Marine Geology 346 (2013) 31–46

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Marine Geology

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Glacial and tectonic control on fjord morphology and sediment deposition in the Magellan region (53°S),

Sonja Breuer a,⁎, Rolf Kilian a, Dirk Schörner b, Wilhelm Weinrebe b, Jan Behrmann b, Oscar Baeza a a University of Trier, Department of Geology, FB VI, Behringstraße, 54286 Trier, Germany b GEOMAR Helmholtz Institute for Ocean Research, Wischhofstraße 1-3, 4148 Kiel, Germany article info abstract

Article history: In the Patagonian Andes erosion by temperate Pleistocene glaciers has produced a deeply incised fjord system in Received 12 October 2011 which glacial and non-glacial sediments were deposited since the Late Glacial glacier retreat. So far, fjord bathym- Received in revised form 6 July 2013 etry and structures in the sediment infill were widely unexplored. Here we report the results of an investigation Accepted 23 July 2013 of morphology and sediment characteristics of a 250 km long fjord transect across the southernmost Andes Available online 1 August 2013 (53°S), using multibeam and parametric echosounder data, and sediment cores. Subaquatic morphology reveals fi Communicated by J.T. Wells continuity of on-land tectonic lineaments mapped using eld and remote sensing data. Our results indicate that glacial erosion and fjord orientation are strongly controlled by three major strike-slip fault zones. Furthermore, Keywords: erosion is partly controlled by older and/or reactivated fracture zones as well as by differential resistance of reflection seismic the basement units to denudation. Basement morphology is regionally superimposed by Late Glacial and Holo- high-resolution bathymetry cene subaquatic moraines, which are associated to known glacier advances. The moraines preferentially occur sediment sequences on basement highs, which constrained the glacier flows. This suggests that the extent of glacier advances was tectonic setting also controlled by basement morphology. Subaquatic mass flows, fluid vent sites as well as distinct Late Glacial Magellan region and Holocene sediment infills have furthermore modified fjord bathymetry. In the western fjord system close Chile to the subaquatic terraces occur in 20 to 30 m water depth, providing an important tag for proglacial lake level during the Late Glacial. © 2013 Elsevier B.V. All rights reserved.

1. Introduction morphostructural provinces: (1) fjords west of the GCN within the Patagonian Batholith, (2) the Gajardo Channel east of the GCN incised The morphology of the previously glaciated southernmost Andes into a Jurassic–Cretaceous metamorphic basement, (3) the western (52°–54°S, Fig. 1A) is characterised by an up to 2000 m deep fjord sys- located in the Magallanes fold-and-thrust belt and (4) the tem, mainly formed by Pleistocene glacial erosion. Orientation of these eastern Seno Skyring and in the Magallanes foreland basin. fjords is more or less controlled by a neotectonic strike-slip fault system For the surveys we used parametric echosounding and high-resolution (Glasser and Ghiglione, 2009; Waldmann et al., 2011), related to the multibeam bathymetry. Additionally, twenty sediment cores were recov- oblique convergence of the Antarctic and South American plates ered from the fjord sediments. (Diraison et al., 1996; Ramos, 1999; Lodolo et al., 2003; Costa et al., 2006; Oliver and Malumian, 2008; Ghiglione et al., 2010). However, 2. Regional setting only a very crude bathymetry existed for most of the fjords, and very lit- tle information has been published regarding subaquatic moraine sys- The major orogeny of the southernmost Patagonian Andes started in tems and other sediments (Kilian et al., 2007a). the Middle to Late Jurassic, forming an island arc and associated back- We have investigated the bathymetry as well as sediment types and arc basin. Extensional faulting and subsidence lead to the evolution of fjord orientations. Present day glaciers originate from the 200 km2 large theRocasVerdemarginalbasin,withaninfill of Jurassic volcaniclastics ice cap (GCN; Fig. 1C), located at the ice divide of and Cretaceous turbidite sequences. In the late Cretaceous and the the southernmost Andes at 53°S (Schneider et al., 2007). The investigat- Palaeogene crustal shortening and thickening caused the closure of ed fjords are aligned along a 250 km long E–W fjord transect that the Rocas Verde basin, and the formation of the Magallanes fold-and- crosses the main lithological units or morphostructural provinces of thrust belt (Fildani et al., 2003; Menichetti et al., 2008; Oliver and the Andean mountain belt (trending N–S; Fig. 1B). We focus on four Malumian, 2008; Ghiglione et al., 2010; Romans et al., 2010). The areas to the west and to the east of the GCN, covering different Magallanes foreland basin developed east of the Andean orogen, filled with an up to 7 km thick sediment sequence (Ramos, 1989; Ghiglione – ⁎ Corresponding author. Tel.: +49 651 2014670; fax: +49 651 2013915. et al., 2009). From Neogene onwards the region was affected by NW E-mail address: [email protected] (S. Breuer). SE sinistral wrench tectonics (Diraison et al., 1996). The left-lateral

0025-3227/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.margeo.2013.07.015 32 S. Breuer et al. / Marine Geology 346 (2013) 31–46

Fig. 1. A) Southern South America with recent tectonic and volcanic setting (Stern and Kilian, 1996), and the Magallanes Basin with its isopachs (Ramos, 1989; Diraison et al., 1996). B) Geological and tectonic features of the Magellan region (SERNAGEOMIN, 2003; modified). C) The investigated 250 km long fjord transect across the Patagonian Andes. The boxes show the key areas that are described in more detail in the text and other figures.

Magallanes–Fagnano fault zone (MFZ; Fig. 2) represents the major structures and drag folds are due to releasing or restraining bends in transform fault system (Lodolo et al., 2003). Some minor NNE trending the fault system (Lodolo et al., 2003). right-lateral strike-slip faults can be interpreted as associated R′ Riedel During the (LGM) an 80 km wide ice field ex- shear zones (Maffione et al., 2010). Elongated pull-apart basins, horst tended from the GCN to the west and east (Kilian et al., 2007a). It and graben structures, pressure ridges, uplifted and faulted flower formed part of a large (2500 km N–S) Patagonian Icefield (Glasser and S. Breuer et al. / Marine Geology 346 (2013) 31–46 33

Fig. 2. False colour Landsat ETM+ satellite image from 2000 of the Magellan fjord region between 52°S to 53.3°S with morphological lineaments representing the three main strike-slip fault directions (MFZ, SCFZ, GFZ) and their associated trendlines. The orientation of all faults and trendlines are plotted in a rose diagram. The fold axes and thrust fault derived from the official geological map (SERNAGEOMIN, 2003; modified). Epicentres of relevant earthquakes in this region. After USGS Earthquake Hazards Program.

