Evolution of the Lake Kivu Rift, East Africa: A Magmatically-Active Extensional Mixed Siliciclastic/Carbonate System

Scholz, Christopher; Wood, Douglas; Zhang, Xuewei; Mburu, Danson

Lake Kivu is one of the ancient great lakes of the western branch of the East African Rift, straddling the border of Rwanda and the Democratic Republic of the Congo. It is nearly 100 km in length, has a maximum width of ~45 km, and is more than 440 m deep at its deepest point. This hydrologically-open rift-lake is set in a humid-tropical climate, and the Ruzizi River, its sole outlet, is a major hydrologic input to Lake Tanganyika. Lake Kivu has a uniquely stratified water column, which is highly charged with dissolved CO2 and methane, the former due to magmatic degassing. The Nyiragongo and Nyamuragira volcanoes of the Virunga chain are among the most active in Africa, and this magmatic system is responsible for discharging gas and hydrothermal waters directly into the lake water column. The resulting solute load facilitates the deposition of lacustrine carbonates not only in Lake Kivu, but also along the margins of Lake Tanganyika downstream.

In February and March 2012 514 km of single- and multi-channel seismic reflection data were acquired in the Rwandan waters of Lake Kivu. The 24-fold multichannel seismic (MCS) data were acquired aboard a modular research vessel, using a 600 m-long hydrophone streamer and single 40 cubic inch airgun. These data are augmented by a set of high-resolution CHIRP seismic reflection data and suite of sediment piston cores, also acquired on the Rwandan side of the lake.

MCS data revel that the extension in the East Kivu basin is largely accommodated along a major N-S striking, east-dipping boundary fault along the eastern edge of Iwawa Island, and which extends for ~40 km along the length of the basin. Intrabasinal normal faults commonly displace the deep lake floor, and influence location and behavior of active sublacustrine channels that deliver siliciclastic sediments to the deepest parts of the basin. The deepest sedimentary reflections observed on the new MCS data are 1.2-1.5 km below lake floor, near the center of the basin and boundary fault. Crystalline basement is not observed in these deepest areas however, suggesting the presence of a substantial sedimentary section below the imaged strata. Three main depositional sequences are observed within the MCS data set and are defined by pronounced angular unconformities which in many cases extend across the entire width and length of the East Kivu basin. These surfaces suggest multiple episodes of exposure and lake level lowering, likely due to volcanic activity, extensional deformation and climatic changes. The latest exposure surface developed in the late-Pleistocene, and likely correlates to the previous lake level low stage that persisted prior to volcanic damming by the Virunga volcanoes north of the lake, and which produced a rise in lake level of nearly 400 m. Several sublacustrine volcanic cones are imaged in the northern part of the study area near the city of Goma, which pose a significant hazard to the high-density riparian population.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013