GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 111-14
Presentation Time: 11:40 AM


LEPRE, Christopher J., Paleomagnetics Laboratory, Lamont-Doherty Earth Observatory, 61 Route 9W, PO Box 100, Palisades, NY 10964-8000,

Megaliths in the Lake Turkana Basin appear to have been constructed during the mid-Holocene, when lake level was several 10’s of m higher than today. However, not a lot is known about why certain places were chosen as build sites and how stone was procured. To help address these questions the paleogeographic context of one mid-Holocene megalith site (GeJi9) found at Lothagam, SW Turkana Basin, is examined. Outcrops exposed along the western Lothagam horst preserve evidence of shore-zone deposition. Northernmost outcrops comprise a poorly sorted assemblage of basalt gravels, ranging in size from pebbles to boulders. Basalt gravels form ~N/S trending beach terraces with east-facing risers that reach heights of ~1 m, are inclined at 15-45o, and consist of boulder clasts. These deposits are interpreted as the products of lakeshore waves eroding, sorting, and winnowing detritus derived from the columnar-jointed flood basalts of the western Lothagam horst. Risers and treads probably represent the normal upper limit of swash and the breaker zone, respectively. Southern outcrops adjacent to the GeJi9 megalith site preserve shoaling-upward, quarzto-feldpathic, sand-dominated successions. These begin with horizontally bedded mudstones accumulated in lake-bottom areas, followed by planar to tangentially bedded sand sets with oscillation marks that suggest deposition below a wave base in a lower shoreface environment, and end at beach bars and upper shoreface deposits comprised of lakeward-dipping, low-angle beds of gravelly sand. Bedding geometries, cross-strata directions, and dip slopes of Mio-Pliocene beds suggest shoaling was accompanied by progradation away from local tectonic structure. On a larger scale, lake water surrounded the north end of the western Lothagam horst, and the basalt outcrops were headlands. Successive risers and treads accumulated toward the east as lake level regressed. Strong wave energy was necessary to pile the boulders into meter-high risers, suggesting turbulent waters evoked by the winds of the Turkana Jet. Rationales for selecting a megalith-building site at Lothagam may have included a relatively quiescent lake margin and closeness to exposures of columnar basalt.