Ghiglione, 2009). Extent of the ice cover is well documented in the area 12.6 and 11.8 kyr BP (e.g. Kilian et al., 2007a). Pollen records from the of Seno Skyring and Magellan Strait (Benn and Clapperton, 2000; GCN area indicate an evolved Magellanic rainforest after 11.2 kyr McCulloch et al., 2005a; Kilian et al., 2007a). Between 18 and 15 kyr (Lamy et al., 2010). BP a major ice recession led to the formation of extended proglacial Seen in a global context, the Chilean fjord system belongs to the lakes, like the Seno Skyring, Seno Otway and the fjord systems of the Southern Fjord Belt (Syvitski et al., 1987). The present-day GCN ice Magellan Strait (Glasser et al., 2008; Rabassa, 2008). Proglacial lakes cap reaches 1700 m (a.s.l.) and feeds temperate valley and tidewa- were successively transformed to fjords due to the global sea level rise ter glaciers (Warren and Aniya, 1999; Benn and Clapperton, 2000; (Kilian et al., 2007b). The orientation of most fjords follows the structur- Rabassa et al., 2008; Koppes et al., 2009). The Chilean tidewater gla- al geological lineaments (Glasser and Ghiglione, 2009; Waldmann et al., ciers extend into the warmest settings, in terms of air and water 2011). The Seno Otway, Seno Skyring and the Eastern Strait of Magellan temperatures (Dowdeswell et al., 1998). In temperate fjords, like cross-cut the fold-and-thrust belt following extensional structures in the southernmost Chilean fjord system, the postglacial deposition (Diraison et al., 2000) oriented perpendicular to the Andean orogen. is dominated by high organic input (Syvitski and Shaw, 1995; Howe The fjords and bays carved out by the glaciers (McCulloch et al., et al., 2010), which can be as well measured in lake sediment cores 2005a) act as depositional environments for glacial, glaciomarine, and fed by streams coming from the GCN region (Breuer et al., 2013). glaciolacustrine sediments. The sediment from the GCN-derived glaciers enters the fjords mainly Five different deglacial ice retreat stages (A to E) are evident in the as hypopycnal flows (Fig. 3B). Several Holocene tephra layers from vol- Magellan region (Clapperton et al., 1995; McCulloch et al., 2005b; canoes of the Austral Volcanic Zone (AVZ; Stern and Kilian, 1996)form Rabassa, 2008). In the western fjord systems the related moraines are important age markers in sediment cores of the investigated fjord area in general subaquatic, like that of stage D in Seno Skyring and stage E (Kilian et al., 2007a), with the most prominent one originating from the within the Gajardo Channel (Kilian et al., 2007a). In the western 4.15 kyr BP Mt. Burney (Figs. 1, 2)eruption(Kilian et al., 2003; Stern, Magellan region the glaciers retreated to present-day extents between 2008). 34 S. Breuer et al. / Marine Geology 346 (2013) 31–46 S. Breuer et al. / Marine Geology 346 (2013) 31–46 35

3. Material and methods 3.4. Structural and lineament analysis

3.1. Core data Lineament analysis includes terrestrial erosional incisions, fjord ori- entations, as well as distinct, straight lines in the morphology which Stratigraphies and sedimentological properties deduced from piston may represent fractures and tectonic faults. We mainly interpreted and gravity cores were used to improve the interpretation of sediment GeoCover Landsat satellite images, recorded in 2000 by NASA, with a layers seen in the seismic profiles. The sediment cores include: CG-1 pixel size of 14.25 m. The Landsat 7 ETM+ comprises the bands 7, 4 (Baeza, 2005; Kilian et al., 2007a); Sky-1 (Kilian et al., 2007a,b; Lamy and 2 and the mosaic is projected in UTM coordinates in the World Geo- et al., 2010). For cores BA-1, LO-1, SG-1 and MD07-3126 a chronology detic System WGS 84. For the structural overview map (Fig. 2) only the was developed based on tephra chronology, 14C dating from macroplant main linear surface features were taken into account, like fjord valleys, remnants (Poznan Radiocarbon Laboratory; Table 1)and210Pb dating coastlines and fracture zones. Straight continuous landforms are related (Table 2; Appleby and Oldfield, 1992). All 14C ages were converted to to strike-slip faults (Glasser and Ghiglione, 2009). For the detailed maps calibrated ages before present (BP 1950) using the Calib 6.0.1 software of the Seno Glaciar (Fig. 3A) and the Gajardo Channel (Fig. 1B) the same and SHCal04 curve for the Southern Hemisphere (Stuiver and Reimer, principles were applied. Smaller faults, fractures or trendlines were 1993; McCormac et al., 2004; Stuiver et al., 2011). All depicted ages analysed on the ice-polished bedrock. Ground-truthing of most faults are means of 1-sigma values. was done during field work in the Seno Glaciar area and in the Gajardo area in 2001. We also included information, like fold axis and thrust 3.2. Acoustic data faults, from the geological map 1:1,000,000 (SERNAGEOMIN, 2003) and major strike-slip faults determined by Glasser and Ghiglione Along the fjord transect 1600 km of echosounder profiles and (2009). A total of 642 lineaments were mapped and plotted in three dif- around 500 km of multibeam data were recorded, processed and ferent rose diagrams (GEOrient; version 9.5.0). interpreted. We used the Parametric Echosounder System (SES 2000) from Innomar (Wunderlich and Müller, 2003), which provides high 4. Results and variable low (4–12 kHz) frequency signals at water depths of 0.5 to 800 m with a vertical resolution of less than 5 cm. The pulse length 4.1. On-land morphology ranges between 0.08 and 1 ms, and the beam width is 1.8 (Wunderlich and Müller, 2003). The fjord bathymetry was calculated from the high fre- Remote sensing data and field mapping show three sets of first- quency signal. To obtain optimal visualization of sediments, we tested low order fault zones (Fig. 2): (1) The sinistral strike-slip Magallanes Fault frequency signals between 4 and 12 kHz to obtain an optimal trade-off of Zone (MFZ) trending 136°. Northward of the Strait of Magellan this penetration and resolution. This allowed a penetration of up to 55 m. fault fabric is less pronounced (Fig. 3); (2) A sinistral fault system However, in some areas gas in the sediment hampered penetration, oriented NNW (165°) along the continental margin northward until at resulting in transparent layers (Fader, 1997). The ELAC-Nautic Seabeam least 50°S. In the study area it appears primarily to the east of the GCN 1180 multibeam sonar system was used for high-resolution seafloor and parallel to the Swett Channel. Thus, we name it the Swett Channel bathymetry in selected areas. The system is designed for water depths Fracture Zone (SCFZ; Fig. 3); (3) A NNE-trending fault system (44°) up to 600 m. However, deeper basins can be imaged with a reduced around the GCN, running parallel to the northern Gajardo Channel swath width. Using a frequency of 180 kHz, it is able to record 126 single (Fig. 6). It is termed, therefore, the Gajardo Fault Zone (GFZ). These par- depth values with a swath of max. 153°. ticularly dextral strike-slip faults are associated to the MFZ (Maffione et al., 2010). 3.3. Data editing and processing 4.2. Fjords to the west of the GCN (Swett Channel and adjacent bays) The echosounder data were edited using the post processing soft- ware ISE (version 2.9) from Innomar. A sound velocity correction was The multibeam-based bathymetry indicates up to 350 m water applied by using several CTD records taken in the fjord system. Acoustic depth in the Swett Channel and other parallel fjords further to the velocities of water-rich sediments may vary by ±2% compared to east. Most subaquatic structures and canyons are related to the SCFZ marine water (Hamilton, 1979). In our case, comparison of the depth (Fig. 3A). The geology of the Swett Channel region is characterised by al- of the prominent 4.15 kyr BP tephra reflector in the echosounder pro- ternating zones of granitoid rocks, mylonitic rocks and orthogneisses files and related sediment cores indicated sound velocities around (Fig. 3A). The mylonites (Menichetti et al., 2008) crop out along two 1500 m/s. The most important layers were digitised and surfaces of NNW-trending zones parallel to the SCFZ direction, and their easy erod- them were calculated by using different geostatistical interpolation ibility leads to the formation of canyons and fjords. The granitoids, resis- models. The ‘Ordinary Kriging’ method was used for restricted areas, be- tant to erosion and denudation (Breuer et al., 2013), form a distinct cause it relies on the spatial correlation structure of the data to determine NNW-trending ridge east of the SCFZ between the mylonitic zones. the weighting values. For the extended Seno Skyring with less coverage of This ridge formed a morphological barrier capable of damming the seismic lines the ‘InverseDistanceWeighted’ method (IDW) was used, GCN glaciers and hampering their westward flow.Onlynarrowpas- which weighted the nearby data points. The multibeam data were sages cross this morphological barrier. The bathymetric high at the processed at the IFM-GEOMAR (Kiel, Germany). This included checking southern end of the Swett Channel (Fig. 3A, B) separating it from up and editing of the navigation, removing of erroneous measurements to 340 m deep basin (Quenca Luca) is part of the granitoid rock ridges. and artefacts by automatic and manual cleaning, conversion of recorded One of the major palaeo-glacier streams coming from the Glacier travel times to positions and depths by complete ray-tracing through Noroeste migrated first southward through the Swett Channel, before the water column taking into account measured sound velocity, depth crossing into the Luca Basin, and flowing westward into the Seno Glaciar profiles, and calculation of digital terrain models. basin (red arrows in Fig. 3B).

