Lethal Sandslides from Eolian Dunes By D. Loope, J. Mason, L. Dingus presented by Megan Simpson Geol 700 2/28/02

Introduction Fossils found in Central Asia’s Upper Cretaceous Djadokhta Formation (Mongolia) Early workers thought fossils were buried alive by drifting sand during violent wind storms, but none was recovered from sediment that showed evidence of being windblown This work shows calcitic zones within dunes allowed for pore water pressure to develop during major storm events which triggered slides Slides turned into viscous fluid, buried anything in its path

Ukhaa Tolgod site of 1000 skeletons and skulls of 20 species

Sandslides usually not a problem Normally, water drains from well-sorted sand Huge rainfall events are rare in the desert On a typical dune, sand accumulates to a critical angle which then avalanches down the slope (lee side) and comes to rest

Where did the calcite come from? Cross-strata with intergranular calcite are common within these sandstones Texture of sand in these formations ranges from coarse to very fine sand, calcite found only in fine/very fine layers Because there is no calcite source within the sand, calcium came from atmosphere (rainfall) Evaporation/transpiration of rain leads to precipitates (gypsum and calcite)

Rate of calcite build-up Because atmospheric accumulation of calcite is a very slow process, these dunes could have been stable for thousands of years Rates range from 0.1 to 51 g/m 2 per yr Calcite layer formed about 0.5 m below surface of dune slope

More calcite buildup Abundance of fine to very-fine sand enhanced likelihood of calcite buildup In dunes with larger grains, most rainfall moves through the sand and recharges the groundwater Here, because of finer grains, no recharge takes place The rate of infiltration is slower which allows for more evaporation Study showed that infiltrated rainwater tends to redistribute parallel to strata which adds to lack of rainwater getting deep into dune

Slope failures During a large storm event, water would buildup on top of the calcite layer below the surface and form a perched water table Calcite layer also reduced the porosity This increased the pore water pressure and caused a mass movement of the overlying sand

Slope stability calculation A continuous saturated zone develops above the calcite layer As pore water pressure increases, frictional strength is reduced

Conclusion During heavy rainfall, perched water tables developed on top of shallow calcite layers that were underneath dune slopes and caused massive slides These slides engulfed animals on the slopes, which became extremely well preserved Calcite shows dunes were stable for thousands of years Dunes were probably at least m high

Two conditions must be met: Need steep slopesShallow barrier to the infiltration of rainwater