Landslide in Seymareh (Saidmarreh), Zagros fold-thrust belt, Iran

 

Figure 1 Saidmarreh Landslide in Saidmarreh, Iran, as seen by Landsat. The top of the Kabir Kuh anticline borders the slide's source area on the southwest. The debris from the avalanche flowed down the anticline's flank, across the Karkheh River, and across the valley floor. Some of the slide's contents were transported 14 kilometers (9 miles).

Luristan is a province of Iran in the southwest bordering Iraq. It is a mountainous region with peaks reaching up to 9000 feet, whose shape is shaped by simple, almost symmetrical folds in which the limestone bodies, which are strong and resistant, serve as feature-making rocks. The strata are arranged in the following order:

Figure 2 Stratigraphic column of Zagros Mountains.

A 15-kilometer-long slab of Tertiary limestone slid off the northern face of Kabir Kuh in The Zagros Mountains of southern Iran more than 10,000 years ago, causing the landslide that spanned two valleys and an intervening mountain, extending 20 kilometers from its source. An earthquake must have initiated the slide, but the limestone block was most likely previously undermined by the Saidmarreh River. The evidence does not support the idea of compressed air lubrication; instead, crushed marl and a sliding surface of gypsum bedrock could explain the long-distance motion.

The slide's surface is crisscrossed by ridges and troughs, as well as large grabens, which are mostly related to collapse over subsurface cavities caused by gypsum bedrock solution after the slide, both during out seepage of dammed lakes and, later, after the lakes were drained by erosion of the outlets. The surface features are similar to those found on drift-mantled stagnant glaciers or karst created by solution on limestone.

The sliding slab had a surface area of roughly 165 kilometers and was around 15 kilometers (9 miles) broad (64 square miles). At the bottom of the slope, debris from the fall crossed the Karkheh River and scattered across the valley floor. Some of the material on the slide had travelled almost 14 kilometers (9 miles).

The debris from the avalanche blocked the Karkheh River, forming a vast lake behind the dam. The lake remained open long enough for up to 150 meters of sediment to build upon its bottom (these sediments currently support several thousand acres of cultivated land). The dam was then breached, allowing the lake to erode a passage through it.

Figure 3 Google Earth image of the landslide area.

This is effectively a dip slope failure on a tectonic ridge, as the landslide broke off along an inclined bedding plane, as shown in the image above. The slip plane stepped from one bedding plane to the next, as is commonly the case, to utilize the weakest areas of the rock mass, which is predominantly limestone with some marls. According to reports, the maximum fall height was around 1600 meters (Harrison and Falcon, 1938).

It's made up of angular limestone blocks, some of which are large enough to be seen in Google Earth imagery: The extremely fragmented form of the deposit, as well as the vast transit distance, indicate that this was a high-velocity, high-energy landslide – a rock avalanche (sometimes called a sturzstrom).

Figure 4 Angular blocks of limestone to the left, along with sturzstorm feature to the right shown in Google Earth image.

The landslide stopped two rivers, resulting in the formation of two lakes, both of which have since dried up. However, the ruins of one of them may be seen on the south side of the landslide, where the deposited sediment produces rich farmland. The lake appears to have burst through the landslide debris, forming a channel that has since corroded. The current river has since found a new path away from the landslide mass. Close to the landslide, this lake deposit is 39 kilometers long and 150 meters thick.

Figure 5 Possible drainage network and lake deposit before and after the landslide.

The landslide's date is unknown, however, there is an ancient ruined Sassanid town bridge on the larger lake's bed. The Sassanid era lasted from 224 to 651 AD, thus the avalanche must have happened much earlier. (Shoaei and Ghayoumian, 1998) claimed date of 10,370+/-120 years BP.