Fig. 3. A) Georeferenciated ortho-image (Schneider et al., 2007) combined with high resolution bathymetry. Morphologically analysed fault and trendlines, core locations and profiles are shown. Moraines, deltas and fluvial sediments are distinguished within the Quaternary sediments. The rose diagram shows the orientation of faults and trendlines in this region. B)False colour Landsat ETM+ satellite image from 2000 and black-and-white ortho-image (Schneider et al., 2007). The bathymetry is derived from a geostatistical interpolation (Ordinary Kriging). The white dots show the localities of the SG-1 and MD07-3126 core. The red dashed lines indicate the former glacier flow directions. The suspension load and transport direction can be seen clearly in the ortho-image. 36 S. Breuer et al. / Marine Geology 346 (2013) 31–46

Table 1 14C and tephra ages for the piston cores LO-1 and BA-1.

Core depth Material Lab. ID 14C age (yr BP) Error (±yr) Calibrated age (yr BP) Sed.-rate (cm/kyr) References

LO-1 760 Plant remain Poz-15817 870 40 730 40 This paper BA-1 157 Plant remain Poz-10379 1800 30 1660 50 This paper 420 Mt. Burney Tephra 3860 50 4150 60 This paper

In the south-eastern sector of the Bahia de los Glaciares (Fig. 3A) with current activity. The uppermost sequence has a draped channel three glaciers enter this fjord area at present. Fig. 4A shows a cross sec- fill pattern with subparallel internal reflectors documenting suspension tion between the glacier tongues of Bahia de los Glaciares along the an- sedimentation. cient glacier flow until Seno Glaciar (A–A′; Fig. 3A). The profile crosses Bahia Arévalo (Fig. 4C) is a shallow (5–6mwaterdepth)smallbay several well-preserved subaquatic moraine systems, faults, basement located 2.1 km NNW of the mouth of the river coming from Glacier highs and intervening small sediment basins (Fig. 4A). The glaciers Noroeste (Fig. 3A). It was investigated by echosounding and by drilling could only erode two narrow and shallow passages into the above de- (core BA-1). The 8.8 m long piston core documented a continuous scribed granitoid ridge. The passages constricted the glacial streams organic matter bearing clayey sedimentation throughout the past and thus hindered the westward flow during the Neoglacial advances, 5 kyr. The age constraints include the 4.15 kyr Mt. Burney tephra at so that moraine formation occurred particularly in these narrow chan- 4.2 m core depth and a calibrated 14C age of 1.7 kyr BP obtained from nels (Fig. 3A; multibeam-derived bathymetry map). In the echosounder macroplant remnants found at 1.6 m core depth (Table 1), indicating profiles the moraines show typical crag-and-tail structures with chaotic relatively constant sedimentation rates of 1 mm/yr. Below 5 m core internal reflectors (Fig. 4A). In small depressions on the top younger, depth (approx. 5 kyr BP), a transition towards a coarser grained clastic stratified sediments with a draped channel fill pattern were deposited. fluvial sedimentation is recorded, probably documenting a less elevated A very pronounced left-lateral strike-slip fault (SCFZ) is characterised coastline which enabled the formation of a shallow lake. This suggests by a deeper fjord incision. coastline uplift by at least 10 m after ~5 kyr BP (Kilian et al., 2011). Sedimentation in the Swett Bay, west of the GCN, is affected by the The echosounder profile in Fig. 4C is next to the BA-1 core location. inflow of up to 500 m3/s fresh water from two large rivers. One of The reflectors of the uppermost sequence are even parallel and have a them drains Lago Muñoz Gamero with its 810 km2 catchment. The draped pattern. The lowermost lake facies with variable and partly other one is a meltwater river originating from the proglacial lake of coarse sediment shows clinoformal reflector patterns (Fig. 4C), typical Glacier Noroeste (Fig. 3A). The latter river carries a high suspension for prograding deltaic sediment systems (Stoker et al., 1997). At the load into the fjord, causing south-westward hypopycnal flow within top of the lowermost deltaic sequence there is an erosional unconformi- the low-salinity surface water layer over several kilometres along the ty. The upper deltaic facies II filled the depressions and levelled the Swett Channel (Fig. 3A, B) and into the adjacent bays (e.g. Bahia subaquatic morphology. The deltaic sequences have an Early to Mid- Lobo). The wind-controlled distribution of the suspended load can be Holocene age. The tephra layer is conformal on the deltaic sequence, seen in several satellite images and aerial photographs (Fig. 3B), and and is overlain by an organic-rich, marine sediment. At present a river was also observed during our cruises at different seasons of the year. enters the bay and satellite images show a recent subaquatic delta. The on-land deltaic fan continues a few hundred metres into the fjord, Bahia Lobo is a larger fjord bay (3 × 0.8 km) 2.5 km south of the as can be seen in the high-resolution bathymetry (Fig. 3A). Related to glacial river coming from the Glacier Noroeste (Fig. 3A). In the aerial the above described glacial clay plumes, relatively high sedimentation images parts of the hypopycnal flow that directly enter the bay can be rates of 1.0 (BA-1 sediment core) to 1.2 mm/yr (SG-1 sediment core) seen. The whole 8.17 m long LO-1 piston core retrieved at 83 m water were found in basins within the western GCN fjord system. The depth consists of silty clays with low amounts of organic material. A cal- echosounder data also show up to 30 m thick sediment deposits ibrated radiocarbon age of 0.73 kyr BP was obtained from macroplant (Fig. 4B) in basins between subaquatic ridges and/or moraines, and in remnants at 761 cm core depth (Table 1) indicating very high sedimen- shallow coastal bays or platforms. tation rates of 10 mm/yr during the past centuries. Fig. 4B shows an echosounder profile across the Swett Bay into the Swett Channel. Three seismic sequences can be differentiated. The basement is characterised by chaotic internal reflectors and shows a 4.2.1. Seno Glaciar, Seno Icy and Bahia Beaufort pronounced morphology. The basin of Swett Bay is separated from a During the last Glacial Maximum there was an extended south- basin in the Swett Channel by a granitoid ridge. Both basins are filled westward ice stream from the GCN along the Swett Channel into Seno with N20 m of well stratified sediments, interpreted as ice distal Glaciar, Bahia Beaufort and Seno Icy, where it merged with the large deposits. This indicates sedimentation under more dynamic conditions westward-migrating ice stream of the western Strait of Magellan north of Tamar Island (Figs. 2, 3A, B). The bathymetry of Seno Glaciar does not show pronounced ridges. The fjord includes a N5 km wide sed- Table 2 iment basin with a water depth of 520 m, which was filled by N50 m 210Pb ages for the core SG-1. The calibrated gamma lines (DL1a standard) of 210Pb, 214Pb, thick sediments during the Late Glacial and Holocene. Sediment thick- 214Bi and 137Cs have been measured at the AWI-Bremerhaven using standard methods fl (Appleby and Oldfield, 1992). ness is constrained by extrapolating seismic re ectors from the basin margin towards its centre where a thick IRD layer hampers acoustic 210 Depth (cm) Pb age Sed-rate (cm/yr) penetration. Two short sediment cores MD07-3126 and SG-1 could 1.5 0 also not penetrate this IRD layer. The 0.48 m long QASQ box core 4.5 11 0.27 (MD07-3126 obtained with RV Marion Dufresne in 2007) and the 7.5 44 0.09 10.5 81 0.08 0.56 m long gravity core (SG-1 obtained with RV Gran Campo II) were 210 14.5 92 0.36 retrieved from the basin. Six Pb ages give an age frame for the last 17.5 104 0.25 ~0.55 kyr (Table 2). The base of the silty to clayey sediment sequence 19.5 162 0.03 contains ice-rafted debris. 210Pb ages for the past 75 years indicate aver- Average sedimentation rate: 0.12. age sedimentation rates of 1.2 mm/yr and suggest that the massive IRD S. Breuer et al. / Marine Geology 346 (2013) 31–46 37

