

4. Depositional mechanisms and development of hyperpycnal CLC systems

Two stages are recognized in the development of the studied CLC systems. The first stage is related to hyperpycnal flows originated during the waxing period of discharge of rivers in flood, and is typified by the lower channel fill facies association and part of the levee facies association.

The lower channel fill facies association includes the coarsest deposits that, depending on sediment concentration and flow velocity, could be structureless or could bear complex, current sedimentary structures. The structureless pebbly sandstones and sandstones could have originated from sustained flows with constant “grain rain” aggradational deposition or through the collapse and “in mass” deposition of a high concentration of suspended load during sudden flow expansions associated to hydraulic jumps. The pebbly sandstones with parallel to diffuse, undulating lamination and armored intraclasts are interpreted as originated dominantly through basal traction transportation in dilute flows, with enough competence as to move pebbly intraclasts as bedload. The common presence of imbricate clasts and orientation of their largest axis normal to current direction also indicate bedload transportation. These depositional features could also be enhanced by irregularities of the sea bottom, e.g. those originated by the localized concentration of large resedimented boulders, up to 1 m in diameter, deposited at the base of the slope of channel margins. These inertial deposits, whose morphologies are similar to those of fluvial bars, form localized obstacles that could subsequently control the sedimentary features of successive deposits. If the flow is at full capacity and with high sediment concentration, after climbing the stoss side of the obstacle it experiences a hydraulic jump due to flow transformation at the lee side of the "bar". If the flow is not at full capacity and with dilute sediment concentration, the smaller intraclasts left behind in the tail of the flow move as bedload. While moving over the obstacle these intraclasts are forced to "copy" the undulating shape of the "bar" resulting in large dune structures. Continuous acceleration during the waxing state of the hyperpycnal flow, together with a diminishing sediment concentration, favors the origination of climbing dunes. The relative high angle of climbing (>30°) suggests prevalence of sediment fallout over traction in the generation of these climbing dunes. Rapid velocity fluctuations in the flow, particularly those initially characterized by sudden increase in velocity, produce erosive structures, usually U-shaped in transversal sections and asymmetric, flute-shaped scours in longitudinal sections. The erosive features are preserved due to rapid sediment filling of the scours during subsequent decrease in velocity of the flow. These cut-and-fill structures are observed recurrently in lateral and vertical transition to climbing dunes; the latter are commonly developed immediately down current of asymmetric scours.

The deposition of part of the levee facies association also occurs during the waxing period of the hyperpycnal flow and thus it is simultaneous with deposition of the lower channel fill facies association. Depending on the magnitude and duration of the hyperpycnal flow, the heterolithic successions within the levee facies association could form prograding (larger and more sustained flows) sand-dominated packages or retrograding (less important and less enduring flows) mud-dominated packages. Small-scale slumping is generated by gravitation instability during the enlargement of the levee.

The second stage includes the upper channel fill facies association and part of the deposits of the levee facies association, particularly those deposits comprising the finest sediment caliber. The fine-grained clastic deposits of both facies associations reflect accumulation during the waning period of hyperpycnal flows generated by rivers in flood. The upper channel fill facies association comprises thick, tabular successions of graded beds, usually with T_a-e Bouma divisions.

5. Conclusions

The main factors controlling the development of the sedimentary filling of the hyperpycnal CLC in the Cabo Viamonte beds are the pulsating and sustained character of the flow and changes in clinoform morphology, particularly those related to changes in the angle of the depositional slope.

Fluctuations in velocity resulting from the inherent pulsating character of hyperpycnal flows, control sediment concentration. The sustained character of the flow promotes the accumulation of thick, sedimentary packages with no distinct rheological boundaries and with recurrent and transitional complex, sedimentary structures.

Diminishing slope angles, particularly in areas located at the base of the depositional slope breaks, promote the generation of hydraulic jumps and the accumulation of inertial deposits, with external morphology similar to fluvial bars.

The lower channel fill facies association includes a family of sedimentary structures that is poorly known in the sedimentological record. The preservation of these structures requires a particular set of conditions, among which are of paramount importance the sustained character of the flow, high sedimentation rates, and marked variations of sediment concentration controlled by velocity fluctuations in the hyperpycnal flow.

Reference

Ponce J.J., Olivero E., and Martinioni D., 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. Schematic section showing main architectural elements of a hyperpycnal channel-levee complex (CLC). 1. Cut-and-fill structures (cf), climbing dunes (cd), and inertial deposits (id) of the lower channel fill facies association. 2. Tabular beds onlapping the channel margin of the upper channel fill facies association. 3. Levee facies association integrated by muddy heterolithics. Note well-defined lenticular bedding structure.
 

AAPG Search and Discovery Article #90079©2008 AAPG Hedberg Conference, Ushuaia-Patagonia, Argentina