JOURNAL OF SEDIMENTARY RESEARCH,VOL. 68, NO. 5, SEPTEMBER, 1998, P. 841–849 Copyright  1998, SEPM (Society for Sedimentary Geology) 1073-130X/98/068-0841/$03.00 THE FROLIKHA FAN: A LARGE PLEISTOCENE GLACIOLACUSTRINE OUTWASH FAN IN NORTHERN LAKE BAIKAL, SIBERIA STEFAN BACK, 1 MARC DE BATIST, 2 PAVEL KIRILLOV, 3 MANFRED R. STRECKER, 1 AND PIETER VANHAUWAERT 2 1 Institut fu ¨r Geowissenschaften, Universita ¨t Potsdam, P.O. Box 601553, 14415 Potsdam, Germany 2 Renard Centre of Marine Geology, University of Gent, 9000 Gent, Belgium 3 Institute of the Earth’s Crust, SB RAS, 664033 Irkutsk, Russia ABSTRACT: New high-resolution seismic reflection data off the NE shore of Lake Baikal indicate intimate relations between recurrent late Pleistocene glaciations onshore and lacustrine sedimentation processes offshore. The basin floors bordering the most extensively glaciated regions of the lake margin are characterized by large sublacustrine fans. Extending over 20 km in diameter, the Frolikha Fan is among the largest fans in northern Lake Baikal. Morphologically the fan con- sists of three parts: the upper fan, which is marked by a prominent single-leveed channel, the middle fan, consisting of smaller distributary channels (without levees) associated with convex fan lobes, and the lower fan, which is poorly developed and characterized by a transition of lobate sedimentary units into basin-plain or bottom-current channel deposits. The extremely steep morphology of the Frolikha Fan, internal seismic characteristics, and in particular the depositional geometry and seismic signature of the upper fan division indicate that the fan formed during glacial periods in a proglacial depositional environment. In fact, large moraines onshore Frolikha Bay document multiple Pleistocene glaciations down to and beyond the present-day shoreline into the lake basin. It is therefore very likely that the Frolikha Fan developed in response to the recurrent late Pleistocene advances of valley glaciers into Lake Baikal. INTRODUCTION Subaqueous fans are present in a variety of settings, from the deep sea to freshwater lakes. Similarly to their subaerial counterparts they can broad- ly be described as accumulations of sediments in the shape of fans or cones along slopes or basin plains (Mitchum 1985). In lakes, subaqueous (sub- lacustrine) fans form important depositional systems that are commonly fed by density underflows originating directly from influent rivers (e.g., Hou- bolt and Jonker 1968; Sturm and Matter 1978; Van Rensbergen 1996). In proglacial and terminoglacial lakes, this underflow activity is enhanced by meltwater effects (Ashley et al. 1985; Weirich 1986). Here, extremely high rates of sediment discharge can trigger rapid accumulation of large glacio- lacustrine fans (e.g., Gustavson et al. 1975). In this study we focus on a large sublacustrine fan off the formerly glaciated NE shore of Lake Baikal, which is interpreted to be related to glacial outwash. Lake Baikal (Figs. 1 and 2) is the world’s deepest (1620 m) and most voluminous (23,000 km 3 ) lake. The drainage basin of the lake (Fig. 2) covers approximately 570,000 km 2 , with over 300 rivers delivering 60 km 3 /yr of water (Galaziy 1993). During the Pleistocene, large parts of this basin were glaciated, with the center of glaciation in the higher mountain ranges around northern Lake Baikal, and in the adjacent North Angara region (Fig. 2). Numerous glacial deposits in the immediate vicinity of the present lakeshore (e.g., Mats 1993) document that Pleistocene glaciation was not restricted to highlands; instead, valley glaciers advanced down to the shore, and locally even beyond the present-day shoreline (Back and Strecker 1998). This observation suggests that recurrent Pleistocene glacial advances into the Baikal basin strongly affected sedimentation processes and resulting facies patterns in northern Lake Baikal. In 1995 and 1996 two seismic surveys were carried out on northern Lake Baikal in order to gain a better understanding of the relation between late Pleistocene glaciations and the lacustrine sedimentary record. About 1500 km of high-resolution seismic profiles were acquired (Fig. 3) using a CEN- TIPEDE-sparker seismic source (frequency range 150–1500 Hz, when op- erated at 500 J) and a single-channel streamer. Because of low-frequency disturbances the effective bandwidth was constrained to 500–2000 Hz. The data were digitally recorded on a DELPH2 system and subsequently pro- cessed using PHOENIX VECTOR software. In this article we present the results of the interpretation of new onshore and offshore data of the Frolikha region, at the NE shore of Lake Baikal (Fig. 3). The onshore data provide detailed information on the glacial his- tory of the study area. The offshore data give a first insight into the lacus- trine record off a former glacial outlet, and allow us to investigate the reaction of the lacustrine depositional system to recurrent Pleistocene gla- ciations onshore. GEOLOGICAL FRAMEWORK The Cenozoic Baikal Rift developed at the southern margin of the Si- berian platform in structurally heterogeneous high-grade metamorphic and granitic basement (Fig. 1). The western margin of the rift consists mainly of Proterozoic amphibolites and gneisses (Melnikov et al. 1994). The east- ern rift flank is dominated by extensive Paleozoic granite intrusions (e.g., Barguzin batholith), partly covered with Mesozoic volcanic and sedimen- tary units (Ermikov 1994). Large faults bound the presently active rift on its western and eastern sides, although the major border faults are generally located at the western rift margin. This causes a pronounced structural asymmetry of the rift, which led to an asymmetric pattern in the Cenozoic rift-basin fill (Hutchinson et al. 1992; Moore et al. 1997). The structural asymmetry of the rift is also very well expressed in the distribution of Pleistocene glacial landforms and deposits in the North Bai- kal Basin (Fig. 4). On the structurally segmented western rift shoulder, Pleistocene glacial advances toward the lake basin were limited by a fault- trellis drainage pattern; only at Tyya valley glaciers did reach the present lakeshore (Fig. 4). In contrast, on the less faulted eastern rift flank, structure and drainage conditions allowed the development of numerous consequent streams providing thruways and accommodation space for recurrent valley glaciations. Thus, the glaciers on the eastern flank could advance beyond the present-day lakeshore into the lake basin in numerous locations (Fig. 4) and influence depositional systems in this sector in an important way. In the following sections, we focus on the region of the former glacial outlet at Frolikha, which serves as a model for the other Pleistocene glacial outlets at the Baikal NE coast onshore as well as offshore. THE FROLIKHA AREA—GLACIAL GEOLOGY AND GEOMORPHOLOGY Pleistocene glacial landforms and deposits dominate the morphologic and stratigraphic setting in the onshore areas of Frolikha Bay (Figs. 5 and 6). Additionally, two paleo-shorelines, 10 and 4 m high, parallel the present shoreline. The 10-m-high lake terrace I has a radiocarbon age of 32 ka (Logatchev 1989), the 4-m-high lake terrace II is dated at 13 ka (Back and Strecker 1998); the deposits associated with both paleo-shorelines are un- conformable with nearshore glacial deposits (Fig. 5). There are two main generations of moraines pre-dating lake terrace I. Situated 40–60 m above the present lake level, the older generation (Mo- raines I) comprises the highest and most extensive moraines in the study