Fig. 4. Bathymetric profile (A–A′) from the Seno Glaciar to the eastern Bahia Glaciares (line in Fig. 5) showing a high seafloor relief with occurring moraine belts, basement highs, basins and faults. The detail figure shows the echosound profile of the moraine belt with its typical reflection pattern and sediment distribution. The echosound profile (B–B′) from the Swett Bay to the Swett Channel with a typical sediment succession of ice distal and organic-rich Holocene sediments. The echosound profile (C–C′) across the Bahia Arevalo next to the BA-1 sediment core showing two different deltaic facies separated by a hiatus and an organic-rich marine facies on top. layer was deposited during an early maximum of the Little Ice Age requires relatively constant shoreline elevations during the Late Glacial between ~0.6 and 0.5 kyr BP. and Early Holocene. This observation can be only explained, if the local Further downstream, the ancient ice flow turned to SSW, running glacial rebound was similar to the global sea-level rise for some 15 km in Bahia Beaufort and Seno Icy parallel to the GFZ (Fig. 1C). The millennia, especially between 14 and 9 kyr BP. This is compatible with crude bathymetry of Seno Icy indicates several ridges perpendicular to previous regional coast levels estimated by Kilian et al. (2007a) and the ancient ice flow, which is related to cross-cutting fractures of the Lamy et al. (2010). No clearly preserved subaquatic moraine remnants MFZ (Figs. 2, 3). A detailed bathymetry was obtained across this up to have been detected bathymetrically along the ancient glacier stream be- 600 m deep U-shaped fjord-valley (Figs. 1C, 5). In the north of this tween Swett Channel and the Seno Icy area, indicating a relatively fast bathymetrical map several subaquatic canyons are visible, parallel to and continuous glacier retreat in this sector. the MFZ and GFZ directions. They are more pronounced on the shallow fjord margins and may have been formed by fluvial erosion after the re- 4.3. Gajardo Channel gional ice retreat after 17 kyr BP (Kilian et al., 2007a), when the coast- line was much lower than at present, due to N100 m lower global sea The 55 km long Gajardo Channel (Figs. 1, 6) was formed by glacier level. The bathymetric data also show subaquatic terraces in 20 to streams originating mainly from the GCN and from mountains on western 30 m water depth on both sides of the valley (Fig. 5). Their formation (Kilian et al., 2007a). Along the ancient glacier flow direction 38 S. Breuer et al. / Marine Geology 346 (2013) 31–46

Fig. 5. A) False colour Landsat ETM+ image from 2000 combined with high-resolution bathymetric map of the Bahia Beaufort next to the Strait of Magellan, with morphologically interpreted fault directions. Submarine terraces occur in 20 to 30 m water depth which can be seen as well in the S–Nprofile (B) across the Bahia Beaufort.

the NNE-trending northern section of the Gajardo Channel stretches over fjord (680 m water depth) is located ca. 6 km southwest of the entrance 30 km from the Angostura de Témpanos at the GCN towards the Seno of the Gajardo Channel. A 1.73 m long gravity core (CG-1) was taken in Skyring (Fig. 6). the 250 m deep basin for which a fjord-parallel echosounder profile is Multibeam bathymetry and selected echosounder cross sections shown in Fig. 6. It is characterised by parallel reflectors with a draped (Fig. 6) show a U-shaped glacial valley with a total relief of up to channel fill pattern. The 4.15 kyr Mt. Burney tephra layer in 5.3 m 2000 m. The present day water depth is up to 680 m. On-land mapping depth in the core produces a distinct reflector within the Holocene in the north-eastern sector around the GCN, interpretation of aerial sequence. The underlying ice distal sediments (Fig. 6)showpounded photos, as well as false colour satellite images show NNE-trending ac- fill patterns with sub-parallel internal reflectors. The CG-1 core shows tive dextral strike-slip faults parallel to the GFZ direction. Faults parallel an overall clayey to silty sediment composition (Kilian et al., 2007a). to the MFZ (NW direction) have been also mapped in the field. The rose The upper 14 cm of the core has been dated by the 210Pb-method. diagram shows a third minor E–W trending fault direction shaping the These ages indicate high sedimentation rates of 1.3 mm/yr. Bahamondes area and Chandler Island. Parallel to this on-land fault A Late Holocene to recent debris fan was detected in the bathymetric zone several lineaments could be detected, which cross the northern map south of Chandler Island (Fig. 6). The associated steep (48°), hill Gajardo Channel diagonally (Fig. 6). slope on land shows clear evidence of a recent debris flow. The sub- The high-resolution bathymetry shows areas with irregular sub- aquatic deposits cover an area of 1.0 × 1.2 km and have a volume of aquatic ridges. Partly these are basement highs of less erodible rocks, around 80 million m3. Two major debris flow events are distinguished, which can be related to terrestrial features, and partly they are sub- indicated by the dashed lines in Fig. 6;B–B′. The most recent abrasion aquatic moraines. The latter are seismically characterised by chaotic to surface is not visible in the Landsat TM 345 false colour image of 1986, contorted reflectors (Kilian et al., 2007a). They were probably formed but was observed in the field in 1999. It is possible that the mass flow at around 14 kyr BP during Late Glacial glacier readvance Stage E is related to the M = 5.0 earthquake which occurred on 31.08.1996 (Fig. 6; McCulloch et al., 2005b; Kilian et al., 2007a). In front of the around 38 km to the SSW, or the 4.7 magnitude earth quake which oc- north-eastern entrance of the Gajardo Channel within the Euston curred on 19.11.1988 around 32 km to the SWW (Fig. 1C). Channel, an older subaquatic moraine system was likely being formed between 17.5 and 15 kyr BP during Stage D (Kilian et al., 2007a). To the north of the Angostura Témpanos and in the southward MFZ- 4.4. Western Seno Skyring oriented deeply incised fjord, moraines of the Little Ice Age have been previously detected (Koch and Kilian, 2005; Kilian et al., 2007a). The At present, the former proglacial lake Seno Skyring (Figs. 1C, 2) rep- high-resolution multibeam map shows several curved terminal moraines resents an estuarine fjord system with restricted connection to the Strait (Fig. 6), which are partly related to on-land moraine remnants. of Magellan through the Gajardo Channel in the west, and Seno Otway, The Gajardo Channel includes several basins (Fig. 6), which are sep- via the Fitz Roy Channel in the east. The E–W oriented fjord has a length arated by subaquatic moraine systems and/or bedrock ridges. The main of around 80 km and is up to 15 km wide. In the west Seno Skyring Holocene sedimentation took place in these basins and in small depres- passes into the Euston Channel, where its deepest part of the basin sions located on top of the morainal ridges. The deepest basin of the (660 m water depth) is located. Further to the east, sediment basins S. Breuer et al. / Marine Geology 346 (2013) 31–46 39

