Identification of the prehistoric catastrophes at the Lajia Ruins using micromorphological analysis within the Guanting Basin,Minhe County,Qinghai Province

The Lajia Ruins is a large Neolithic settlement of the Qijia Culture that is dated to 4200–3950 bp within the Guanting Basin along the upper Yellow River. Archaeological excavations have exposed extraordinary pictures of prehistoric catastrophes that ruined this settlement and caused great loss of human life. The causation of the prehistoric catastrophes is an arguable topic in the field of environmental archaeology in China and over the world. Our fieldwork investigations indicated that the palaeo-ground of the Qijia Culture at the Lajia Ruins was broken by several groups of earthquake fissures. A layer of conglomerated red clay buried the Qijia Culture at the Lajia Ruins and filled in these earthquake fissures. Determining the macro- and microscale morphological features of the conglomerated red clay provides an alternative reference for understanding the cause of the prehistoric catastrophes at the Lajia Ruins. The macrofeatures of the conglomerated red clay has a rolling surface and wavy structure, with human bodies of the Qijia Culture, stones, loess soil clods, pottery shards, burnt earth, charcoal and ash from the palaeo-ground entrapped inside. And the micromorphological features of the conglomerated red clay is characterized by rolling and wavy structure along with no directional alignment of grains within horizons and a large numbers of pores with complicated shapes, including large regular pores that can be characterized as sub-round, elliptical and cloud-like pores, as well as small irregular pores. Thus, the conglomerated red clay is classified as dense mudflow deposits. And then, we can conclude that the large Neolithic settlement of the Qijia Culture at the Lajia Ruins was buried and devastated by immense mudflows that accompanied a major earthquake.