Morphology and Curvature of Delta Slopes in Swiss Lakes: Lessons for the Interpretation of Clinoforms in Seismic Data
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Sedimentology (2001) 48, 661±679 Morphology and curvature of delta slopes in Swiss lakes: lessons for the interpretation of clinoforms in seismic data ERWIN W. ADAMS*, WOLFGANG SCHLAGER* and FLAVIO S. ANSELMETTI *Institute of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands (E-mail: [email protected]) Geologisches Institut, Swiss Federal Institute of Technology ETH, Sonneggstrasse 5, CH-8092 Zurich, Switzerland ABSTRACT Seismic surveys were conducted and bathymetric data obtained from four alpine lakes in Switzerland. The curvature of the delta slopes was analysed with mathematical equations. Linear or exponential pro®les are observed, representing planar or concave morphologies respectively. Planar pro®les are interpreted to represent sediment that rests at the angle-of-repose. The slope angle of these pro®les shows a correlation with sediment calibre. Exponential pro®les do not show a clear correlation between sediment calibre and slope angle; they do not rest at the angle-of-repose, and different kinds of sediment can rest at the same slope angle. At the transition from lower slope to toe- of-slope, the exponential equation fails to predict the present-day morphology. The toe-of-slope lies above the predicted trend. This is attributed to a drastic increase in turbidite deposition that provides additional sediment and raises the basin-¯oor pro®le above the predicted trend. The breaks between delta plain and slope are sharp, re¯ecting an abrupt change from transport by river ¯ow and waves to gravity-driven transport. In these lakes, the base-level ¯uctuations relative to supply are small and insuf®cient to alter this sharp topographic break. The absence of sigmoidal pro®les on the Swiss deltas is attributed to the high rate of progradation coupled with small ¯uctuations in base level. Keywords Deltas, glacial lakes, lacustrine sedimentation, morphology, slopes, Switzerland. INTRODUCTION break is ®xed (Adams et al., 1998) are not disturbed at the shelf break by this interference, The explanation of the origin of seismic re¯ection and will therefore have a concave or planar slope patterns is a recurrent problem in seismic inter- morphology (Fig. 1). In the present report, this pretation. An important issue is the interpretation hypothesis is tested by studying the morphology of the geometry of clinoforms in seismic sections and curvature of delta slopes in Swiss lakes. in terms of style of deposition. Adams & Schlager Since Gilbert (1890) published his classical (2000) proposed a hypothesis that a sigmoidally model of a prograding delta, numerous studies curved slope pro®le results from the interference have focused on the depositional architecture of between wave-dominated shelf transport and deltas (e.g. Postma, 1990), the processes acting in gravity-driven slope transport on the upper por- and on delta regimes (Nemec, 1990a; Prior & tion of the slope. This interference might be the Bornhold, 1990) and the in¯uence of sediment result of base-level ¯uctuations. Strongly pro- grain size on deltaic processes (e.g. Orton & grading systems or systems in which the shelf Reading, 1993). Kenyon & Turcotte (1985) were Ó 2001 International Association of Sedimentologists 661 662 E. W. Adams et al. Fig. 1. Schematic cross-sections of prograding deltas. Sigmoidal pro- ®les represent regular planar or concave-upward curvatures whose upper parts have been disturbed at the shelf break during base-level ¯uctuations. Wave-dominated transport on the shelf interferes with gravity-driven transport on the slope. (A) Rapidly prograding deltas exceed the ¯uctuations in base level and have concave-upward curva- tures in cross-section. (B) Fluctu- ation in base level exceeds progradation rates, resulting in sigmoidal pro®les. among the ®rst to model delta fronts numerically slope morphology of lacustrine deltas and to and to compare results with the geological record. study sedimentary processes acting in delta and Extensive modelling of the stratigraphic sequence basin environments; they can be viewed as geometry of clinoforms has been carried out (Ross analogues for small ocean basins (Hsu & Kelts, et al., 1995; Pirmez et al., 1998; Driscoll & Karner, 1985) that are easy to monitor and study. Study- 1999). However, quantitative and empirical stud- ing glacial lakes has the following advantages: ies on slope morphology have received less 1. Alpine lakes, formed by glacial erosion, are attention. ®lled with sediments at the end of a glacial period Alpine lakes in Switzerland (Fig. 2) are ideal and during interglacial periods; sedimentation systems within which to analyse the subaqueous Fig. 2. Map of Switzerland showing the distribution of lakes and major rivers. The boxes indicate the lakes studied in this report. Ó 2001 International Association of Sedimentologists, Sedimentology, 48, 661±679 complete mixing in winter. in turbidity currents (Sturminfusion & of Matter, oxygen-rich 1972). surface waters entrained ponded basin ¯oor. the lake basins. much to the total amountlateral of sediment delivered input to scree-apron points, deltas (Nemec, which 1990b) are do formed not by in¯ux. contribute These input points provide1890) most are of formed the in sediment front of theout¯ows. main tributaries. steep lateral mountain slopes and axial deltas or of geomorphological parameters): is more or less similarconsidered (see here. Table The 1 morphology forKelts, of a these summary 1976), basins lakes only before those theAlthough W of sediments Holocene wereof the age deposited last glacial periods into are subsequent and interglacial sedimentation these periods. during thepresent ®nal condition to stages glacial erosion1984). of In valleys general, and however, these basins owe their Ó is variable (forThe genesis a of discussion, alpine lake see basins Finckh inSETTING Switzerland submarine slopes is discussed. cance ofslope these morphology lacustrine isreviewed, slopes examined the as and in¯uencein models the of of and signi®- sedimentmorphology on calibre is on this examined, depositional the environment processesand are acting earthquakes) is well known in these areas. tions. setting, such as major windsince and 1870. currentof direc- theation. lake-level Average position daily, monthly have and been yearlyrapidly values recorded prograding delta sequences. rates are high in these alpine settings, producing 2001 International Association of Sedimentologists, 6. 5. 4. 3. 2. 1. In the present paper, the origin of delta4. slope 3. 2. Thermal strati®cation occurs in summer, The lakes are oligotrophicThe as main part a of these lakes result consists of of a ¯at Along the steep lateral mountain slopes, Distinct alluvial Gilbert-type deltas (Gilbert, Alpine lakes have an elongate shape with The recent history (including major slides There is good control on the `oceanographic' There is a good record of lake-level ¯uctu- urm glacial period (Finckh & et al. Sedimentology , Morphology and curvature of delta slopes , 48, Table 1. Geomorphological parameters of four Swiss lakes. 661±679 Length Width Depth Surface Volume Altitude Quaternary sedi- Major Major Average water (km) (km) (m) (km2) (km3) (m) ment thickness (m) out¯ows in¯ows Delta ¯ow (m3 s)1) Lake Uri* 11 1±3 200 21 2á9 434 120±200 Branch of Reuss Yes 45 Lake Muota Yes 19 Lucerne Lake Brienzà 14 2±3 260 30 5á2 564 350±550 Aare Aare Yes 33 Lutschine Yes 19 Lake Thun§ 18 2±3 215 48 6á6 558 200±300 Aare Aare No ± Kander Yes 40 Lake Constance±$ 45 7±14 250 450 49á4 396 170 Rhine Rhine Yes 224 * Data from Finckh et al. (1984). Data from Siegenthaler & Sturm (1991). à Data from Matter et al. (1973). § Data from Matter et al. (1971). ± Data from Muller & Gees (1970). $ Data from Muller (1966). 663 664 E. W. Adams et al. 7. Strong thermal winds (6±11 m s)1) blow bandpass ®ltered (1400±6500 Hz) and printed from the alpine range (Foehn winds). This is not with a graphic recorder (EPC 1086). The un®l- the dominant wind direction but is one of the tered analogue seismic signal was converted by strongest wind systems in the Alps. the seismic processing unit into digital format at a sampling rate of 42 ls and stored on 8 mm Exabyte tapes in SEG-Y format. GPS co-ordinates were recorded and stored into the trace header. METHODS On a workstation, the digitally recorded data were processed using processing software for Seismic pro®ling SPW Macintosh. Navigation was performed by map High-resolution seismic pro®ling was conducted and GPS readings. with a 3á5-kHz single-channel pinger containing four piezoelectric transducers. This pinger was loaded between two parallel in¯atable hulls and RESULTS pulled at a distance of 30 m from an in¯atable Zodiac boat. Shot intervals were set at 0á5s, The four lakes studied are Lake Uri (Urnersee), triggered by a seismic processor (Octopus Marine which is the south-eastern branch of Lake 360). After A/D conversion, the signal was gained, Lucerne (VierwaldstaÈttersee; Fig. 3A), Lake Fig. 3. Bathymetric maps. (A) Map of Lake Uri with the Reuss delta in the south and the Muota delta in the north. Note the tracklines of the seismic pro®les. (B) Map of Lake Brienz with the Aare delta in the north-east and the Lutschine delta in the south-west. Note the trackline of the seismic pro®le. Contour lines are given in metres above sea-level. Ó 2001 International Association of Sedimentologists, Sedimentology, 48, 661±679 Morphology and curvature of delta slopes 665 Brienz (Brienzersee; Fig. 3B), Lake Thun (Thu- Lake Uri nersee; Fig. 4A) and Lake Constance (Bodensee; Fig. 4B). Seismic surveys were conducted on Lake Uri (Fig. 3A) is 11 km long, 1±3 km wide Lake Uri and Lake Brienz; bathymetric data were and has a maximum depth of 200 m. Two main obtained from Lake Thun and Lake Constance. rivers, the Reuss in the south and the Muota in This presentation of results begins with the the north, provide most of the sediment in¯ux description of seismic and bathymetric data, and make distinct deltas in Lake Uri.