Fig. 6. Landsat ETM+ image combined high resolution bathymetric map of the Gajardo Channel. The two main fault and trendline directions are plotted in the rosediagram.Several moraine systems, basement highs, basins and one pronounced debris flow could be detected within the fjord bathymetry. The profiles (1–3, upper left corner) show the typical U-shaped form. The echosound profile (A–A′) shows the typical sediment succession of the isolated basins within the channel. The profile (B–B′) shows the suspected original seafloor, and the dashed lines indicating the assumed border between the two events.

are developed in-between perpendicular oriented ridges, which belong 2011;KS1minFig. 7), which was more resistant to glacial erosion. Sed- to the Magallanes fold-and-thrust belt. iment core ES-1 was retrieved at 78 m water depth in the 2.5 km wide A bathymetric map of the Seno Skyring (Fig. 1C) was generated by bay within U-shaped coastline of Escarpada Island. The core shows the geostatistical interpolation from the echosounder profiles shown in 4.15 kyr BP Mt. Burney layer at 3.2 m sediment depth, indicating sedi- Fig. 7. In some areas, e.g. the Euston Channel, not enough data were mentation rates of 0.78 mm/yr (Baeza, 2005). available for a good interpolation. Eastward of Escarpada Island the basin of the former Skyring glacier In general, the water depth decreases towards east. The 16 km2 incised the Magellanic fold-and-thrust belt (Figs.1,2,7), where Upper sized Escarpada basin with a maximum water depth of 440 m is located Cretaceous and Lower Tertiary rock units of the former Magellan Basin in the central sector of Seno Skyring. Echosounder profiles show a more crop out (SERNAGEOMIN, 2003). Erosionally resistant sedimentary than 35 m thick sediment fill with parallel reflectors, which belong to rock layers form N-S-trending ridges, which can be easily identified in Holocene and Late Glacial ice distal sequences. Escarpada Island has a satellite images, are subaquatically preserved. Some of them define U-shape (Fig. 1C), due to selective erosion of a shallowly northward small islands or peninsulas. The most important subaquatic ridge plunging fold of the Cretaceous Cerro Toro Formation (Romans et al., extends from Altamirana Bay southward to Riesco Island across the 40 S. Breuer et al. / Marine Geology 346 (2013) 31–46

Fig. 7. False colour Landsat ETM+ image with the N-S trending Magallanes fold-and-thrust belt. The main fold axes, faults and rock units are taken from the geological map (SERNAGEOMIN, 2003;modified). The multibeam bathymetry shows the submarine continuation of a weathering resistant fold limb. The white dashed lines show the bathymetry of the Seno Skyring derived from interpolated echosound data. The corresponding echosound routes are indicated by the yellow dashed lines.

whole Seno Skyring (Fig. 7). The ridge stands up to 210 m above the gla- sector from 76 m water depth. Four tephra layers with well-known cially abraded basement, and subdivides Seno Skyring into two main ages (Kilian et al., 2007a,b; Lamy et al., 2010) were identified. IRD layers basins. in this core indicate ice retreat as early as 17 kyr BP. The sedimentation Fig. 8 shows a coast-parallel echosounder E–Wprofile across Seno rates of 0.19 mm/yr throughout the Holocene and late Glacial are much Skyring, including the above described subaquatic ridge. Four different lower than further west. sediment sequences can be distinguished. A lowermost Sequence I of Fig. 9 shows a representative 6.5 km long echosounder profile ice proximal deposits (5–10 m thickness) with subparallel internal re- (A–A′) from the eastern part of the Seno Skyring. Local reflectors are flectors and partly chaotic reflectors documents coarse sedimentation disturbed, indicating gas within the sediments that could be derived (IRD), above the folded basement. Above this layer, a non-stratified Se- from deeper sources (Fig. 9A). This layer blanking mainly affects the quence II occurs in isolated lenses often related to fault-like structures deepest sequence of the ice proximal deposits. We suspect that the (Fig. 8). These may represent local subaquatic debris flow deposits gas ascends along faults from the subjacent hydrocarbon-bearing either triggered by fault activity, or by reworking of IRD deposits of Cretaceous and Lower Tertiary rock units of the Magallanes Basin. The the Late Glacial (Kilian et al., 2007a). We prefer an interpretation of lowermost ice proximal deposits have an uneven reflector signature; the wedge-shaped acoustically transparent bodies, lenticular in cross- in gas-influenced areas the reflectors are disrupted. The top of the se- section, as debris flows (Syvitski et al., 1987; Whittington and Niessen, quence has a subdued relief, but the transition to the ice distal sedi- 1997; Gipp, 2003; Howe et al., 2003). The well-stratified Sequence III ments is concordant. In general, the ice distal facies is characterised by further up-section is interpreted as ice distal deposits (4–10 m thickness) even sub-parallel reflectors, in some areas (from 2.0 to 2.5 km profile aboveabasalunconformity.Theuppermostsequenceconsistsofwell length) with a wavy parallel pattern. Occasional synsedimentary faults stratified, organic-rich Holocene sediments (2–8 m thickness) and in- cause offsets in the range of some decimetres up to 1 m. This indicates cludes the 4.15 kyr Mt. Burney tephra layer. The transition to the under- that some of the faults were active during the Holocene. In a few cases lying ice distal sediments is mostly concordant. The sediment thickness an erosional unconformity separates the Holocene from the underlying of each sediment sequence reflects basement morphology, with higher ice distal sediments. One pronounced reflector within the Holocene sediment thickness in basins. Faulting leads to block rotations of the base- represents the 4.15 kyr BP Mt. Burney tephra layer. In the SKY-E1 core ment (Fig. 8). The sediment cover is only partly faulted, with low displace- this tephra layer occurs in 1.18 m core depth with a thickness of ments. Most of these faults cause a sediment deformation and the minor 22 cm. Within the seismic profile (Fig. 9A) the tephra layer appears at faults die out upwards. Analysis of all seismic profilesacrossSenoSkyring around 1.5 m depth. indicates that Holocene sediment thickness decreases towards the east. A gas-related feature appears at 5 km of the profile and extends to- The shallow (b80 m water depth) eastern part of Seno Skyring is the wards the east from there. It is characterised by dome-shaped struc- area of the ancient glacier tongue. Core Sky-1 was recovered in this tures, which deformed and uplifted the whole overlying sedimentary S. Breuer et al. / Marine Geology 346 (2013) 31–46 41

