Todd M. Thornburg

Sedimentation in the Chile Trench: Depositional morphologies, lithofacies, and stratigraphy

The depositional bodies of the Chile Trench (trench fans, the axial channel, sheeted basins, ponded basins, and axial sediment lobes) control the spatial distribution of modem lithofacies in the basin. Sheeted basins (south of 41°S) are presumably fed by closely spaced submarine gullies that approximate a line of source of sediment supply along the base of the slope. Trench fans (41°S−33°S) are built at the mouths of major submarine canyon systems which act as point sources of sediment supply. The axial channel follows the northward gravitational gradient, draining the distributary networks of trench fans into the longitudinal transport system. Down-gradient (northern) fan lobes are severely dissected by erosional processes; evidently, periods of proximal deposition alternate with periods of massive sediment remobilization and progradation of the axial dispersal system into more distal environments. Channelized basins in the canyon-mouth areas yield to sheeted basins (depositional surface maintains an axial gradient) or ponded basins (depositional surface is strictly flat) in inter-canyon areas. Tectonic disruption of the oceanic basement can locally augment the axial gradient and stimulate flow channelization, or reverse the gradient and induce sediment ponding. A large, margin-parallel sediment lobe is built at the base of a high axial escarpment near 33°S, where the axial channel crosses a transverse discontinuity at the convergent plate boundary. Five lithofacies are defined by Q-mode factor analysis of sediment textures and hydro-dynamic structures in 27 cores from the Chile Trench. The Channel facies (thick, amalgamated sand, massive to laminated or cross-bedded) is deposited by high-energy processes within the coarse-grained bedload of turbidity currents; it forms in distributary and axial channels. The Levee facies (rhythmically bedded, internally structureless, graded sand and graded silt) is deposited from concentrated sediment suspensions that quickly lose momentum during channel spillover; it forms on channel flanks, although constructional levees are not always present. The Basin-1 facies (more complete Bouma sequences, both upper and lower flow regime structures) forms in ponded basins where flows are confined by a high-relief, seaward trench wall. The Basin-2 facies (graded and laminated silt, lower flow regime structures) forms in low-energy environments, such as interchannel areas, distal basins, trench walls, and elevated topographic features. The Contourite facies (silt and sand laminations winnowed from hemipelagic muds and distal turbidites) is best developed in sediment-starved basins where geostrophic currents are constricted and accelerated between the steep inner and outer trench walls. The trench wedge records a coarsening-upward sequence as the oceanic plate migrates toward and into the trench during plate convergence, and becomes more proximal to sediment sources along the base of the continental margin. Near canyon mouths, prograding trench fans drive the axial channel seaward into the trench wedge, and the coarsening-upward sequence is truncated by a time-transgressive erosional unconformity. Abandoned axial channel deposits are carried landward beneath prograding fans to record a fining-upward sequence above the basal unconformity. Channel migration and lobe aggradation may produce fining- and coarsening-upward sequences on depositional fan lobes, but sequences on the erosional lobes are fragmented by numerous truncation surfaces.

Submarine-fan development in the southern Chile Trench: A dynamic interplay of tectonics and sedimentation

Submarine fans are well developed in the southern Chile Trench, from 33°S to 41°S latitude. SeaMARC-II side-scan sonar and seismic reflection records image steep erosional escarpments, as much as 400 m in relief, extending seaward across the trench basin from the mouths of submarine canyons. The scarps bisect trench fans into paired lobes of contrasting morphology where gravity flows either follow or oppose the gradient of the axial trough. Fanlobes are depositional and constructional up-gradient (south) from the canyon mouths. They are composed of aggraded channel/levee complexes, smooth and conformable sediment drapes, and crescentic levees rimming the headwalls of erosional scarps. Fanlobes are carved and dissected by erosional processes down-gradient (north) from the canyon mouths. They exhibit amalgamated lag pavements, composite sediment lobes, longitudinal furrows, braided channels, and canyon-mouth bars. Thick, massive-to-laminated sand and gravel with abundant scour surfaces were sampled from the erosional fanlobes, whereas fine-grained turbidites with expanded hemipelagic intervals typify the depositional fanlobes. The texture and composition of the sediment supply, the onshore climate, and the tectonic perturbations of the axial gradient affect the morphologic development of trench fans. Stratigraphic intervals recording periods of intensified fan erosion and progradation of the axial channel are manifested by channeled, high-amplitude seismic facies: reflective lenticular bodies, truncation and scour surfaces, planar-amalgamation, and sigmoidaccretion structures. The severe erosional dissection of down-gradient fanlobes and the northerly encroachment of the axial channel are best developed in the surficial strata of the trench basin. Extensive sediment remobilization and efficient longitudinal transport sculpted the present fan surface and are correlated with the last glacial maximum. The trench fans may have been deposited on progressively steeper trench gradients, because the buoyant Chile Rise migrates northward as it subducts beneath the Andean continental margin. Fan distributary channels, axial channels, slump scars, and erosional gullies are largely localized along structural features. Normal faults propagate through the sedimentary cover and create elongate depressions on the sea floor that capture the high-velocity mainstreams of turbidity currents. Orthogonal fault sets within the deposits of the trench basin, which parallel the spreading and transform structures of the extinct Pacific-Farallon Rise and the Chile Rise, are evidently reactivated during subduction by flexure of the oceanic basement along the outer wall of the trench basin. Uplifted thrust ridges, generally restricted to a narrow zone along the base of the deformation front, are dissected by distributary channels, and channel courses are locally deflected seaward of these propagating structures. Transform-oriented basement ridges, associated with strike displacements of the axial channel and vertical faults in the trench basin, may accommodate renewed strike-slip motion as they enter the subduction zone and thereby influence the debouchment points of submarine canyons to the trench basin.

