Geomorphological studies along a transect from the taiga to the desert in Central Mongolia—evolution of landforms in the mid-latitude continental interior as a function of climate and vegetation

Abstract

Variations of drainage systems in the central Mongolian steppe was investigated along a N–S transect covering all zones of vegetation and precipitation from the taiga to the outer reaches of the Gobi Desert. Geomorphological studies supplemented by sedimentological, mineralogical (heavy minerals, lithoclasts) and chemical analyses (arenaceous deposits) resulted in the delineation of five lithological groups which were subdivided into 23 rock types using rock strength, geomorphological forms/geometry, lithology, clast size, sorting, stratification, rock contacts and grain shape. To demonstrate the relationship between landforms and the climatic as well as morphological processes, a process-product approach was taken and eight rock type associations were established. The transport regime along the slopes is characterized by mass flows that pass upslope into solifluction/gelifluction sheets and soil creep. Towards the thalweg, mass flows grade into coarse-grained gravel deposition of highly sinuous and braided fluvial streams. On vegetated high-altitude peneplains and mid-slopes, as well as scarcely vegetated desert steppe plains, unconfined flow prevails over confined flow. Arenaceous deposits of aeolian origin gave rise to dune fields and sand sheets. Chemical weathering is moderate in the steppe of the continental interior and the pH value of meteoric fluids is straddling around neutral. Due to the intermediate position of the steppe between polar and warm deserts, salt efflorescence and calcretes came into being. The major characteristics of the steppe depositional environments will be discussed and summarized in order to provide a key for the interpretation of paleosteppe settings in the ancient sedimentary record.

Introduction

Mongolia covers an area of 1,566,000 km² with only little more than 2 million inhabitants (Jargalsaikan, 1998) (Fig. 1). Throughout the Mesozoic and the Cenozoic, basins subsided into the basement rocks, especially in the southern parts of the country (Marinov et al., 1973). Quaternary beds have been under investigation in Mongolia for many years. These continental depositional environments record in an excellent way periodic and episodic climatic changes in this central Asian region which is characterized by the highest degree of seasonal contrast on Earth. Considering the global distribution of morphoclimatic zones of Tricart and Cailleux (1972), the study area in Mongolia forms part of the dry continental zone. Attribution to this morphoclimatic zone means that the average annual temperature is in the range of 0–10 °C and the mean annual precipitation lies between 100 and 400 mm (Hilbig, 1995). The northernmost area under consideration lies close to the periglacial zone with average temperatures well below the freezing point and an annual precipitation of more than 400 mm according to the data reported by Hilbig (1995)- (Fig. 2). According to the map of vegetation zones drafted by Hilbig (1995), the majority of the area under consideration forms part of the mountain forest steppe which passes northward into what is called the taiga and towards the south into the steppe sensu stricto which gives way further south to the desert steppe (Fig. 2).

The fluvial drainage systems have attracted much attention and were therefore intensively studied by many geoscientists (Schumm, 1977, Bristow and Best, 1993, Miall, 1996, Galloway and Hobday, 1996). Sedimentological studies have been conducted in numerous modern and ancient sedimentary environments. The steppe, however, a mid-latitude region which is transitional between the desert and the taiga across Central Asia, has attracted only little attention as far as the drainage systems and landforms are concerned. Only lakes were the target during some research operations (Grunert et al., 2000, Williams et al., 2001, Peck et al., 2002). Some papers have been published on the climatic evolution, dealing mainly with the interplay of glaciation and lake-level fluctuation in this central Asian region which has been secluded for so long (Lehmkuhl, 1998- see further literature thereunder). An overview of the drainage system in this area has not been given so far and a transect covering the various landforms of the steppe has not yet been worked through. Central Mongolia, showing the highest degree of seasonal climatic contrast on Earth, is an excellent study area for any investigation of steppe rivers. A north-south transect through Central Mongolia covers all zones of climate and vegetation and, thus, yields a full picture of the various types of the steppe and the various rock types which characterize its drainage systems (Table 1). Geographic studies focused on the present-day landforms and their sedimentary structures may help create a picture as close as possible of the paleogeography of continental depositional environments in ancient deposits (Table 1).