Fig. 8. Echosound profile across the Magallanes fold-and-thrust belt with the Late Glacial and Holocene sediment succession and main faults cutting the sediments. Four different sediment types are distinguished due to their seismic character. Within the Holocene sediments the 4.15 kyr Mt. Burney tephra occur as a distinct reflector. sequence (Fig. 9A). Westerly-induced currents in the upper 70 m of the water-escape features occur in the profile as V-shaped structures water column (Kilian et al., 2007b) eroded the uplifted sediments by (Fig. 9B), refilled with sediment. The water-escape features start within bottom transport processes. These eroded sediments were deposited the ice proximal sequence (Fig. 9;B–B′), where coarser clastic sedi- further east, above the normally deposited Holocene sediments with a ments occur. After Syvitski (1997) these events are typical for ice prox- clear unconformity (Fig. 9A). The reflectors of the redeposited sedi- imal areas. Water-escape events may have been triggered by periodical ments have an oblique tangential pattern typical for prograding sys- earthquakes in this region (Mills, 1983), but the mechanisms are not ob- tems. In contrary to this, in debris flows no internal reflectors can be vious by analysing the morphologies (Morretti and Ronchi, 2010). Rapid seen (Fig. 9A). In the eastern part of the Seno Skyring the thickness of sedimentation can be also a trigger mechanism (Postma, 1983; Syvitski, the Holocene sequence is on average 10 m in basin areas, and thins to 1997). It is obvious that the water-escape structures only occur up to the 2 m at morphological highs. unconformable transition to the uppermost sediment sequence (Fig. 9B). This erosional unconformity cuts the reflectors of the Glacial II sequence 4.5. Seno Otway (at 5.5 km profile length). The overlying relatively thin (1.5 to 2 m thick- ness) Holocene sequence has sub-parallel reflectors and levels the surface Seno Otway (Fig. 9B) is an 87 km long and 30 km wide estuarine bay between the basin and the structural highs of the bedrock (at 3.3 km and former proglacial lake, connected to the Magellan Strait in the west profile length). by the Jeronimo Channel. Most of the measured seismic data were obtained in the north-eastern and central parts (Fig. 1). Where the Magellanic fold-and-thrust belt intersects Seno Otway, 5. Discussion pockmarks occur frequently. Strata below the pockmarks are strongly disturbed, likely due to gas expulsion. Here we concentrate our descrip- Cenozoic subduction and associated upper-plate deformation (Ramos, tion on the easternmost sector of Seno Otway (Fig. 1) to the northeast of 1999; Diraison et al., 2000; Polonia et al., 2007; Glasser and Ghiglione, the major pockmark field. In the echosounder profile (Fig. 9) four 2009) have strongly impacted on the basin and mountain topography, sequences are evident. The oldest one is weakly reflective and chaotic, and the structure and erodibility of rocks in the southernmost Andes. probably representing the bedrock or older glacial deposits (Glacial I; Our investigation of fjord and on-land lineament orientation also in- Fig. 9;B–B′). The transition to the ice proximal deposits is clearly visible dicate a predominant control of the erosion by a complex framework as well as the basin-like structure. In this sequence (till) the internal re- of at least three major fault zone systems, producing a complex fjord flectors are chaotic and contorted (DaSilva et al., 1997). Above, there is a network. In this framework we have attempted to identify other impor- concordant transition into well-stratified ice distal sediments. The inter- tant mechanisms that have modified on-land morphology and bathyme- nal reflectors are even parallel with a draped pattern and indicate try of the fjord system. These include formation of lateral and/or terminal homogeneous, fine-clastic sedimentation under fairly calm conditions. morainesaswellasdebrisflow deposits. Irregular morphological struc- This Glacial II sequence is secondarily deformed by water-escape struc- tures like basement ridges, bathymetric highs, or narrow passages pro- tures, which equally affected the underlying Glacial I sediments. The foundly influence fjord currents and related sediment transport. In the 42 S. Breuer et al. / Marine Geology 346 (2013) 31–46