Sedimentation in the Chile Trench: Petrofacies and Provenance

ABSTRACT The Chile Trench study area extends over 2,000 km along the Andean continental margin, from 23°S to 42°S latitude. The QFL composition of Chile Trench sands encompasses the average compositions of modern sands from strike-slip, continental arc, oceanic backarc, and oceanic forearc types of active margins (Valloni and Maynard 1981), and spans the complete spectrum from dissected to undissected magmatic arc settings (Dickinson et al. 1983). Trench sands along North Chile, Central Chile, and the glaciated archipelago of South Chile are derived from a more plutonic source region, or dissected magmatic arc; these sands exhibit abnormally low lithic and calcic plagioclase content, but high quartz and alkali feldspar. In Central Chile, the plutonic provenance is correlated with an ab ence of Quaternary volcanism. Along the Chilean archipelago, Pleistocene glaciation has carved into the magmatic roots of the Andes, effecting arc dissection in spite of active volcanism. In arid North Chile, the volcanic debris of the High Cordillera is trapped onshore in longitudinal forearc basins, allowing an exaggerated contribution from crystalline rocks of the Coast Range. The light and heavy, mineral and lithic composition of trench sands is described by four ideal petrologic assemblages, isolated by Q-mode factor analysis. The Basic Magmatic Arc petrofacies (volcanic lithics, calcic plagioclase, olivine, clinopyroxene, and orthopyroxene), the most chemically unstable and least differentiated assemblage, is derived exclusively from basalts and basaltic andesites in the Quaternary volcanic arc of South Chile. The Acidic Magmatic Arc petrofacies (volcanic lithics, alkali feldspar, biotite, hornblende, and magnetite) is derived from andesitic to rhyolitic volcanics, and granodioritic plutons emplaced during various Mesozoic and Cenozoic magmatic episodes in the history of the Andean margin. The Metamorphic Magmatic Arc petrofacies (mica, alkali feldspar, hlorite, actinolite, epidote, and blue-green hornblende) is derived from ferromagnesian rocks altered to the greenschist facies in the basal marine section of the Andean eugeosyncline and in the Paleozoic basement. The Cratonic Block petrofacies (quartz and the accessory minerals andalusite, garnet, and tourmaline) is derived from mature platform sediments, late-stage granitic intrusions, and high-grade regional metamorphics associated with the South American craton; it is most prominent along northern Chile where the continental crust is thickest and presumably truncated by subduction erosion. Actual petrofacies in the Chile Trench are typically mixtures of the four ideal petrofacies. Their compositions can be related to 1) the lithology and erodibility of the source terranes, 2) the distribution and composition of modern arc volcanism, 3) the prevailing climate in the provenance region, and 4) the structure and morphology of the continental margin and trench. In many cases contemporary volcanism inundates and dominates the petrologic assemblage, diluting contributions from all other source-rock types and forming a low-diversity petrofacies. At high latitudes, where glaciation was extensive, a variety of source-rock terranes were actively denuded and petrofacies of high diversity were deposited in the adjacent trench. The linear frequency of sediment-supply points along the base of the continental slope controls the spatial heterogeneity of petrofacies on the depositional surface in the trench basin. The most homogenous petrofacies, derived from major submarine canyon systems and extensive subaerial drainage basins, may extend hundreds of kilometers along-margin due to axial transport processes within the trench.