Fig. 9. The echosound profile (A–A′) shows the typical sediment units in the eastern Seno Skyring. Important features like gas intrusion and redeposition are marked. The detail figure gives a close-up of the internal seismic structure of the redeposition feature. The echosound profile (B–B′) from the eastern Seno Otway with the interpreted sediment facies and their typical seismic character. Water escape structures occur between 4.0 and 4.5 km profile length. An erosional unconformity separates the ice distal deposits from the Holocene sequence. following chapters we discuss these aspects in the light of all the data are partly superimposed by a Quaternary detritus cover (Fig. 3A), presented. often represented by subaerial moraines or deltas. In some areas, like around Mt. Burney volcano (Fig. 1) the Holocene volcaniclastic deposits 5.1. Tectonic control mask the glacially shaped fjord morphology. Such postglacial deposits complicate the interpretation of the satellite image. A tectonic control on the fjord orientation has been previously pro- Fjord orientations are consistently linked to the main tectonic direc- posed by Glasser and Ghiglione (2009), based on the investigation of tions, as described above. Rock lithology, however, has a strong local in- fjord orientation and fault zones without bathymetrical information. fluence on landscape and fjord morphology. The basement of the Swett The detailed fjord bathymetry presented here, and the investigated Channel area is dominated by silicic magmatic rocks of the Patagonian relationship to mapped on-land tectonic lineaments, however, improve Batholith and the Tobifera Formation (SERNAGEOMIN, 2003), which the understanding of subaquatic erosion, sediment transport and mass are relatively resistant to glacial erosion. However, fracturing along movements. fault zones and tectonic lineaments greatly facilitates erosion, leading The rose diagram in Fig. 2 shows the three main directions of the to a fjord landscape with steep slopes. The bathymetric map shows sev- MFZ, SCFZ and GFZ fault systems. From the detailed maps of Swett eral subaquatic basement highs, which can be directly related to the Channel (Fig. 3A) and Gajardo Channel (Fig. 6) it becomes obvious subaerial morphology of continuing ridges along strike. These basement that the MFZ direction is present in each area, but the SCFZ direction highs and the subaquatic moraine systems subdivide the area into is lacking in Gajardo Channel area and vice versa. In the Swett Channel uneven-sized basins. area zones of mylonitic rocks occur parallel to the SCFZ, probably origi- The Gajardo region is mainly affected by dextral strike-slip faults nating from strike-slip faulting in Late Cretaceous or Tertiary times (Fig. 6) and some thrust faults, which belong to the Magallanes fold- (Menichetti et al., 2008). These present-day sinistral faults of SCFZ ori- and-thrust belt (Fig. 2). The uniform basement of the Gajardo Channel entation may represent inverted or reactivated older faults. area consists of allochthonous rocks of the Tobifera Formation, and Beside the SCFZ orientation no clear other preferred direction can be metamorphed sedimentary rocks of Devonian and Carboniferous age seen in the rose diagram of the Swett Channel (Fig. 3A). When taking a (SERNAGEOMIN, 2003). In contrast to the magmatic rocks of the closer look at the satellite image (Fig. 3A) it is obvious that almost every Patagonian Batholith, metasedimentary rocks are generally more erod- peninsula has its own main trendline direction. Whether this relates to ible (Fig. 1). Therefore the Gajardo Channel was formed as a uniform fabric genesis of the different rock units (e.g. Patagonian Batholith, U-shape glacial valley with less subaquatic basement highs, but was Tobifera Formation) separate in time and space, and a later juxtaposi- later bathymetrically modified by subaquatic moraine systems (Fig. 6). tion at terrane boundaries during the Tertiary, is not entirely clear. An Further to the east, the Seno Skyring is subdivided in two main improved interpretation would require extensive radiometric dating basins by the N–S trending folds of the Magallanes fold-and-thrust and structural mapping. The tectonically controlled erosion features belt (Figs. 1C, 7). These steeply dipping and weathering resistant rocks S. Breuer et al. / Marine Geology 346 (2013) 31–46 43 form typical ridges in the subaerial landscape (Fig. 7), which continue triggered by the same factors and the water-saturated soils enhance into the fjords. These structures and active faults may have played a the subaerial debris flows (Fig. 6B). role in creating very irregular coast lines (Figs. 8, 9A) and there is evi- dence that the Glacial and Holocene sediments in the fjord were affect- 5.3. Sediment structures in fjord basins ed by very young faulting (Fig. 9A). The locations of gas expulsion structures may also be controlled by faults and their activity. In general, we distinguish two main glacial units due to their seismic The whole research area is a tectonically active region, documented expression: the ice proximal deposits and the ice distal deposits. The by recent earthquakes, with epicentres shown in Fig. 2. Along the SCFZ heterogeneous composition of the ice proximal sediments often has northwest (Fig. 2) of our research area the epicentres of the three earth- no clear internal seismic reflection pattern. Ice proximal sediments quakes of magnitudes 5.3 to 6.6 are located. These events document occur in different thicknesses, but often the base of the units cannot active wrench faulting in this area. Other earthquakes of the last de- be clearly identified owing to the decreasing quality of the echosounder cades (Fig. 2) are also located close to the main fault zones, or within data with depth. Above a mainly concordant transition the ice distal de- the foreland fold-and-thrust belt. Generally, the earthquakes in south- posits succeed with a variable thickness. The ice distal sediments often ernmost South America and Tierra del Fuego are characterised by shal- fill depressions or small basins and smooth the submarine morphology low hypocentres (b40 km) and by intermediate magnitudes (M = (Figs. 6, 9B). In other cases, like the examples of the Skyring (Figs. 8, 9A), 36), with higher magnitudes along MFZ and including the Lago Fagnano the ice distal sediments closely follow the seafloor morphology. Their area (Lodolo et al., 2003; Waldmann et al., 2011). One of the recent earth- draped character suggests sedimentation under more tranquil condi- quakes in the GCN area may have triggered the large debris flow in the tions, like suspension sedimentation. The seismic character of the ice Gajardo Channel. distal sediments is given by continuously parallel to sub-parallel, medi- um to high-amplitude reflections (Cai et al., 1997; Stoker et al., 1997). In most areas the transition to overlying organic-rich Holocene sed- 5.2. Subaquatic moraines iments is conformal, like e.g. Skyring (Figs. 8, 9A) and Gajardo Channel (Fig. 6). In the Seno Otway (Fig. 9B) a hiatus occurs between the ice dis- In addition to the basement highs, subaquatic moraine systems are tal and the Holocene sediments. The hiatus contains an unknown responsible for the development of isolated basins and therefore for timespan and is associated with an erosional unconformity. Probably the specific sediment distribution and deposition. the hiatus marks the catastrophic outflow event of the former proglacial The cross section A–A′ through the Bahia de los Glaciares shows lake of the Seno Otway. The Holocene sequence in the entire research Neoglacial moraine ridges which were formed during the most extend- area is consistently characterised by even and parallel internal reflectors ed Holocene glacier advance at around 2.3 to 2.2 kyr (Arz et al., 2011). and a draped pattern, which is typical for suspension sedimentation. These moraines are preferentially located on top of basement highs In the larger embayments such as Seno Skyring and Seno Otway, we (Fig. 4A), suggesting that fjord morphology is also controlled by the ex- found a homogeneous and conformable sedimentation of both se- tent of glacier advances. Post-neoglacial sedimentation is restricted to quences. The highest sediment thicknesses were found in large basin- small depressions with a higher thickness (~11 m) or on top of the mo- like structures like e.g. in the 440 m deep Escarpada and in the 660 m raines with a lower thickness (~3 m). deep Euston basin within the Seno Skyring area (Fig. 1) and in the The stage E moraines in the Gajardo Channel (Fig. 6) are not located on 680 m deep basin of the north–north-eastern Gajardo Channel. In basement highs. They were formed on the nearly flat basement floor, like these depressions the reference layer of the 4.15 kyr BP Mt. Burney the other Neoglacial moraine systems in the Gajardo Channel. Within the tephra is overlain by up to 8 m of sediments (Figs. 4B, 6, 8). The Holo- irregular moraine belts sedimentation is mainly restricted to small basins cene sequence thins out to the basin margins. In most of the smaller ba- and depressions in the moraine system. The Late Glacial and the sins and shallower coastal bays of the inner fjord region the 4.15 kyr Mt. Neoglacial moraines show the same sedimentation characteristics. The Burney tephra layer appears at 1–3 m sediment depth. In the narrow draped character of the Holocene sediments and the predominant fjords and bays and in areas with high seafloor relief (basement highs hypopycnal sediment flow with following suspension sedimentation or subaquatic moraines) the sediments are not spatially continuous. lead to a uniform sediment deposition. Seismic character of the morainal systems is dominated by chaotic internal reflectors with a lower am- 5.4. Comparison with other fjord regions plitude and often characterised by strong sea bottom multiples (Carlson, 1989). Howe et al. (2003) described similar features from the The Chilean fjord system belongs to the world's southern fjord belt Kongsfjorden area, and Sexton et al. (1992) characterised moraines as (Syvitski et al., 1987; Howe et al., 2010). The southernmost Chilean structureless seismic bodies. fjords described here are located in an area with a temperate climate. Some small debris or mass flows were detected at the steep slopes of The annual mean temperature is about 5.7 °C at sea level (Schneider subaquatic moraines. Following Ottesen et al. (2008), Carlson (1989) or et al., 2007) and the annual precipitation rates are about 2000– Cai et al. (1997) we interpret these as glacigenic debris flows. These 8000 mm yr−1. The precipitation shows a positive correlation with alti- mass movements affected the older ice distal sediments and partly the tude in the GCN region and a negative correlation with the distance Holocene sediments. The older debris flows affecting the ice distal de- from the Pacific towards the steppe landscapes in the east. Despite the posits are embedded in the normal sediment succession. Slumping at relatively mild climate, the glaciers in the GCN region reach the sea morainal ridges and at sediments deposited at basement highs and level and form tidewater glaciers. The high precipitation rates lead to along the steep fjord walls is often caused by an unstable sediment con- high glacier accumulation rates in the GCN region (Möller et al., figuration due to rapid sediment accumulation (Solheim and Pfirman, 2007). In some cases the glacier retreat was stronger in the last years, 1985; Syvitski and Shaw, 1995; Stoker et al., 2010). The slumping pro- so that the glaciers do not longer calve into the fjords and the fjords cesses modified the original shape and structure of sedimentation. The are now under a glacio-fluvial regime. It is important to note, that the slumps and subaqueous gravity flows (Figs. 6B, 8) may be generated salinity of water at the glacier terminus has no influence on the calving by earthquakes (Gipp, 2003; Stoker et al., 2010) or by expulsion of bio- rates of the glacier (e.g. Venteris, 1999). genic gas (Fig. 9). The marine sediments are often unconsolidated, and The first order controls on glacimarine sedimentation named by when shaken by large earthquakes, they thought to move en masse Powell and Molnia (1989) are firstly tectonism, which creates accumu- (Powell and Molnia, 1989). High sedimentation rates create slope lation areas for glaciers and provides sources for glacial debris. From over-steepening, and often lead to gravitational mass transfer (Powell their convergent tectonic setting the fjord regions of Alaska and Chile and Molnia, 1989). In southernmost Chile the debris flows are likely are correlatable. Tectonism at convergent margins causes mountain 44 S. Breuer et al. / Marine Geology 346 (2013) 31–46 building and under favourable continental orientation to ocean water flocculation and a decreasing velocity (Zajaczkowski, 2008; Hill et al., masses and air masses, glaciations can be predominately accumulation 2009). The high freshwater input derived from glacial meltwater and controlled (Warren and Sugden, 1993). This setting allows glaciers in precipitation may affect the fjord water salinity, which will affect the SE Alaska and in Chile at relatively low latitudes to extend down to flocculation process and therefore the sedimentation rate (Cowan, sea level (Powell and Molnia, 1989). Furthermore, the tectonic move- 1992). From the Svalbard archipelago Pfirman and Solheim (1989) re- ments have a strong influence on the amount of sediment production, ported, that the suspended material extends up to 15 km away from and therefore for high sedimentation rates in the accommodation area the glacier front. These values are comparable to the Seno Swett area, (Hallet et al., 1996). For example, the tectonic uplift combined with lo- where the turbidity current can be traced for 12 km (Fig. 3B). cally fractured and metasomatised rocks creates continuously renew- Sedimentation architecture is strongly influenced by the fjord bed able sources for glacial erosion and debris production (Powell and morphology (e.g. Elverøi et al., 1983; Dowdeswell et al., 1998). In the re- Molnia, 1989). This is reflected by glacial erosion at the Bahia de los search area, where basins are separated by basement and morainal Glaciares and at the Swett Channel (Figs. 3, 4A). The investigated fjord ridges, like in the Gajardo Channel, Seno Glaciar and Seno Swett area, area is an excellent example how the tectonic setting controls the orien- the main sedimentation occurs in deep troughs and basins, and only a tation of fjord systems and in particular changes in flow direction thin sedimentary cover appears on subaquatic highs (Figs.4,6,8,9; through time (Gipp, 2003). Powell and Molnia (1989) describe from see also Syvitski and Shaw, 1995,orSexton et al., 1992) If the depres- the southeast Alaskan margin those active and dormant strike-slip faults sions are partially filled with Holocene clayey silt, it is an indicator and their alignments separate tectonostratigraphic terranes of the mar- that they trap suspended sediment (Powell and Molnia, 1989). Seismic gin. Thus major valleys and fjords are aligned with these fault systems sections from the Gajardo Channel (A–A′ in Fig. 6) and from Seno and other tectonic elements such as thrust faults. Pfirman and Solheim Skyring (Figs. 8, 9B) document this type of sedimentation. (1989) have shown that the tidewater glaciers from Nordaustlandet, Svalbard, follow topographic depressions. 6. Conclusions The second first order control mentioned by Powell and Molnia (1989) is the climate. It produces high rates of snow accumulation The on-land and subaquatic morphology of the investigated area in and/or high rainfall and glacier ablation, which cause high runoff and the southernmost Andes is controlled by three main strike-slip fault high sediment discharge. In southeast Alaska precipitation increases zone directions. These are the WNW trending Magallanes Fault Zone, with altitude, like in the southernmost Patagonian Andes. High freshwa- the Swett Channel fault zone (SCFZ), and the right-lateral Gajardo Chan- ter discharge entering the fjord systems of southeast Alaska and south- nel Fracture Zone, which is conjugated to the Magallanes Fault Zone ernmost Chile is due to a combination of high precipitation and glacial (Maffione et al., 2010). meltwater (Powell and Molnia, 1989). Second-order controls are com- The basement highs consisting of erosion-resistant rocks partly con- fl posed of the glaciers themselves and the marine- uvial interaction trolled the extent of glacier advances, and thereby the formation of the (Powell and Molnia, 1989). subaquatic moraine systems. The Late Glacial and Neoglacial moraine The fjords of southernmost Chile are dominated by siliciclastic depo- systems prevent a spatially continuous sedimentation, which is mainly sitional systems, and thus comparable to the fjords of Alaska (Powell restricted to isolated fjord basins. Due to the predominant suspension and Molnia, 1989). Sediment supply in the southernmost Chilean fjords sediment transport in the upper freshwater-dominated water column is carried by glacial meltwater, like in the fjords of Spitsbergen (Elverøi with wind-induced currents, the subaquatic geomorphology has no di- et al., 1980), of the north-western Barents Sea (Dowdeswell et al., rect influence on the sediment deposition. However, fjord morphology 1998), and of southeast Alaska (Cowan, 1992). In contrast, in the fjords may have controlled locally high salinity bottom currents and their of East Greenland and of the Antarctic Peninsula the high numbers of mixing with low-salinity surface water. This could have enhanced floc- icebergs play a more significant role in transport and sedimentation culation and settling of clay particles. Besides areas with stronger water (Dowdeswell et al., 1998). In temperate glacier setting like in Chile currents, like in the Strait of Magellan or directly connected fjords, and in SE Alaska the precipitation is an important factor in glacimarine where sediments could be eroded, the basement highs form sediment sedimentation (Cowan, 1992; Dowdeswell et al., 1998), while in more traps with increased sediment accumulation. Local sediment redistribu- fl river-in uenced fjords the circulation and sediment transport have tion processes like debris flows, water escape structures and gas expul- stronger ties to the hydrological cycle (Syvitski et al., 1987). sion along faults are important and may have been triggered by seismic fi Among the former processes ne-grained meltwater plume sedi- events in the southernmost Patagonian Andes. mentation is dominant in these temperate environments (Fig. 3B). Two types of sedimentation from meltwater plumes can be distin- guished: first, slow sedimentation from mainly fine-grained sediment Acknowledgements derived from the meltwater plume (Syvitski and Murray, 1981; Pfirman and Solheim, 1989); and, secondly faster deposition of the This study is funded by Grant Ki-456/11 of the German Research coarse-grained bed load of the glacier streams forming ice proximal Foundation (Deutsche Forschungsgemeinschaft, DFG). Jana Friedrich is 210 deltas (Fig. 3A; Dowdeswell et al., 1998). The ice distal sediments in thanked for Pb dating at AWI Bremerhaven. We also thank Marcelo our research area are mainly characterised by fine grained input, like Arévalo for his encouragement during the different cruises since 2002, silts and clay transported by glacial meltwater, a fact that is documented and the catchment survey. We are most grateful for sponsorship from in different sediment cores taken in the research area (Kilian et al., Gran Campo Transportes Ltda. We thank the reviewers and the editor 2007a,b). Such suspension-controlled deposits are reported from the for useful comments and suggestions, which sharpened our thinking fjord system of Svalbard during the Holocene (Elverøi et al., 1980; and helped to improve the paper considerably. Dowdeswell et al., 1998). Beyond the major depocenters of tidewater fronts and deltas, most sedimentation in the Chilean fjords is controlled References by the hypopycnal flows. Particle settling from turbid plumes in upper fi water layers is dependent on plume dispersal, turbulence, sediment Appleby, P.G., Old eld, F., 1992. Application of lead-210 to sedimentation studies. In: Ivanovich, M., Harmon, R.S. (Eds.), Uranium Series Disequilibrium: Application to concentrations and sizes (Syvitski et al., 1987). The suspension sedi- Earth, Marine and Environmental Sciences. Clarendon Press, London, pp. 731–778. mentation is enhanced by flocculation, agglomeration and pelletisation Arz, H., Kilian, R., Lamy, F., Kaiser, J., Baeza, O., Breuer, S., Caniupan, M., Möller, M., (Syvitski and Murray, 1981; Pfirman and Solheim, 1989). These pro- Schneider, C., 2011. New Constrains on Neoglacial Glacier Advances in the Superhumid Southernmost Andes (50–53°S) and Evaluation of Their Control Mecha- cesses are dependent on the salinity of fjord water. With increasing sa- nism Based on New Palaeoclimate Records and Modelling. INQUA Congress, Bern, linity the settling of suspended material is faster, due to an increasing Switzerland. S. Breuer et al. / Marine Geology 346 (2013) 31–46 45

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